Abstract

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 (ΔV_t) and apical membrane voltage (ΔV_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 ΔV_t and ΔV_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 ΔV_a, whereas the baselineV_a and V_t were unchanged. The early effect of luminal Cd-MT was simulated by addition of 10⁻⁴ M phloretin. Addition of 10⁻⁴ 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.