The Journal of General Physiology, Vol 84, 753-770, Copyright © 1984 by The Rockefeller University Press
Bicarbonate-water interactions in the rat proximal convoluted tubule. An effect of volume flux on active proton secretion
RJ Alpern
The effect of volume absorption on bicarbonate absorption was examined in
the in vivo perfused rat proximal convoluted tubule. Volume absorption was
inhibited by isosmotic replacement of luminal NaCl with raffinose. In
tubules perfused with 25 mM bicarbonate, as raffinose was increased from 0
to 55 to 63 mM, volume absorption decreased from 2.18 +/- 0.10 to 0.30 +/-
0.18 to -0.66 +/- 0.30 nl/mm X min, respectively, and bicarbonate
absorption decreased from 131 +/- 5 to 106 +/- 8 to 91 +/- 13 pmol/mm X
min, respectively. This bicarbonate-water interaction could not be
attributed to dilutional changes in luminal or peritubular bulk phase
bicarbonate concentrations. Inhibition of active proton secretion by
acetazolamide abolished the effect of volume flow on bicarbonate
absorption, which implies that the bicarbonate reflection coefficient is
close to 1 and eliminates the possibility of solvent drag across the tight
junction. When the luminal bicarbonate concentration was varied, the
magnitude of the bicarbonate-water interaction increased with increasing
luminal bicarbonate concentration. The largest interaction occurred at high
luminal bicarbonate concentrations, where the rate of proton secretion has
been previously shown to be independent of luminal bicarbonate
concentration and pH. The results thus suggest that a peritubular and/or
cellular compartment exists that limits bicarbonate diffusion, and where pH
changes secondary to bicarbonate-water interactions (solute polarization)
alter the rate of active proton secretion.
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