[Survival of acclimatized and hypothyroid rats at 40 degrees C (author's transl)].

The survival of rats acclimatized to either 5 degrees C or 30 degrees C, treated or not with propylthiouracil (P.T.U.), was determined at 40 degrees C. Changes in metabolic rate and rectal temperature were followed throughout exposure. Total water loss was estimated at the end of the experiment. The survival time of rats accclimatized to 5 degrees C was less than that of rats acclimatized to 30 degrees C, and that of controls was less than that of treated animals. The differences reported seem to be independent of heat transfer capacities, which are similar in all treatment conditions and possibly limited by the experimental set up. A high thermogenic rate would appear to limit resistance to heat under conditions where external heat stress approaches heat transferability, such that the animal's temperature overcomes the lethal threshold, and leads to heat stroke. Results of P.T.U. treatment suggest the involvement of thyroid hormones in heat stress-induced thermogenesis and in cellular lability to elevated internal temperatures.