Manganous chloride stimulation of adenylate cyclase responsiveness in ocular ciliary process membranes

Manganous chloride was compared with magnesium chloride in supporting maximal stimulations of the adenylate cyclase system in ocular ciliary process membranes by isoproterenol, vasoactive intestinal peptide, sodium fluoride, guanosine 5'-(beta, gamma-imino) triphosphate (GppNHp), or forskolin, and in supporting synergism between isoproterenol and forskolin. The primary effect of Mn2+ (2 mM) was due to an interaction at the catalytic unit. Mn2+ had no significant effect on the function of the GTP-binding stimulatory G-protein (Gs) which couples beta-adrenergic receptors to the catalytic unit of adenylate cyclase. However, Gs-protein function was impaired by Mn2+ relative to Mg2+ when GppNHp was used instead of GTP as the ligand for the Gs-protein. Compared with Mg2+, Mn2+ caused a 4-5.5-fold increase in adenylate cyclase responsiveness to all the activators tested (except GppNHp, where the increase was 2.5-3.5-fold). Thus Mn2+ ions appeared to be intrinsically more effective at divalent cation binding sites on the catalytic unit that control its enzymatic activity. Ciliary process membranes differ from erythrocyte and S49 lymphoma cell membranes where 2 mM Mn2+ strongly inhibits hormone-Gs-protein-mediated stimulations of adenylate cyclase. Divalent cations bound to the catalytic unit also affected the degree of synergism between hormone-activated Gs and forskolin to stimulate adenylate cyclase activity. In the presence of MgCl2 all effective doses of isoproterenol and forskolin in combination showed marked synergism. In contrast, with MnCl2 there was no synergism with high-dose isoproterenol-forskolin combinations, which gave only additive responses.