THE CHLORIDE BASE EXCHANGER IN THE BASOLATERAL CELL-MEMBRANE OF RABBIT RENAL PROXIMAL TUBULE S3 SEGMENT REQUIRES BICARBONATE TO OPERATE

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 (pHi) was measured in response to sudden substitution of bath Cl- by gluconate. In control solutions containing 25 mmol/l HCO3 pHi increased initially by 5.0±0.3 × 10-3 unit/s but after perfusion with CO2/HCO3--free solutions pHi 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 (ΔpHi) 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. © 1990 Springer-Verlag.