Calcium-activated ATPase activity in a plasma membrane-rich preparation of rat pancreas.

1. Calcium-stimulated ATPase activity was studied in a plasma membrane rich fraction of rat pancreas homogenate. 2. The enzyme is stimulated to the same maximum rate of ATP hydrolysis by either calcium or magnesium, but the apparent requirement for calcium is five-fold lower than for magnesium. 3. Maximum hydrolytic activity of the enzyme is not increased when additional magnesium is added to the optimal amount of calcium. 4. The enzyme does not require Na+ or K+ for its activation by Ca2+ and is not inhibited by ouabain or 2,4-dinitrophenol. 5. Pancreozymin, in concentrations which evoke secretion of zymogen protein, inhibits the calcium-stimulated ATPase. 6. Carbachol and dibutyryl cyclic AMP, in concentrations which increase the release of digestive proteins, do not alter the activity of the calcium-stimulated enzyme. 7. It is suggested that the plasma membrane calcium-activated enzyme, is not involved in the active calcium extrusion, previously reported to occur with use of the various pancreatic secretagogues tested.