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


CONCLUSIONS
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.
Department of Pharmacology, Jichi Medical University, Shimotsuke, Tochigi, Japan. tsuru@md.tsukuba.ac.jp
Shuichi
S
Tsuruoka
Erik R
ER
Swenson
Akio
A
Fujimura
Masashi
M
Imai
D000086:Acetazolamide / Q000494:pharmacology; D000818:Animals; D001639:Bicarbonates / Q000378:metabolism; D001693:Biological Transport, Active / Q000187:drug effects / Q000502:physiology; D019256:Cadmium Chloride / Q000633:toxicity*; D002257:Carbonic Anhydrase Inhibitors / Q000494:pharmacology; D002256:Carbonic Anhydrases / Q000187:drug effects / Q000494:pharmacology / Q000502:physiology*; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism*; D003404:Creatinine / Q000097:blood; D004305:Dose-Response Relationship, Drug; D004594:Electrophysiology; D005260:Female; D005947:Glucose / Q000378:metabolism / Q000494:pharmacology; D006029:Glycosuria / Q000139:chemically induced / Q000517:prevention & control; D006863:Hydrogen-Ion Concentration; D007267:Injections; D007687:Kidney Tubules, Proximal / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D008668:Metallothionein / Q000378:metabolism* / Q000633:toxicity; D009924:Organ Culture Techniques; D011817:Rabbits; D051247:Sodium-Glucose Transport Proteins / Q000037:antagonists & inhibitors* / Q000378:metabolism; D013997:Time Factors; D018675:Toxicity Tests
D001639:Bicarbonates; D002257:Carbonic Anhydrase Inhibitors; D051247:Sodium-Glucose Transport Proteins; C116509:cadmium-metallothionein complex; D008668:Metallothionein; D003404:Creatinine; D002256:Carbonic Anhydrases; D005947:Glucose; D019256:Cadmium Chloride; D000086:Acetazolamide