Mz Quantitative Sudomotor Axon Reflex Test in Tropical Africans Compared to Temperate Koreans

Tropical inhabitants possess the ability of heat-tolerance through permanent residence in the tropics. Previously, we have shown that tropical African and Thai subjects regulate the core temperature with less amount of sweat against heat compared to temperate Koreans subjects and that suppression of sweating in tropical subjects was attributed to suppression in both central and peripheral sudomotor mechanisms. To elucidate the peripheral mechanisms of the suppressed thermal sweating in tropical natives, sweating responses to acetylcholine(ACh), a primary transmitter of the sudomotor innervation, were compared between Koreans and Africans. ACh was iontophoretically administered on the forearm. Directly activated and axon reflex-mediated sweat responses were evaluated by quantitative sudomotor axon reflex test. The sweat onset-time was 0.70min shorter (P＜0.01) and the sweat volumes were 75%-79% higher (P＜0.01) in the Koreans than the Africans. Iodine-impregnated paper method revealed that sweat gland density was 60.6% higher (P＜0.001) and sweat gland output per single gland was 25.4% larger (P＜0.001) in the Koreans compared to the Africans. ACh iontophoresis did not produce any influences on oral temperature, but increased the local skin temperature in both the Koreans and the Africans. These results indicate that suppressed thermal sweating in Africans is, at least in part, attributed to the suppressed glandular sensitivity to ACh through both recruitment of sweat glands and sweat output per each gland.