On the role of metallothionein in cadmium absorption by rat jejunum in situ.

The role of metallothionein (MT) in the mechanism of cadmium absorption from the jejunum was studied in 7--9 week-old-male rats exposed to 50 ppm of cadmium in drinking water for 9 days. Exposed animals contained an average of 144 micrograms MT/g of mucosal tissue, compared to 40 micrograms in control anaimls. During jejunal perfusion in situ with 5 mM glucose-saline containing 10--20 nM CdCl2 the increased MT content of mucosa exerted no effect either on cadmium absorption from the lumen (step I), or on its further transport into the body (step II). Immediately after perfusion, essentially all cadmium removed from the lumen was fully recovered in the intestinal mucosa. About 50% of the mucosal cadmium was found in the sediment after homogenization and centrifugation; a large portion of this cadmium may be assigned to the membrane fraction. The binding of freshly absorbed cadmium in the mucosal cytosol was not restricted to low molecular weight protein, although cadmium binding capacity in the MT fraction of controls as well as of exposed animals greatly exceeded actual binding of newly absorbed cadmium. Our results offer no support for the view that MT in the jejunal mucosa serves as determinant of cadmium absorption.