The role of bicarbonate in proximal tubular sodium chloride transport

The role of bicarbonate in proximal tubular sodium chloride transport. Microperfusion studies of single proximal tubules in rat kidney cortex slices show a progressive increase in the rate of reabsorption of isosmotic fluid and NaCl as the bathing medium bicarbonate concentration is raised from 0 to 12 mEq/liter while the pH is maintained at 7.3 to 7.4. When bathing medium bicarbonate was 25 mEq/liter, tubules maintained a significantly lower intraluminal NaHCO3 concentration: 15 mEq/liter. At the same time luminal NaCl concentration was equivalently elevated above that of the bathing medium. Reabsorptive rates were decreased when acetazolamide was added either to the bathing medium or to the tubular perfusate. Intratubular carbonic anhydrase reversed the effects of acetazolamide. Active transport of NaHCO3 stimulates NaCl reabsorption by generating and maintaining concentration gradients favoring passive reabsorption of this salt. The proximal tubule also possesses mechanisms for active NaCl reabsorption