Activation of autonomic thermoeffectors preceding the decision to behaviorally thermoregulate in resting humans

We tested the hypothesis that acute increases in metabolic heat production and sweating precede the initiation of thermoregulatory behavior in resting humans exposed to cool and warm environments. Twelve healthy young subjects passively moved between 17 °C and 40 °C rooms when they felt 'too cool' (CW) or 'too warm' (WC). Skin and internal (intestinal) temperatures, metabolic heat production, local sweat rate (forearm, chest), and cutaneous vascular conductance (CVC; forearm, fingertip) were measured continually. Compared to pre-test Baseline (31.8 ± 0.3 °C), skin temperature was higher at CW (32.0 ± 0.7 °C, P = 0.01) and WC (34.5 ± 0.5 °C, P < 0.01). Internal temperature did not differ (P = 0.12) between Baseline (37.2 ± 0.3 °C), CW (37.2 ± 0.3 °C) or WC (37.0 ± 0.3 °C). Metabolic heat production was not different from Baseline (40 ± 9 W/m²) at CW (39 ± 7 W/m², P = 0.50). Forearm (0.06 ± 0.01 mg/cm²/min) and chest (0.04 ± 0.02 mg/cm²/min) sweat rate at WC did not differ from Baseline (forearm: 0.05 ± 0.02 mg/cm²/min, chest: 0.04 ± 0.02 mg/cm²/min, P ≥ 0.23). Forearm CVC was not different from Baseline (0.30 ± 0.21 PU/mmHg) at CW (0.24 ± 0.11, P = 0.17), but was higher at WC (0.65 ± 0.33 PU/mmHg, P < 0.01). Fingertip CVC was different from Baseline (2.6 ± 2.0 PU/mmHg) at CW (0.70 ± 0.42 PU/mmHg, P < 0.01) and WC (4.49 ± 1.66 PU/mmHg, P < 0.01). Thermoregulatory behavior at rest in cool and warm environments is preceded by changes in vasomotor tone in glabrous and non-glabrous skin, but not by acute increases in metabolic heat production or sweat rate.

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