Role of basolateral carbonic anhydrase in proximal tubular fluid and bicarbonate absorption.

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Membrane-bound carbonic anhydrase (CA) is critical to renal acidification. The role of CA activity on the basolateral membrane of the proximal tubule has not been defined clearly. To investigate this issue in microperfused rabbit proximal straight tubules in vitro, we measured fluid and HCO(3)(-) absorption and cell pH before and after the extracellular CA inhibitor p-fluorobenzyl-aminobenzolamide was applied in the bath to inhibit only basolateral CA. This inhibitor was 1% as permeant as acetazolamide. Neutral dextran (2 g/dl, molecular mass 70,000) was used as a colloid to support fluid absorption because albumin could affect CO(2) diffusion and rheogenic HCO(3)(-) efflux. Indeed, dextran in the bath stimulated fluid absorption by 55% over albumin. Basolateral CA inhibition reduced fluid absorption ( approximately 30%) and markedly decreased HCO(3)(-) absorption ( approximately 60%), both reversible when CA was added to the bathing solution. In the presence of luminal CA inhibition, which reduced fluid ( approximately 16%) and HCO(3)(-) ( approximately 66%) absorption, inhibition of basolateral CA further decreased the absorption of fluid (to 74% of baseline) and HCO(3)(-) (to 22% of baseline). CA inhibition also alkalinized cell pH by approximately 0.2 units, suggesting the presence of an alkaline disequilibrium pH in the interspace, which would secondarily block HCO(3)(-) exit from the cell and thereby decrease luminal proton secretion (HCO(3)(-) absorption). These data clearly indicate that basolateral CA has an important role in mediating fluid and especially HCO(3)(-) absorption in the proximal straight tubule.