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The aim of the present study was to quantitatively investigate the age and sex-related differences in sudomotor function in healthy humans. The quantitative sudomotor axon reflex test (QSART) with iontophoresis (2 mA for 5 min) and 10% acetylcholine (ACh) was performed to determine axon reflex-mediated (AXR), with and without iotophoresis (AXR(1) and AXR(2), respectively), and directly activated (DIR) sweating. All experiments were conducted under thermoneutral conditions (temperature 24.0 ± 0.5°C; relative humidity 40 ± 3%). In general, men had enhanced values of onset time of AXR, sweat rates, activated sweat gland density (SGD) and activated sweat gland output (SGO) than women, but not in all cases. The onset time of AXR (r(2)  = 0.567; P < 0.001) was positively correlated with advancing age, whereas sweat rates of AXR(1) and AXR(2) (r(2)  = 0.571 and r(2)  = 0.486, respectively; P < 0.0001), DIR (r(2)  = 0.594; P < 0.0001), SGD (r(2)  = 0.496; P < 0.0001) and SGO (r(2)  = 0.551; P < 0.0001) were negatively correlated in both men and women with advancing age. The results demonstrate that an attenuation of sudomotor function occurs with aging in both sexes. Moreover, the findings showed a progressive increase in onset time and a decrease in sweat rates, SGD and SGO with increasing age in both sexes. A variation in sweat function was found between sexes, but not in all age groups.

Tropical inhabitants are able to tolerate heat through permanent residence in hot and often humid tropical climates. The goal of this study was to clarify the peripheral mechanisms involved in thermal sweating pre and post exposure (heat-acclimatization over 10 days) by studying the sweating responses to acetylcholine (ACh), a primary neurotransmitter of sudomotor activity, in healthy subjects (n=12). Ten percent ACh was administered on the inner forearm skin for iontophoresis. Quantitative sudomotor axon reflex testing, after iontophoresis (2 mA for 5 min) with ACH, was performed to determine directly activated (DIR) and axon reflex-mediated (AXR) sweating during ACh iontophoresis. The sweat rate, activated sweat gland density, sweat gland output per single gland activated, as well as oral and skin temperature changes were measured. The post exposure activity had a short onset time (p＜ 0.01), higher active sweat rate [(AXR (p＜ 0.001) and DIR (p＜ 0.001)], higher sweat output per gland (p ＜ 0.001) and higher transepidermal water loss (p＜ 0.001) compared to the pre-exposure measurements. The activated sweat rate in the sudomotor activity increased the output for post-exposure compared to the pre-exposure measurements. The results suggested that post-exposure activity showed a higher active sweat gland output due to the combination of a higher AXR (DIR) sweat rate and a shorter onset time. Therefore, higher sudomotor responses to ACh receptors indicate accelerated sympathetic nerve responsiveness to ACh sensitivity by exposure to environmental conditions.

The quantitative sudomotor axon reflex testing (QSART) is a classic test of routine postganglionic sudomotor function. We investigated sudomotor function by QSART after summer (July 2012) and winter (January 2013) seasonal acclimation (SA) in the Republic of Korea. QSART with acetylcholine (ACh) iontophoresis were performed to determine directly activated (DIR) and axon reflex-mediated (AXR1, 2) sweating rate. Onset time of axon reflex, activated sweat gland density (ASGD), activated sweat gland output (ASGO), tympanic and skin temperatures (Tty, Tsk), basal metabolic rate (BMR), and evaporative loss volume changes were measured. Tympanic and mean body temperature (b; calculated from Tty, Tsk) were significantly lower after summer-SA than that of winter-SA. Sweat onset time was delayed during winter-SA compared to that after summer-SA. BMR, AXR(1), AXR(2), and DIR sweat rates, ASGD and ASGO, and evaporative loss volume were significantly diminished after winter-SA relative to after summer-SA. In conclusion, changes in sweating activity measured by QSART confirmed the involvement of the peripheral nervous system in variation of sudomotor activity in seasonal acclimation.