RESUMEN
KEY POINTS: With the advent of more frequent extreme heat events, adaptability to hot environments will be crucial for the survival of many species, including humans. However, the mechanisms that mediate human heat adaptation have remained elusive. We tested the hypothesis that heat acclimation improves the neural control of body temperature. Skin sympathetic nerve activity, comprising the efferent neural signal that activates heat loss thermoeffectors, was measured in healthy adults exposed to passive heat stress before and after a 7 day heat acclimation protocol. Heat acclimation reduced the activation threshold for skin sympathetic nerve activity, leading to an earlier activation of cutaneous vasodilatation and sweat production. These findings demonstrate that heat acclimation improves the neural control of body temperature in humans. ABSTRACT: Heat acclimation improves autonomic temperature regulation in humans. However, the mechanisms that mediate human heat adaptation remain poorly understood. The present study tested the hypothesis that heat acclimation improves the neural control of body temperature. Body temperatures, skin sympathetic nerve activity, cutaneous vasodilatation, and sweat production were measured in 14 healthy adults (nine men and five women, aged 27 ± 5 years) during passive heat stress performed before and after a 7 day heat acclimation protocol. Heat acclimation increased whole-body sweat rate [+0.54 L h-1 (0.32, 0.75), P < 0.01] and reduced resting core temperature [-0.29°C (-0.40, -0.18), P < 0.01]. During passive heat stress, the change in mean body temperature required to activate skin sympathetic nerve activity was reduced [-0.21°C (-0.34, -0.08), P < 0.01] following heat acclimation. The earlier activation of skin sympathetic nerve activity resulted in lower activation thresholds for cutaneous vasodilatation [-0.18°C (-0.35, -0.01), P = 0.04] and local sweat rate [-0.13°C (-0.24, -0.01), P = 0.03]. These results demonstrate that heat acclimation leads to an earlier activation of the neural efferent outflow that activates the heat loss thermoeffectors of cutaneous vasodilatation and sweating.