Volume expansion predominantly inhibits proximal reabsorption of NaCl rather than NaHCO3

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Abstract
The present study quantitated the effect of extracellular volume expansion on absolute proximal bicarbonate, chloride, and water reabsorption in 10 Sprague-Dawley rats. Paired free-flow micropuncture measurements were made in the plasma-replete euvolemic state and following 10% body wt isohydric, colloid-free volume expansion. Single nephron glomerular filtration rate and glomerular ultrafiltrate concentration of bicarbonate and chloride were similar in the two conditions. Volume expansion caused absolute proximal reabsorption of bicarbonate to fall by only 7% (from 987 +/- 39 to 920 +/- 53 pmol/min, P less than 0.05). This decrement in bicarbonate reabsorption was compatible with the known increase in bicarbonate permeability and backleak induced by expansion. In contrast, a marked 26% fall in absolute proximal chloride reabsorption occurred (from 1,601 +/- 147 to 1,180 +/- 149 peq/min, P less than 0.005), attributed to alteration in transcellular rather than passive sodium chloride transport. Thus, the decrease in absolute proximal volume reabsorption (from 19.7 +/- 1.0 to 16.7 +/- 1.2 nl/min, P less than 0.005) induced by volume expansion was predominantly a consequence of inhibition of sodium chloride rather than sodium bicarbonate reabsorption.

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