Intracellular pH regulates Na(+)-independent Cl(-)-base exchange in JTC-12 (proximal tubule) cells

The role of an anion exchange pathway in the regulation of intracellular pH (pHi) under alkaline load and steady-state conditions and the modulation of this transporter by pHi was investigated in confluent monolayers of cloned JTC-12 cells, derived from monkey kidney proximal tubule. Regulation of pHi was fluorometrically monitored using the pH-sensitive probe, 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Monolayers in which pHi was rapidly elevated by removal of HCO3(-)-CO2 from the bathing medium demonstrated an absolute requirement for Cl- to recover toward base-line pHi. The recovery process proceeded in the absence of Na+, was inhibited 80% by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid, and was unaffected by 10(-4) M amiloride. When extracellular pH (pHo) was serially lowered from 7.4 to 6.7, recovery from an alkaline load induced by removal of HCO3(-)-CO2 from the medium occurred only when pHi was elevated above approximately 7.25. Below pHi approximately 7.25 no recovery toward initial pHi was observed. When pHi was elevated above approximately 7.25 with pHo maintained at 6.7, the recovery process ceased at pHi approximately 7.25 despite favorably oriented Cl- and OH- chemical gradients. Consistent with these observations, removal of Cl- from the medium of cells buffered with 25 mM HCO3(-)-5% CO2 at pHo 7.4 (in the absence of Na+) resulted in reversible elevation of pHi, whereas in a solution buffered to pHo 6.7 with 5 mM HCO3(-)-5% CO2, removal of Cl- failed to elevate pHi. Under steady-state conditions in the presence of 25 mM HCO3(-)-5% CO2 at pHo 7.4, pHi was 7.40 +/- 0.02 and reversibly decreased to 7.23 +/- 0.01 on removal of Na+ (in the presence of amiloride) from the bathing medium, indicating that the Cl(-)-base exchanger is operative under basal conditions and functions as a base extruder. In summary the JTC-12 cell possesses a Na(+)-independent Cl(-)-base exchange mechanism that is operative under alkaline load and steady-state conditions. pHi but not pHo modulates the activity of this transport pathway, and below pHi approximately 7.25 the exchanger is quiescent.