Diurnal variations in plasma concentrations of cortisol, prolactin, growth hormone and glucose in the fetal sheep and pregnant ewe during late gestation

We have measured fetal and maternal plasma concentrations of cortisol, prolactin, GH and glucose in samples collected during a 24-h period in 14 animals between 127 and 142 days of gestation. There was a significant increase in both the mean daily plasma cortisol concentration and mean daily coefficient of variation (C.V.) of plasma cortisol concentrations after 135 days of gestation. There was also a significant variation in the fetal plasma cortisol concentrations with a peak occurring at 19.00 h. There was a significant sinusoidal diurnal rhythm in the plasma prolactin concentrations in both the fetal sheep and pregnant ewe and the maximal prolactin concentrations occurred between 19.00 and 23.00 h (fetal) and 21.00 and 01.00 h (maternal). Although no significant diurnal variation was detected in fetal plasma GH concentrations, there was a significant sinusoidal diurnal rhythm in the plasma GH concentrations of the pregnant ewe and the maximal maternal GH concentrations occurred between 21.00 and 01.00 h. Both the fetal and maternal plasma glucose concentrations showed a significant sinusoidal diurnal rhythm. The maximal maternal and fetal glucose concentrations were measured between 21.00 and 01.00 h and between 23.00 and 03.00 h respectively. We have therefore established that diurnal variations in plasma cortisol and prolactin concentrations exist prenatally. Whether the presence of such hormonal rhythms reflects the activity of an endogenous fetal circadian pacemaker remains to be established.     

The availability of cortisol in amniotic fluid to the fetus and chorionic and amniotic membranes.

This study examined whether cortisol, present in amniotic fluid (AF), could reach the fetal blood, and might be available to various fetal organs and to the fetal membranes and placenta. [3H]Cortisol ([3H]F) was injected into the AF of five sheep at 130--143 days gestation. Samples of AF and fetal blood were collected for 120 min, at which time the fetuses were delivered and fetal organs and fluid were collected for measurement of the [3H]F content. [3H]F disappeared from AF with an initial half-life of about 40 min and was detected in fetal blood by 30 min after intraamniotic injection. At 120 min after injection, [3H]F was widely distributed within the pregnant uterus, and the highest concentrations of radioactivity were found in allantoic fluid, fetal membranes, and placental tissue. The concentration of [3H]F in the umbilical vein was higher than in umbilical arterial blood in four of five sheep. [3H]F and [3H]cortisone were found in several fetal tissues, including adrenal, kidney, pancreas, lung, and liver. [3H]F in tracheal and stomach fluid was generally less than in fetal blood. We conclude that 1) cortisol can reach the fetus from AF, 2) a major route of entry may be via the vasculature of the fetal membranes, 3) a considerable proportion of [3H]F is retained by the fetal membranes, and 4) interconversion of [3H]F and [3H]cortisone may occur in some fetal tissues.     

Lack of effect of melatonin on myometrial electromyographic activity in the pregnant sheep at 138-142 days gestation (term = 147 days gestation)

A 24-h rhythm has been demonstrated in fetal and maternal melatonin plasma concentrations in pregnant sheep in the last third of gestation. Melatonin in the maternal circulation can cross the placenta and is the major source of melatonin in the fetal circulation. Melatonin has been postulated to act as a prostaglandin (PG) synthetase inhibitor in the uterus. PG synthetase inhibitors decrease myometrial contractility. To assess transplacental passage of melatonin and potential influences of melatonin on uterine contractility, we infused melatonin continuously into the maternal jugular vein in seven pregnant sheep at 138-142 days gestation (term in our instrumented animals is 147 days gestation) at three infusion rates for successive 1-h periods during the late morning to late afternoon. There was no change in the total time during which the myometrium was active, as indicated by myometrial electromyographic activity or the myometrial contracture frequency during the 3 h before and after melatonin infusions and for each hour of the infusions. The MCR for melatonin in the ewe was 4128 +/- 410 ml/min (mean +/- SE; n = 7; weight, 50-70 kg). The resting maternal to fetal melatonin concentration ratio was 0.8; this ratio was maintained at 2.28 during melatonin infusion to the ewe at a wide range of maternal melatonin concentrations. Melatonin concentrations in the range of 3-200 times normal had no effect on the maternal plasma PGF2 alpha metabolite concentration, but caused a 40.4% fall in fetal plasma PGE2 (P less than 0.05). We conclude that changes in maternal and fetal plasma melatonin concentrations within the physiological range observed throughout the day do not alter myometrial contractility, but do alter fetal PGs.     

Effect of morphine and naloxone on plasma prolactin concentrations in the fetal sheep and pregnant ewe during late gestation

This study investigated the effects of intrafetal morphine or naloxone administration on fetal and maternal plasma prolactin concentrations in the sheep during late pregnancy (117-143 days gestation). After intravenous morphine (3 mg/kg) there was a significant increase (p less than 0.05) in fetal plasma prolactin concentrations which was sustained for 180 min post-injection. There was no significant effect of either gestational age (125-133 days compared to 134-143 days gestation) or repeated administration (up to 3 treatments) of morphine on the fetal prolactin response to morphine. Intrafetal administration of naloxone (3.8 mg/kg bolus + 9.9 mg/kg/60 min), blocked the fetal prolactin response to morphine. Maternal plasma concentrations of prolactin were significantly increased (p less than 0.05) at 180 min after the intrafetal morphine bolus. When naloxone alone was infused, there was no change in fetal plasma prolactin concentrations, but there was a significant fall (p less than 0.05) in maternal plasma prolactin from 25 min after the start of the naloxone infusion. Thus, acute administration of morphine is associated with fetal and maternal hyperprolactinaemia. Although the endogenous opioids do not appear to mediate basal prolactin secretion in the fetus, they may have a role in the control of prolactin release in the pregnant ewe during late gestation.     

The clearance of ACTH from the plasma of adult and fetal sheep.

The disappearance of 125-I-ACTH from the circulation of 4 pregnant and fetal sheep has been followed after a single injection to the ewe or fetus. The mean metabolic clearance rate for the fetus and the ewe was 55 and 34 ml/min/kg respectively, giving a half-life in each case of about 1 min. The higher fetal than maternal arterial plasma ACTH concentration has been ascribed to a higher rate of secretion rather than a reduced rate of clearance compared with the ewe. There was no evidence of placental transfer of immunologically reactive ACTH.     

Nutritional regulation of insulin-like growth factor-binding protein gene expression in the ovine fetus and pregnant ewe.

The factors controlling the synthesis and degradation of the insulin-like growth factor-binding proteins (IGFBPs) during pregnancy are poorly understood. To clarify the roles of nutritional factors in the regulation of fetal and maternal IGFBP production, we examined the effects of fasting, refeeding, and glucose administration on plasma IGFBP concentrations and hepatic IGFBP mRNA levels in fetal lambs and pregnant ewes (n = 24). Maternal fasting for 3 days in late gestation stimulated a 50-100% increase in maternal plasma BP-1 concentrations (P < 0.05) and a 2- to 3-fold increase in fetal plasma BP-1 (P < 0.05), as determined by densitometric analysis of Western ligand blots. Fasting also stimulated a 40-70% increase in maternal plasma BP-2 concentrations (P < 0.05), but had no significant effect on fetal plasma BP-2 levels. Levels of hepatic BP-1 mRNA in the fetus and pregnant ewe during fasting paralleled plasma BP-1 levels, suggesting that fasting modulates fetal and maternal plasma BP concentrations at least in part through effects on hepatic gene expression. The effects of fasting on both mRNA and plasma levels of BP-1 and BP-2 were reversed by 3 days of refeeding and were prevented by glucose infusion during fasting. When ewes were made hyperglycemic by the infusion of hypertonic glucose, plasma BP-1 and BP-2 concentrations varied inversely with blood glucose concentrations. In addition, hyperglycemia reduced maternal liver BP-1 and BP-2 mRNA levels and fetal BP-1 mRNA levels by 50-65%. Direct administration of hypertonic glucose to the fetus decreased fetal plasma BP-1 levels acutely and reduced fetal BP-1 mRNA levels by 57%, but had no effect on fetal plasma BP-2 or fetal hepatic BP-2 mRNA levels. These findings indicate that glucose and other nutritional factors regulate gene expression and plasma levels of BP-1 and BP-2 in the pregnant ewe and BP-1 in the fetal lamb. The changes in expression of these IGFBPs during fasting and hyperglycemia may play roles in adaptation of the pregnant mother and fetus to metabolic stress.     

Maternal pinealectomy abolishes the diurnal rhythm in plasma melatonin concentrations in the fetal sheep and pregnant ewe during late gestation

We have investigated the effect of pinealectomy of ewes in pregnancy on the presence of the diurnal rhythm in fetal and maternal plasma concentrations of melatonin. Six ewes were pinealectomized between 104 and 118 days of gestation. Fetal and maternal blood samples were collected during 24-h periods between 125 and 140 days of gestation in the pinealectomized ewes and in an intact control (n = 4). There was a significant diurnal rhythm in both fetal and maternal plasma concentrations of melatonin in the control group. In this group, the fetal and maternal plasma melatonin concentrations were significantly higher in the dark (128.4 +/- 6.2 and 192.2 +/- 10.7 pmol/l respectively) than in the light (46.2 +/- 4.2 and 25.8 +/- 2.1 pmol/l respectively). However there was no diurnal rhythm in either the fetal or maternal plasma melatonin concentrations in the pinealectomized group between 125 and 140 days of gestation. In contrast to the control animals, there was also no light-dark difference in the fetal or maternal plasma melatonin concentrations in four pinealectomized animals sampled frequently in the 3-7 days preceding delivery (mean length of gestation 146.5 days). However, in the pinealectomized sheep there was a gradual increase in the combined light-dark fetal plasma melatonin concentrations during late gestation from 27.9 +/- 2.8 pmol/l (at 15-20 days before delivery) to 95.2 +/- 14.1 pmol/l on the day of delivery. We have therefore demonstrated that the maternal pineal is the major source of the diurnal rhythm in maternal and fetal plasma melatonin concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)     

Androstenedione metabolism in the late gestation sheep fetus

We have determined metabolic parameters for androstenedione (A) in chronically catheterized late gestation (day 130) sheep fetuses. The MCR (MCRA) was 3210 +/- 229 (SEM, n = 12) ml/min, the fetal arterial whole blood concentration of A [A] was 65 +/- 5 pg/ml, and the blood production rate (PRA) was 204 +/- 20 ng/min. Pulsatile administration of ACTH in amounts that raised fetal arterial plasma cortisol concentrations by 5- to 7-fold increased [A] to 154 +/- 20 pg/ml and PRA to 471 +/- 31 ng/min with no change in MCRA. In the presence of metopirone to block fetal adrenal cortisol output, ACTH treatment still provoked elevations in [A] (to 198 +/- 23 pg/ml) and PRA (539 +/- 158 ng/min), without altering MCRA. The major radiolabeled product in blood of infused [3H]A was [3H]testosterone; smaller amounts of phenolic steroids were formed. Extensive metabolism of [3H]A occurred in whole blood in vitro. The major product was [3H]testosterone; the 17-oxidoreductase activity was associated with the red blood cells. Umbilical vein [A] was greater than umbilical artery [A]; ACTH treatment increased [A] in both vessels. Concomitant metopirone abolished the arteriovenous difference by eliminating the ACTH-induced increase in venous [A], although arterial [A] rose significantly. The venous [A] and the arteriovenous gradient were restored with exogenous glucocorticoid treatment to the fetus. Collagenase-dispersed fetal adrenal cells secreted A. Adrenal cells from fetuses pretreated with ACTH in vivo had higher basal and ACTH-induced output of A in vitro than cells from fetuses pretreated with saline in vivo. We conclude that the MCRA in fetal sheep is extremely high, in part due to conversion of A to testosterone in fetal blood. The elevated PRA after ACTH plus metopirone and the lack of an umbilical arteriovenous gradient of [A] in this, but not other groups of fetuses, suggests a source of A production independent of the cortisol-induced changes in the placenta. Direct evidence is provided for fetal adrenal secretion of A which is enhanced by ACTH pretreatment of the fetus in vivo and for the utilization of circulating A in the fetus as a precursor for estrogen in both fetal and maternal compartments.     

Ontogenic differences in the nutritional regulation of circulating IGF binding proteins in sheep plasma.

Well-fed castrated male sheep (N = 3) and 125 days gestation pregnant ewes (N = 6) with chronically catheterized fetuses were fasted for 72 h. Insulin-like growth factor-binding protein (IGFBP) levels in fed and starved fetal, maternal and castrated male sheep plasma were measured using ligand blot analysis. IGFBPs in adult and fetal sheep differed in distribution both before and after 72 h starvation. IGFBP-3 was the major postnatal binding protein, while in the fetus IGFBP-2, IGFBP-3 and the circulating IGF type 2 receptor fragment each contributed 25-30% of total IGF binding capacity. After starvation, total IGF binding capacity and IGFBP-3 fell in plasma of maternal and castrated male sheep (p less than 0.05). Total IGF binding capacity rose with starvation in fetal plasma (p less than 0.05) as a result of an increase in IGFBP-1 (p less than 0.01) and IGFBP-2 (p less than 0.05). The different nutritional control of the IGFBPs in the fetus and the adult may reflect ontogenic differences in the regulation and function of circulating IGFs and their binding proteins.     

Fetal and maternal thyroid hormone responses to ethanol exposure during the third trimester equivalent of gestation in sheep

BACKGROUND: Abnormal thyroid hormone system function in the mother or fetus during pregnancy can result in brain defects, some of which resemble those found in children with fetal alcohol syndrome. It has been hypothesized that ethanol may act to mediate alcohol-related birth defects in part by altering thyroid hormone system function. We investigated whether a binge pattern of maternal ethanol consumption over the last trimester equivalent of gestation in sheep results in an alteration in fetal or maternal thyroid function. METHODS: Pregnant ewes received saline or ethanol beginning on day 109 of gestation (term, 145 days) for three consecutive days per week followed by 4 days without exposure. The fetuses were surgically instrumented on day 113, and experiments were performed on days 118 or 132. Fetal and maternal blood samples were collected, and plasma tri-iodothyronine (T3), thyroxine (T4), and free T4 concentrations were measured. RESULTS: Fetal T3 and T4 on day 118, fetal T3 on day 132, and maternal T3 on day 132 were lower in response to ethanol. Fetal and maternal free T4 and maternal T4 did not change in response to ethanol. Fetal thymus and adrenal weights were reduced in response to ethanol. CONCLUSIONS: We conclude that, in sheep, maternal ethanol exposure during the third trimester equivalent resulting in blood ethanol concentrations that are commonly achieved by ethanol abusers decreases circulating thyroid hormone concentrations in the mother and fetus and fetal thyroid and thymus mass.     

Fetal plasma insulin and thyroid hormone levels during acute in utero ethanol exposure in a maternal-fetal sheep model

The effects of acute in utero ethanol (ETOH) treatment on basal and stimulated thyroid and insulin levels in fetal plasma were studied in chronically cannulated fetal sheep. In test situations, pregnant ewes (0.78-0.88 gestation) which were chronically cannulated received 2 g/kg ETOH [25% (vol/vol) in isotonic saline] for 2 h; this was followed by a maintenance iv infusion of 0.13 g/kg ETOH. Control animals received isovolemic infusions of isotonic saline. Fetal arterial plasma samples were obtained after the 2-h infusion, and basal levels of T3, T4, glucose, and insulin were measured. The 2-h ETOH infusion did not influence fetal basal plasma T3, T4, insulin, or glucose. Fetal thyroid responses to an intraarterial injection of 0.01, 0.10, 1.00, or 10.00 micrograms/kg TRH or of 5 mU/kg TSH through the fetal catheters were studied in the presence or absence of high plasma ETOH concentrations. Fetal T4 or T3 levels during the 4 h following any of these stimuli were not significantly different in ethanol-treated and control animals. The effects of acute ETOH exposure on insulin responses to a glucose challenge were studied in six chronically cannulated ewes and their fetuses using a cross-over experimental design. After the 2-h ETOH infusion, ewes received a bolus injection of 600 mg/kg 50% glucose, followed by a 1-h infusion of 624 mg/kg 50% glucose and 0.13 g/kg ETOH. In control situations, ewes received saline plus glucose. Acute ETOH treatment did not influence maternal or fetal plasma glucose levels at any time, but enhanced both maternal and fetal insulin responses to glucose. Total insulin release, as measured by the area under the insulin response curve, was greater during ETOH exposure in both mother (ETOH, 4740 +/- 1475 microU/ml X min; control, 2807 +/- 766 microU/ml X min; P = 0.05) and fetus (ETOH, 562 +/- 94 microU/ml X min; control, 363 +/- 46 microU/ml X min; P less than 0.05). Thus acute in utero ETOH exposure does not diminish plasma levels of either thyroid hormones or insulin, two important hormones for fetal growth and development. However, ethanol exposure enhances the insulin response to increases in blood glucose in both mother and fetus.     

Persistence of fetal zone function in the infant rhesus monkey adrenal gland

Plasma dehydroepiandrosterone sulfate (DHAS) concentrations increase markedly in the rhesus monkey fetus at the end of gestation. A further increase occurs in the infant. To determine whether the changes in plasma concentration between the fetus and infant represent maintenance of DHAS production by the infant adrenal gland, we measured the t1/2, distribution volume (VD), MCR, and production rate of DHAS in the late gestation rhesus monkey fetus (129-155 days gestation; term is 165 days) and infant (14-42 days of age). A single bolus dose of [3H]DHAS was injected into five fetuses and four infants, and blood samples were collected serially from 5 min to 24 h after the injection. The amount of [3H]DHAS in the circulation was measured after solvolysis, extraction, and Celite chromatography. The concentration of DHAS in each sample was measured by RIA. DHAS was cleared significantly more rapidly in the fetus than in the infant [MCR in fetus, 2.4 +/- 0.4 (+/- SE); MCR in infant, 0.6 +/- 0.2 liters day-1 kg-1]. The t1/2 of DHAS was shorter in the fetus than in the infant (1.0 +/- 0.1 vs. 3.3 +/- 0.7 h). Absolute VD values were larger in the fetus than in the infant (231 +/- 29 and 143.8 +/- 11.6 ml kg-1); however, they were similar when the fetal VD was calculated including placental weight as a component of fetal weight. The production rate of DHAS, calculated as the product of MCR and integrated plasma DHAS concentration for the duration of the experiment, was not significantly different between the fetus and the infant (1.0 +/- 0.2 and 3.3 +/- 1.2 mg kg-1 day-1) in spite of the marked differences in plasma DHAS concentrations (445.8 +/- 103.8 ng ml-1 in the fetus and 5165 +/- 1296 ng ml-1 in the infant). These results indicate that the adrenal of the infant rhesus monkey continues to secrete DHAS at a rate at least as high as that in the late gestation fetus. Since the infant maintains DHAS production similar to that of the fetus in the absence of the placenta, a corollary of these studies is that the elevated DHAS secretion in the rhesus infant is independent of the placenta or the hormonal milieu of pregnancy. The maintenance of a functional fetal zone in the adrenal gland makes the rhesus infant a suitable model to use in studying the regulation of DHAS secretion and fetal zone morphology.     

Placental transport of low molecular weight heparin in the pregnant sheep

Standard heparin, an effective treatment for antepartum thromboembolic disease, is thought to be safe for the fetus since it does not cross the placenta. Recently, a number of low molecular weight heparins have been prepared which have been shown to produce less bleeding than standard heparin for an equivalent antithrombotic effect in experimental animals. These observations suggest that the low molecular weight heparins may also provide superior antithrombotic therapy in antepartum thromboembolic disease. However, it is not known whether the low molecular weight heparins cross the placenta. To determine this, we examined the pharmacokinetics of 125I-labelled standard heparin and a low molecular weight heparin, and their anticoagulant effects in mother and fetus, using a pregnant sheep model. Catheters were inserted into maternal and fetal femoral arteries at 108-119 d gestation (term: 147 d). 1-3 days later the mothers were given a bolus i.v. injection of 5000 anti-Xa units of 125I-labelled standard heparin or low molecular weight heparin, CY 222. Nine serial blood samples were collected over 4 h from both mother and fetus for measurements of radioactivity, anti-Xa activity (chromogenic) and activated partial thromboplastin times. When therapeutic levels of standard and CY 222 heparins were achieved in the mother, there was no detectable radioactivity or anticoagulant effect in the fetus. We conclude that standard heparin and the low molecular weight CY 222 do not cross the placenta in the pregnant sheep.     

Total and free serum thyroid hormone concentrations in fetal and adult pregnant and nonpregnant guinea pigs

Information on total and free serum thyroid hormone concentrations in the adult and fetal guinea pig (Cavia porcellus) is limited. These variables were studied in adult male and female guinea pigs and in pregnant guinea pigs and their fetuses at various times during gestation. Total serum T4 levels in adult males, nonpregnant females, and pregnant females did not differ significantly [range, 2.5 +/- 0.3 to 3.2 +/- 0.8 micrograms/dl (mean +/- SD)]. Similarly, there were no significant differences in the percent free T4 (0.046-0.068%), free T4 (1.26-2.03 ng/dl), total T3 (39-44 ng/dl), the percent free T3 (0.521-0.638%), and free T3 (0.221-0.260 ng/dl) among adult males, nonpregnant females, and pregnant females. rT3 was undetectable in adult male, nonpregnant female, and pregnant female guinea pig serum. T4 values were similar and those for T3 were lower in fetal compared to maternal serum at 45 days of gestation, whereas serum rT3 was detectable in fetal serum. Between 45 and 62 days of gestation, fetal serum T4 increased from 2.5 to 0.3 to 4.3 +/- 1.3 micrograms/dl (mean +/- SD, P less than 0.01), fetal serum T3 remained unchanged, and fetal serum rT3 increased from 5.2 +/- 3.3 to 25.0 +/- 11.4 ng/dl (P less than 0.01). Near term, fetal serum total and free T4 and total rT3 concentrations were significantly higher and total and free T3 concentrations were significantly lower than the corresponding values in maternal serum. Total serum T4 is higher in the guinea pig than in the rabbit, is similar to values in the rat, and is lower than values in man. The free T4 concentration in guinea pig serum is similar to those in humans and rats. The ontogenesis of thyroid hormones differs strikingly in the guinea pig fetus compared to that in the rat fetus and shares many similarities with sheep and human fetal thyroid development.     

Fetal and maternal endocrine responses to reduced uteroplacental blood flow.

In clinical practice, falling or reduced maternal estrogen levels are commonly regarded as indicators of fetal distress. However, experimental studies in primate animal models demonstrate that changes in maternal estradiol concentrations vary in response to reduced uteroplacental blood flow and are elevated during fetal hypoxemic stress, suggesting an increase in fetal adrenal precursor steroids. We studied the effects of graded reductions in maternal distal aortic blood flow (Qda) on the fetal MCR of dehydroepiandrosterone (D; MCR-D), the fetal production rate of D (PR-D), and changes in maternal and fetal plasma concentrations of D, D sulfate, cortisol, androstenedione, estrone (E1), and estradiol (E2) and in fetal plasma ACTH, PRL, and LH. A continuous i.v. infusion of [7-3H]D was administered to fetuses in five pregnant baboons (Papio anubis) at 155-165 days gestation (term, 184 days) for 270 min. A 50% reduction in mean distal aortic blood flow was imposed after 60 min by means of partial occlusion of the aorta with a snare device, which was released at 180 min. Maternal and fetal blood samples were collected at 10-min intervals from 30-60, 120-180, and 240-270 min. Equilibrium concentrations of [3H]D in fetal plasma were determined, and the MCR-D and PR-D were calculated for each of the three levels of Qda corresponding to the control, occlusion, and release intervals. Concentrations of steroid and peptide hormones in maternal and fetal plasma were determined by RIA, and arterial blood pH, pO2, and pCO2 were measured. Control fetal PR-D (mean +/- SE, 4.4 +/- 2.0 mg/day) rose significantly during aortic occlusion accompanied by fetal hypoxemia (11.8 +/- 3.1 mg/day; P less than 0.05) and remained elevated with release of the aortic constriction (13.8 +/- 2.9 mg/day). Changes in fetal MCR-D were variable and not statistically significant. Among the maternal plasma steroids, only E1 and E2 increased significantly, doubling from control values during aortic occlusion and increasing by another 50% after release (P less than 0.05). There was a significant correlation between fetal PR-D and maternal plasma E2 and E1 concentrations (r2 = 0.76 and 0.71, respectively; P less than 0.01). Fetal hypoxemia was associated with dramatic increases in fetal plasma D, D sulfate, androstenedione, E1, and E2. No significant change occurred in fetal plasma cortisol, which tended to decline throughout the study. We observed a dramatic and sustained increase in fetal plasma ACTH during the period of reduced Qda and for 90 min thereafter, but no change in PRL or LH.(ABSTRACT TRUNCATED AT 400 WORDS)     

Vitamin D metabolism in pregnant rabbits: differences between the maternal and fetal response to administration of large amounts of vitamin D3

Maternal and fetal metabolism of vitamin D was examined in term pregnant rabbits fed a normal diet and in those supplemented with a large amount of vitamin D3. Term pregnant rabbits (27--30 days of gestation) fed the normal diet showed lower levels of plasma calcium, 25-hydroxyvitamin D3 (250HD3), and 24,25-dihydroxyvitamin D3 [24,25-(OH)2D3] and higher plasma 1 alpha, 25-dihydroxyvitamin D3 [1 alpha, 25-(OH)2D3] levels than age-matched nonpregnant female rabbits. Kidney homogenates from pregnant rabbits produced mainly 1 alpha 25-(OH)2D3, while those from nonpregnant animals produced 24,25-(OH)2D3 primarily. Plasma concentrations of calcium and phosphorus were significantly higher in fetuses than in mothers. Plasma levels of 250HD3 and 24,25-(OH)2D3 in fetuses were almost identical to those in mothers, whereas 1 alpha,25-(OH)2D3 levels in plasma were significantly higher in mothers than in their fetuses. A daily administration of 650 nmol vitamin D3 for 3 days to term pregnant rabbits caused a significant increase in calcium, phosphorus, 25OHD3, and 24,25-(OH)2D3 in maternal plasma, and in 25OHD3 and 24,25-(OH)2D3, but not calcium and phosphorus in fetal plasma. Treatment with large amounts of vitamin D3 also induced a marked suppression of 1 alpha-hydroxylase activity and a concomitant increase of 24-hydroxylase activity in the maternal but not in the fetal kidney. Plasma concentrations of 1 alpha,25-(OH)2D3 were not affected by treatment with large amounts of vitamin D3 in either the fetuses or the mothers. These results clearly indicate that the renal 25OHD3 metabolism in the fetus is regulated independently of that in the mother.     

Fetal and maternal sheep hypothalamus pituitary adrenal axis responses to chronic binge ethanol exposure during the third trimester equivalent

BACKGROUND: We tested the hypothesis that in utero ethanol exposure results in changes in fetal and maternal adrenocorticotropin (ACTH) and cortisol during the third trimester equivalent, by using a chronically instrumented fetal sheep model. METHODS: Pregnant ewes received saline or ethanol intravenously 3 consecutive days per week from day 109 to day 132 of gestation. Fetal and maternal blood samples were collected on days 118 and 132. RESULTS: Maternal and fetal ACTH and cortisol values increased on days 118 and 132 of gestation in response to ethanol infusions that created blood ethanol concentrations (BECs) that are easily achievable by human drinkers. Peak ACTH and cortisol values were detected 30 to 60 min after peak BECs were achieved. CONCLUSIONS: Chronic ethanol exposure during the third trimester equivalent in sheep resulted in repeated activation of the hypothalamus-pituitary-adrenal axis in both the mother and fetus. Temporally, the patterns of maternal and fetal responses to ethanol infusion were similar. We conclude that ovine maternal ethanol exposure during the third trimester equivalent increases fetal ACTH and cortisol concentrations, hormonal responses that may play a role in mediating alcohol-related birth defects.     

Effects of altered photoperiod or maternal melatonin administration on plasma prolactin concentrations in fetal lambs

During winter (December to March), when late-pregnant ewes were maintained under an artificial long-day photoperiod (16 h light) for 3 weeks or more before insertion of fetal vascular cannulae between 118 and 120 days of gestation (full term, 147 days), plasma prolactin concentrations in their fetal lambs were significantly increased throughout the last 3 weeks of gestation in comparison with values in similar aged fetuses from ewes experiencing only the natural short-day (less than 9 h light) winter photoperiod. When additional lighting was given only after vascular cannulation, fetal plasma prolactin increased steadily from low values, characteristic of winter pregnancies, to high values, characteristic of long-day (16 h light:8 h darkness) pregnancies. Maternal plasma prolactin concentrations changed in a similar way. During summer pregnancies (greater than 16 h light), plasma prolactin in fetal lambs was significantly reduced within 48 h when ewes were given melatonin by i.v. infusion for 14 h each night to simulate the winter duration of the nocturnal increase in plasma melatonin. Maternal plasma prolactin concentrations also decreased significantly when melatonin was given for 3 weeks, but not in a shorter experiment. Increases in fetal plasma prolactin were proportional to the basal prolactin concentration in fetuses injected i.v. with TRH or a dopaminergic antagonist, metoclopramide, to assess how photoperiod influenced the responsiveness of prolactin secretion to acute stimulation. The results confirm that photoperiod, rather than developmental maturity, is the principal determinant of plasma prolactin in the fetal lamb during the last third of gestation, and provide evidence that photoperiodic information is transmitted to the fetus through the diurnal rhythm of melatonin in the ewe.     

Regulation of basal adrenocorticotropin and cortisol secretion by arginine vasopressin in the fetal sheep during late gestation.

This study tested the hypothesis that arginine vasopressin (AVP) is involved in the regulation of basal ACTH secretion in the ovine fetus near term. In five fetuses challenged with AVP (1 microgram/ml, iv bolus) plasma ACTH concentrations increased to an 8-fold peak within 10 min of the preceding baseline (55 +/- 6 to 403 +/- 241 pg/ml). Cortisol in fetal circulation subsequently increased 2-fold (11 +/- 1 to 28 +/- 5 ng/ml) within 15 min of the AVP injection. The AVP-induced rise in plasma ACTH and cortisol concentrations was blocked when the fetus was pretreated with the AVP V1 receptor antagonist d(Ch2)5Tyr(Me)AVP. In a total of seven studies, antagonist (10 micrograms/kg estimated BW, iv bolus) was administered to three fetuses, aged 137-147 days gestation, followed 40 min later by the exogenous AVP challenge, as described above. After AVP antagonist treatment, basal ACTH and cortisol concentrations were not significantly different from the preinjection baseline levels (P greater than 0.05, by analysis of variance). Moreover, plasma ACTH and cortisol remained unchanged after the AVP challenge. To further define the role of endogenous AVP in basal ACTH and cortisol secretion, the AVP antagonist was administered (five studies in two fetuses) at 30-min intervals for a total of three injections per fetus. This extended AVP antagonist regimen also failed to alter fetal circulating concentrations of ACTH or cortisol (P greater than 0.05). Cortisol in the maternal circulation was not affected by any of the fetal AVP or AVP antagonist treatments. Lambs were born at 146 +/- 2 days gestation (n = 5), within the range for the normal duration of pregnancy. These data do not support the hypotheses that AVP is involved in the regulation of basal ACTH secretion in the fetal sheep during the 10 days preceding parturition. Rather, the ability of AVP antagonist to block the AVP-induced rise in plasma ACTH and cortisol in the fetus suggests that basal and stimulated ACTH secretion are under separate regulatory mechanisms.     

Differential effects of maternal dexamethasone treatment on circulating thyroid hormone concentrations and tissue deiodinase activity in the pregnant ewe and fetus

Clinically, treatment of pregnant women at risk of preterm delivery with synthetic glucocorticoids accelerates fetal maturation. This study investigated the effect of maternal dexamethasone treatment, in clinically relevant doses, on plasma thyroid hormone concentrations and tissue deiodinase activities (D1, D2, and D3) in ewes and their fetuses. From 125 d of gestation (term 145 +/- 2 d), pregnant ewes were injected twice im with either saline (2 ml of 0.9% NaCl, n = 11) or dexamethasone (2 x 12 mg in 2 ml of saline, n = 10) at 24-h intervals. Maternal dexamethasone treatment increased plasma T(3) and reverse T(3) (rT(3)), but not T(4), concentrations in the fetuses. In the dexamethasone-exposed fetuses, hepatic D1 activity was higher, and renal and placental D3 activities were lower, than in the saline-exposed fetuses. In the ewes, plasma concentrations of T(3) and T(4) were reduced, and rT(3) increased, by dexamethasone treatment without any change in tissue deiodinase activity. Therefore, maternal dexamethasone treatment has different effects on the thyroid hormone axis of the pregnant ewe and fetus. In the fetus, the dexamethasone-induced rise in circulating T(3) may be due to both increased hepatic production of T(3) from T(4), and reduced clearance of T(3) by the kidney and placenta. Changes in T(3) bioavailability may mediate some of the maturational effects of antenatal glucocorticoid treatment in the preterm fetus.