Stimulus‐Secretion Coupling in Isolated Adrenal Chromaffin Cells: Calcium Channel Activation and Possible Role of Cytoskeletal Elements

Abstract

The catecholamine secretory function of a preparation of isolated bovine adrenal chromaffin cells was characterized under conditions designed to elucidate the mechanism of Ca channel activation and the possible role of cytoskeletal elements in stimulus-secretion coupling. Three related sets of data were obtained: differences in kinetics, Ca dependence, strength and additivity of the secretory response to acetylcholine (ACh) versus excess K; the effects on secretion of the Ca channel-blocking agents, Ni, Mg and verapamil; and the Ca dependence of vinblastine action on ACh- and K-evoked secretion. A major portion of the Ca influx required for catecholamine release may enter the cell via voltage-dependent Ca channels, with some additional Ca influx via the ACh receptor channel. Comparison of data with corresponding known electrophysiological properties of isolated chromaffin cells provides added evidence for a role of chromaffin cell action potentials in regulation of Ca influx and the secretory response. Elevated Ca concentrations enhanced K-evoked secretion to levels comparable to that of ACh, but did not induce a vinblastine block of K-evoked release. This provides further evidence against a role of microtubules in the common exocytosis event per se. A role of cytoskeletal elements in directing the movement of secretory granules, or an action of vinblastine at cholinergic receptors, remain distinct possibilities.