Calcium and secretion: distinction between two pools of glucose-sensitive calcium in pancreatic islets.

D-Glucose, but not L-glucose or 3-O-methyl-D-glucose, stimulates 45Ca2+ uptake by both lanthanum-displaceable and lanthanum-nondisplaceable pools in pancreatic islets. The nondisplaceable pool probably represents secretory granules, while the displaceable pool may be located in the beta-cell membrane. Kinetic studies with isotopically labeled islets suggest that only the displaceable pool participates in the short-term coupling of the glucose stimulus with secretion.