Factors affecting proximal tubular acidification of non-bicarbonate buffer in the rat

The effect of peritubular PCO2 and pH changes within the physiological range on proximal tubular acidification of non-bicarbonate (phosphate) buffer was evaluated with and without carbonic anhydrase inhibition by stopped-flow microperfusion and Sb micro-electrode techniques. Luminal steady-state pH was reduced from 6.69 to 6.37 and H ion fluxes (JH+) increased from 0.63 to 1.57 nmol cm-2 s-1 by increasing capillary CO2 from 0 to 9.6% at pH 7.2. After acetazolamide a marked, although attenuated, effect of CO2 on acidification was still observed; JH+ increased from 0.088 nmol cm-2 s-1 at 0% CO2 to 0.78 at 9.6% CO2. Most of this effect can be explained by titration of luminal buffer by CO2, uncatalysed CO2 hydration and H2CO3 recirculation. An increase in capillary CO2 reduced acidification half-times (t/2), which, according to an analogue circuit model, may be due to increased H ion access to the pump. Peritubular pH changes at 0% CO2 also modified tubular acidification, increasing JH+ from 0.73 nmol cm-2 s-1 at pH 7.6 to 0.99 at pH 7.0. After acetazolamide, JH+ still increased from 0.11 nmol cm-2 s-1 at pH 7.6 to 0.57 at pH 7.0. In conclusion, both peritubular CO2 changes at constant pH and pH changes at 0% CO2 were effective to modify JH+, in the presence and absence of carbonic anhydrase activity. In the studied range, capillary CO2 induced larger changes in JH+ than pH. The data show substrate (H ion) is a limiting factor for tubular H ion secretion.