Synergistic Enhancement of Glutathione S–Transferase Placental Form–positive Hepatic Foci Development in Diethylnitrosamine–treated Rats by Combined Administration of Five Heterocyclic Amines at Low Doses

Potential synergism among 5 heterocyclic amines at low doses in the induction of glutathione S–transferase placental form (GST–P)–positive liver cell foci was examined in an 8–week experiment using male rats initially given diethylnitrosamine (200 mg/kg, ip). The heterocyclic amines applied were 3–amino–l–methyl‐5H–pyrido[4,3–b]indole (500 ppm), 2–amino–6–methyldipyrido[l,2–a:3′,2′–d]–imidazole (500 ppm), 2–amino–3–methyl–9H–pyrido[2,3‐b]indole (800 ppm), 2–amino‐9H–pyrido[2,3–b]indole (800 ppm), and 2–amino–l–methyl–6–plienyliniidazo[4,5–b]pyridine (PhIP, 400 ppm). Separate groups received each chemical at the dose used in earlier carcinogenicity assays (above doses), at 1/5 or 1/25 of these, or all 5 chemicals together, each at the 1/5 or 1/25 levels. The numbers and areas of GST–P–positive foci were significantly increased with all chemicals, except for PhIP, at the highest dose, the results being consistent with the reported liver carcinogenicity. In the combined treatment at the 1/5 dose levels, synergistic enhancement occurred; the numbers and areas of foci were significantly increased above the sums of individual data. However, this was not the case for the 1/25 dose groups. Although the synergism between pyrolysis products in liver carcinogenesis depended on the dose and combination of chemicals, the findings, together with those from a previous experiment using 5 different heterocyclic amines, are of particular significance since several heterocyclic amines might be simultaneously generated during cooking of foodstuffs.