Acetazolamide inhibition of basolateral Cl⁻/HCO ⁻₃ exchange in rabbit renal proximal tubule S3 segment

Abstract

Cell pH (pH_i) and cell membrane potential (V _b) were measured in isolated S3 segments of rabbit renal proximal tubule with double-barrelled microelectrodes to search for a possible effect of the carbonic anhydrase inhibitor, acetazolamide (ACZ), on Cl⁻/HCO ⁻₃ exchange in the basolateral cell membrane. ACZ was found to retard and reduce the pH_i response to bath Cl⁻ removal reversibly with half-maximal inhibition at 0.42 mmol/l and a rather flat concentration dependence (Hill coefficient ≈ 0.36). To determine whether the retardation resulted from inhibition of cytoplasmic carbonic anhydrase, which might have delayed the attainment of HCO ⁻₃ /CO₂ equilibrium, we have measured the response of pH_i to step changes in PCO₂ in the presence and absence of ACZ. ACZ greatly retarded the pH_i response to CO₂ steps; however, the concentration dependence differed (half-maximal inhibition at 18 μmol/l) and even at maximal ACZ concentrations the response to CO₂ steps was more than twice as fast as the response to Cl⁻ replacement. Since, in addition, the ACZ inhibition of Cl⁻/HCO ⁻₃ exchange could not be overcome by increasing PCO₂ we conclude that the ACZ effect on Cl⁻/HCO ⁻₃ exchange in rabbit proximal tubule S3 segments does not result from inhibition of cytosolic or membrane-bound carbonic anhydrase, but from a direct interaction with the exchanger molecule.