A dual mechanism for intracellular pH regulation by leech neurones.

Neutral‐carrier pH‐sensitive micro‐electrodes were used to investigate intracellular pH (pHi) in leech neurones. When used in snail neurones such electrodes gave very similar pHi values to those recorded simultaneously by recessed‐tip glass micro‐electrodes. Leech Retzius neurones superfused with a pH 7.4 HCO3‐‐free physiological saline were found to have a pHi of 7.3, too high to be explained by a passive distribution of H+ or OH‐. To investigate pHi regulation the pHi was decreased by one of three methods: by exposure to propionate, by adding and then removing NH4Cl or by exposure to CO2. Acidification by any method was followed by a recovery to normal pHi values within 15‐20 min. In HCO3‐‐free solutions, pHi recovery from acidification was blocked by removing external Na or by amiloride (2 mM). In solutions buffered with 2% CO2 and 11 mM‐HCO3‐, amiloride slowed but did not block pHi recovery. The anion exchange inhibitor SITS (4‐acetamido‐4'‐isothiocyanato‐stilbene‐2,2'‐disulphonic acid) also slowed pHi recovery in the presence of HCO3‐. In CO2/HCO3‐ solution the removal of external Na either slowed or blocked pHi recovery, and blocked it completely in the presence of amiloride. We conclude that in HCO3‐‐free solutions pHi regulation is by a Na‐H exchange system; but in the presence of HCO3‐ there is an additional mechanism which is probably a Na‐dependent Cl‐HCO3 exchanger.