The response of previously irradiated mouse skin to heat alone or combined with irradiation: influence of thermotolerance.

The skin of the mouse foot was used to study the effects of previous irradiation on the response to hyperthermia (44 degrees C), to irradiation, or to irradiation combined with hyperthermia (43 degrees C or 44 degrees C). Hyperthermia was applied by immersing the mouse foot into a hot waterbath and irradiation was performed using a 250 kV X-ray generator. Previous irradiation of the feet of mice 90 days before, with 20 Gy, increased the subsequent response to heat alone, or when combined with irradiation, as well as to irradiation alone. It had little effect on the thermal enhancement ratio's for both acute and late skin reactions. Memory of the previous irradiation treatment could be masked when the temperature of the subsequent heat treatment alone, or when combined with irradiation, was 44 degrees C. A priming heat treatment induced resistance both to a subsequent heat treatment and to a subsequent combined irradiation-heat treatment in normal skin as well in previously irradiated skin. This 'resistance' is probably mainly the result of thermotolerance induced in cells in the skin by the priming heat treatment. In thermotolerant skin a 'memory' of the previous irradiation was always evident when the reaction after heat alone or heat combined with irradiation was measured. When the late skin reaction was considered, a larger 'memory' of the previous irradiation treatment was always evident, compared to the acute skin reaction: the 'remembered' dose in the late skin reaction was about two times the 'remembered dose' in the acute skin reaction.