Mechanisms for Cadmium Lumen-to-Cell Transport by the Luminal Membrane of the Rabbit Proximal Tubule

The lumen-to-cell transport, cellular accumulation, and toxicity of ionic cadmium (Cd) and cadmium-cysteine conjugate (Cys-S-Cd-S-Cys) were studied in isolated perfused S2 segments of the proximal tubule of the rabbit kidney. All perfusion solutions were HEPES buffered and contained H-L-glucose which functioned as a volume and leak marker along with 250 nM FD & C Green dye as a vital dye. When ionic cadmium, 0.73μM Cd, or 0.73μM cadmium-cysteine conjugate (Cys-S-Cd-S-Cys) containing solution was perfused through the lumen of the tubule there was no visual evidence of toxicity such as blebbing of the luminal membrane, cellular vital dye uptake, and cellular swelling. Ionic Cd transport was temperature dependent (87% reduction at 22°C and 100% at 11°C) and inhibited by FeCl2 (42% reduction at 10μM) and ZnCl2 (48% reduction at 20μM), and high Ca concentrations (27% reduction at 1.95mM and 69% at 2.6mM). The ionic Cd transport was not affected by verapamil and diltiazem. The cadmium conjugate (Cys-S-Cd-S-Cys) transport was also temperature dependent (76% reduction at 22°C and 100% at 11°C) and inhibited by the amino acids L-cystine and Larginine (55% and 50% respectively), stimulated by L-methionine (56%), but not affected by L-aspartate, L-glutamate and Gly-Sar. 2, 3-Dimercaptopropane-1-Sulfonate (DMPS) co-perfused with Cd decreased absorption of 20μM Cd (39% reduction at 30 μM and 94.6% reduction at 200 μM), while DMPS added to the bathing solution has no effect on the luminal transport of Cd. DMPS co-perfused with 20 μM Cys-S-Cd-S-Cys substantially reduced Cd transport (62% reduction at 30 μM). We conclude that cadmium can be transported at the luminal membrane of the S2 segment of the proximal tubule by multiple mechanisms, depending on the form which it is presented to membrane. Ionic cadmium appears to be transported by iron (DCT1), zinc (ZTL1) transporters and some kind of calcium-selective channel while cadmium conjugate of Lcysteine appears to be transported by L-cystine transporters (system b). Dipeptide transporter is not involved in the transport of cadmium. DMPS appears to be a chelator for cadmium. INDEX WORDS: Cadmium, Luminal Membrane, Transport, Proximal Tubule, Kidney, Iron, Calcium, Zinc, Sulfhydryl-Containing Amino Acids, Cysteine, Cystine, Dipeptide, DMPS, Isolated Perfused Tubule MECHANISMS FOR CADMIUM LUMEN-TO-CELL TRANSPORT BY THE LUMINAL MEMBRANE OF THE RABBIT PROXIMAL TUBULE