Transplacental passage and fetal deposition of mercury after low-level exposure to methylmercury--effect of seleno-L-methionine.

Previous experimental studies on transplacental passage and possible fetotoxicity of methylmercury have almost exclusively used a single dosage or 2-4 repeated doses of mercury on specific days during gestation and often used at relatively high dose levels. In previous studies, selenium supplementation considerably increased the concentration of mercury in the blood of offspring after maternal exposure of rats to methylmercury, whereas whole-body retention and organ deposition of mercury in mice were unaffected. The present study in mice, which involved exposure for 5 weeks to a low dose of methylmercury in the drinking water (1 nmol/ml) before and during pregnancy, demonstrates that mercury is deposited in offspring both in utero and during lactation, and that transplacentally absorbed mercury is not, or only very slowly, excreted. Seleno-L-methionine increased the deposition of mercury in offspring, but the effect was due to slightly higher deposition in the dams. Selenomethionine significantly reduced the kidney deposition of mercury in offspring, whereas liver deposition of mercury was unaffected. These results indicate that interactions between organo-selenium compounds and methylmercury orally administered at low doses is of less importance than previously believed on the basis of experiments with higher doses of selenite injected or supplemented to the diet.