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

The removal of extracellular HCO3- together with a decrease in pCO2, 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 Ca2+ and abolished at low extracellular Na+ concentration. It failed to occur in glucose-stimulated islets, whether in the presence or absence of extracellular Ca2+. The removal of HCO3- and decrease in the pCO2 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 Ca2+ from islet cells as mediated by Na+ -Ca2+ countertransport and the inflow of Ca2+ by gated Ca2+ 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.