Direct evaluation of acidification by rat proximal tubule: role of carbonic anhydrase

Abstract

Luminal acetazolamide inhibits bicarbonate reabsorption by > 85%. The mechanism by which this large inhibition occurs assumes importance, since cytoplasmic and luminal carbonic anhydrase play a central role in bicarbonate reabsorption. To examine this mechanism and the role of these enzymes, intraluminal pH was directly measured while rat proximal tubules were perfused in vivo with artificial ultrafiltrate containing 1 of 3 carbonic anhydrase inhibitors, which were selected because of their varying cell permeabilities (benzolamide, low permeability; acetazolamide and methazolamide, high permeability). Each of the 3 drugs inhibited bicarbonate reabsorption by > 90%. With respect to intraluminal pH the drug effects differed. During benzolamide perfusion a significant decrease in pH in the loop nearest the perfusion pipette was observed (from 7.19-6.68), and the pH remained acid along the entire tubule length. Neither acetazolamide nor methazolamide caused an acid pH early in the tubule, although pH decreased with increasing tubule length despite minimal bicarbonate reabsorption. The PCO2 in the tubules perfused with inhibitors did not differ from the surrounding cortex, demonstrating that H2CO3 not CO2 is the source of the acid luminal pH. H+ secretion may be a mechanism of proximal tubule acidification. Benzolamide apparently completely inhibits only luminal carbonic anhydrase, whereas methazolamide and acetazolamide inhibit both luminal and cytoplasmic forms.