Seasonal Acclimatization in Summer versus Winter to Changes in the Sweating Response during Passive Heating in Korean Young Adult Men

We investigated the sweating response during passive heating (partial submersion up to the umbilical line in 42+/-0.5degrees C water, 30 min) after summer and winter seasonal acclimatization (SA). Testing was performed in July during the summer, 2011 [summer-SA; temp, 25.6+/-1.8degrees C; relative humidity (RH), 82.1+/-8.2%] and in January during the winter, 2012 (winter-SA; temp, -2.7+/-2.9degrees C; RH, 65.0+/-13.1%) in Cheonan (126degrees52'N, 33.38'E), Republic of Korea. All experiments were carried out in an automated climatic chamber (temp, 25.0+/-0.5degrees C: RH, 60.0+/-3.0%). Fifteen healthy men (age, 23.4+/-2.5 years; height, 175.0+/-5.9 cm; weight, 65.3+/-6.1 kg) participated in the study. Local sweat onset time was delayed during winter-SA compared to that after summer-SA (p< 0.001). Local sweat volume, whole body sweat volume, and evaporative loss volume decreased significantly after winter-SA compared to those after summer-SA (p<0.001). Changes in basal metabolic rate increased significantly after winter-SA (p< 0.001), and tympanic temperature and mean body temperature were significantly lower after summer-SA (p<0.05). In conclusion, central sudomotor acitivity becomes sensitive to summer-SA and blunt to winter-SA in Rebubic of Korea. These results suggest that the body adjusts its temperature by economically controlling the sweating rate but does not lower the thermal dissipation rate through a more effective evaporation scheme after summer-SA than that after winter-SA.

Economical Sweating Function in Africans: Quantitative Sudomotor Axon Reflex Test

People in tropics have the ability to tolerate heat by residential permanence in the tropics. Previously, we have shown that African and Thai subjects who lived for whole their lives in only their respective countries sweat less under hot conditions than South Koreans who also lived whole their lives in Korea. The difference in sweating responses was attributed to the dissimilar central and peripheral sweating mechanisms operating in people from both groups. In the present study, acetylcholine (ACh), the primary transmitter for the sudomotor functions, was iontophoretically administered to South Koreans and Africans to determine the characteristic sudorific responses of their acclimatized biologic make-up to their respective environments. Using quantitative sudomotor axon reflex test (QSART), direct (DIR) and axon reflex (AXR) responses were evaluated. The findings revealed that the sweat onset-time among South Koreans was 0.91 min earlier than among Africans (P< 0.01). The axon reflex sweat volume of nicotine receptor activity AXR (1) and sweat volume of muscarinic receptor activity DIR (2) among South Koreans were 79% and 53% greater (P< 0.01), respectively. These results indicate that the reduced thermal sweating among Africans is at least in part attributed to the diminished sensitivity of sweat glands to ACh.