Variability of the adaptive response to ionizing radiations in humans.

Human lymphocytes exposed to low doses of ionizing radiations from incorporated tritiated thymidine ([3H]dThd) or from X-rays become less susceptible to the induction of chromatid aberrations by high doses of X-rays. This indicates that low doses of ionizing radiation can produce an effect similar to the adaptive response observed with alkylating agents in prokaryotes, animal and plant cells. To determine whether there is individual variability in the adaptive response to ionizing radiations we exposed human lymphocytes from 18 different healthy donors to 'adapting' doses of [3H]dThd (0.01 microCi/ml) or X-rays (0.01 Gy) and subsequently to a 'challenge' treatment of 0.75 Gy of X-rays delivered 2 h before fixation. Four of the 18 donors did not show an adaptive response; in some cases in these individuals a synergistic response of increased, rather than decreased, damage was found. Two of these 4 donors showed no adaptive response in 3 subsequent experiments separated by 4-month intervals. This suggests that the human population exhibits a heterogeneity in the adaptive response to ionizing radiations which might be, at least in part, genetically determined.