The plasma membrane calcium ATPase (PMCA) of neurones is electroneutral and exchanges 2 H+ for each Ca2+ or Ba2+ ion extruded

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Abstract
The coupling between Ca2+ extrusion and H+ uptake by the ubiquitous plasma membrane calcium ATPase (PMCA) has not been measured in any neurone. I have investigated this with Ca2+‐ and pH‐sensitive microelectrodes in large voltage‐clamped snail neurones, which have no Na+–Ca2+ exchangers. The recovery of [Ca2+]i and surface pH after a brief depolarization or Ca2+ injection was not slowed by hyperpolarization to −90 mV from a holding potential of −50 mV, consistent with a 1 Ca2+ : 2 H+ coupling ratio. Since Ca2+ injections proved difficult to quantify, and Ca2+ currents through channels were obscured by K+ currents, Ba2+ was used as a substitute. When the cell was bathed in Ca2+‐free Ba2+ Ringer solution, the K+ currents were blocked and large inward currents were revealed on depolarization. The Ca2+‐sensitive microelectrodes were sensitive to intracellular Ba2+ as well as Ca2+. With equal depolarizations Ba2+ entry appeared larger than Ca2+ entry and generated similar but slower pH changes. Ba2+ extrusion was insensitive to hyperpolarization, blocked by eosin or high pH, and about 5 times slower than Ca2+ extrusion. The ratio of the pH change caused by the extrusion of unit charge of Ba2+ influx to that caused by unit charge of H+ injection was 0.85 ± 0.08 (s.e.m., n= 8), corresponding to a Ba2+ : H+ ratio of 1 : 1.7. Both this ratio and the electroneutrality of the PMCA suggest that the Ca2+ : H+ ratio is 1 : 2, ensuring that after a Ca2+ influx [Ca2+]i recovery is not influenced by the membrane potential and maximizes the conversion of Ca2+ influxes into possible pH signals.

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