Acetazolamide inhibits acidification by the turtle bladder independent of cell pH

Acetazolamide (ACZL) inhibits luminal acidification by the turtle urinary bladder, a process thought to be mediated by the drug's ability to inhibit carbonic anhydrase (CA) and thus elevate cell pH. To test the hypothesis that these transport changes are actually mediated by changes of cell pH, we measured this parameter in single, identifiable mucosal cells using 4-methylumbelliferone and fluorescence microscopy. In control bladders 5 X 10(-4) M ACZL inhibited proton transport by 80 +/- 6%, and alkalinized cell pH, especially in a subpopulation of CA cells. A much larger cell alkalinization was induced by serosal HCO3- but proton transport fell only 30 +/- 7%. When cell pH was clamped at approximately 7.0 using 50 mM dimethyloxazolidinedione, or when cell pH was acidified using 7.5 mM propionate, transport rates still declined by 74 +/- 2, and 100 +/- 12%, respectively, in response to ACZL. In propionate-acidified bladders, 1 mM sodium azide blocked the inhibition of transport seen with 5 X 10(-4) M ACZL and reversed the inhibition with 10(-5) M ACZL. The apical endocytosis rate was increased by ACZL in normal and propionate-acidified bladders, but was not stimulated by alkalinizing the cell with NH4Cl. We conclude that ACZL can induce cellular alkalinization in this tissue, but that this pH change is not required for the inhibition of transport, or the ACZL-associated stimulation of endocytosis. The drug's ability to inhibit acidification appears to be the result of an azide-sensitive mechanism that has yet to be defined.