Glucose-stimulated and La3+-nondisplaceable Ca2+ pool in pancreatic islets

To study intracellular pools of calcium tissue specimens from noninbred ob/ob mice were labeled with 45Ca2+ and subsequently washed with La3+. D-glucose, 20 mM, enhanced the labeling of the La3+-nondisplaceable calcium in pancreatic islets but not in pieces of exocrine pancreas or liver. The disappearance of 45Ca2+ from labeled islets was accelerated by the ionophore, X-537A, but not by dibutyryl cyclic AMP, theophylline, or pentobarbital. On fractionation of 45Ca2+-labeled islets, the greatest radioactivity per unit of protein occured in a fraction rich in insulin secretory granules. The radioactivity of this fraction was higher after the islets had been loaded with 45Ca2+ in the presence of 20 mM glucose as compared to 3 mM glucose. It is concluded that the secretory granules make up a considerable part of the glucose-sensitive calcium store in the beta-cells.