Studies of the regulation of the hypothalamic-pituitary-adrenal axis in sheep with hypothalamic-pituitary disconnection. II. Evidence for in vivo ultradian hypersecretion of proopiomelanocortin peptides by the isolated anterior and intermediate pituitary.

Studies were performed to determine whether the isolated ovine anterior and intermediate pituitary might rhythmically secrete three POMC peptides, ACTH, ir-beta-endorphin (ir-beta-EP), and ir-alpha-melanocyte stimulating hormone (ir-alpha-MSH) in vivo. When blood was taken at 10-min intervals from four ewes with hypothalamo-pituitary-disconnection (HPD), a distinct POMC-peptide and cortisol ultradian rhythm was noted. A comparison of the four HPD ewes with five nonstressed hypothalamopituitary-intact (HPI) ewes revealed that the mean plasma levels of the three POMC-peptides and cortisol were increased, the mean ACTH and ir-alpha-MSH pulse amplitudes were increased, and the mean ir-beta-EP and ir-alpha-MSH interpulse intervals were decreased. When four HPI ewes were subjected to a mild stress, plasma POMC-peptide and cortisol levels increased significantly when compared with the five unstressed HPI animals. In addition, the ACTH and cortisol pulse amplitudes increased and the ir-beta-EP and ir-alpha-MSH interpulse intervals decreased. Although plasma ACTH levels in the stressed HPI and HPD ewes were comparable, mean plasma cortisol levels were 2-fold greater in the stressed HPI animals. To determine whether the ACTH hypersecretion in the HPD ewe might reflect a net reduction in hypothalamic inhibitory influence over ACTH secretion, we examined the effects of dopamine (DA), somatostatin (SS-14), and rat atrial natriuretic peptide [rANF(1-28)] on the secretion of ACTH from cultured ovine anterior pituitary cells. DA and SS-14 did not exert a discernible effect on basal, CRF-, or arginine vasopressin (AVP)-stimulated ACTH secretion. Although basal ACTH secretion was unaffected by rANF(1-28) (10(-12)-10(-8) M), a significant inhibition of CRF- and AVP-stimulated ACTH release was observed.(ABSTRACT TRUNCATED AT 400 WORDS)