Phenobarbital promotion in diethylnitrosamine-initiated infant B6C3F1 mice: influence of gender.

Phenobarbital (PB) promotes hepatic tumorigenesis when chronically administered to male B6C3F1 mice after initiation with diethylnitrosamine (DENA) at 30 days of age. In contrast, when male B6C3F1 mice were initiated with DENA at 15 days of age, an inhibition of hepatic tumorigenesis occurred. The present study was undertaken to evaluate the influence of gender on the inhibiting ability of PB in the 15 day old DENA-initiated B6C3F1 mouse. Mice were injected with either DENA (5 micrograms/g) or saline at 15 days of age. At weaning mice were given either PB (500 p.p.m.) containing drinking water or deionized drinking water for 24 weeks. Male mice treated with DENA and PB demonstrated a significant decrease in the number of hepatocellular adenomas compared to males receiving DENA only. In contrast, females exposed to DENA and PB exhibited an enhancement of hepatic adenoma number compared to those receiving only DENA. In an additional experiment, individual preneoplastic foci from male and female B6C3F1 mice initiated with DENA at 15 days of age were examined for their responsiveness to the mitogenic stimuli of PB. Mice were exposed to either PB-containing or PB-free drinking water for 7 days. In non-PB treated males and females, preneoplastic hepatocytes demonstrated higher rates of DNA synthetic labelling compared to normal hepatocytes with no gender difference noted. Males exposed to PB exhibited increased levels of DNA synthesis in normal cells but not in preneoplastic foci. Females treated with PB, however, demonstrated significant increases in DNA synthesis in both preneoplastic and normal hepatocytes compared to non-PB treated females and PB-treated males. These findings suggest that in male mice initiated with DENA at 15 days of age, the preneoplastic foci are refractory to the proliferative effects of PB which may account for the observed inhibition of hepatic tumorigenesis by PB in this mouse strain.