Abstract
A pronounced circadian rhythm has been observed in mitotic indices from cheek pouch epithelium of Chinese hamsters. Interaction of this rhythm with cytokinetic effects of x-radiation in single exposures of 100 and 300 R and repeated 100-R exposures at 24-hour intervals has been investigated. Thymidine-3 H was administered 30 minutes before single or final radiation exposure. Altered rhythmic mitotic activity persisted with repeated irradiation. Early mitotic inhibition appeared to vary directly with total dose, whether or not it was fractionated. Recovery as indicated by a wave of increased mitotic indices 12-24 hours postirradiation occurred earlier after each succeeding 100-R exposure and was virtually absent after 300-R single exposure. Proportion of labeled mitosis (PLM) curves indicated 4-hour prolongation of G2 by both 100- and 300-R single exposure, but only 2 hours by two and three repeated 100-R exposures. These experiments demonstrated the requirement, imposed by the circadian rhythm, of relatin...
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