Tropical natives are reported to exhibit suppressed sweating during heat exposures compared to temperate natives, however the underlying mechanisms are not fully known. The aim of this work is to determine the difference in sudomotor responses to heat challenge and acetylcholine (ACh), iontophoresis (IP) in long-term tropical and temperate adapted men. Quantitative sudomotor axon reflex test (QSART) was performed to quantitatively evaluate glandular ACh-sensitivity. ACh was iontophoretically administered on the forearm. Directly activated and axon reflex-mediated sweat responses were evaluated by QSART. Ten healthy male Japanese and 10 healthy male Africans volunteered for this study. African subjects (from Nigeria, Tanzania, Senegal, Ghana, Kenya and Chad) were Japan International Cooperation Agency (JICA) trainees and academic researchers who stayed at the Institute of Tropical Medicine, Nagasaki University between October 1997 and January 2001. Japanese subjects were students and staff of Nagasaki University. Experiments on the Africans were conducted within 1-2 weeks of residence in Japan. To do this, sweating responses to ACh 10% IP (QSART of Experimental 1) and hot water (HW, 43°C of Experimental 2) leg immersions were compared between Japanese (TEMP) and Africans (TROP). All experiments were (24±0.5℃, relative humidity 40± 3%). Directly activated (muscarinic receptors) and axon reflex-mediated (nicotinic receptor) sweat responses during IP were evaluated by quantitative sudomotor axon reflex test and sweating responses in the HW test were measured by capacitance hygrometer. The sweat onset-time (SOT) during IP was 0.72 min earlier (P < 0.01), and the sweat volume 56% higher and SOT during HW test was 8.04 min earlier and sweat volume 135% higher in the TEMP compared to the TROP. Sweat gland density was 73-112% higher and sweat output per single gland was 20.4-66% in the TEMP compared to the TROP. These results indicate that suppressed thermal sweating in tropical subjects is, at least in part, attributed to the suppressed glandular sensitivity to sweating through both reduced recruitment of sweat glands and sweat output per each gland.
Source: Korean J Physiol Pharmacol.2001 Dec;5(Suppl II):S93
|