An apical K⁺-dependent HC0₃^- transport pathway opposes transepithelial HC0₃^- absorption in rat medullary thick ascending limb

Absorption of HCO₃^- in the medullary thick ascending limb (MTAL) is mediated by apical membrane Na⁺/H⁺ exchange. The identity and function of other apical acid-base transporters in this segment have not been defined. The present study was designed to examine apical membrane HCO₃^-/OH^-/H⁺ transport pathways in the rat MTAL and to determine their role in transepithelial HCO₃^- absorption. MTALs were perfused in vitro in Na⁺- and Cl^--free solutions containing 25 mM HCO₃^-, 5% CO₂. Lumen addition of either 120 mM Cl^- or 50 mM Na⁺ (50 μM EIPA present) had no effect on intracellular pH (pH_i). Lumen Cl^- addition also had no effect on pH_i in the presence of 145 mM Na⁺ or in the nominal absence of HCO₃^-/CO₂. Thus there was no evidence for apical Cl^-/HCO₃^- (OH^-) exchange, Na⁺-dependent Cl^-/HCO₃^- exchange, or Na⁺-HCO₃^- cotransport. In contrast, in tubules studied in Na⁺- and Cl^--free solutions containing 25 mM HCO₃^-, 5% CO₂ and 120 mM K⁺, removal of luminal K⁺ induced a rapid and pronounced decrease in pH_i (ΔpH_i = 0.56 ± 0.06 pH U). pH_i recovered following lumen K⁺ readdition. The initial rate of net base efflux induced by lumen K⁺ removal was decreased 85% at the same pHi in the nominal absence of HCO₃ ^-/CO₂, indicating a dependence on HCO₃ ^-/CO₂ and arguing against apical K⁺/H ⁺ exchange. A combination of the apical K⁺ channel blockers quinidine (0.1 mM) and glybenclamide (0.25 mM) had no effect on the lumen K⁺-induced pH_i changes, arguing against electrically coupled K⁺ and HCO₃^- conductances. The effect of lumen K⁺ on pH_i was inhibited by 1 mM H ₂DIDS. In addition, lumen addition of DIDS increased transepithelial HCO₃^- absorption from 10.7 ± 0.7 to 14.9 ± 0.7 pmol·min^-1·mm^-1 (P < 0.001) and increased pH_i slightly in MTAL studied in physiological solutions (25 mM HCO₃^- and 4 mM K⁺). Lumen DIDS stimulated HCO₃^- absorption in the absence and presence of furosemide. These results are consistent with an apical membrane K ⁺-dependent HCO₃^- transport pathway that mediates coupled transfer of K⁺ and HCO₃^- from cell to lumen in the MTAL. This mechanism, possibly an apical K ⁺-HCO₃^- cotransporter, functions in parallel with apical Na⁺/H⁺ exchange and opposes transepithelial HCO₃^- absorption.