Thermotolerance in the mouse foot estimated at various levels of normal tissue damage.

The effect of fractionated 43.7 degrees C water bath heating on the skin of CDF1 mice was investigated. The normal tissue damage was scored at five levels (from slight redness and oedema to loss of a toe or greater damage) according to an arbitrary score system. The heating time to induce a given level of damage in half of the treated animals (RD50) was used as an end point. The feet were exposed either to a single treatment at 43.7 degrees C for different time periods or to a priming treatment of 30 min. at 43.7 degrees C followed at different intervals by a second graded heat treatment at 43.7 degrees C. In all treatment schedules, the score level increased proportionally with heating time, and the score system offered a good description of the acute skin damage following hyperthermia. The priming heat treatment induced thermotolerance with a time course independent of the score level chosen to estimate the heat response. The thermotolerance developed rapidly, reached a maximum within a 24 hr. interval, and then decayed slowly. The degree of thermotolerance was calculated by means of two previously described formulas for the thermotolerance ratio (TTR). The kinetics of thermotolerance in the skin of mice was independent of the TTR formula, whereas the degree of thermotolerance depended on both the score level and the TTR formula used.