Mechanism of Cd-induced inhibition of Na-glucose cotransporter in rabbit proximal tubule cells: roles of luminal pH and membrane-bound carbonic anhydrase.

BACKGROUND/AIMS: We have previously reported that a complex of cadmium-metallothionein (Cd-MT) directly affects the apical Na-glucose cotransporter on the luminal side in proximal tubules, suggesting that Cd-MT is more toxic than CdCl(2) in causing tubulopathy. To find the potential mechanisms, we evaluated the effect of luminal pH alteration and carbonic anhydrase (CA) inhibition on Cd-MT-induced reduction of glucose-dependent transmural voltage in rabbit S2 segments perfused in vitro.

METHODS: Before and after the addition of Cd-MT (1 microg Cd/ml) to the lumen, the deflections of transmural voltage upon the elimination of glucose from the perfusate (DeltaVt(glu)) were measured as a parameter of activity of the Na-glucose cotransporter.

RESULTS: During perfusion with a control solution of pH 7.4, the DeltaVt(glu) significantly decreased after addition of Cd-MT for 10 min. A reduction in pH to 6.8 significantly shortened the time needed to reduce the DeltaVt(glu) to <5 min, whereas an increase of pH to 7.7 significantly prolonged the time to >20 min. Furthermore, simultaneous addition of acetazolamide with control perfusate prevented the reduction. P-Fluorobenzyl-aminobenzolamide (pFB-ABZ), a membrane-impermeable CA inhibitor, added to the lumen also completely prevented the reduction in DeltaVt(glu). In rabbits with chronic Cd exposure, acetazolamide prevented the glucosuria.

CONCLUSION: Cd-MT-induced inhibition of Na-glucose cotransporter activity in the S2 segment strongly depends on luminal pH, and that an increase in pH by inhibition of luminal membrane-bound CA is useful to prevent renal Cd toxicity.