Effects of carbon tetrachloride administration on initiation of liver cell foci by the non-hepatocarcinogens N-methyl-N-nitro-N-nitrosoguanidine (MNNG) and benzo(a)pyrene (B(a)P)

In a development trial for an initiation bioassay system, cell proliferation kinetics after partial hepatectomy (PH) or CCl₄ administration (1 ml/kg b.w., i.g.) and the effect of administration time after PH or CCl₄ treatment on liver cell foci induction by the direct and indirect non-hepatocarcinogens, N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and benzo(a)pyrene (B(a)P) were investigated. Male F344 rats were killed 12, 18, 24, 36, 48, 72 or 96 h after PH or CCl₄ treatment and liver cell proliferation was examined with the bromodeoxiuridine (BrdU) labeling method. Appreciable increase in the BrdU labeling index was observed 18-36 h after PH with a peak at 24 h, and 18-72 h following treatment with CCl₄ with a peak at 48 h. MNNG (SO mg/kg i.g.) or B(a)P (100 mg/kg i.g.) were administered to 7-week-old male F344 rats at various times after PH or CCl₄ treatment and lesion induction was assessed using the resistant hepatocyte model. MNNG caused significant numbers of glutathione S-transferase placental form (GST-P)-positive liver cell foci in rats when given 12-36 h after PH, with a peak at 24 h. In contrast, the numbers of foci induced by B(a)P were maximal with exposure at 12 h after PH. In the CCl₄ study, both MNNG and B(a)P induced significant increase in GST-P-positive liver cell foci when given 12-72 h after CCl₄ treatment, with a peak at 48 h, the results being directly in line with the changes in BrdU labeling. From these findings, it is concluded that initiation assay protocols with a CCl₄ proliferative stimulus to hepatocytes may prolong the appropriate administration period for effective detection of the initiation potential of both direct and indirect carcinogens targeting sites other than the liver.