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.