Role of Na(+)-H+ and Cl(-)-HCO3- antiports in the regulation of cytosolic pH near neutrality.

In Vero cells, Na(+)-H+ antiport as well as Na(+)-coupled and Na(+)-independent Cl(-)-HCO3- antiport are involved in regulation of cytosolic pH (pHi) after large (unphysiological) deviations from neutrality. In this paper we have studied to which extent each of the three antiports is involved in regulation of pHi after small deviations from neutrality expected to occur under physiological conditions. At physiological extracellular pH (pHo), inhibition of Na(+)-H+ exchange by amiloride did not alter pHi. At neutral and alkaline pHo, pHi was found to be lower in the presence of HCO3- than in its absence, whereas at acidic pHo, pHi was higher in the presence of HCO3- than in its nominal absence. Above pHi 6.5, the activity of the Na(+)-coupled Cl(-)-HCO3- antiport was higher than the Na(+)-H+ antiport. After a small reduction of pHi, the recovery of steady-state pHi was entirely dependent on Na(+)-coupled Cl(-)-HCO3- antiport, whereas after more pronounced acidification, also Na(+)-H+ exchange contributed to the acid extrusion. The Na(+)-independent Cl(-)-HCO3- antiport, which acts as an acidifying mechanism, was strongly activated at pHi greater than 7.1. The results indicate that at physiological pHo the steady-state pHi is largely determined by the activity of the two Cl(-)-HCO3- antiports, and they suggest that Na(+)-H+ exchange does not influence the resting pHi under these conditions.