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Abstract
To determine the peripheral mechanisms involved in thermal sweating during the hot summers in July before acclimatization and after acclimatization in September, we evaluated the sweating response of healthy subjects (n=10) to acetylcholine (ACh), a primary neurotransmitter involved in peripheral sudomotor sensitivity. The quantitative sudomotor axon reflex test (QSART) measures sympathetic C fiber function after iontophoresed ACh evokes a measurable reliable sweat response. The QSART, at 2 mA for 5 min with 10% ACh, was applied to determine the directly activated (DIR) and axon reflex-mediated (AXR) sweating responses during ACh iontophoresis. The AXR sweat onset-time by the axon reflex was 1.50±0.32 min and 1.84±0.46 min before acclimatization in July and after acclimatization in September, respectively (p<0.01). The sweat volume of the AXR(1) [during 5 min 10% iontophoresis] by the axon reflex was 1.45±0.53 mg/cm2 and 0.98±0.24 mg/cm2 before acclimatization in July and after acclimatization in September, respectively (p<0.001). The sweat volume of the AXR(2) [during 5 min post-iontophoresis] by the axon reflex was 2.06±0.24 mg/cm2 and 1.39±0.32 mg/cm2 before and after acclimatization in July and September, respectively (p<0.001). The sweat volume of the DIR was 5.88±1.33 mg/cm2 and 4.98±0.94 mg/cm2 before and after acclimatization in July and September, respectively (p<0.01). These findings suggest that lower peripheral sudomotor responses of the ACh receptors are indicative of a blunted sympathetic nerve response to ACh during exposure to hot summer weather conditions.
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Fig. 1.
Monthly mean ambient temperatures in Cheonan (126 52'N, 33.38'E; Republic of Korea). Cheonan is located in a temperate zone, with cold winters and hot summers. The mean ambient temperature of the experimental period was −1.9°C in the winter (January) and 25.8°C in the summer (August) of 2007.
Fig. 2.
Sweat onset-time of axon reflex was 1.50±0.32 min and 1.84±0.46 min in human subjects before acclimatization in July and after acclimatization in September, respectively. The experimental period was during the hottest months of the summer (between July and August; Fig. 1). Values are presented as means (n=10)±SD. Statistical significance was set at p<0.001.
Fig. 3.
The AXR (1) sweat volume of the axon reflex was 1.46±0.53 mg/cm2 and 0.98±0.24 mg/cm2 in human subjects before acclimatization in July and after acclimatization in September, respectively. Comparison of the AXR (2) sweat volume of the axon reflex was 2.06±0.24 mg/cm2 and 1.39±0.32 mg/cm2 in human subjects before acclimatization in July and after acclimatization in September, respectively. 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 means (n=10)±SD. Statistical significance was set at ∗∗∗p<0.001.
Fig. 4.
The DIR sweat volume (directly activated 6~11 min) was 5.88±1.33 mg/cm2 and 4.98±0.94 mg/cm2 in human subjects before acclimatization in July and after acclimatization in September, respectively. DIR=directly activated sweating during (muscarinic receptor-mediated sweating activity). Values are presented as means (n=10)±SD. Statistical significance was set at p<0.01.
Fig. 5.
Sweating activity (forearm local skin evaporative loss volume) was 15.96±1.33 μg/cm2/min and 11.90±2.50 μg/cm2/min in human subjects before acclimatization in July and after acclimatization in September, respectively. Values are presented as the means (n=10)±SD. Statistical significance was set at ∗∗∗p<0.001.
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