The mechanism by which low pH sensitizes cells to hyperthermic treatments is unknown. In this report we have examined the relationship between the extracellular and intracellular pH of Chinese hamster ovary (CHO) cells after 45.0°C heating. The intracellular pH ( pH i) was measured with a fluorescence technique developed for flow cytometric instrumentation using the dye 1,4-diacetoxy-2,3-dicyanobenzene. Cellular esterase activity was modified by heating, requiring 20 min incubation time with the dye to achieve stable intracellular pH measurements. The intracellular pH of CHO cells incubated under normal pH (7.3) conditions was not perturbed significantly with up to 20 min of heating. Heating from 5 to 15 min induced increases in the pH i of CHO cells incubated under low pH (6.6) conditions. Cells heated in either normal pH or low pH medium became acidified with prolonged heating times. However, the distributions of intracellular pH showed that 20 to 30% of the population heated for up to 60 min at either pH 7.3 or 6.6 overlapped with the pH i distributions of normal cells. A decrease in intracellular pH does not seem to be the principal reason for the heat sensitization caused by low extracellular pH.

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