Exposure to pretreatment hypothermia as a determinant of heat killing

When Chinese hamster ovary (CHO) cells were exposed to 22°C for 2 hr prior to 42.4°C hyperthermia, neither the shoulder region of the survival curve nor the characteristic development of thermotolerance after 3-4 hr of heating were observed. Absolute cell survival after 4 hr at 42.4°C was decreased by a factor of between 10 and 100 (depending on the rate of heating of nonprecooled controls). Conditioning at 30°C for 2 hr, 26°C for 2 hr, or 22°C for 20 min followed by heating to 42.4°C over 30 min did not result in sensitization. Prolonged (16 hr) conditioning at 30°C, however, increased the cytotoxicity of immediate exposure to 41.4 or 45° C with maximum sensitization to 45°C occurring after 6 hr at 30°C. Both 3- and 18-hr pretreatments at 30°C similarly increased the cytotoxicity of 45-41.5°C step-down heating (D₀ = 28 min in precooled versus 40 min in nonprecooled cells).