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 x 10(-3) unit/s but after perfusion with CO2/HCO3(-)-free solutions pHi of the same cells increased only by 1.3 +/- 0.2 x 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 x 10(-12) mol/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 (delta pHi) 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.