Effect of the absence of bicarbonate upon intracellular pH and calcium fluxes in pancreatic islet cells

Abstract

The removal of extracellular HCO − 3 together with a decrease in pCO 2, in order to maintain a normal extracellular pH, caused a sustained increase of intracellular pH in rat pancreatic islets. This increase was more marked in glucose-deprived than in glucose-stimulated islets, and was associated with a facilitation of 45Ca efflux from the glucose-deprived islets. Such a facilitation was slightly reduced in the absence of extracellular Ca 2+ and abolished at low extracellular Na + concentration. It failed to occur in glucose-stimulated islets, whether in the presence or absence of extracellular Ca 2+. The removal of HCO − 3 and decrease in the pCO 2 also reduced the magnitude of both the secondary rise in 45Ca efflux and stimulation of insulin release normally evoked by an increase in glucose concentration. These findings suggest that changes in intracellular pH affect both the outflow of Ca 2+ from islet cells as mediated by Na +-Ca 2+ countertransport and the inflow of Ca 2+ by gated Ca 2+ channels. The experimental data are also compatible with the view that islet cells are equipped with an active process of bicarbonate-chloride exchange involved in the regulation of intracellular pH.