Thermotolerance: a review of observations and possible mechanisms.

The effects of hyperthermia on the response of cells and tissues to subsequent heating given alone or in combination with X-rays or drugs are considered. Two types of such induced thermotolerance are described: Type I results from continuous heating at temperatures below about 43 degrees C; type II results from a prior heating at a higher temperature, with subsequent incubation at about 37 degrees C. The time courses of the various types of thermotolerance are considered in relation to the magnitude of the priming treatment and whether the test is hyperthermia alone or in combination with X-rays or drugs. Reference is made to in vivo results wherever possible as well as to in vitro data. Factors which influence the development of thermotolerance are discussed, and some tentative hypotheses are made concerning the mechanisms leading to the phenomenon.