Persistence of Thermotolerance in Slowly Proliferating Plateau-Phase Cells

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Abstract
Thermotolerance decay rates were determined under rapidly proliferating exponential growth conditions and under high-cell-density plateau-phase growth conditions. Thermotolerance was induced by a marginally toxic initial heat treatment of 10 min at 44.degree. C in Chinese hamster ovary cells. Second dose-response curves were obtained at 24-h intervals for up to 96 h after the initial heat treatment. Resistance to 2nd heat treatment had disappeared by 72 h after the initial heat treatment in proliferating cells, but was evident in plateau-phase cells at 96 h. Regardless of the proliferative status, thermotolerance decayed in an exponential manner. The half-times of tolerance decay were .apprx. 13 h in proliferating cells and 30 h in plateau-phase cells. [Relevance to the planning of hyperthermia dose schedules for treating human malignancies was discussed.].