Influence of low pH on the development and decay of 42 degrees C thermotolerance in CHO cells

The development, magnitude and decay of thermotolerance in Chinese hamster ovary cells heated to 42 degrees C at pH 6.7 was examined. The cells were exposed to single or fractionated heat treatments with 0 to 168 hours elapsing between the treatments. Administration of a specified heat treatment in two fractions at low pH substantially reduced the lethal effect of heat. The rate of hyperthermic cell killing was reduced by a factor of approximately 2.5 in preheated cells compared to cells not receiving prior heat treatment. Tolerance to second heat treatments was apparent when 3 hours at 37 degrees C separated the treatments, and was near maximum when 6 hours separated the treatments. Beginning about 96 hours after the initial heat treatment, resistance to second heat treatments began to decline, and was not evident when 168 hours at 37 degrees C separated the treatments. In contrast to these results, a sparing effect as a result of dose fraction was not observed at pH 7.4. At pH 7.4, cells developed thermotolerance during the initial heat treatment and additional culturing at 37 degrees C was without additional effect. Nevertheless, the sensitivity of cells to hyperthermia was greater at pH 6.7 than at pH 7.4. This pH sensitizing effect was more pronounced in cells exposed to single heat treatments than in cells exposed to fractionated treatments at 42 degrees C.