Calcium-stimulated ATPase activity in plasma membrane vesicles from pancreatic acinar cells

Plasma membrane vesicles were prepared from rat pancreatic acinar cells using nitrogen cavitation and magnesium precipitation. The vesicles exhibited ATPase activity that was stimulated by submicromolar concentrations of free calcium and was dependent upon the presence of magnesium. This enzyme activity was localized to the cytoplasmic surface of the plasma membrane by two criteria. First, no activity was observed when intact cells replaced the membrane vesicles in the assay. Second, right side-out and inside-out vesicles were separated using concanavalin A sepharose-B. The calcium-stimulated, magnesium-dependent ATPase activity per mg protein in the inside-out fraction was 60% greater than that occurring in the mixed vesicle preparation. These results indicate that the plasma membrane of pancreatic acinar cells has a calcium-stimulated, magnesium-dependent ATPase located on its cytoplasmic surface and that this enzyme is stimulated by submicromolar concentrations of free calcium.