The effect of chronic and acute heat conditioning on the development of thermal tolerance.

Survival studies with Chinese hamster ovary cells showed that thermal tolerance, which developed during chronic heating (treatment times greater than or equal to 1 hr) or after acute heating (treatment times less than 1 hr) involves similar mechanisms. For example, cells that expressed thermal tolerance during a 6-14 hr chronic heat treatment at 41.5 degrees C or 42 degrees C also expressed thermal tolerance to a subsequent acute treatment at 45.5 degrees C. Also, cells heated acutely for 10 min at 45.5 degrees C and incubated at 37 degrees C for 12 hr showed tolerance to both 45.5 degrees C acute and 42 degrees C chronic hyperthermia. Finally, thermal tolerance developed between fractionated acute heat treatment at 45.5 degrees C and fractionated chronic heat treatments at 42.5 degrees C. These data indicate that when cells are tolerant to chronic hyperthermia they are also tolerant to acute hyperthermia and that the reverse is also true.