Lowering of cytosolic free Ca2+ by carbachol, a muscarinic cholinergic agonist, in clonal pituitary cells (GH3 cells).

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.