抄録
Last year we have reported that when immature spermatocytes with active DNA repair function were irradiated with low dose, low dose-rate X-rays, mutation frequency became lower than control. Here we present the results of further studies with high and/or low doses and dose-rates. A high dose (10Gy) irradiation at a high dose-rate (500mGy/min) made the mutation frequency significantly higher than control. A 10Gy irradiation at a low dose-rate (50mGy/min) made the mutation frequency higher than control but lower than the high dose-rate experiment. The dose-rate effect was evident. With a low dose (200mGy) irradiation at a high dose-rate, the mutation was not significantly more frequent than control. A 200mGy irradiation at a 50mGy dose-rate resulted in a significant decrease in the mutation frequency. These results suggest that the dose response relationship is non-linear for a low dose-rate irradiation. A possible mechanism of the existence of the threshold is that the DNA repair function is activated by the low dose-rate irradiation and it repaired not only radiation induced damage but also spontaneous one. The background mutation frequency in this experiment was 10 to the 6th/gene which corresponds to that induced by a 4.2Gy irradiation. If a considerable fraction of the spontaneous damage is repaired without error, the background mutation frequency should be reduced, which compensates the increase in the induced mutation and thus forms a practical threshold. Some results using a repair defective strain will also be presented.
