Sudomotor modifications by acclimatization of stay in temperate Japan of Malaysian native tropical subjects

Tropical subjects regulate core temperature with less amount of sweat against heat compared to temperate subjects through long-term heat-acclimatization. The purpose of the study is to determine whether acclimatization in tropical subjects decay during a stay in temperate area. Local sweating response activated by acetylcholine (ACh) applied iontophoretically among Malaysians (n=26) with varying duration of stay in Japan and Japanese residents subjects (J-R n=30). Based on their length of stay, Malaysian subjects were divided into three groups, group M-S (n=9) with a duration of stay oflto 12 months, M-M (n=7) with 13 to 36 months and M-L (n=10) with 37 to 72 months. ACh, the primary transmitter for sudomotor innervation, was iontophoretically administered on the forearm. Sweating response elicited directly (DIR) and indirectly via axon reflex (AXR) were evaluated by quantitative sudomotor axon reflex test. The onset-time of AXR was shortened with the longer duration of stay among Malaysian subjects (P <0.01). DIR, sweat volume directly (ACh muscarinic receptors) induced by ACh for 6-11 min, was 0.68±0.35 mg/cm2, 1.02±0.64 mg/cm2, 1.45±0.71 mg/cm2 and 2.39±0.47 mg/cm2 in M-S, M-M, M-L and J-R, respectively. These were statistically different (P <0.01). From these results, suppressed neuroglandular response to ACh was confirmed in Malaysians. It is suggested that long-term heat-acclimatization acquired in tropical subjects may decay after immigration to temperate area.