H+ current response to CO2 and carbonic anhydrase inhibition in turtle bladder

To evaluate the role of CO2 and carbonic anhydrase (CA) in H+ transport (JH) by turtle urinary bladder the effect of CO2 addition, with and without addition of CA inhibitiors, was examined on JH. Since in the presence of exogenous CO2 and HCO3- the pH stat-measured rate of mucosal (M) acidification underestimates JH by the rate of electroneutral HCO3- secretion, the reverse short-circuit current (RSCC) applied across ouabain-treated bladders was used to estimate JH. That the RSCC is a measure of JH was demonstrated by: 1) in the absence of added CO2 and HCO3- the rate of M acidification totally accounted for the RSCC, and 2) increases in RSCC with CO2 addition occurred without changes in Na+ and K+ fluxes or the coupled ration of HCO3- secretion for Cl-absorption. When serosal (S) percent CO2 was progressively progressively increased JH achieved a maximum rate of 64 +/- 3 muA (SE) with 4.5% CO2. At higher S percent CO2 JH did not change, suggesting that factors other than the rate of CO2 hydration were rate limiting. The maximum rate of JH was not decreased by low concentrations of CA inhibitors (acetazolamide, 5 X 10(-5) M), although the percent CO2 at which this maximum rate occurred increased to 8.5%. The increased percent CO2 requirement for the maximum rate of JH with low concentrations of CA inhibitors suggests that these agents alter JH by decreasing the rate of enzymatic CO2 hydration. At high concentrations (acetazolamide, 5 X 10(-4) M) these inhibitors decrease the maximum rate of JH in the presence of CO2, implying that these inhibitors at higher concentrations directly interfere with the H+ transport system.