The chloride/base exchanger in the basolateral cell membrane of rabbit renal proximal tubule S3 segment requires bicarbonate to operate

Abstract

Isolated microperfused S3 segments of rabbit renal proximal tubule were investigated with pH-sensitive double-barrelled intracellular microelectrodes to determine whether the Cl⁻/base exchanger, which we have previously identified in the basolateral cell membrane of this segment requires HCO₃ ⁻ or can also work in CO₂/HCO₃ ⁻ free conditions. Cell pH (pH_i) was measured in response to sudden substitution of bath Cl⁻ by gluconate. In control solutions containing 25 mmol/l HCO₃ pH_i increased initially by 5.0±0.3 × 10⁻³ unit/s but after perfusion with CO₂/HCO₃ ⁻-free solutions pH_i of the same cells increased only by 1.3±0.2 × 10⁻³ unit/s in response to Cl⁻ substitution. From measurements of the cellular buffering power it was calculated that the control base flux had fallen drastically from 3.7±0.3 to 0.3±0.1 × 10⁻¹² mols/s·cm tubule length. To test whether the remaining flux might have resulted from metabolic CO₂, oxidative metabolism was poisoned with cyanide (5 mmol/l). This abolished the pH change (ΔpH_i) in CO₂/HCO₃ ⁻-free solutions, but did not affect the pH shift in the presence of HCO₃ ⁻. The data indicate that basolateral Cl⁻/base exchange in S3 segment requires HCO₃ ⁻ to operate. A model in which HCO₃ ⁻ absorption proceeds in form of OH⁻ and CO₂ can be largely excluded.