The Effect of Low pH on Thermotolerance Induction Using Fractionated 45°C Hyperthermia

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Abstract
The effect of low pH on thermotolerance induction was studied in Chinese hamster ovary [CHO] cells in culture. An exposure of 10 min at 45.degree. C resulted in maximal thermotolerance 7 h later. The thermotolerance ratio (TTR), defined as the ratio of the Do [mean lethal dose] of the 2nd (thermotolerant) heat survival curve to the Do of the initial (single-dose) heat survival curve, was 4.2 .+-. 0.2 for pH 7.2. The pH was reduced during both heating and induction intervals, and the TTR was reduced proportionately with pH to a lowest value of 1.7 at pH 6.3. The Do values for the single-dose heat survival curves fell from 3.6 min at pH 7.2 to 1.3 min at pH 6.3, while those of the thermotolerant heat survival curves fell from 14.8 min at pH 7.2 to 2.4 min at pH 6.3. The data are discussed in light of the potential application of hyperthermia as a treatment modality in tumor therapy.