Abstract
Female C57BL/6N mice (5 week-old, 35-36 in each group) were irradiated with 137Cs gamma rays at 0.35, 0.70 or 1.2 mGy/hr for 34 days. The mice were then injected (s.c) with 0.05 mg of methylcholanthrene (MC) dissolved in olive oil and the irradiation was continued. The levels of oxidative DNA damage product, 8-OhdG, in urine of all of mice were measured every month. The cumulative tumor incidence after 220 days following the MC injection in non-irradiated control group was 23 %. There was a delay in the incidence of tumor in the irradiated groups. The cumulative tumor incidence was also suppressive in the irradiated groups: 22 % in 0.35-mGy/hr group, 11 % in 0.70-mGy/hr, and 6 % in 1.2-mGy/hr. No difference was formed in the level of 8-OhdG between the non-irradiated control group and the irradiated groups, or among the irradiated groups. These results suggested that the inhibitory effects on the tumor incidence by the low-dose rate irradiation were due to certain mechanism offer than the suppression of endogenous oxidative damage.
