INDUCTION OF THERMAL TOLERANCE OF RAT SCIATIC-NERVE BY MILD HYPERTHERMIA

A 5 mm segment of the rat sciatic nerve was treated at 38 or 43-degrees-C for 30 min using a brass thermode. This pretreatment was followed by a test heat treatment at 45-degrees-C. Different intervals between the pretreatment and test treatment were studied. The effect of fractionated hyperthermia on the motor function of rat sciatic nerve was evaluated using a functional assay, the toe-spreading test. Both pretreatments led to thermal resistance of the nerve, which was maximal 24 h after the pretreatment. Thermal resistance, induced at 38-degrees-C, did not show any decay over a period of 6 weeks. Thermal resistance, induced at 43-degrees-C, decayed slowly, but after a 6-week interval between priming and test heat treatment thermal resistance was still observed. As the resistance induced by a mild heat pretreatment is transient, we considered this to be thermotolerance. We accounted for the thermal resistance induced by the 38-degrees-C pretreatment in the calculation of the thermal tolerance ratio (TTR) after mild heat treatment at 43-degrees-C. Maximal thermal tolerance was observed 24 h after mild heat with a TTR of 3.4+/-0.6. The TTR after a 6-week interval had declined to 1.4+/-0.3.