EVIDENCE FOR DIVERGENT GLUCOSE EFFECTS ON CALCIUM-METABOLISM IN PANCREATIC BETA-CELLS AND ALPHA-2-CELLS

Elucidation of the role of Ca2+ in the secretion of insulin and glucagon is complicated by the presence of different cell types in the pancreatic islets. Visualization of Ca in sections of guinea pig pancreas with the histochemical reagent glyoxal bis-2-hydroxyanil revealed the most intense staining in the endocrine part but no differences between various islet cell types. A procedure for eliminating the majority of the .beta.-cells by streptozotocin injection in the guinea pig enabled a comparison of collagenase-isolated islets rich in .alpha.2-cells with islets from untreated animals rich in .beta.-cells. The latter islets contained 24.6 .+-. 2.4 mmol Ca/kg dry wt, as estimated by flameless atomic absorption spectrophotometry. This is twice as much as noted for the exocrine pancreas or the islets rich in .alpha.2-cells. After storage for 3 days in culture medium, the 2 types of islets contained similar amounts of Ca. The cultured islets displayed differences related to cellular composition when measuring the incorporation of 45Ca into a La-nondisplaceable (intracellular) pool. In the presence of 3 mM glucose, more 45Ca was incorporated into the islets rich in .alpha.2-cells. Increasing the glucose concentration to 20 mM with or without further addition of 30 units/l bovine insulin had no effect on the 45Ca uptake into the islets rich in .alpha.2-cells but stimulated that into islets rich in .beta.-cells. The different Ca dependence on glucose in the 2 types of islets may indicate that increased uptake of Ca2+ is a component of the mechanism for the secretion of both insulin and glucagon.