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NEUROHUMORAL CONTROL OF CIRCULATION AND HYPERTENSION

-Adrenergic vascular responsiveness to sympathetic nerve activity is intact after head-down bed rest in humans

¹Department of Cardiovascular Dynamics, National Cardiovascular Center Research Institute, Osaka 565-8565; ²Department of Autonomic Neuroscience, Research Institute of Environmental Medicine, Nagoya University, Nagoya 464-8601; and ³Core Laboratory, Nagoya City University Graduate School of Medical Sciences, Nagoya 467-8601, Japan

Submitted 7 November 2002 ; accepted in final form 3 September 2003

Space-flight and its ground-based simulation model, 6° 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 -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 -adrenergic vascular responsiveness to sympathetic nerve activity is preserved in the supine position after HDBR in humans.

microgravity; spaceflight; vascular contractility; vasoconstriction

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