The natural history and dose-response characteristics of enzyme-altered foci in rat liver following phenobarbital and diethylnitrosamine administration.

Enzyme-altered foci (EAF) were induced in the liver of female rats by 70% partial hepatectomy (PH), followed by a single intragastric administration of diethylnitrosamine (DEN) at a dose of 10 mg/kg. The stability and response of these foci to various doses of the hepatic promoting agent, phenobarbital (PB), were studied. The number of gamma-glutamyltranspeptidase-positive (GGT+) EAF resulting from PH/DEN followed by PB (0.05%) administration for 1 week, 2 weeks, 1 month, 2 months, 3 months or 4 months did not significantly change when the administration of the promoting agent was followed by a 6-month period of a diet containing no PB. These data demonstrate the stability of the foci induced by the PH/DEN/PB regimen and indicate that the increased number of foci resulting from PB promotion in the absence of overt hepatic necrosis are not reversible on removal of the promoting stimulus. Chronic administration of dose levels of PB below 0.001% in the diet failed to demonstrate an increase in the number of EAF over the number in the control animals not promoted with PB. A linear increase in the number of EAF was observed when rats were chronically fed doses of PB ranging between 0.001% and 0.05% in the diet, whereas diet concentrations of PB greater than 0.05% did not result in any further increase in the number of EAF. The number of EAF resulting from PH/DEN followed by 0.05% PB in the diet increased during the first 3-4 months of promotion. Thereafter, the number of foci did not change despite the continued administration of PB for as long as 8 months. These data suggest the presence of an apparent threshold (no effect level) for promotion by PB and demonstrate the presence of a maximal response of EAF to this promoting agent after initiation by a single dose of DEN.