Exercise thermoregulation and hyperprolactinaemia

The anterior pituitary hormone prolactin (PRL), measured in the peripheral blood circulation, reflects alterations in central brain 5-hydroxytryptamine (serotonin) and dopaminergic activity and is used as a marker of 'central fatigue' during active heat exposure. Significant correlations have consistently been found between PRL and core temperature (T(CORE)) during prolonged exercise. There has been no investigation into the relationship between PRL and other key thermoregulatory variables during exercise, such as weighted mean skin (T(SK)) and mean body temperature (T(B)), heat storage (HS), thermal gradient (T(GRAD)), heart rate (HR) and skin blood flow (cutaneous vascular conductance, CVC). Therefore, the aim of this study was to ascertain if a significant relationship exists between PRL and these thermoregulatory variables during prolonged exercise. Nine active male subjects conducted three trials of approximately 60% VO(2peak) at 70-80 rpm for 45 min on a semi-recumbent cycle ergometer at three different ambient temperatures [6 degrees C (Cold), 18 degrees C (Neutral) and 30 degrees C (Hot)] to elicit varying levels of thermoregulatory stress during exercise. Significant differences existed in T(SK), T(B), HS, T(GRAD) and CVC across the environmental conditions (p < 0.001). Core temperature (T(CORE)), HR and PRL were significantly elevated only in Hot (p < 0.05). Moderate correlations were found for T(CORE), T(SK), T(B), HS, T(GRAD), HR and CVC with post-exercise PRL (rho = 0.358-0.749). The end-of-exercise <38.0 degrees C T(CORE) responses were not (rho = -0.129, p > 0.05) but the >38.0 degrees C T(CORE) responses were (rho = 0.845, p < 0.001) significantly related to their corresponding PRL responses. The significant relationships between PRL release and T(SK), T(B), HS, T(GRAD), HR and CVC have extended previous research on T(CORE) and PRL release and indicate an association between these thermoregulatory variables, as well as T(CORE), and serotonergic/dopaminergic activity during prolonged exercise.