The in vivo-in vitro hepatocyte assay for assessing DNA repair and DNA replication: studies in the CD-1 mouse.

The in vivo-in vitro hepatocyte assay, a short-term test that is useful for detecting potential rodent hepatocarcinogens, has been extended to the CD-1 mouse. Male CD-1 mice, 50-100 days old, were dosed orally with N-nitrosodimethylamine (NDMA), trichloroethylene (TCE), 2-acetylaminofluorene (2-AAF), 4-acetylaminofluorene (4-AAF), phenobarbital (PB) or a vehicle. At 3, 16, 24 and 48 hr after dosing, hepatocytes were isolated by an in situ perfusion procedure, incubated in the presence of [3H]thymidine and fixed. Unscheduled as well as scheduled DNA synthesis was assessed by quantitative autoradiography. Unscheduled DNA synthesis represents DNA repair, while scheduled DNA synthesis (S phase) represents DNA replication. No mortality or morbidity was observed in the animals dosed in this study. The vehicles used, distilled water and corn oil, did not induce DNA repair or increase the percentage of hepatocytes in S phase at any of the time points examined. The higher dose of NDMA (10 mg/kg) induced DNA repair and increased S phase, whereas the lower dose (2 mg/kg) induced DNA repair but did not increase S phase. TCE (1000 mg/kg), PB (100 mg/kg) and 4-AAF (200 mg/kg) all markedly increased the percentage of hepatocytes in S phase (5-10 times the control) but all failed to induce DNA damage and repair. 2-AAF (200 mg/kg) caused a slight (twofold) increase in S phase but did not induce DNA repair. This assay appears useful for detecting potential CD-1 mouse hepatocarcinogens and, in addition, may provide information on the mode of action of known hepatocarcinogens.