X-Ray Sensitivity during the Cell Generation Cycle of Cultured Chinese Hamster Cells

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Abstract
The X-ray sensitivity of cultured Chinese hamster lung cells as a function of stage in the generation cycle was described. Previous work with a cell population partially synchronized by selection of cells at or near division was improved by removing from stages other than the S period, resistant S cells. An exposure of about 1 hr. to 20 [mu]Cl of high-specific-activity H3-thymidine/ml was used. A modification of the procedure also enabled the sensitivity of mitotic cells to be determined. A more accurate description of the variation in X-ray sensitivity throughout the cell generation cycle is now possible. The data presented here show that mitosis and G2 are the most sensitive stages, followed in order of decreasing sensitivity by Gi, then by early S, and finally by late S. The rate of change of sensitivity of cells entering S is much less than the rate of change of sensitivity of cells leaving S. The survival curves indicate that not only the shoulder but also the slope varies throughout but in a similar manner. One of the most interesting new features is the almost complete absence of any shoulder in the survival curves of G2 and mitotic cells. The use of H3TdR increases the variability of results from experiment to experiment. Results were still consistent with those obtained in asynchronous populations. In spite of an apparent association between X-ray survival and DNA synthesis, no direct correlation between number of cells in synthesis or rate of DNA synthesis with survival was noted.