Thermoregulatory changes induced during heat acclimatization by controlled hypothermia in man.

1. Heat acclimatization has been carried out by a controlled hyperthermia procedure, and induced thermoregulatory changes have been investigated for nine fit young men.2. During the experiments two types of thermal responses became apparent. The subjects tolerating the rise in the central temperature during hyperthermia displayed a marked improvement of their sweating capacity and their sweating performances, and a parallel shift towards lower body temperatures of the curves relating sweat rate to tympanic, rectal and mean skin temperatures.3. This shift occurred without any change in the slope, that is without any change in the gain of the central control system. It was accompanied by a concomitant shift of the threshold of sweating onset, so that for a given central temperature there was a higher sweat rate following treatment.4. The sweat decline appeared late in tolerant subjects. This decline was early in the intolerant subjects and appeared as early as the first day of heat treatment. Moreover, these subjects displayed no increase of sweat output with repeated sessions of heat treatment.5. The core-periphery temperature gradient measured during the cooling stage did not change in the tolerant subjects when acclimatization had developed. This gradient increased in the intolerant subjects because deep temperature decreased slowly whilst skin temperature decreased rapidly. The core of intolerant subjects retained the stored heat and dissipated it slowly. The tolerant subjects seemed on the contrary to display in the same conditions increased tissue conductance enabling rapid dissipation of the stored heat.