Possible involvement of GTP-binding protein in the mechanism for Ca2+ -dependent exocytosis in adrenal chromaffin cells

In order to elucidate the possible involvement of GTP-binding protein in the mechanism for exocytosis, we studied effects of pertussis toxin (islet activating protein; IAP) on the secretory function of cultured bovine adrenal chromaffin cells. Treatment of chromaffin cells with IAP resulted in an increase in both basal release of catecholamine and evoked-release by either acetylcholine (ACh) or high K^+ . In the dose-response curve for ACh-evoked release IAP-treatment produced an increase of the maximal response without affecting the half-maximal concentration of ACh. When the cells were permeabilized with digitonin after IAP-pretreatment, Ca^2+ -dependent exocytosis was markedly increased where the affinity of exocytosis for Ca^2+ was augmented. On the other hand, exposure of permeabilized cells to GTP-γ-S (nonhydrolyzable GTP analogue) inhibited Ca^2+ -dependent exocytosis by reducing the affinity for Ca^2+ . These findings suggest that IAP-sensitive GTP-binding protein (or proteins) directly controls the Ca^2+ -triggered process in the machinery of exocytosis by modulating the affinity for Ca^2+ .