Previous investigations of the role of Ca in stimulus-secretion coupling have been undertaken in populations of adrenal chromaffin cells. In the present study, the simultaneous detection of intracellular Ca, with the fluorescent probe fura-2, and catecholamine release, using a carbon-fiber microelectrode, are examined at single chromaffin cells in culture. Results from classic depolarizing stimuli, high potassium (30-140 mM) and 1,1-dimethyl-4-phenylpiperazinium (3-50 µM), show a dependence of peak cytosolic Ca concentration and catecholamine release on secretagogue concentration. Catecholamine release induced by transient high K stimulation increases logarithmically with K concentration. Continuous exposure to veratridine (50 µM) induces oscillations in intracellular Ca and at higher concentrations (100 µM) concomitant fluctuation of cytosolic Ca and catecholamine secretion. Mobilization of both caffeine- and inositol trisphosphate-sensitive intracellular Ca stores is found to elicit secretion with or without extracellular Ca. Caffeine-sensitive intracellular Ca stores can be depleted, refilled, and cause exocytosis in medium without Ca. Single cell measurement of exocytosis and the increase in cytosolic Ca induced by bradykinin-activated intracellular stores reveal cell to cell variability in exocytotic responses which is masked in populations of cells. Taken together, these results show that exocytosis of catecholamines can be induced by an increase in cytosolic Ca either as a result of transmembrane entry or by release of internal stores.

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