Adaptive response in different mitotic cycles after irradiation

The frequency of cells with chromosome aberrations and the number of aberrations per cell by metaphase analysis have been studied in the nonirradiated progeny of irradiated human blood lymphocytes. The DNA fragmentation (DNA double strand breaks) have simultaneously been investigated by the DNA comet assay. PHA stimulated lymphocytes have been irradiated in the adaptive dose 0.05 Gy 24 h and in the challenge dose 1 Gy 48 h after stimulation to study the adaptive response (AR). 5-bromodeoxyuridine have been added for the identification the first--the fourth mitoses. It has been discovered that the frequency of chromosome aberrations is increased is all mitotic cycles after challenge irradiation, the level of double strand breaks is increased too. The adaptive response in induced by the adaptive and challenge irradiation in the first and the second mitotic cycles (fixation 48 and 72 h after stimulation) for the most parts of individuals, but it is absent in the third and the fourth mitosis. Only chromatid aberrations are observed in the first mitosis, but chromosome aberrations--in the following mitosis. Investigation by the DNA comet assay have showed the adaptive response is noticed 48-72 h after stimulation but it is insignificant 96 h. The conclusion is that the genomic instability is observed in nonirradiated progeny irradiated lymphocytes; the adaptive response is manifested up to third mitosis and is explained by the decreasing of the number of the chromatid and chromosome aberrations and DNA fragmentation. We can suppose that double strand DNA breaks can be signaling damage for the adaptive response induction.