Studies on the dual effects of glucose on 45Ca++ efflux from isolated rat islets

45Ca++ efflux studies were performed on rat islets of Langerhans which were loaded to isotopic equilibrium during 48 h in tissue cultures. 45Ca++ loading was 50% complete at 1 h, 80% at 4 h, and reached, at equilibrium, a content equal to 10-11 pmol/islet. The islets responded to glucose stimulation with a rapid and markedly biphase insulin release. Under normal conditions, glucose stimulated 45Ca++ efflux with an initial surge (simultaneous with the first peak of insulin release), which declined rapidly to 50% of the peak value and then slowly declined for the remainder of the glucose stimulation. Special conditions were required to uncover an early inhibition of 45Ca++ efflux; these were the lowering of the temperature of the perifusate from 37 C to 30 C or below, or reduction of the medium Ca++ concentration to 0.1 mM or less. Under zero calcium conditions the glucose inhibition of 45Ca++ efflux can be rigorously interpreted as an inhibition of calcium efflux. The studies at low temperature or low Ca++ concentrations revealed two effects of glucose on 45Ca++ efflux: an initial inhibition followed by a stimulation, the inhibitory effect was obscured by the rapidity of onset of the stimulatory effect under normal conditions. At low temperature it was also possible to inhibit glucose-stimulated insulin release, although the stimulated 45Ca++ efflux remained unchanged. At 30 C or in experiments with 0.3 mM Ca++, glucose-stimulated insulin release preceded the stimulation of 45Ca++ efflux. It therefore, is, concluded that the stimulated 45Ca++ efflux is a consequence, rather than a determinant, of stimulus-secretion coupling. The stimulated efflux is dependent on the presence of Ca++ in the medium and is independent of emiocytosis. This latter finding excludes the secretory granules as a significant source of glucose-stimulated 45Ca++ extrusion.