Evidence for an upper temperature limit for thermotolerance development in L1A2 tumour cells in vitro

The response of L1A2 cells in vitro to water bath hyperthermia was investigated. In previous studies in the temperature range 40.5-45.0 degrees C, an Arrhenius plot for heat killing of L1A2 cells was linear, and thermotolerance did not develop during continuous heating. The present Arrhenius analysis was extended to the range 38-45 degrees C, and the Arrhenius plot was biphasic with an inflection point at 40.5 degrees C, below which the activation energy was significantly increased from 724 to 923 kJ/mol (P less than 0.001). In order to test for the development of thermotolerance during continuous heating, the cells were heated in the range 38-41 degrees C for 10 h, followed immediately by a graded test treatment at 42 degrees C. Thermotolerance developed below 40.5 degrees C as shown by an increased D0 of the 42 degrees C survival curve, but not at 40.5 and 41 degrees C. Preheating for 90 min at 42 degrees C followed by a 10 h incubation at 37 degrees C resulted in maximal thermotolerance with a thermotolerance ratio of approximately 4.3, a ratio also obtained if the cells were incubated for 10 h at temperatures of 38-40 degrees C. No thermotolerance was observed at incubation temperatures of 40.5 degrees C and above. Thus, in the L1A2 cells 40 degrees C is the upper temperature permissive for thermotolerance development, and the data support the assumption that the inflection point on the Arrhenius plot reflects the upper limit for thermotolerance development.