Ontogeny and induction of hepatic isometallothioneins in immature rats

It is well established that hepatic concentrations of metallothioneins (MTs) are very high in developing rats. In this study, the ontogeny of hepatic isometallothioneins (MT-I and MT-II) was determined. MTs were separated and quantitated with a high-performance liquid chromatography (HPLC) method. MT-I and MT-II, first detected in fetal liver on gestation Day 18, increased coordinately during late gestation and reached maximum hepatic concentrations of approximately 400 and 500 micrograms/g, respectively, which were maintained from postpartum Days 1 through 7. Thereafter, MT concentrations decreased, reaching concentrations similar to adult levels (approximately 10 micrograms/g liver) between 28 and 35 days of age. MT-II was significantly higher than MT-I in 14- and 21-day-old rats. In 35-day-old rats, MT-I was below the detection limit. MT-I and MT-II mRNA levels were quantitated by Northern blot hybridization with mouse cRNA probes. In contrast to protein concentrations, MT mRNA reached maximal levels during late gestation which were maintained throughout the first 3 weeks postpartum. Additional experiments indicated that Cd (1-30 mumol/kg), Zn (100-3000 mumol/kg), and dexamethasone (0.3-10 mumol/kg) increased hepatic concentrations of MT-I and MT-II and their respective mRNAs in 14-day-old rats, despite the preexisting high levels of protein and mRNA at this time. These results indicate that hepatic concentrations of MT-I and MT-II isoproteins and mRNA, which increase coordinately during fetal development, decline differently during neonatal development. In 14-day-old neonates, MT-I and MT-II genes are responsive to Cd, Zn, and dexamethasone when constitutive levels of both MT mRNA and isoproteins are high.