Cell cycle kinetics of irradiated synchronous and asynchronous tumor cells with DNA distribution analysis and BrdUrd‐Hoechst 33258‐technique

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Abstract
Radiation‐induced progression delay in S‐phase and G2‐block, both depending on time of irradiation in the cell cycle, were measured in synchronized Ehrlich ascites tumor cells in vitro using flow cytometric analysis of DNA content in single cells. Similar results could be obtained by the BrdUrd‐Hoechst 33258 technique after irradiating asynchronous cells. BrdUrd replaces thymidine in newly synthesized DNA which is not stainable by the thymidine‐specific dye Hoechst 33258. The temporal development of the fluorescence distributions after addition of BrdUrd to the medium has been measured in the flow cytometer. In asynchronous cells the mean durations of the cell cycle phases and their perturbations after irradiation could be calculated, and the data agreed with the results in synchronized cells. Division delay after irradiation consists of a small progression delay in S‐phase and a block in G2‐phase. Both effects depend on time of irradiation during the cell cycle with delay in S‐phase increasing in irradiated late S‐cells and G2‐block increasing in irradiated G2‐cells. Data from both types of experiments were used with a simple model describing the fractions of cells in the cell cycle phases after irradiation had been applied.