Evidence for divergent glucose effects on calcium metabolism in pancreatic beta- and alpha 2-cells

Elucidation of the role of Ca2+ in the secretion of insulin and glucagon is complicated by the presence of different types of cells in the pancreatic islets. Visualization of calcium 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 calcium/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 two types of islets contained similar amounts of calcium. The cultured islets displayed differences related to cellular composition when measuring the incorporation of 45Ca into a lanthanum-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 U/liter bovine insulin was without effect on the 45Ca uptake into the islets rich in alpha 2-cells but stimulated that into islets rich in beta-cells. The different calcium dependence on glucose in the two types of islets may indicate that increased uptake of Ca2+ is a component of the mechanism for the secretion of both insulin and glucagon.