Evidence for a Disequilibrium pH in the Proximal Tubule of Rat Kidney.

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Abstract
HCO3 reabsorption by the kidney is thought to be mediated by secretion of H+, which would react with filtered HCO3" to form H2CO3. Excess H2CO3 would therefore accumulate and intratubular pH would be lower than that calculated by the Henderson-Hasselbalch equation using plasma pCO2 and lumi-nal concentration of HCO3". An opposing view which postulates that HCO3 ions are reabsorbed directly would not involve excess H2CO3 formation. Therefore the intratubular pH would be equal to that predicted by the Henderson-Hasselbalch equation. In the present experiments the in vivo intratubular pH of single proximal tubules perfused with a solution containing 60 mM NaHCO3 was 6.3, or 1.5 pH units be-low the calculated equilibrium pH of 7.8. This pH difference was obliterated by adding carbonic anhydrase to the perfusion solution, proving that it was due to excess H2CO3. These results afford the first direct evidence that HCO3 reabsorption in the proximal tubule is mediated via H+ secretion.

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