Heat-induced thermal resistance and its relationship to lysosomal response

Two separate effects of hyperthermia on mouse splenic lysosomes have been reported, dependent on the severity of the treatment. Heating to temperatures below 42.5 degrees C causes a transient increase in lysosomal acid phosphatase activity which can be correlated with the ability of moderate hyperthermia to potentiate X-ray damage. Heating to temperatures above 42.5 degrees C results in an immediate increase in lysosomal membrane permeability which may be involved in tissue necrosis. By giving a priming heat treatment at 41.8 degrees C, induced thermal resistance was demonstrated for the lysosomal membrane effect, but not for the enzyme activation. The degree of induced thermal resistance observed is similar to that reported for the cell-killing effect of heat on tissues in vivo and cells in vitro and occurs over a similar time course. The relevance of these results to the understanding of fractionated hyperthermia in cancer therapy is discussed.