Mechanism of calcium efflux from isolated bovine adrenal chromaffin cells.

Chromaffin cells isolated from bovine adrenals and maintained in culture were loaded with 45Ca, and the mechanism of 45Ca efflux from these cells was investigated. During exposure to a Ca-deficient, Mg (1 mM)-containing medium, replacement of NaCl with sucrose, choline Cl, or Tris Cl reduced the rate of 45Ca efflux by approx. 30-40%, showing the presence of a Na-dependent 45Ca efflux mechanism. In contrast to the previous findings, however, Li can partially replace Na in maintaining 45Ca efflux. Reintroduction of Ca to a Ca-deficient medium increased the rate of 45Ca efflux depending on the concentrations of Ca, showing the presence of a Ca-dependent Ca efflux mechanism. The marked but transient rise in the rate of 45Ca efflux in response to Ca reintroduction was observed under conditions in which the concentration of internal Na was raised either by inhibiting Na pumping activity (ouabain or K removal) or by removal of Mg from the medium. The increased 45Ca efflux was further potentiated by simultaneous Na removal and was markedly inhibited by Ca channel blockers, suggesting that the transiently increased Ca influx in exchange for internal Na facilitated the rate of 45Ca efflux. However, other conditions which evoke catecholamine secretion by stimulating Ca influx also increased the rate of 45Ca efflux, suggesting that the increased Ca influx via any route increases 45Ca efflux. These results thus confirmed the presence of both Na-dependent and Ca-dependent Ca efflux mechanisms in chromaffin cells.