The frequencies of chromosome aberrations and micronuclei were evaluated to assess the induction of adaptive response to low dose ionizing radiation in each of the blood samples collected from eight different individuals. Following stimulation with phytohemagglutinin, the cells were exposed to an adaptive dose of 1 cGy X-radiation at 24 hours and a challenge dose of 150 cGy gamma radiation at 48 hours. Lymphocytes were fixed at 54 hours to examine the incidence of chromosome aberrations and at 72 hours to examine the frequency of micronuclei in cytokinesis-blocked binucleated cells. Lymphocytes from five donors, i.e., "responder", exhibited the induction of adaptive response; their lymphocytes, which were pre-treated with 1 cGy had significantly fewer chromosome aberrations and micronuclei induced by the challenge dose at 150 cGy gamma radiation, as compared to the cells which did not receive the pre-treatment with 1 cGy. Such an induction of adaptive response was not observed in the remaining three donors, i.e., "non-responders"; the incidence of chromosome aberrations and micronuclei induced by the challenge dose of 150 cGy was not significantly different between the cells which were pre-exposed and un-exposed to 1 cGy. In all eight individuals, there was a strong positive correlation between the incidence of chromosome aberrations and micronuclei. Hence, whether or non an individual is a "responder" or "non-responder" could be assessed using either chromosome aberrations or micronuclei as the end-point. The overall pattern of response confirms the heterogeneity in adaptive response between individuals to ionizing radiation, which may in part be genetically controlled. Because of the simplicity of the technique and rapid assessment of the binucleated cells, we suggest the use of the micronucleus test as an alternative procedure in large scale population studies related to the adaptive response.
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