Potential mechanisms involved in the absorptive transport of cadmium in isolated perfused rabbit renal proximal tubules

a b s t r a c t Lumen-to-cell transport, cellular accumulation, and toxicity of cadmium as ionic cadmium (Cd 2+ ) or as the l-cysteine (Cys) or d,l-homocysteine (Hcy) S-conjugate of cadmium (Cys-S-Cd-S-Cys, Hcy-S-Cd-S-Hcy) were studied in isolated, perfused rabbit proximal tubular segments. When Cd 2+ (0.73 M) or Cys-S-Cd-S-Cys (0.73 M) was perfused through the lumen of S 2 segments of the proximal tubule, no visual evidence of cellular pathological changes was detected during 30 min of study. Cd 2+ -transport was temperature-dependent and was inhibited by Fe 2+ , Zn 2+ , and elevated concentrations of Ca 2+ . Luminal uptake of Cys-S-Cd-S-Cys was also temperature-dependent and was inhibited by the amino acids lcystine and l-arginine, while stimulated by l-methionine. Neither l-aspartate, l-glutamate, the synthetic dipeptide, Gly-Sar nor Zn 2+ had any effect on the rate of Cys-S-Cd-S-Cys transport. Conclusions: When delivered to the luminal compartment, Cd 2+ appears to be capable of utilizing certain transporter(s) of Zn 2+ and some transport systems sensitive to Ca 2+ and Fe 2+ . In addition, Cys-S-Cd-S-Cys and Hcy-SCd-S-Hcy appear to be transportable substrates of one or more amino acid transporters participating in luminal absorption of the amino acid l-cystine (such as system b 0,+ ). These findings indicate that multiple mechanisms could be involved in the luminal absorption of cadmium (Cd) in proximal tubular segments depending on its form. These findings provide a focus for future studies of Cd absorption in the proximal tubule

Amino acid transporters involved in luminal transport of mercuric conjugates of cysteine in rabbit proximal tubule.

ABSTRACT The primary aim of the present study was to test the hypothesis that amino acid transport systems are involved in absorptive transport of dicysteinylmercury (cysteine-Hg-cysteine). Luminal disappearance flux [J D , fmol min Ϫ1 (mm tubular length) Ϫ1 ] of inorganic mercury (Hg 2ϩ ), in the form of dicysteinylmercury, was measured in isolated perfused S 2 segments with various amino acids or amino acid analogs in the luminal compartment under one of two conditions, in the presence or absence of Na ϩ . The control perfusion fluid contained 20 M dicysteinylmercury. Replacing Na ϩ in both the bathing and perfusing solutions with N-methyl-D-glucamine reduced the J D of Hg 2ϩ by about 40%. Nine amino acids and two amino acid analogs were coperfused individually (at millimolar concentrations) with dicysteinylmercury. The amino acids and amino acid analogs that had the greatest effect on the J D of Hg 2ϩ were L-cystine, L-serine, Lhistidine, L-tryptophan, and 2-(Ϫ)-endoamino-bicycloheptane-2-carboxylic acid. The greatest reduction (76%) in the total J D of Hg 2ϩ occurred when L-cystine was coperfused with dicysteinylmercury in the presence of Na ϩ . Overall, the current findings indicate that Hg 2ϩ is transported from the lumen into proximal tubular epithelial cells via amino acid transporters that recognize dicysteinylmercury. In addition, the data indicate that multiple amino acid transporters are involved in the luminal uptake of dicysteinylmercury, including the Na ϩ -dependent low-affinity L-cystine, B 0 , and ASC systems and the Na ϩ -independent L-system. Furthermore, the transport data obtained when L-cystine was added to the luminal fluid indicate strongly that dicysteinylmercury is likely transported as a molecular homolog of L-cystine