Control of adrenal androgen secretion.

The human adrenal gland secretes large amounts of androgens and androgenic precursors as compared with the adrenal glands of other species. In part, this pattern of secretion is regulated by ACTH, analogous to the control of cortisol. However, in many instances, including adrenarche, puberty, aging, and severe illness, secretion of adrenal androgens and cortisol diverge for reasons which are not clear. Factors endogenous to the adrenal gland may have a role in the modulation of adrenal androgen secretion. These include the centripetal blood supply of the adrenal gland, availability of cofactors of steroidogenic enzymes, and intrinsic properties of adrenal cells or enzymes. However, these mechanisms may, in turn, be modified by factors exogenous to the adrenal gland. Possible evidence for a regulatory role for such known substances as growth hormone, IGF-1, gonadotropins, estrogens, angiotensins, prostaglandins, insulin, EGF, and POMC-related peptides is discussed. Furthermore, pituitary or extrapituitary substances, in addition to known hormones, may have a role in the control of adrenal androgen secretion. Several lines of evidence include the inability of ACTH to maintain a normal adrenal androgen/cortisol ratio in adrenally suppressed patients, or experimentally, in hypophysectomized chimpanzees. Evidence for such factors derived from fetal and adult human pituitary gland is discussed.     

Role of hCG in regulation of the fetal zone of the human fetal adrenal gland.

It has been suggested that hCG is a trophic hormone for the fetal zone of the human fetal adrenal gland. To test this hypothesis, the isolated fetal zones of adrenals from eight fetuses (12-17-week gestation age) were superfused in the presence or absence of hCG. Dehydroepiandrosterone sulfate (DHAS) was measured in the superfusion effluent. A significant increase in DHAS production was observed in the presence of hCG. DHAS secretion decreased during the first 60 min in the control and experimental superfusions from 83 +/- 10.0 (mean +/- SE) to 71 +/- 8.0, and from 90 +/- 9.0 to 70 +/- 6.0 ng/100 mg/ml, respectively. In the presence of hCG (250 ng/ml), DHAS secretion increased significantly (P less than 0.01) over the controls to 116 +/- 12.0 at 120 min, and remained above the controls thereafter. These results support the hypothesis that hCG is one of the regulators of DHAS production by the human fetal adrenal gland early in gestation. As we found that ACTh stimulated DHAS secretion in a previous study and as there is indirect evidence for a role of ACTH in DHAS regulation late in pregnancy, these observations suggest dual regulation by hCG and ACTH early in pregnancy, and a possible transition to ACTH regulation of the fetal zone of the human fetal adrenal after midgestation.     

Steroid production by definitive and fetal zones of the human fetal adrenal gland.

This study was performed to assess the relative contributions of the fetal and definitive zones of the human fetal adrenal gland to "corticoid" (cortisol and perhaps other corticosteroids) and dehydroepiandrosterone sulfate (DHAS) production, and the possible regulatory role of ACTH and the fetal pituitary in the secretion of of these steroids. Corticoid and radioimmunoassayable DHAS or total aromatizable androgen secretion by the isolated definitive and fetal zones of the human fetal adrenal gland between 10-20 weeks gestation has been studied in a superfusion system. Different functional capacities of the two zones were seen; corticoids were found to be secreted primarily by the definitive zone, while DHAS was found to be the main secretory product of the fetal zone. Addition of ACTH (250 ng/ml) or fetal pituitary homogenate produced a 2- to 5-fold stimulation of corticoid production by the definitive zone at all gestational ages studied. DHAS secretion by the fetal zone was also stimulated by ACTH. These results indicate that the definitive and fetal zones of the human fetal adrenal gland at midgestation have the capacity to respond to ACTH with increased corticoid or DHAS secretion, respectively.     

The effects of ACTH on steroid metabolomic profiles in human adrenal cells

The adrenal glands are the primary source of mineralocorticoids, glucocorticoids, and the so-called adrenal androgens. Under physiological conditions, cortisol and adrenal androgen synthesis are controlled primarily by ACTH. Although it is well established that ACTH can stimulate steroidogenesis in the human adrenal gland, the effect of ACTH on overall production of different classes of steroid hormones has not been defined. In this study, we examined the effect of ACTH on the production of 23 steroid hormones in adult adrenal primary cultures and 20 steroids in the adrenal cell line, H295R. Liquid chromatography/tandem mass spectrometry analysis revealed that, in primary adrenal cell cultures, cortisol and corticosterone were the two most abundant steroid hormones produced with or without ACTH treatment (48 h). Cortisol production responded the most to ACTH treatment, with a 64-fold increase. Interestingly, the production of two androgens, androstenedione and 11β-hydroxyandrostenedione (11OHA), that were also produced in large amounts under basal conditions significantly increased after ACTH incubation. In H295R cells, 11-deoxycortisol and androstenedione were the major products under basal conditions. Treatment with forskolin increased the percentage of 11β-hydroxylated products, including cortisol and 11OHA. This study illustrates that adrenal cells respond to ACTH through the secretion of a variety of steroid hormones, thus supporting the role of adrenal cells as a source of both corticosteroids and androgens.     

Steroid hormone receptors in the adrenal glands of fetal and adult rhesus monkeys.

Sex steroid hormone receptors have been identified in the adrenal glands of rodents and may have a role in adrenal function. The highly estrogenic environment during pregnancy has been proposed to influence steroidogenesis by the fetal zone of the primate fetal adrenal gland. In order to determine whether these effects involve receptor-mediated mechanisms, we have examined the concentration and distribution of estrogen receptor (ER), androgen receptor, and progesterone receptor (PR) in the adrenal glands of fetal, immature, and adult rhesus monkeys. Monoclonal antibodies were used for immunocytochemistry (ICC), and in a gradient shift assay, for determination of receptor distribution and concentrations, respectively. There was no difference between the concentrations of ER in the adrenal glands from male and female adult animals (12.4 +/- 2.2, n = 3 and 13.2 +/- 2.0 fmol/mg DNA, n = 7; respectively); however, the concentration of ER in the fetal adrenal glands was markedly lower than in the adults (combined adult 12.7 +/- 1.6, n = 10, and fetal 0.9 +/- 0.4 fmol/mg DNA, n = 7; P less than 0.01). The concentration of ER in the adrenal glands of immature animals was also lower compared to adult animals (6.1 +/- 1.6, n = 6, P less than 0.05). In the adult, ICC revealed that staining for ER was restricted to the cell nucleus and was most dense in the zone fasciculata, with lesser staining in the zona glomerulosa and zona reticularis, and with no detectable staining in the medulla. ER staining was virtually absent in the fetal zone which comprises the major portion of the fetal gland; however, some staining was observed in the narrow definitive zone. The distribution of androgen receptor was similar to that of ER, whereas there was no detectable staining for PR in the adrenals of either adult or fetal animals. We conclude: 1) that the lower concentration of ER in fetal adrenal glands is due to the absence of ER in the fetal zone; 2) the lack of ER and PR in the fetal zone suggests that estrogens and progestins do not influence the growth or function of the fetal zone by receptor-mediated mechanisms; 3) estrogens and androgens may influence the function of the adult adrenal cortex.     

Stimulation of adrenocorticotropin secretion by insulin-induced hypoglycemia in the developing rat involves arginine vasopressin but not corticotropin-releasing factor.

In the neonatal rat, the response of the hypothalamo-pituitary-adrenal axis to stressful stimuli is markedly decreased during the first 2 weeks of life. This peculiar period was named "stress hyporesponsive period." In this report, we studied the effect of insulin-induced hypoglycemia, known as a strong stimulator of the corticotroph function in the adult rat. Rats (8- or 20-day-old) were injected ip with 3 IU/kg synthetic insulin and were killed at various times. In 20-day-old rats, hypoglycemia induced a rapid drop in blood glucose concentrations accompanied by a stimulation of ACTH and corticosterone secretion which reached maximal values within 30 min. On the opposite, in 8-day-old rats, despite a rapid decrease in blood glucose levels, insulin injection induced a gradual rise of plasma ACTH and corticosterone concentrations which peaked at 90 min. This delayed response of the hypothalamo-pituitary-adrenal axis to hypoglycemia in the youngest rats does not seem to be due to a difference of sensitivity to insulin-induced hypoglycemia since injection of increasing doses of insulin (0.3, 0.75, or 3 IU/kg body wt) induced a dose-related decrease of blood glucose concentrations and a rise in plasma ACTH and corticosterone levels, comparable in the two age group studied. Basal or hypoglycemia-stimulated absolute corticosterone values were much lower in 8-day-old rats than in 20-day-old animals, suggesting an immaturity of the adrenal glands in the youngest animals. Daily ACTH injection, starting 3 days before the experiment, had a trophic effect on the adrenal glands leading to a more important increase of corticosterone levels after hypoglycemia in 8-day-old rats. Our results confirm that there is an immaturity of the adrenal glands in young rats, probably due to the low plasma ACTH levels during the neonatal period. To determine the respective role of the two major hypothalamic ACTH secretagogues, we studied the effect of passive immunization against CRF or arginine vasopressin (AVP) on plasma ACTH response after hypoglycemia. Passive immunization against AVP decreased significantly hypoglycemia-stimulated ACTH secretion in both 8- and 20-day-old rats, while no change of plasma ACTH response to insulin injection was observed after passive immunization against CRF. This results suggest that CRF does not seem to be involved in the regulation of ACTH secretion after hypoglycemia in the young rat while AVP seems to be the main hypothalamic stimulatory factor for anterior pituitary corticotrophs response to hypoglycemia during the postnatal period.(ABSTRACT TRUNCATED AT 400 WORDS)     

Effect of brain monoamines on the secretion of adrenocorticotrophic hormone

The role of brain monoamines (5-HT, NA and DA) in the secretion of adrenocorticotrophic hormone (ACTH) was studied in view of contradictory reports. Plasma corticosterone levels and the rate of synthesis of corticosterone in vitro by the adrenal gland were estimated in albino rats and have been taken as the index of ACTH activity. These estimations were done in unstressed and stressed, and in untreated and treated rats. Drugs were administered intracerebroventricularly to the rats to cause selective degeneration of tryptaminergic, noradrenergic or dopaminergic neurons. The results show that plasma corticosterone levels and the rate of synthesis of corticosterone were significantly decreased after selective degeneration of tryptaminergic neurons in unstressed rats. After selective degeneration of either tryptaminergic or noradrenergic neurons, the acute increase in the plasma corticosterone levels and rate of synthesis of corticosterone in vitro by adrenal glands in stressed rats were significantly inhibited. These results have been interpreted to suggest that the central tonic control on adrenal glands may be 5-HT mediated and that during stress ACTH secretion may be both 5-HT and NA mediated. DA does not seem to have significant role in the regulation of ACTH secretion.     

Steroidogenesis in the human fetal adrenal: a role for cholesterol synthesized de novo

Primary monolayer cultures of human fetal adrenal cells maintained in either lipoprotein-depleted or lipoprotein-supplemented media responded chronically to ACTH treatment with similarly increased steroid secretion. The principal steroid secreted into each medium was dehydroepiandrosterone sulfate. The presence of human low density lipoprotein (hLDL) in the medium enhanced the secretion of nonsulfoconjugated steroids, especially dehydroepiandrosterone. The secretion rate of 11 beta-hydroxyandrostenedione was similar to that of cortisol. In the absence of hLDL, ACTH increased cholesterologenesis to maintain the high rates of steroid secretion. After ACTH treatment, increased accumulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase, a rate-determining enzyme of cholesterol biosynthesis, was found. Immunoblot analysis demonstrated that this enzyme was a 97K protein in human fetal adrenal cells. Interestingly, the content of this enzyme in cells treated with ACTH in lipoprotein-depleted medium was similar to that in adrenal fetal zone tissue. This finding suggests that cholesterologenesis de novo in addition to plasma LDL is important as a source of steroid precursor in vivo in the human fetal adrenal gland.     

Regulation of baboon fetal adrenal androgen formation by pituitary peptides at mid- and late gestation

A short term incubation of baboon fetal adrenal cells obtained at midgestation and near term was used to determine whether a change in the regulation of androgen formation occurs with advancing gestation. Adrenal glands were removed from baboon (Papio anubis) fetuses on day 100 (mid; n = 7) or day 170 (late; n = 5) of gestation, and cells were dispersed with 0.2% collagenase. Cells (10(5] were incubated at 37 C for 3 h in medium 199 in the presence or absence of 10 nM ACTH, 10 nM ovine PRL, 10 nM ovine GH, 50 nM hCG, or 50 nM human chorionic somatomammotropin. Dehydroepiandrosterone (DHA), DHA sulfate (DHAS), cortisol (F), and androstenedione concentrations were determined in the medium by RIA. At midgestation, ACTH, PRL, and GH elevated (P less than 0.05) DHA (168%, 169%, and 178%, respectively) and DHAS (142%, 210%, and 197%, respectively) formation. Near term, ACTH and PRL retained their ability to stimulate (P less than 0.05) DHA (307% and 220%, respectively), but not DHAS, synthesis. The fetal adrenal at late gestation, however, lost its ability to respond to treatment with GH. hCG and human chorionic somatomammotropin did not stimulate steroidogenesis at either time of gestation. F formation at midgestation was less (P less than 0.05) than that at term and not responsive to ACTH. ACTH stimulated (P less than 0.05) F secretion by 68% in fetal adrenal cells obtained near term. The secretion of androstenedione was not affected by any peptide treatment at either stage of gestation. These data indicate that the responsivity of the baboon fetal adrenal to various pituitary peptides is different at two developmentally distinct stages of gestation. We conclude, therefore, that the regulation of fetal adrenal steroidogenesis changes with advancing gestation.     

Adenylate cyclase activity in neocortex and fetal zone membrane fractions of the human fetal adrenal gland

Whether peptide hormones other than ACTH may be responsible for the difference in size or rate and pattern of steroidogenesis of the fetal zone (FZ) compared to those of the neocortex (NC) of the human fetal adrenal gland is controversial. In the present investigation, the activity of adenylate cyclase in membrane fractions of separated zones of the human fetal adrenal gland was determined. Basal adenylate cyclase activity was 2- to 3-fold greater in NC than in FZ membrane fractions. The addition of ACTH-(1-24) stimulated adenylate cyclase activity in both zones, but the activity was more sensitive to ACTH (10(-10) M) in NC fractions than in FZ fractions (10(-7) M). In addition to ACTH-(1-24), the effect of other ACTH-related peptides on the activity of adenylate cyclase in the separated zones of the adrenal gland was investigated. 16K fragments 2-36, gamma 3MSH, alpha MSH, beta-endorphin, leu-enkephalin, and met-enkephalin, as well as hCG, FSH, prostaglandin E2, prostaglandin F2 alpha, epinephrine, and norepinephrine did not stimulate adenylate cyclase activity in either zone. It is concluded that basal and ACTH-(1-24)-stimulated adenylate cyclase activities are greater in NC than in FZ membrane fractions. In addition, the results of the present investigation do not support the concept that other ACTH-related peptides or peptide or protein hormones increase steroidogenesis by stimulating adenylate cyclase activity in the human fetal adrenal gland.     

Changes in protein kinase activities in lamb adrenals at late gestation and early postnatal stages

Cytosols prepared from adrenal glands of ovine fetuses (110-144 days of gestation) and of newborn lambs (1-6 days post-partum) were analysed for their protein kinase activities. Two major peaks of casein kinase activities and two major peaks of histone kinase activities were observed in all cytosols of both fetal and neonatal adrenal glands. They were characterized as cAMP-independent casein kinases of type A and type G, and as cAMP-dependent histone kinase isoenzymes of type I and type II. The specific activity of each enzyme increased 2-fold between 118 days of gestation and 6 days post-partum. Casein kinase of the G type was 4-fold higher than casein kinase of the A type; in contrast, the mean ratio of type II to type I histone kinase activity varied between 5- and 12-fold. Infusion of ACTH1-24 (100 micrograms/day) for 5 days to 118- to 128-day-old ovine fetuses in utero increased their plasma corticosteroid levels and the responsiveness of their adrenal cells to stimulation by ACTH1-24 in vitro. In addition, such treatment doubled the specific activity of casein kinases A and G, but had no significant effect on cAMP-dependent histone kinase activities. In relation to current concepts of the role of protein kinases in adult adrenal cells, the present results suggest that casein kinase activities are involved in cell multiplication and differentiation in the fetal adrenal gland. In addition, they show that neither cytosolic histone kinase of type I nor that of type II is likely rate-limiting in the steroidogenic response to ACTH of ovine fetal adrenal cells.     

Inhibition of replication of normal adrenocortical cells in culture by adrenocorticotropin.

Adrenocorticotropic hormone (ACTH) inhibited [3H]thymidine incorporation in normal adrenocortical cells of adult rats in culture, with a concomitant increase in corticosterone production and a characteristic retraction of cells. Both dibutyryl cyclic AMP and an analog of ACTH, which produces virtually no cyclic AMP, inhibited DNA synthesis and stimulated steroid production. ACTH inhibited the proliferation of adrenocortical cells obtained from suckling rats as well as the cells obtained from the capsular tissue of adult rat adrenal glands, whereas insulin caused a stimulation of DNA synthesis. These results suggest that the major role of ACTH is to induce the transformation of the undifferentiated cells of the adrenal gland into functional fasciculata cells and that the proliferation of adrenocortical cells may be under control of factors other than ACTH.     

The role of serum lipoproteins in steroidogenesis by the human fetal adrenal cortex.

In the present investigation it was found that human fetal adrenal tissue maintained in organ culture secreted appreciable quantities of dehydroisoandrosterone sulfate (DS) and cortisol. Pregnenolone was also secreted in significant amounts, principally as the sulfate ester. The highest rate of secretion of these steroids by fetal adrenal tissue occurred when both ACTH and whole human serum were present in the culture medium. In the absence of ACTH, steroid secretion was low. When whole serum was replaced by lipoprotein-poor serum, the steroidogenic response to ACTH was markedly attenuated but not abolished. On the basis of these findings, it is concluded (1) that the human fetal adrenal can synthesize steroid hormones de novo from cholesterol, (2) that ACTH is an important stimulant of steroidogenesis by the human fetal adrenal, and (3) that plasma lipoproteins are a major source of the cholesterol utilized by the human fetal adrenal for steroidogenesis. Hence, it is likely that factors which regulate the production of fetal plasma lipoproteins are important determinants of fetal adrenal steroidogenic activity.     

Influence of the thymus on steroidogenesis by rat ovarian cells in vitro

Thymic hormones and factors have been shown to modulate the function of other endocrine glands including the gonads. Absence of the thymus during development results in ovarian dysgenesis characterized by a decrease in the number of follicles and corpora lutea, bringing about severe changes in reproductive function. To examine whether thymic secretions might affect ovarian activity, whole dispersed ovarian cells obtained from immature rats pretreated with pregnant mare serum gonadotrophin were exposed to a thymus fraction of approximately 28 kDa and also to the media from incubated thymuses (TIM) and the conditioned media from cultured thymic reticuloepithelial cells (TCM). The thymic fraction caused a dose-dependent decrease in human chorionic gonadotrophin (hCG)-stimulated production of progesterone, oestradiol and testosterone, but had no effect on their synthesis in the absence of hCG. Similarly, hCG-induced production of these steroids was decreased by TIM and TCM. Progesterone secretion was the most markedly affected. These results suggest: (1) that the thymus contains a factor with a molecular weight of approximately 28 kDa which interacts with hCG in ovarian cells, (2) that the thymus can release active substances which modify steroid secretion by the ovary in vitro and (3) that the reticuloepithelial cells of the thymus are involved in the secretion of factors which modulate the stimulation by hCG of steroidogenesis in ovarian cells.     

The vasopressin receptor system in the neonatal pituitary gland: evidence for reduced binding capacity and signal transmission

The present study was aimed at evaluating the capacity of anterior pituitary cells from neonatal rats to bind arginine vasopressin (AVP) and show AVP-receptor-mediated signal transmission. We found that in cultures of pituitary cells of 10-day-old pups, in contrast to cultures of cells of adults, AVP was unable to trigger sustained adrenocorticotropin (ACTH) secretion and, in addition, was also less potent in synergizing with the effect of corticotropin-releasing factor (CRF) on both ACTH output and cyclic AMP formation. Binding studies revealed the existence of a much lower number of AVP receptor sites in membranes of neonatal pituitary gland than in those of adult tissue (32.3 +/- 9.0 and 137.6 +/- 6.2 fmol/mg protein, respectively), although the binding of agonists and the apparent molecular weight (Mr about 120,000) of the receptors were similar. Activation by phorbol ester PMA of protein kinase C, a messenger involved in AVP action, resulted in a dose-related enhancement of ACTH secretion that was 2-3 times smaller for immature corticotrophs than for mature ones. Importantly, PMA treatment allowed AVP to significantly stimulate ACTH secretion from neonatal cells, while it failed to similarly affect AVP-evoked hormone output from adult tissue. Our results indicate that pituitary corticotrophs of rat pups fail to properly transduce AVP-receptor-mediated signalling and, thereby, suggest an explanation for the postnatal 'stress nonresponsive period'.     

Influence of adrenocorticotropin and adrenalectomy on gonadotropin secretion in immature rats

We examined the effects and mechanisms of action of ACTH and ACTH fragments on gonadotropin secretion in immature rats. ACTH administered by daily injection or continuous infusion (osmotic minipumps) attenuated the postcastration rise in serum LH. Pituitary LH concentration was either unchanged or increased in ACTH-treated rats and pituitary sensitivity to gonadotropin-releasing hormone (GnRH) was reduced by ACTH treatment. A fragment of ACTH (ACTH 4-10), which is less steroidogenic, did not alter levels of serum LH, and ACTH did not reduce LH secretion in adrenalectomized castrates. Serum and pituitary concentrations of prolactin were normal in ACTH-treated animals. These studies demonstrate that the suppression of gonadotropin secretion by ACTH is mediated by the adrenal gland. This mechanism causes a decreased pituitary sensitivity to GnRH, but LH synthesis does not appear to be affected. Prolactin does not play a role in this mechanism.     

Basic fibroblast growth factor expression is regulated by corticotropin in the human fetal adrenal: a model for adrenal growth regulation.

Human fetal adrenal growth after midgestation is very rapid and appears to be dependent upon pituitary adrenocorticotropin (ACTH) in vivo. We hypothesized that the regulation of fetal adrenal growth by ACTH is mediated by ACTH-stimulated local growth factor production. As we have found basic fibroblast growth factor (bFGF) to be a potent mitogen for human fetal adrenal cells in culture, we conducted studies to determine whether bFGF is synthesized by the human fetal adrenal gland and whether bFGF gene expression in primary cultures of human fetal adrenal cells is regulated by ACTH. Bioassayable bFGF-like activity was detected in extracts of whole human fetal adrenal glands and primary cultures of human fetal adrenal cells. Northern blot analysis of total RNA from whole human fetal adrenal glands revealed a characteristic 7-kilobase bFGF mRNA, indicating that the fetal adrenal bFGF bioactivity was most likely due to local synthesis. Slot blot and ribonuclease protection analysis showed that bFGF mRNA was present in very low amounts in total RNA from primary cultures of unstimulated human fetal adrenal cells but was increased 2- to 3-fold in cells exposed to 10 nM ACTH-(1-24) or 1 mM 8-bromoadenosine 3',5'-cyclic monophosphate for 24 hr. bFGF mRNA was localized to adrenocortical cells and not fibroblasts by in situ hybridization. bFGF mRNA was barely detectable in unstimulated cells, whereas it was markedly increased in cells exposed to either ACTH or 8-bromoadenosine 3',5'-cyclic monophosphate. These data support our hypothesis that the regulation of human fetal adrenal growth by ACTH at midgestation may be mediated by the stimulation of local growth factor production, and we suggest that bFGF may play a major role in this process.     

Responses of rat fetal adrenals to synthetic adrenocorticotrophic hormone and alpha-melanocyte-stimulating hormone in-vivo and in-vitro studies

The response of the adrenals from rat fetuses at 16, 18 and 20 days of gestation to 1-24 ACTH and alpha-melanocyte-stimulating hormone (alpha-MSH) was studied in vitro. The response to 1-24 ACTH increased as gestation progressed. By the end of fetal life, corticosterone release induced by ACTH from whole adrenals was greater than that observed with adrenal tissue from non-pregnant adult female rats. High doses of alpha-MSH also stimulated adrenal activity but the response to ACTH was always higher than that to alpha-MSH. The effect of 1-24 ACTH and alpha-MSH on fetal adrenal growth was also compared in vivo. The adrenal atrophy induced by fetal hypophysectomy on day 17 of gestation could be prevented by i.m. administration of 10 microgram 1-24 ACTH or alpha-MSH. However, the adrenal growth was greater in ACTH-treated fetuses than in alpha-MSH treated ones. Later in gestation, between days 19 and 20, 1-24 ACTH but not alpha-MSH was able to prevent atrophy induced by fetal hypophysectomy. These findings are discussed in relation to the literature on levels of ACTH and alpha-MSH in the plasma and pituitary glands of the rat throughout the last third of gestation. High levels of ACTH in the fetal circulation contrast sharply with very weak or undetectable concentrations of alpha-MSH. Since the present data suggest that both trophic and steroidogenic activities of ACTH were greater than those of alpha-MSH, it may be concluded that ACTH but not alpha-MSH plays a major physiological role during gestation in the regulation of both fetal adrenal growth and function in the rat.