Function of proximal tubule carbonic anhydrase defined by selective inhibition

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Abstract
The specific role of luminal carbonic anhydrase in bicarbonate reabsorption by the proximal tubule has not been established because it has been difficult to inhibit selectively the luminal enzyme without simultaneous inhibition of the cytoplasmic enzyme. The present experiments employed in vivo microperfusion, microcalorimetry, and microelectrode techniques to determine the effects of luminal application of a dextran-bound carbonic anhydrase inhibitor (DBI) on bicarbonate reabsorptive rate (JtCO2) and intraluminal pH in the rat proximal convoluted tubule. Tubules were perfused at 20 nl/min with an artificial ultrafiltrate. Aminoethyl dextran (AED) with no enzyme-inhibitor activity was added to the control perfusate, and the effects of the parent inhibitor STZ [2-succinylamido-1,3,4-thiadiazole-5-sulfonamide] not bound to dextran were also determined. Both DBI and STZ significantly reduced JtCO2 from 138 .+-. 10 pmol .cntdot. mm-1 .cntdot. min-1 (control) to 30 .+-. 4 and 30 .+-. 9, respectively. In contrast to the indistinguishable effects on JtCO2, intraluminal pH measured close to the site of perfusion was 6.80 .+-. 0.02 during DBI perfusion, whereas with STZ perfusion the pH was 7.24 .+-. 0.04 (P < 0.001). Using the collected perfusate total CO2 concentration and a renal cortical PCO2 [partial pressure of CO2] of 60 mm Hg, the calculated equilibrium pH for this solution was 7.27. DBI inhibited only luminal carbonic anhydrase. Luminal carbonic anhydrase is in functional contact with proximal tubule fluid and is necessary for at least 80% of bicarbonate reabsorption by this segment. In the presence of DBI, an acid disequilibrium pH is generated as a result of continued H+ secretion from a cellular H+ source.