The effect of reduced pH on the induction of thermotolerance.

The effect of low pH on thermotolerance induction was studied in Chinese hamster ovary cells. The thermotolerance ratio (TTR), defined as the ratio of the Do of a subsequent heat survival curve following heat challenge to the Do of the single-dose heat survival curve, was found at pH 7.2 to be maximal at 3.6 seven hours following a 10-min 45 degrees initial heat dose. When heating and induction times were at pH 6.6, the TTR ws reduced to 3.1. When the pH was reduced to 6.6 only during the induction period, the TTR was even lower at 2.5. Heat sensitization at low pH was distinguishable from inhibition of thermotolerance induction. Both the heat sensitization and reduced thermotolerance induction at low pH may enhance the thermal response of tumors relative to normal tissues.