Stimulation of adrenocorticotropin secretion from AtT-20 cells by the calcium channel activator, BAY-K-8644, and its inhibition by somatostatin and carbachol

Abstract

Somatostatin and carbachol receptors are believed to be negatively coupled to adenylate cyclase in AtT-20 mouse pituitary tumor cells by an inhibitory guanine nucleotide-binding regulatory subunit. Activation of these receptors causes inhibition of cyclic AMP synthesis and adrenocorticotropin (ACTH) secretion stimulated by a variety of hormones. Secretion in response to several pharmacological agents, which do not increase AtT-20 cyclic AMP levels, is also antagonized by both somatostatin and carbachol. Inasmuch as ACTH secretion in response to all stimulants is dependent on extracellular calcium, the possibility that somatostatin and carbachol block calcium entry was investigated by observing the effects of these agents on the activity of the calcium channel activator, BAY-K-8644 [methyl-1,4-dihydro-2,6-dimethyl-3-nitro-4- (2-trifluoromethylphenyl)-pyridine-5-carboxy-late] in AtT-20 cells. In first characterizing the effect of BAY-K-8644, it was noted that the channel agonist at 10(-10) to 10(-6) M itself rapidly increased basal ACTH secretion; higher concentrations (10(-4) M) reduced basal, (-)-isoproterenol, phorbol ester, 8-Br-cAMP and K+-stimulated secretion. BAY-K-8644 did not alter basal formation of cyclic AMP. The secretory response to BAY-K-8644 was dependent on extracellular calcium, and was inhibited by the calcium channel antagonist, nifedepine. When coapplied with (-)-isoproterenol, phorbol ester and 8-Br-cAMP, at a concentration which optimally stimulated ACTH secretion, BAY-K-8644 had an additive effect; the secretory responses to K+ (50 mM) or the calcium ionophore, A-23187, on the other hand, were potentiated.(ABSTRACT TRUNCATED AT 250 WORDS)