Suppression of the acetylcholine sensitivity in tropical Malaysians

Abstract

Tropical natives are reputed to tolerate high ambient temperatures. This results from their long-term residence in the frequently hot and often humid tropical climates. This study was designed to compare the peripheral mechanisms of thermal sweating in tropical natives to their temperate counterparts. Fifty five healthy male subjects (25 Koreans, 30.4±4.7 yr and 30 Malaysians, 28.3±4.0 yr) gave their informed written consent to be used as subjects in this study. A 10% solution of acetylcholine (ACh) was iontophoretically administered into the forearm skin. The sweat onset-time was 0.96 min shorter (P<0.01) and directly activated and axon reflex mediated total sweat output for 5 min were 1.08±0.19 mg/cm2/min and 0.42±0.11 mg/cm2/min in the temperate natives and 0.68±0.14 mg/cm2/min and 0.15±0.19 mg/cm2/min tropical natives, respectively. These values being 100% (P<0.01) and 150% (P<0.01) higher in the temperate natives compared to the tropical natives. These results indicate that suppressed thermal sweating in tropical natives is, at least in part, due to suppressed sweat glandular sensitivity to ACh through both recruitment of sweat glands and sweat output per each gland. [Jpn J Physiol 54 Suppl:S235 (2004)]