1. Although α-adrenoceptor antagonists have been shown to induce core hypothermia in animals, it is unclear whether the primary mechanism is increased heat loss or decreased heat production. Furthermore, studies have not been performed in humans to determine the role of α-adrenoceptors in the maintenance of core temperature. 2. α-Adrenoceptor blockade was achieved with three doses of phentolamine given by random assignment on three different study days in five male and five female healthy subjects. Core temperature, mean skin-surface temperature, fingertip capillary blood flow and metabolic heat production were measured. Dose-response curves were plotted for all measured variables, and males and females were compared to identify potential gender differences. 3. Core temperature decreased with all doses of phentolamine. At the completion of the phentolamine infusion, the decrease in core temperature was more significant with high-dose (0.3 ± 0.1°C, P = 0.03) and with medium-dose (0.2 ± 0.0°C P = 0.05) phentolamine than with low-dose phentolamine (0.1 ± 0.0°C), The maximum core temperature decrease during the study was more significant with high dose (0.6 ± 1°C) than with medium (0.3 ± 1°C, P = 0.04) or low (0.3 ± 1°C, P = 0.005) doses. Mean skin-surface temperature was increased with all doses. Fingertip blood flow was increased (approximately 60% above baseline) with the medium and high doses, but was unchanged with the low dose. Total body oxygen consumption was unchanged regardless of dose. Although females had a higher core temperature at baseline, changes in core temperature, skin-surface temperature and fingertip blood flow were similar in males and females. 4. These findings suggest that α-adrenergicantagonist-induced hypothermia results from a dose-dependent redistribution of heat from the core to the periphery, and not from a decrease in metabolic heat production. This leads us to conclude that baseline α-adrenergic 'tone' serves to maintain core temperature by constraining heat to the core thermal compartment.
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