AJP - Renal Physiology, Vol 245, Issue 2 272-F275, Copyright © 1983 by American Physiological Society
Volume expansion predominantly inhibits proximal reabsorption of NaCl rather than NaHCO3
M. G. Cogan
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.
