Tetanus toxin and Clostridium perfringens enterotoxin as tools for the study of exocytosis

The role of calmodulin in exocytotic secretion was studied using digitonin-permeabilized bovine adrenal chromaffin cells to examine the effect of calmodulin directly introduced into the cells and using tetanus toxin as a specific inhibitor of exocytotic secretion. Addition of calmodulin to the permeabilized cells increased Ca(2+)-dependent norepinephrine release in a dose-dependent manner. The enhancement of release by calmodulin was specific to calmodulin: bovine serum albumin, actin, and caldesmon had no such effect. Enhancement of release by calmodulin occurred at Ca2+ concentrations of more than 10(-6) M and increased with an increase of Mg2+ concentration. The release of norepinephrine enhanced by calmodulin was inhibited by tetanus toxin. These results indicate directly that calmodulin plays an important role in exocytotic secretion from chromaffin cells. Exocytosis is known to occur by fusion of plasma membrane with limiting membranes of secretory vesicles following an increase in intracellular Ca2+. We used the enterotoxin of Clostridium perfringens type A as a specific tool to modify plasma membrane permeability to induce calcium influx. Multigranular exocytosis was recognized electron-microscopically in addition to the single-granular exocytosis in rat anterior pituitary cells and pancreatic acinar cells treated with the enterotoxin in the presence of extracellular Ca2+. The treatment with the enterotoxin did not induce any drastic change in the fine membrane structures of both types of cells. The enterotoxin-treated anterior pituitary cells and pancreatic acinar cells should provide a useful system for studying the molecular mechanism of fusion of membranes in exocytosis.