Colonic heating patterns and the variation of thermal resistance among rats

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Abstract
The colonic heating patterns of rats were examined in relation to exposure temperature and humidity, hypertension and dehydration. In addition, heat resistance was determined between 0800-2000 h. Three basic patterns of heating were discerned. Type 1 individuals heated from the resting to the lethal temperature in a linear fashion. Type 2 individuals exhibited a 2-stage heating curve with a decrease in the rate of heating during the 2nd segment of the curve. Type 3 animals showed a 3-stage heating curve with attainment of a true equilibrium temperature and a marked decrease in the rate of core temperature elevation during segment 2 which was in turn followed by an increase in the rate of heating that continued to the lethal temperature. Where total survival time differed among the treatment groups, it was generally noted that groups which showed decreased thermal resistance had a higher percentage of type 1 and 2 individuals. Segmental analysis of the heating curves indicated that decreased survival time in groups exposed to similar conditions was related to a reduction in segment 2 duration. The mechanism of the increase observed in thermal resistance between morning and evening determinations was not understood but was coincident with differences in the preexposure state of hydration. An unexplained depression of the lethal temperature was also noted in afternoon determinations.

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