Cadmium accumulation and metallothionein concentrations after 4-week dietary exposure to cadmium chloride or cadmium-metallothionein in rats.

The distribution of cadmium was examined in rats fed diets containing either cadmium-metallothionein (CdMt) or cadmium chloride (CdCl2) for 4 weeks. The test diets contained 3, 10, or 30 mg Cd/kg diet (3, 10, or 30 ppm) as CdMt or 30 mg Cd/kg diet (30 ppm) as CdCl2. A second study was performed to establish the Cd content in liver and kidneys after exposure to low doses of both CdMt and CdCl2 (1.5 and 8 ppm Cd). The feeding of CdMt resulted in a dose- and time-dependent increase of the Cd concentration in liver, kidneys, and intestinal mucosa. Rats fed 30 ppm CdMt consistently showed less Cd accumulation in liver and intestinal mucosa than did rats fed 30 ppm CdCl2. However, renal accumulation in rats fed 30 ppm was similar until Day 28 regardless of Cd form. At lower dietary Cd levels (1.5 and 8 ppm), relatively more Cd is deposited in the kidneys, although even at these doses the kidney/liver ratio of Cd is still higher with CdMt than with CdCl2. Tissue metallothionein (Mt) levels in the intestinal mucosa were relatively constant but always higher after CdCl2 exposure than after CdMt exposure. Mt levels in both liver and kidney increased after CdCl2 or CdMt exposure during the course of study. Although Mt levels in liver were higher after CdCl2 intake (30 ppm) than after CdMt intake (30 ppm), renal Mt concentrations were the same for both groups. In fact on Day 7, CdMt administration resulted in slightly higher Mt levels than CdCl2 administration, suggesting a direct accumulation of exogenous CdMt in the kidneys. In conclusion, after oral exposure to CdMt in the diet there is a relatively higher Cd accumulation in the kidneys. However, the indirect renal accumulation via redistribution of Cd from the liver might be lower than after CdCl2 exposure. Which of these two phenomena is decisive in the eventual level of renal toxicity of Cd after long-term oral intake could determine the toxicological risk of the chronic intake of biologically incorporated Cd.