Suppression of the Sweat Gland Sensitivity to Acetylcholine Applied Iontophoretically in Tropical Africans Compared to Temperate Japanese

Tropical inhabitants possess the ability of heat-tolerance through permanent residence in the tropics. Previously, we had shown that tropical African and Thai subjects regulate the core temperature with less amount of sweat against heat compared to temperate Japanese 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 Japanese (20 healthy males) and Africans (10 healthy males). 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 on-set-time was 0.72 min shorter (P<0.01) and the sweat volumes were 72% - 110% higher (P<0.01) in the Japanese than the Africans. Iodine-impregnated paper method revealed that sweat gland density was 50.6% higher (P<0.001) and sweat gland output per single gland was 20.4% larger (P<0.001) in the Japanese compared to the Africans. The Japanese showed the a 0.17℃ higher oral temperature and a 0.30℃ higher forearm skin temperature compared to the Africans (P<0.05, respectively) at rest under a thermoneutral condition. ACh iontophoresis did not produce any influences on oral temperature, but increased the local skin temperature in both the Japanese 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.