Influence of time and temperature on the kinetics of thermotolerance in L1A2 cells in vitro.

The overall importance of the primary heat treatment temperature and heating time for the degree and kinetics of thermotolerance was investigated in L1A2 cells in vitro. The degree and time course of thermotolerance developed following primary heating were independent of the priming temperature (in the range 41-44 degrees), if the heating time was adjusted to give identical survival levels. A pretreatment at these temperatures with a survival level of approximately 8%, equivalent to 90 min at 42 degrees, resulted in maximal thermotolerance at a 10-hr interval with a thermotolerance ratio (TTRmax) of approximately 4.3. This was also found irrespective of the temperature (in the range 41-45 degrees) of the second heat treatment. Preheating of cells at 42 degrees for 45, 90, 110, or 135 min corresponding to survival levels of approximately 40, 8, 3.2, and 1.8%, respectively, induced a subsequent delay of 0.8 to 5.8 hr in the onset of thermotolerance. In addition, with more severe primary heat treatments, the delay period, the TTRmax, and the time interval at 37 degrees necessary to develop TTRmax increased. Maximal thermotolerance was obtained at an interval of 6, 10, 13, and 16 hr, respectively, with TTRmax's of 2.9, 4.2, 5.3, and 5.9, respectively. In contrast, the rate of both development and decay of thermotolerance was independent of the primary heating time. These data indicate that the degree and kinetics of thermotolerance in L1A2 cells depend on the survival level following the primary heating irrespective of the treatment temperature and heating time used to obtain this survival level.