Different Effects of Regenerative and Direct Mitogenic Stimuli on the Growth of Initiated Cells in the Resistant Hepatocyte Model

The possible mechanism(s) responsible for the different effects exerted by proliferative stimuli of different nature on the appearance of enzyme‐altered hepatic foci, were investigated in male Wistar rats. Rats given an initiating dose of diethylnitrosamine (150 mg/kg body weight) were fed a diet containing 0.03% acetylaminofluorene for 2 weeks. Between the first and the second week, cell proliferation was induced by a proliferative stimulus of compensatory type (partial hepatectomy) or by a direct mitogenic stimulus (lead nitrate, 100 μmol/kg). The effect of the two different proliferative stimuli on the appearance of γ‐glutamyl transferase‐positive foci was monitored by killing the rats for examination at 1, 2, 3, 5, and 6 days after the induction of cell proliferation. The results indicate that while enzyme‐altered hepatocytes can be observed as early as 3 days after partial hepatectomy and are characterized by a rapid growth, direct hyperplasia did not exert any effect on the growth capacity of initiated cells. No effect of lead nitrate‐induced hyperplasia was observed following three administrations of the mitogen. When platelet‐poor plasma taken from animals exposed to the different proliferative stimuli was tested in primary cultures of hepatocytes, it was found that it induced a significant increase in the labeling index of normal hepatocytes. However, while serum taken 6 days after partial hepatectomy was still able to induce a significant increase in the labeling index, platelet‐poor plasma from lead‐treated rats had lost part of its effect at 5 days after treatment. The inability of direct hyperplasia to stimulate the development of enzyme‐altered hepatic foci was not unique to lead nitrate since the same phenomenon was observed when three other hepatomitogens, nafenopin, cyproterone acetate, and ethylene dibromide, were used.