The effect of pH on 86Rubidium efflux from pancreatic islet cells

The influence of pH on the K+ permeability of pancreatic islet cells was investigated by measuring 86Rb+ fluxes in isolated rat islets perifused or incubated in the absence of glucose. Acidification of the medium (to pH 6.5), by decreasing the concentration of HCO-3 or by increasing pCO2 at constant HCO-3 reversibly reduced the rate of 86Rb+ efflux from perifused islets. Alkalinization of the medium (to pH 7.8), by decreasing the pCO2, reversibly increased 86Rb+ efflux. Similar changes were recorded upon alteration of the pH in a bicarbonate-free, Hepes-buffered medium with or without calcium. Alteration of the CO2 level at constant external pH - in order to modify internal pH - produced only a small and transient increase in efflux rate when CO2 was lowered, and a decrease in efflux rate when CO2 was raised. NH4Cl reversibly augmented 86Rb+ efflux in the presence and in the absence of HCO-3. At low pH (6.5), 86Rb+ uptake by islet cells was reduced after 10 min (25%) and 30 min (11%), but not after 60 min, of incubation; it was not significantly affected by high pH (7.8). Calcium uptake and insulin release were reduced at low pH and increased at high pH. These results show that the K+ permeability of islet cells is affected by changes in extracellular but not in intracellular pH. They suggest that the endogenous production of protons that accompanies glucose stimulation of islet cells is not the mediator of the decrease in K+ permeability induced by the sugar.