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

Abstract

Reabsorption of filtered HCO₃ ^- by the kidney is thought to be mediated by secretion of cellular H⁺(1,2), resulting in the following intratubular reaction:

Since dehydration of H₂CO₃ is not instantaneous, excess H₂CO₃ would accumulate in tubular fluid and would not be in equilibrium with plasma CO₂. Consequently, intratubular pH would be lower than that calculated by the Henderson-Hasselbalch equation using plasma pCO₂ and luminal concentration of HCO₃ ^-(3).

An opposing view postulates that HCO₃ ^- ions per se are reabsorbed directly (4). Were this the mechanism involved, excess H₂CO₃ would not be generated, H₂CO₃ and CO₂ would be in continuous equilibrium, and 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 below the calculated equilibrium pH of 7.8. These results afford the first direct evidence that HCO₃ ^- reabsorption in the proximal tubule is mediated via H⁺ secretion.

Methods. Sprague-Dawley rats were anesthetized and prepared for micropuncture as previously described (5). The exposed surface of the kidney was covered with Ringer's bicarbonate solution continuously equilibrated with 5% CO₂-95% O₂.

Single proximal tubules were perfused by first injecting a drop of silicone oil into the lumen through a micropipette. The oil drop was then split by injecting with a second micropipette a solution containing 90 mM NaCl and 60 mM NaHCO₃. The pH of this solution when equilibrated with 5% CO₂ was 7.8.

The in vivo intratubular pH was estimated by adding various acid-base indicators to the perfusion fluid in a concentration of 100 mg %. The color changes in the column of injected fluid were observed by direct microscopic visualization.