Stimulation of Cl^- self exchange by intracellular HCO₃^- in rabbit cortical collecting duct

In rabbit cortical collecting duct, Cl^- self exchange accounts for most of the transepithelial Cl^- tracer rate coefficient, K(Cl) (nm/s); a small fraction is effected by Cl^--HCO₃^- exchange and Cl^- diffusion. We previously reported that changing from a CO₂-free N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES) bath to a 5% CO₂-25 mM HCO₃^- bath stimulates Cl^- self exchange. Here, we examine in further detail the individual components of the CO₂-HCO₃^- system that stimulate K(Cl). Addition of 0.5% CO₂ to a HEPES bath (final pH = 7.24) stimulated K(Cl) by 70 ± 19 nm/s, a ΔK(Cl) comparable to that induced by 1% CO₂ (pH 7.12), 6% CO₂ (pH 6.6), or 6% CO₂-25 mM HCO₃^- (pH 7.4). The roles of intracellular pH (pH(i)) and HCO₃^- concentration were examined by clamping pH(i) using high K⁺ and nigericin. Increasing pH(i) from 6.9 to 7.6 in solutions without exogenous CO₂ or HCO₃^- increased K(Cl) by 71 ± 17 nm/s. These results suggest that pH(i) might regulate anion exchange. However, during such a pH(i)-shift experiment, metabolically derived CO₂ produces a concomitant change in intracellular HCO₃^- concentration ([HCO₃^-](i)). To determine whether an increase in [HCO₃^-](i) could stimulate Cl^- self exchange, we replaced HEPES with 6% CO₂-5 mM HCO₃^- isohydrically (pH(i) clamped at 6.9). With this increase in [HCO₃^-](i) at constant pH(i), K(Cl) increased by 51 ± 10 nm/s. These maneuvers had negligible effects on Cl^- diffusion and Cl^--HCO₃^- exchange. These experiments demonstrate that increases in cell [HCO₃^-] (or perhaps CO₂) can stimulate transepithelial anion exchange. Although pH(i) may also have an effect on the anion exchange process, under the present conditions, the HCO₃^- effect seems to be more potent and is evident at low concentrations of HCO₃^- and CO₂.