INCREMENTAL CA2+ MOBILIZATION BY INOSITOL TRISPHOSPHATE RECEPTORS IS UNLIKELY TO BE MEDIATED BY THEIR DESENSITIZATION OR REGULATION BY LUMINAL OR CYTOSOLIC CA2+

The kinetics of Ins(1,4,5)P-3 (InsP(3))-stimulated Ca2+ release from i ntracellular stores are unusual in that submaximal concentrations of I nsP(3) rapidly release only a fraction of the InsP(3)-sensitive Ca2+ s tores. By measuring unidirectional Ca-45(2+) efflux from permeabilized rat hepatocytes, we demonstrate that such quantal responses to InsP(3 ) occur at all temperatures between 2 and 37 degrees C, but at much lo wer rates at the lower temperatures. Preincubation with submaximal con centrations of InsP(3), which themselves evoked quantal Ca2+ release, had no effect on the sensitivity of the stores to further additions of InsP(3). The final Ca2+ content of the stores was the same whether th ey were stimulated with two submaximal doses of InsP(3) or a single ad dition of the sum of these doses. Such incremental responses and the p ersistence of quantal behaviour at 2 degrees C indicate that InsP(3)-e voked receptor inactivation is unlikely to be the cause of quantal Ca2 + mobilization. Reducing the Ca2+ content of the intracellular stores by up to 45% did not affect their sensitivity to InsP(3), but substant ially reduced the time taken for each submaximal InsP(3) concentration to exert its full effect. These results suggest that neither luminal nor cytosolic Ca2+ regulation of InsP(3) receptors are the determinant s of quantal behaviour. Our results are not therefore consistent with incremental responses to InsP(3) depending on mechanisms involving att enuation of InsP(3) receptor function by cytosolic or luminal Ca2+ or by InsP(3) binding itself. We conclude that incremental activation of Ca2+ release results from all-or-nothing emptying of stores with heter ogeneous sensitivities to InsP(3). These characteristics allow rapid g raded recruitment of InsP(3)-sensitive Ca2+ stores as the cytosolic In sP(3) concentration increases.