CALCIUM HYDROGEN-EXCHANGE BY THE PLASMA-MEMBRANE CA-ATPASE OF VOLTAGE-CLAMPED SNAIL NEURONS

The submicromolar levels of free Ca2+ ions in animal cells are believe d to be maintained in the long term by two different plasma membrane t ransport mechanisms. These are Na-Ca exchange, driven by the sodium gr adient, and a Na-independent Ca pump, driven by ATP. There is good evi dence from red blood cells, and indirect evidence from other non-neuro nal preparations, that the Ca-ATPase exchanges internal Ca2+ for exter nal H+. Although Ca extrusion from nerve cells is inhibited by high ex ternal pH, there as yet is no evidence for the counter-transport of H. We have used both pH- and calcium-sensitive microelectrodes on the c ell surface, and the Ca indicator fura-2 intracellularily, to investig ate how snail neurons regulate cytoplasmic free Ca2+. We now report th at in snail neurons the recovery of intracellular Ca2+ after an increa se coincides with both the expected increase in surface Ca2+ and a dec rease in surface H+. Recovery of intracellular Ca and the changes in s urface pH and Ca are all blocked by intracellular vanadate. We conclud e that snail neurons regulate intracellular Ca mainly by a Ca-H ATPase , and suggest that this Ca-H exchange may account for many of the repo rted extracellular pH changes seen with neuronal excitation.