Low pH inhibits compensatory endocytosis at a step between depolarization and calcium influx

Cell function can be modulated by the insertion and removal of ion channels from the cell surface. The mechanism used to keep channels quiescent prior to delivery to the cell surface is not known. In eggs, cortical vesicle exocytosis inserts voltage-gated calcium channels into the cell surface. Calcium influx through these channels triggers compensatory endocytosis. Secretory vesicles contain high concentrations of calcium and hydrogen ions. We propose that lumenal hydrogen ions inhibit vesicular calcium channel gating prior to exocytosis, discharge of lumenal protons upon vesicle-plasma membrane fusion enables calcium channel gating. Consistent with this hypothesis we find that cortical vesicle lumens are acidic, and exocytosis releases lumenal hydrogen ions. Acidic extracellular pH reversibly blocks endocytosis, and the windows of opportunity for inhibition with a calcium-channel blocker or hydrogen ions are indistinguishable. Calcium ionophore treatment circumvents the low pH block, suggesting that calcium influx, or an upstream step, is obstructed. Inhibition of calcium influx by preventing membrane depolarization is unlikely, as elevation of the extracellular potassium concentration failed to overcome the pH block, and low extracellular pH was found to depolarize the membrane potential. We conclude that low pH inhibits endocytosis at a step between membrane depolarization and calcium influx.