Enhancement of urethan-induced adenoma formation in Swiss mice exposed to methylmercury.

Female Swiss mice were exposed to methylmercury in the drinking water for 15 weeks. The mice were administered concentrations of methylmercury ranging from 0 to 2.0 micrograms/mL mercury. After three weeks of the 15 week exposure period, the mice were administered urethan (1.5 mg/g) intraperitoneally. Pulmonary adenoma formation was evaluated 12 weeks later. Methylmercury exposures of 0.2 and 0.5 micrograms/mL did not affect the number of adenomas, but 2.0 micrograms/mL mercury caused a significant increase in adenoma production. A dose-dependent increase in the mean tumor diameter was seen at methylmercury exposures up to 0.5 micrograms/mL. No further increase in diameter was seen at higher exposures (2.0 micrograms/mL). The changes in adenoma production were seen at exposure levels of methylmercury which did not cause any clinical manifestations. Animal weight gains and water consumption were not affected. In addition, urethan-induced sleeping times which reflect the rate of urethan metabolism or excretion remained unchanged.