Exercise-induced hyperthermia and hormonal responses to exercise.

Changes in plasma hormonal concentrations during exercise have been ascribed to the type, duration, and intensity of exercise, physical fitness of subjects, oxygen availability and debt, and acid-base balance. However, relatively few studies have examined the possible role of exercise-induced hyperthermia. This paper reviews previous studies on this subject and describes a series of experiments carried out in our laboratories to define the role of changes in body temperature in the release of hormones during exercise. In a first series of experiments, we studied the relationship between thermoregulatory and growth hormone responses to severe exercise at 23 degrees C for 2 h in fit euhydrated subjects, controlling the core temperature increase to a maximum of 40 degrees C by varying wind speed. Exponential relationships were found between increases in core temperature and plasma growth hormone, prolactin, and catecholamines during exercise, suggesting the existence of a thermal threshold for stimulation of hormonal release during exercise. The effect of endurance exercise with and without a thermal clamp (immersion in cold and warm water) on hormonal and leukocyte responses was examined. Again, a significant exponential relationship was found between increases in core temperature and hormonal responses. Thermal clamping significantly diminished the hormonal and the leukocytic responses to exercise, suggesting that an exercise-induced thermal threshold of approximately 38 degrees C exists where hormonal responses are observed. Therefore, core temperature increases may be integrated in the controlling system of hormonal and leukocytic responses to exercise.