Effect of luminal pH and HCO3- on phosphate reabsorption in the rabbit proximal convoluted tubule

Luminal pH in the proximal convoluted tubule may alter phosphate reabsorption in a variety of acid-base disturbances and in normal conditions as luminal pH decreases along the length of the proximal tubule. These studies address the influence of luminal pH on phosphate reabsorption in isolated perfused proximal convoluted tubules from normal rabbits. Initial perfusates were either pH 6.2 or 7.4, and the bath pH was 7.4. All solutions contained 10 mM total phosphate. The first experiments used HCO3-/CO2-free solutions to examine the effect of luminal pH independent of changes in [HCO3-] or Pco2. Lumen-to-bath phosphate flux increased from 0.63 +/- 0.23 with alkaline perfusate to 2.04 +/- 0.35 pmol X mm-1 X min-1 with acid perfusate. In a separate group of tubules, acetazolamide had no qualitative effect on this result. With HCO3-/CO2-containing solutions, phosphate reabsorption increased from 4.53 +/- 1.46 with alkaline perfusate to 9.67 +/- 1.77 pmol X mm-1 X min-1 with acid perfusate. Thus, an acid luminal fluid can enhance proximal phosphate reabsorption independent of the presence or absence of HCO3-/CO2. To examine the specificity of this effect, the influence of luminal pH on another solute (glucose), reabsorbed via a Na+-coupled mechanism, was studied. Lumen-to-bath glucose flux increased in the same direction: from 52.78 +/- 4.91 with alkaline perfusate to 57.13 +/- 4.70 pmol X mm-1 X min-1 with acid perfusate. The mechanism of the influence of luminal pH on phosphate and glucose reabsorption is not explained but could be direct or indirect from changes in intracellular pH, Na+ activity, metabolism, or basolateral transport. Since an acid luminal pH is expected to inhibit or decrease Na+-H+ exchange at the luminal membrane, these results are also consistent with a competition for the available Na+ gradient between phosphate and glucose transport and the Na+-H+ exchanger in the proximal tubule.