Blood pressure and heart rate in the ovine fetus: ontogenic changes and effects of fetal adrenalectomy

The Yin and Yang of the oxytocin and stress systems: opposites, yet interdependent and intertwined determinants of lifelong health trajectories

During parturition and the immediate post-partum period there are two opposite, yet interdependent and intertwined systems that are highly active and play a role in determining lifelong health and behaviour in both the mother and her infant: the stress and the anti-stress (oxytocin) system. Before attempting to understand how the environment around birth determines long-term health trajectories, it is essential to understand how these two systems operate and how they interact. Here, we discuss together the hormonal and neuronal arms of both the hypothalamic-pituitary-adrenal (HPA) axis and the oxytocinergic systems and how they interact. Although the HPA axis and glucocorticoid stress axis are well studied, the role of oxytocin as an extremely powerful anti-stress hormone deserves more attention. It is clear that these anti-stress effects depend on oxytocinergic nerves emanating from the supraoptic nucleus (SON) and paraventricular nucleus (PVN), and project to multiple sites at which the stress system is regulated. These, include projections to corticotropin releasing hormone (CRH) neurons within the PVN, to the anterior pituitary, to areas involved in sympathetic and parasympathetic nervous control, to NA neurons in the locus coeruleus (LC), and to CRH neurons in the amygdala. In the context of the interaction between the HPA axis and the oxytocin system birth is a particularly interesting period as, for both the mother and the infant, both systems are very strongly activated within the same narrow time window. Data suggest that the HPA axis and the oxytocin system appear to interact in this early-life period, with effects lasting many years. If mother-child skin-to-skin contact occurs almost immediately postpartum, the effects of the anti-stress (oxytocin) system become more prominent, moderating lifelong health trajectories. There is clear evidence that HPA axis activity during this time is dependent on the balance between the HPA axis and the oxytocin system, the latter being reinforced by specific somatosensory inputs, and this has long-term consequences for stress reactivity.

The reproductive stress hypothesis

In this paper, we propose the reproductive stress hypothesis that describes the pregnant females response to reproductive events based upon the activation of the hypothalamic–pituitary–adrenal axis and sympathetic adrenomedullary system. The main components of the reproductive stress hypothesis can be summarized as follows: (1) events unique to reproduction including empathema, pregnancy, parturition and lactation cause non-specific responses in females, called active reproductive stress; (2) the fetus is a special stressor for pregnant females where endocrine hormones, including corticotropin-releasing hormones and fetal glucocorticoids secreted by the fetus and placenta, enter the maternal circulatory system, leading to another stress response referred to as passive reproductive stress and (3) response to uterine tension and intrauterine infection is the third type of stress, called fetal intrauterine stress. Appropriate reproductive stress is a crucial prerequisite in normal reproductive processes. By contrast, excessive or inappropriate reproductive stress may result in dysfunctions of the reproductive system, such as compromised immune function, leading to susceptibility to disease. The novel insights of the reproductive stress hypothesis have important implications for deciphering the pathogenesis of certain diseases in pregnant animals, including humans, which in turn may be applied to preventing and treating their occurrence.

Mechanisms of in utero cortisol effects on the newborn heart revealed by transcriptomic modeling

We have identified effects of elevated maternal cortisol (induced by maternal infusion 1 mg·kg-1·day-1) on fetal cardiac maturation and function using an ovine model. Whereas short-term exposure (115-130-day gestation) increased myocyte proliferation and Purkinje fiber apoptosis, infusions until birth caused bradycardia with increased incidence of arrhythmias at birth and increased perinatal death, despite normal fetal cortisol concentrations from 130 days to birth. Statistical modeling of the transcriptomic changes in hearts at 130 and 140 days suggested that maternal cortisol excess disrupts cardiac metabolism. In the current study, we modeled pathways in the left ventricle (LV) and interventricular septum (IVS) of newborn lambs after maternal cortisol infusion from 115 days to birth. In both LV and IVS the transcriptomic model indicated over-representation of cell cycle genes and suggested disruption of cell cycle progression. Pathways in the LV involved in cardiac architecture, including SMAD and bone morphogenetic protein ( BMP) were altered, and collagen deposition was increased. Pathways in IVS related to metabolism, calcium signaling, and the actin cytoskeleton were altered. Comparison of the effects of maternal cortisol excess to the effects of normal maturation from day 140 to birth revealed that only 20% of the genes changed in the LV were consistent with normal maturation, indicating that chronic elevation of maternal cortisol alters normal maturation of the fetal myocardium. These effects of maternal cortisol on the cardiac transcriptome, which may be secondary to metabolic effects, are consistent with cardiac remodeling and likely contribute to the adverse impact of maternal stress on perinatal cardiac function.

Glucocorticoids, antenatal corticosteroid therapy and fetal heart maturation

Glucocorticoids are essential in mammals to mature fetal organs and tissues in order to survive after birth. Hence, antenatal glucocorticoid treatment (termed antenatal corticosteroid therapy) can be life-saving in preterm babies and is commonly used in women at risk of preterm birth. While the effects of glucocorticoids on lung maturation have been well described, the effects on the fetal heart remain less clear. Experiments in mice have shown that endogenous glucocorticoid action is required to mature the fetal heart. However, whether the potent synthetic glucocorticoids used in antenatal corticosteroid therapy have similar maturational effects on the fetal heart is less clear. Moreover, antenatal corticosteroid therapy may increase the risk of cardiovascular disease in adulthood. Here, we present a narrative review of the evidence relating to the effects of antenatal glucocorticoid action on the fetal heart and discuss the implications for antenatal corticosteroid therapy.

Use of radiotelemetry to assess perinatal cardiac function in the ovine fetus and newborn

The late gestation fetal ECG (fECG) has traditionally been difficult to characterize due to the low fECG signal relative to high maternal noise. Although new technologies have improved the feasibility of its acquisition and separation, little is known about its development in late gestation, a period in which the fetal heart undergoes extensive maturational changes. Here, we describe a method for the chronic implantation of radiotelemetry devices into late gestation ovine fetuses to characterize parameters of the fECG following surgery, throughout late gestation, and in the perinatal period. We found no significant changes in mean aortic pressure (MAP), heart rate (HR), or ECG in the 5 days following implantation; however, HR decreased in the first 24 h following the end of surgery, with associated increases in RR, PR, and QRS intervals. Over the last 14 days of fetal life, fetal MAP significantly increased, and HR significantly decreased, as expected. MAP and HR increased as labor progressed. Although there were no significant changes over time in the ECG during late gestation, the duration of the PR interval initially decreased and then increased as birth approached. These results indicate that although critical maturational changes occur in the late gestation fetal myocardium, the mechanisms that control the cardiac conduction are relatively mature in late gestation. The study demonstrates that radiotelemetry can be successfully used to assess fetal cardiac function, in particular conduction, through the process of labor and delivery, and may therefore be a useful tool for study of peripartum cardiac events.

Development of Mechanical and Failure Properties in Sheep Cerebral Arteries

Traumatic brain injury (TBI) is a devastating problem for people of all ages, but the nature of the response to such injury is often different in children than in adults. Cerebral vessel damage and dysfunction are common following TBI, but age-dependent, large-deformation vessel response has not been characterized. Our objective was to investigate the mechanical properties of cerebral arteries as a function of development. Sheep middle cerebral arteries from four age groups (fetal, newborn, juvenile, and adult) were subjected to biaxial loading around physiological conditions and then to failure in the axial direction. Results show little difference among age groups under physiological loading conditions, but response varied significantly with age in response to large axial deformation. Vessels from all age groups reached the same ultimate stretch level, but the amount of stress carried at a given level of stretch increased significantly with age through the developmental period (fetal to juvenile). Our results are the first to identify changes in cerebral vessel response to large deformations with age and may lead to new insights regarding differences in response to TBI with age.

The critical importance of the fetal hypothalamus-pituitary-adrenal axis

The fetal hypothalamus-pituitary-adrenal (HPA) axis is at the center of mechanisms controlling fetal readiness for birth, survival after birth and, in several species, determination of the timing of birth. Stereotypical increases in fetal HPA axis activity at the end of gestation are critical for preparing the fetus for successful transition to postnatal life. The fundamental importance in fetal development of the endogenous activation of this endocrine axis at the end of gestation has led to the use of glucocorticoids for reducing neonatal morbidity in premature infants. However, the choice of dose and repetition of treatments has been controversial, raising the possibility that excess glucocorticoid might program an increased incidence of adult disease (e.g., coronary artery disease and diabetes). We make the argument that because of the critical importance of the fetal HPA axis and its interaction with the maternal HPA axis, dysregulation of cortisol plasma concentrations or inappropriate manipulation pharmacologically can have negative consequences at the beginning of extrauterine life and for decades thereafter.

ANG II modulation of cardiac growth and remodeling in immature fetal sheep

ANG II increases fetal blood pressure and stimulates fetal heart growth; however, little is known regarding its direct effects on cardiomyocytes in vivo. We sought to determine whether ANG II stimulates heart growth and cardiomyocyte hypertrophy and/or hyperplasia in utero in the immature fetal heart independent of the effects on cardiac afterload. In twin gestation, fetal sheep at ∼100 days gestation (term 145 days), one fetus received a chronic (6 days) infusion of ANG II alone (50 μg·kg(-1)·min(-1)) or ANG II plus nitroprusside (NTP) to attenuate the increase in blood pressure; noninstrumented twins served as controls. ANG II alone, but not ANG II + NTP resulted in a significant increase in heart mass (left and right ventricle + septum, corrected for body weight) compared with controls. ANG II, but not ANG II+NTP, also significantly increased cardiomyocyte area compared with control and increased the percentage of binucleated myocytes. ANG II with or without concomitant infusion of NTP increased cardiac PCNA expression, a marker of proliferation. Steady-state protein expression of terminal mitogen-activated protein kinases, cyclin B1, cyclin E1, and p21 were similar among groups. We conclude that in vivo, ANG II increases fetal cardiac mass via cardiomyocyte hypertrophy, differentiation, and to a lesser extent hyperplasia. The effects of ANG II on hypertrophy appear dependent upon the increase in blood pressure (mechanical load), whereas effects on proliferation are load-independent.

Effects of maternal nutrient restriction, intrauterine growth restriction, and glucocorticoid exposure on phosphoenolpyruvate carboxykinase-1 expression in fetal baboon hepatocytes in vitro

BACKGROUND

The objective of this study was to develop a cell culture system for fetal baboon hepatocytes and to test the hypotheses that (i) expression of the gluconeogenic enzyme phosphoenolpyruvate carboxykinase-1 (PEPCK-1) is upregulated in hepatocytes isolated from fetuses of nutrient-restricted mothers (MNR) compared with ad libitum-fed controls (CTR), and (ii) glucocorticoids stimulate PEPCK-1 expression.

METHODS

Hepatocytes from 0.9G CTR and MNR fetuses were isolated and cultured. PEPCK-1 protein and mRNA levels in hepatocytes were determined by Western blot and quantitative PCR, respectively.

RESULTS

Fetuses of MNR mothers were intrauterine growth restricted (IUGR). Feasibility of culturing 0.9G fetal baboon hepatocytes was demonstrated. PEPCK-1 protein levels were increased in hepatocytes isolated from IUGR fetuses, and PEPCK-1 mRNA expression was stimulated by glucocorticoids in fetal hepatocytes.

CONCLUSIONS

Cultured fetal baboon hepatocytes that retain their in vivo phenotype provide powerful in vitro tools to investigate mechanisms that regulate normal and programmed hepatic function.

Glucocorticoid exposure of sheep at 0.7 to 0.75 gestation augments late-gestation fetal stress responses

OBJECTIVES

Exposure to glucocorticoid levels inappropriately high for current maturation alters fetal hypothalamo-pituitary-adrenal axis (HPAA) development. In an established fetal sheep model, we determined whether clinical betamethasone doses used to accelerate fetal lung maturation have persistent effects on fetal HPAA hypotensive-stress responses.

STUDY DESIGN

Pregnant ewes received saline (n = 6) or betamethasone (n = 6); 2 × 110 μg/kg body weight doses injected 24 hours apart (106/107 and 112/113 days' gestational age, term 150 days). Basal adrenocorticotropin (ACTH) and cortisol and responses to fetal hypotension were measured before and 5 days after the first course and 14 days after the second course.

RESULTS

Basal ACTH and cortisol were similar with treatment. HPAA responses to hypotension increased after the second but not first course and ACTH/cortisol ratio increased indicating central HPAA effects.

CONCLUSIONS

Results demonstrate latency in the emergence of fetal HPAA hyperresponsiveness following betamethasone exposure that may explain hyperresponsiveness in full-term but not preterm neonates.

Transfusion effects on cardiomyocyte growth and proliferation in fetal sheep after chronic anemia

Chronic fetal anemia results in significant cardiac remodeling. The capacity to reverse these effects is unknown. We examined the effects of transfusion on cardiomyocyte adaptations after chronic anemia in fetal sheep subjected to daily hemorrhage beginning at 109-d GA (term ∼145 d). After 10 d of anemia, one group was killed for comparison with age-matched controls. A separate group of anemic fetuses was transfused with red blood cells at 119-d GA for comparison with controls at 129-d GA. Anemia significantly increased the heart-to-body weight ratio, an effect partially ameliorated after transfusion. Cardiomyocyte dimensions were similar among all groups, suggesting an absence of hypertrophy. The percentages of mono- and binucleated cardiomyocytes were similar between groups at 119-d GA, although the percentage of binucleated cells was significantly less in transfused fetuses compared with controls at 129-d GA. Protein levels of mitogen-activated protein kinases and protein kinase B were similar between controls and their respective intervention groups, except for a significant increase in phosphorylated c-Jun N-terminal kinase 1/2 in transfused fetuses. Thus, cardiomyocyte proliferation but not hypertrophy contributes to cardiac enlargement during fetal anemia. Transfusion results in slowing but not cessation of cardiac growth after anemia.

Adrenal glands are essential for activation of glucogenesis during undernutrition in fetal sheep near term

In adults, the adrenal glands are essential for the metabolic response to stress, but little is known about their role in fetal metabolism. This study examined the effects of adrenalectomizing fetal sheep on glucose and oxygen metabolism in utero in fed conditions and after maternal fasting for 48 h near term. Fetal adrenalectomy (AX) had little effect on the rates of glucose and oxygen metabolism by the fetus or uteroplacental tissues in fed conditions. Endogenous glucose production was negligible in both AX and intact, sham-operated fetuses in fed conditions. Maternal fasting reduced fetal glucose levels and umbilical glucose uptake in both groups of fetuses to a similar extent but activated glucose production only in the intact fetuses. The lack of fasting-induced glucogenesis in AX fetuses was accompanied by falls in fetal glucose utilization and oxygen consumption not seen in intact controls. The circulating concentrations of cortisol and total catecholamines, and the hepatic glycogen content and activities of key gluconeogenic enzymes, were also less in AX than intact fetuses in fasted animals. Insulin concentrations were also lower in AX than intact fetuses in both nutritional states. Maternal glucose utilization and its distribution between the fetal, uteroplacental, and nonuterine maternal tissues were unaffected by fetal AX in both nutritional states. Ovine fetal adrenal glands, therefore, have little effect on basal rates of fetal glucose and oxygen metabolism but are essential for activating fetal glucogenesis in response to maternal fasting. They may also be involved in regulating insulin sensitivity in utero.

Epigenetic modification of fetal baboon hepatic phosphoenolpyruvate carboxykinase following exposure to moderately reduced nutrient availability

Decreased maternal nutrient availability during pregnancy induces compensatory fetal metabolic and endocrine responses. Knowledge of cellular changes involved is critical to understanding normal and abnormal development. Several studies in rodents and sheep report increased fetal plasma cortisol and associated increased gluconeogenesis in response to maternal nutrient reduction (MNR) but observations in primates are lacking. We determined MNR effects on fetal liver phosphoenolpyruvate carboxykinase 1 (protein, PEPCK1; gene, PCK1 orthologous/homologous human chromosomal region 20q13.31) at 0.9 gestation (G). Female baboon social groups were fed ad libitum (control, CTR) or 70% CTR (MNR) from 0.16 to 0.9G when fetuses were delivered by caesarean section under general anaesthesia. Plasma cortisol was elevated in fetuses of MNR mothers (P < 0.05). Immunoreactive PEPCK1 protein was located around the liver lobule central vein and was low in CTR fetuses but rose to 63% of adult levels in MNR fetuses. PCK1 mRNA measured by QRT-PCR increased in MNR (2.3-fold; P < 0.05) while the 25% rise in protein by Western blot analysis was not significant. PCK1 promoter methylation analysis using bisulfite sequencing was significantly reduced in six out of nine CpG-dinucleotides evaluated in MNR compared with CTR liver samples. In conclusion, these are the first data from a fetal non-human primate indicating hypomethylation of the PCK1 promoter in the liver following moderate maternal nutrient reduction.

The role of the neural sympathetic and parasympathetic systems in diurnal and sleep state-related cardiovascular rhythms in the late-gestation ovine fetus

The efferent mechanisms mediating the well-known diurnal cardiovascular rhythms in the late-gestation fetus are only partially understood. In the present study, we evaluated the contribution of the parasympathetic and sympathetic nervous systems (SNS) to these rhythms. Chronically instrumented fetal sheep at a mean (SE) of 122 ( 1 ) days gestation (term is 147 days) underwent either chemical sympathectomy with 6-hydroxydopamine the day after surgery ( n = 8), vagotomy at surgery ( n = 8), or were sham controls ( n = 8). Fetal heart rate (HR), fetal HR variability (HRV), mean arterial blood pressure (MAP), carotid blood flow (CaBF), electrocorticogram (ECoG) activity, and nuchal activity were measured continuously for 24 h. Changes between sleep states were determined in a 6-h interval. Control fetal sheep showed consistent diurnal rhythms in fetal HR, HRV, MAP, and CaBF, with maximal activity in the evening, but not in nuchal activity. Sympathectomy was associated with a significant reduction of both fetal HR and HRV, while vagotomy was associated with a fall in fetal HRV ( P < 0.05) but no change in HR. Despite this, most animals in the two intervention groups still showed diurnal rhythms for fetal HR, HRV, MAP, and CaBF, although peak HR may have been delayed in the sympathectomy group (mean 02:22 vs. 23:54 h in controls, P = 0.06). There was no effect of either intervention on sleep state cycling, although state-related cardiovascular rhythms were significantly modulated. These data indicate that, neither the SNS nor vagal activity, in isolation at least, is essential for generating cardiovascular diurnal rhythms in the late-gestation fetus.

Impact of endothelial cells and mechanical conditioning on smooth muscle cell extracellular matrix production and differentiation

The aim of the current study was to explore the separate and coupled effects of endothelial cell (EC) presence and mechanical conditioning on smooth muscle cell (SMC) responses by combining bilayered poly(ethylene glycol) diacrylate (PEGDA) hydrogels with a pulsatile flow bioreactor. Each construct was composed of an outer PEGDA layer containing SMC and an inner PEGDA layer, either with or without EC. After an initial 3 days of static culture, EC(+) and EC(-) constructs were each further divided into two subgroups, half of which received mechanical conditioning mimetic of late gestation (mean pressures of approximately 50 mmHg and peak-to-trough pressure differentials of approximately 20 mmHg at approximately 140-180 beats/min) and half of which were cultured statically. After 18 additional days of culture, the SMC-containing layer of each construct was harvested, and western blots and quantitative histology were conducted to compare collagen type I, collagen type III, and elastin levels among treatment groups. SMC differentiation was evaluated by focusing on SMC marker calponin h1 and direct regulators of its gene expression-the transcription factor serum response factor (SRF) and two of its binding partners, myocardin and Elk-1. Combined EC and pulsatile flow conditioning increased elastin production, but decreased collagen type I deposition. Further, combined EC presence and mechanical stimulation increased SRF levels and the ratio of myocardin to active, phosphorylated Elk-1. This modulation of SRF and its binding partners appeared to result in a net increase in SMC differentiation, as evidenced by an associated increase in calponin h1 levels.

Effects of maternal antenatal glucocorticoid treatment on apoptosis in the ovine fetal cerebral cortex

We examined the effects of single and multiple maternal glucocorticoid courses on apoptosis in the cerebral cortices of ovine fetuses (CC). Ewes received single dexamethasone or placebo courses at 104-106 or 133-135 days or multiple courses between 76-78 and 104-106 days gestation. In the single-course groups, ewes received four 6 mg dexamethasone or placebo injections every 12 hr for 48 hr. Multiple-course groups received the same treatment once per week for 5 weeks. Neuronal and nonneuronal apoptotic cell numbers per square millimeter were determined with TUNEL and NeuN staining and with caspase-3 enzyme activity on CC tissues harvested at 106-108 (70%) or 135-137 (90%) days of gestation. Apoptotic cell numbers and caspase-3 activity were 50% lower (P < 0.02) after single placebo courses at 90% than 70% gestation; 90% of apoptotic cells were (P < 0.01) nonneuronal at both ages. Nonneuronal apoptotic cells and caspase-3 activity were 40% and 20% lower (P < 0.02) after single dexamethasone than placebo courses at 70%, but not 90%, gestation. Caspase-3 activity was 20% lower (P < 0.01) after multiple dexamethasone than placebo courses, but apoptotic cell number did not differ. We conclude that nonneuronal apoptosis represents the major form of apoptosis in the CC at both 70% and 90% of gestation. Apoptosis in nonneuronal cells decreases with maturity and after a single course of dexamethasone at 70%, but not at 90%, gestation and not after multiple courses at 70% gestation. We speculate that a single course of glucocorticoids exerts maturational changes on the rate of apoptosis in the cerebral cortex of preterm ovine fetuses.

Cardiac corticosteroid receptors mediate the enlargement of the ovine fetal heart induced by chronic increases in maternal cortisol

Previous studies have demonstrated that modest, physiologically relevant increases in maternal cortisol in late gestation result in enlargement of the fetal heart. In this study, we investigated the role of mineralocorticoid receptor (MR) or glucocorticoid receptor (GR) in this enlargement. Ewes with single fetuses were randomly assigned at approximately 120 days of gestation to one of four groups: maternal cortisol infusion (1 mg/kg per day, cortisol); maternal cortisol infusion with fetal intrapericardial infusion of the MR antagonist (MRa) potassium canrenoate (600 microg/day; cortisol+MRa); maternal cortisol infusion with fetal intrapericardial infusion of the GR antagonist (GRa) mifepristone (50 microg/day, cortisol+GRa); and maternal saline infusion (control). At approximately 130 days of gestation, fetal heart to body weight ratio and right ventricular (RV) and left ventricular (LV) free wall thicknesses were increased in the cortisol group when compared with control group. Fetal hearts from the cortisol+MRa group weighed significantly less, with thinner LV, RV, and interventricular septum walls, when compared with the cortisol group. Fetal hearts from the cortisol+GRa group had significantly thinner RV walls than the cortisol group. Fetal arterial pressure and heart rate were not different among groups at 130 days. Picrosirius red staining of fetal hearts indicated that the increased size was not accompanied by cardiac fibrosis. These results suggest that physiologic increases in maternal cortisol in late gestation induce fetal cardiac enlargement via MR and, to a lesser extent, by GR, and indicate that the enlargement is not secondary to an increase in fetal blood pressure or an increase in fibrosis within the fetal heart.

Fetal body weight and the development of the control of the cardiovascular system in fetal sheep

Reduced birth weight predisposes to cardiovascular diseases in later life. We examined in fetal sheep at 0.76 (n = 18) and 0.87 (n = 17) gestation whether spontaneously occurring variations in fetal weight affect maturation of autonomic control of cardiovascular function. Fetal weights at both gestational ages were grouped statistically in low (LW) and normal weights (NW) (P < 0.01). LW fetuses were within the normal weight span showing minor growth dysproportionality at 0.76 gestation favouring heart and brain, with a primary growth of carcass between 0.76 and 0.87 gestation (P < 0.05). While twins largely contributed to LW fetuses, weight differences between singletons and twins were absent at 0.76 and modest at 0.87 gestation, underscoring the fact that twins belong to normality in fetal sheep not constituting a major malnutritive condition. Mean fetal blood pressure (FBP) of all fetuses was negatively correlated to fetal weight at 0.76 but not 0.87 gestation (P < 0.05). At this age, FBP and baroreceptor reflex sensitivity were increased in LW fetuses (P < 0.05), suggesting increased sympathetic activity and immaturity of circulatory control. Development of vagal modulation of fetal heart rate depended on fetal weight (P < 0.01). These functional associations were largely independent of twin pregnancies. We conclude, low fetal weight within the normal weight span is accompanied by a different trajectory of development of sympathetic blood pressure and vagal heart rate control. This may contribute to the development of elevated blood pressure in later life. Examination of the underlying mechanisms and consequences may contribute to the understanding of programming of cardiovascular diseases.

Development of the ovine fetal cardiovascular defense to hypoxemia towards full term

We tested the hypothesis that fetal cardiovascular responses to hypoxemia change close to full term in relation to the prepartum increase in fetal basal cortisol and investigated, in vivo, the neural and endocrine mechanisms underlying these changes. Fetal heart rate and peripheral hemodynamic responses to 1 h of hypoxemia were studied in 25 chronically instrumented sheep within three narrow gestational age ranges: 125–130 ( n = 13), 135–140 ( n = 6), and >140 ( n = 6) days (full term ∼145 days). Chemoreflex function and plasma concentrations of vasoconstrictor hormones were measured. Reductions in fetal arterial Po2 during hypoxemia were similar at all ages. At 125–130 days, hypoxemia elicited transient bradycardia, femoral vasoconstriction, and increases in plasma concentrations of catecholamines, neuropeptide Y (NPY), AVP, ACTH, and cortisol. Close to full term, in association with the prepartum increase in fetal basal cortisol, there was a developmental increase in the magnitude and persistence of fetal bradycardia and in the magnitude of the femoral constrictor response to hypoxemia. The mechanisms mediating these changes close to full term included increases in the gain of chemoreflex function and in the magnitudes of the fetal NPY and AVP responses to hypoxemia. Data combined irrespective of gestational age revealed significant correlations between fetal basal cortisol and fetal bradycardia, femoral resistance, chemoreflex function, and plasma AVP concentrations. The data show that the fetal cardiovascular defense to hypoxemia changes in pattern and magnitude just before full term because of alterations in the gain of the neural and endocrine mechanisms mediating them, in parallel with the prepartum increase in fetal basal cortisol.

Kinetics of betamethasone and fetal cardiovascular adverse effects in pregnant sheep after different doses

OBJECTIVE

To study the pharmacokinetics of different betamethasone doses and preparations used to enhance fetal lung maturation in the maternal and fetal circulation of sheep and the adverse effects on fetal blood pressure.

METHODS

Doses of 170 (n = 6) and 110 microg/kg (n = 6) betamethasone phosphate equivalent to 12 or 8 mg, respectively, administered to a 70 kg pregnant woman or 170 microg/kg (n = 6) of a depot formulation (50% betamethasone phosphate and 50% betamethasone acetate) were injected intramuscularly to chronically instrumented pregnant sheep.

RESULTS

Both betamethasone preparations produced highest maternal concentrations after 15 min followed by an exponential decline with a t(1/2) of about 3 hours. The drug fell below the limit of detection at 8 to 12 hours. Betamethasone was first detectable in the fetal circulation at 1 hour, peaked at 3 hours, and decreased below the limit of detection at 8 hours independently of the dose or preparation. Maternal and fetal betamethasone concentrations achieved with the phosphate and acetate formulation were one half of those obtained with betamethasone phosphate, suggesting that very little betamethasone is released from the acetate within the first 8 hours when the effect on lung maturation is needed. Betamethasone led to a maximal increase of mean fetal blood pressure from 42+/-1 to 51+/-1 mm Hg (P < .05) and did not differ between the doses and preparations, although plasma concentrations showed a clear dose-concentration relationship.

CONCLUSION

The doses of betamethasone used in obstetrics are supramaximal in terms of cardiovascular effects in sheep. Risk-benefit studies are needed to find the effective steroid dose with the least adverse effects.

Development of baroreflex function and hind limb vascular reactivity in the horse fetus

This study investigated, in vivo, the mechanisms underlying the development of cardiovascular function in the horse fetus, with particular relevance to baroreflex function and hind limb vascular arterial reactivity to constrictor agonists. Under general anaesthesia, vascular catheters were inserted and a Transonic flow probe was implanted around one of the metatarsal arteries of 13 horse fetuses, either at 0.6 of gestation (n= 6) or at 0.9 of gestation (n= 7, term approximately 335 days). At least 5 days after surgery, pressor, vasoconstrictor and cardiac chronotropic responses to exogenous bolus doses of phenylephrine, angiotensin II and arginine vasopressin were recorded. Fetal cardiac baroreflex slopes were obtained using the peak pressor and heart rate responses to increasing doses of phenylephrine. Fetal treatment with phenylephrine, angiotensin II and vasopressin produced significant changes in arterial blood pressure, hind limb vascular resistance and heart rate. Pressor and vasopressor responses to all agonists were greater at 0.9 than at 0.6 of gestation; however, fetal cardiac baroreflex sensitivity decreased with advancing gestational age. Correlation analysis revealed that fetal plasma cortisol rather than gestational age was a greater determinant of pressor and vasopressor reactivity. In contrast, gestational age rather than cortisol better determined heart rate and baroreflex responsiveness in the equine fetus. The data show that development of cardiovascular function in the equine fetus occurs via cortisol-dependent and -independent pathways.

Development of baroreflex and endocrine responses to hypotensive stress in newborn foals and lambs

OBJECTIVE

The aims of this study were to compare and contrast the development of the cardiac baroreflex and endocrine responses to acute hypotensive stress in healthy newborn pony foals and lambs during the first two weeks of postnatal life.

METHODS

Under general anaesthesia, seven Welsh pony foals and six Welsh Mountain lambs were catheterised with hind limb artery and vein catheters. Following post-surgical recovery, at 1 week and 2 weeks of age, blood pressures of the animals were raised and lowered acutely by intravenous infusion of phenylephrine and sodium nitroprusside, respectively. During hypotension, blood samples were taken for measurement of plasma hormones associated with activation of the stress axis.

RESULTS

Basal arterial blood pressure increased significantly (P<0.05) between week 1 and week 2 in the absence of any significant change in basal heart rate in foals and with a significant reduction in basal heart rate in lambs. In foals, the slope of the heart rate-blood pressure relationship decreased in response to acute hypertension, and it increased in response to acute hypotension, from week 1 to week 2 (all P<0.05). In contrast, in lambs, the slope of the heart rate-blood pressure relationship decreased with both acute hypertension and acute hypotension from week 1 to week 2 (all P<0.05). In foals, there were significant increases in plasma concentrations of noradrenaline, neuropeptide Y (NPY), vasopressin, adrenocorticotrophic hormone (ACTH) and cortisol in response to hypotension (P<0.05). In lambs, there were also significant increases in plasma concentrations of ACTH and cortisol during hypotension. Plasma concentrations of noradrenaline, NPY and vasopressin were not measured during hypotension in lambs. In foals, although the magnitude of the ACTH response to hypotension was smaller at week 2 than week 1, the increment in plasma cortisol was similar in the two age groups. In contrast, in lambs, the profile of both the ACTH and cortisol responses was similar at week 1 and week 2.

CONCLUSION

These data suggest that the increase in basal arterial blood pressure in the foal and the lamb during the first 2 weeks of postnatal life is accompanied by differential maturational changes in the vagal and sympathetic components of the cardiac baroreflex between the two species. These developmental cardiac baroreflex changes occur together with increased adrenocortical responsiveness to acute hypotensive stress, which appears comparatively more mature in lambs than in foals.

Development of cardiovascular function in the horse fetus

In mammals, the mechanisms regulating an increase in fetal arterial blood pressure with advancing gestational age remain unidentified. In all species studied to date, the prepartum increase in fetal plasma cortisol has an important role in the maturation of physiological systems essential for neonatal survival. In the horse, the prepartum elevation in fetal cortisol and arterial blood pressure are delayed relative to other species. Hence, the mechanisms governing the ontogenic increase in arterial blood pressure in the horse fetus may mature much closer to term than in other fetal animals. In the chronically instrumented pony mare and fetus, this study investigated how changes in fetal peripheral vascular resistance, in plasma concentrations of noradrenaline, adrenaline and vasopressin, and in the maternal-to-fetal plasma concentration gradient of oxygen and glucose relate to the ontogenic changes in fetal arterial blood pressure and fetal plasma cortisol concentration as term approaches. The data show that, towards term in the horse fetus, the increase in arterial blood pressure occurs together with reductions in metatarsal vascular resistance, elevations in plasma concentrations of cortisol, vasopressin, adrenaline and noradrenaline, and falls in the fetal : maternal ratio of blood P(a,O(2)) and glucose concentration. Correlation analysis revealed that arterial blood pressure was positively related with plasma concentrations of vasopressin and noradrenaline, but not adrenaline in the fetus, and inversely related to the fetal : maternal ratio of blood P(a,O(2)), but not glucose, concentration. This suggests that increasing vasopressinergic and noradrenergic influences as well as changes in oxygen availability to the fetus and uteroplacental tissues may contribute to the ontogenic increase in fetal arterial blood pressure towards term in the horse.

Betamethasone effects on fetal sheep cerebral blood flow are not dependent on maturation of cerebrovascular system and pituitary-adrenal axis

Synthetic glucocorticoids are administered to pregnant women in premature labour to accelerate fetal lung maturation at a time when fetal cerebrovascular and endocrine systems are maturing. Exposure to glucocorticoids at 0.8-0.9 of gestation increases peripheral and cerebrovascular resistance (CVR) in fetal sheep. We examined whether the increase of CVR and its adverse effect on cerebral blood flow (CBF) depend on the current level of maturation of the pituitary-adrenal axis and the cerebrovascular system. Using fluorescent microspheres, regional CBF was measured in 11 brain regions before and 24 h and 48 h after the start of 3.3 microg kg(-1) h(-1) betamethasone (n = 8) or vehicle (n = 7) infusions to fetal sheep at 0.73 of gestation. Hypercapnic challenges were performed before and 24 h after the onset of betamethasone exposure to examine betamethasone effects on cerebrovascular reactivity. Betamethasone exposure decreased CBF by approximately 40% in all brain regions after 24 h of infusion (P < 0.05). The decline in CBF was mediated by a CVR increase of 111 +/- 16% in the cerebral cortex and 129 +/- 29% in subcortical regions (P < 0.05). Hypercapnic cerebral vasodilatation and associated increase in CBF were blunted (P < 0.05). Fetal CBF recovered after 48 h of betamethasone administration. There were no differences in glucocorticoid induced CBF and CVR changes compared with our previous findings at 0.87 of gestation. We conclude that the cerebrovascular effects of antenatal glucocorticoids are independent of cerebrovascular maturation and preparturient increase in activity of the fetal pituitary-adrenal axis.

Effects of gestational age and cortisol treatment on ovine fetal heart function in a novel biventricular Langendorff preparation

Structural and functional maturation of a number of fetal organs and physiological systems occurs in the immediate period prior to term, in association with the prepartum increase in fetal plasma cortisol concentration. At present, little is known about how myocardial sensitivity to adrenergic and muscarinic cholinergic stimulation changes as the fetus approaches term, nor the role of the prepartum increase in plasma cortisol concentration in mediating these changes. This study used a novel Langendorff, biventricular, ovine fetal heart preparation to investigate the effects of advancing gestation and cortisol treatment on myocardial sensitivity to adrenergic (isoprenaline) and muscarinic cholinergic (carbachol) stimulation. It was hypothesized that cortisol infusion would fully mimic developmental changes in myocardial responsiveness to adrenergic and cholinergic stimulation. Sixteen Welsh Mountain sheep fetuses were surgically prepared under general anaesthesia with vascular catheters. At 125 +/- 1 days gestational age (dGA; term, 145 dGA) fetuses were infused with saline vehicle (n= 7; Premature Control) or with cortisol (n= 4; 2-3 mg kg(-1) d(-1)i.v.; Premature Cortisol) for 5 days. The Term Control group (n= 5) comprised fetuses that were surgically prepared at 130 dGA and infused with vehicle for 5 days prior to delivery (n= 2), or that received no surgery (n= 3). Under terminal anaesthesia, Premature Control and Premature Cortisol fetuses were delivered at 130 dGA and Term Control fetuses between 135 and 143 dGA. Following exsanguination under anaesthesia, fetal hearts were mounted in the Langendorff preparation, allowing measurement of left ventricular (LV) developed pressure and right ventricular (RV) developed pressure, heart rate (HR), coronary perfusion pressure and perfusate distribution to the myocardium. Cortisol infusion elevated fetal plasma cortisol concentrations to values similar to those measured close to term (45.0 +/- 7.1 ng ml(-1)). Advancing gestational age, but not cortisol treatment, enhanced fetal LV developed pressure, RV developed pressure and HR responses to carbachol (P < 0.05). Advancing gestational age, but not cortisol treatment, suppressed fetal LV developed pressure, RV developed pressure and HR responses to isoprenaline (P < 0.05). Maximum doses of either carbachol or isoprenaline had no effect on coronary perfusate distribution. Changes in myocardial responsiveness to adrenergic and muscarinic cholinergic stimulation with advancing gestation provide mechanisms that contribute to the maturation of the cardiovascular system as the ovine fetus approaches term. These changes in myocardial responsiveness are not solely dependent on preparturient elevations in fetal plasma cortisol concentration.

The effect of intrafetal infusion of metyrapone on arterial blood pressure and on the arterial blood pressure response to angiotensin II in the sheep fetus during late gestation

While the impact of exogenous glucocorticoids on the fetal cardiovascular system has been well defined, relatively few studies have characterised the role of endogenous fetal glucocorticoids in the regulation of arterial blood pressure (BP) during late gestation. We have therefore infused metyrapone, an inhibitor of cortisol biosynthesis, into fetal sheep from 125 days gestation (when fetal cortisol concentrations are low) and from 137 days gestation (when fetal cortisol concentrations are increasing) and measured fetal plasma cortisol, 11-desoxycortisol and ACTH, fetal systolic, diastolic and mean arterial BP, heart rate, and the fetal BP responses to increasing doses of angiotensin II (AII). At 125 days gestation, there was a significant increase in fetal plasma ACTH and 11-desoxycortisol by 24 h after (+24 h) the start of the metyrapone infusion, and plasma cortisol concentrations were not different at +24 h when compared with pre-infusion values. Whilst the initial fall in circulating cortisol concentrations may have been transient, systolic, diastolic and mean arterial BP were ~5-6 mmHg lower (P < 0.05) in metyrapone- than in vehicle-infused fetuses at 24-48 h after the start of the infusion. When metyrapone was infused from 137/138 days gestation, there was a significant decrease in plasma cortisol concentrations by +6 h, which was followed by an increase back to pre-infusion values. While cortisol concentrations decreased, there was no change in fetal mean arterial BP during the first 24 h after the start of metyrapone infusion. Mean fetal arterial BP values at 137-139 days gestation were not different in fetuses that had been infused with either vehicle or metyrapone from 125 days gestation or with metyrapone from 137/138 days gestation. At 137-139 days gestation, however, arterial BP responses to increasing doses of AII were significantly blunted in fetuses that had been infused with metyrapone from 125 days gestation, when compared with fetuses that had been infused with metyrapone from 137/138 days gestation or with vehicle from 125 days gestation. The dissociation of the gestational age increase in arterial BP and the effects of intrafetal AII on fetal arterial BP indicates that increase in fetal BP with gestational age is not entirely a result of an increased vascular responsiveness to endogenous AII. Furthermore there may be a critical window during late gestation when the actions of cortisol contribute to the development of vascular responsiveness to AII.

Periconceptional nutrition programs development of the cardiovascular system in the fetal sheep

It has been proposed that fetal adaptations to intrauterine nutrient deprivation permanently reprogram the cardiovascular system. We investigated the impact of restricted periconceptional nutrition and/or restricted gestational nutrition on fetal arterial blood pressure (BP), heart rate, rate pressure product, and the fetal BP responses to ANG II and the angiotensin-converting enzyme inhibitor captopril during late gestation. Restricted periconceptional nutrition resulted in an increase in fetal mean arterial BP between 115 and 125 days gestation (restricted 41.5 +/- 2.8 mmHg, n = 12; control 38.5 +/- 1.5 mmHg, n = 13) and between 135 and 147 days gestation (restricted 50.5 +/- 2.2 mmHg, n = 8; control 42.5 +/- 1.9 mmHg, n = 10) as well as an increase in the rate pressure product in twin, but not singleton, fetuses between 115 and 147 days gestation. Mean BP and fetal plasma ACTH were also positively correlated in twin, but not singleton, fetuses. This is the first demonstration that maternal undernutrition during the periconceptional period results in an increase in fetal arterial BP. This increase occurs concomitantly with an increase in fetal ACTH but is not dependent on activation of the fetal renin-angiotensin system.

Effects of fetal ovine adrenalectomy on sympathetic and baroreflex responses at birth

Studies were performed to test the hypothesis that the absence of adrenal glucocorticoids late in gestation alters sympathetic and baroreflex responses before and immediately after birth. Fetal sheep at 130-131 days gestation (term 145 days) were subjected to bilateral adrenalectomy before the normal prepartum increase in plasma cortisol levels. One group of fetuses (n = 5) received physiological cortisol replacement with a continuous infusion of hydrocortisone (2 mg x day(-1) x kg(-1) for 10 days), whereas the other group received 0.9% NaCl vehicle (n = 5). All animals underwent a second surgery 48 h before the study for placement of a renal nerve recording electrode. Heart rate (HR), mean arterial blood pressure (MABP), renal sympathetic nerve activity (RSNA), and baroreflex control of HR and RSNA were studied before and after cesarean section delivery. At the time of study (140-141 days gestation), fetal plasma cortisol concentration was undetectable in adrenalectomized (ADX) fetuses and 58 +/- 9 ng/ml in animals receiving cortisol replacement (ADX + F). Fetal and newborn MABP was significantly greater in ADX + F relative to ADX animals. One hour after delivery, MABP increased 13 +/- 3 mmHg and RSNA increased 91 +/- 12% above fetal values in ADX + F (both P < 0.05) but remained unchanged in ADX lambs. The midpoint pressures of the fetal HR and RSNA baroreflex function curves were significantly greater in ADX + F (54 +/- 3 and 56 +/- 3 mmHg for HR and RSNA curves, respectively) than ADX fetuses (45 +/- 2 and 46 +/- 3 mmHg). After delivery, the baroreflex curves reset toward higher pressure in ADX + F but not ADX lambs. These results suggest that adrenal glucocorticoids contribute to cardiovascular regulation in the late-gestation fetus and newborn by modulating arterial baroreflex function and sympathetic activity.

Cortisol-mediated potentiation of uterine artery contractility: effect of pregnancy

During pregnancy, maternal plasma cortisol concentrations approximately double. Given that cortisol plays an important role in the regulation of vascular reactivity, the present study investigated the potential role of cortisol in potentiation of uterine artery (UA) contractility and tested the hypothesis that pregnancy downregulated the cortisol-mediated potentiation. In vitro cortisol treatment (3, 10, or 30 ng/ml for 24 h) produced a dose-dependent increase in norepinephrine (NE)-induced contractions in both nonpregnant and pregnant (138-143 days gestation) sheep UA. However, this cortisol-mediated response was significantly attenuated by approximately 50% in pregnant UA. The 11 beta-hydroxysteroid dehydrogenase (11-beta HSD) inhibitor carbenoxolone did not change the effect of cortisol in nonpregnant UA but abolished its effect in pregnant UA by increasing the NE pD(2) in control tissues from 6.20 +/- 0.05 to 6.59 +/- 0.11. The apparent dissociation constant value of NE alpha(1)-adrenoceptors was not changed by cortisol in pregnant UA but was decreased in nonpregnant UA. There was no difference in glucocorticoid receptor density between nonpregnant and pregnant UA. Cortisol significantly decreased endothelial nitric oxide (NO) synthase protein levels and NO release in both nonpregnant and pregnant UA, but the effect of cortisol was attenuated in pregnant UA by approximately 50%. Carbenoxolone alone had no effects on NO release in nonpregnant UA but was decreased in pregnant UA. These results suggest that cortisol potentiates NE-mediated contractions by decreasing NO release and increasing NE-binding affinity to alpha(1)-adrenoceptors in nonpregnant UA. Pregnancy attenuates UA sensitivity to cortisol, which may be mediated by increasing type-2 11-beta HSD activity in UA.

A novel method for controlled and reversible long term compression of the umbilical cord in fetal sheep

1. In fetal sheep during late gestation the aims of the present study were to (1) develop a technique for inducing prolonged but reversible periods of controlled compression of the umbilical cord and (2) characterise the cardiovascular, endocrine and metabolic responses to this challenge. 2. Under 1-2 % halothane anaesthesia, 16 Welsh Mountain sheep fetuses were chronically instrumented at 118 +/- 2 days of gestation (term is ca 145 days) with an inflatable occluder cuff around the umbilical cord, amniotic and femoral vascular catheters and with transit-time flow probes around the contra-lateral femoral artery and an umbilical artery. At 125 days, umbilical blood flow was reduced by 30 % from a pre-determined 24 h baseline for 3 days by automated servo-controlled inflation of the occluder cuff (n = 8). The occluder was then deflated allowing return of umbilical blood flow to baseline. The remaining eight fetuses were used as sham-operated controls in which the occluder was not inflated throughout the protocol. Fetal cardiovascular variables were recorded at 8 s intervals and arterial blood samples taken for measurement of blood gases, glucose and lactate and plasma adrenaline, noradrenaline and vasopressin concentration throughout the study. 3. Automated servo-controlled inflation of the occluder cuff, programmed to reduce umbilical blood flow by 30 % from baseline, reduced umbilical blood flow by 30.2 +/- 1.7 %, with a coefficient of variation during compression of 6.5 +/- 1.1 %. Sustained partial compression of the umbilical cord produced falls in fetal arterial pH, P(a,O2), percentage O(2) saturation of haemoglobin, and hindlimb oxygen delivery, and increases in P(a,CO2), haemoglobin concentration, arterial blood oxygen carrying capacity and in blood glucose and lactate concentrations. While the reductions in P(a,O2), percentage saturation of haemoglobin and hindlimb oxygen delivery and the increase in P(a,CO2) were sustained throughout compression, the reduction in arterial pH and the increase in arterial oxygen carrying capacity had returned towards baseline values by 48 h compression. Fetal blood lactate concentrations reached a peak at 8 h of compression and, thereafter, were maintained at an elevated level relative to baseline. 4. Partial compression of the umbilical cord produced fetal hypertension, a reduction in femoral blood flow and, consequently, an increase in calculated fetal femoral vascular resistance for the duration of the challenge. In addition, the fall in heart rate measured in sham control fetuses by the end of the study, did not occur in cord-compressed fetuses. Cosinor analysis on 24 h rhythms of cardiovascular data indicated a significant increase in the amplitude of the 24 h rhythm in heart rate in cord-compressed fetuses relative to sham controls during the period of compression or sham-compression. Furthermore, cord compression led to an increase in fetal plasma noradrenaline, but not adrenaline and vasopressin concentrations relative to sham control fetuses. 5. In conclusion, a novel reversible method for controlled, long-term compression of the umbilical cord in sheep has been developed. The data show that sustained, partial compression of the umbilical cord produced moderate but sustained asphyxia, which resolved after the end of the compression period, and induced changes in fetal cardiovascular, endocrine and metabolic functions.

Development of fetal vascular responses to endothelin-1 and acetylcholine in the sheep

Responses to K(+), endothelin-1 (ET-1), and acetylcholine (ACh) of isolated adrenal, femoral, middle cerebral, and renal arteries from fetal [110--145 days gestational age (dGA, term approximately 148 dGA)] and 0- to 24-h newborn (NB) lambs were evaluated using the technique of wire myography. Responses at distinct developmental ages for each vascular bed were compared. In all arteries sensitivity to K(+)-induced vasoconstriction was similar at all fetal age points examined. In contrast, sensitivity to ET-1 increased with increasing fetal age in arteries from all vascular beds. The magnitude of the maximal vasoconstriction was positively correlated with GA for K(+) in adrenal, femoral, and cerebral arteries and for ET-1 in femoral, cerebral, and renal arteries. Cerebral arteries showed a greater sensitivity when compared with the other systemic arteries to K(+) and ET-1 at all fetal ages and to K(+) in NB. ACh evoked relaxatory responses in fetal and NB femoral and adrenal arteries. However, renal arteries relaxed comparatively less in response to ACh, and no vasodilation was noted in middle cerebral arteries at any age points examined. For femoral arteries ACh-induced vasorelaxation decreased with increasing GA but was restored in arteries from NB lambs. In summary, the responsiveness of isolated resistance arteries varies with developmental age in the fetal and perinatal sheep and these effects are both agonist and vascular bed specific. The augmented sensitivity in response to ET-1 of middle cerebral compared with other systemic arteries may reflect the importance of cerebral blood flow control during this critical developmental period.

Effects of betamethasone administration to the fetal sheep in late gestation on fetal cerebral blood flow

Glucocorticoid administration to women at risk of preterm delivery to accelerate fetal lung maturation has become standard practice. Antenatal glucocorticoids decrease the incidence of intraventricular haemorrhage as well as accelerating fetal lung maturation. Little is known regarding side effects on fetal cerebral function. Cortisol and synthetic glucocorticoids such as betamethasone increase fetal blood pressure and femoral vascular resistance in sheep. We determined the effects of antenatal glucocorticoid administration on cerebral blood flow (CBF) in fetal sheep. Vehicle (n = 8) or betamethasone (n = 8) was infused over 48 h via the jugular vein of chronically instrumented fetal sheep at 128 days gestation (term 146 days). The betamethasone infusion rate was that previously shown to produce fetal plasma betamethasone concentrations similar to human umbilical vein concentrations during antenatal glucocorticoid therapy. Regional CBF was measured in 10 brain regions, using coloured microspheres, before and 24 and 48 h after onset of treatment, and during hypercapnic challenges performed before and 48 h after onset of betamethasone exposure. Betamethasone exposure decreased CBF in all brain regions measured except the hippocampus after 24 h of infusion (P < 0.05). The CBF decrease was most pronounced in the thalamus and hindbrain (45-50% decrease) and least pronounced in the cortical regions (35-40% decrease). It was mediated by an increase in cerebral vascular resistance (CVR, P < 0.05) and led to a decrease in oxygen delivery to subcortical and hindbrain structures of 30-40%, to 8.6 +/- 1.1 ml x (100 g)(-1) x min(-1), and 40-45 %, to 11.0 +/- 1.6 ml x 100 g(-1) x min(-1), respectively (P < 0.05). After 48 h of betamethasone treatment, the reduction in CBF was diminished to about 25-30 %, but was still significant in comparison to vehicle-treated fetuses in all brain regions except three of the five measured cortical regions (P < 0.05). CVR and oxygen delivery were unchanged in comparison to values at 24 h of treatment. The CBF increase in response to hypercapnia was diminished (P < 0.05). These observations demonstrate for the first time that glucocorticoids exert major vasoconstrictor effects on fetal CBF. This mechanism may protect the fetus against intraventricular haemorrhage both at rest and when the fetus is challenged. Betamethasone exposure decreased the hypercapnia-induced increase in CBF (P < 0.05) due to decreased cerebral vasodilatation (P < 0.05).

Developmental changes in plasma catecholamine concentrations during normoxia and acute hypoxia in the chick embryo

In the mammalian fetus, the cardiovascular responses to acute hypoxaemia include a redistribution of the cardiac output away from the periphery towards the adrenal, myocardial and cerebral circulations. A component of the peripheral vasoconstriction is mediated by increased release of catecholamines into the fetal circulation during acute hypoxaemia. Previously, we have shown that the chick embryo also shows an increase in peripheral vascular resistance during acute hypoxaemia and that this response becomes progressively larger towards the end of the incubation period. However, the ontogeny of the catecholaminergic response to acute hypoxaemia has not been investigated in this species. Fertilised chicken eggs were studied on days 10, 13, 16 and 19 of incubation (hatching is at 21 days). At each stage of incubation, blood samples were obtained from the chorioallantoic artery of the chick embryos during normoxia and after 5 min of hypoxaemia for measurement of plasma concentrations of adrenaline and noradrenaline by HPLC. Basal plasma adrenaline and noradrenaline concentrations by the end of the incubation period were much higher in the chick embryo than values reported for mammalian fetuses during late gestation. During normoxia, basal plasma noradrenaline concentration remained unchanged during development but plasma adrenaline concentration showed a developmental increase from < 25.1 pmol l-1 at day 10 to 3 nmol l-1 at day 19 of incubation. Acute hypoxaemia caused an increase in plasma noradrenaline and adrenaline from day 13 and day 16 of incubation, respectively. In addition, the increase in plasma adrenaline and noradrenaline and in the ratio of plasma adrenaline to noradrenaline during acute hypoxaemia became progressively larger by the end of the incubation period. These data show an ontogenic increase in basal plasma catecholamines and in the catecholaminergic response to acute hypoxaemia in the chick embryo during the last third of the incubation period.

Effect of cortisol on blood pressure and the renin-angiotensin system in fetal sheep during late gestation

The effects of cortisol on blood pressure and the circulating components of the renin-angiotensin system (RAS) were investigated in sheep fetuses during late gestation and after exogenous cortisol infusion. Plasma cortisol concentration was greater in fetuses at 140 +/- 1 days of gestation (term 145 +/- 2 days) compared to those studied earlier in gestation (128 +/- 1 days), although, because of wide inter-animal variation, no differences were observed in blood pressure or plasma angiotensin II (AII), renin or angiotensinogen (Ao) concentrations. At 129 +/- 1 days of gestation, an infusion of cortisol for 5 days (2-3 mg kg(-1) day(-1) i.v.) increased plasma cortisol concentration to a value normally seen close to term. This rise in plasma cortisol was accompanied by increases in blood pressure and plasma concentrations of AII, renin and Ao. When observations from all fetuses were considered, plasma cortisol concentration correlated with plasma AII and renin, and blood pressure correlated with plasma cortisol and AII concentrations. Intravenous administration of an AII type 1 (AT(1))-specific receptor antagonist (3 mg kg(-1) GR138950) caused a reduction in blood pressure in all fetuses; the hypotensive response was greatest in fetuses studied near term and in the cortisol-treated fetuses. Overall, the magnitude of the hypotension induced by GR138950, and the concomitant rise in plasma renin, both correlated with the plasma cortisol concentration before GR138950 treatment. These findings show that, in the sheep fetus during late gestation, the RAS becomes more important in the maintenance of resting blood pressure when plasma cortisol concentration is elevated, whether endogenously or exogenously.

Increased myometrial contracture frequency at 96-140 days accelerates fetal cardiovascular maturation

Fetal cardiovascular responses to an altered intrauterine environment of increased myometrial contractures induced by oxytocin (OT) pulses to the ewe over the final 50 days of gestation were studied in chronically instrumented sheep. Ewes received saline (Cntl) or long-term OT treatment (LTOT, 600 microU x kg(-1) x min(-1) in 5-min pulses every 20 min) from 96 days gestational age. Fetal baroreflex responses to sodium nitroprusside (SNP) and phenylephrine (PE) were studied at 133 days gestation. OT increased contractures in LTOT ewes. Fetal blood pressure (FBP) was higher, and fetal heart rate (FHR) and slope of daily change in FBP and FHR were lower in LTOT fetuses. Fetal SNP-induced hypotension resulted in a narrow R-R interval variation range in LTOT fetuses; Cntl fetuses showed early breakdown in compensation. Baroreflex response slope during PE-induced fetal hypertension was lower in LTOT than in Cntl fetuses. Although the cortisol-to-ACTH ratio was lower in LTOT fetuses, fetal plasma ACTH and cortisol changes were similar in control and LTOT fetuses. We hypothesize that contracture-induced alterations in the intrauterine environment accelerate fetal cardiovascular development through mild hypoxemia, repetitive fetal pituitary-adrenal stimulation, and/or physical stimulation.