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 b ovine chromaffin cells has been correlated with Ca2+ influx and intrac ellular Ca2+ concentrations. Nicotine and high K+ caused prompt secret ion of catecholamines from cells. Coincidently, both secretagogues evo ked (45)[Ca2+] influx with a parallel increase in free intracellular C a2+ concentration, as determined by Quin 2 fluorescence. However, the rate of return of Ca2+ level to baseline after nicotine stimulation wa s more rapid than after K+ stimulation. In comparison, stimulation wit h 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 ind uced a substantial catecholamine release, which was larger than that o btained after nicotine or K+ stimulations. The Ca2+ ionophore A23187 ( 1 mu M) induced a pronounced increase in intracellular Ca2+ levels, bu t did not evoke any significant catecholamine release. Finally, additi on of the Ca2+ channel blocker verapamil following stimulation, at a t ime when intracellular Ca2+ concentration was at its peak level, did n ot affect the rate of decline in intracellular free Ca2+ concentration but promptly blocked Ca2+ uptake and catecholamine secretion. These f indings suggest that the rate of Ca2+ influx, rather than the absolute level of intracellular Ca2+ concentration, determines the rate and ex tent of catecholamine release.