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