Variation in Sensitivity to Heat Shock during the Cell-cycle of Chinese Hamster Cellsin Vitro

SummaryWhen asynchronous cells were exposed for 4–100 min to heat shocks of 43·5–46·5°c, a sigmoidal relationship between survival and duration of heat treatment was observed. From an Arrhenius plot, an activation energy of 140 800 cal/mole was calculated, which is similar to that reported for the heat denaturation of several proteins.Heating of synchronous cells, obtained by the selective removal of mitotic cells from an asynchronous population, revealed that the mitotic and S phases were the most sensitive, primarily indicated by the smaller shoulders on the survival curves compared with the curve for cells heated in the resistant G1 phase. The mitotic cells failed to complete cytokinesis and appeared in the next division as tetraploid cells. For heat treatments which reduced the survival to only 50 per cent, the mitotic delay was about 11 hours (regardless of the phase heated). These results for heat contrast sharply with results for x-irradiation.