Sensitization to hyperthermia induced in a normal tissue by step-down heating

The effect of step-down heating was investigated in the skin of the CDF1 mouse foot. Step-down heating was induced with a 44.7 degrees C/10 min pretreatment followed by a test treatment at a lower temperature for variable time. Step-up heating, that is, a test treatment followed by a 44.7 degrees C/10 min treatment, and single heating were used as controls. The normal tissue reaction was scored at five levels of damage (from slight redness and oedema to loss of a toe or greater reaction), and the heating time to induce each level in 50% of the animals, RD50, was used as the endpoint. The effect of step-down heating was quantified by the step-down ratio, calculated as the ratio of test heating times to obtain the endpoint. A significant reduction of the RD50 was seen at all score levels when the 44.7 degrees C/10 min was given in a step-down heating schedule, and the effect increased with decreasing test treatment temperature. In contrast, the heat sensitivity was only marginally influenced by step-up heating. An analysis of the time-temperature relationship demonstrated a log-linear relationship between temperature and RD50 for single heating in the range 42.2-44.7 degrees C and for step-down heating in the range 41.7-44.7 degrees C. The curve for step-down heating showed a lesser slope indicating a decrease of the activation energy. The kinetics of the SDH effect were investigated by inserting an interval between a primary 44.7 degrees C/10 min treatment and a test treatment performed at 42.2 degrees C. The effect of step-down heating was maximal with no interval between the priming treatment and the test treatment. As the interval was increased to 1.5 hr the step-down sensitization disappeared, and with even longer intervals thermotolerance developed. From a clinical point of view, the present data indicate that step-down heating may increase the extent of both reversible and irreversible heat damage in the normal tissue.