Effects of passive hyperthermia versus exercise-induced hyperthermia on immune responses: hormonal implications

Different stress hormones are released during prolonged exercise and passive hyperthermia. We hypothesized that these different hormonal responses could contribute to the different changes in the immune response during these two challenges.

Eight subjects completed three trials in a randomized order. In the control trial (C), the subjects remained in a sitting posture for three hours in thermoneutral conditions. In the exercise hyperthermia trial (E), they exercised for two hours on a treadmill at 65% max in thermoneutral conditions, followed by 1-h recovery in thermoneutral conditions; in the passive hyperthermia trial (PH), the subjects remained in a semi-recumbent position in a climatic chamber for two hours in hot conditions, followed by 1-h recovery in thermoneutral conditions. During the E and PH trials, wind speed and thermal conditions were modulated to reach a rectal temperature (Tre) of 38.5 degrees C at 60 min and 39 degrees C at 120 min. The subjects did not drink during the experiments. Blood samples (10 mL) were taken at 0, 60, 120 and 180 min of each trial. The total white cell count and its subsets were measured; plasma catecholamines, cortisol and prolactin were assayed. In a whole blood assay, blood leukocytes were stimulated by lipopolysaccharide (LPS) or phytohemagglutinin (PHA) for 24 and 48 hours, respectively. Cytokines, such as TNF-alpha, IL-10 and INF-gamma were measured in the culture supernatant.

The plasma levels of catecholamines were increased only during E, prolactin was increased only during PH, and cortisol was increased in both E and PH. Only the exercise caused a mobilization of blood leukocytes and leukocyte subsets. The INF-gamma and TNF-alpha production by PHA- and LPS-stimulated blood, respectively, were inhibited in a substantial way in both E and PH compared to control when Tre reached 39 degrees C. Only LPS-induced IL-10 production was enhanced during the exercise. The effects of the challenges were increased with 39 degrees C compared to 38.5 degrees C.

Catecholamines play a major role in the mobilization of immunocompetent cells and the production of IL-10 during exercise. Prolactin and catecholamines have adverse role on the immune response, whereas cortisol exerts similar effects during both trials. The consequence could be a protection against inflammatory overshooting.