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

Abstract

New Findings r 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? r 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.

Which comes first in human temperature regulation: the physiological or the behavioural response?

“…Schlader and colleagues have been at the forefront of addressing this important gap in the literature and, in the current issue of Experimental Physiology, expand on their previous work by addressing the question of whether changes in metabolic heat production and sweating precede thermal behaviour (Schlader et al 2009(Schlader et al , 2013(Schlader et al , 2016a. Using a novel experimental design, in which participants were free to move between cool and warm rooms, Schlader et al (2016a) found that initiation of thermal behaviour precedes any changes in metabolic heat production when going from a cool to a warm room as well as changes in sweating when going from a warm to a cool room.…”

“…Conversely, initiation of thermal behaviour appeared to coincide with changes in cutaneous vasomotor tone such that there was substantial cutaneous vasoconstriction when moving from a cool to a warm room and maximal withdrawal of vasoconstrictor tone when moving from warm to cool (and perhaps engagement of the active vasodilator system; Schlader et al 2016a). Inasmuch as changes in skin blood flow can affect skin temperature, the findings of Schlader et al (2016a) appear to support the notion that changes in skin temperature are the primary determinants of thermal behaviour (Cabanac et al 1972;Schlader et al 2013). If, as proposed above, we accept the concept that thermal behaviour is a mechanism to maintain thermal homeostasis and, perhaps, to minimize our reliance on physiology, these findings make sense.…”

“…Using a novel experimental design, in which participants were free to move between cool and warm rooms, Schlader et al (2016a) found that initiation of thermal behaviour precedes any changes in metabolic heat production when going from a cool to a warm room as well as changes in sweating when going from a warm to a cool room. Conversely, initiation of thermal behaviour appeared to coincide with changes in cutaneous vasomotor tone such that there was substantial cutaneous vasoconstriction when moving from a cool to a warm room and maximal withdrawal of vasoconstrictor tone when moving from warm to cool (and perhaps engagement of the active vasodilator system; Schlader et al 2016a). Inasmuch as changes in skin blood flow can affect skin temperature, the findings of Schlader et al (2016a) appear to support the notion that changes in skin temperature are the primary determinants of thermal behaviour (Cabanac et al 1972;Schlader et al 2013).…”

Physiological benefits likely underlie the systematic recruitment of thermoeffectors

“…We have previously identified that the initiation of thermal behavior temporally precedes the initiation of sweating in the heat and changes in metabolism in the cold [3]. However, thermal behavior is consistently preceded by changes in skin blood flow [3].…”

“…When given the opportunity to behaviorally thermoregulate, the initiation of thermal behavior is mediated by changes in skin temperature, largely preventing changes in core temperature [3]. We have previously identified that the initiation of thermal behavior temporally precedes the initiation of sweating in the heat and changes in metabolism in the cold [3].…”