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Physiology and Pharmacology of Temperature Regulation

Modification of cutaneous vasodilator response to heat stress by daytime exogenous melatonin administration

¹Department of Physiology, University of Texas Health Science Center at San Antonio, San Antonio, Texas; and ²Department of Hygiene/Space Medicine, Nihon University School of Medicine, Tokyo, Japan

Submitted 15 February 2006 ; accepted in final form 27 April 2006

In humans, the nocturnal fall in internal temperature is associated with increased endogenous melatonin and with a shift in the thermoregulatory control of skin blood flow (SkBF), suggesting a role for melatonin in the control of SkBF. The purpose of this study was to test whether daytime exogenous melatonin would shift control of SkBF to lower internal temperatures during heat stress, as is seen at night. Healthy male subjects (n = 8) underwent body heating with melatonin administration (Mel) or without (control), in random order at least 1 wk apart. SkBF was monitored at sites pretreated with bretylium to block vasoconstrictor nerve function and at untreated sites. Cutaneous vascular conductance, calculated from SkBF and arterial pressure, sweating rate (SR), and heart rate (HR) were monitored. Skin temperature was elevated to 38°C for 35–50 min. Baseline esophageal temperature (T_es) was lower in Mel than in control (P < 0.01). The T_es threshold for cutaneous vasodilation and the slope of cutaneous vascular conductance with respect to T_es were also lower in Mel at both untreated and bretylium-treated sites (P < 0.05). The T_es threshold for the onset of sweating and the T_es for a standard HR were reduced in Mel. The slope of the relationship of HR, but not SR, to T_es was lower in Mel (P < 0.05). These findings suggest that melatonin affects the thermoregulatory control of SkBF during hyperthermia via the cutaneous active vasodilator system. Because control of SR and HR are also modified, a central action of melatonin is suggested.

circadian rhythm; skin blood flow; sweating; thermoregulation

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