SMALL-INTESTINAL ABSORPTION OF CADMIUM AND THE SIGNIFICANCE OF MUCOSAL METALLOTHIONEIN

1 Although food intake is among the most important routes of Cd exposu re, not many details are known about the intestinal absorption mechani sms of Cd. In this respect Cd is representative of most other nonessen tial, merely toxic metals. 2 Based on a concept of two distinguishable steps, intestinal absorption of Cd is characterized by high accumulat ion within the intestinal mucosa and a low rate of diffusive transfer into the organism. 3 After uptake into the mammalian organism, Cd is s equestered into hepatic metallothionein (MT). It is assumed that hepat ic Cd-MT then gradually redistributes Cd to the kidney, which is the m ain target organ for chronic Cd toxicity. 4 When feeding low levels of dietary CdCl2, however, Cd accumulates preferentially in the kidney a nd to a lesser degree in the liver, a distribution pattern also found after intravenous and peroral administration of the Cd-MT complex itse lf. As dietary Cd induces intestinal MT, intestinal Cd-MT complexes co uld be at least partly responsible for the renal accumulation of dieta ry Cd. 5 For this mechanism, however, serosal release of mucosal Cd-MT is required. In fact, in vitro findings in rats reveal a concentratio n-dependent release of intestinal MT to the serosal side of the small intestine. These results indicate that endogenous intestinal MT may de liver Cd-MT to other inner organs, thus contributing to the preferenti al renal accumulation of ingested Cd.