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The in vivo effect of vitamin C or beta-carotene on sister chromatid exchange (SCE) radio-induction was determined in murine bone marrow cells sensitized by BrdU incorporation. Pre- or post-treatment with 100 mg/kg body wt vitamin C did not cause a significant reduction in SCE induced by the exposure to 0.63 Gy gamma-rays. Treatment with a double dose of vitamin C and with 0.45 Gy radiation did not cause a significant reduction in SCE frequency. However, due to the fact that vitamin C per se is capable of SCE induction, if an additive effect of radiation and vitamin C is considered, the expected frequency is higher than that observed. This implies that vitamin C could have a slight radioprotective activity. With regard to beta-carotene, it has been demonstrated that 50 mg/kg body wt causes a statistically significant increase per se, although pre- and post-treatment with the same dose has an additive effect on SCE frequency induced by 0.62 Gy radiation. This indicates that beta-carotene does not have radioprotective activity under the conditions used in the present study.

The aims of the study were to establish the kinetics of the early adaptive response and to determine the minimum adaptive dose of gamma rays capable of inducing this response. The minimum adaptive dose was determined by exposing groups of five BALB/c male mice to an adaptive dose of 0.005 or of 0.02 Gy gamma rays from a 137Cs source and challenge with 1.0 Gy 60 min later. The kinetics of adaptive response induction was established by exposing mice to an adaptive dose of 0.01 Gy, and subsequently to a challenge dose of 1.0 Gy at different times. Blood samples were collected from the tail immediately after exposure to the challenge dose, and the percentage of DNA-damaged cells and the extent of damaged were determined by single cell gel electrophoresis in 300 leukocytes per animal in five mice. The results confirms the capability of an in vivo induction of an early radioprotective process against the DNA-damage produced by gamma rays in murine leukocytes, and allows us to conclude that the minimum adaptive dose lies between 0.005 and 0.01 Gy of gamma rays, and the early adaptive response is induced as early as 30 min after the exposure and persists for at least 18 hr.