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

摘要

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