Experiments were performed to determine whether diurnal variations in apoptosis in the mouse small intestine after irradiation with 2.5 Gy γ rays depended on the time of day that the mice were irradiated, the time of day that the mice were sacrificed or the interval between irradiation and sacrifice. Experiments were performed with a 12-h light:dark regimen with the light period from 6:00 to 18:00 h. With fixed intervals of 6 h and 24 h between irradiation and sacrifice, a peak in induced apoptosis (16%) was observed in mice sacrificed at 8:00 h, two times higher than the nadir of response at 23:00 h (8%). When variable intervals were used between irradiation and measurement of apoptosis, i.e. sacrifice, at 8:00 h or 23:00 h, the induced apoptosis was dependent on the interval, with a peak for 18-h intervals. However, the level of apoptosis was always about twofold higher when measured at 8:00 h than at 23:00 h. No correlation was observed between diurnal variations in apoptosis and survival of mouse intestinal crypts. The diurnal variations in apoptosis after irradiation can be interpreted either in terms of expression of apoptosis during the G2/ M phase of the cell cycle in partially synchronized cells, or in terms of a systemic mechanism such as diurnal variation in the neurohormone melatonin.

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