Fetal cardiovascular, metabolic and endocrine responses to acute hypoxaemia during and following maternal treatment with dexamethasone in sh
1 Department of Physiology, University of Cambridge, Downing Street, Cambridge CB2 3EG, UK
2 ICSM Endocrine Unit, Hammersmith Hospital, London W12 0NN, UK
Abstract
In sheep, direct fetal treatment with dexamethasone alters basal cardiovascular function and the cardiovascular response to acute hypoxaemia. However, in human clinical practice, dexamethasone is administered to the mother, not to the fetus. Hence, this study investigated physiological responses to acute hypoxaemia in fetal sheep during and following maternal treatment with dexamethasone in doses and at dose intervals used in human clinical practice. Under anaesthesia, 18 fetal sheep were instrumented with vascular and amniotic catheters, a carotid flow probe and a femoral flow probe at 118 days gestation (term ca 145 days). Following 6 days recovery at 124 days gestation, 10 ewes received dexamethasone (2 x 12 mg daily I.M. injections in saline). The remaining animals were saline-injected as age-matched controls. Two episodes of hypoxaemia (H) were induced in all animals by reducing the maternal FIO2for 1 h (H1, 8 h after the second injection; H2, 3 days after the second injection). In fetuses whose mothers received saline, hypoxaemia induced significant increases in fetal arterial blood pressure, carotid blood flow and carotid vascular conductance and femoral vascular resistance, significant falls in femoral blood flow and femoral vascular conductance and transient bradycardia. These cardiovascular responses were accompanied by a fall in arterial pH, increases in blood glucose and blood lactate concentrations and increased plasma concentrations of catecholamines. In fetuses whose mothers were treated with dexamethasone, bradycardia persisted throughout hypoxaemia, the magnitude of the femoral vasoconstriction, the glycaemic, lactacidaemic and acidaemic responses and the plasma concentration of neuropeptide Y (NPY) were all enhanced during H1. However, during H2, all of these physiological responses were similar to saline controls. In dexamethasone fetuses, the increase in plasma adrenaline was attenuated during H1 and the increase in carotid vascular conductance during hypoxaemia failed to reach statistical significance both during H1 and during H2. These data show that maternal treatment with dexamethasone in doses and intervals used in human obstetric practice modified the fetal cardiovascular, metabolic and endocrine defence responses to acute hypoxaemia. Furthermore, dexamethasone-induced alterations to these defences depended on whether the hypoxaemic challenge occurred during or following maternal dexamethasone treatment.
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Introduction
Acute hypoxaemia elicits integrated cardiovascular, metabolic and endocrine responses that facilitate fetal survival during a period of reduced oxygen availability (Giussani et al. 1994b). In the ovine fetus during late gestation, the cardiovascular defence responses to an episode of acute hypoxaemia include transient bradycardia, a gradual increase in arterial blood pressure (Boddy et al. 1974; Giussani et al. 1993) and redistribution of the fetal combined ventricular output to preferentially perfuse hypoxia sensitive organs, such as the adrenal gland, heart and brain, at the expense of perfusion of peripheral vascular beds (Boddy et al. 1974; Cohn et al. 1974; Peeters et al. 1979; Jansen et al. 1979; Giussani et al. 1993; Giussani et al. 1994b; Jensen & Hanson, 1995; Bennet et al. 1999). The initial bradycardia and increase in peripheral vascular resistance result from carotid chemoreflex-mediated activation of the parasympathetic and sympathetic nervous systems, respectively (Itskovitz et al. 1991; Giussani et al. 1993, 1994b; Bartelds et al. 1993). Activation of the parasympathetic system triggers the fall in fetal heart rate in response to acute hypoxaemia, since bradycardia may be prevented by vagotomy (Boddy et al. 1974) and by muscurinic cholinergic antagonism using atropine (Berman et al. 1976; Parer et al. 1980; Ikenoue et al. 1981; Giussani et al. 1993). Activation of the sympathetic nervous system increases peripheral vascular tone via -adrenergic mediated vasoconstriction of peripheral circulations, such as the femoral vascular bed, since sympathectomy (Iwamoto et al. 1983) and -adrenergic blockade (Reuss et al. 1982; Giussani et al. 1993) prevent the vasoconstriction. Once initiated, peripheral vasoconstriction is maintained by the release of vasomotor agents into the fetal circulation, including catecholamines and NPY (Jones et al. 1988; Fletcher et al. 2000a). Increased circulating levels of catecholamines also stimulate the myocardium, and fetal heart rate subsequently returns towards baseline values (Court et al. 1984). Acute hypoxaemia also induces increased circulating concentrations of glucose and lactate (Jones, 1977; Jones et al. 1983). Hyperglycaemia during acute hypoxaemia results from reduced fetal glucose consumption in the first instance, followed by increased glucose production, which increases circulating glucose availability to the hypoxic tissues (Jones, 1977; Jones et al. 1983). Lactic acidaemia results from increased glucose availability as well as from decreased oxygen delivery to peripheral tissues and subsequent anaerobic metabolism of glucose (Boyle et al. 1992).
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The discovery of Liggins and Howie that cortisol has a maturational role in the fetus during late gestation (Liggins, 1969; Liggins & Howie, 1972) has led to routine use of antenatal glucocorticoid therapy in human obstetric practice to treat pregnant women at risk of preterm delivery (NIH Consensus Development Conference, 1995). In 1994, the National Institutes of Health in the United States recommended the use of synthetic glucocorticoids in all pregnancies in which delivery prior to 34 weeks is threatened or inevitable, for example due to preterm labour, preterm premature rupture of the membranes, preeclampsia, diabetes mellitus, third trimester bleeding or fetal distress. Clinical treatment involves maternal intramuscular (I.M.) injection of synthetic glucocorticoid by one of two recommended dose regimens: two I.M. injections of 12 mg synthetic glucocorticoid 24 h apart or four I.M. injections of 6 mg synthetic glucocorticoid 12 h apart (NIH Consensus Development Conference, 1995). A meta-analysis of randomised placebo-controlled trials from 1972 to 1994 showed that antenatal glucocorticoid therapy has resulted in a significant reduction in neonatal mortality, respiratory distress syndrome, intraventricular haemorrhage, and necrotizing enterocolitis associated with premature delivery (Crowley, 1995).
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Since antenatal glucocorticoids are administered to pregnant women most at risk of preterm delivery, episodes of acute fetal hypoxaemia, which are common during labour and delivery (Huch et al. 1977), are likely to occur during, or shortly after, steroid treatment. A previous study in our laboratory showed that direct treatment of the ovine fetus with dexamethasone modifies fetal cardiovascular, metabolic and endocrine responses to an episode of acute hypoxaemia during late gestation (Fletcher et al. 2000b, 2003b). Furthermore, the effects of dexamethasone exposure on the fetal responses to acute hypoxaemia differed depending on whether the hypoxaemic challenge occurred during fetal infusion with dexamethasone, or 2 days after cessation of dexamethasone treatment (Fletcher et al. 2003b). However, in human clinical practice dexamethasone is administered to the mother and not to the fetus. Therefore, the aim of the present study was to determine the cardiovascular, metabolic and endocrine responses to an episode of acute hypoxaemia occurring during and following maternal dexamethasone treatment in the ovine fetus during late gestation, using doses and dose intervals relevant to human clinical practice.
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Methods
Surgical preparation
Eighteen Welsh Mountain ewes carrying fetuses of known gestational age were used in the study. All procedures were performed under the UK Animals (Scientific Procedures) Act 1986. All food, but not water, was withheld from the animals for 24 h prior to surgery.
Surgery was performed under aseptic conditions at 118 ± 1 days of gestation (dGA; term is ca 145 dGA, mean ±S.E.M.). Anaesthesia was induced with sodium thiopentone (20 mg kg–1I.V. Intraval Sodium; Rhone Mérieux, Dublin, Ireland) and maintained with 1–2% halothane in 50: 50 O2–N2O. In brief, following abdominal and uterine incisions, the fetal head was exteriorized for insertion of a catheter (i.d. 0.86 mm, o.d. 1.52 mm; Critchly Electrical Products, NSW, Australia) into a carotid artery with the tip of the catheter extended to the ascending aorta. An ultrasonic flow transducer (2R or 2S with silicone flange; Transonics Inc., Ithaca, USA) was implanted around the contra-lateral carotid artery. The catheter was filled with heparinized saline (100 i.u. heparin ml–1 in 0.9% NaCl) and plugged with a brass pin, and the uterine incision closed in layers. The fetal hindlimbs were exteriorized via a second uterine incision for insertion of a femoral artery catheter (i.d. 0.86 mm, o.d. 1.52 mm, Critchly Electrical Products, NSW, Australia), which was advanced into the descending aorta. A transit-time flow transducer (2R or 2S with silicone flange; Transonics Inc., Ithaca, USA) was placed around the contra-lateral femoral artery. A further catheter was anchored onto the fetal hind limb in the amniotic cavity for recording of amniotic pressure. The catheters were filled with heparinized saline (100 i.u. heparin ml–1 in 0.9% NaCl), plugged with sterile brass pins and the uterine incision was closed in layers.
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In 12 ewes (6 saline and 6 dexamethasone) a Teflon catheter was inserted into the maternal femoral artery and advanced to the descending aorta. Antibiotics were administered to the fetus (I.A. 300 mg ampicillin; Penbritin, SmithKline-Beecham Animal Health, Surrey, UK) and into the amniotic cavity (300 mg ampicillin). All catheters were exteriorized through a key-hole incision in the maternal flank and housed in a pouch sutured onto the maternal skin.
Post-operative care
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Animals were housed in individual pens with access to hay and water ad libitum. Concentrates were fed twice daily (100 g; Sheep Nuts No. 6; H & C Beart Ltd, Kings Lynn, UK). All ewes received antibiotics (0.20–0.25 mg kg–1 I. Depocillin; Mycofarm, Cambridge, UK) immediately after surgery and daily for 3 days. Patency of fetal vascular catheters was maintained by a slow continuous infusion of heparinized saline (100 i.u. heparin ml–1 at 0.1 ml h–1 in 0.9% NaCl) containing antibiotic (10 mg ml–1 benzylpenicillin; Crystapen, Schering-Plough, Animal Health Division, Welwyn Garden City, UK).
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Experimental procedure
Six days after surgery, at 124 ± 1 dGA, the animals were divided randomly into two experimental groups. Ten ewes received 2 x 12 mg injections of dexamethasone (dexamethasone sodium phosphate; Merck Sharpe, Dohme Ltd, Herts, UK) in 2 ml saline (0.9% NaCl) 24 h apart. Eight ewes received two injections of 2 ml of saline 24 h apart to serve as age-matched controls. Clinical treatment involves one of two recommended dose regimens: two I.M. injections of 12 mg synthetic glucocorticoid 24 h apart (usually betamethasone) or four I.M. injections of synthetic glucocorticoid 12 h apart (usually dexamethasone; NIH Consensus Development Conference, 1995). In this study, the regimen of two I.M. injections of 12 mg dexamethsone 24 h apart was chosen for practicality reasons.
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All ewes and their fetuses were subjected to two separate episodes of acute hypoxaemia, induced by reducing the maternal FIO2, at 125 ± 1 dGA (8 h after the second maternal injection; Hypoxaemia 1, H1) and again at 128 ± 1 dGA (3 days after the second dexamethasone injection; Hypoxaemia 2, H2). The rationale for the first episode of acute hypoxaemia was to determine if physiological responses to acute stress were affected when dexamethasone was present in the maternal and fetal circulations. The acute hypoxaemic challenge was performed 8 h after the second injection at a time when fetal plasma concentrations of dexamethasone were similar to those measured during direct fetal treatment (Fletcher et al. 2000b). The rationale for the second episode of acute hypoxaemia was to determine the long-term effects of glucocorticoid exposure after the dexamethasone had cleared from both the maternal and fetal circulations (Jellyman et al. 2004). The protocol for acute hypoxaemia involved a 3 h experiment consisting of 1 h of normoxia, 1 h of hypoxaemia and 1 h of recovery as previously described in detail (Giussani et al. 1993). In brief, air was passed for 1 h at a rate of ca 40 l min–1 into a large, transparent, polythene bag placed over the ewes' head. Following this control period, maternal and fetal hypoxaemia was induced by changing the concentrations of gases breathed by the ewe to 9% O2 in N2 with 2–3% CO2. This mixture was designed to reduce maternal and fetal arterial Pa,O2 to ca 35 and 12 mmHg, respectively, while maintaining maternal and fetal arterial isocapnia. Following the 1-h period of hypoxaemia the ewe was returned to breathing air for the 1 h recovery period.
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Measurements and biochemical analyses
During the hypoxaemia protocols, arterial blood samples (4 ml) were collected at 15 and 45 min of normoxia, after 15 and 45 min of hypoxaemia and after 45 min of recovery. The descending aortic blood samples (0.4 ml) were drawn into sterile syringes and analysed for measurement of arterial blood gases, percentage saturation of O2 in haemoglobin, haemoglobin concentration and acid/base status using an ABL5 blood gas analyser and OSM2 haemoximeter (Radiometer, Copenhagen, Denmark). Measurements in maternal and fetal blood using the ABL5 blood gas analyser were corrected to 38 and 39.5°C, respectively. Blood glucose and lactate concentrations were also measured using an automated analyser (Yellow Springs 2300 Stat Plus Glucose/Lactate Analyser; YSI Ltd, Farnborough, UK). Haematocrit was calculated as the ratio of the volume of red cells to the volume of whole blood. All blood samples for hormone analysis were collected into K+/EDTA-treated tubes, kept on ice and centrifuged at 1200 g for 4 min at 4°C. Plasma samples were stored at –70°C until analyses.
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Dexamethasone. Plasma dexamethasone levels were measured after ether extraction using tritium-labelled dexamethasone as tracer, as previously described (Fletcher et al. 2000b). All values were corrected for recovery (86%). The interassay coefficients of variation for three control plasma pools (1.8, 5.4 and 26.7 nmol l–1) were 14.6, 9.3 and 8.2%, respectively. The lower detection limit of the assay was 0.2 nmol l–1. The antidexamethasone antiserum showed a 1.6% cross-reactivity against cortisol and cross-reactivities of less than 0.5% against 11-deoxycortisol, corticosterone, testosterone, progesterone and oestriol.
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Catecholamines. Plasma noradrenaline and adrenaline concentrations were measured by HPLC with electrochemical detection as previously described in detail (Silver et al. 1982). Samples were prepared by absorption of 250 μl of plasma onto acid-washed alumina and 20 μl aliquots of the 100 μl perchloric acid elutes were injected onto the column. Dihydroxybenzylamine was added as the internal standard to each plasma sample before absorption. Recovery ranged from 63 to 97% and all catecholamine values were corrected for their respective recovery. The interassay coefficients of variation for noradrenaline and adrenaline were 6.2% and 7.3%, respectively, and the minimum detectable concentration was 10 pg ml–1.
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NPY. Plasma concentrations of NPY were measured by radioimmunoassay as previously described in detail (Fletcher et al. 2000a). The NPY assay could detect less than 1 pmol l–1 (95% confidence interval). The interassay coefficient of variation was 6.8% and there was no detectable cross-reactivity of the anti-NPY antiserum with peptide YY.
Data collection and analyses
Analog signals for calibrated ascending and descending aortic blood pressures, heart rate, carotid and femoral blood flows were recorded continuously during the experimental protocol using a data acquisition system. The signals were digitized, displayed and subsequently stored at 1 s intervals on disk by custom software (NI-DAQ, National Instruments, Austin, Texas) running on a PC. Files were subsequently analysed using Microsoft Excel spreadsheets. Fetal ascending and descending aortic blood pressures were corrected for amniotic pressure. Carotid vascular conductance was calculated every minute by dividing mean carotid blood flow by mean ascending aortic arterial blood pressure. Femoral vascular resistance was calculated every minute by dividing mean descending aortic blood pressure by mean femoral blood flow.
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One dexamethasone and two saline fetuses were instrumented with carotid flow probes only, four dexamethasone and two saline fetuses were instrumented with femoral flow probes only, and five dexamethasone and four saline fetuses were instrumented with both carotid and femoral flow probes to give a total of 10 dexamethasone and eight saline fetuses. One carotid flow probe (1 saline) and three femoral flow probes (3 dexamethasone) developed acoustic errors by H2. However, flow measurements from at least five fetuses, and in most instances six fetuses, from any one group were obtained and used for statistical comparisons.
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Statistical analyses
Values for all variables are expressed as means ±S.E.M. All measured variables were first analysed for normality of distribution. All data obtained were parametric and were compared using two-way ANOVA with repeated measures (SigmaStat; Systat Software Inc., Point Richmond, CA, USA) followed by an appropriate post hoc test. For all comparisons, statistical significance was accepted when P < 0.05.
Results
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Dexamethasone immunoreactivity was below detectable levels in all animals prior to maternal injection, and remained undetectable throughout the experimental protocol in saline injected ewes and fetuses. The plasma concentration of dexamethasone was significantly elevated in dexamethasone fetuses (3.8 ± 0.3 nmol l–1) during H1, but was below the detection limit of the assay during H2.
Blood gas and acid/base status
Maternal. During the normoxic period of H1 and H2, values for arterial blood gas and acid/base status were similar in saline- and dexamethasone-treated ewes (Tables 1 and 2). Acute hypoxaemia induced similar falls in Pa,O2 and percentage saturation of haemoglobin and increases in haemoglobin concentrations without alterations in arterial pH or PCO2 in all ewes. Although haematocrit tended to increase in all ewes during hypoxaemia this fell outside of statistical significance in saline-treated ewes (Tables 1 and 2). During recovery all variables returned towards baseline values (Tables 1 and 2).
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Fetal. Values for fetal arterial blood gas and acid/base status were similar in dexamethasone and saline groups during the normoxic periods of H1 and H2 (Tables 3 and 4). Acute hypoxaemia induced similar falls in fetal Pa,O2 and percentage saturation of haemoglobin, increases in haemoglobin concentrations without alterations in Pa,CO2 or haematocrit in all fetuses during H1 and H2 (Tables 3 and 4). Acute hypoxaemia induced progressive acidaemia, but the magnitude of the fall in arterial pH was significantly greater in dexamethasone than saline groups during H1 only (Tables 3 and 4).
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Metabolic status
Maternal. Basal blood glucose concentrations were significantly elevated in dexamethasone- compared with saline-treated ewes during the normoxic period of H1 (Table 1). In contrast, during the normoxic period of H2 maternal blood glucose concentrations were similar in saline- and dexamethasone-treated ewes (Table 2). Basal blood lactate concentrations were similar in all ewes during the normoxic periods of H1 and H2 (Tables 1 and 2). Although acute hypoxaemia did not induce significant alterations in maternal blood concentrations of glucose or lactate during H1 or H2, there was a tendency for maternal glucose concentrations to increase during H1 only (Tables 1 and 2).
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Fetal. Blood glucose concentrations were significantly elevated in dexamethasone fetuses during the normoxic period of H1, but not H2 (Tables 3 and 4). Acute hypoxaemia induced increases in blood concentrations of glucose and lactate in all fetuses. The magnitude of the increase in both blood glucose and lactate were significantly greater in dexamethasone compared with saline fetuses during H1, but not H2. The ratio of glucose concentration in the fetal: maternal blood was significantly increased by 45 min of hypoxaemia in all groups. However, the magnitude of the increase in the ratio was significantly greater in dexamethasone-treated fetuses during H2 only (Fig. 1).
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Arterial blood samples were collected at 15 (N15) and 45 min (N45) of normoxia (baseline), at 15 (H15) and 45 min (H45) of hypoxaemia, and at 15 (R15) and 45 min (R45) of recovery during H1 and H2. Values are means ±S.E.M. for saline- (, n= 8) and dexamethasone- (, n= 10) treated ewes and their fetuses. Statistical differences are P < 0.05: a, normoxia versus hypoxaemia or recovery; b, saline versus dexamethasone ewes (two way RM ANOVA with Tukey's test).
Fetal cardiovascular variables
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During the normoxic period of H1, basal ascending aortic blood pressure, basal carotid flow and basal carotid vascular conductance were similar in dexamethasone (48 ± 4 mmHg, 56 ± 8 ml min–1, 1.17 ± 0.17 ml min–1 mmHg–1) and saline (43 ± 3 mmHg, 57 ± 5 ml min–1, 1.35 ± 0.16 ml min–1 mmHg–1) groups. Basal values for fetal ascending aortic blood pressure, carotid blood flow, and carotid vascular conductance were similar in dexamethasone (47 ± 4 mmHg, 72 ± 7 ml min–1, 1.57 ± 0.16 ml min–1 mmHg–1) and saline (38 ± 1 mmHg, 56 ± 6 ml min–1, 1.50 ± 0.16 ml min–1 mmHg–1) groups during the normoxic period of H2.
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In saline fetuses, acute hypoxaemia elicited an increase in ascending aortic blood pressure, carotid blood flow and carotid vascular conductance (Figs 2 and 3). However, in fetuses whose mothers received dexamethasone, the increases in carotid vascular conductance failed to reach statistical significance during H1 and H2 (Figs 2 and 3).
Fetal ascending aortic blood pressure, descending aortic blood pressure, heart rate, carotid blood flow, carotid vascular conductance, femoral blood flow and femoral vascular resistance during acute hypoxaemia (H1) during maternal dexamethasone treatment. Values represent either mean changes from baseline ±S.E.M. calculated every minute during 1 h of normoxia, 1 h of hypoxaemia and 1 h of recovery (line graph) or the mean ±S.E.M. area under the curve for each hour of the experimental protocol (bar graphs) in saline (; H1: n= 6, H2, n= 5) or dexamethasone (; H1: n= 6, H2: n= 6) fetuses. Statistical differences are P < 0.05: a, normoxia versus hypoxaemia or recovery; b, saline versus dexamethasone ewes (area under the curve; two way RM ANOVA with Tukey's test).
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Fetal ascending aortic blood pressure, descending aortic blood pressure, heart rate, carotid blood flow, carotid vascular conductance, femoral blood flow and femoral vascular resistance during acute hypoxaemia (H2) following maternal dexamethasone treatment. Values represent either mean changes from baseline ±S.E.M. calculated every minute during 1 h of normoxia, 1 h of hypoxaemia and 1 h of recovery (line graph) or the mean ±S.E.M. area under the curve for each hour of the experimental protocol (bar graphs) in saline (; H1: n= 6, H2, n= 5) or dexamethasone (; H1: n= 6, H2: n= 6) fetuses. Statistical differences are P < 0.05: a, normoxia versus hypoxaemia or recovery (area under the curve; two way RM ANOVA with Tukey's test).
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Acute oxygen deprivation elicited bradycardia, increases in fetal descending aortic blood pressure and femoral vascular resistance, as well as falls in femoral blood flow in all fetuses during H1 and H2 (Figs 2 and 3). In saline-treated fetuses, bradycardia was transient and returned towards baseline values within 20 min of the onset of the hypoxaemic challenge (Figs 2 and 3). In contrast, bradycardia persisted throughout acute hypoxaemia in dexamethasone-treated fetuses during H1 and H2 (Figs 2 and 3). The magnitudes of the fetal bradycardia, pressor and vasopressor responses were significantly greater in dexamethasone fetuses during H1, but not H2 (Figs 2 and 3).
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Fetal endocrine variables
Fetal plasma concentrations of catecholamines and NPY were similar in saline and dexamethasone fetuses during the normoxic periods of H1 and H2 (Fig. 4). In saline fetuses, acute hypoxaemia induced significant increases in plasma concentrations of noradrenaline and adrenaline (Fig. 4). In fetuses whose mothers were treated with dexamethasone, the increase in plasma concentrations of adrenaline was significantly attenuated during H1, but not H2 (Fig. 4). In contrast, plasma NPY concentrations were significantly enhanced in dexamethasone compared with saline fetuses at 45 min of hypoxaemia during H1, but not H2 (Fig. 4).
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Arterial blood samples were collected at 15 (N15) and 45 min (N45) of normoxia (baseline), at 15 (H15) and 45 min (H45) of hypoxaemia, and at 45 min (R45) of recovery during H1 and H2. Values are means ±S.E.M. for fetuses of saline- (; n= 6) and dexamethasone- (; n= 6) treated ewes. Statistical differences are P < 0.05: a, normoxia versus hypoxaemia or recovery; b, saline versus dexamethasone ewes (two Way RM ANOVA and Tukey's test).
Discussion
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The data in the present study show that in fetuses whose mothers received saline, acute hypoxaemia induced significant increases in fetal arterial blood pressure, carotid blood flow, carotid vascular conductance significant falls in femoral blood flow and femoral vascular resistances, bradycardia in all animals. These cardiovascular responses were accompanied by a fall in arterial pH, increases in blood glucose and blood lactate concentrations and increased plasma concentrations of catecholamines. In fetuses whose mothers were treated with dexamethasone, the bradycardia persisted throughout hypoxaemia, and the magnitude of the femoral vasoconstriction, the glycaemic, lactacidaemic and acidaemic responses, and the plasma NPY concentration were all enhanced when the acute hypoxaemic episode occurred during dexamethasone treatment. However, when the acute hypoxaemia protocol occurred 3 days after the second maternal dexamethasone injection, all of these alterations in cardiovascular, metabolic and endocrine responses were similar to saline controls. In dexamethasone fetuses, the increase in plasma adrenaline concentration was attenuated during treatment and the increase in carotid vascular conductance during hypoxaemia failed to reach statistical significance both during and after dexamethasone treatment.
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The persistence of the bradycardic response throughout hypoxaemia in dexamethasone fetuses when the challenge occurred during exposure to the steroid is a similar finding to that reported in a previous study which addressed the effects of direct fetal infusion with low doses of dexamethasone on fetal cardiovascular function (Fletcher et al. 2000b). However, in contrast to the findings of Fletcher and colleagues, bradycardia did not persist during hypoxaemia when the glucocorticoid was no longer detectable in the fetal circulation. The mechanism accounting for persisting bradycardia in dexamethasone fetuses may include increased negative chronotropic and/or decreased positive chronotropic effects on the fetal heart. Dexamethasone may increase the activity or gain of the carotid chemoreflex during acute hypoxaemia, since ontogenic changes in the set point and sensitivity of the arterial chemoreceptors (Blanco et al. 1984) and increases in the gain of the cardiac and femoral vasoconstrictor components of the fetal chemoreflex (Fletcher et al. 2003b) both occur in association with the prepartum increase in endogenous fetal cortisol (Fowden, 1995). Synthetic glucocorticoids may also enhance cholinergic signal transduction at the myocardium since endogenous glucocorticoids increase muscarinic ACh receptor affinity in rat myocardial cells in vivo (Jacobsson et al. 1983) and in vitro (Ransnas et al. 1987). A decrease in positive chronotropic effects on the fetal heart by dexamethasone may result from depression of circulating catecholamines and/or a reduction in myocardial sensitivity to -adrenergic receptor agonists. Fetal basal plasma concentrations of catecholamines were suppressed during fetal infusion (Derks et al. 1997) and 3 h after delivery in lambs treated with betamethasone in utero (Ervin et al. 2000). Moreover, the increment in fetal plasma adrenaline during acute hypoxaemia was attenuated during dexamethasone infusion of the ovine fetus (Fletcher et al. 2003b) and during, but not following, maternal treatment with dexamethasone in the present study. Dexamethasone has also been reported to decrease myocardial cell sensitivity to -adrenergic stimulation in rats treated prenatally (Hou & Slotkin, 1989), or postnatally (Lau & Slotkin, 1981).
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During acute hypoxaemia, the fetal combined ventricular output is redistributed towards central and away from peripheral circulations (Cohn et al. 1974; Giussani et al. 1993). The cerebral circulation is thus protected during hypoxaemia at the expense of the peripheral vascular beds, which, in turn, act as a reservoir of vascular resistance to aid the maintenance of perfusion in hypoxia-sensitive circulations. Although fetal carotid blood flow increased in all fetuses during acute hypoxaemia, the increase tended to be delayed in dexamethasone fetuses. Similarly, the increase in fetal cerebral blood flow in response to a hypercapnic challenge was diminished due to decreased cerebral vasodilatation during betamethasone infusion into the ovine fetus (Schwab et al. 2000; Lohle et al. 2005). Moreover, the depressive effect of dexamethasone on the hyperaemic response of the carotid vascular bed during acute hypoxaemia is similar to the effects of dexamethasone on haemodynamic responses in other essential vascular beds, such as the umbilical circulation. In another study, maternal dexamethasone abolished the normal increase in umbilical blood flow during acute hypoxaemic challenges occurring either during or 3 days after treatment in ovine fetuses during late gestation (Jellyman et al. 2004).
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In contrast to essential circulations, dexamethasone significantly enhanced the fall in femoral blood flow and the increase in femoral vascular resistance in the present study, similar to a study of responses to acute hypoxaemia during direct infusion of the ovine fetus with low doses of dexamethasone (Fletcher et al. 2003b). Synthetic glucocorticoids may enhance the fetal sympathetic outflow, since maternal dexamethasone treatment increased renal sympathetic nerve activity in lambs at birth (Segar et al. 1998) and enhanced the increase in plasma concentrations of NPY during acute hypoxaemia (Fletcher et al. 2000a). NPY is colocalized and coreleased with noradrenaline at sympathetic nervous terminals (Lundberg et al. 1983; Ekblad et al. 1984) and unlike noradrenaline lacks re-uptake mechanisms (Lundberg, 1996), making measurement of changes in its circulating concentration a useful measure of sympathetic nervous system activity. The adrenal medulla is unlikely to contribute to circulating levels of NPY in rats (Mormede et al. 1990), calves (Bloom et al. 1988) or lambs (Bloom et al. 1989). Hence, increased plasma concentrations during acute stress most likely reflect overspill from sympathetic nervous terminals.
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NPY may also contribute to the enhanced femoral vasoconstriction in dexamethasone-treated fetuses by endocrine and neuromodulator mechanisms. In adult animals NPY elicits potent long-lasting vasoconstriction by direct actions on NPY-Y1 receptors in the peripheral vasculature and/or by potentiating the effects of circulating vasoconstrictors, such as catecholamines (Edvinsson et al. 1984; Franco-Cereceda et al. 1985; Corder et al. 1986; Waeber et al. 1990; Balasubramaniam, 2003). Accordingly, fetal plasma NPY concentrations during basal and acute hypoxaemic conditions showed a significant positive correlation with the increase in femoral vascular resistance in ovine fetuses during late gestation (Fletcher et al. 2000a).
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In addition to altering the balance of circulating plasma concentrations of constrictors and dilators, dexamethasone may alter vascular sensitivity to vasomotor agents. However, exposure to glucocorticoids in utero did not alter vascular sensitivity to the vasoconstrictors noradrenaline or angiotensin II in fetal sheep (Fletcher et al. 2002) or lambs (Dodic et al. 1998). Conversely, studies using in vitro wire myography to examine isometric vascular responses of small femoral arterial branches from fetal sheep during late gestation suggest that betamethasone significantly increased the maximum vasoconstriction to noradrenaline and decreased endothelium-independent responses to both bradykinin and forskolin (Anwar et al. 1999). Similarly, the maximum endothelin-1-induced vasoconstriction was greater in femoral arteries taken from dexamethasone-infused fetuses (Docherty et al. 2001), probably due to down-regulation of endothelial NO release (Molnar et al. 2003).
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It is possible that dexamethasone could have induced alterations in components of vascular structure, such as tropoeleastin (Pierce et al. 1995) or collagen and non-collagen proteins (Leitman et al. 1984). However, dexamethasone-induced vascular hypertrophy or hyperplasia is unlikely to account for the enhanced femoral vasoconstrictor responses during acute hypoxaemia since they were reversible following treatment.
Hence, past and present data suggest that contrasting effects of dexamethasone on essential and peripheral vascular beds appear to be mediated via similar mechanisms, decreasing the gain of dilator mechanisms and increasing the gain of constrictor mechanisms. Thus, the magnitude of the response in circulations which normally dilate during hypoxaemia is diminished, such as in the carotid and umbilical vascular beds. In contrast, the magnitude of the response in circulations which normally constrict during hypoxaemia is enhanced, such as in the femoral vascular bed.
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In accordance with previous studies, hypoxaemia-induced increases in fetal blood glucose concentrations (Jones, 1977; Hooper et al. 1995) were enhanced by dexamethasone treatment (Fletcher et al. 2000b). Whilst a greater increase in fetal blood glucose concentrations during, but not following, maternal glucocorticoid treatment suggests that enhancement of the glycaemic response may be dependent on the continued presence of the synthetic glucocorticoid in the fetal circulation, differential mechanisms may account for the glucogenic responses at these times. Increased fetal blood glucose may reflect increased transplacental glucose flux, decreased fetal glucose utilization and/or increased stimulation of endogenous fetal glucose production. In agreement with previous studies, hypoxaemia increased the ratio of glucose in the fetal: maternal circulation by 45 min in all animals, indicative of elevations in glucose concentration of fetal origin (Jones et al. 1983). In the present study it is not possible to distinguish between decreased fetal glucose utilization and increased endogenous fetal glucose production. Similar fetal: maternal blood glucose ratio between groups suggest that increased basal and stimulated fetal blood glucose concentrations during H1 probably resulted from increased transplacental glucose transport in dexamethasone fetuses. Conversely, a greater increment in the fetal: maternal glucose ratio at 45 min of hypoxaemia during H2 suggests that dexamethasone may have matured the fetal glucogenic capacity by 3 days after treatment. Glucocorticoids are known to increase the capacity for glucose production by increasing hepatic glycogen content (Barnes et al. 1978) and hepatic gluconeogenic enzyme activities (Fowden et al. 1990) in the ovine fetus during late gestation. Recent studies in our laboratory suggest that dexamethasone increases glycogenolytic but not gluconeogenic capacity in the fetus. Maternal dexamethasone significantly increased the activity of glucose-6-phosphatase, the final enzyme in both the gluconeogenic and glycogenolytic pathways, irrespective of whether enzyme activities were expressed per gram wet tissue weight or per gram of protein. However, dexamethasone did not affect activity of phosphoenolpyruvate carboxykinase, a rate-limiting enzyme specific to the gluconeogenic pathway (K. L. Franko, D. A. Giussani & A. L. Fowden, unpublished observations).
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In the present study, the acidaemic and lactic acidaemic responses to acute hypoxaemia in fetuses whose mothers were treated with dexamethasone were significantly enhanced during, but not following, glucocorticoid treatment. These data contrast with a previous study in which the increment in fetal blood lactate concentrations in dexamethasone-infused fetuses were similar to saline controls during hypoxaemia despite an enhanced glycaemic response (Fletcher et al. 2000b). The fetal hind limb (Boyle et al. 1992), but not the placenta (Gu et al. 1985; Hooper et al. 1995), may represent a source of anaerobic lactate output due to decreased oxygen delivery and subsequent anaerobic metabolism during fetal hypoxaemia. Lactate is an important metabolic substrate in the ovine fetus during basal conditions (Fisher et al. 1982; Gleason et al. 1990) and dexamethasone may promote cerebral lactate utilization, since lactate crosses the blood brain barrier in sheep fetuses near term, is oxidized and at elevated concentrations can complement glucose as an oxidative substrate (Turbow et al. 1995).
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The greater fall in arterial pH in dexamethasone- compared with saline-treated fetuses may have occurred secondary to the greater increment in blood lactate, which is the principal factor governing acidaemia in the fetus (Lawrence et al. 1982). Furthermore, acidaemia may contribute to the enhanced cardiovascular responses to hypoxaemia since Gardner et al. (2002) showed that changes in fetal arterial pH, rather than Pa,O2, were a greater determinant of fetal femoral vasoconstriction in response to acute hypoxaemia in chronically acidaemic fetuses. However, the extent to which dexamethasone-induced fetal acidaemia is a cause or a consequence of the enhanced cardiovascular responses to acute hypoxaemia in this and previous studies remains unknown. Whatever the mechanism involved, a positive feed-forward cycle between increased acidosis, enhanced peripheral vasoconstriction and augmented lactic acidaemia in the fetus may help augment the gain of the fetal cardiovascular responses to acute hypoxaemia during fetal exposure to dexamethasone.
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In the ovine fetus during late gestation heart rate deceleration during acute hypoxaemia has several functional advantages. Fisher et al. (1982) suggested that the product of the heart rate and blood pressure (the rate-pressure product) is a useful correlate of myocardial oxygen consumption as it reflects myocardial workload. In untreated fetal sheep during late gestation, the rate–pressure product decreases during acute hypoxaemia, thereby decreasing myocardial workload and cardiac oxygen consumption. Fetal bradycardia reduces coronary flow, which enhances myocardial oxygen extraction (Fisher et al. 1982; Thornburg & Reller, 1999) and prolongs end-diastolic filling times, which increases ventricular end-diastolic volume, promoting a greater force of ventricular contraction through the Frank-Starling mechanism and increasing stroke volume (Anderson et al. 1986). During acute hypoxaemia fetal cardiac output is thereby maintained despite a pronounced fall in heart rate. In fetuses exposed to dexamethasone, prolonged bradycardia will maintain these advantages throughout the period of hypoxaemia. In addition, maintenance of cardiac function during anaerobic metabolism is dependent on the glycolytic flux from myocardial glycogen stores or from circulating blood glucose (Hoerter, 1976; Jarmakani et al. 1978; Hoerter & Opie, 1978; Schwab et al. 2000). We suggest that in contrast to the short-lived advantages conveyed by maternal dexamethasone on peripheral vascular resistance, cardiac output and its distribution, dexamethasone-induced enhancement of the fetal capacity for glucose production in the fetus may provide a real advantage. Increased capacity for glucose production in the fetus will improve glucose delivery to hypoxia-sensitive circulations, such as the myocardial and cerebral vascular beds. The increased glucogenic capacity in the fetus will, at the very least, maintain glucose delivery to the brain, despite a smaller increment in carotid perfusion during and following dexamethasone treatment.
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Comparison of the findings in the present study with previous studies revealed that maternal dexamethasone treatment had broadly similar effects on the fetal cardiovascular, metabolic and endocrine responses to an episode of acute hypoxaemia (Fletcher et al. 2000b; Fletcher et al. 2003b). However, the cardiovascular effects of direct fetal treatment with steroids appear to persist after steroid clearance from the fetal circulation whereas those of maternal treatment are short-lived. The mechanisms accounting for the differential effects of maternal and fetal treatment with dexamethasone on fetal physiology are unknown, but may reflect differences in concentrations of dexamethasone achieved in the fetal circulation, duration of exposure to the synthetic glucocorticoid, the subsequent secretion of natural cortisol and/or the contribution of effects secondary to changes in maternal physiology induced by glucocorticoid treatment.
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In conclusion, maternal treatment with dexamethasone in doses and dose intervals used in human clinical practice modified the fetal cardiovascular, metabolic and endocrine defence responses to acute hypoxaemia in the late gestation ovine fetus. Furthermore, dexamethasone-induced alterations to these defences depended on whether the hypoxaemic challenge occurred during or following maternal dexamethasone treatment. These findings have important implications for antenatal glucocorticoid therapy in current human obstetric practice.
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References
Anderson PA, Glick KL, Killam AP & Mainwaring RD (1986). The effect of heart rate on in utero left ventricular output in the fetal sheep. J Physiol 372, 557–573.
Anwar MA, Schwab M, Poston L & Nathanielsz PW (1999). Betamethasone-mediated vascular dysfunction and changes in hematological profile in the ovine fetus. Am J Physiol 276, H1137–H1143.
Balasubramaniam A (2003). Neuropeptide Y (NPY) family of hormones: progress in the development of receptor selective agonists and antagonists. Curr Pharm Des 9, 1165–1175.
, http://www.100md.com
Barnes RJ, Comline RS & Silver M (1978). Effect of cortisol on liver glycogen concentrations in hypophysectomized, adrenalectomized and normal foetal lambs during late or prolonged gestation. J Physiol 275, 567–579.
Bartelds B, van Bel F, Teitel DF & Rudolph AM (1993). Carotid, not aortic, chemoreceptors mediate the fetal cardiovascular response to acute hypoxemia in lambs. Pediatr Res 34, 51–55.
Bennet L, Kozuma S, McGarrigle HH & Hanson MA (1999). Temporal changes in fetal cardiovascular, behavioural, metabolic and endocrine responses to maternally administered dexamethasone in the late gestation fetal sheep. Br J Obstet Gynaecol 106, 331–339.
, 百拇医药
Berman W, Goodlin RC, Heymann MA & Rudolph AM (1976). Relationships between pressure and flow in the umbilical and uterine circulations of the sheep. Circ Res 38, 262–266.
Blanco CE, Dawes GS, Hanson MA & McCooke HB (1984). The response to hypoxia of arterial chemoreceptors in fetal sheep and new-born lambs. J Physiol 351, 25–37.
Bloom SR, Edwards AV & Jones CT (1988). The adrenal contribution to the neuroendocrine responses to splanchnic nerve stimulation in conscious calves. J Physiol 397, 513–526.
, 百拇医药
Bloom SR, Edwards AV & Jones CT (1989). Neuroendocrine responses to stimulation of the splanchnic nerves in bursts in conscious, adrenalectomized, weaned lambs. J Physiol 417, 79–89.
Boddy K, Dawes GS, Fisher R, Pinter S & Robinson JS (1974). Foetal respiratory movements, electrocortical and cardiovascular responses to hypoxaemia and hypercapnia in sheep. J Physiol 243, 599–618.
Boyle DW, Meschia G & Wilkening RB (1992). Metabolic adaptation of fetal hindlimb to severe, nonlethal hypoxia. Am J Physiol 263, R1130–R1135.
, 百拇医药
Cohn HE, Sacks EJ, Heymann MA & Rudolph AM (1974). Cardiovascular responses to hypoxemia and acidemia in fetal lambs. Am J Obstet Gynecol 120, 817–824.
Corder R, Lowry PJ, Wilkinson SJ & Ramage AG (1986). Comparison of the haemodynamic actions of neuropeptide Y, angiotensin II and noradrenaline in anaesthetised cats. Eur J Pharmacol 121, 25–30.
Court DJ, Parer JT, Block BS & Llanos AJ (1984). Effects of beta-adrenergic blockade on blood flow distribution during hypoxaemia in fetal sheep. J Dev Physiol 6, 349–358.
, http://www.100md.com
Crowley PA (1995). Antenatal corticosteroid therapy: a meta-analysis of the randomized trials, 1972–94. Am J Obstet Gynecol 173, 322–335.
Derks JB, Giussani DA, Jenkins SL, Wentworth RA, Visser GH, Padbury JF & Nathanielsz PW (1997). A comparative study of cardiovascular, endocrine and behavioural effects of betamethasone and dexamethasone administration to fetal sheep. J Physiol 499, 217–226.
Docherty CC, Kalmar-Nagy J, Engelen M, Koenen SV, Nijland M, Kuc RE, Davenport AP & Nathanielsz PW (2001). Effect of in vivo fetal infusion of dexamethasone at 0.75 GA on fetal ovine resistance artery responses to ET-1. Am J Physiol Regul Integr Comp Physiol 281, R261–R268.
, 百拇医药
Dodic M, May CN, Wintour EM & Coghlan JP (1998). An early prenatal exposure to excess glucocorticoid leads to hypertensive offspring in sheep. Clin Sci (Lond) 94, 149–155.
Edvinsson L, Ekblad E, Hakanson R & Wahlestedt C (1984). Neuropeptide Y potentiates the effect of various vasoconstrictor agents on rabbit blood vessels. Br J Pharmacol 83, 519–525.
Ekblad E, Edvinsson L, Wahlestedt C, Uddman R, Hakanson R & Sundler F (1984). Neuropeptide Y co-exists and co-operates with noradrenaline in perivascular nerve fibers. Regul Pept 8, 225–235.
, 百拇医药
Ervin MG, Padbury JF, Polk DH, Ikegami M, Berry LM & Jobe AH (2000). Antenatal glucocorticoids alter premature newborn lamb neuroendocrine and endocrine responses to hypoxia. Am J Physiol Regul Integr Comp Physiol 279, R830–R838.
Fisher DJ, Heymann MA & Rudolph AM (1982). Fetal myocardial oxygen and carbohydrate consumption during acutely induced hypoxemia. Am J Physiol 242, H657–H661.
Fletcher AJ, Edwards CM, Gardner DS, Fowden AL & Giussani DA (2000a). Neuropeptide Y in the sheep fetus: effects of acute hypoxemia and dexamethasone during late gestation. Endocrinology 141, 3976–3982.
, 百拇医药
Fletcher AJW, McGarrigle HHG, Edwards CMB, Fowden AL & Giussani DA (2002b). Effects of low dose dexamethasone treatment on basal cardiovascular and endocrine function in fetal sheep during late gestation. J Physiol 545, 649–660.
Fletcher AJ, Gardner DS, Edwards CM, Fowden AL & Giussani DA (2003b). Cardiovascular and endocrine responses to acute hypoxaemia during and following dexamethasone infusion in the ovine fetus. J Physiol 549, 271–287.
, http://www.100md.com
Fletcher AJ, Goodfellow MR, Forhead AJ, Gardner DS, McGarrigle HH, Fowden AL & Giussani DA (2000b). Low doses of dexamethasone suppress pituitary-adrenal function but augment the glycemic response to acute hypoxemia in fetal sheep during late gestation. Pediatr Res 47, 684–691.
Fowden AL (1995). Endocrine regulation of fetal growth. Reprod Fertil Dev 7, 351–363.
Fowden AL, Coulson RL & Silver M (1990). Endocrine regulation of tissue glucose-6-phosphatase activity in the fetal sheep during late gestation. Endocrinology 126, 2823–2830.
, 百拇医药
Franco-Cereceda A, Lundberg JM & Dahlof C (1985). Neuropeptide Y and sympathetic control of heart contractility and coronary vascular tone. Acta Physiol Scand 124, 361–369.
Gardner DS, Fletcher AJ, Bloomfield MR, Fowden AL & Giussani DA (2002). Effects of prevailing hypoxaemia, acidaemia or hypoglycaemia upon the cardiovascular, endocrine and metabolic responses to acute hypoxaemia in the ovine fetus. J Physiol 540, 351–366.
, 百拇医药
Giussani DA, Spencer JAD & Hanson MA (1994b). Fetal cardiovascular reflex responses to hypoxaemia. Fetal Maternal Med Rev 6, 17–37.
Giussani DA, Spencer JA, Moore PJ, Bennet L & Hanson MA (1993). Afferent and efferent components of the cardiovascular reflex responses to acute hypoxia in term fetal sheep. J Physiol 461, 431–449.
Gleason CA, Hamm C & Jones MD Jr (1990). Effect of acute hypoxemia on brain blood flow and oxygen metabolism in immature fetal sheep. Am J Physiol 258, H1064–H1069.
, http://www.100md.com
Gu W, Jones CT & Parer JT (1985). Metabolic and cardiovascular effects on fetal sheep of sustained reduction of uterine blood flow. J Physiol 368, 109–129.
Hoerter J (1976). Changes in the sensitivity to hypoxia and glucose deprivation in the isolated perfused rabbit heart during perinatal development. Pflugers Arch 363, 1–6.
Hoerter JA & Opie LH (1978). Perinatal changes in glycolytic function in response to hypoxia in the incubated or perfused rat heart. Biol Neonate 33, 144–161.
, 百拇医药
Hooper SB, Walker DW & Harding R (1995). Oxygen, glucose, and lactate uptake by fetus and placenta during prolonged hypoxemia. Am J Physiol 268, R303–R309.
Hou QC & Slotkin TA (1989). Effects of prenatal dexamethasone or terbutaline exposure on development of neural and intrinsic control of heart rate. Pediatr Res 26, 554–557.
Huch A, Huch R, Schneider H & Rooth G (1977). Continuous transcutaneous monitoring of fetal oxygen tension during labour. Br J Obstet Gynaecol 84 (Suppl. 1), 1–39.
, http://www.100md.com
Ikenoue T, Martin CB Jr, Murata Y, Ettinger BB & Lu PS (1981). Effect of acute hypoxemia and respiratory acidosis on the fetal heart rate in monkeys. Am J Obstet Gynecol 141, 797–806.
Itskovitz J, LaGamma EF, Bristow J & Rudolph AM (1991). Cardiovascular responses to hypoxemia in sinoaortic-denervated fetal sheep. Pediatr Res 30, 381–385.
Iwamoto HS, Rudolph AM, Mirkin BL & Keil LC (1983). Circulatory and humoral responses of sympathectomized fetal sheep to hypoxemia. Am J Physiol 245, H767–H772.
, http://www.100md.com
Jacobsson BA, Bergh CH & Hjalmarson A (1983). Corticosteroid modulation of muscarinic receptors in rat myocardial membranes. Biochim Biophys Acta 760, 77–83.
Jansen CA, Krane EJ, Thomas AL, Beck NF, Lowe KC, Joyce P, Parr M & Nathanielsz PW (1979). Continuous variability of fetal PO2 in the chronically catheterized fetal sheep. Am J Obstet Gynecol 134, 776–783.
Jarmakani JM, Nagatomo T, Nakazawa M & Langer GA (1978). Effect of hypoxia on myocardial high-energy phosphates in the neonatal mammalian heart. Am J Physiol 235, H475–H481.
, 百拇医药
Jellyman JK, Gardner DS, Fowden AL & Giussani DA (2004). Effects of dexamethasone on the uterine and umbilical vascular beds during basal and hypoxemic conditions in sheep. Am J Obstet Gynecol 190, 825–835.
Jensen A & Hanson MA (1995). Circulatory responses to acute asphyxia in intact and chemodenervated fetal sheep near term. Reprod Fertil Dev 7, 1351–1359.
Jones CT (1977). The development of some metabolic responses to hypoxia in the foetal sheep. J Physiol 265, 743–762.
, http://www.100md.com
Jones CT, Ritchie JW & Walker D (1983). The effects of hypoxia on glucose turnover in the fetal sheep. J Dev Physiol 5, 223–235.
Jones CT, Roebuck MM, Walker DW & Johnston BM (1988). The role of the adrenal medulla and peripheral sympathetic nerves in the physiological responses of the fetal sheep to hypoxia. J Dev Physiol 10, 17–36.
Lau C & Slotkin TA (1981). Maturation of sympathetic neurotransmission in the rat heart. VII. Suppression of sympathetic responses by dexamethasone. J Pharmacol Exp Ther 216, 6–11.
, 百拇医药
Lawrence GF, Brown VA, Parsons RJ & Cooke ID (1982). Feto-maternal consequences of high-dose glucose infusion during labour. Br J Obstet Gynaecol 89, 27–32.
Leitman DC, Benson SC & Johnson LK (1984). Glucocorticoids stimulate collagen and noncollagen protein synthesis in cultured vascular smooth muscle cells. J Cell Biol 98, 541–549.
Liggins GC (1969). Premature delivery of foetal lambs infused with glucocorticoids. J Endocrinol 45, 515–523.
, 百拇医药
Liggins GC & Howie RN (1972). A controlled trial of antepartum glucocorticoid treatment for prevention of the respiratory distress syndrome in premature infants. Pediatrics 50, 515–525.
Lohle M, Muller T, Wicher C, Roedel M, Schubert H, Witte OW, Nathanielsz PW & Schwab M (2005). Betamethasone effects on fetal sheep cerebral blood flow are not dependent on maturation of cerebrovascular system and pituitary-adrenal axis. J Physiol 564, 575–588.
, 百拇医药
Lundberg JM (1996). Pharmacology of cotransmission in the autonomic nervous system: integrative aspects on amines, neuropeptides, adenosine triphosphate, amino acids and nitric oxide. Pharmacol Rev 48, 113–178.
Lundberg JM, Terenius L, Hokfelt T & Goldstein M (1983). High levels of neuropeptide Y in peripheral noradrenergic neurons in various mammals including man. Neurosci Lett 42, 167–172.
Molnar J, Howe DC, Nijland MJ & Nathanielsz PW (2003). Prenatal dexamethasone leads to both endothelial dysfunction and vasodilatory compensation in sheep. J Physiol 547, 61–66.
, http://www.100md.com
Mormede P, Castagne V, Rivet JM, Gaillard R & Corder R (1990). Involvement of neuropeptide Y in neuroendocrine stress responses. Central and peripheral studies. J Neural Transm Supplement 29, 65–75.
NIH Consensus Development Conference (1995). Effect of corticosteroids for fetal maturation on perinatal outcomes. NIH Consensus Development Panel on the Effect of Corticosteroids for Fetal Maturation on Perinatal Outcomes. JAMA 273, 413–418.
, 百拇医药
Parer JT, Krueger TR & Harris JL (1980). Fetal oxygen consumption and mechanisms of heart rate response during artificially produced late decelerations of fetal heart rate in sheep. Am J Obstet Gynecol 136, 478–482.
Peeters LL, Sheldon RE, Jones MD Jr, Makowski EL & Meschia G (1979). Blood flow to fetal organs as a function of arterial oxygen content. Am J Obstet Gynecol 135, 637–646.
Pierce RA, Mariencheck WI, Sandefur S, Crouch EC & Parks WC (1995). Glucocorticoids upregulate tropoelastin expression during late stages of fetal lung development. Am J Physiol 268, L491–L500.
, 百拇医药
Ransnas L, Hjalmarson A, Sabler E & Jacobsson B (1987). Effect of corticosteroids on muscarinic receptors on cultured myocardial cells. Pharmacol Toxicol 61, 107–110.
Reuss ML, Parer JT, Harris JL & Krueger TR (1982). Hemodynamic effects of alpha-adrenergic blockade during hypoxia in fetal sheep. Am J Obstet Gynecol 142, 410–415.
Schwab M, Roedel M, Anwar MA, Muller T, Schubert H, Buchwalder LF, Walter B & Nathalielsz W (2000). Effects of betamethasone administration to the fetal sheep in late gestation on fetal cerebral blood flow. J Physiol 528, 619–632.
, http://www.100md.com
Segar JL, Lumbers ER, Nuyt AM, Smith OJ & Robillard JE (1998). Effect of antenatal glucocorticoids on sympathetic nerve activity at birth in preterm sheep. Am J Physiol 274, R160–R167.
Silver M, Barnes RJ, Comline RS & Burton GJ (1982). Placental blood flow: some fetal and maternal cardiovascular adjustments during gestation. J Reprod Fertil Supplement 31, 139–160.
Thornburg KL & Reller MD (1999). Coronary flow regulation in the fetal sheep. Am J Physiol 277, R1249–R1260.
Turbow RM, Curran-Everett D, Hay WW Jr & Jones MD Jr (1995). Cerebral lactate metabolism in near-term fetal sheep. Am J Physiol 269, R938–R942.
Waeber B, Burnier M, Nussberger J & Brunner HR (1990). Role of atrial natriuretic peptides and neuropeptide Y in blood pressure regulation. Horm Res 34, 161–165., http://www.100md.com(J. K. Jellyman, D. S. Gar)
2 ICSM Endocrine Unit, Hammersmith Hospital, London W12 0NN, UK
Abstract
In sheep, direct fetal treatment with dexamethasone alters basal cardiovascular function and the cardiovascular response to acute hypoxaemia. However, in human clinical practice, dexamethasone is administered to the mother, not to the fetus. Hence, this study investigated physiological responses to acute hypoxaemia in fetal sheep during and following maternal treatment with dexamethasone in doses and at dose intervals used in human clinical practice. Under anaesthesia, 18 fetal sheep were instrumented with vascular and amniotic catheters, a carotid flow probe and a femoral flow probe at 118 days gestation (term ca 145 days). Following 6 days recovery at 124 days gestation, 10 ewes received dexamethasone (2 x 12 mg daily I.M. injections in saline). The remaining animals were saline-injected as age-matched controls. Two episodes of hypoxaemia (H) were induced in all animals by reducing the maternal FIO2for 1 h (H1, 8 h after the second injection; H2, 3 days after the second injection). In fetuses whose mothers received saline, hypoxaemia induced significant increases in fetal arterial blood pressure, carotid blood flow and carotid vascular conductance and femoral vascular resistance, significant falls in femoral blood flow and femoral vascular conductance and transient bradycardia. These cardiovascular responses were accompanied by a fall in arterial pH, increases in blood glucose and blood lactate concentrations and increased plasma concentrations of catecholamines. In fetuses whose mothers were treated with dexamethasone, bradycardia persisted throughout hypoxaemia, the magnitude of the femoral vasoconstriction, the glycaemic, lactacidaemic and acidaemic responses and the plasma concentration of neuropeptide Y (NPY) were all enhanced during H1. However, during H2, all of these physiological responses were similar to saline controls. In dexamethasone fetuses, the increase in plasma adrenaline was attenuated during H1 and the increase in carotid vascular conductance during hypoxaemia failed to reach statistical significance both during H1 and during H2. These data show that maternal treatment with dexamethasone in doses and intervals used in human obstetric practice modified the fetal cardiovascular, metabolic and endocrine defence responses to acute hypoxaemia. Furthermore, dexamethasone-induced alterations to these defences depended on whether the hypoxaemic challenge occurred during or following maternal dexamethasone treatment.
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Introduction
Acute hypoxaemia elicits integrated cardiovascular, metabolic and endocrine responses that facilitate fetal survival during a period of reduced oxygen availability (Giussani et al. 1994b). In the ovine fetus during late gestation, the cardiovascular defence responses to an episode of acute hypoxaemia include transient bradycardia, a gradual increase in arterial blood pressure (Boddy et al. 1974; Giussani et al. 1993) and redistribution of the fetal combined ventricular output to preferentially perfuse hypoxia sensitive organs, such as the adrenal gland, heart and brain, at the expense of perfusion of peripheral vascular beds (Boddy et al. 1974; Cohn et al. 1974; Peeters et al. 1979; Jansen et al. 1979; Giussani et al. 1993; Giussani et al. 1994b; Jensen & Hanson, 1995; Bennet et al. 1999). The initial bradycardia and increase in peripheral vascular resistance result from carotid chemoreflex-mediated activation of the parasympathetic and sympathetic nervous systems, respectively (Itskovitz et al. 1991; Giussani et al. 1993, 1994b; Bartelds et al. 1993). Activation of the parasympathetic system triggers the fall in fetal heart rate in response to acute hypoxaemia, since bradycardia may be prevented by vagotomy (Boddy et al. 1974) and by muscurinic cholinergic antagonism using atropine (Berman et al. 1976; Parer et al. 1980; Ikenoue et al. 1981; Giussani et al. 1993). Activation of the sympathetic nervous system increases peripheral vascular tone via -adrenergic mediated vasoconstriction of peripheral circulations, such as the femoral vascular bed, since sympathectomy (Iwamoto et al. 1983) and -adrenergic blockade (Reuss et al. 1982; Giussani et al. 1993) prevent the vasoconstriction. Once initiated, peripheral vasoconstriction is maintained by the release of vasomotor agents into the fetal circulation, including catecholamines and NPY (Jones et al. 1988; Fletcher et al. 2000a). Increased circulating levels of catecholamines also stimulate the myocardium, and fetal heart rate subsequently returns towards baseline values (Court et al. 1984). Acute hypoxaemia also induces increased circulating concentrations of glucose and lactate (Jones, 1977; Jones et al. 1983). Hyperglycaemia during acute hypoxaemia results from reduced fetal glucose consumption in the first instance, followed by increased glucose production, which increases circulating glucose availability to the hypoxic tissues (Jones, 1977; Jones et al. 1983). Lactic acidaemia results from increased glucose availability as well as from decreased oxygen delivery to peripheral tissues and subsequent anaerobic metabolism of glucose (Boyle et al. 1992).
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The discovery of Liggins and Howie that cortisol has a maturational role in the fetus during late gestation (Liggins, 1969; Liggins & Howie, 1972) has led to routine use of antenatal glucocorticoid therapy in human obstetric practice to treat pregnant women at risk of preterm delivery (NIH Consensus Development Conference, 1995). In 1994, the National Institutes of Health in the United States recommended the use of synthetic glucocorticoids in all pregnancies in which delivery prior to 34 weeks is threatened or inevitable, for example due to preterm labour, preterm premature rupture of the membranes, preeclampsia, diabetes mellitus, third trimester bleeding or fetal distress. Clinical treatment involves maternal intramuscular (I.M.) injection of synthetic glucocorticoid by one of two recommended dose regimens: two I.M. injections of 12 mg synthetic glucocorticoid 24 h apart or four I.M. injections of 6 mg synthetic glucocorticoid 12 h apart (NIH Consensus Development Conference, 1995). A meta-analysis of randomised placebo-controlled trials from 1972 to 1994 showed that antenatal glucocorticoid therapy has resulted in a significant reduction in neonatal mortality, respiratory distress syndrome, intraventricular haemorrhage, and necrotizing enterocolitis associated with premature delivery (Crowley, 1995).
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Since antenatal glucocorticoids are administered to pregnant women most at risk of preterm delivery, episodes of acute fetal hypoxaemia, which are common during labour and delivery (Huch et al. 1977), are likely to occur during, or shortly after, steroid treatment. A previous study in our laboratory showed that direct treatment of the ovine fetus with dexamethasone modifies fetal cardiovascular, metabolic and endocrine responses to an episode of acute hypoxaemia during late gestation (Fletcher et al. 2000b, 2003b). Furthermore, the effects of dexamethasone exposure on the fetal responses to acute hypoxaemia differed depending on whether the hypoxaemic challenge occurred during fetal infusion with dexamethasone, or 2 days after cessation of dexamethasone treatment (Fletcher et al. 2003b). However, in human clinical practice dexamethasone is administered to the mother and not to the fetus. Therefore, the aim of the present study was to determine the cardiovascular, metabolic and endocrine responses to an episode of acute hypoxaemia occurring during and following maternal dexamethasone treatment in the ovine fetus during late gestation, using doses and dose intervals relevant to human clinical practice.
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Methods
Surgical preparation
Eighteen Welsh Mountain ewes carrying fetuses of known gestational age were used in the study. All procedures were performed under the UK Animals (Scientific Procedures) Act 1986. All food, but not water, was withheld from the animals for 24 h prior to surgery.
Surgery was performed under aseptic conditions at 118 ± 1 days of gestation (dGA; term is ca 145 dGA, mean ±S.E.M.). Anaesthesia was induced with sodium thiopentone (20 mg kg–1I.V. Intraval Sodium; Rhone Mérieux, Dublin, Ireland) and maintained with 1–2% halothane in 50: 50 O2–N2O. In brief, following abdominal and uterine incisions, the fetal head was exteriorized for insertion of a catheter (i.d. 0.86 mm, o.d. 1.52 mm; Critchly Electrical Products, NSW, Australia) into a carotid artery with the tip of the catheter extended to the ascending aorta. An ultrasonic flow transducer (2R or 2S with silicone flange; Transonics Inc., Ithaca, USA) was implanted around the contra-lateral carotid artery. The catheter was filled with heparinized saline (100 i.u. heparin ml–1 in 0.9% NaCl) and plugged with a brass pin, and the uterine incision closed in layers. The fetal hindlimbs were exteriorized via a second uterine incision for insertion of a femoral artery catheter (i.d. 0.86 mm, o.d. 1.52 mm, Critchly Electrical Products, NSW, Australia), which was advanced into the descending aorta. A transit-time flow transducer (2R or 2S with silicone flange; Transonics Inc., Ithaca, USA) was placed around the contra-lateral femoral artery. A further catheter was anchored onto the fetal hind limb in the amniotic cavity for recording of amniotic pressure. The catheters were filled with heparinized saline (100 i.u. heparin ml–1 in 0.9% NaCl), plugged with sterile brass pins and the uterine incision was closed in layers.
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In 12 ewes (6 saline and 6 dexamethasone) a Teflon catheter was inserted into the maternal femoral artery and advanced to the descending aorta. Antibiotics were administered to the fetus (I.A. 300 mg ampicillin; Penbritin, SmithKline-Beecham Animal Health, Surrey, UK) and into the amniotic cavity (300 mg ampicillin). All catheters were exteriorized through a key-hole incision in the maternal flank and housed in a pouch sutured onto the maternal skin.
Post-operative care
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Animals were housed in individual pens with access to hay and water ad libitum. Concentrates were fed twice daily (100 g; Sheep Nuts No. 6; H & C Beart Ltd, Kings Lynn, UK). All ewes received antibiotics (0.20–0.25 mg kg–1 I. Depocillin; Mycofarm, Cambridge, UK) immediately after surgery and daily for 3 days. Patency of fetal vascular catheters was maintained by a slow continuous infusion of heparinized saline (100 i.u. heparin ml–1 at 0.1 ml h–1 in 0.9% NaCl) containing antibiotic (10 mg ml–1 benzylpenicillin; Crystapen, Schering-Plough, Animal Health Division, Welwyn Garden City, UK).
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Experimental procedure
Six days after surgery, at 124 ± 1 dGA, the animals were divided randomly into two experimental groups. Ten ewes received 2 x 12 mg injections of dexamethasone (dexamethasone sodium phosphate; Merck Sharpe, Dohme Ltd, Herts, UK) in 2 ml saline (0.9% NaCl) 24 h apart. Eight ewes received two injections of 2 ml of saline 24 h apart to serve as age-matched controls. Clinical treatment involves one of two recommended dose regimens: two I.M. injections of 12 mg synthetic glucocorticoid 24 h apart (usually betamethasone) or four I.M. injections of synthetic glucocorticoid 12 h apart (usually dexamethasone; NIH Consensus Development Conference, 1995). In this study, the regimen of two I.M. injections of 12 mg dexamethsone 24 h apart was chosen for practicality reasons.
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All ewes and their fetuses were subjected to two separate episodes of acute hypoxaemia, induced by reducing the maternal FIO2, at 125 ± 1 dGA (8 h after the second maternal injection; Hypoxaemia 1, H1) and again at 128 ± 1 dGA (3 days after the second dexamethasone injection; Hypoxaemia 2, H2). The rationale for the first episode of acute hypoxaemia was to determine if physiological responses to acute stress were affected when dexamethasone was present in the maternal and fetal circulations. The acute hypoxaemic challenge was performed 8 h after the second injection at a time when fetal plasma concentrations of dexamethasone were similar to those measured during direct fetal treatment (Fletcher et al. 2000b). The rationale for the second episode of acute hypoxaemia was to determine the long-term effects of glucocorticoid exposure after the dexamethasone had cleared from both the maternal and fetal circulations (Jellyman et al. 2004). The protocol for acute hypoxaemia involved a 3 h experiment consisting of 1 h of normoxia, 1 h of hypoxaemia and 1 h of recovery as previously described in detail (Giussani et al. 1993). In brief, air was passed for 1 h at a rate of ca 40 l min–1 into a large, transparent, polythene bag placed over the ewes' head. Following this control period, maternal and fetal hypoxaemia was induced by changing the concentrations of gases breathed by the ewe to 9% O2 in N2 with 2–3% CO2. This mixture was designed to reduce maternal and fetal arterial Pa,O2 to ca 35 and 12 mmHg, respectively, while maintaining maternal and fetal arterial isocapnia. Following the 1-h period of hypoxaemia the ewe was returned to breathing air for the 1 h recovery period.
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Measurements and biochemical analyses
During the hypoxaemia protocols, arterial blood samples (4 ml) were collected at 15 and 45 min of normoxia, after 15 and 45 min of hypoxaemia and after 45 min of recovery. The descending aortic blood samples (0.4 ml) were drawn into sterile syringes and analysed for measurement of arterial blood gases, percentage saturation of O2 in haemoglobin, haemoglobin concentration and acid/base status using an ABL5 blood gas analyser and OSM2 haemoximeter (Radiometer, Copenhagen, Denmark). Measurements in maternal and fetal blood using the ABL5 blood gas analyser were corrected to 38 and 39.5°C, respectively. Blood glucose and lactate concentrations were also measured using an automated analyser (Yellow Springs 2300 Stat Plus Glucose/Lactate Analyser; YSI Ltd, Farnborough, UK). Haematocrit was calculated as the ratio of the volume of red cells to the volume of whole blood. All blood samples for hormone analysis were collected into K+/EDTA-treated tubes, kept on ice and centrifuged at 1200 g for 4 min at 4°C. Plasma samples were stored at –70°C until analyses.
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Dexamethasone. Plasma dexamethasone levels were measured after ether extraction using tritium-labelled dexamethasone as tracer, as previously described (Fletcher et al. 2000b). All values were corrected for recovery (86%). The interassay coefficients of variation for three control plasma pools (1.8, 5.4 and 26.7 nmol l–1) were 14.6, 9.3 and 8.2%, respectively. The lower detection limit of the assay was 0.2 nmol l–1. The antidexamethasone antiserum showed a 1.6% cross-reactivity against cortisol and cross-reactivities of less than 0.5% against 11-deoxycortisol, corticosterone, testosterone, progesterone and oestriol.
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Catecholamines. Plasma noradrenaline and adrenaline concentrations were measured by HPLC with electrochemical detection as previously described in detail (Silver et al. 1982). Samples were prepared by absorption of 250 μl of plasma onto acid-washed alumina and 20 μl aliquots of the 100 μl perchloric acid elutes were injected onto the column. Dihydroxybenzylamine was added as the internal standard to each plasma sample before absorption. Recovery ranged from 63 to 97% and all catecholamine values were corrected for their respective recovery. The interassay coefficients of variation for noradrenaline and adrenaline were 6.2% and 7.3%, respectively, and the minimum detectable concentration was 10 pg ml–1.
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NPY. Plasma concentrations of NPY were measured by radioimmunoassay as previously described in detail (Fletcher et al. 2000a). The NPY assay could detect less than 1 pmol l–1 (95% confidence interval). The interassay coefficient of variation was 6.8% and there was no detectable cross-reactivity of the anti-NPY antiserum with peptide YY.
Data collection and analyses
Analog signals for calibrated ascending and descending aortic blood pressures, heart rate, carotid and femoral blood flows were recorded continuously during the experimental protocol using a data acquisition system. The signals were digitized, displayed and subsequently stored at 1 s intervals on disk by custom software (NI-DAQ, National Instruments, Austin, Texas) running on a PC. Files were subsequently analysed using Microsoft Excel spreadsheets. Fetal ascending and descending aortic blood pressures were corrected for amniotic pressure. Carotid vascular conductance was calculated every minute by dividing mean carotid blood flow by mean ascending aortic arterial blood pressure. Femoral vascular resistance was calculated every minute by dividing mean descending aortic blood pressure by mean femoral blood flow.
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One dexamethasone and two saline fetuses were instrumented with carotid flow probes only, four dexamethasone and two saline fetuses were instrumented with femoral flow probes only, and five dexamethasone and four saline fetuses were instrumented with both carotid and femoral flow probes to give a total of 10 dexamethasone and eight saline fetuses. One carotid flow probe (1 saline) and three femoral flow probes (3 dexamethasone) developed acoustic errors by H2. However, flow measurements from at least five fetuses, and in most instances six fetuses, from any one group were obtained and used for statistical comparisons.
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Statistical analyses
Values for all variables are expressed as means ±S.E.M. All measured variables were first analysed for normality of distribution. All data obtained were parametric and were compared using two-way ANOVA with repeated measures (SigmaStat; Systat Software Inc., Point Richmond, CA, USA) followed by an appropriate post hoc test. For all comparisons, statistical significance was accepted when P < 0.05.
Results
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Dexamethasone immunoreactivity was below detectable levels in all animals prior to maternal injection, and remained undetectable throughout the experimental protocol in saline injected ewes and fetuses. The plasma concentration of dexamethasone was significantly elevated in dexamethasone fetuses (3.8 ± 0.3 nmol l–1) during H1, but was below the detection limit of the assay during H2.
Blood gas and acid/base status
Maternal. During the normoxic period of H1 and H2, values for arterial blood gas and acid/base status were similar in saline- and dexamethasone-treated ewes (Tables 1 and 2). Acute hypoxaemia induced similar falls in Pa,O2 and percentage saturation of haemoglobin and increases in haemoglobin concentrations without alterations in arterial pH or PCO2 in all ewes. Although haematocrit tended to increase in all ewes during hypoxaemia this fell outside of statistical significance in saline-treated ewes (Tables 1 and 2). During recovery all variables returned towards baseline values (Tables 1 and 2).
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Fetal. Values for fetal arterial blood gas and acid/base status were similar in dexamethasone and saline groups during the normoxic periods of H1 and H2 (Tables 3 and 4). Acute hypoxaemia induced similar falls in fetal Pa,O2 and percentage saturation of haemoglobin, increases in haemoglobin concentrations without alterations in Pa,CO2 or haematocrit in all fetuses during H1 and H2 (Tables 3 and 4). Acute hypoxaemia induced progressive acidaemia, but the magnitude of the fall in arterial pH was significantly greater in dexamethasone than saline groups during H1 only (Tables 3 and 4).
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Metabolic status
Maternal. Basal blood glucose concentrations were significantly elevated in dexamethasone- compared with saline-treated ewes during the normoxic period of H1 (Table 1). In contrast, during the normoxic period of H2 maternal blood glucose concentrations were similar in saline- and dexamethasone-treated ewes (Table 2). Basal blood lactate concentrations were similar in all ewes during the normoxic periods of H1 and H2 (Tables 1 and 2). Although acute hypoxaemia did not induce significant alterations in maternal blood concentrations of glucose or lactate during H1 or H2, there was a tendency for maternal glucose concentrations to increase during H1 only (Tables 1 and 2).
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Fetal. Blood glucose concentrations were significantly elevated in dexamethasone fetuses during the normoxic period of H1, but not H2 (Tables 3 and 4). Acute hypoxaemia induced increases in blood concentrations of glucose and lactate in all fetuses. The magnitude of the increase in both blood glucose and lactate were significantly greater in dexamethasone compared with saline fetuses during H1, but not H2. The ratio of glucose concentration in the fetal: maternal blood was significantly increased by 45 min of hypoxaemia in all groups. However, the magnitude of the increase in the ratio was significantly greater in dexamethasone-treated fetuses during H2 only (Fig. 1).
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Arterial blood samples were collected at 15 (N15) and 45 min (N45) of normoxia (baseline), at 15 (H15) and 45 min (H45) of hypoxaemia, and at 15 (R15) and 45 min (R45) of recovery during H1 and H2. Values are means ±S.E.M. for saline- (, n= 8) and dexamethasone- (, n= 10) treated ewes and their fetuses. Statistical differences are P < 0.05: a, normoxia versus hypoxaemia or recovery; b, saline versus dexamethasone ewes (two way RM ANOVA with Tukey's test).
Fetal cardiovascular variables
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During the normoxic period of H1, basal ascending aortic blood pressure, basal carotid flow and basal carotid vascular conductance were similar in dexamethasone (48 ± 4 mmHg, 56 ± 8 ml min–1, 1.17 ± 0.17 ml min–1 mmHg–1) and saline (43 ± 3 mmHg, 57 ± 5 ml min–1, 1.35 ± 0.16 ml min–1 mmHg–1) groups. Basal values for fetal ascending aortic blood pressure, carotid blood flow, and carotid vascular conductance were similar in dexamethasone (47 ± 4 mmHg, 72 ± 7 ml min–1, 1.57 ± 0.16 ml min–1 mmHg–1) and saline (38 ± 1 mmHg, 56 ± 6 ml min–1, 1.50 ± 0.16 ml min–1 mmHg–1) groups during the normoxic period of H2.
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In saline fetuses, acute hypoxaemia elicited an increase in ascending aortic blood pressure, carotid blood flow and carotid vascular conductance (Figs 2 and 3). However, in fetuses whose mothers received dexamethasone, the increases in carotid vascular conductance failed to reach statistical significance during H1 and H2 (Figs 2 and 3).
Fetal ascending aortic blood pressure, descending aortic blood pressure, heart rate, carotid blood flow, carotid vascular conductance, femoral blood flow and femoral vascular resistance during acute hypoxaemia (H1) during maternal dexamethasone treatment. Values represent either mean changes from baseline ±S.E.M. calculated every minute during 1 h of normoxia, 1 h of hypoxaemia and 1 h of recovery (line graph) or the mean ±S.E.M. area under the curve for each hour of the experimental protocol (bar graphs) in saline (; H1: n= 6, H2, n= 5) or dexamethasone (; H1: n= 6, H2: n= 6) fetuses. Statistical differences are P < 0.05: a, normoxia versus hypoxaemia or recovery; b, saline versus dexamethasone ewes (area under the curve; two way RM ANOVA with Tukey's test).
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Fetal ascending aortic blood pressure, descending aortic blood pressure, heart rate, carotid blood flow, carotid vascular conductance, femoral blood flow and femoral vascular resistance during acute hypoxaemia (H2) following maternal dexamethasone treatment. Values represent either mean changes from baseline ±S.E.M. calculated every minute during 1 h of normoxia, 1 h of hypoxaemia and 1 h of recovery (line graph) or the mean ±S.E.M. area under the curve for each hour of the experimental protocol (bar graphs) in saline (; H1: n= 6, H2, n= 5) or dexamethasone (; H1: n= 6, H2: n= 6) fetuses. Statistical differences are P < 0.05: a, normoxia versus hypoxaemia or recovery (area under the curve; two way RM ANOVA with Tukey's test).
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Acute oxygen deprivation elicited bradycardia, increases in fetal descending aortic blood pressure and femoral vascular resistance, as well as falls in femoral blood flow in all fetuses during H1 and H2 (Figs 2 and 3). In saline-treated fetuses, bradycardia was transient and returned towards baseline values within 20 min of the onset of the hypoxaemic challenge (Figs 2 and 3). In contrast, bradycardia persisted throughout acute hypoxaemia in dexamethasone-treated fetuses during H1 and H2 (Figs 2 and 3). The magnitudes of the fetal bradycardia, pressor and vasopressor responses were significantly greater in dexamethasone fetuses during H1, but not H2 (Figs 2 and 3).
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Fetal endocrine variables
Fetal plasma concentrations of catecholamines and NPY were similar in saline and dexamethasone fetuses during the normoxic periods of H1 and H2 (Fig. 4). In saline fetuses, acute hypoxaemia induced significant increases in plasma concentrations of noradrenaline and adrenaline (Fig. 4). In fetuses whose mothers were treated with dexamethasone, the increase in plasma concentrations of adrenaline was significantly attenuated during H1, but not H2 (Fig. 4). In contrast, plasma NPY concentrations were significantly enhanced in dexamethasone compared with saline fetuses at 45 min of hypoxaemia during H1, but not H2 (Fig. 4).
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Arterial blood samples were collected at 15 (N15) and 45 min (N45) of normoxia (baseline), at 15 (H15) and 45 min (H45) of hypoxaemia, and at 45 min (R45) of recovery during H1 and H2. Values are means ±S.E.M. for fetuses of saline- (; n= 6) and dexamethasone- (; n= 6) treated ewes. Statistical differences are P < 0.05: a, normoxia versus hypoxaemia or recovery; b, saline versus dexamethasone ewes (two Way RM ANOVA and Tukey's test).
Discussion
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The data in the present study show that in fetuses whose mothers received saline, acute hypoxaemia induced significant increases in fetal arterial blood pressure, carotid blood flow, carotid vascular conductance significant falls in femoral blood flow and femoral vascular resistances, bradycardia in all animals. These cardiovascular responses were accompanied by a fall in arterial pH, increases in blood glucose and blood lactate concentrations and increased plasma concentrations of catecholamines. In fetuses whose mothers were treated with dexamethasone, the bradycardia persisted throughout hypoxaemia, and the magnitude of the femoral vasoconstriction, the glycaemic, lactacidaemic and acidaemic responses, and the plasma NPY concentration were all enhanced when the acute hypoxaemic episode occurred during dexamethasone treatment. However, when the acute hypoxaemia protocol occurred 3 days after the second maternal dexamethasone injection, all of these alterations in cardiovascular, metabolic and endocrine responses were similar to saline controls. In dexamethasone fetuses, the increase in plasma adrenaline concentration was attenuated during treatment and the increase in carotid vascular conductance during hypoxaemia failed to reach statistical significance both during and after dexamethasone treatment.
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The persistence of the bradycardic response throughout hypoxaemia in dexamethasone fetuses when the challenge occurred during exposure to the steroid is a similar finding to that reported in a previous study which addressed the effects of direct fetal infusion with low doses of dexamethasone on fetal cardiovascular function (Fletcher et al. 2000b). However, in contrast to the findings of Fletcher and colleagues, bradycardia did not persist during hypoxaemia when the glucocorticoid was no longer detectable in the fetal circulation. The mechanism accounting for persisting bradycardia in dexamethasone fetuses may include increased negative chronotropic and/or decreased positive chronotropic effects on the fetal heart. Dexamethasone may increase the activity or gain of the carotid chemoreflex during acute hypoxaemia, since ontogenic changes in the set point and sensitivity of the arterial chemoreceptors (Blanco et al. 1984) and increases in the gain of the cardiac and femoral vasoconstrictor components of the fetal chemoreflex (Fletcher et al. 2003b) both occur in association with the prepartum increase in endogenous fetal cortisol (Fowden, 1995). Synthetic glucocorticoids may also enhance cholinergic signal transduction at the myocardium since endogenous glucocorticoids increase muscarinic ACh receptor affinity in rat myocardial cells in vivo (Jacobsson et al. 1983) and in vitro (Ransnas et al. 1987). A decrease in positive chronotropic effects on the fetal heart by dexamethasone may result from depression of circulating catecholamines and/or a reduction in myocardial sensitivity to -adrenergic receptor agonists. Fetal basal plasma concentrations of catecholamines were suppressed during fetal infusion (Derks et al. 1997) and 3 h after delivery in lambs treated with betamethasone in utero (Ervin et al. 2000). Moreover, the increment in fetal plasma adrenaline during acute hypoxaemia was attenuated during dexamethasone infusion of the ovine fetus (Fletcher et al. 2003b) and during, but not following, maternal treatment with dexamethasone in the present study. Dexamethasone has also been reported to decrease myocardial cell sensitivity to -adrenergic stimulation in rats treated prenatally (Hou & Slotkin, 1989), or postnatally (Lau & Slotkin, 1981).
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During acute hypoxaemia, the fetal combined ventricular output is redistributed towards central and away from peripheral circulations (Cohn et al. 1974; Giussani et al. 1993). The cerebral circulation is thus protected during hypoxaemia at the expense of the peripheral vascular beds, which, in turn, act as a reservoir of vascular resistance to aid the maintenance of perfusion in hypoxia-sensitive circulations. Although fetal carotid blood flow increased in all fetuses during acute hypoxaemia, the increase tended to be delayed in dexamethasone fetuses. Similarly, the increase in fetal cerebral blood flow in response to a hypercapnic challenge was diminished due to decreased cerebral vasodilatation during betamethasone infusion into the ovine fetus (Schwab et al. 2000; Lohle et al. 2005). Moreover, the depressive effect of dexamethasone on the hyperaemic response of the carotid vascular bed during acute hypoxaemia is similar to the effects of dexamethasone on haemodynamic responses in other essential vascular beds, such as the umbilical circulation. In another study, maternal dexamethasone abolished the normal increase in umbilical blood flow during acute hypoxaemic challenges occurring either during or 3 days after treatment in ovine fetuses during late gestation (Jellyman et al. 2004).
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In contrast to essential circulations, dexamethasone significantly enhanced the fall in femoral blood flow and the increase in femoral vascular resistance in the present study, similar to a study of responses to acute hypoxaemia during direct infusion of the ovine fetus with low doses of dexamethasone (Fletcher et al. 2003b). Synthetic glucocorticoids may enhance the fetal sympathetic outflow, since maternal dexamethasone treatment increased renal sympathetic nerve activity in lambs at birth (Segar et al. 1998) and enhanced the increase in plasma concentrations of NPY during acute hypoxaemia (Fletcher et al. 2000a). NPY is colocalized and coreleased with noradrenaline at sympathetic nervous terminals (Lundberg et al. 1983; Ekblad et al. 1984) and unlike noradrenaline lacks re-uptake mechanisms (Lundberg, 1996), making measurement of changes in its circulating concentration a useful measure of sympathetic nervous system activity. The adrenal medulla is unlikely to contribute to circulating levels of NPY in rats (Mormede et al. 1990), calves (Bloom et al. 1988) or lambs (Bloom et al. 1989). Hence, increased plasma concentrations during acute stress most likely reflect overspill from sympathetic nervous terminals.
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NPY may also contribute to the enhanced femoral vasoconstriction in dexamethasone-treated fetuses by endocrine and neuromodulator mechanisms. In adult animals NPY elicits potent long-lasting vasoconstriction by direct actions on NPY-Y1 receptors in the peripheral vasculature and/or by potentiating the effects of circulating vasoconstrictors, such as catecholamines (Edvinsson et al. 1984; Franco-Cereceda et al. 1985; Corder et al. 1986; Waeber et al. 1990; Balasubramaniam, 2003). Accordingly, fetal plasma NPY concentrations during basal and acute hypoxaemic conditions showed a significant positive correlation with the increase in femoral vascular resistance in ovine fetuses during late gestation (Fletcher et al. 2000a).
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In addition to altering the balance of circulating plasma concentrations of constrictors and dilators, dexamethasone may alter vascular sensitivity to vasomotor agents. However, exposure to glucocorticoids in utero did not alter vascular sensitivity to the vasoconstrictors noradrenaline or angiotensin II in fetal sheep (Fletcher et al. 2002) or lambs (Dodic et al. 1998). Conversely, studies using in vitro wire myography to examine isometric vascular responses of small femoral arterial branches from fetal sheep during late gestation suggest that betamethasone significantly increased the maximum vasoconstriction to noradrenaline and decreased endothelium-independent responses to both bradykinin and forskolin (Anwar et al. 1999). Similarly, the maximum endothelin-1-induced vasoconstriction was greater in femoral arteries taken from dexamethasone-infused fetuses (Docherty et al. 2001), probably due to down-regulation of endothelial NO release (Molnar et al. 2003).
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It is possible that dexamethasone could have induced alterations in components of vascular structure, such as tropoeleastin (Pierce et al. 1995) or collagen and non-collagen proteins (Leitman et al. 1984). However, dexamethasone-induced vascular hypertrophy or hyperplasia is unlikely to account for the enhanced femoral vasoconstrictor responses during acute hypoxaemia since they were reversible following treatment.
Hence, past and present data suggest that contrasting effects of dexamethasone on essential and peripheral vascular beds appear to be mediated via similar mechanisms, decreasing the gain of dilator mechanisms and increasing the gain of constrictor mechanisms. Thus, the magnitude of the response in circulations which normally dilate during hypoxaemia is diminished, such as in the carotid and umbilical vascular beds. In contrast, the magnitude of the response in circulations which normally constrict during hypoxaemia is enhanced, such as in the femoral vascular bed.
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In accordance with previous studies, hypoxaemia-induced increases in fetal blood glucose concentrations (Jones, 1977; Hooper et al. 1995) were enhanced by dexamethasone treatment (Fletcher et al. 2000b). Whilst a greater increase in fetal blood glucose concentrations during, but not following, maternal glucocorticoid treatment suggests that enhancement of the glycaemic response may be dependent on the continued presence of the synthetic glucocorticoid in the fetal circulation, differential mechanisms may account for the glucogenic responses at these times. Increased fetal blood glucose may reflect increased transplacental glucose flux, decreased fetal glucose utilization and/or increased stimulation of endogenous fetal glucose production. In agreement with previous studies, hypoxaemia increased the ratio of glucose in the fetal: maternal circulation by 45 min in all animals, indicative of elevations in glucose concentration of fetal origin (Jones et al. 1983). In the present study it is not possible to distinguish between decreased fetal glucose utilization and increased endogenous fetal glucose production. Similar fetal: maternal blood glucose ratio between groups suggest that increased basal and stimulated fetal blood glucose concentrations during H1 probably resulted from increased transplacental glucose transport in dexamethasone fetuses. Conversely, a greater increment in the fetal: maternal glucose ratio at 45 min of hypoxaemia during H2 suggests that dexamethasone may have matured the fetal glucogenic capacity by 3 days after treatment. Glucocorticoids are known to increase the capacity for glucose production by increasing hepatic glycogen content (Barnes et al. 1978) and hepatic gluconeogenic enzyme activities (Fowden et al. 1990) in the ovine fetus during late gestation. Recent studies in our laboratory suggest that dexamethasone increases glycogenolytic but not gluconeogenic capacity in the fetus. Maternal dexamethasone significantly increased the activity of glucose-6-phosphatase, the final enzyme in both the gluconeogenic and glycogenolytic pathways, irrespective of whether enzyme activities were expressed per gram wet tissue weight or per gram of protein. However, dexamethasone did not affect activity of phosphoenolpyruvate carboxykinase, a rate-limiting enzyme specific to the gluconeogenic pathway (K. L. Franko, D. A. Giussani & A. L. Fowden, unpublished observations).
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In the present study, the acidaemic and lactic acidaemic responses to acute hypoxaemia in fetuses whose mothers were treated with dexamethasone were significantly enhanced during, but not following, glucocorticoid treatment. These data contrast with a previous study in which the increment in fetal blood lactate concentrations in dexamethasone-infused fetuses were similar to saline controls during hypoxaemia despite an enhanced glycaemic response (Fletcher et al. 2000b). The fetal hind limb (Boyle et al. 1992), but not the placenta (Gu et al. 1985; Hooper et al. 1995), may represent a source of anaerobic lactate output due to decreased oxygen delivery and subsequent anaerobic metabolism during fetal hypoxaemia. Lactate is an important metabolic substrate in the ovine fetus during basal conditions (Fisher et al. 1982; Gleason et al. 1990) and dexamethasone may promote cerebral lactate utilization, since lactate crosses the blood brain barrier in sheep fetuses near term, is oxidized and at elevated concentrations can complement glucose as an oxidative substrate (Turbow et al. 1995).
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The greater fall in arterial pH in dexamethasone- compared with saline-treated fetuses may have occurred secondary to the greater increment in blood lactate, which is the principal factor governing acidaemia in the fetus (Lawrence et al. 1982). Furthermore, acidaemia may contribute to the enhanced cardiovascular responses to hypoxaemia since Gardner et al. (2002) showed that changes in fetal arterial pH, rather than Pa,O2, were a greater determinant of fetal femoral vasoconstriction in response to acute hypoxaemia in chronically acidaemic fetuses. However, the extent to which dexamethasone-induced fetal acidaemia is a cause or a consequence of the enhanced cardiovascular responses to acute hypoxaemia in this and previous studies remains unknown. Whatever the mechanism involved, a positive feed-forward cycle between increased acidosis, enhanced peripheral vasoconstriction and augmented lactic acidaemia in the fetus may help augment the gain of the fetal cardiovascular responses to acute hypoxaemia during fetal exposure to dexamethasone.
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In the ovine fetus during late gestation heart rate deceleration during acute hypoxaemia has several functional advantages. Fisher et al. (1982) suggested that the product of the heart rate and blood pressure (the rate-pressure product) is a useful correlate of myocardial oxygen consumption as it reflects myocardial workload. In untreated fetal sheep during late gestation, the rate–pressure product decreases during acute hypoxaemia, thereby decreasing myocardial workload and cardiac oxygen consumption. Fetal bradycardia reduces coronary flow, which enhances myocardial oxygen extraction (Fisher et al. 1982; Thornburg & Reller, 1999) and prolongs end-diastolic filling times, which increases ventricular end-diastolic volume, promoting a greater force of ventricular contraction through the Frank-Starling mechanism and increasing stroke volume (Anderson et al. 1986). During acute hypoxaemia fetal cardiac output is thereby maintained despite a pronounced fall in heart rate. In fetuses exposed to dexamethasone, prolonged bradycardia will maintain these advantages throughout the period of hypoxaemia. In addition, maintenance of cardiac function during anaerobic metabolism is dependent on the glycolytic flux from myocardial glycogen stores or from circulating blood glucose (Hoerter, 1976; Jarmakani et al. 1978; Hoerter & Opie, 1978; Schwab et al. 2000). We suggest that in contrast to the short-lived advantages conveyed by maternal dexamethasone on peripheral vascular resistance, cardiac output and its distribution, dexamethasone-induced enhancement of the fetal capacity for glucose production in the fetus may provide a real advantage. Increased capacity for glucose production in the fetus will improve glucose delivery to hypoxia-sensitive circulations, such as the myocardial and cerebral vascular beds. The increased glucogenic capacity in the fetus will, at the very least, maintain glucose delivery to the brain, despite a smaller increment in carotid perfusion during and following dexamethasone treatment.
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Comparison of the findings in the present study with previous studies revealed that maternal dexamethasone treatment had broadly similar effects on the fetal cardiovascular, metabolic and endocrine responses to an episode of acute hypoxaemia (Fletcher et al. 2000b; Fletcher et al. 2003b). However, the cardiovascular effects of direct fetal treatment with steroids appear to persist after steroid clearance from the fetal circulation whereas those of maternal treatment are short-lived. The mechanisms accounting for the differential effects of maternal and fetal treatment with dexamethasone on fetal physiology are unknown, but may reflect differences in concentrations of dexamethasone achieved in the fetal circulation, duration of exposure to the synthetic glucocorticoid, the subsequent secretion of natural cortisol and/or the contribution of effects secondary to changes in maternal physiology induced by glucocorticoid treatment.
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In conclusion, maternal treatment with dexamethasone in doses and dose intervals used in human clinical practice modified the fetal cardiovascular, metabolic and endocrine defence responses to acute hypoxaemia in the late gestation ovine fetus. Furthermore, dexamethasone-induced alterations to these defences depended on whether the hypoxaemic challenge occurred during or following maternal dexamethasone treatment. These findings have important implications for antenatal glucocorticoid therapy in current human obstetric practice.
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References
Anderson PA, Glick KL, Killam AP & Mainwaring RD (1986). The effect of heart rate on in utero left ventricular output in the fetal sheep. J Physiol 372, 557–573.
Anwar MA, Schwab M, Poston L & Nathanielsz PW (1999). Betamethasone-mediated vascular dysfunction and changes in hematological profile in the ovine fetus. Am J Physiol 276, H1137–H1143.
Balasubramaniam A (2003). Neuropeptide Y (NPY) family of hormones: progress in the development of receptor selective agonists and antagonists. Curr Pharm Des 9, 1165–1175.
, http://www.100md.com
Barnes RJ, Comline RS & Silver M (1978). Effect of cortisol on liver glycogen concentrations in hypophysectomized, adrenalectomized and normal foetal lambs during late or prolonged gestation. J Physiol 275, 567–579.
Bartelds B, van Bel F, Teitel DF & Rudolph AM (1993). Carotid, not aortic, chemoreceptors mediate the fetal cardiovascular response to acute hypoxemia in lambs. Pediatr Res 34, 51–55.
Bennet L, Kozuma S, McGarrigle HH & Hanson MA (1999). Temporal changes in fetal cardiovascular, behavioural, metabolic and endocrine responses to maternally administered dexamethasone in the late gestation fetal sheep. Br J Obstet Gynaecol 106, 331–339.
, 百拇医药
Berman W, Goodlin RC, Heymann MA & Rudolph AM (1976). Relationships between pressure and flow in the umbilical and uterine circulations of the sheep. Circ Res 38, 262–266.
Blanco CE, Dawes GS, Hanson MA & McCooke HB (1984). The response to hypoxia of arterial chemoreceptors in fetal sheep and new-born lambs. J Physiol 351, 25–37.
Bloom SR, Edwards AV & Jones CT (1988). The adrenal contribution to the neuroendocrine responses to splanchnic nerve stimulation in conscious calves. J Physiol 397, 513–526.
, 百拇医药
Bloom SR, Edwards AV & Jones CT (1989). Neuroendocrine responses to stimulation of the splanchnic nerves in bursts in conscious, adrenalectomized, weaned lambs. J Physiol 417, 79–89.
Boddy K, Dawes GS, Fisher R, Pinter S & Robinson JS (1974). Foetal respiratory movements, electrocortical and cardiovascular responses to hypoxaemia and hypercapnia in sheep. J Physiol 243, 599–618.
Boyle DW, Meschia G & Wilkening RB (1992). Metabolic adaptation of fetal hindlimb to severe, nonlethal hypoxia. Am J Physiol 263, R1130–R1135.
, 百拇医药
Cohn HE, Sacks EJ, Heymann MA & Rudolph AM (1974). Cardiovascular responses to hypoxemia and acidemia in fetal lambs. Am J Obstet Gynecol 120, 817–824.
Corder R, Lowry PJ, Wilkinson SJ & Ramage AG (1986). Comparison of the haemodynamic actions of neuropeptide Y, angiotensin II and noradrenaline in anaesthetised cats. Eur J Pharmacol 121, 25–30.
Court DJ, Parer JT, Block BS & Llanos AJ (1984). Effects of beta-adrenergic blockade on blood flow distribution during hypoxaemia in fetal sheep. J Dev Physiol 6, 349–358.
, http://www.100md.com
Crowley PA (1995). Antenatal corticosteroid therapy: a meta-analysis of the randomized trials, 1972–94. Am J Obstet Gynecol 173, 322–335.
Derks JB, Giussani DA, Jenkins SL, Wentworth RA, Visser GH, Padbury JF & Nathanielsz PW (1997). A comparative study of cardiovascular, endocrine and behavioural effects of betamethasone and dexamethasone administration to fetal sheep. J Physiol 499, 217–226.
Docherty CC, Kalmar-Nagy J, Engelen M, Koenen SV, Nijland M, Kuc RE, Davenport AP & Nathanielsz PW (2001). Effect of in vivo fetal infusion of dexamethasone at 0.75 GA on fetal ovine resistance artery responses to ET-1. Am J Physiol Regul Integr Comp Physiol 281, R261–R268.
, 百拇医药
Dodic M, May CN, Wintour EM & Coghlan JP (1998). An early prenatal exposure to excess glucocorticoid leads to hypertensive offspring in sheep. Clin Sci (Lond) 94, 149–155.
Edvinsson L, Ekblad E, Hakanson R & Wahlestedt C (1984). Neuropeptide Y potentiates the effect of various vasoconstrictor agents on rabbit blood vessels. Br J Pharmacol 83, 519–525.
Ekblad E, Edvinsson L, Wahlestedt C, Uddman R, Hakanson R & Sundler F (1984). Neuropeptide Y co-exists and co-operates with noradrenaline in perivascular nerve fibers. Regul Pept 8, 225–235.
, 百拇医药
Ervin MG, Padbury JF, Polk DH, Ikegami M, Berry LM & Jobe AH (2000). Antenatal glucocorticoids alter premature newborn lamb neuroendocrine and endocrine responses to hypoxia. Am J Physiol Regul Integr Comp Physiol 279, R830–R838.
Fisher DJ, Heymann MA & Rudolph AM (1982). Fetal myocardial oxygen and carbohydrate consumption during acutely induced hypoxemia. Am J Physiol 242, H657–H661.
Fletcher AJ, Edwards CM, Gardner DS, Fowden AL & Giussani DA (2000a). Neuropeptide Y in the sheep fetus: effects of acute hypoxemia and dexamethasone during late gestation. Endocrinology 141, 3976–3982.
, 百拇医药
Fletcher AJW, McGarrigle HHG, Edwards CMB, Fowden AL & Giussani DA (2002b). Effects of low dose dexamethasone treatment on basal cardiovascular and endocrine function in fetal sheep during late gestation. J Physiol 545, 649–660.
Fletcher AJ, Gardner DS, Edwards CM, Fowden AL & Giussani DA (2003b). Cardiovascular and endocrine responses to acute hypoxaemia during and following dexamethasone infusion in the ovine fetus. J Physiol 549, 271–287.
, http://www.100md.com
Fletcher AJ, Goodfellow MR, Forhead AJ, Gardner DS, McGarrigle HH, Fowden AL & Giussani DA (2000b). Low doses of dexamethasone suppress pituitary-adrenal function but augment the glycemic response to acute hypoxemia in fetal sheep during late gestation. Pediatr Res 47, 684–691.
Fowden AL (1995). Endocrine regulation of fetal growth. Reprod Fertil Dev 7, 351–363.
Fowden AL, Coulson RL & Silver M (1990). Endocrine regulation of tissue glucose-6-phosphatase activity in the fetal sheep during late gestation. Endocrinology 126, 2823–2830.
, 百拇医药
Franco-Cereceda A, Lundberg JM & Dahlof C (1985). Neuropeptide Y and sympathetic control of heart contractility and coronary vascular tone. Acta Physiol Scand 124, 361–369.
Gardner DS, Fletcher AJ, Bloomfield MR, Fowden AL & Giussani DA (2002). Effects of prevailing hypoxaemia, acidaemia or hypoglycaemia upon the cardiovascular, endocrine and metabolic responses to acute hypoxaemia in the ovine fetus. J Physiol 540, 351–366.
, 百拇医药
Giussani DA, Spencer JAD & Hanson MA (1994b). Fetal cardiovascular reflex responses to hypoxaemia. Fetal Maternal Med Rev 6, 17–37.
Giussani DA, Spencer JA, Moore PJ, Bennet L & Hanson MA (1993). Afferent and efferent components of the cardiovascular reflex responses to acute hypoxia in term fetal sheep. J Physiol 461, 431–449.
Gleason CA, Hamm C & Jones MD Jr (1990). Effect of acute hypoxemia on brain blood flow and oxygen metabolism in immature fetal sheep. Am J Physiol 258, H1064–H1069.
, http://www.100md.com
Gu W, Jones CT & Parer JT (1985). Metabolic and cardiovascular effects on fetal sheep of sustained reduction of uterine blood flow. J Physiol 368, 109–129.
Hoerter J (1976). Changes in the sensitivity to hypoxia and glucose deprivation in the isolated perfused rabbit heart during perinatal development. Pflugers Arch 363, 1–6.
Hoerter JA & Opie LH (1978). Perinatal changes in glycolytic function in response to hypoxia in the incubated or perfused rat heart. Biol Neonate 33, 144–161.
, 百拇医药
Hooper SB, Walker DW & Harding R (1995). Oxygen, glucose, and lactate uptake by fetus and placenta during prolonged hypoxemia. Am J Physiol 268, R303–R309.
Hou QC & Slotkin TA (1989). Effects of prenatal dexamethasone or terbutaline exposure on development of neural and intrinsic control of heart rate. Pediatr Res 26, 554–557.
Huch A, Huch R, Schneider H & Rooth G (1977). Continuous transcutaneous monitoring of fetal oxygen tension during labour. Br J Obstet Gynaecol 84 (Suppl. 1), 1–39.
, http://www.100md.com
Ikenoue T, Martin CB Jr, Murata Y, Ettinger BB & Lu PS (1981). Effect of acute hypoxemia and respiratory acidosis on the fetal heart rate in monkeys. Am J Obstet Gynecol 141, 797–806.
Itskovitz J, LaGamma EF, Bristow J & Rudolph AM (1991). Cardiovascular responses to hypoxemia in sinoaortic-denervated fetal sheep. Pediatr Res 30, 381–385.
Iwamoto HS, Rudolph AM, Mirkin BL & Keil LC (1983). Circulatory and humoral responses of sympathectomized fetal sheep to hypoxemia. Am J Physiol 245, H767–H772.
, http://www.100md.com
Jacobsson BA, Bergh CH & Hjalmarson A (1983). Corticosteroid modulation of muscarinic receptors in rat myocardial membranes. Biochim Biophys Acta 760, 77–83.
Jansen CA, Krane EJ, Thomas AL, Beck NF, Lowe KC, Joyce P, Parr M & Nathanielsz PW (1979). Continuous variability of fetal PO2 in the chronically catheterized fetal sheep. Am J Obstet Gynecol 134, 776–783.
Jarmakani JM, Nagatomo T, Nakazawa M & Langer GA (1978). Effect of hypoxia on myocardial high-energy phosphates in the neonatal mammalian heart. Am J Physiol 235, H475–H481.
, 百拇医药
Jellyman JK, Gardner DS, Fowden AL & Giussani DA (2004). Effects of dexamethasone on the uterine and umbilical vascular beds during basal and hypoxemic conditions in sheep. Am J Obstet Gynecol 190, 825–835.
Jensen A & Hanson MA (1995). Circulatory responses to acute asphyxia in intact and chemodenervated fetal sheep near term. Reprod Fertil Dev 7, 1351–1359.
Jones CT (1977). The development of some metabolic responses to hypoxia in the foetal sheep. J Physiol 265, 743–762.
, http://www.100md.com
Jones CT, Ritchie JW & Walker D (1983). The effects of hypoxia on glucose turnover in the fetal sheep. J Dev Physiol 5, 223–235.
Jones CT, Roebuck MM, Walker DW & Johnston BM (1988). The role of the adrenal medulla and peripheral sympathetic nerves in the physiological responses of the fetal sheep to hypoxia. J Dev Physiol 10, 17–36.
Lau C & Slotkin TA (1981). Maturation of sympathetic neurotransmission in the rat heart. VII. Suppression of sympathetic responses by dexamethasone. J Pharmacol Exp Ther 216, 6–11.
, 百拇医药
Lawrence GF, Brown VA, Parsons RJ & Cooke ID (1982). Feto-maternal consequences of high-dose glucose infusion during labour. Br J Obstet Gynaecol 89, 27–32.
Leitman DC, Benson SC & Johnson LK (1984). Glucocorticoids stimulate collagen and noncollagen protein synthesis in cultured vascular smooth muscle cells. J Cell Biol 98, 541–549.
Liggins GC (1969). Premature delivery of foetal lambs infused with glucocorticoids. J Endocrinol 45, 515–523.
, 百拇医药
Liggins GC & Howie RN (1972). A controlled trial of antepartum glucocorticoid treatment for prevention of the respiratory distress syndrome in premature infants. Pediatrics 50, 515–525.
Lohle M, Muller T, Wicher C, Roedel M, Schubert H, Witte OW, Nathanielsz PW & Schwab M (2005). Betamethasone effects on fetal sheep cerebral blood flow are not dependent on maturation of cerebrovascular system and pituitary-adrenal axis. J Physiol 564, 575–588.
, 百拇医药
Lundberg JM (1996). Pharmacology of cotransmission in the autonomic nervous system: integrative aspects on amines, neuropeptides, adenosine triphosphate, amino acids and nitric oxide. Pharmacol Rev 48, 113–178.
Lundberg JM, Terenius L, Hokfelt T & Goldstein M (1983). High levels of neuropeptide Y in peripheral noradrenergic neurons in various mammals including man. Neurosci Lett 42, 167–172.
Molnar J, Howe DC, Nijland MJ & Nathanielsz PW (2003). Prenatal dexamethasone leads to both endothelial dysfunction and vasodilatory compensation in sheep. J Physiol 547, 61–66.
, http://www.100md.com
Mormede P, Castagne V, Rivet JM, Gaillard R & Corder R (1990). Involvement of neuropeptide Y in neuroendocrine stress responses. Central and peripheral studies. J Neural Transm Supplement 29, 65–75.
NIH Consensus Development Conference (1995). Effect of corticosteroids for fetal maturation on perinatal outcomes. NIH Consensus Development Panel on the Effect of Corticosteroids for Fetal Maturation on Perinatal Outcomes. JAMA 273, 413–418.
, 百拇医药
Parer JT, Krueger TR & Harris JL (1980). Fetal oxygen consumption and mechanisms of heart rate response during artificially produced late decelerations of fetal heart rate in sheep. Am J Obstet Gynecol 136, 478–482.
Peeters LL, Sheldon RE, Jones MD Jr, Makowski EL & Meschia G (1979). Blood flow to fetal organs as a function of arterial oxygen content. Am J Obstet Gynecol 135, 637–646.
Pierce RA, Mariencheck WI, Sandefur S, Crouch EC & Parks WC (1995). Glucocorticoids upregulate tropoelastin expression during late stages of fetal lung development. Am J Physiol 268, L491–L500.
, 百拇医药
Ransnas L, Hjalmarson A, Sabler E & Jacobsson B (1987). Effect of corticosteroids on muscarinic receptors on cultured myocardial cells. Pharmacol Toxicol 61, 107–110.
Reuss ML, Parer JT, Harris JL & Krueger TR (1982). Hemodynamic effects of alpha-adrenergic blockade during hypoxia in fetal sheep. Am J Obstet Gynecol 142, 410–415.
Schwab M, Roedel M, Anwar MA, Muller T, Schubert H, Buchwalder LF, Walter B & Nathalielsz W (2000). Effects of betamethasone administration to the fetal sheep in late gestation on fetal cerebral blood flow. J Physiol 528, 619–632.
, http://www.100md.com
Segar JL, Lumbers ER, Nuyt AM, Smith OJ & Robillard JE (1998). Effect of antenatal glucocorticoids on sympathetic nerve activity at birth in preterm sheep. Am J Physiol 274, R160–R167.
Silver M, Barnes RJ, Comline RS & Burton GJ (1982). Placental blood flow: some fetal and maternal cardiovascular adjustments during gestation. J Reprod Fertil Supplement 31, 139–160.
Thornburg KL & Reller MD (1999). Coronary flow regulation in the fetal sheep. Am J Physiol 277, R1249–R1260.
Turbow RM, Curran-Everett D, Hay WW Jr & Jones MD Jr (1995). Cerebral lactate metabolism in near-term fetal sheep. Am J Physiol 269, R938–R942.
Waeber B, Burnier M, Nussberger J & Brunner HR (1990). Role of atrial natriuretic peptides and neuropeptide Y in blood pressure regulation. Horm Res 34, 161–165., http://www.100md.com(J. K. Jellyman, D. S. Gar)