Possible role of uterine contractions in the short-term fluctuations of plasma ACTH concentration in fetal sheep

Central nervous system prostaglandin endoperoxide synthase-1 and -2 responses to oestradiol and cerebral hypoperfusion in late-gestation fetal sheep

Previous work in this laboratory has demonstrated that cerebral hypoperfusion increases the expression of prostaglandin endoperoxide synthase-2 (PGHS-2) in ovine fetal brain regions. Endogenously produced prostaglandins, in turn, partially mediate the fetal hypothalamus- pituitary-adrenal (HPA) axis response to arterial hypotension. In separate experiments, we have found that oestradiol stimulates fetal HPA axis activity. The present experiments were designed to test the hypothesis that oestradiol increases the expression of PGHS isoforms, and that oestradiol augments the PGHS response to cerebral hypoperfusion. Sixteen fetal sheep of known gestational ages (124-128 days' gestation at the time of study) were subjected to chronic catheterization and implantation of extravascular occluder around the brachiocephalic artery. Eight fetuses were subjected to subcutaneous implantation of a pellet containing 17beta-oestradiol (release rate 5 mg (21 days)-1). Brachiocephalic occlusion (BCO) stimulated adrenocorticotropin (ACTH), cortisol and arginine vasopressin (AVP) secretion, responses that were augmented by oestradiol. One hour after the beginning of a 10 min period of BCO, PGHS-1 mRNA was increased in fetal brainstem and hypothalamus, and PGHS-2 mRNA was increased in fetal brainstem. Oestradiol, by itself, increased the abundance of PGHS-2 mRNA in brainstem and cerebellum, and augmented the PGHS-2 mRNA response to BCO in brainstem. In contrast, oestradiol had no significant effect on the abundance of PGHS-1 mRNA in any brain region. PGHS-1 and PGHS-2 protein levels did not reflect the changes in the respective mRNAs. The abundance of both proteins was increased in cerebral cortex in response to BCO, and the abundance of PGHS-2 protein was increased by both oestradiol and BCO in the hippocampus. The results of this study confirm and extend the results of our previous studies, demonstrating an effect of cerebral hypoperfusion on the expression of both isoforms of PGHS. We conclude that oestradiol increases the expression of PGHS-2 in specific fetal brain regions, and that there is an interaction between oestradiol and BCO in the control of PGHS-2 expression in the fetal brainstem. We expect that at later time-points, the changes in mRNA would be followed by similar changes in enzyme abundance at the protein level. We speculate that at least a part of the effect of oestradiol on fetal HPA axis function is mediated by an interaction between oestradiol and prostaglandin biosynthesis in the fetal brain.

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.

Alteration of fetal oxygenation and responses to acute hypoxemia by increased myometrial contracture frequency produced by pulse administration of oxytocin to the pregnant ewe from 96 to 131 days' gestation

OBJECTIVES

This study examined the effects of increased myometrial contracture frequency from 96 to 131 days' gestation on ovine fetal oxygen transport and physiologic response to acute hypoxemia.

STUDY DESIGN

Ten pregnant ewes received either saline solution (control, n = 5) or long-term administration of oxytocin (600 microU. kg-1. min-1) in 5-minute pulses every 20 minutes to the maternal jugular vein beginning at 96 days' gestation (n = 5). Maternal tracheal tubes, fetal carotid artery and jugular vein catheters, and electrocorticographic and electromyographic electrodes were placed with the animals under halothane general anesthesia at 122 days' gestation. At 131 days' gestation fetal hypoxemia was induced for 1 hour. Maternal and fetal arterial blood gas samples were obtained at 60, 15, and 5 minutes before the start of hypoxemia and at 5, 10, 20, 30, 40, 60, and 120 minutes after the start of hypoxemia.

RESULTS

Baseline PO2 before hypoxemia was significantly lower and oxygen content was significantly higher in fetuses in the long-term oxytocin group than in control fetuses. At the end of hypoxemia the fetal pH, oxygen saturation, and oxygen content were significantly higher in the long-term oxytocin group than in the control group, although PO2 did not differ between groups. The fetal blood oxygen dissociation curve was shifted to the left in the long-term oxytocin group. During hypoxemia the absolute fetal blood pH was higher and the blood pH variation was lower in long-term oxytocin group than in the control group. Lower baseline and hypoxia-induced fetal plasma cortisol concentrations were observed in fetuses in the long-term oxytocin group than in fetuses of control ewes.

CONCLUSION

Increased contracture frequency during a period of 35 days shifts the fetal oxygen dissociation curve toward the left and alters fetal response to acute hypoxemia.

Regional adrenal blood flow responses to adrenocorticotropic hormone in fetal sheep

To determine whether adrenocorticotropic hormone (ACTH) at plasma concentrations measured during mild hypoxemia and at term affects adrenal blood flow, we measured regional blood flows in five unanesthetized normoxemic fetuses (125-130 days gestation) during a 24-h intravenous infusion of ACTH-(1-24) in isotonic saline solution. Another five fetuses received an infusion of vehicle. Blood flows were determined before the infusion, at 2 and 24 h from its onset, and 24 h afterward using radionuclide-labeled microspheres. Blood flow to the adrenal medulla was fivefold greater than that to the adrenal cortex. Adrenal blood flow rose 99% at 24 h of the ACTH infusion. There was a large increase in adrenal cortical blood flow of 272% at this time but medullary blood flow did not change significantly during ACTH infusion. The rise in cortical blood flow was attributable to decreased vascular resistance. No significant alterations occurred in fetal arterial blood pressure and heart rate, or in blood flow to other lower body organs of the fetus or to the placental cotyledons. These findings are consistent with the hypothesis that the increase in adrenal blood flow observed during fetal hypoxia is associated with changes in plasma ACTH concentration. They are also indicative of selective regulation of cortical and medullary blood flows in the sheep fetus at this stage of gestation.

Oxytocin given in a pulsatile manner to the ewe at 120 to 140 days' gestational age increases fetal sheep plasma cortisol

A single intravenous injection of oxytocin into pregnant sheep (123 to 144 days' gestational age) causes a bout of myometrial activity and an increase in fetal plasma corticotropin levels. We hypothesized that a sustained increased frequency of myometrial contractures accelerates the normal increase in fetal adrenal secretion in sheep in the last 3 weeks of gestation. To test this hypothesis, pulses of saline solution (group 1, 9 ewes and 10 fetuses) or oxytocin (group 2, 11 ewes and 12 fetuses) 600 or 960 microU/kg/min for 5 of every 30 minutes were infused into the maternal jugular vein for 6 days beginning at day 128 +/- 2 (mean +/- SD) days' gestational age. Total myometrial activity increased to 160% of baseline in group 2 by day 6. Myometrial activity did not change throughout the study in group 1. Maternal plasma cortisol concentrations did not rise during oxytocin infusion when compared with their own baselines (group 2) and were not different from concentrations in saline-infused ewes (group 1). By day 4 of oxytocin administration fetal plasma cortisol concentrations had risen significantly above baseline in group 2 (p less than 0.05). Fetal plasma cortisol concentrations did not rise in group 1. Corticotropin levels were not elevated in the fetal carotid arterial plasma of either group. A small but significant decrease occurred in fetal carotid arterial PO2 in group 2 by day 6 but not in group 1. In conclusion, increased myometrial activity produced by pulsed oxytocin is accompanied by increased fetal plasma cortisol concentrations, demonstrating that long-term alteration of myometrial activity affects fetal adrenal function over several days at this critical period of gestation.

Cocaine in pregnancy: the effect of maternal administration of cocaine on the maternal and fetal pituitary-adrenal axes

The effects of cocaine on the maternal and fetal pituitary-adrenal axis in vivo during pregnancy have not been reported. Six pregnant ewes and their fetuses underwent instrumentation at 113 to 119 days' gestation. Ewes were assigned to receive an intravenous bolus injection of vehicle or cocaine (0.5, 1.0, or 2.0 mg.kg-1) at 124 to 136 days' gestation. Maternal arterial blood gases, fetal pH and fetal PCO2 were unchanged after injection of cocaine or vehicle. After administration of 2.0 mg.kg-1 cocaine, arterial fetal PO2 fell 3.2 +/- 1.72 mm Hg (p less than 0.05) at +5 minutes, returning to baseline by +15 minutes. Maternal and fetal adrenocorticotropin levels rose within 5 minutes after the highest cocaine dose (p less than 0.05). There was a significant (p less than 0.05) increase in maternal cortisol at all doses of cocaine and in fetal cortisol at +15 minutes after the 2.0 mg.kg-1.