Dose-response relationships for initiation of rat liver tumours by diethylnitrosamine and promotion by phenobarbitone or alcohol.

Small doses of initiators, such as the nitrosamines, are present in the diet and man is also exposed to promoters, such as phenobarbitone or alcohol. A simple two-stage model of rat hepatocarcinogenesis with a single ip dose of diethylnitrosamine (DEN) as the initiator and sodium phenobarbitone or ethanol given in the drinking-water for 12-18 months as the promoter was used to investigate dose-response relationships for initiation and promotion. Phenobarbitone given alone for 12 months had no carcinogenic effect on rat liver. Low doses of DEN, given prior to phenobarbitone promotion, resulted in the formation of hyperplastic nodules, but only the high dose of DEN (30 mg/kg) resulted in carcinoma formation. Basophilic foci showed a dose-response relationship with DEN and it is suggested that these may be more important than nodules in carcinoma formation and that they represent a heterogeneous group. Only the top concentration of phenobarbitone (1000 micrograms/ml) promoted carcinoma initiated by 30 mg DEN/kg although lower doses of phenobarbitone produce significant enzyme induction. A weak enzyme inducer, 5% ethanol, was as effective a promoter as 1000 micrograms phenobarbitone/ml. Enzyme induction and tumour formation are therefore not directly related. Commencing top-dose phenobarbitone promotion 10 months after the single dose of DEN led to tumour formation, demonstrating the persistence of DEN-initiated cells. The significance to man of the apparent thresholds for tumour initiation and promotion is discussed.