Mechanism of cyclic AMP facilitation of stimulation-evoked catecholamine release in adrenal chromaffin cells—I. Evidence for enhancement of evoked increase in cytosolic free Na+ concentration by cyclic AMP elevation

Abstract

The mechanism by which cyclic AMP facilitates the secretagogue-stimulated catecholamine (CA) release from cultured bovine adrenal chromaffin cells was investigated. Both 8-Br cyclic AMP and forskolin alone showed no detectable change in cytosolic free Ca 2+ concentration ([Ca 2+ ]i) but markedly potentiated acetylcholine (ACh)-, nicotine- or excess KCl- induced rise in [Ca 2+ ]i. Verapamil, nicardipine and diltiazem antagonized ACh-evoked CA release. 8-Br cyclic AMP still enhanced CA release elicited by ACh even in the presence of Ca 2+ channel blockers. Potentiation of stimulation-evoked CA release and 45 Ca uptake induced by cyclic AMP or forskolin was antagonized by a Na + channel blocker, tetrodotoxin (TTX), or by the removal of Na + from the medium, or by amiloride. Enhancement of secretagogue-stimulated CA release and of 45 Ca uptake induced by ouabain was also antagonized by TTX. Ouabain and veratrine mimicked the effect of cyclic AMP and these agents counteracted each other. Forskolin alone did not increase 22 Na uptake but enhanced 22 Na uptake induced by ACh. TTX markedly reduced 22 Na uptake induced by ACh with forskolin or by ACh with ouabain. The present study suggests that cyclic AMP induced potentiation of stimulation-evoked CA release from cultured bovine chromaffin cells primarily by enhancing the accumulation of intracellular free Na + . From the analogous results obtained using ouabain and veratrine, and from the counteraction of the effects of cyclic AMP by treatments which elevate the intracellular Na + , the possibility that cyclic AMP may inhibit Na + , K + -ATPase in plasma membrane and produce the accumulation of Na + , thereby increasing [Ca 2+ ]i through the mechanism of Na + /Ca 2+ exchange is discussed.