Effect of the Heat-exposure on Peripheral Sudomotor Activity Including the Density of Active Sweat Glands and Single Sweat Gland Output

Abstract

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.

Seasonal acclimation in sudomotor function evaluated by QSART in healthy humans

“…Central components are the preoptic area and anterior hypothalamus (PO/AH) and peripheral components are postganaglionic sympathetic nerve fibers and sweat glands [14]. Acetylcholine (ACh) is the primary neurotransmitter involved in this process [1516].…”

Age‐ and sex‐related differences in sudomotor function evaluated by the quantitative sudomotor axon reflex test (QSART) in healthy humans

“…Central sudomotor mechanisms integrate inputs from the core and skin thermoreceptors and produce signals to control the sweat glands . Efferent sweat fibres originate in the preoptic area and anterior hypothalamus and descend to the post‐ganglionic sympathetic plexus (sudomotor neurons) through the ipsilateral brain stem and medulla . The sweat glands generate a sweat response to central sudomotor nerve activity .…”

“…The sudomotor response can be tested using several tools, such as thermoregulatory sweat testing, quantitative sudomotor axon reflex test (QSART), silicone impressions, the sympathetic skin response, the acetylcholine (ACh) sweat‐spot test and quantitative direct and indirect axon reflex testing . The QSART evaluates the post‐ganglionic sympathetic sudomotor axons, measuring the axonal reflex‐mediated (AXR) and directly activated (DIR) sweat response over time, following stimulation of sweat glands with 10% ACh . In the QSART, sweat glands are stimulated by ACh iontophoresis, which activates both sudomotor fibres and primary afferent nociceptors .…”

“…This leads to sudomotor and vasodilator axon reflexes . Moreover, ACh is the first neurotransmitter involved in the sweat mechanism and induces direct sudomotor activity as a result of activation of muscarinic receptors, as well as indirect sudomotor activity as a result of activation of nicotinic receptors . Therefore, sweat gland activity depends on ACh sensitivity, which influences sweat gland response and output .…”

“…Moreover, ACh is the first neurotransmitter involved in the sweat mechanism and induces direct sudomotor activity as a result of activation of muscarinic receptors, as well as indirect sudomotor activity as a result of activation of nicotinic receptors . Therefore, sweat gland activity depends on ACh sensitivity, which influences sweat gland response and output . Furthermore, the QSART is used to detect C‐fibre dysfunction and sympathetic nerve function and to evaluate post‐ganglionic sympathetic cholinergic sudomotor function.…”