The phenotypic stability of altered hepatic foci: effects of withdrawal and subsequent readministration of phenobarbital.

Previous studies from this and a number of laboratories have demonstrated that the majority of both focal and nodular lesions which occur during the early phase of multistage hepatocarcinogenesis in the rodent disappear or 'remodel' upon removal of the promoting agent. Utilizing techniques of quantitative stereology we have found that many if not the majority of altered hepatic foci induced by initiation with diethylnitrosamine/partial hepatectomy and promoted for 4 months with phenobarbital disappear with little or no morphologic evidence of remodeling within 10 days after withdrawal from phenobarbital and do not reappear for a 6-month period thereafter. However, administration of phenobarbital in the diet following an interval period of 10, 20, 30 or 90 days in the absence of the promoting agent resulted in the reappearance of these focal lesions at the same or an increased numerical level. The volume occupied by these focal lesions, which reflects the total numbers of altered cells present in the liver, increased somewhat even during the interval absence of phenobarbital but increased dramatically upon readministration of the promoting agent. Rats fed only a semi-purified diet for 180 days following a 4-month period of phenobarbital promotion of initiated animals exhibited a greater number but only half the total volume of altered hepatic foci, as did rats fed a cereal-based diet only for 180 days. The cereal-based diet (NIH-07) may contain some unknown initiating and/or promoting factors responsible for this phenomenon. These studies present further evidence both for the reversible nature of the stage of promotion of hepatocarcinogenesis and the irreversible characteristic of the stage of initiation, as well as the importance of the composition of the diet utilized in models of multistage hepatocarcinogenesis.