A quantitative histochemical assay has been used to investigate the effects of hyperthermia on lysosomal acid phosphatase activity in mouse spleen. The results showed a rapid increase in activity which was dependent on the increase in temperature. The activity level returned to normal within about 24 hr, the time course being similar to the decay in thermal enhancement ratio of some tissues for X rays given with time after heating. The variation in enzyme activity observed corresponded very well with the potential thermal enhancement of radiation damage with respect to both temperature dependence and the temporal relationship between heating and irradiation. These results suggest an involvement of lysosomal enzyme activation in the transient ability of hyperthermia to potentiate X-ray damage.

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