Endogenous metallothionein as determinant of intestinal cadmium absorption: a reevaluation

Isolated segments of rat jejunum were perfused in situ with glucosesaline containing varying amounts of CdC12. At low Cd concentrations the endogenous Cd-binding capacity sufficed to trap almost all Cd taken up in the mucosa. At higher Cd levels, induction of metallothionein synthesis by Zn increased fractional trapping of Cd. Pretreatment with Zn did not induce protein synthesis in any fraction other than the low molecular weight heatstable cytosol fraction. Acutely Zn did not affect Cd extrusion from the mucosa. Finally, Cd-metallothionein itself crosses the mucosa only very slowly. An earlier conclusion that metallothionein does not contribute to Cd trapping was based on low Cd concentrations in the perfusate; at higher concentrations metallothionein appears to be a major determinant of mucosal Cd retention.