The role of short-lived lesions in the induction of micronuclei in rat liver by ethylnitrosourea and methyl methanesulphonate: the importance of experimental design.

Rats received single injections of ethylnitrosourea (ENU) or methyl methanesulphonate (MMS) at the peak of DNA synthesis after partial hepatectomy. Hepatocytes were isolated 1-4 days later, and analysed for presence of micronuclei. With both chemicals, frequencies of micronucleated hepatocytes were increased in a dose-dependent manner, but ENU proved to be both more effective (6 times; based on molar dose) and more efficient (18 times; based on total DNA alkylation) than MMS. In general, the micronucleus frequency was relatively low at 1 day after injection, then increased to reach a maximum at days 2 or 3 (depending on the dose), after which it decreased strongly in case of MMS or remained stable in the case of ENU. The result with ENU is interpreted as a balance between loss and/or dilution of micronucleated hepatocytes and simultaneous formation of new ones. The present observations are in line with our earlier conclusion that ENU, in contrast with MMS, is able to induce persistent preclastogenic lesions in rat hepatocytes. ENU also proved to be more effective and efficient than MMS with respect to the formation of micronuclei in bone marrow cells. Our results with ENU and MMS indicate that administration of the genotoxin at the peak of DNA synthesis after partial hepatectomy, instead of before hepatectomy, increases the sensitivity of the liver micronucleus assay at least in the case of directly acting chemicals.