Vol. 292, Issue 2, 769-777, February 2000

Acute Effect of Cadmium-Metallothionein on Glucose and Amino Acid Transport across the Apical Membrane of the Rabbit Proximal Tubule Perfused In Vitro

Shuichi Tsuruoka, Koh-ichi Sugimoto, Shigeaki Muto, Kazuo Nomiyama, Akio Fujimura and Masashi Imai

Departments of Clinical Pharmacology (S.T., K.-I.S., A.F.), Nephrology (S.M.), Hygiene (K.N.), and Pharmacology (M.I.), Jichi Medical School, Tochigi, Japan.

Acute as well as chronic exposure of cadmium (Cd) leads to proximal tubule injury. The exact cellular mechanism of this disorder and whether there is a contribution of cadmium-metallothionein (Cd-MT), a binding protein of Cd, remain unclear. We perfused isolated S2 segments of rabbit nephron, and the deflections of transmural voltage (V_t) and apical membrane voltage (V_a) on elimination of glucose or alanine from the perfusate were measured for the parameters of activity of Na⁺-glucose and Na⁺-amino acid cotransporters. The effects of Cd-MT or CdCl₂ to either bath or lumen for 10 min on these parameters were examined. We also measured the lumen-to-bath [¹⁴C]glucose flux. Addition of Cd-MT to lumen suppressed glucose- or alanine-dependent V_t and V_a, as well as baseline V_t and basolateral membrane voltage (V_b), at approximately 10 min. [¹⁴C]glucose flux was inhibited by Cd-MT to lumen. The effects of Cd-MT to bath and CdCl₂ to either lumen or bath were 100-fold less potent than that of Cd-MT to lumen. Luminal Cd-MT immediately suppressed the glucose-dependent V_a, whereas the baseline V_a and V_t were unchanged. The early effect of luminal Cd-MT was simulated by addition of 10⁴ M phloretin. Addition of 10⁴ M ouabain to the bath simulated the later effect of Cd-MT. The protection of SH group by dithiothreitol prevented the early effect of Cd-MT, but not the later effect. We concluded that Cd-MT initially acts directly on Na⁺-glucose and Na⁺-amino acid cotransporters from the lumen by attacking SH group, followed by the later inhibition of Na⁺-K⁺-ATPase after entering the cell from the apical membrane.