AbstractAt normal membrane potential (Em), steady-state intracellular pH (pHi), which is the balance between acid loading and acid extrusion mechanisms, is more acid in CO2/HCO3- containing (25 mM HCO-3 5% CO2) than in CO2/HCO-3 free (HEPES-buffered) solutions of the same pH (± 7.4) (Ellis & Thomas, 1976). In squid axon, Boron & De Weer (1976) suggested that the extra acid load in CO2/HCO-3 solution results from CO2/HCO-3 shuttle movement: the negative Em would drive intracellular HCO3- out and cause therebys further dissociation of intracellular carbonic acid with liberation of protons. In a previous contribution (de Hemptinne, 1981) we found that in high K+-depolarized fibres, in which the driving force on HCO-3 is reduced, the extra acid load in CO2/HCO3- solution is apparently decreased and the steady-state pHi reached in CO2/HCO-3 and HEPES tends to become equal. In the present experiments the effect of electrical depolarization on the difference in steady-state pH, was investigated. pHi and Em were ...