Cadmium-Metallothionein nephropathy: relationships between ultrastructural/biochemical alterations and intracellular cadmium binding.

Cadmium metallothionein (CdMT) nephrotoxicity was studied in rats injected i.p. with a single nonlethal dose of CdMT (0.6 mg of Cd per kg). Within 8 hr of CdMT injection, urine volume and urine sodium excretion were increased and sodium dodecyl sulfate gel electrophoresis of urine proteins showed that elevated levels of low molecular weight proteins were present in the urines of CdMT-treated rats. Urine RNAase activity was also elevated, approximately 7-fold, by CdMT but not by zinc metallothionein (ZnMT) or lysozyme at equivalent protein doses, demonstrating that a proteinuria indicative of proximal tubule cell dysfunction develops as an early response to CdMT exposure. Ultrastructural alterations were also present in animals injected with CdMT but not ZnMT or lysozyme. The earliest alterations occurred in the lysosome compartment of the cell. By 1 hr, the number of small lysosomes in renal proximal convoluted tubule cells increased significantly with no changes in other organelle compartments. By 4 and 8 hr, there was a further increase in lysosome number with a concomitant decrease in size and a marked increase in the number of small clear apical vacuoles. Lysosomal cathepsin D activity was decreased at 4 and 8 hr after CdMT injection, and in vitro studies indicated that this effect was not due to a direct inhibition of the enzyme by Cd++ or CdMT. Thus, both lysosome size and protease activity were rapidly altered by CdMT exposure. Studies of Cd binding in the kidney suggest that non-MT-bound Cd is an important factor in CdMT-associated toxicity. Approximately 97% of the Cd present in the cytoplasm at 1 hr was non-MT-bound. Prior induction of renal MT by treatment with zinc (20 mg of Zn per kg as ZnSO4, i.p. 16 hr before CdMT injection) markedly reduced non-MT binding of Cd++ in kidneys of treated animals and inhibited the alterations in urine volume and low molecular weight protein reabsorption induced by CdMT. These data suggest that acute CdMT exposure provides an excellent system for studying the mechanism of cadmium tubular proteinuria and that the intracellular renal MT pool plays a key role in regulating this process.