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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.
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Fig. 1.
(A) Possible functional pathway for sudomotor nerves under dynamometry stimulation: efferent sweat fibers originate centrally in the areas regulating sweating, the preoptic area and anterior hyopothalamus (PO/AH), and peripherally by ACh. The fibers descend through the ipsilateral brainstem and medulla to synapse with the intermediolateral cell column neurons. The preganglionic fibers emerge from the anterior roots to reach (via the white rami communicantes) the chain ganglia. Unmyelinated postganglionic sympathetic class C fibers arise from the sympathetic ganglia to join the major peripheral nerves to reach the sweat glands, providing them with cholinergic innervations (Figure modified from Vetrugno et al. 2003). (B) Diagram of the sudomotor axon reflex. The sudomotor axon reflex. Cholinergic agonists with neurotransmitters (ACh) applied through iontophoresis bind to muscarinic receptors causing local sweat production (DIR sweating). The cholinergic agonist simultaneously binds to nicotinic receptors on nerve terminals of sudomotor fibers and an impulse travels antidromically. At branch points this impulse travels orthodromically to a neighboring population of eccrine sweat glands causing an indirect axon mediated sweat response [AXR (1) and AXR (2) sweating].
Fig. 2.
Comparison of sweating activity between PRE (0.29±0.16 mg/cm2/min) and POST (0.47±0.19 mg/cm2/min) exposure (43°C water immersion of leg, 60 min/day, during 10 days) in subjects with the AXR (1) sweat response. Comparison of sweating activity between PRE (0.43±0.14 mg/cm2/min) and POST (0.63±0.17 mg/cm2/min) exposure (43°C water immersion of leg, 60 min/day, during 10 days) in subjects with the AXR (2) sweat response. AXR=axon reflex-mediated (indirectly activated) sweating during (nicotinic receptor mediated sweating activity), AXR (1)=0∼5 min and AXR (2)=6∼11 min. Values are presented as the means±SD. Statistical significance at ∗∗∗p<0.001.
Fig. 3.
Comparison of the DIR (muscarinic receptor mediated sweating activity) sweat rate (directly activated 6∼11 min) was 0.94±0.21 mg/cm2/min and 1.18±0.24 mg/cm2/min in the subjects PRE and POST exposure (43°C water immersion of leg, 60 min/day, during 10 days), respectively. Values are presented as the means±SD. Statistical significance d at ∗∗∗p<0.001.
Fig. 4.
Comparison of the active sweat gland density was 99.2± 15.61 count/cm2 and 108.4±19.44 count/cm2 in the subjects PRE and POST exposure (43°C water immersion of leg, 60 min/day, during 10 days), respectively. Values are presented as the means±SD.
Fig. 5.
Comparison of the active sweat output per gland was 9.45±1.08 μg/min/single gland and 10.93±1.33 μg/min/single gland in the subjects by PRE and POST exposure (43°C water immersion of leg, 60 min/day, during 10 days), respectively. Values are presented as the means±SD. Statistical significance at ∗∗∗p<0.001.
Fig. 6.
Comparison of the sweating activity (volume of skin evaporative loss) was 10.51±1.91 μg/cm2/min and 13.67±2.12 μg/cm2/min in the subjects PRE and POST exposure (43°C water immersion of leg, 60 min/day, during 10 days), respectively. Values are presented as the means±SD. Statistical significance at ∗∗∗p<0.001.
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