Low doses of gamma-radiation induce nonlinear dose responses in Mammalian and plant cells

The percentage of cells with chromosome aberrations or micronuclei induced by low doses of acute (dose rate of 47 cGy/min) or chronic (dose rate of 0.01 cGy/min) gamma-irradiation was studied in vitro in Chinese hamster fibroblasts, human lymphocytes, and Vicia faba seeds and seedlings. The sensitivity of the indicated biological entities to low doses was greater than expected based on linear extrapolation from higher doses. The dose-response curves for cytogenetic damage that were obtained were nonlinear when evaluated over the full range of the doses used. At very low doses, the dose-response curves appeared linear, followed by a plateau region at intermediate doses. At high doses the dose response curves again appeared linear with a slope different from that for the low-dose region. There was no statistically significant difference between the yields of cells with micronuclei induced by low doses of acute versus chronic irradiation. Similar data were obtained both for human lymphocyte culture and for roots and seeds of Vicia faba. Our experiments revealed that the dose range over which the plateau occurs depends on the type of cells irradiated. We have also shown that the modifying effects of the repair inhibitor caffeine and the radioprotector mercaptoethylenamine (MEA) are absent at low doses of gamma irradiation and that caffeine increased the number of cells with cytogenetic damage when evaluated over the plateau region. In the presence of MEA, the upper end of the plateau region was extended from just above 1 Gy to about 2 Gy. We therefore provide direct evidence that a plateau exists in the dose-response curve for the indicated radiation-induced stochastic effects. Furthermore, our results suggest that, for low linear energy transfer radiation, the induction of DNA repair occurs only after a threshold level of cytogenetic damage and that the higher yield of cytogenetic damage per unit dose at low radiation doses is attributable to an insignificant contribution or the absence of DNA repair processes.