Persistence of thermotolerance in slowly proliferating plateau-phase cells.

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 degrees C in Chinese hamster ovary cells. Second dose-response curves were obtained at 24-hr intervals for up to 96 hr after the initial heat treatment. Resistance to second heat treatment had disappeared by 72 hr after the initial heat treatment in proliferating cells, but was evident in plateau-phase cells at 96 hr. Regardless of the proliferative status, thermotolerance decayed in an exponential manner. The half-times of tolerance decay were approximately 13 hr in proliferating cells and 30 hr in plateau-phase cells.