The effect of pH on cell lethality induced by hyperthermic treatment.

Abstract

Chinese hamster ovary (CHO) cells were subjected to hyperthermic treatment (41–45.5°C) in either acidic or alkaline medium. Consistent with previous reports, thermal sensitivity was increased as the range of the environmental pH level was decreased from 7.4–7.5 to 6.6–6.7. The increase in sensitivity was expressed as a decrease in the shoulder and an increase in the slope of the survival curve. Incubation at 37°C in acidic medium for 24 hours prior to hyperthermic treatment at 45.5°C neither enchanced nor diminished thermal sensitivity. However, thermal damage acquired during heating (42.5–45.5°C) in acidic or alkaline medium could be augmented by incubating the cells at 37°C in acidic medium after hyperthermic treatment. This augmentation of thermal damage was expressed as a further increase in the final slope of the survival curve. An Arrhenius plot of the rate of heat inactivation as a function of the inverse of absolute temperature indicated that heating in acidic medium increased the inactivation rate compared to the rate which occurred for cells heated in alkaline medium. However, the slope of the Arrhenius curve (i.e., activation energy) was fairly independent of the environmental pH level. When CHO cells were treated in alkaline medium, the Arrhenius curve changed slope at 43°C. But when cells were treated in acidic medium, the temperature at which the curve changed slope depended upon the duration of treatment in acidic medium after the hyperthermic exposure. Heating at 42°C or below resulted in the appearance of a thermotolerant population. The onset of thermotolerance occurred after 200–300 minutes of heating, regardless of the environmental pH level at the time of heating. However, the percentage of the population that expressed thermotolerance when cells were heated at a given temperature decreased when cells were heated in acidic medium. The most interesting observations were: 1 that heat treatments at low pH decreased the number of thermotolerant cells; and 2) that when cells were maintained at low pH after hyperthermic treatment, thermal damage was augmented. These observations may have important implications in the clinic.