Increase in metallothionein mRNA accumulation during Cd challenge in oysters preexposed to Cd

We investigated the effect of sublethal Cd challenge on branchial metallothionein (MT) mRNA accumulation in the mollusc Crassostrea virginica preexposed to a range of low Cd concentrations. MT mRNA concentrations were measured with quantitative ribonuclease protection assays, Initial treatments of control (0.0003 mu M) and 0.0044, 0.044, and 0.44 mu M Cd exposure for 21 days were followed by challenge with 0.44 mu M Cd, the highest preexposure Cd concentration tested. MT mRNA accumulation during preexposure exhibited concentration-and time-dependence, increasing at all preexposure concentrations, At 21 days, mean MT mRNA concentrations ranged from 16 to 563 attomol/mu g total RNA. During the challenge, MT mRNA increased in all samples and the concentration-dependence of MT mRNA in relation to preexposure concentrations was maintained. MT mRNA concentrations during the challenge were significantly increased in oysters preexposed to the two highest Cd concentrations in comparison with the controls. A lower limit to this response was indicated since, during challenge, MT mRNA concentrations in gills of oysters preexposed to the lowest concentration of 0.0044 mu M Cd were not different from control values. Since MT mRNA concentrations are correlated with the rate of translation in previous studies, these data indicate greater MT synthesis in individuals preexposed to low concentrations of Cd, The findings implicate new MT synthesis in acquired tolerance to Cd toxicity.