Induced thermal resistance in the mouse ear

Abstract

The mouse ear (pinna) was used to investigate the effect of 2 hyperthermic treatments [commonly used in clinical practice]. Heating was by immersion in hot water at 43.5.degree. C. A single treatment of about 50 min was required to cause necrosis in 50% of the ears heated. When heat treatment was given in 2 equal fractions the total heating time had to be increased if the interval between fractions was greater than 4 h. By 24 h a total treatment of about 100 min was required, indicating almost complete recovery from the 1st heating. Priming treatments at 43.5.degree. C induced thermal resistance to a 2nd heat treatment at 43.5.degree. C. Maximum resistance was observed 1 day after a 20 min priming and 2 days after a 40 min priming, when the heating time had to be increased to 120 min, an increase by a factor of 2.4. Shorter priming treatments induced less resistance, the minimum heating time to produce an effect being 2 min. In all cases the effect decreased during the next 4-5 days. The reduced response of tissues to fractionated hyperthermia is probably due to the repair of sublethal heat damage and induction of thermal resistance.