Pertussis toxin, PT, abolishes inhibitory regulation of adenylate cyclase by cell surface receptors. Inhibitors of adenylate cyclase in GH, cells, namely somatostatin and the muscarinic cholinergic agonist carbachol, lower the cytosolic free Ca2+ concentration.[Ca2+], and cause hyperpolarization. These responses are selectively abolished by PT. It is concluded that the effects of somatostatin and carbachol to lower [Ca'+]i and to hyperpolarize are secondary to their inhibitory action on adenylate cyclase. In contrast, PT does not impair the TRH induced rise in [Ca'+]i in GH, cells demonstrating that the coupling of TRH receptors to Ca2+ mobilization is not mediated by a P'I' substrate.

Muscarinic cholinergic agonists have been shown to inhibit PRL secretion in normal and tumor-derived pituitary cells. Evidence from experiments with the fluorescent Ca2+ probe quin 2 shows that carbachol, acting through muscarinic acetylcholine receptors, lowers the cytosolic free Ca2+ concentration ([Ca2+]i), in GH3 cells. A decrease in [Ca2+]i is observed rapidly after carbachol addition, the lowered steady state [Ca2+]i is maintained, and upon the addition of atropine [Ca2+]i returns to the initial basal value. The lowering from a basal [Ca2+]i, averaging 110 +/- 2 nM (+/- SEM, n = 9), to a steady state [Ca2+]i of 63 +/- 4 nM (+/- SEM, n = 5) at 10 micron carbachol is dose dependent, a significant decrease from basal [Ca2+]i being observed at 0.1 micron. Carbachol does not prevent TRH-induced mobilization of Ca2+ but attenuates the resulting rise in [Ca2+]i. The lowering of steady state [Ca2+]i and the attenuation of the rise in [Ca2+]i provoked by stimulators of PRL secretion could explain the inhibition of both basal and stimulated PRL secretion. Concomitantly with the action on [Ca2+]i, carbachol causes hyperpolarization of GH3 cells. Together with the established inhibition of adenylate cyclase by muscarinic cholinergic agonists, these findings suggest a relation between changes in trans-membrane Ca2+ fluxes and cAMP generation.