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

Abstract

What is the central question of this study? Do increases in metabolic heat production and sweat rate precede the initiation of thermoregulatory behaviour in resting humans exposed to cool and warm environments? What is the main finding and its importance? Thermoregulatory behaviour 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. These findings suggest that sweating and shivering are not obligatory for thermal behaviour to be initiated in humans. We tested the hypothesis that acute increases in metabolic heat production and sweating precede the initiation of thermoregulatory behaviour in resting humans exposed to cool and warm environments. Twelve healthy young subjects passively moved between 17 and 40°C rooms when they felt ‘too cool’ (C→W) or ‘too warm’ (W→C). Skin and internal (intestinal) temperatures, metabolic heat production, local sweat rate (forearm and chest) and cutaneous vascular conductance (CVC; forearm and fingertip) were measured continually. Compared with pretest baseline (31.8 ± 0.3°C), skin temperature was higher at C→W (32.0 ± 0.7°C; P = 0.01) and W→C (34.5 ± 0.5°C; P < 0.01). Internal temperature did not differ (P = 0.12) between baseline (37.2 ± 0.3°C), C→W (37.2 ± 0.3°C) and W→C (37.0 ± 0.3°C). Metabolic heat production was not different from baseline (40 ± 9 W m−2) at C→W (39 ± 7 W m−2; P = 0.50). Forearm (0.06 ± 0.01 mg cm−2 min−1) and chest (0.04 ± 0.02 mg cm−2 min−1) sweat rate at W→C did not differ from baseline (forearm, 0.05 ± 0.02 mg cm−2 min−1 and chest, 0.04 ± 0.02 mg cm−2 min−1; P ≥ 0.23). Forearm CVC was not different from baseline (0.30 ± 0.21 perfusion units (PU) mmHg−1) at C→W (0.24 ± 0.11 PU mmHg−1; P = 0.17), but was higher at W→C (0.65 ± 0.33 PU mmHg−1; P < 0.01). Fingertip CVC was different from baseline (2.6 ± 2.0 PU mmHg−1) at C→W (0.70 ± 0.42 PU mmHg−1; P < 0.01) and W→C (4.49 ± 1.66 PU mmHg−1; P < 0.01). Thermoregulatory behaviour 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.