Cell Loss and Proliferation Induced by A^I-FIuorenylacetamide in the Rat Liver in Relation to Hepatoma Induction1

and the total hepatic DNA were determined. In Experiment 3, the incidence of hepatic carcinoma was determined for various exposure durations to 0.03, 0.01, 0.003, and 0.001% 2-FAA. At 42 days after the start of administration of 0.03% 2-FAA, the uptake of TdR-3H in the hepatic DNA increased markedly. With 0.03% 2-FAA, the pulse labeling index increased progressively between 42 and 120 days, while the pulse labeling index for 0.01% 2-FAA showed no increase until 120 days. With 0.03% 2-FAA, the prelabeled DNA decreased markedly between 42 and 72 days, while the total hepatic DNA increased by about 15 to 20% by 72 days. The tumor data showed that a 365-day exposure to 0.01% 2-FAA was approximately equivalent to a 112-day exposure to 0.03% 2-FAA, i.e., an equivalent tumor yield for equal total doses. However, 28and 56-day exposures to 0.03% 2-FAA were far less effective than 112-day exposures to 0.01% 2-FAA. The data suggest that, at 0.03%, the carcinogen produced an increase in the parenchymal cell replication rate that correlated with the tumor incidence and that, when the carcinogen was stopped early enough to prevent tumors, very little of the original DNA was lost and the replication rate of the parenchymal cells was only slightly increased.