Abstract

Accumulation of cyclic AMP in response to cholera toxin was studied in Ca²⁺-depleted and Ca²⁺-restored C6 glial tumor cells. Accumulation was retarded when the cells were depleted of Ca²⁺, but the maximal cyclic AMP content observed with toxin in Ca²⁺-restored cells was eventually attained in Ca²⁺-depleted cells. The concentration of cholera toxin which elicited half-maximal cyclic AMP accumulation (apparent K_act) was not altered by Ca²⁺. Micromolar free Ca²⁺ concentrations in the extracellular medium were sufficient to overcome the retardation of toxin-stimulated cyclic AMP accumulation in Ca²⁺-depleted cells. The effect of Ca²⁺ on cyclic AMP accumulation in response to toxin was rapid and was reversible upon the addition of ethylene glycol bis(β-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) in excess of the cation. Accumulation of cyclic AMP by intact cells correlated with increases in the adenylate cyclase activities of Ca²⁺-depleted and Ca²⁺-restored cells exposed to cholera toxin. Ca²⁺, therefore, appears to enhance the rate of adenylate cyclase activation by cholera toxin. Trifluoperazine retarded cholera toxin-dependent cyclic AMP accumulation in a manner similar to cellular Ca²⁺ depletion. Cyclic AMP accumulation in both Ca²⁺-depleted and Ca²⁺-restored preparations was slowed by the drug at micromolar concentrations. Once maximal cell cyclic AMP was attained, however, neither trifluoperazine nor EGTA in excess of Ca²⁺ affected the nucleotide contents of cells or their extracellular fluids. Binding of ¹²⁵I-labeled cholera toxin by intact cells was not altered by Ca²⁺ or by trifluoperazine. The Ca²⁺ dependence of cyclic AMP accumulation in response to norepinephrine was not affected by pretreatment of cells with cholera toxin, although the extent of the response to the catecholamine was reduced. Similar effects of Ca²⁺ on cyclic AMP accumulation in cholera toxin-treated cells were identified in neuroblastoma, neuroblastoma-glioma, pituitary tumor, and Chinese hamster ovary cells.