Differential thermal behavioral response to heat vs. cold stress in humans

Abstract

PurposeData from non‐human endotherms indicate that thermal behavioral responses to increases in core temperature (Tcore) are more robust when compared to similar reductions in Tcore. These observations have been interpreted as a protective response, owing to the proximity of the endothermic Tcore to the upper lethal limit compared to the lower lethal limit. In resting humans, thermal behavior is mostly initiated by changes in skin temperature (Tskin). There is some evidence that thermal behavior in humans is more sensitive to increases in Tskin vs. decreases. On the contrary, it is often posited that skin cooling evokes a greater thermoregulatory response compared to equivalent increases in Tskin because of the greater density of cold receptors in the skin. Based on this uncertainty, we tested the hypothesis that thermal behavior is more sensitive during heat vs. cold stress.MethodsWearing a water perfused suit, twelve healthy adults (23 ± 2 y, 6 F) underwent 60 min of heat (HEAT) and cold (COLD) stress that induced gradual changes in mean body temperature (Tbody). Using a custom‐made device, subjects controlled the temperature of their dorsal neck to perceived thermal comfort. Thus, neck temperature provided an index of thermal behavior. Neck temperature, weighted Tskin (6 site), and Tcore (intestinal) were measured continually. Tbody, the integrated stimulus for thermal behavior, was calculated as the unweighted average of Tskin and Tcore. Neck temperature was analyzed using segmental regression analysis, providing an analysis of thermal behavior relative to changes in Tbody. This analysis provided two parameters: a) threshold, which is indicative of the activation of thermal behavior as a function of Tbody, and b) gain, which is the slope of the thermal behavioral response to continued changes in Tbody after the threshold. To permit direct comparisons between HEAT and COLD, data were analyzed as absolute values and the gain was normalized to the maximum change within a trial and subject.ResultsIn HEAT, Tskin increased by 3.7 ± 0.4°C through 36 min (P<0.01) and an increase in Tskin of 4.1 ± 0.2°C was maintained thereafter. Elevations in Tcore occurred at 36 min, with Tcore maximally increasing by 0.9 ± 0.3°C (P<0.01). In HEAT, neck temperature was decreased at 40 min (P=0.03) with a maximal reduction of 7.9 ± 4.6°C. In COLD, Tskin decreased throughout, dropping by 6.0 ± 0.6°C (P<0.01), while Tcore did not change (P=0.10). In COLD, neck temperature was increased at 32 min (P=0.01) with a maximal increase of 4.3 ± 1.9°C. The threshold change in Tbody upon which thermal behavior was initiated was lower in COLD (0.8 ± 0.7 vs. 1.3 ± 0.7°C, P=0.04). However, the relative gain of the response after the threshold was greater in HEAT (3.8 ± 3.1 vs. 1.4 ± 0.9 a.u., P=0.02).ConclusionsCompared to cold stress, thermal behavior during heat stress is initiated following greater changes in Tbody. However, after activation the relative sensitivity of thermal behavior to continued changes in Tbody is greater during heat stress. These findings are consistent with a higher density of cold receptors in the skin, yet also support that thermal behavior more robustly defends increases in Tbody after thermal behavior is initiated.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.