Abstract

In the previous paper [Mol. Pharmacol. 23:671-680 (1983)] it was shown that ouabain enhances the exocytotic release of catecholamines from isolated bovine adrenal medullary cells. This effect is dependent upon extracellular sodium, but persists in the nominal absence of calcium. In this paper the study has been extended to include an investigation of the effects of ouabain on the fluxes of 86Rb, 42K, 24Na, and 45Ca in these cells. The basic features of the chromaffin cell sodium pump are characterized, and it is shown for the first time that both the pump itself (i.e., the kinetics and properties of transport) and its inhibition by ouabain resemble those of squid axons and erythrocytes. However, serious doubts are cast upon the often-stated possibility that there is a direct link between sodium pump inhibition and exocytotic secretion because parallel measurements of both phenomena have, for example, shown that while the secretory effect of ouabain is sodium-dependent, pump inhibition is not. Instead, an entirely different explanation is suggested by the discovery that ouabain produces a marked decrease in the rate of active calcium extrusion from chromaffin cells, under all conditions in which catecholamine secretion is enhanced. This inhibition seems not to be accompanied by any change in calcium influx, and may therefore provide a direct explanation for the rise in free calcium which is required to stimulate exocytosis in this tissue.