Persistence of preclastogenic damage in hepatocytes of rats exposed to ethylnitrosourea, diethylnitrosamine, dimethylnitrosamine and methyl methanesulphonate. Correlation with DNA O-alkylation.

Male Wistar rats received a single injection of ethylnitrosourea (ENU; 140 mg/kg), dimethylnitrosamine (DMN; 10 mg/kg) or methyl methanesulphonate (MMS; 80 mg/kg). After 1, 6, 28 and 56 days liver cells were assayed for the presence of preclastogenic lesions. This was done by analysis of micronuclei in hepatocytes isolated at 2, 3 and 4 days after partial hepatectomy. Treatment with MMS did not give rise to a statistically significant increased micronucleus frequency after the two time intervals tested (1 and 6 days). In contrast, frequencies of micronuclei were significantly enhanced at all time intervals after treatment with either ENU or DMN. These results support our previous conclusion that only those alkylating agents which give rise to substantial DNA O-alkylation are able to induce long-lived preclastogenic damage in rat liver cells. Both after ENU and DMN treatment, frequencies of micronuclei were significantly higher at day 6 than at days 1, 28 and 56. It is postulated that part of the primary preclastogenic lesions (supposed to be DNA adducts) are converted into secondary preclastogenic lesions during the first period after exposure to the chemicals. Apparently, both primary and secondary preclastogenic lesions are gradually lost after day 6. This loss is rather low (less than 25%) after ENU, but more impressive after DMN (approximately 60%). The persistent nature of preclastogenic damage induced by diethylnitrosamine was demonstrated in an experiment which showed that a single dose of this agent (50 mg/kg) had the same clastogenic effect as a fractionated dose (five weekly injections of 10 mg/kg). This result also implies that a threshold dose for induction of micronuclei, if present, must be very low, appreciably below 10 mg/kg.