Correlation between secretagogue-induced Ca²⁺ influx, intracellular Ca²⁺ levels and secretion of catecholamines in cultured adrenal chromaffin cells

Catecholamine secretion induced by various secretagogues in cultured bovine chromaffin cells has been correlated with Ca²⁺ influx and intracellular Ca²⁺ concentrations. Nicotine and high K⁺ caused prompt secretion of catecholamines from cells. Coincidently, both secretagogues evoked ⁴⁵[Ca²⁺] influx with a parallel increase in free intracellular Ca²⁺ concentration, as determined by Quin 2 fluorescence. However, the rate of return of Ca²⁺ level to baseline after nicotine stimulation was more rapid than after K⁺ stimulation. In comparison, stimulation with veratridine produced a slow and prolonged Ca²⁺ influx accompanied by lower levels of intracellular Ca²⁺ than those observed after nicotine or K⁺ stimulation. Yet, during 15 min of stimulation, veratridine induced a substantial catecholamine release, which was larger than that obtained after nicotine or K⁺ stimulations. The Ca²⁺ ionophore A23187 (1 μM) induced a pronounced increase in intracellular Ca²⁺ levels, but did not evoke any significant catecholamine release. Finally, addition of the Ca²⁺ channel blocker verapamil following stimulation, at a time when intracellular Ca²⁺ concentration was at its peak level, did not affect the rate of decline in intracellular free Ca²⁺ concentration but promptly blocked Ca²⁺ uptake and catecholamine secretion. These findings suggest that the rate of Ca²⁺ influx, rather than the absolute level of intracellular Ca²⁺ concentration, determines the rate and extent of catecholamine release.