Effects of three days of dry immersion on muscle sympathetic nerve activity and arterial blood pressure in humans

The present study was performed to determine how sympathetic function is al tered by simulated microgravity, dry immersion for 3 days, and to elucidate the mechanism of post-spaceflight orthostatic intolerance in humans. Six h ealthy men aged 21-36 years old participated in the study. Before and after the dry immersion, subjects performed head-up tilt (HUT) test to 30 degree s and 60 degrees (5 min each) with recordings of muscle sympathetic nerve a ctivity (MSNA, by microneurography), electrocardiogram, and arterial blood pressure (Finapres). Resting MSNA was increased after dry immersion from 23 .7+/-3.2 to 40.9+/-3.0 bursts/min (p<0.005) without significant changes in resting heart rate (HR). MSNA responsiveness to orthostasis showed no signi ficant difference but KR response was significantly augmented after dry imm ersion (p<0.005). A significant diastolic blood pressure fall at 5th min of 60 degrees HUT was observed in five orthostatic tolerant subjects despite enough MSNA discharge after dry immersion. A subject suffered from presynco pe at 2 min after 60 degrees HUT. He showed gradual blood pressure fall 10 s after 60 degrees HUT with initially well-maintained MSNA response and the n with a gradually attenuated MSNA, followed by a sudden MSNA withdrawal an d abrupt blood pressure drop. In conclusion, dry immersion increased MSNA w ithout changing MSNA response to orthostasis, and resting HR, while increas ing the HR response to orthostasis. Analyses of MSNA and blood pressure cha nges in orthostatic tolerant subjects and a subject with presyncope suggest ed that not only insufficient vasoconstriction to sympathetic stimuli, but also a central mechanism to induce a sympathetic withdrawal might play a ro le in the development of orthostatic intolerance after microgravity exposur e. (C) 2000 Elsevier Science B.V. All rights reserved.