Effects of calcium manipulation and glucose stimulation on a histochemically detectable mobile calcium fraction in isolated rat pancreatic islets

Pancreatic B-cell calcium as histochemically detectable with glyoxal bis (2-hydroxyanil) = GBHA was studied in isolated islets of fed rats. GBHA has previously been shown by us to detect an ionized or readily ionizable Ca-fraction (GBHA-Ca). In the presence of Ca++ (2.5 mM), high glucose (15 mM) induced a rapid decrease (30%) of islet GBHA-Ca followed by a rise between 30 and 60 min to levels above the initial value. At low glucose (0 or 2.5 mM) GBHA-Ca showed a slight and gradual decline under these conditions. Omission of Ca++ at low glucose rapidly decreased (30%) islet GBHA-Ca. This decrease was markedly inhibited by high glucose, although glucose did not induce insulin secretion under these conditions. Preincubation in the absence of Ca++ (15 min) depleted islet GBHA-Ca, but partial restoration occurred during subsequent incubation with Ca++ at low glucose. By contrast, high glucose completely restored GBHA-Ca within 5 min, followed by a decline and a subsequent rise. Reintroduction of Ca++ also rapidly restored the glucose-induced insulin secretion. These results indicate that islet GBHA-Ca represents a mobile Ca-fraction which is dependent on extracellular Ca++ and which responds very rapidly to glucose stimulation. It is suggested that changes of GBHA-Ca in the B-cells may reflect changes in the Ca pool involved in the insulin secretory mechanism.