Fetal growth restriction: adaptations and consequences

A range of pathophysiological factors can result in a perturbation or restriction of fetal growth, and the cardiovascular, neuroendocrine and metabolic adaptations of the fetus to these stimuli will depend on their nature, timing and intensity. The critical importance of these physiological adaptations for both immediate survival and long-term health outcomes has provided an impetus for experimental studies of the nature and consequences of specific fetal adaptations to a poor intrauterine environment. This review summarizes data from recent studies that have focused on the responses of the fetal cardiovascular, sympathoadrenal, hypothalamo-pituitary-adrenal and renin-angiotensin systems to experimental restriction of placental function in the sheep and discusses the consequences of these adaptations for fetal, neonatal and adult health.

Restriction of fetal growth has a differential impact on fetal prolactin and prolactin receptor mRNA expression

Prolactin is present in the fetal circulation and prolactin receptors are expressed in a wide range of fetal tissues. The factors which regulate the synthesis and secretion of prolactin, and the expression of its receptors before birth, are poorly understood. We have investigated whether experimental restriction of placental growth in the sheep has an impact on the prolactin axis in the growth restricted fetus. The majority of uterine endometrial caruncles were removed before pregnancy in 10 ewes (placental restriction; PR group). Placental, fetal liver and kidney weights were reduced in the PR compared to the control group (n = 10). The ratio of fetal prolactin mRNA : 18S rRNA was significantly lower (P < 0.01) in the PR group (1.83 +/- 0.45, n = 6) than in the control group (4.11 +/- 0.54, n = 6). The ratio of prolactin mRNA : 18S rRNA in the fetal pituitary was positively correlated with fetal and with placental weight. Using stepwise linear regression, it was determined that the level of fetal prolactin mRNA : 18S rRNA expression was best described (as judged by the maximum adjusted R2) by prolactin mRNA: 18 S rRNA = - 3.0378 + 0.17 PO2 + 2.772 glucose (adjusted R2 = 0.765, F = 17.53, P < 0.001). Fetal plasma prolactin concentrations were significantly reduced (P < 0.05) in the PR group compared to control animals between 109 and 141 days gestation. Fetal prolactin receptor (PRLR) mRNA transcripts encoding long (PRLR1) and short forms (PRLR2) of PRLR were present in the liver and kidney of animals in the PR and control groups at 140-141 days gestation. PR did not alter the levels of PRLR1 or PRLR2 mRNA in the fetal liver or kidney. The suppression of the synthesis and secretion of prolactin in the growth restricted fetus may limit the action of prolactin on the growth and metabolism of key fetal organs during suboptimal intrauterine conditions

Differential effects of placental restriction on IGF-II, ACTH receptor and steroidogenic enzyme mRNA levels in the foetal sheep adrenal

We have investigated the effects of restriction of placental growth on foetal adrenal growth and adrenal expression of mRNAs for Insulin-like Growth Factor II (IGF-II), the IGF binding protein IGFBP-2, Steroidogenic Factor 1 (SF-1) and adrenocorticotrophic hormone (ACTH) receptor (ACTH-R) and the steroidogenic cytochrome P-450 enzymes: cholesterol side chain cleavage (CYP11A1), 17alpha-hydroxylase (CYP17) and 21-hydroxylase (CYP21A1); and 3beta-hydroxysteroid dehydrogenase/Delta5Delta4 isomerase (3betaHSD). Endometrial caruncles were removed from non-pregnant ewes before mating (placental restriction group; PR). The total adrenal: foetal weight ratio was higher in PR (n=6 foetuses) than in control foetuses (n=6 foetuses). There was no difference in plasma ACTH concentrations between the PR and control foetuses between 130 and 140 days gestation. Adrenal IGF-II mRNA levels were lower (P<0.05) in the PR group, however, adrenal IGFBP-2 mRNA levels were not different between the PR and control groups. Adrenal ACTH-R mRNA levels were also lower whilst CYP11A1 mRNA levels were increased (P<0.005) in the PR group. We conclude that foetal adrenal growth and steroidogenesis are stimulated as a consequence of foetal growth restriction and that factors other than ACTH are important in foetal adrenal activation during chronic, sustained hypoxaemia.

Effects of placental insufficiency on the ovine fetal renin-angiotensin system

We postulated that chronic placental insufficiency would be associated with reduced expression of renal renin and angiotensinogen genes in the fetal sheep. Placental development was restricted in ewes by removing the majority of caruncles prior to mating (placentally restricted (PR) group). The weights of PR fetuses were significantly reduced (P < 0.05, 2.98 +/- 0.33 kg) compared to control fetuses (4.20 +/- 0.30 kg). Kidney weights were also significantly reduced in the PR fetuses (P < 0.05, 8.4 +/- 0.9 g) compared with control fetuses (12.2 +/- 1.3 g). The ratios of renal renin/-actin mRNA levels were significantly reduced in PR fetuses (P < 0.001, 0.35 +/- 0.02) when compared to control animals (0.98 +/- 0.13). The renal angiotensinogen mRNA/18S rRNA ratio was significantly lower (P < 0.05, 0.28 +/- 0.13) in PR fetuses compared with control fetuses (0.72 +/- 0.10), while hepatic angiotensinogen was unaffected. There was a positive correlation between renal renin mRNA and renal angiotensinogen mRNA levels (r = 0.65, P < 0.05, n = 12). It is unlikely that these changes in renal angiotensinogen and renin mRNA were due to the small increment in plasma cortisol levels (< 5 nmol l-1). There was, however, a positive correlation between arterial PO2 and renal renin mRNA (r2 = 0.77, P < 0.01). Plasma renin levels were not different between the two groups. Thus, restriction of nutrient and oxygen supply throughout fetal life was associated with suppression of renal renin and renal angiotensinogen gene expression, with no effect on hepatic angiotensinogen mRNA levels. This specific suppression of fetal renal renin and angiotensinogen expression could alter the activity of the intrarenal RAS and so affect growth and development of the kidney.

Restriction of placental and fetal growth in sheep alters fetal blood pressure responses to angiotensin II and captopril

1. We have measured arterial blood pressure between 115 and 145 days gestation in normally grown fetal sheep (control group; n = 16) and in fetal sheep in which growth was restricted by experimental restriction of placental growth and development (PR group; n = 13). There was no significant difference in the mean gestational arterial blood pressure between the PR (42.7 +/- 2.6 mmHg) and control groups (37.7 +/- 2.3 mmHg). Mean arterial blood pressure and arterial PO2 were significantly correlated in control animals (r = 0.53, P < 0.05, n = 16), but not in the PR group. 2. There were no changes in mean arterial blood pressure in either the PR or control groups in response to captopril (7.5 microg captopril min-1; PR group n = 7, control group n = 6) between 115 and 125 days gestation. After 135 days gestation, there was a significant decrease (P < 0.05) in the fetal arterial blood pressure in the PR group but not in the control group during the captopril infusion (15 microg captopril min-1; PR group n = 7, control group n = 6). 3. There was a significant effect (F = 14.75; P < 0.001) of increasing doses of angiotensin II on fetal diastolic blood pressure in the PR and control groups. The effects of angiotensin II were different (F = 8.67; P < 0.05) in the PR and control groups at both gestational age ranges. 4. These data indicate that arterial blood pressure may be maintained by different mechanisms in growth restricted fetuses and normally grown counterparts and suggests a role for the fetal renin-angiotensin system in the maintenance of blood pressure in growth restricted fetuses.

Differential effects of increasing gestational age and placental restriction on tyrosine hydroxylase, phenylethanolamine N-methyltransferase, and proenkephalin A mRNA levels in the fetal sheep adrenal

We have demonstrated that there are differential changes in the levels of tyrosine hydroxylase (TH), phenylethanolamine N-methyltransferase (PNMT), and proenkephalin A (Pro Enk A) mRNA in the fetal sheep adrenal during late gestation. Adrenal TH mRNA:18S rRNA ratios increased between gestational days 100 (0.98 +/- 0.13; n = 6) and 125 (1.40 +/- 0.15; n = 6) and then decreased, whereas adrenal PNMT mRNA:18S rRNA ratios increased regularly between gestational days 100 (0.08 +/- 0.01) and 146 (0.17 +/- 0.03). The ratio of adrenal Pro Enk A mRNA to 18S rRNA was higher at gestational day 125 (0.085 +/- 0.005) than at either 80-100 days (0.038 +/- 0.007) or 140-146 days of gestation (0.055 +/- 0.013). In 12 ewes, the growth and development of the placenta were restricted (placental restriction group) from conception. The ratio of adrenal PNMT mRNA to 18S rRNA was significantly reduced in the placental restriction group of fetal sheep (0.003 +/- 0.002) compared with controls (0.011 +/- 0.002), and there was a significant correlation between the ratio of adrenal PNMT mRNA to 18S rRNA and the mean arterial PO2 (r = 0.88, p < 0.0005). In contrast, TH mRNA and Pro Enk mRNA were unaffected by placental restriction. Adrenaline and noradrenaline syntheses are therefore differentially regulated in the adrenal during late gestation and in response to chronic intrauterine hypoxemia.

Impact of placental restriction on the development of the sympathoadrenal system

We have investigated the impact of chronic restriction of placental function on circulating catecholamine concentrations and responses to the indirectly acting, sympathomimetic amine, tyramine, in the fetal sheep in late gestation. In 10 ewes, endometrial caruncles or placental placentation sites were removed before conception (placental restriction (PR) group). Fetal sheep in the PR group were hypoxemic throughout late gestation and growth-restricted (3.02 +/- 0.35 kg) when compared with control fetal sheep (4.30 +/- 0.29 kg; n = 8) at 140 d of gestation. Fetal plasma concentrations of noradrenaline and adrenaline were higher (p < 0.05) in the PR (7.06 +/- 3.17 pmol/mL and 2.89 +/- 2.01 pmol/mL, respectively) than in the control group (3.55 +/- 0.54 pmol/mL and 1.30 +/- 0.48 pmol/mL, respectively) throughout late gestation. Plasma noradrenaline, but not adrenaline concentrations, increased significantly between 110 and 140 d of gestation in both the PR and control group, and there was a significant inverse relationship between plasma noradrenaline and arterial PO2 in the PR and control groups (plasma noradrenaline = 12.34 - 0.40 PO2). In the PR group, plasma noradrenaline increased (p < 0.05) after tyramine infusion from 4.51 +/- 1.28 pmol/mL to a peak of 19.40 +/- 3.56 pmol/mL. In the control group, noradrenaline increased from 2.08 +/- 0.30 pmol/mL to a peak of 12.23 +/- 1.67 pmol/mL after tyramine infusion. There was no difference, however, in the maximal proportional changes in plasma noradrenaline concentrations in the PR (319 +/- 55%) and control (449 +/- 100%) groups after tyramine. We conclude that the most likely source of the increased plasma catecholamines in the PR group is enhanced catecholamine synthesis and secretion from developing sympathetic neurons.

Placental restriction alters the functional development of the pituitary-adrenal axis in the sheep fetus during late gestation

We have experimentally restricted placental growth in the sheep to investigate the impact of reduced substrate delivery on fetal pituitary proopiomelanocortin (POMC) mRNA levels and on circulating ACTH 1-39, immunoreactive ACTH, and cortisol concentrations during late gestation. Endometrial caruncles were removed in nine ewes before mating to reduce the number of placentomes formed [placental restriction group (PR)]. Fetal arterial PO2 and O2 saturation were reduced in the PR group (2.0 +/- 0.1 kPa and 42.8 +/- 1.1%, n = 9) when compared with control fetuses (3.1 +/- 0.1 kPa and 66.4 +/- 0.9%, n = 10). The ratio of anterior pituitary POMC mRNA:18 S ribosomal RNA was also lower (p < 0.05) in the PR group (0.49 +/- 0.05) when compared with the control group (0.80 +/- 0.12) after 140 d of gestation. In contrast, plasma concentrations of ACTH 1-39 and immunoreactive ACTH were similar in the PR and control groups throughout late gestation. Plasma ACTH 1-39 concentrations increased (p < 0.006) between 128 and 134 d of gestation, in both the PR (122-128 d: 2.70 +/- 0.34 pmol/L: 134-141 d; 7.07 +/- 1.57 pmol/L) and control (122-128 d; 3.36 +/- 0.56 pmol/L: 134-141 d; 10.78 +/- 2.88 pmol/L) groups. Combined adrenal weight was higher (p < 0.005) in the PR group (130 +/- 10 mg/kg) compared with controls (80 +/- 1 mg/kg) at 140 d of gestation, and plasma cortisol concentrations were also higher (p < 0.02) in PR than control fetuses between 127 and 141 d of gestation. These changes imply that the fetal hypothalamopituitary-adrenal axis is operating at a new central set point in the growth-restricted fetus.

Plasma catecholamine and met-enkephalin-Arg6-Phe7 responses to hypoxaemia after adrenalectomy in the fetal sheep

We have investigated whether removal of the fetal adrenal glands alters the effect of acute hypoxaemia on the circulating concentrations of noradrenaline, adrenaline, Met-Enkephalin and Met-Enkephalin-Arg6-Phe7 (MERF) in the late gestation sheep fetus. Ewes of 6 adrenalectomized and 9 intact fetuses were subjected to a 30 min period of hypoxaemia and a 30 min period of normoxaemia between 135 and 141 days gestation. Removal of the fetal adrenals abolished the fetal adrenaline response and significantly reduced the fetal noradrenaline response to hypoxaemia. There was no significant increase in circulating Met-Enkephalin during either hypoxaemia or normoxaemia in either the intact and adrenalectomized groups. During basal conditions, plasma concentrations of MERF were significantly greater in the adrenalectomized group (1.64 +/- 0.07 ng/ml) than in the intact fetal sheep (1.04 +/- 0.05 ng/ml). There were significantly greater changes (p < 0.05) in plasma MERF concentrations during hypoxaemia than during normoxaemia in both the intact and adrenalectomized groups. Plasma concentrations of MERF were inversely correlated with arterial PO2 (r = -0.44, p < 0.01) in the adrenalectomized but not the intact group of fetal sheep during hypoxaemia. We have demonstrated therefore that the fetal adrenal is the major source of circulating catecholamines, but not Met-Enkephalin or MERF during hypoxaemia in late gestation. The increase in circulating MERF concentrations after adrenalectomy may reflect compensatory changes in the synthesis and/or secretion of MERF in developing sympathetic neurones.

Changes in renal renin gene expression in fetal sheep

1. Renin gene expression was investigated in kidneys from 13 foetal and four adult sheep. 2. The levels of renal renin mRNA in foetuses were greater than those in adult sheep (P < 0.001). 3. There was no significant difference in renin mRNA levels between foetuses aged 91 and 134 days (term 150 days). 4. The levels of renin gene expression were higher (178 +/- 5 units; P < 0.001) in foetuses at 142 days than levels in 91 (144 +/- 4 units) and 134 days old foetuses (146 +/- 6 units). 5. It is concluded that the high level of renin gene expression in the foetal kidney is responsible for the high levels of renin in the foetal circulation.

The non-neurogenic catecholamine response of the fetal adrenal to hypoxia is dependent on activation of voltage sensitive Ca2+ channels

We have investigated the cellular mechanisms underlying the catecholamine response of the fetal sheep adrenal to hypoxia before and after the development of adrenal innervation. Adrenals were collected before (80-100 days gestation: n = 7) and after (135-146 days gestation: n = 10) development of innervation and retrogradely perfused with oxygenated Krebs bicarbonate buffer in vitro via the renal vein. Adrenal hypoxia was induced by perfusion with hypoxic Krebs buffer (pO2 = 46.7 +/- 2.4 mm Hg) for 30 min periods in the presence and absence of hexamethonium (500 microM), Ca2+ (2.5 mM), nifedipine (1 microM) and KCl (10 mM). Hypoxia stimulated an increase (P < 0.001) in the output of noradrenaline at 80-100 days (3 min pre hypoxia, 0.18 +/- 0.07 nmol/3 min; 20 min hypoxia, 0.74 +/- 0.22 nmol/3 min) and at 135-146 days (3 min pre hypoxia, 0.53 +/- 0.20 nmol/3 min; 20 min hypoxia, 1.71 +/- 0.85 nmol/3 min). Adrenaline output was also higher (P < 0.001) than basal values (80-100 days, 0.11 +/- 0.06 nmol/3 min; 135-146 days, 0.53 +/- 0.15 nmol/3 min) after 20 min hypoxia (0.41 +/- 0.20 nmol/3 min and 1.35 +/- 0.56 nmol/3 min respectively). The catecholamine responses to hypoxia were abolished by removal of Ca2+ from the adrenal perfusate. There was a reduction (P < 0.05) in the catecholamine secretory response to hypoxia in the presence of nifedipine. Noradrenaline output decreased from 4.33 +/- 0.84 nmol/30 min to 0.16 +/- 0.49 nmol/30 min and adrenaline output decreased from 3.16 +/- 1.66 nmol/30 min to -0.01 +/- 0.24 nmol/30 min in the presence of nifedipine. The fetal adrenal secretes catecholamines by a direct or non-neurogenic mechanism in response to hypoxia. This secretory response is dependent on the activation of voltage sensitive Ca2+ channels in the chromaffin cell membrane.

The role of the pituitary-thyroid axis in the developmental regulation of enkephalin-containing peptides in the superior cervical ganglia and adrenal of the fetal sheep

We have postulated that sympathetic neurones may be one source of circulating enkephalins in the fetus. Therefore, we have investigated the effects of increasing gestational age on the content of the enkephalin-containing peptides in the superior cervical ganglia (SCG) of the fetal sheep during late gestation. We have also investigated the role of the pituitary-thyroid axis in the coordinate regulation of enkephalin-containing peptides within the fetal sympathoadrenal system. We found an increase (P < 0.05) in the content of Free Met-O-Enkephalin between 85 and 105 days (d) (0.46 +/- 0.12 nmoles/SCG) and 116 and 125 d (0.93 +/- 0.12 nmoles/SCG), and the SCG content of Free Met-O-Enkephalin remained elevated throughout the remainder of gestation. The fetal SCG content of Total Met-O-Enkephalin also increased significantly between 85 and 105 d (2.8 +/- 0.5 nmoles/SCG) and 116 and 125 d (7.1 +/- 1.2 nmoles/SCG). There was, therefore, no change in the ratio of Total:Free Met-O-Enkephalin content of the SCG between 85 and 145 d. There were no significant effects of fetal hypophysectomy with or without thyroxine (T4) replacement on the content of Free Met-O-Enkephalin (intact, 1.11 +/- 0.18 nmoles; hypophysectomy (HX) + Sal, 1.41 +/- 0.25 nmoles; HX + T4, 1.08 +/- 0.27 nmoles) or Total Met-O-Enkephalin (intact, 3.03 +/- 0.90 nmoles; HX + Sal, 4.03 +/- 0.22 nmoles; HX + T4, 3.53 +/- 0.88 nmoles) in the fetal SCG. The adrenal contents of Free and Total Met-O-Enkephalin were, however, significantly reduced after fetal hypophysectomy and these effects were not altered by replacement with T4 after fetal hypophysectomy. In summary, we have demonstrated for the first time that enkephalin-containing peptides are present in the SCG of the fetal sheep and that the SCG enkephalin content increases between 85 and 125 days gestation. We have found no evidence that there is a change in the post-translational processing of the enkephalin precursor to Free Met-Enkephalin within the SCG during the second half of gestation. In contrast to the fetal SCG, maintenance of adrenal enkephalin content in late gestation is dependent on the presence of an intact fetal pituitary, but is independent of the circulating concentrations of thyroxine.

Thyroxine replacement after hypophysectomy alters the pattern of enkephalin localisation in the adrenal medulla of the fetal sheep

It has been suggested that a pituitary-derived or -dependent factor may suppress enkephalin peptide synthesis in the central noradrenaline- containing cells of the sheep adrenal medulla in late gestation. We have investigated the effect of thyroxine (T4) replacement after fetal hypophysectomy on the localisation of enkephalin peptides within the peripheral adrenaline- and central noradrenaline-containing regions of the adrenal medulla of the fetal sheep. Fetal hypophysectomy (HX) was performed in 12 fetal sheep at 105-108 days gestation (term = 145 +/- 3 days gestation). T4 (40 micrograms/kg/24h) (HX + T4) or saline (HX + Sal) were infused between 110 and 140 days gestation. Adrenal glands were collected from the HX + T4, HX + Sal groups and from a group of intact fetal sheep (n = 4) for immunohistochemistry using anti- met-enkephalin and an avidin-biotin staining system. In adrenals from intact fetal sheep, there was intense positive staining for met-enkephalin in the peripheral adrenaline-containing region and sparse staining for enkephalins in the central noradrenaline-containing region. In contrast, in the HX + Sal group, enkephalin staining was uniformly present throughout the peripheral and central regions of the adrenal medulla. In the HX + T4 group, however, the staining density of met-enkephalin was higher within the adrenaline cells of the peripheral rim of the medulla than in the central medullary region. We have demonstrated that T4 replacement after fetal hypophysectomy restores the normal ontogenetic pattern of localisation of enkephalin peptides within the fetal adrenal. We postulate that T4 acts either indirectly via neural mechanisms or directly at the noradrenaline-containing cells to mediate the suppression of enkephalin staining within the central noradrenaline-containing region of the adrenal medulla of the fetal sheep in late gestation.

Impact of gestational age on the catecholamine responses of the fetal sheep adrenal to cholinergic stimulation in vitro

We have used the retrograde perfused adrenal preparation to investigate the catecholamine responses of the fetal sheep adrenal to increasing doses of acetylcholine (ACh) and excess potassium. Adrenal glands were collected from fetal sheep between 84 and 99 days gestation (before innervation; n = 7), between 103 and 113 days gestation (during innervation; n = 8) and between 137 and 144 days (after innervation; n = 9). Whilst the basal output of noradrenaline (NA) did not change between 84 and 144 days gestation, there was a significant increase in the adrenal output of adrenaline (Adr) between 84 and 144 days. The NA response to submaximal doses of ACh (10-10(3) microM) was significantly greater (P < 0.05) between 84 and 99 days gestation (mean NA response: 49.6 +/- 11.4 nmol/(30 min)) than after 137 days gestation (mean NA response: 27.3 +/- 8.3 nmol/(30 min)). Similarly, when the NA response to 10-10(3) microM ACh was expressed as a proportion of the maximal NA response to 10(4) microM ACh, the proportional NA response was greater at 84-99 days (48.9 +/- 11.3% of maximal) than at 137-144 days gestation (27.2 +/- 8.3%). The adrenal NA and Adr responses to 10(3) microM ACh between 84 and 137 days were reduced by up to 99% after the addition of hexamethonium. It appears, therefore, that the fetal adrenal is responsive to ACh before the development of innervation of the gland. There is also a decrease in sensitivity of the NA-secreting cells to submaximal doses of ACh as gestation progresses, which is not associated with a decrease in the size of the releasable pool of NA in the fetal adrenal. We speculate, therefore, that innervation of the fetal adrenal may be associated with an increase in the excitation threshold of the NA cells to nicotinic stimulation.

Thyroid hormone replacement restores circulating enkephalin concentrations in hypophysectomized fetal sheep

We have investigated the effects of fetal hypophysectomy (HX) with or without thyroxine (T4)replacement on the plasma concentrations of free methionine-enkephalin (free Met-Enk), noradrenaline, and adrenaline in late gestation sheep fetus. Plasma adrenaline concentrations were significantly higher in intact fetal sheep (1.05 +/- 0.12 pmol/L) between 125 and 140 days of gestation when compared with the HX + saline (0.64 +/- 0.10 pmol/L) and HX + T4 (0.61 +/- 0.08 pmol/L) groups. During the first 15 days of the T4 or saline infusion, the plasma concentrations of free Met-Enk were significantly higher in the HX + T4 group (392 +/- 40 pmol/L) than in the HX + saline group (299 +/- 43 pmol/L). At this stage of gestation, however, circulating concentrations of free Met-Enk were significantly higher in intact fetal sheep (556 +/- 51 pmol/L) than in either of the HX groups. Between 125 and 140 days of gestation, plasma free Met-Enk concentrations were similar and significantly higher in the intact and HX + T4 groups than those measured in the HX + saline fetal sheep. We conclude that the decrease in circulating free Met-Enk concentrations after removal of the fetal pituitary is primarily a consequence of functional athyroidism.

Effect of hypophysectomy on plasma catecholamines and enkephalins in fetal sheep

The aim of this study was to determine the effect of fetal hypophysectomy on the plasma concentrations of catecholamines and enkephalins in the fetal sheep between 120 and 140 days gestation under basal and hypoxic conditions. During basal conditions, there was no difference in the plasma concentrations of norepinephrine and epinephrine between intact and hypophysectomized groups. Fetal plasma norepinephrine concentrations were significantly increased during hypoxia in intact fetal sheep (7.2 +/- 2.2 pmol/ml, -15 min; 20.2 +/- 7.7 pmol/ml, 30 min) between 130 and 140 days, but after fetal hypophysectomy there was no significant norepinephrine response to hypoxia at this gestational age (4.7 +/- 1.3 pmol/ml, -15 min; 8.8 +/- 2.8 pmol/ml, 30 min). In contrast, fetal plasma epinephrine concentrations were significantly increased during hypoxia in both the intact (1.5 +/- 0.5 pmol/ml, -15 min; 3.3 +/- 1.7 pmol/ml, 30 min) and hypophysectomized groups (1.8 +/- 0.6 pmol/ml, -15 min; 6.8 +/- 4.1 pmol/ml, 30 min) between 130 and 140 days. During basal conditions, plasma concentrations of free Met-Enk were significantly less in hypophysectomized fetal sheep (170.8 +/- 34.3 pg/ml; 120-140 days) than in intact fetal sheep (305.6 +/- 47.3 pg/ml). There were no differences, however, in the fetal plasma concentrations of total Met-Enk between the intact (18.0 +/- 1.9 ng/ml) and hypophysectomized (16.9 +/- 2.6 ng/ml) groups. During hypoxia, there were no changes in the fetal plasma concentrations of either free or total Met-Enk in the intact or hypophysectomized groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Fetal adrenalectomy does not affect circulating enkephalins in the sheep fetus during late gestation

We have measured circulating catecholamines and enkephalins in intact and bilaterally adrenalectomised fetal sheep between 115 and 144 days of gestation using specific radioimmunoassays for total Met-enk-containing peptides (total Met-Enk), free Met-Enk and Met-Enk-arg6-phe7 (MERF). In the intact group fetal plasma concentrations of noradrenaline increased from 1.7 +/- 0.4 (115-124 days) to a peak of 3.7 +/- 0.6 pmol/ml (135-144 days; all results expressed as means +/- standard error of the mean). The mean plasma concentration and the gestational age profile of noradrenaline were the same in the intact and adrenalectomised fetal sheep. We observed no change in the fetal plasma concentrations of adrenaline between 115 and 144 days of gestation and there was also no effect of removal of both fetal adrenal glands on plasma adrenaline concentrations. In the intact fetal sheep there was a significant increase in the circulating concentration of free Met-Enk between 115-119 (497.7 +/- 128.4 pmol/l) and 125-129 days of gestation (647.8 +/- 59.5 pmol/l). There was a similar increase in plasma MERF concentrations between 115-119 (850.4 +/- 170.4 pmol/l) and 130-134 days (1,525.1 +/- 227.0 pmol/l). There was no change, however, in the plasma concentrations of total Met-Enk across this gestational age range. The mean circulating concentrations and the gestational age profiles of plasma total and free Met-Enk and MERF were the same in the intact and adrenalectomised fetal sheep across the age range studied. We have demonstrated therefore that during unstressed conditions the fetal adrenal medulla is not the major source of circulating enkephalins.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of feeding on the diurnal rhythm of plasma cortisol and adrenocorticotrophic hormone concentrations in the pregnant ewe and sheep fetus

The effects of two different feeding regimes on the 24 h profiles of maternal and fetal plasma cortisol and adrenocorticotrophic hormone (ACTH) concentrations were studied in eight pregnant ewes between 123 and 144 days of gestation. Once daily-fed ewes (n = 4) received 1 kg of lucerne-chaff at 11.00 h, and multi-fed ewes (n = 4) received 100-200 g of lucerne-chaff at 09.00, 11.00 and 13.00 h and then 150 g until 09.00 h the following day. There were significant differences between the two feeding groups in the 24 h profile of maternal plasma osmolality; once daily feeding at 11.00 h was associated with a peak in maternal plasma osmolality at 15.00 h whereas maternal plasma osmolality reached plateau levels at around 17.00 h in the multi-fed group. There were also differences between the two feeding groups in the 24 h profiles of maternal and fetal plasma glucose. Maternal and fetal plasma glucose reached peak concentrations at 19.00 h in the once daily-fed ewes in contrast to the multi-fed group, where a plateau in maternal and fetal plasma glucose was reached between 19.00 h and 09.00 h the following day. A significant diurnal variation in the plasma concentrations of cortisol was present in the once daily-fed ewes from 123 days gestation and in their fetuses after, but not before, 135 days gestation. Plasma cortisol peaked at 11.00 h in the ewes and at 13.00 h in the fetuses of this group. In the once daily-fed group there was also a significant diurnal variation in maternal and fetal plasma ACTH; plasma ACTH concentrations were highest at 11.00 h in the ewes aged between 123 and 144 days and in fetuses after 135 days gestation. In the multi-fed group, whilst ACTH was highest at 09.00 h in the ewes and at 13.00 h in the fetuses, there was no significant diurnal variation in the plasma concentrations of cortisol in the ewes or fetuses of this group at any stage between 123 and 144 days gestation.