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Exercise-induced growth hormone secretion and hemorheology during exercise in elite athletes.

Sweating is induced during hyperthermia by cholinergic sympathetic inputs triggered by thermoreceptor-mediated CNS activation. In this process, plasma filtration is increased in cutaneous sweat glands [1–3]. This water loss results in a decrease in both blood volume and blood fluidity [4–8] and is thus likely to exert negative effects on performance. Recent research suggests that growth hormone (GH) may be involved in the mechanism of sweating. Studies in GH-deficient subjects compared to controls demonstrate a reduced sweating rate during exercise that could increase the risk of hyperthermia [9–14]. On the other hand, acromegalic patients suffer from excess sweating [15]. Specific receptors for GH have been evidenced in the epithelium of sweat glands [16,17]. However, studies on the effects of GH on sweating are difficult for several reasons. First, animal models are not relevant to this chapter of human physiology [18]. For instance, exercise reduces GH release in rats while it increases it in humans. On the other hand, experimental alterations in GH status cannot be performed in humans for ethical reasons, unless in deficient subjects undergoing treatment. Actually, there is a wide physiological range of GH responses and GH-related growth factors levels in humans according to the training status and body composition. Furthermore, sweating rates are quite different among humans of various fitness and training conditions [19–22]. To our knowledge, a correlational study of sweat, hemorheology, and GH has not yet been reported. Thus, the scope of this work was to investigate interrelationships among water loss, blood rheology, and GH response during exercise in healthy athletes.