Arterial pressure oscillation and muscle sympathetic nerve activity after 20 days of head-down bed rest

Both spectral power within the low-frequency component, i.e., 0.04 to 0.15 Hz, of systolic pressure and muscle sympathetic nerve activity are increased during head-up tilt. The nerve activity during tilt is altered after space flight and exposure to simulated microgravity. In the present study, correlations of the low-frequency component and the nerve activity were analyzed before and after 20 days of -6 degrees of head-down bed rest. Measurements were performed at -6 degrees head-down bed rest, 0 degrees (flat), and 30 degrees and 60 degrees head-up tilt (HUT). Mean arterial pressure during HUT was not different between pre- and post-bed rest, but muscle sympathetic nerve activity in post-bed rest significantly increased at tilt angles of -6 degrees, 0 degrees, 30 degrees, and 60 degrees compared with those during pre-bed rest. The low-frequency component of systolic pressure also significantly increased during post-bed rest compared with pre-bed rest at tilts of 0 degrees, 30 degrees, and 60 degrees. The nerve activity and the frequency component were linearly correlated for individual (r(2) = 0.51-0.88) and averaged (r(2) = 0.60) values when the values included both pre- and post-bed rest. Thus, the low-frequency component of systolic pressure could be an index of the muscle sympathetic nerve activity during tilt during pre- and post-bed rest. (C) 2013 Elsevier B.V. All rights reserved.