Effect of pH on single or fractionated heat treatments at 42-45 degrees

The temperature dependency of the pH-sensitizing effect was determined in cells exposed to single or fractionated heat treatments over the temperature range of 42-45 degrees. Chinese hamster ovary cells were exposed to single graded heat treatments or to an initial heat treatment followed 10 hr later by second heat treatments. Sensitization was quantitated by comparing survival curve terminal slopes of cells heated at pH 7.4 or pH 6.7. Reduction in pH increased the sensitivity of cells exposed to single or fractionated treatments. The magnitude of this sensitizing effect was most pronounced at 42 degrees, regardless of the fractionation scheme and decreased with increasing temperature. At survival levels below approximately 0.15, low pH sensitization was greater by a factor of 2 in cells exposed to fractionated compared to single heat treatments over the temperature range of 43-45 degrees. The increased sensitivity of cells exposed to fractionated heat treatments occurred as a result of reduction in medium pH between heat treatments which inhibited the development of thermoltolerance.