Acetazolamide inhibition of basolateral base exit in rabbit renal proximal tubule S2 segment

The influence of the carbonic anhydrase inhibitor acetazolamide (ACZ) was investigated on HCO₃^–transport mechanisms in the basolateral cell membrane of rabbit renal proximal tubule. Experiments were performed on isolated S2 segments using double-barrelled microelectrodes to measure cell membrane potential (V_b) and cell pH (pH_i) during step changes in bath perfusate ion concentrations. Peritubular application of ACZ (1 mmol/l) reduced the initial V_b response to 101 reduction of bath HCO₃^–concentration only slightly, from +53.8±4.2 mV to+49.1±0.3 mV (n=5), but caused an intermittent overshooting repolarization in the secondary V_b response. In conjunction with these effects it left the initial pH_i response virtually unchanged but induced a secondary slow acidification. These observation indicate that — under the present experimental conditions — ACZ does not block the Na⁺-HCO₃^–cotransporter but acts via inhibition of cytosolic carbonic anhydrase. This was confirmed by studying the effect of elevated intracellular HCO₃^–concentrations under reduced flux conditions and by comparing the concentration dependence of the V_b response with the inhibition kinetics of cytosolic carbonic anhydrase. In contrast, peritubular ACZ inhibited Na⁺-independent Cl^–/HCO₃^–exchange in the basolateral cell membrane of S2 segments directly in a similar way to that described in the preceding publication for S3 segments.