Different radioprotective action mechanisms of CeO2 nanoparticles in vitro and in vivo are demonstrated and discussed.

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The aim of the present study was to compare genotoxicity induced by high- versus very low dose-rate exposure of mice to gamma-radiation within a dose range of 5 to 61 cGy using the single-cell gel electrophoresis (comet) assay and the micronucleus test. CBA/lac male mice were irradiated at a dose rate of 28.2 Gy/h (high dose rate) or 0.07 mGy/h (very low dose rate). The comet assay study on spleen lymphocytes showed that very low dose-rate irradiation resulted in a statistically significant increase in nucleoid relaxation (DNA breaks), starting from a dose of 20 cGy. Further prolongation of exposure time and, hence, increase of a total dose did not, however, lead to further increase in the extent of nucleoid relaxation. Doses of 20 and 61 cGy were equal in inducing DNA breaks in mouse spleen lymphocytes as assayed by the comet assay. Of note, the level of DNA damage by 20-61 cGy doses of chronic irradiation (0.07 mGy/h) was similar to that an induced by an acute (28.2 Gy/h) dose of 14 cGy. The bone marrow micronucleus test revealed that an increase in polychromatic erythrocytes with micronuclei over a background level was induced by very low-level gamma-irradiation with a dose of 61 cGy only, with the extent of the cytogenetic effect being similar to that of 10 cGy high-dose-rate exposure. In summary, presented results support the hypothesis of the nonlinear threshold nature of mutagenic action of chronic low dose-rate irradiation.

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Biophysics, Pushchino, Moscow Region, Russia The action of chronic irradiation (dose rate 2.9 Gy/day) on human lymphocyte culture was investigated. Whole blood was irradiated at 370C. Aliquots (0.2 ml) of whole blood were cultivated by the standard method. A medium containing phytohemagglutinin was added immediately after irradiation. All structural chromosome-and chromatid-type changes were recorded. The experimental data showed that the conditions of irradiation of lymphocytes affected neither the background level of chromosome damage nor their radiosensitivity. The obtained dose-response curve of chromosome aberrations was described by a linear regression, which then became a plateau. There is no statistically significant difference between the results for the low doses (10-50 cGy) of chronic and acute radiation.