Circadian Variation in Lung Tumor Induction with X-rays in Mice

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Many studies on the acute deaths of rodents with ionizing radiation given at various times of day exist in the literatures. The mean survival time of mice exposed to X-rays at night has been reported to be shorter than that of mice exposed in the daytime. The reason for this fact has been revealed by two hypotheses that (1) rodents are most sensitive during periods of increased bone marrow mitotic activity, and (2) a daily variation of the content of endogenous radioprotectors is included in the variation of sensitivity. Both hypotheses imply that not only bone marrow cells but also other cells are sensitive at night. On the other hand, circadian rhythm in tumorigenicity has not been studied so far. In the present study we investigated circadian variation in tumorigenesis with X-rays. The C^3H lung tumor system was used. Circadian variation of alveolar injury by X-rays was also studied. The alveolar injury was judged by the degree of proliferation of epithelial cells, since proliferation of epithelial cells is a common response to alveolar injury. ^3H-labeled thymidine (^3H-TdR) was utilized to measure the proliferation of alveolar cells.
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