Received June 10, 2005
Revised July 6, 2005
Accepted after revision September 12, 2005

¹ Institute for Exercise and Environmental Medicine
² Sheffield Hallam University
³ Liverpool John Moores University

^* To whom correspondence should be addressed. E-mail: dbloke2004{at}yahoo.com.

	Abstract

The aim of this study was to compare the prolactin and blood pressure responses at identical core temperatures during active and passive heat stresses, using prolactin as an indirect marker of central fatigue. Twelve male subjects cycled to exhaustion at 60% VO_{2 peak} in a 33° C room (active). In a second trial they were passively heated (passive) in a water bath (41.56 ± 1.65° C) until core temperature was equal to the core temperature observed at exhaustion during the active trial. Blood samples were taken from an indwelling venous cannula for the determination of serum prolactin during active heating and at corresponding core temperatures during passive heating. Core temperature was not significantly different between the two methods of heating and averaged 38.81 (± 0.53) °C and 38.82 (± 0.70) °C at exhaustion during active and at the end of passive heating, respectively (p > 0.05). Mean arterial blood pressure was significantly lower throughout passive heating [active, 73 (± 9), passive, 62 (± 12) mm Hg, p < 0.01]. Despite similar core temperatures, but significantly reduced blood pressure responses, the prolactin response was the same during both forms of heating [active, 14.9 (± 12.6), passive, 13.3 (± 9.6) ng ml^-1, p > 0.05]. These results suggest that thermoregulatory, i.e. core temperature and not cardiovascular afferents, provide the key stimulus for prolactin release, an indirect marker of central fatigue, during exercise in the heat.

Key Words: Exercise, Hyperthermia, Prolactin

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