Somatostatin inhibits vasoactive intestinal peptide-stimulated cyclic adenosine monophosphate accumulation in GH pituitary cells

Abstract

Somatostatin (SRIF) has previously been shown to inhibit both basal and hormone-stimulated PRL secretion from GH4C1 cells, a clonal strain of rat pituitary tumor cells. In this study we examined the ability of SRIF to modulate cAMP accumulation in GH4C1 cells to determine whether such alterations mediate its biological effects. SRIF did not cause statistically significant changes in basal cAMP accumulation. Of six PRL secretagogues examined, only vasoactive intestinal peptide (VIP) increased cAMP accumulation significantly: TRH, bombesin, epidermal growth factor, insulin, and the tumor promoter, phorbol-12,13-dibutyrate were without effect. When SRIF was added simultaneously with VIP, it inhibited maximal VIP-stimulated cAMP accumulation (55 +/- 3%, mean +/- SE) without changing the ED50 for VIP (3.0 +/- 0.2 nM). Inhibition by SRIF was not due to altered kinetics of VIP stimulation, since the half-time for VIP-stimulated cAMP accumulation was 2 min both in the absence and presence of 100 nM SRIF. SRIF did not inhibit isobutylmethylxanthine-stimulated cAMP accumulation, and the presence of 0-10 mM isobutylmethylxanthine did not alter the inhibitory effect of SRIF on VIP-stimulated cAMP accumulation. Therefore, SRIF must act primarily to modulate VIP activation of adenylate cyclase activity. Inhibition of VIP-stimulated cAMP accumulation occurred at concentrations of SRIF (ID50 = 1.2 +/- 0.1 nM) close to the equilibrium dissociation constant for receptor binding (Kd = 0.6 +/- 0.2 nM). Furthermore, the potencies of a series of SRIF analogs to inhibit VIP-stimulated cAMP accumulation correlated with the apparent Kd of each peptide for binding to the SRIF receptor. In addition, SRIF did not reduce VIP-stimulated cAMP accumulation in GH(1)2C1 cells, which lack SRIF receptors. We conclude that SRIF inhibits VIP-stimulated cAMP accumulation by a receptor-mediated process that may be causally related to the ability of SRIF to inhibit VIP-dependent PRL secretion.