Vasomotor sympathetic nerve activity in men during bed rest and on orthostasis after bed rest.
BACKGROUND
Alterations in autonomic function are commonly seen during and after spaceflight, and its ground-based analog, 6 degrees head-down bed rest (HDBR). They may include peripheral vascular regulation, but vasomotor sympathetic efferent nerve discharges to peripheral vasculatures have not been examined. The aim of our study was to examine changes in vasomotor sympathetic nerve activity during HDBR and under orthostasis after HDBR.


METHODS
We performed 6 d of HDBR on six male subjects, and measured muscle sympathetic nerve activity (MSNA) together with plasma norepinephrine concentrations in the supine position before HDBR and in 6 degrees head-down position on the sixth day (HDBR6) of HDBR. We also measured MSNA in head-up tilt (HUT) test before and after HDBR.


RESULTS
On HDBR6, MSNA burst rate was the same (17+/-4 bursts x min(-1)) as that in supine position before HDBR (15+/-2 bursts min(-1)), but plasma norepinephrine concentrations were decreased to 1.14+/-0.10 pmol x ml(-1) compared with the supine value before HDBR (1.56+/-0.20 pmol x ml(-1), p<0.05). After HDBR, supine MSNA burst rate significantly increased by 58% to 24+/-4 bursts x min(-1). MSNA increment in response to HUT was similar between before (34+/-3 bursts min(-1) x sin HUT(-1)) and after (40+/-6 bursts x min(-1) x sin HUT(-1)) HDBR.


CONCLUSIONS
Our findings suggest that: a) the relationship between MSNA and plasma norepinephrine concentrations was altered on the sixth day during HDBR; b) the vasomotor sympathetic nerve activity was enhanced after HDBR; and c) the augmentation of vasomotor sympathetic outflow to muscles under orthostasis was preserved after HDBR.
Kamiya
A
A
Department of Autonomic Neuroscience, Research Institute of Environmental Medicine, Nagoya University, Japan.
Iwase
S
S
Sugiyama
Y
Y
Mano
T
T
Sudoh
M
M
D000328:Adult; D009320:Atrial Natriuretic Factor / Q000097:blood; D001510:Bed Rest / Q000009:adverse effects*; D018475:Head-Down Tilt / Q000009:adverse effects* / Q000502:physiology; D006339:Heart Rate; D006801:Humans; D007024:Hypotension, Orthostatic / Q000175:diagnosis / Q000209:etiology* / Q000503:physiopathology*; D008297:Male; D009638:Norepinephrine / Q000097:blood; D013026:Space Flight; D016683:Supine Position / Q000502:physiology; D013997:Time Factors; D014666:Vasomotor System / Q000503:physiopathology*
D009320:Atrial Natriuretic Factor; D009638:Norepinephrine