CRH AND AVP-INDUCED CHANGES IN SYNTHESIS AND RELEASE OF ACTH FROM THEOVINE FETAL PITUITARY IN-VITRO - NEGATIVE INFLUENCES OF CORTISOL

During late gestation in sheep, fetal plasma adrenocorticotrophin (ACT H) and cortisol levels increase, and these are associated with increas ed pro-opiomelanocortin (POMC) mRNA levels in the anterior pituitary. Corticotrophin-releasing hormone (CRH) and vasopressin (AVP) are the p rimary hypophysiotrophic factors regulating ACTH secretion From the fe tal sheep pituitary corticotroph, but previous reports with term fetal tissue have failed to show effects on levels of POMC mRNA. The object ives of the present study were to establish the effects of CRH and AVP on both synthesis and secretion of ACTH before term, and to determine how cortisol affects these responses. Fetal pituitaries were removed at d 138 of gestation (term similar to d 147), the anterior pituitary was separated, and the cells dispersed and placed in monolayer tissue culture. After 4 d, cells were treated for 18 h with several different concentrations (10(-6)-10(-9) M) and combinations of CRH, AVP, and co rtisol. Following incubation, the medium was removed for ACTH analysis , and the cells fixed for POMC mRNA measurement and immunoreactive (ir )-ACTH localization. Separately, CRH and AVP significantly (p < 0.05) stimulated ACTH secretion in a dose-dependent manner. Simul-taneous tr eatment of maximally stimulating levels of CRH and AVP augmented (p < 0.05) the output of ACT Fl. Cortisol d id not affect basal (nonstimula ted) ACTH output, but attenuated the neuropeptide-induced increases in ACTH secretion. This effect of cortisol was more pronounced in cells treated with CRH than in cells treated with AVP. POMC mRNA levels were increased by both CRH and AVP treatments in a dose-dependent manner, though there was no further increase in POMC mRNA when CRH and AVP wer e added together. Cortisol attenuated (p < 0.05) the neuropeptide-indu ced increases in POMC mRNA, though AVP-stimulated POMC mRNA levels wer e significantly higher than in cells treated with cortisol alone. Cort isol failed to alter non-stimulated POMC mRNA levels. We conclude that in late gestation: 1) Fetal pituitary corticotrophs respond to CRH an d AVP by increasing POMC mRNA levels and ACTH secretion 2) AVP is more potent than CRH at the level of ACTH secretion, but not POMC transcri ption 3) Cortisol attenuates the synthetic and secretory responses to CRH and AVP, but has little effect in the nonstimulated state.