Evidence for Distinct Glucocorticoid and Guanine 3′,5′-Monophosphate-Effected Inhibition of Stimulated Adrenocorticotropin Releasein Vitro*

Abstract

Previous work has shown that corticosterone, cell-membrane permeant analogs of cGMP, as well as activators of guanylyl cyclase inhibit secretagogue-stimulated ACTH release. In the present study we have examined whether cGMP mediates the inhibitory effect of corticosterone in perifused isolated rat anterior pituitary cells. A brief 22.5-min exposure to corticosterone strongly inhibited ACTH secretion evoked by arginine vasopressin (AVP), 48 mM KC1, and two types of combined stimuli, i.e. 41-residue CRF and AVP (0.05 and 0.5 nM, respectively; CRF/AVP), or ionomycin and phorbol-dibutyrate (200 and 10 nM, respectively; PdBu/IM). The time course of inhibition by corticosterone was similar in all cases; a rapid approximately 30% reduction in ACTH was evident within 25 min, which increased to 60% by 50-70 min and will be referred to as the delayed effect. The corticosteroid inhibition of PdBu/IM-induced ACTH release was fully antagonized by the glucocorticoid/progestin antagonist RU 38486, indicating that it is exerted through type II glucocorticoid receptors. In contrast to corticosterone, the cGMP derivative 8-bromo-cGMP failed to suppress ACTH release evoked by PdBu/IM, whereas it effectively inhibited the action of CRF/AVP. Furthermore, ionomycin reversed the reduction of CRF/AVP-stimulated ACTH release by 8-bromo-cGMP, but had no effect on the delayed inhibition caused by corticosterone. These data indicate that there are two distinct cellular pathways of inhibiting stimulus-evoked ACTH secretion in vitro. One of these is activated by corticosterone, whereas the other involves cGMP as a cellular messenger.