pH recovery from intracellular alkalinization in Retzius neurones of the leech central nervous system.

1. Neutral‐carrier pH‐sensitive microelectrodes were used to investigate intracellular pH (pHi) recovery from alkalinization in leech Retzius neurones in Hepes‐ and in CO2‐HCO3(‐)‐buffered solution. The Retzius neurones were alkaline loaded by the addition and subsequent removal of 16 mM acetate, by changing from 5% CO2‐27 mM HCO3‐ to 2% CO2‐11 mM HCO3‐ or by changing from CO2‐HCO3(‐)‐ to Hepes‐buffered solution. 2. In Hepes‐buffered solution (pH 7.4) the mean pHi was 7.29 +/‐ 0.11 and the mean membrane potential ‐44.7 +/‐ 5.9 mV (mean +/‐ S.D.; n = 83). 3. The rate of pHi recovery from alkalinization increased with decreasing pH of the bathing medium (pHb). pHi changed about 0.30 pH units for a pHb unit change. 4. A decrease of extracellular buffer concentration (Hepes concentration lowered from 20 to 5 mM) caused an acidification of extracellular and intracellular pH and an acceleration of pHi recovery from alkalinization. 5. A depolarization of the Retzius cell membrane‐induced by increasing the K+ concentration of the bathing medium from 4 to 20 mM (delta Em = 16.5 +/‐ 5.5 mV) or from 4 to 40 mM (delta Em = 24.8 +/‐ 3.5 mV)‐‐evoked a decrease of pHi and an acceleration of pHi recovery from alkalinization. 6. The H+ current blocker Zn2+ (0.5 mM) inhibited pHi recovery from alkalinization at resting membrane potential as well as during depolarization. The inhibition was more pronounced during depolarization. 7. In Cl(‐)‐free, CO2‐HCO3(‐)‐buffered solution pHi recovery from an alkaline load by changing from 5% CO2‐27 mM HCO3‐ to 2% CO2‐11 mM HCO3‐ was slowed by 48‐71%. The rate of pHi recovery from an alkaline load induced by changing from CO2‐HCO3‐ to Hepes buffer was reduced by 33‐56% in Cl(‐)‐free solution. The removal of external Cl‐ did not affect pHi recovery in Hepes‐buffered solution. 8. The pHi recovery from alkalinization was DIDS‐insensitive in CO2‐HCO3(‐)‐ as in Hepes‐buffered solutions and was not slowed in the absence of external Na+. 9. It is concluded that in Retzius neurones pHi recovery from alkalinization is mediated by a passive voltage‐dependent H+ influx along the electrochemical proton gradient. In the presence of CO2‐HCO3‐ buffer a DIDS‐insensitive Cl(‐)‐HCO3‐ exchanger additionally regulates pHi after an intracellular alkaline load. It cannot be excluded that intracellular processes (e.g. H+ release from organelles, metabolic H+ production) are also involved in pHi recovery from alkalinization.