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

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 ra bbit proximal straight tubules in vitro, we measured fluid and HCO3- absorp tion and cell pH before and after the extracellular CA inhibitor p-fluorobe nzyl-aminobenzolamide was applied in the bath to inhibit only basolateral C A. 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 absorpti on because albumin could affect CO2 diffusion and rheogenic HCO3- efflux. I ndeed, dextran in the bath stimulated fluid absorption by 55% over albumin. Basolateral CA inhibition reduced fluid absorption (similar to 30%) and ma rkedly decreased HCO3- absorption (similar to 60%), both reversible when CA was added to the bathing solution. In the presence of luminal CA inhibitio n, which reduced fluid (similar to 16%) and HCO3- (similar to 66%) absorpti on, inhibition of basolateral CA further decreased the absorption of fluid (to 74% of baseline) and HCO3- (to 22% of baseline). CA inhibition also alk alinized cell pH by similar to0.2 units, suggesting the presence of an alka line disequilibrium pH in the interspace, which would secondarily block HCO 3- exit from the cell and thereby decrease luminal proton secretion (HCO3- absorption). These data clearly indicate that basolateral CA has an importa nt role in mediating fluid and especially HCO3- absorption in the proximal straight tubule.