Diurnal variation in the response of gamma-ray-induced apoptosis in the mouse intestinal epithelium

The appearance of apoptosis has been used as an index of carcinogeninduced damage incurred by the intestinal epithelium. Here we report that the cytotoxic response elicited by gamma radiation in the duodenal and colonic epithelium of the mouse is markedly affected by the time of day at which the animals are irradiated or sampled. The diurnal pattern in radiosensitivity of the crypt cells, as measured by the appearance of apoptosis, is reflective of circadian rhythms in mouse intestinal cells. Circadian effects should therefore be considered when interpreting apoptosis data obtained at one time point only or when comparing data obtained at different times of the day.