Tumor promotion in rat liver.

An initiation/promotion bioassay for chemical carcinogens and tumor promoters has been developed in rat liver using presumed preneoplastic lesions, foci of gamma-glutamyltranspeptidase (GGTase)-positive hepatocytes, as the endpoint. To evaluate the tumor-promoting activity of phenobarbital, rats were administered diethylnitrosamine (DENA), 2.0 mmole/kg, followed by 500 ppm phenobarbital in their drinking water. After 6 weeks of phenobarbital promotion, the rats had an increased incidence of foci as compared to nonphenobarbital-treated rats. By 50 weeks, the number of foci in the nonpromoted animals equaled the number observed with promotion. The stability and progression of GGTase-positive foci was determined in rats that received a 2/3 partial hepatectomy, followed 24 hours later by DENA administration (0.3 mmole/kg). The rats then received 500 ppm phenobarbital in the drinking water for 7 weeks. After 7 weeks, half of the rats were continued on phenobarbital and the other half were removed from phenobarbital treatment. The number of foci observed in rats continued on phenobarbital treatment leveled off after 10 weeks of promotion, while in rats taken off phenobarbital it did not regress but increased at a slower rate, and, by 56 weeks, approached the number observed in rats subjected to continuous promotion. At 56 weeks, the size of foci was larger after continuous promotion. At 81 weeks, all 6 (100%) of the rats on continuous promotion had liver tumors, while only 3 of 6 (50%) of the rats removed from promotion had tumors. Promotion by phenobarbital stimulated the growth and decreased the time required for the appearance of GGTase-positive foci and liver tumors.