We have compared the biological (B) and immunological (I) ACTH-like activities (ALA) released by short term cultured pituitary cells from fetal (63-144 days old) and young lambs (7-150 days old) both under basal conditions, and when stimulated by ovine(o)CRF-(1-41) or arginine vasopressin (AVP), separately or in combination. The bioassay was based on the production of corticosteroids by cultured adrenal cells from adult sheep. The standard used in both the RIA and the bioassay was synthetic ACTH-(1-24). Under every condition, curves for dilution of the incubation media were parallel to the standard curves in both the RIA and the bioassay. When expressed per 5 X 10(5) pituitary cells, B-ALA output increased with gestational age and remained high after birth. During fetal life, B-ALA released was always lower than I-ALA. However, the B/I ratios increased with gestational age from 0.29 +/- (SE) 0.04 at 63 days to 0.70 +/- 0.04 at 144 days. Conversely, for postpartum animals, the B/I ratios were not different from 1, except in the case of stimulation by AVP alone, when they were significantly higher than 1.A 4-day treatment of pituitary cells from 120- to 126-day-old fetuses by AVP or cortisol resulted in an 1.5-fold enhancement of the B/I ratio of the ALA secreted under AVP stimulation. These results indicate that during development of the sheep I-ACTH levels are probably not representative of the corticotropic activity released by the pituitary. Furthermore, they suggest that the prepartum increase in fetal cortisol does result, in part, from an enhanced tropic drive to the fetal adrenal.

Corticotropin-releasing factor-like immunoreactivity (CRF-LI), arginine vasopressin-like immunoreactivity (AVP-LI) and ACTH-releasing bioactivity of hypothalamic tissue from 63- to 143-day-old ovine fetuses and from 7- and 150-day-old lambs have been assessed. CRF-LI and AVP-LI contents and concentrations increased steadily between 63 and 138 days of gestation, decreased at day 143 then rose again in postpartum animals. In terms of concentration, the AVP-LI/CRF-LI ratio remained close to 5 between 63 and 123 days of intrauterine life, decreased steadily in late gestation down to 1.2 at day 143 increased again in lambs. The total hypothalamic ACTH-releasing bioactivity increased 26-fold between 63 and 138 days of gestation and remained constant until day 143. The highest values were observed in 150-day-old-lambs. These data support the view that AVP and oCRFι_4i might be important hypothalamic factors involved in the regulation of ACTH release by the pituitary gland of the sheep fetus. In addition, they suggest that AVP is more important in young fetuses and lambs than in prepartum animals.

The proportions of different molecular weight forms of adrenocorticotropic hormone (ACTH) present in pituitary extracts, cultured pituitary cell extracts and cell-derived incubation media from fetal and postnatal sheep have been determined. When pituitary extracts, cell extracts, and incubation media were subjected to gel filtration on Sephadex G50 fine, three forms of ACTH were always observed. ‘Big’ ACTH eluted in the void volume of the column; ‘intermediateACTH eluted between ‘big’ ACTH and human ACTH_1–39, and ‘little’ ACTH eluted in the same fractions as human ACTH_1–39. High amounts of ‘big’ and ‘intermediate’ ACTH were observed in extracts of cultured pituitary cells from both fetuses and newborn lambs, whereas very few ‘little’ ACTH was found. Conversely, in incubation media, the predominant form of ACTH was always ‘little’ ACTH. The relative proportion of these three forms of ACTH were rather similar in pituitary extracts and in cell-derived incubation media from newborn lambs. However, for fetuses the proportions of ‘big’ and ‘intermediate’ ACTH were higher, whereas that of ‘little’ ACTH was lower in pituitary extracts than in cell-derived incubation media. Neither corticotropin-releasing factor (oCRF_1–41) nor arginine vasopressin (AVP), separately or in combination, was able to modify the relative proportion of the three forms of ACTH released by pituitary cells from newborns. Conversely, for fetal cells, the proportion of ‘little’ ACTH released in the medium was higher in the presence of oCRF_1-41 and/or AVP than under control conditions. The proportion of ‘little’ ACTH released by pituitary cells in the absence of stimulating factor increased linearly between 63 days of gestation and 150 days postpartum. Reciprocally the proportions of ‘big’ and ‘intermediate’ ACTH decreased. When cells were incubated in the presence of oCRF_1-41 such a relationship was no longer observed. Fetal pituitary cells cultured for 4 days in the absence of secretagogues released a lower proportion of ‘little’ ACTH in response to AVP than cells cultured for only 2 days. However, when cells were cultured in the presence of 10^–9M AVP or 10^–7M cortisol, the proportion of ‘little’ ACTH released under acute stimulation by 10^–7M AVP was higher than for cells cultured without those factors (but stimulated by AVP), and similar to that of cells cultured for only 2 days under basal conditions. ‘Big’ ACTH had only a weak steroidogenic activity whereas both ‘intermediate’ and ‘little’ ACTH highly stimulated steroidogenesis of cultured adrenal cells. These results indicate that the proportions of the different forms of ACTH present in fetal pituitary extracts are not representative of those released by fetal pituitary cells. Moreover, they suggest that the lower proportion of ‘little’ ACTH in fetal than in postnatal cell-derived culture media results from a deficiency in the hypothalamic trophic drive to the fetal corticotrophs under these in vitro circums...

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Introduction.