Synergistic induction of altered hepatocyte foci by combined gamma radiation and diethylnitrosamine administered to neonatal rats.

To investigate mechanisms underlying the formation of carcinogen-induced altered hepatocyte foci, we histochemically examined the livers of 150-day-old rats that had been treated neonatally with single doses of gamma radiation (75, 150 or 300 rad, whole body) and i.p.-injected diethylnitrosamine (DEN, 0.15 mumol/g body wt) either separately or in combination. Three focus phenotypes, showing the elevated gamma-glutamyl transpeptidase [GG(+)] and/or the iron-exclusion [Fe(-)] histochemical marker(s) were quantitated through the use of serial frozen sectioning techniques and computer-assisted image analysis. DEN and gamma radiation each induced foci when given separately but total focus yield per cm3 of liver was more than 10-fold greater in DEN-treated than in irradiated rats and approximately 3-fold higher in females than in males. Combining the DEN and radiation treatments synergistically increased total focus yields for both sexes, although this response declined with increasing radiation dosage. For rats receiving DEN alone, the sex-dependent differential in total focus yield was due to the higher frequencies in females of the iron-excluding phenotypes [Fe(-) alone and Fe(-) + GG(+)]; the frequencies of foci showing only GG(+) were similar in both sexes. In contrast, the enhancement in total focus yield resulting from the combined DEN--radiation treatments was primarily a consequence of increases in the foci with GG(+) alone. The results suggest that (a) qualitatively different types of genetic damage (carcinogen-induced point mutations and radiation-induced rearrangements) may interact synergistically in the induction of phenotypically altered cells and (b) separate genetic loci are involved in the sex-mediated modulation of focus production and the synergistic enhancement of focus production by DEN--gamma radiation interactions.