CO2 along the proximal tubules in the rat kidney

The proximal intratubular pH of the rat kidney was measured in vivo with an antimony electrode system. PCO2 and bicarbonate concentration of the proximal tubular fluid were determined with an ultramicro equilibration system. The tubular fluid to plasma inulin concentration ratio was evaluated by a microscope fluorometric method. The acid-base parameters and the inulin concentrations were determined under control conditions and during acetazolamide treatment. The intratubular PCO2 was higher than the PCO2 of the systemic arterial blood under control conditions and the difference in PCO2 was increased during acetazolamide treatment. In acetazolamide treated rats the rate of fractional bicarbonate reabsorption was decreased in the early part of the proximal tubule, while it was of about the same in the middle and late parts as compared with control rats. The total bicarbonate reabsorption in the proximal tubule was reduced by 50% due to the carbonic anhydrase inhibition. It seems possible that the bicarbonate is still reabsorbed as CO2 after carbonic anhydrase inhibition, as hydrogen ion secretion is not totally stopped by this treatment. The increase in intratubular PCO2 after acetazolamide treatment is assumed to be due to an inhibition of the carbonic anhydrase facilitating effect on outward diffusion of CO2 from the tubular lumen across the cell wall.