The role of bicarbonate, chloride and sodium ions in the regulation of intracellular pH in snail neurones

Abstract

Intracellular pH (pHi), Cl- and Na+ levels were recorded in snail [Helix aspersa] neurons using ion-sensitize micro-electrodes, and the mechanism of pHi recovery from internal acidification investigated. Reducing external HCO3- concentration greatly inhibited the rate of pHi recovery from HCl injection. Reducing external Cl- did not inhibit pHi recovery, but reducing internal Cl-, by exposing the cell to sulfate Ringer, inhibited pHi recovery from CO2 application. During pHi recovery from CO2 application, internal Cl- concentration decreased. The measured fall in internal Cl- concentration averaged about 25% of the calculated increase in internal HCO3-. Removal of external Na inhibited pHi recovery from CO2 application or HCl injection. During pHi recovery from acidification there was an increase in the internal Na+ concentration ([Na+]i) which was larger than that occurring when the Na pump was inhibited by K-free Ringer. The increase in [Na+]i that occurred during pHi recovery from an HCl injection was about half that produced by a similar injection of NaCl. The inhibitory effects of Na-free Ringer and of the anion exchange inhibitor SITS on pHi recovery after HCl injection were not additive. The pHi regulating system involves tightly linked Cl--HCO3- and Na+-H+ exchange, with Na entry down its concentration gradient probably providing the energy to drive inward movement of HCO3- and the outward movement of Cl- and H+.