Dimethylnitrosamine genotoxicity in rat liver primary cell cultures with low cytochrome P‐450 levels

Liver primary cell cultures (LPCC) with decreasing concentrations of cytochrome P‐450 were used to investigate the genotoxicity of the hepatic carcinogen dimethylnitrosamine (DMN) and the correlation between DMN genotoxicity and cytochrome P‐450 levels. Hepatocytes were isolated from partially hepatectomized rats and incubated with [ 3 H]thymidine; single‐strand DNA molecular weight was determined by alkaline sucrose sedimentation. The molecular weight of DNA decreased 50% in LPCC plated either 2 or 24 h before being treated for 24 h with 70 μm DMN. Cytochrome P‐450 content was 188 pmol per mg protein in freshly isolated hepatocytes, whereas it was 70 and 32 pmol per mg protein in hepatocytes that had been cultured 24 and 48 h, respectively. Incorporation of 14 C into acid‐insoluble material was the same in LPCC exposed 24 h to [ 14 C]DMN starting either 2 or 24 h after cell plating. At non‐toxic concentrations (0.01‐1 μM), SKF 525‐A, an inhibitor of mixed‐function oxidase enzymes, inhibited approximately 20% of the binding of 14 C from [ 14 C]DMN to acid‐insoluble material in LPCC plated either 2 or 24 h before they were exposed to DMN for 24 h. Hepatocyte cultures exposed to the direct‐acting alkylating agent N ‐methyl‐ N ‐nitro‐ N ‐nitrosoguanidine (at concentrations ranging between 6.8 × 10 −8 and 6.8 × 10 −5 M) starting 2 and 24 h after plating, exhibited significant unscheduled DNA synthesis. These results indicate that DMN genotoxicity was similar in LPCC differing considerably in cytochrome P‐450 levels, and they suggest that DMN genotoxicity in these cultures is due mainly to similar DMN activation than to decreased DNA repair.