Low-doses of radiation induced radioresistance in mice

Cells irradiated with low doses of ionizing radiation become less susceptible to the induction of harmful effects by subsequent irradiation with high doses. Similar responses have been observed in the ICR strain of mice preirradiated with 5 or 10 cGy of X-rays 2 months before the sublethal exposure. Significant and reproducible improvements in 30-day survival were observed when they were compared with those of the control mice not exposed to a low dose beforehand. When the ICR mice preirradiated with 5 cGy were sublethally exposed after various interval between 1 day and 5 months, the radioresistance appeared 2 months later and lasted for about 2 weeks. However, there might be a narrow 'window' of response in the induction of radioresistance, since it did not appear within 1.5 months and disappeared 3 months after the preirradiation. Furthermore, we found that preirradiation with both 30 and 50 cGy resulted in the induction of radioresistance 2 weeks later, while no radioresistance could be induced 2-5 weeks after preirradiation with 20 cGy. Irradiation either with 2.5 cGy or with 15 cGy also failed to induce the radioresistance 2 months later. Therefore, so far as the induction of radioresistance is concerned, low dose radiation may be distinguished with the following four ranges in ICR mice: (1)below 2.5 cGy: no acquired radioresistance 2 months later, (2)5-10 cGy: significant radioresistance 2-2.5 months later, (3)15-20 cGy: no acquired radioresistance 2 weeks - 2 months later, and (4)30-50 cGy: significant radioresistance 2 weeks later. Three other strains of mice were examined to determine whether the induction of radioresistance. C57BL showed the radioresistance both 2 months after the exposure to 5 cGy and 2 weeks after the exposure to 50 cGy. On the contrary, neither preirradiation could induce radioresistance in BALB/c and BDF1. Therefore, it is likely that the induction of radioresistance might be regulated by certain genetical factors. (J.P.N.)