Abstract
Multiple injections of nontoxic levels of cadmium to a rat result in much higher level of metallothionein (MT) production in the liver than does the single injection. In order to understand the underlying mechanisms we have quantitated and compared the metallothionein-specific messenger RNA contents in the livers following the two induction regimens. Cell-free translation assays coupled with specific immunoprecipitation of MT revealed that MT-mRNA activity in livers of animals multiply injected with Cd is 7- to 10-fold higher than that in livers 4 h after a single Cd-induction. By oligo(dT)-cellulose chromatography, sucrose density gradient centrifugation, and methylmercuric hydroxide-agarose gel electrophoresis this mRNA has been enriched approximately 100-fold from the total RNA. The size of the mRNA is about 400 nucleotides. Hybridization assays with a complementary DNA probe synthesized against the enriched MT-mRNA showed a 4-fold difference in the level of MT-mRNA between the two induction regimens in agreement with the results obtained by the cell-free translation assays. The possible mechanisms for these observations in consideration of the short lived nature of MT-mRNA are discussed.