Effect of chronically induced thermotolerance on thermosensitization in CHO cells.

Chronic thermotolerance was induced in Chinese hamster ovary (CHO) cells by pretreatment at 40 degrees C for various times ranging from 15 min to 16 h. The thermotolerant cells were either exposed to single heat treatments at 43 degrees C or subjected to step-down heating consisting of a priming treatment at 43 degrees C for 90 min immediately followed by a graded test treatment at 40 degrees C. The results showed that chronic thermotolerance affected the thermal sensitivity of step-down-heated CHO cells in two ways: by lowering the effectiveness of the priming treatment at 43 degrees C and by reducing the response to the test treatment at 40 degrees C. The effect on the priming treatment corresponds to a reduction in the effective heating time, i.e. the thermotolerant cells respond as if they were exposed to 43 degrees C for times shorter than 90 min. It was further shown that, for a given conditioning treatment, the effectiveness of both the priming and the test treatment was reduced by the same factor; the thermotolerance ratios determined for 43 degrees C and 40 degrees C showed an identical dependence on the duration of the thermotolerance-inducing conditioning treatment. Since thermotolerance development did not reverse heat sensitization by step-down heating, it is concluded that thermotolerance and thermosensitization are distinct phenomena which act independently.