Body temperature and plasma prolactin and norepinephrine relationships during exercise in a warm environment: effect of dehydration

The effects of euhydration (Eh) and light (Dh₁) and moderate (Dh₂) dehydrations on plasma prolactin (PRL) levels were studied in 5 young male volunteers at rest and during exercise to exhaustion (50% of\(\dot V_{{\text{O}}_{{\text{2}} {\text{max}}} } \)) in a warm environment (Tdb=35° C, rh=20–30%). Light and moderate dehydrations (loss of 1.1 and 1.8% body respectively) were obtained before exercise by controlled hyperthermia. Compared to Eh, time for exhaustion was reduced in Dh₁ and Dh₂ (p<0.01) and rectal temperature (Tre) rose faster in Dh₂ (p<0.05). Both venous plasma PRL and norepinephrine (NE) increased during exercise at any hydration level (p<0.05). Plasma PRL reached higher values after 40 and 60 min in Dh₂ and Dh₁ (p<0.05). Plasma NE values were higher in Dh₂ at rest and at the 40th min during exercise (p<0.05). Plasma PRL was linearly correlated to Tre and plasma NE (p<0.001) but unrelated to plasma volume variation and osmolality. Our results provide further evidence for the major effect of body temperature in exercise-induced PRL changes. Moreover, the plasma PRL-NE relationship suggests that these changes may result from central noradrenergic activation.