| Research Abstract |
To examine effects of long-term bed rest on autonomic control of cardiovascular function, we made 14 days of 6° head-down bed rest(HDBR, a ground-based simulation model of microgravity)in 25 young healthy male volunteers(age, mean 23 years). Each subject kept always and strictly the 6° HD posture during HDBR.Dietary intake was about 2,100 kcal per day and fluid intake from daily drinks was ad iibitum ; the average was about 1,200 ml per day. Before and after HDBR, we performed 60° head-up tilt(HDUT)test, ±10° passive postural change test, exercise test(isometric plantar flexion at 70% of maximal voluntary contraction for 2 minutes, isometric handgrip at 30% of maximal voluntary contraction to fatigue), mental stress test(mental arithmetic for 10 minutes)and whole body heating test(with waterperfused suit through which 41℃ water was circulated). Post-HDBR induced orthostatic hypotension was associated with a reduced arterial baroreflex gain and a reduction of muscle sympathetic nerve activity(MSNA, measured by microneurography). The ratio of changes in MSNA and changes in peripheral venous pressure during ±10° passive postural change test was significantly less after HDBR compared with before HDBR, suggesting that cardiopulmonary baroreflex gain was attenuated after HDBR.Increase in MSNA duning exercise was identical in handgrip but was significantly less in plantar flexion after HDBR while maximal force was reduced by 12% in plantar flexion but not in handgrip after HDBR.Increase in MSNA during mental stress test was significantly augmented after HDBR while vasodilation during mental stress was attenuated. Threshold core temperatures for increases in sweating and cutaneus vascular conductance were significantly increased during whole body heating after HDBR while gains of these increases were significantly increased, suggesting that thermoregulatory function during heating was impaired after HDBR.
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