The involvement of metallothionein in the intestinal absorption of cadmium in mice.

Female mice were exposed to a single dose of 0.005 to 5 microg Cd/kg body wt., in order to test the hypothesis that once the Cd-binding capacity of intestinal metallothionein is saturated. Cd becomes more readily available for transfer from the mucosa to the circulatory system, causing an increase in Cd absorption. In this case the binding of Cd to MT would act as a barrier against Cd absorption, thus protecting the organism from accumulation and toxic effects of Cd in target organs such as the kidney. In mice the fractional Cd uptake (% of dose) in the duodenum, which was the main site of Cd uptake in the intestine, was not influenced by the Cd dose 6 h after dosage. However, the percentage of cytosolic Cd associated to MT in the duodenum decreased when the Cd dosage increased from 0.005 or 0.025 microg/kg to 5 microg/kg. Concomitantly, the percentage bound to low-molecular-weight (LMW) ligands increased, indicating saturation of the Cd-binding capacity of MT. Nevertheless, the fractional absorption was not dosage dependent in the dosage interval studied. Moreover, there was no statistically significant correlation between the percentage of cytosolic Cd bound to MT and the percentage of Cd absorbed. Thus, our results do not support the hypothesis that the intestinal Cd absorption is increased when the Cd-binding capacity of intestinal MT is saturated.