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NEW FINDINGS What is the central question of this study? To examine, in male lowlanders, whether, and to what extent, hypoxia would modulate thermoregulatory effector responses during repeated whole-body cold stress encountered in a single day. What is the main finding and its importance? A ∼10-h sustained exposure to hypoxia appears to mediate a gradual upregulation of endogenous heat production, preventing the progressive hypothermic response prompted by the serial cold stimuli. Also, hypoxia progressively degraded mood, and compounded the perceived thermal discomfort, and sensations of fatigue and coldness. ABSTRACT We examined whether hypoxia would modulate thermoeffector responses during repeated cold stress encountered in a single day. Eleven men completed two ∼10-h sessions, while breathing, in normobaria, either normoxia or hypoxia (PO2 : 12 kPa). During each session, subjects underwent sequentially three 120-min immersions to the chest in 20°C water (CWI), interspersed by 120-min rewarming. In normoxia, the final drop in rectal temperature (Trec ) was greater in the third (∼1.2°C) than in the first and second (∼0.9°C) CWIs (P < 0.05). The first hypoxic CWI augmented the Trec fall (∼1.2°C; P = 0.002); but the drop in Trec did not vary between the three hypoxic CWIs (P = 0.99). In normoxia, the metabolic heat production (M) was greater during the first half of the third CWI than during the corresponding part of the first CWI (P = 0.02); yet the difference was blunted during the second half of the CWIs (P = 0.89). In hypoxia, by contrast, the increase in M was augmented by ∼25% throughout the third CWI (P < 0.01). Regardless of the breathing condition, the cold-induced elevation in MAP was blunted in the second and third CWI (P < 0.05). Hypoxia aggravated the sensation of coldness (P = 0.05) and thermal discomfort (P = 0.04) during the second half of the third CWI. Present findings therefore demonstrate that prolonged hypoxia mediates, in a gradual manner, metabolic and thermoperceptual sensitization to repeated cold stress. This article is protected by copyright. All rights reserved.
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