Involvement of the regulatory protein (Ns) in the maturation of ACTH-sensitive adenylate cyclase of ovine fetal adrenal during late gestation

Infusion of ACTH stimulates expression of adrenal ACTH receptor and steroidogenic acute regulatory protein mRNA in fetal sheep

The late-gestation plasma cortisol surge in the sheep fetus is critical for stimulating organ development and parturition. Increased adrenal responsiveness is one of the key reasons for the surge; however, the underlying mechanisms are not fully understood. Our recent studies suggest that ACTH-mediated increased expression of ACTH receptor (ACTH-R) and steroid acute regulatory protein (StAR) may play a role in enhancing responsiveness. Hence, we examined effects of ACTH infusion in fetal sheep on mRNA expression of these two mediators of adrenal responsiveness and assessed the functional consequences of this treatment in vitro. Fetuses of approximately 118 and 138 days of gestational age (dGA) were infused with ACTH-(1-24) for 24 h. Controls received saline infusion. Arterial blood was sampled throughout the infusion. Adrenals were isolated and analyzed for ACTH-R and StAR mRNA, or cells were cultured for 48 h. Cells were stimulated with ACTH, and medium was collected for cortisol measurement. Fetal plasma ACTH and cortisol concentrations increased over the infusion period in both groups. ACTH-R mRNA levels were significantly higher in ACTH-infused fetuses in both the 118 and 138 dGA groups. StAR mRNA increased significantly in both the 118 and 138 dGA groups. Adrenal cells from ACTH-infused fetuses were significantly more responsive to ACTH stimulation in terms of cortisol secretion than those from saline-infused controls. These findings demonstrate that increases in circulating ACTH levels promote increased expression of ACTH-R and StAR mRNA and are coupled to heightened adrenal responsiveness.

Endocrine and paracrine regulation of birth at term and preterm

We have examined factors concerned with the maintenance of uterine quiescence during pregnancy and the onset of uterine activity at term in an animal model, the sheep, and in primate species. We suggest that in both species the fetus exerts a critical role in the processes leading to birth, and that activation of the fetal hypothalamic-pituitary-adrenal axis is a central mechanism by which the fetal influence on gestation length is exerted. Increased cortisol output from the fetal adrenal gland is a common characteristic across animal species. In primates, there is, in addition, increased output of estrogen precursor from the adrenal in late gestation. The end result, however, in primates and in sheep is similar: an increase in estrogen production from the placenta and intrauterine tissues. We have revised the pathway by which endocrine events associated with parturition in the sheep come about and suggest that fetal cortisol directly affects placental PGHS expression. In human pregnancy we suggest that cortisol increases PGHS expression, activity, and PG output in human fetal membranes in a similar manner. Simultaneously, cortisol contributes to decreases in PG metabolism and to a feed-forward loop involving elevation of CRH production from intrauterine tissues. In human pregnancy, there is no systemic withdrawal of progesterone in late gestation. We have argued that high circulating progesterone concentrations are required to effect regionalization of uterine activity, with predominantly relaxation in the lower uterine segment, allowing contractions in the fundal region to precipitate delivery. This new information, arising from basic and clinical studies, should further the development of new methods of diagnosing the patient at risk of preterm labor, and the use of scientifically based strategies specifically for the management of this condition, which will improve the health of the newborn.

Steroidogenesis in isolated adrenocortical cells during development in rats

After postnatal day 1 (d1), the hypothalamo-pituitary-adrenal axis of neonatal rats becomes less responsive to certain stimuli for up to 2 weeks. The present study was designed to quantify the development of adrenocortical cell responsiveness to its normal secretagogue, adrenocorticotropic hormone (ACTH), and to better localize intracellular sites of adrenal cell hyporesponsivity. Maximum steroidogenic responses of collagenase-dispersed adrenocortical cells (using two isolation methods) to ACTH varied significantly in the order adult > d1 > d10. The response pattern to dibutyryl cAMP ((Bu)2cAMP) was identical to that observed for ACTH (adult > d1 > d10), suggesting that neonatal adrenal responsiveness is limited by a site distal to cAMP formation. Sensitivity (EC50) of adult cells to ACTH was approximately 3-fold greater than in neonatal cells, but there was no age-dependent shift in sensitivity to (Bu)2cAMP. 20 alpha-Hydroxycholesterol (20 alpha-OHCHOL), a membrane permeable analog of cholesterol, also failed to normalize the d10 adrenal response to ACTH. This result indicates that one site of refractoriness is apparently distal to cholesterol transport, and strongly suggests possible differential cytochrome P450 enzyme expression or activity in neonatal rat adrenal cells. Finally, although stimulated secretion was lower in neonatal cells, basal corticosterone secretion was significantly greater in neonatal adrenals, suggesting that constitutive activity of neonatal adrenal cells is high compared to that of adult cells.

Effects of repeated maternal separations on the adrenocortical response to stress of preweanling rats

Previous data indicate that the infant rat shows a marked increase in adrenocortical responsiveness to stress immediately following prolonged maternal separation. In Experiment 1 we studied the immediate effects of repeated maternal deprivation. Our results indicate that the increase in basal as well as stress-induced corticosterone levels is a direct function of the length of the deprivation period immediately preceding testing, and is not cumulative. In Experiment 2 we examined the long-term consequences of maternal deprivation on adrenal responsivity. Four days following a single 24-h period of maternal deprivation, pups remained hyperresponsive to stress, although their basal levels of corticosterone had returned to control values. Shorter periods of deprivation (which did result in increased responsivity immediately following deprivation) did not have persistent effects. Our data suggest: 1) short periods of deprivation do not have a cumulative effect, and 2) there is a critical length of deprivation beyond which persistent changes in adrenocortical responsivity ensue.

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.

Adrenal adenylate cyclase and steroidogenic activities of 63 day old ovine fetuses: in vitro effects of ACTH1-24 and forskolin

This study examines the activity of the adenylate cyclase system and that of some enzymes of the steroidogenic pathway of adrenal cells from 62-63 day old ovine fetuses. Synthetic corticotropin (ACTH1-24), cholera toxin and forskolin stimulated both cAMP and corticoid productions by freshly isolated adrenal cells. The cAMP response to ACTH1-24 was lower than that to forskolin. However, forskolin-induced steroidogenesis was significantly lower than the ACTH1-24-induced steroid output. Freshly isolated cells metabolized quickly [14C]-labeled pregnenolone mainly through the 17-deoxy pathway. The amounts of cortisol and of corticosterone formed, in the presence of exogenous pregnenolone, were roughly 15-fold higher than under maximal stimulation by ACTH1-24. When the cells were cultured for 6 days in the absence or presence of ACTH1-24 (10(-8) M) or forskolin (10(-5) M), a small development of the cAMP response to these factors was observed in the course of the experiment. However, the mechanism of this development appeared different, according to the conditions of culture. The amounts of corticosterone secreted on day 6 by ACTH1-24- or forskolin-treated cells were 2- to 4-fold higher than on day 1, whereas cortisol outputs were much lower on day 6 than on day 1. The response to ACTH1-24 of cells maintained in ACTH-free media decreased dramatically during the culture in terms of both cortisol and of corticosterone. On day 6 of the experiment, the metabolism of [14C]pregnenolone was lower than on day 1 under all 3 conditions of culture. Only the 3 beta-hydroxysteroid dehydrogenase/isomerase activity could be maintained by continuous treatment with forskolin. However, both ACTH1-24 and forskolin enhanced the production of pregnenolone from an endogenous substrate. In conclusion, these results present evidence that: 1) the adenylate cyclase system is not a bottleneck in the steroidogenic response to ACTH1-24 of freshly isolated adrenal cells from 62-63 day old ovine fetuses; 2) the main rate-limiting step for steroidogenesis by these cells is the availability of pregnenolone; 3) neither ACTH1-24 nor forskolin is able to maintain the activity of most enzymes involved in the metabolization of pregnenolone by cultured cells while increasing pregnenolone availability; 4) some inhibiting factors are involved in the loss of adrenal cells responsiveness to ACTH between days 50 and 100 of gestation, and they probably act mainly on the adenylate cyclase system.

Clonal variation in response to adrenocorticotropin in cultured bovine adrenocortical cells: relationship to senescence

During cellular senescence, non-clonal cultures of bovine adrenocortical cells show a continuous decline in the rate of production of cyclic AMP (cAMP) stimulated by adrenocorticotropin (ACTH), without changes in the rate of forskolin- or prostaglandin E1-stimulated cAMP production. We investigated the possible mechanisms for loss of response to ACTH by examining the properties of clones of bovine adrenocortical cells. ACTH-stimulated cAMP production rates were measured in clones immediately after isolation, during long-term growth following isolation, and after subcloning. ACTH-stimulated rates were compared with cAMP production in response to forskolin, which acts directly on the catalytic subunit of adenylate cyclase. The results show that cloning is not necessarily associated with a loss of response to ACTH, but that clones with high ACTH response can give rise to subclones with low response. Clones of adrenocortical cells, at the same approximate population doubling level (PDL), showed ACTH response levels that ranged from 12 to 135 pmol cAMP/10(6) cells/min, whereas mass cultures at this PDL showed approximately 50 pmol/10(6) cells/min. Forskolin-stimulated cAMP production rates in clones varied only over the range of 59-119 pmol/10(6) cells/min and showed no correlation with the ACTH-stimulated rates. All clones were adrenocortical cells, as shown by mitogenic response to angiotensin II and cAMP-inducible 17 alpha-hydroxylase activity. The replicative potential of clones varied widely, and there was no apparent correlation between ACTH response levels and growth potential. The level of ACTH response in each clone was stable during proliferation through at least 25 PD beyond the stage at which the clone was isolated. When clones were subcloned, a clone with a high ACTH response level produced sister subclones that had ACTH response levels ranging from 3% of that of the parent clone to a level slightly greater than that of the parent clone. The growth potential of sister subclones varied widely, as for the parent clones, and there was no obvious correlation between growth potential and ACTH response. Two subclones were cloned; in sub-subclones, levels of ACTH response were again different from each other and also from the parent subclone; in one case, the level of ACTH response was approximately eight-fold higher than that of the parent subclone. These experiments show that clonal variation in the extent of expression of a differentiated property may occur in a normal differentiated cell in culture. The loss of ACTH response and the loss of proliferative potential appear to be independent stochastic events.