Bicarbonate-water interactions in the rat proximal convoluted tubule. An effect of volume flux on active proton secretion.

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Abstract
The effect of volume absorption on HCO3- 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 HCO3-, as raffinose was increased from 0 to 55 to 63 mM, volume absorption decreased from 2.18 .+-. 0.01-0.30 .+-. 0.18-066 .+-. 0.30 nl/mm .cntdot. min, respectively, and HCO3- absorption decreased from 131 .+-. 5 to 106 .+-. 8 to 91 .+-. 13 pmol/mm .cntdot. min, respectively. This HCO3--water interaction could not be attributed to dilutional changes in luminal or peritubular bulk phase HCO3- concentrations. Inhibition of active proton secretion by acetazolamide abolished the effect of volume flow on HCO3- absorption, which implies that the HCO3- 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 HCO3--water interaction increased with increasing luminal HCO3- concentration. The largest interaction occurred at high luminal HCO3- concentrations, where the rate of proton secretion was previously shown to be independent of luminal HCO3- concentration and pH. Apparently, a peritubular and/or cellular compartment exists that limits HCO3- diffusion, and where pH changes secondary to HCO3-water interactions (solute polarization) alter the rate of active proton secretion.