PROTEIN-KINASE-C ENHANCES EXOCYTOSIS FROM CHROMAFFIN CELLS BY INCREASING THE SIZE OF THE READILY RELEASABLE POOL OF SECRETORY GRANULES

We have used membrane capacitance measurements to assay Ca2+-triggered exocytosis in single bovine adrenal chromaffin cells. Brief applicati on of phorbol ester (PMA) enhances depolarization-evoked exocytosis se veralfold while actually decreasing the Ca2+ current. Ca2+ metabolism is unchanged. Three different protocols were used to show that PMA inc reases the size of the readily releasable pool of secretory granules. PMA treatment leads to a large increase in amplitude, but little chang e in the time course of the exocytic burst that results from rapid ele vation of [Ca2+](i) upon photolysis of DM-Nitrophen. Thus, PKC appears to affect a late step in secretion but not the Ca2+ sensitivity of th e final step.