Todd, Diane M., M.S., Department of Pharmacology/Toxicology, Wright State University, 2003. The Two Isoforms of Rat Metallothionein are Coordinately Regulated In Vivo. Metallothionein (MT) is an inducible protein whose unique structure contributes to its many functions. One of MT’s functions is the detoxification of heavy metals. Cadmium (Cd), which is an environmental pollutant and a hazard to both humans and animals, is detoxified by MT. There are four known MT isoforms (MT I-IV) and this study focuses on only MT I and MT II. Even though a lot is known about the effect of Cd on MT induction little has been reported about the MT isoforms and their pattern of mRNA expression. It is the aim of this study to investigate the hypothesis that the two isoforms of rat metallothionein are coordinately regulated in vivo. To test this hypothesis, rats were dosed intraveneously via the lateral tail vein with 0.0, 0.5, 1.0 or 2.0 mg/kg Cd acetate and the mRNA levels in the liver tissue were determined by Real-Time PCR at various time points (0, 0.5, 1, 3, 6, 9, 12, 18 and 24 hours). The fold increase in mRNA expression was determined using the comparative CT method and plotted versus time. MT I mRNA exhibits a similar induction pattern at the 0.5, 1.0 and 2.0 mg/kg doses up to the 6 hour time point. By 12 hours both the 0.5 mg/kg and 1.0 mg/kg doses decline to baseline levels whereas the 2.0 mg/kg dose remains elevated. The pattern of induction for MT II mRNA is similar for all three doses up to the 9 hour time point. At the 0.5 mg/kg and 1.0 mg/kg doses MT II mRNA declines to baseline at the 12 hour time point and then increases at 24 hours. At the 2.0 mg/kg dose, MT II mRNA remains elevated at times greater than 12 hours. In comparing MT I and MT II mRNA expression, the results demonstrate that at all doses, MT II peaks first followed by a decline at 3-6 hours at which time MT I peaks. MT I mRNA decreases between 6 and 12 hours, while MT II mRNA peaks at 9 hours. At the 0.5 mg/kg and 1.0 mg/kg dose both MT I and MT II decrease at the 12 hour time point and then increase at 24 hours while at the 2.0 mg/kg dose, both mRNAs remain elevated between 12 and 24 hours. By comparing MT I and MT II mRNA patterns of induction it is evident that the two isoforms are not coordinately regulated in vivo.