Current investigation in this project has focused on the regulation and actions of corticotropin releasing factor (CRF) receptors, and the interactions of CRF with other ACTH regulators including vasopressin (VP), angiotensin II (AII), norepinephrine and glucocorticoids. A. Pituitary CRF receptor regulation. We have previously shown that the increases in plasma ACTH after adrenalectomy are accompanied by pituitary 34F receptor down-regulation and desensitization. Further studies in rats receiving CRF infusion demonstrated that sustained exposure of the pituitary to CRF causes CRF receptor loss and a specific decrease in CRF-stimulated adenylate cyclase activity, which could partially account for the changes following adrenalectomy. B. CRF receptors in the nervous system. Similar to previous findings in the rat, studies in the monkey demonstrated the presence of CRF receptors in the cerebral cortex and limbic system related areas in the primate brain. In the peripheral nervous system, the importance of CRF receptors in the adrenal medulla was emphasized by studies in isolated bovine chromaffin cells which demonstrated the ability of CRF to stimulate catecholamine and met-enkephalin secretion. C. Interactions between ACTH regulators and mechanism of action. In addition to the cyclic AMP-dependent mechanisms by which CRF stimulates the corticotroph, activators of protein kinase C such as phorbol esters, synthetic diacylglycerol and phospholipase C were found to stimulate ACTH secretion. This effect was additive to the stimulatory effect of CRF, but not to those of VP, AII and norepinephrine, suggesting the involvement of protein kinase C in the action of cyclic AMP-independent stimuli. In regard to glucocorticoid feedback, experiments in isolated pituitary cells demonstrated that the biphasic inhibitory pattern of ACTH secretion observed in vivo also occurs in invitro in the corticotroph. The two inhibitory components have different kinetics and sensitivity to corticosterone and probably involve different mechanisms of action of glucocorticoids in the corticotroph.
