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.

Physiological changes in maternal cortisol do not alter expression of growth-related genes in the ovine placenta.

OBJECTIVES: The aim of this study was to investigate the effect of cortisol on growth-related genes in the ovine placenta. STUDY DESIGN: Ewes carrying singleton pregnancies were operated on between 112 and 116 days of gestation (115 ± 0.4, term = 147 days) and randomly assigned into three groups: six control animals, five ewes that were administered cortisol by continuous intravenous infusion (1 mg/kg/day, high cortisol), and five ewes that were adrenalectomized and replaced with 0.5-0.6 mg cortisol/kg/day and 3 µg aldosterone/kg/day to produce cortisol concentrations equivalent to pre-pregnancy values (low cortisol). At necropsy (130 ± 0.2 days of gestation), placental tissue was frozen and stored at -80 °C for mRNA analysis. MAIN OUTCOME MEASURES: To assess potential molecular mechanisms by which cortisol alters placental structure and function and fetal growth. RESULTS: Cortisol levels did not significantly affect 11ß-hydroxysteroid dehydrogenase 1 and 2 enzymes, glucocorticoid receptor, mineralocorticoid receptor and angiotensin II receptor, type 1 (AT1R) expression levels. Gene expression levels of AT2R were increased in the high cortisol group for type B placentomes. There was little effect of cortisol on the insulin-like growth factor (IGF) axis. There was significantly more IGF-I mRNA in B versus A type and more IGFBP-2 mRNA in B and C type versus A type placentomes regardless of treatment (p < 0.05). CONCLUSIONS: These data suggest that cortisol increases placental AT2R expression at high concentrations whereas it has little effect on the placental IGF axis.

Reduction of maternal adrenal steroids results in increased VEGF protein without increased eNOS in the ovine placenta.

Fetal sheep studies have shown that reduced maternal cortisol or aldosterone levels alter placental morphology, with a reduction in placental blood flow. We have now tested the hypothesis that changes in placental morphology with relative adrenal hypoadrenalism are associated with changes in vascular endothelial growth factor (VEGF) and endothelial nitric oxide synthase (eNOS). Four groups of late gestation pregnant ewes with singleton fetuses were studied; controls (intact adrenals), normal cortisol and aldosterone (ewes adrenalectomized and replaced with normal cortisol and aldosterone levels), low cortisol (ewes adrenalectomized and replaced with low cortisol levels), and low aldosterone (ewes adrenalectomized and replaced with low aldosterone levels). The placenta was categorized into A, B, C or D type placentomes. There were significantly more B and C type placentomes in the adrenalectomized groups than in controls. Overall, B types had more VEGF mRNA than A types. VEGF protein levels corresponding to a 23 kDa band were highest in low aldosterone animals in A and C type placentomes. VEGF protein levels corresponding to a 47 kDa band were higher in C type placentomes than A types; protein levels were also higher overall in low cortisol animals compared to controls. Fetoplacental eNOS protein levels were lower in the adrenalectomized groups than in controls. In conclusion, our results indicate that increases in cotyledonary VEGF(164) protein were associated with fetal tissue overgrowth in the placenta when the pregnancy-induced increase in adrenal steroids was prevented in the ewe. However, cotyledonary eNOS protein was suppressed with reduced maternal adrenal steroids, which is consistent with the reduced placental perfusion previously observed in this model.

Pregnancy alters cortisol feedback inhibition of stimulated ACTH: studies in adrenalectomized ewes.

These studies test the hypothesis that pregnancy alters the feedback effects of cortisol on stimulated ACTH secretion. Ewes were sham-operated (Sham), or adrenalectomized (ADX) at approximately 108 days gestation and replaced with aldosterone (3 microg x kg(-1) x day(-1)) and with cortisol at either of two doses (ADX + 0.6 and ADX + 1 mg x kg(-1) x day(-1)); ewes were studied during pregnancy and postpartum. Mean cortisol levels produced in ADX ewes were similar to normal pregnant ewes (ADX+1) or nonpregnant ewes (ADX+0.6), respectively. Plasma ACTH concentrations in response to infusion of nitroprusside were significantly increased in the pregnant ADX+0.6 ewes (1,159 +/- 258 pg/ml) relative to pregnant Sham ewes (461 +/- 117 pg/ml) or the ADX+1 ewes (442 +/- 215 pg/ml) or the same ewes postpartum (151 +/- 69 pg/ml). Plasma ACTH concentrations were not significantly different among the groups postpartum. Increasing plasma cortisol to 20-30 ng/ml for 24 h before hypotension produced similar inhibition of ACTH in all groups. Pregnancy appears to decrease the effectiveness of low concentrations of cortisol to inhibit ACTH responses to hypotension.

Nitric oxide reduces pressor responsiveness during ovine hypoadrenocorticism.

1. Hypoadrenocorticism frequently results in critical hypotension, hypovolaemia, hyponatraemia and hyperkalaemia. Perhaps even more important, hypoadrenocorticoid humans experience decreased vasoconstriction in response to exogenous administration of vasoconstrictors, such as noradrenaline. 2. We studied chronically adrenalectomized adult sheep to test the hypothesis that the reduction in pressor responsiveness is the result of increased production of nitric oxide (NO) during hypoadrenocorticism. 3. Withdrawal of steroid replacement resulted in reduced blood pressure, reduced pressor responsiveness, as well as hyperkalaemia and hyponatraemia. 4. Inhibition of NO production by NG-nitro-L-arginine methyl ester in the hypoadrenocorticoid ewes restored mean arterial pressure and pressor responsiveness response to normal values. 5. The results of these experiments support the hypothesis that reduced pressor responsiveness in the hypoadrenocorticoid state is mediated by the overproduction of NO.

Estrogen and androgen influence hypothalamic AVP and CRF concentrations in fetal and adult sheep.

The present study tested the hypothesis that action of sex steroids on the hypothalamus-pituitary-adrenal (HPA) axis is measurable in the hypothalamus. Late-gestation fetal sheep were treated (5 mg/21 days) with either estradiol, androstenedione, or tamoxifen and compared to age-matched control fetuses. Tamoxifen significantly increased hypothalamic corticotropin releasing factor (CRF) and arginine vasopressin (AVP) concentrations, and androstenedione significantly decreased hypothalamic CRF concentration. Adult sheep were treated with estrone (10 mg/21 days), and responded with significant increases in hypothalamic AVP concentration, but not in immunoreactive ACTH concentration or processing within the pituitary. The results demonstrate that the effect of estrogen on the HPA axis is measurable in the hypothalamus, and is therefore not primarily at the anterior pituitary.

Effects of a simulated estrous cycle on sodium, volume, ACTH, and AVP in sheep.

The studies were designed to test for effects of acute increases in estradiol and progesterone, similar in magnitude and duration to those in the ovine estrous cycle, on adrenocorticotropic hormone (ACTH) and plasma vasopressin (AVP) under resting conditions and in response to hypotension. Ewes (7 per group) were studied as intact, ovariectomized, ovariectomized and treated with progesterone for 7-8 days, or subsequently treated with estradiol. During progesterone treatment plasma sodium and AVP were increased significantly. However, neither plasma volume nor blood pressure was altered. Plasma AVP responses to hypotension were not altered by either progesterone or estradiol treatment. The peak plasma ACTH response to hypotension was not altered by steroid treatment; however, the duration of the response was greater in progesterone-treated ewes than in intact ewes. The results indicate that changes in gonadal steroids similar to those in the ovine estrous cycle cause a small increase in plasma sodium that stimulates AVP, but do not alter regulation of blood pressure or volume or AVP or ACTH responses to hypotension.

Differential effects of pregnancy on mineralocorticoid and glucocorticoid receptor availability and immunoreactivity in cortisol feedback sites.

The suppression of plasma adrenocorticotropic hormone by very low levels of cortisol is reduced in pregnant adrenalectomized ewes, suggesting that pregnancy reduces the efficacy of the high-affinity corticosteroid receptor. This study was designed to determine the effects of pregnancy on the availability, immunoreactivity, and affinity of both corticosteroid receptors: the high-affinity mineralocorticoid receptor (MR) and the lower-affinity glucocorticoid receptor (GR). Availability was measured in the hypothalamus, pituitary, hippocampus and kidney using a saturation point radioligand binding assay. GR availability was significantly decreased in hippocampal cytosols obtained from pregnant ewes, but did not significantly change in other tissues. This finding is consistent with increased GR activation due to elevated circulating concentrations of cortisol. MR availability significantly increased from undetectable levels in hippocampal cytosols obtained from nonpregnant ewes to 2.8 +/- 1.6 fmol/mg protein in pregnant ewes, suggesting a reduced MR activation in the hippocampus during pregnancy. MR availability tended to be greater in other tissues during pregnancy, but these differences were not significant. The amount of immunoreactive MR (iMR) and GR (iGR) protein was estimated by quantifying Western blots. iGR significantly increased in the pituitary, but did not significantly change in other tissues. In contrast, iMR was significantly increased during pregnancy in all tissues assayed, suggesting that an increased cytosolic MR protein amount contributes to the observed increase in MR availability. Since studies suggest that progesterone is a potent anticorticosteroid, we tested for evidence of endogenous inhibition of binding to MR and/or GR during pregnancy by determining MR and GR affinity in pituitary cytosols obtained from nonpregnant and pregnant ewes. Although there was a tendency towards a decreased affinity of the MR in pregnant ewes, there was no significant change in the K(D) of the pituitary MR or GR during pregnancy. We hypothesize that an alteration in activation and/or autoregulation of the MR during pregnancy, particularly in the hippocampus, may contribute to the observed changes in receptor availability and immunoreactivity and increase basal plasma cortisol levels during pregnancy.

ACTH responses to CRF and AVP in pregnant and nonpregnant ewes.

During both ovine and human pregnancy plasma cortisol is increased. In human pregnancy the placenta secretes corticotropin-releasing factor (CRF), but pituitary responses to CRF are decreased. However, in ovine pregnancy there is no measurable placental secretion of CRF. This study tests for changes in pituitary responsiveness to CRF or AVP. Pregnant and nonpregnant ewes were infused with saline or CRF at three doses (3, 9, 45 microg/h), with or without coinfusion of AVP (9 microg/h). AVP infusion increased plasma AVP to approximately 250 pg/ml. CRF infusions increased plasma CRF from approximately 25 to 50, 150, and 850 pg/ml. ACTH was significantly increased by the infusion of AVP and by all infusions of CRF. Within-animal comparisons revealed a potentiation of the ACTH response to CRF in the presence of AVP. The ACTH responses to AVP and/or CRF were During both ovine and human pregnancy plasma cortisol is increased. In human pregnancy the placenta secretes corticotropin-releasing factor (CRF), but pituitary responses to CRF are decreased. However, in ovine pregnancy there is no measurable placental secretion of CRF. This study tests for changes in pituitary responsiveness to CRF or AVP. Pregnant and nonpregnant ewes were infused with saline or CRF at three doses (3, 9, 45 microg/h), with or without coinfusion of AVP (9 microg/h). AVP infusion increased plasma AVP to approximately 250 pg/ml. CRF infusions increased plasma CRF from approximately 25 to 50, 150, and 850 pg/ml. ACTH was significantly increased by the infusion of AVP and by all infusions of CRF. Within-animal comparisons revealed a potentiation of the ACTH response to CRF in the presence of AVP. The ACTH responses to AVP and/or CRF were

ACTH responses to hypotension and feedback inhibition of ACTH increased by chronic progesterone treatment.

During pregnancy, arterial pressure, baroreceptor sensitivity, and adrenocorticotropic hormone (ACTH) responses to hypotension are decreased. Basal ACTH and cortisol are increased in pregnancy, suggesting a reduction in cortisol feedback inhibition of ACTH. Acute treatment with progesterone decreases arterial pressure, baroreflex-mediated responses, and corticosteroid feedback effects on ACTH. These experiments test the hypothesis that chronic increases in progesterone produce changes in arterial pressure, ACTH responses to stress, and feedback inhibition of ACTH similar to pregnancy. Ewes were treated with progesterone for 60-80 days. This increase in plasma progesterone (to 7.6 +/- 0.4 ng/ml) did not alter basal ACTH, cortisol, arterial pressure, or heart rate. However, ACTH and AVP responses to hypotension were augmented in progesterone-treated ewes compared with untreated ewes. Chronic progesterone treatment resulted in greater inhibition of ACTH by cortisol. Because chronic progesterone treatment did not decrease the ACTH response to hypotension or attenuate the feedback control of ACTH secretion, these results suggest that the changes in pituitary-adrenal control during pregnancy do not reflect a simple effect of progesterone alone.

Evidence for reset of regulated cortisol in pregnancy: studies in adrenalectomized ewes.

These studies test the hypothesis that the increased adrenocorticotropic hormone (ACTH) and cortisol in pregnancy reflect a reset of regulated plasma cortisol concentrations. Ewes were sham operated (Sham) or adrenalectomized (ADX) at approximately 108 days gestation. Adrenalectomized ewes were replaced with aldosterone (3 micrograms.kg-1.day-1) and with cortisol at either of two doses (ADX + 0.6 and ADX + 1.0 mg.kg-1.day-1); the ewes were also studied postpartum. Plasma cortisol concentrations in ADX + 0.6 ewes (5.3 +/- 1.3 ng/ml) were similar to the Sham ewes postpartum (5.5 +/- 0.6 ng/ml), whereas ADX + 1.0 concentrations (8.9 +/- 1.0 ng/ml) were similar to pregnant Sham ewes (9.5 +/- 1.9 ng/ml). Plasma ACTH concentrations were significantly increased in the pregnant ADX + 0.6 ewes (273 +/- 44 pg/ml) relative to pregnant Sham ewes (84 +/- 9 pg/ml) or the same ewes postpartum (42 +/- 9 pg/ml). Plasma ACTH concentrations were not different among the groups postpartum. Acute increases in plasma cortisol to 15-25 ng/ml produced similar inhibition in all groups. These results suggest that pregnancy resets the basal cortisol concentration required for normalization of basal ACTH concentration.

Sheep model for study of maternal adrenal gland function during pregnancy.

Our goal was to develop a model for the study of maternal adrenal gland regulation and the effects of maternal cortisol secretion on fetal homeostasis. At about 108 days of gestation, before the time of rapid fetal growth or fetal adrenocortical maturation, ewes, under halothane anesthesia with controlled ventilation and positioned in sternal recumbency, were adrenalectomized. Ewes were treated with aldosterone by intravenous infusion (3 micrograms/kg of body weight per day) to induce normal late-gestation aldosterone concentration. Ewes were also treated with cortisol; for 2 postoperative days, this infusion (1 to 2 micrograms/kg per min) induced plasma concentration similar to that associated with stress. Thereafter, the dosage of cortisol was reduced to induce plasma values similar to normal late-gestation cortisol concentration in ewes (1 mg/kg per day), or to values in nonpregnant ewes (0.6 mg/kg per day). Administration of cortisol and aldosterone was required to prevent electrolyte imbalance and signs of hypoadrenocorticism. With steroid replacement, plasma protein, electrolyte, and glucose concentrations in adrenalectomized ewes were not different from those in sham-operated pregnant ewes. Of 11 adrenalectomized ewes, one died as a result of failure of the infusion pump, and one died as a result of inappropriate treatment for hypoglycemia. Of the remaining ewes, two aborted fetuses, three ewes each delivered one live and one dead fetus, two delivered live singleton fetuses, and two delivered twins. Therefore, this model of relative hypoadrenocorticism in pregnancy is feasible and practical for studying the influence of maternal cortisol concentration on maternal and fetal homeostasis.

Progesterone rapidly reduces arterial pressure in ewes.

Chronic progesterone treatment decreases mean arterial pressure (MAP) and expands plasma volume. Evidence now suggests that progesterone metabolites have rapid nongenomic actions on the baroreflex. This experiment tests for a rapid effect of progesterone on MAP,Na+, arginine vasopressin, and baroreflex sensitivity. Ewes were studied during 2-h infusions of vehicle or progesterone at 3 and 6 micrograms.kg-1.min-1. Infusion of progesterone at 3 micrograms.kg-1.min-1 resulted in progesterone levels characteristic of ovine pregnancy and significantly reduced MAP by the 17th minute, suggesting a nongenomic mechanism. However, progesterone infusion at 6 micrograms.kg-1.min-1 produced supraphysiological progesterone levels and failed to modify MAP. Overall baroreflex sensitivity was not altered by either dose of progesterone, but the slope of the tachycardic response to hypotension tended to be attenuated after infusion of progesterone at 6 micrograms.kg-1.min-1. We speculate that the lack of a simple, linear dose-response effect of progesterone on blood pressure and baroreflex sensitivity can be explained by progesterone action at multiple receptor populations.

Effects of progesterone on blood pressure, plasma volume, and responses to hypotension.

Gonadal steroids have been implicated in the control of blood pressure and fluid homeostasis. These experiments test the effect of progesterone in ovariectomized ewes on blood pressure, volume, and hormone responses to hypotension. Eight ewes were each studied in four conditions: ovariectomized, progesterone and estrogen replaced, progesterone replaced, or sham treated. During each treatment mean arterial pressure (MAP), plasma volume (PV), baroreflex responsiveness, and adrenocorticotropic hormone (ACTH), arginine vasopressin (AVP), and renin responses to hypotension were determined. Progesterone treatment significantly reduced resting MAP and increased PV compared with ovariectomy or sham treatments. Heart period at 85 mmHg was reduced with progesterone treatment. There was no effect of progesterone treatment on ACTH, AVP, or renin or heart rate responses to hypotension. Basal AVP levels were increased, and angiotensin II concentrations were decreased, by estrogen and progesterone and by sham treatments; plasma Na+ also tended to be increased during these treatments. These results suggest that a small increase in progesterone can reset resting MAP and PV without altering reflex heart rate or endocrine responses to hypotension.

Inhibition of stimulated and basal ACTH by cortisol during ovine pregnancy.

In pregnant ewes, as in pregnant women, plasma adrenocorticotropic hormone (ACTH) and cortisol concentrations are increased. Inhibition of free cortisol concentrations by dexamethasone, a synthetic glucocorticoid, is reduced in pregnant women compared with nonpregnant women. These experiments were designed to test the hypothesis that basal and stimulated ACTH concentrations are less sensitive to negative feedback inhibition by cortisol in pregnant ewes than in nonpregnant ewes. Ewes were infused with vehicle and with cortisol at two different rates (1 and 2 micrograms.kg-1.min-1) for 1 h; plasma ACTH concentrations during and after the infusion and after subsequent stimulation by hypotension were measured. Basal plasma ACTH concentrations during a 2-h infusion of cortisol (2 micrograms.kg-1.min-1) were also measured in undisturbed ewes. Cortisol significantly inhibited both stimulated and basal ACTH. The degree of suppression of ACTH was not reduced in the pregnant ewes compared with the nonpregnant ewes. The results indicate that both basal and stimulated ACTH are sensitive to negative feedback inhibition by cortisol during ovine pregnancy.

Ontogeny and ultradian rhythms of adrenocorticotropin and cortisol in the late-gestation fetal horse.

Fetal maturation and the timing of parturition in both sheep and primates are thought to be controlled by the hypothalamic-pituitary-adrenal axis but little is known about the endocrinology of the equine fetus. We investigated the ontogeny of plasma concentrations of adrenocorticotropic hormone (ACTH), cortisol and corticosteroid binding capacity in the late-gestation fetal horse. We also wished to determine whether there is ultradian rhythmic release of ACTH and cortisol in fetal horses and we compared fetuses to maternal and non-pregnant adult horses. Six fetuses, 278-304 days gestation (term approximately 335), were catheterized and sampled daily until delivery. Mean (+/- S.E.M.) ACTH concentrations increased significantly from 159 +/- 21 to 246 +/- 42 pg/ml over the last 2 days before parturition. Fetal cortisol increased significantly from 3.1 +/- 1.0 to 13.4 +/- 3.7 ng/ml (mean +/- S.E.M.) over the last 9 days before delivery. The slope of regressions for ACTH and cortisol concentrations with respect to time were positive in all subjects and statistically significant in 3 of 6 for ACTH and 5 of 6 for cortisol. Fetal corticosteroid binding capacity declined from 49.5 +/- 20.5 to 16.1 +/- 2.2 ng/ml (mean +/- S.E.M.) over the last 10 days before parturition. However, the greatest changes in ACTH, cortisol and corticosteroid binding capacity occurred very late in gestation, during the last 48 to 72 h before parturition.(ABSTRACT TRUNCATED AT 250 WORDS)

Reflex regulation of hormonal responses during pregnancy.

1. During pregnancy in most species, the resting levels of plasma angiotensin II, plasma ACTH (corticotropin) are increased. The concentration of vasopressin is also increased relatively to the osmolality in rats and in humans. 2. In the pregnant state mean arterial pressure is decreased, despite an increase in blood volume. Vasopressin and ACTH responses to hypotension are altered in pregnant ewes; the relationship between mean arterial pressure and vasopressin or ACTH response is shifted to the left, consistent with a change in set-point for regulation of mean arterial pressure. The vasopressin and cortisol responses to hypotensive haemorrhage are also altered in the pregnant dog; in this case the slope of the relation between mean arterial pressure and hormone response is decreased. 3. The decrease in hormone responses to hypotension is stimulus-specific; ACTH responses to hypoglycaemia are increased in the pregnant ewe and AVP responses to hyperosmolality are not altered in the pregnant ewe. 4. The heart rate responses to hypotension are also decreased in pregnant ewes, consistent with the observation that baroreflex responses are decreased in the pregnant rat. 5. The data suggest that a change in regulation of arterial pressure alters the hormonal responses to hypotension in the pregnant state.

Characterization of 11 beta-hydroxysteroid dehydrogenase activity in fetal and adult ovine tissues.

11 Beta-hydroxysteroid dehydrogenase (11 beta-HSD) converts 11-hydroxycorticosteroids to 11-oxocorticosteroids, thereby influencing the availability of bioactive cortisol or corticosterone in target tissues. The activity of this enzyme was investigated in sheep by: (1) measuring relative 11 beta-HSD activities in kidney, liver and placenta, and in various areas of the brain (hypothalamus, hippocampus, and brainstem); (2) characterizing the optimum pH of activities in the tested tissues; (3) investigating the possible effect of gonadal steroids on 11 beta-HSD activity in adult hypothalamus and kidney; and (4) investigating possible developmental changes in activities in the tested tissues. The optimum pH in liver and placenta was pH 9-10, whereas the optimum pH in kidney was pH 7-8. In tissues from adult ewes, 11 beta-HSD activity was highest in liver (84.6 +/- 3.8%) and kidney (49.8 +/- 11.6%), lower but measurable in pituitary (38.8 +/- 3.7%), and near the limit of detection in hypothalamus and hippocampus (2.7 +/- 0.9% and 3.2 +/- 0.8% respectively). Liver, kidney and pituitary from late-gestation fetal sheep contained activities which were similar to those in the adult (76.9 +/- 4.5%, 66.0 +/- 6.7% and 26.3 +/- 3.0% respectively). Activity in the pituitary was not related to fetal gestational age. Placenta also contained measureable 11 beta-HSD activity (21.4 +/- 4.7%). However, no activity was detected in hypothalamus (-1.7 +/- 0.2%), hippocampus (-0.2 +/- 0.6%) or brainstem (-1.0 +/- 0.6%) in late-gestation fetal or neonatal sheep. Enzyme activities in kidney and hypothalamus did not change significantly when the circulating concentrations of ovarian steroids were altered over a 1-3-week period. It is concluded that the ovine kidney, liver and placenta, but not hypothalamus or cerebral cortex, contain 11 beta-HSD activity. In addition, there is no change in 11 beta-HSD activity between late-gestation fetal life and adult life, and the relative activities are not altered by the ovarian steroid milieu.

Prolonged absence of ovarian hormones in the ewe reduces the adrenocorticotropin response to hypotension, but not to hypoglycemia or corticotropin-releasing factors.

The ACTH responses to hypotension, hypoglycemia, CRF, arginine vasopressin (AVP), and the combination of CRF and AVP were compared to determine whether there was a general decrease in ACTH responses to these stimuli in ovariectomized ewes compared to intact animals. The ovariectomized ewes were studied either 2-4 weeks post-ovariectomy (acute) or 4-7 months post-ovariectomy (chronic). Each ewe was subjected to saline control infusion, nitroprusside-induced hypotension (100 micrograms/min for 10 min), insulin-induced hypoglycemia (25 U porcine insulin), CRF (1 microgram/min for 60 min), AVP (0.2 microgram/min), and a combination of CRF plus AVP. In each experiment, plasma ACTH concentrations were measured at 10-min intervals for 1 h. The peak ACTH concentrations were significantly lower in response to hypotension in the chronic ewes compared to those in either the intact or acute group. The ACTH response to hypoglycemia was not significantly reduced in either ovariectomized group. The ACTH responses to CRF, AVP, and the combination of both were not significantly reduced in either ovariectomized group. The results suggest that the effect of ovariectomy on the ACTH response to stress occurs at a site within the brain and does not involve altered pituitary responsiveness to CRF or AVP.

Corticotropin responses to hypoglycemia and hypotension during ovine pregnancy.

The adrenocorticotropic hormone (ACTH) responses to hypoglycemia and to hypotension were compared in pregnant and nonpregnant ewes. In the first study pregnant and nonpregnant ewes were each subjected to hypoglycemia induced by injection of 0.05, 0.10, or 0.25 U regular insulin/kg body wt or to saline infused as a control. In the second study pregnant and nonpregnant ewes were subjected to hypotension induced by the infusion of 2.5, 5.0, or 10.0 micrograms nitroprusside.kg-1.min-1 or to dextrose infused as a control. ACTH responses to hypoglycemia were significantly increased in the pregnant ewes, and the relation between plasma glucose and plasma ACTH was shifted to the right, indicating greater ACTH responses for a given level of hypoglycemia in the pregnant state. The mean ACTH response to infusion of nitroprusside was reduced during pregnancy, despite significantly lower mean arterial blood pressure in the pregnant ewes. When the relation between mean arterial pressure and ACTH was compared in the two groups of ewes, the relation was significantly shifted to the left in the pregnant ewes, indicating lower ACTH responses to a given level of pressure during pregnancy. The results suggest that pregnancy does not uniformly alter ACTH responses to stimuli, suggesting multiple, stimulus-specific effects of pregnancy on the hypothalamo-pituitary-adrenal axis.