Calcium ion-elicited accumulations of cyclic GMP in guinea pig cerebellar slices.

Abstract Ca 2+ elicited a marked accumulation of cyclic GMP in guinea pig cerebellar slices. This calcium-elicited accumulation of cyclic GMP in slices did not appear dependent on endogenous release of the following putative transmitters: norepinephrine, adenosine, acetylcholine, glycine, glutamate, or γ-aminobutyric acid, since antagonists for such transmitters did not reduce the calcium response. The antagonists also had no inhibitory effect on the accumulation of cyclic GMP elicited in guinea pig cerebellar slices by a combination of a depolarizing agent, veratridine, and Ca 2+ . A variety of calcium antagonists were relatively ineffective in preventing calcium-elicited accumulations of cyclic GMP in cerebellar slices. Mg 2+ , cocaine, tetracaine, chlorpromazine, imipramine, diphenyhydramine, (+)- and (−)-brompheniramine, morphine, neomycin, ethanol, and dimethylsulfoxide caused a partial inhibition of the calcium response while verapamil and sodium pentobarbital were ineffective. Verapamil actually augmented calcium-elicited accumulation of cyclic GMP in cerebellar slices and elevated cyclic AMP in cortical and cerebellar slices. Mg 2+ at 20 mM were somewhat more effective in inhibiting veratridine plus calcium-elicited accumulations of cyclic GMP in cerebellar slices, suggesting that a calcium-linked excitation-secretion process is involved in the veratridine response. Promethazine was the most effective and potent antagonist of calcium-elicited accumulation of cyclic GMP with an IC 50 of μ M. Promethazine, chlorpromazine, imipramine, and diphenhydramine were quite ineffective as inhibitors of guanylate cyclases in cerebellar homogenates. The calcium ionophore, A23187, had only a small and variable stimulatory effect on calcium-elicited accumulations of cyclic GMP in cerebellar slices even when the normal response to calcium was inhibited by dimethylsulfoxide. The presence of the ionophore, X-537A, inhibited accumulations of both cyclic AMP and cyclic GMP. Phosphodiesterase inhibitors elevated levels of cyclic GMP both in the presence of Ca 2+ and in calcium-free media. Isobutylmethylxanthine elevated cyclic GMP levels by 3–4-fold, while RO 20–1724, diazepam and papaverine had lesser effects. Theophylline and SQ 20,009 had no significant effect on cyclic GMP levels. The effects of the phosphodiesterase inhibitors on cerebellar cyclic AMP levels were somewhat different then their effects on cyclic GMP levels. RO 20–1724 and to a lesser extent diazepam and isobutylmethylxanthine significantly elevated cyclic AMP. The other inhibitors had no effect; although theophylline reduced the levels of cyclic AMP. In the presence of isobutylmethylxanthine, neither norepinephrine, veratridine nor adenosine augmented the calcium-elicited accumulation of cyclic GMP. The results are consistent witht he following: calcium influx through channels and/or binding at specific sites activates cyclic GMP-generating systems in guinea pig cerebellar slices. This calcium-dependent activation is antagonized most effectively by promethazine and less effectively by chlorpromazine, imipramine, and diphenhydramine. Depolarization perhaps through release of neuromodulators increases the calcium-elicited activivation of guanylate cyclase. Phosphodiesterases associated with cyclic AMP-generating systems would appear most effectively inhibited by RO 20–1724 while those associated with cyclic GMP systems would appear most effectively inhibited by isobutylmethylxanthine.