Mean body temperature does not modulate eccrine sweat rate during upright tilt

Conflicting reports exist about the role of baroreflexes in efferent control of eccrine sweat rate. These conflicting reports may be due to differing mean body temperatures between studies. The purpose of this project was to test the hypothesis that mean body temperature modulates the effect of head-up tilt on sweat rate and skin sympathetic nerve activity (SSNA). To address this question, mean body temperature (0.9 · internal temperature + 0.1 · mean skin temperature), SSNA (microneurography of peroneal nerve, n = 8), and sweat rate (from an area innervated by the peroneal nerve and from two forearm sites, one perfused with neostigmine to augment sweating at lower mean body temperatures and the second with the vehicle, n = 12) were measured in 13 subjects during multiple 30° head-up tilts during whole body heating. At the end of the heat stress, mean body temperature (36.8 ± 0.1 to 38.0 ± 0.1 °C) and sweat rate at all sites were significantly elevated. No significant correlations were observed between mean body temperature and the change in SSNA during head-up tilt (r = 0.07; P = 0.62), sweating within the innervated area (r = 0.06; P = 0.56), sweating at the neostigmine treated site (r = 0.04; P = 0.69), or sweating at the control site (r = 0.01; P = 0.94). Also, for each tilt throughout the heat stress, there were no significant differences in sweat rate (final tilt sweat rates were 0.69 ± 0.11 and 0.68 ± 0.11 mg · cm^-2 · min^-1 within the innervated area; 1.04 ± 0.16 and 1.06 ± 0.16 mg · cm^-2 · min^-1 at the neostigmine-treated site; and 0.85 ± 0.15 and 0.85 ± 0.15 mg · cm^-2 · min^-1 at the control site, for supine and tilf, respectively). Hence, these data indicate that mean body temperature does not modulate eccrine sweat rate during baroreceptor unloading induced via 30° head-up tilt.