ESSENTIAL ROLE OF MYOSIN LIGHT-CHAIN KINASE IN THE MECHANISM FOR MGATP-DEPENDENT PRIMING OF EXOCYTOSIS IN ADRENAL CHROMAFFIN CELLS

Ca2+-induced exocytosis in chromaffin cells now seems to consist of at least two distinct steps: MgATP-dependent Ca2+-enhanced priming of th e secretory apparatus, and Ca2+-dependent MgATP-independent step that triggers exocytosis (Bittner and Holt, 1992). Recently we found that a specific inhibitor of myosin light chain kinase (MLCK), wortmannin, i nhibits Ca2+-induced catecholamine release from digitonin-permeabilize d chromaffin cells, suggesting an implication of MLCK in the mechanism s of Ca2+-induced exocytosis (Imaizumi et al., 1992b). To elucidate fu rther the implication of MLCK in the mechanism of exocytosis, we studi ed the effects of wortmannin and a peptide inhibitor (SM-1) correspond ing to the pseudosubstrate domain of MLCK on MgATP-dependent and MgATP -independent release in digitonin-permeabilized chromaffin cells. Ca2-induced exocytosis from the permeabilized cells in the presence of Mg ATP was inhibited by both SM-1 and wortmannin. Inhibitory effect of wo rtmannin on the rate of release induced by 10 mu M Ca2+ in the presenc e of MgATP was much prominent in the later phase (1-10 min), although the initial rate was also decreased. SM-1 strongly inhibited ATP-depen dent release without affecting Ca2+-dependent ATP-independent release at all. In addition, priming effect of MgATP that underlies Ca2+-depen dent ATP-independent release was remarkably reduced by both wortmannin and SM-1. These results suggest that MLCK plays an essential role in ATP-dependent priming of Ca2+-induced exocytosis in chromaffin cells.