Effect of luminal bicarbonate concentration on proximal acidification in the rat.

The effect of luminal bicarbonate concentration on proximal tubular acidification was studied. Rat proximal convoluted tubules were perfused in vivo with solutions of varying bicarbonate concentration, and bicarbonate absorption was measured using microcalorimetry. Bicarbonate absorption was found to increase linearly with mean luminal bicarbonate concentrations up to 45 mM, but above this level it showed evidence of partial saturation. Bicarbonate permeability was measured and found to be 2.6 +/- 0.3 x 10(-7) cm2/s. Using this permeability, net bicarbonate absorption could be divided into two parallel components, both sensitive to luminal bicarbonate concentration: 1) proton secretion and 2) a passive bicarbonate leak. Proton secretion, when examined as a function of luminal bicarbonate concentration, exhibited saturation kinetics with an apparent Km of 16 mM and a Vmax of 200 pmol . mm-1 . min-1.