Alpha-adrenergic vascular responsiveness to sympathetic nerve activity is intact after head-down bed rest in humans.
Space-flight and its ground-based simulation model, 6 degrees head-down bed rest (HDBR), cause cardiovascular deconditioning in humans. Because sympathetic vasoconstriction plays a very important role in circulation, we examined whether HDBR impairs alpha-adrenergic vascular responsiveness to sympathetic nerve activity. We subjected eight healthy volunteers to 14 days of HDBR and before and after HDBR measured calf muscle sympathetic nerve activity (MSNA; microneurography) and calf blood flow (venous occlusion plethysmography) during sympathoexcitatory stimulation (rhythmic handgrip exercise). HDBR did not change the increase in total MSNA (P = 0.97) or the decrease in calf vascular conductance (P = 0.32) during exercise, but it did augment the increase in calf vascular resistance (P = 0.0011). HDBR augmented the transduction gain from total MSNA into calf vascular resistance, assessed as the least squares linear regression slope of vascular resistance on total MSNA (0.05 +/- 0.02 before HDBR, 0.20 +/- 0.06 U.min-1.burst-1 after HDBR, P = 0.0075), but did not change the transduction gain into calf vascular conductance (P = 0.41). Our data indicate that alpha-adrenergic vascular responsiveness to sympathetic nerve activity is preserved in the supine position after HDBR in humans.
Kamiya
Atsunori
A
Department of Cardiovascular Dynamics, National Cardiovascular Center Research Institute, 5-7-1 Fujishirodai, Suita, Osaka 565-8565, Japan. kamiya@ri.ncvc.go.jp
Michikami
Daisaku
D
Iwase
Satoshi
S
Hayano
Junichiro
J
Kawada
Toru
T
Sugimachi
Masaru
M
Sunagawa
Kenji
K
D000328:Adult; D001510:Bed Rest; D018737:Hand Strength / Q000502:physiology; D018475:Head-Down Tilt; D006801:Humans; D016018:Least-Squares Analysis; D007866:Leg; D008297:Male; D018482:Muscle, Skeletal / Q000098:blood supply; D011942:Receptors, Adrenergic, alpha / Q000502:physiology*; D012039:Regional Blood Flow / Q000502:physiology; D013564:Sympathetic Nervous System / Q000502:physiology*; D014655:Vascular Resistance / Q000502:physiology; D014666:Vasomotor System / Q000502:physiology*
D011942:Receptors, Adrenergic, alpha