Radioadaptive response in mouse survival

Conditioning exposure to low doses (0.05-0.1 Gy) of X-rays to mice (6 week-old, ICR strain) induced a significant radioresistance, measured by the increment of 30-day survival rate after acute exposure to 7.75 Gy, 2 months after the preirradiation. On the other hand, preirradiation with 0.3-0.5 Gy resulted in the radioresistance 2 weeks, but not 2 months thereafter. The mice did not acquire any radioresistance when they were preirradiated with 0.2 Gy. These findings reveal that some biological effects must be distinct within the two dose ranges, i.e. 0.05-0.1 Gy and 0.3-0.5 Gy. These radioresistances were also observed in C57BL strain mice, but not in BALB/c. Genetic control may act on the acquired radioresistance, since BALB/c mice did not acquire the radioresistance even when the feces of ICR strain were added in the cages 3 times before the preirradiation. Partial-body preirradiation with 0.5 Gy showed that mice acquire the radioresistance by the trunk (head shielded) exposure. This indicates that conditioning irradiation may act on blood forming tissues, but not on the central nervous system. On the other hand, in case of the radioresistance acquired after irradiation with 0.05 Gy, whole-body preirradiation was necessary. Cooperation of central nervous system and blood forming system after the preirradiation may be essential in this case. (author)