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NEUROHUMORAL CONTROL OF CIRCULATION AND HYPERTENSION

Bed rest attenuates sympathetic and pressor responses to isometric exercise in antigravity leg muscles 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; ³Department of Hygiene and Space Medicine, Nihon University School of Medicine, Tokyo 173-8610; and ⁴Core Laboratory, Nagoya City University Graduate School of Medical Sciences, Nagoya 467-8601, Japan

Submitted 29 August 2003 ; accepted in final form 24 December 2003

Although spaceflight and bed rest are known to cause muscular atrophy in the antigravity muscles of the legs, the changes in sympathetic and cardiovascular responses to exercises using the atrophied muscles remain unknown. We hypothesized that bed rest would augment sympathetic responses to isometric exercise using antigravity leg muscles in humans. Ten healthy male volunteers were subjected to 14-day 6° head-down bed rest. Before and after bed rest, they performed isometric exercises using leg (plantar flexion) and forearm (handgrip) muscles, followed by 2-min postexercise muscle ischemia (PEMI) that continues to stimulate the muscle metaboreflex. These exercises were sustained to fatigue. We measured muscle sympathetic nerve activity (MSNA) in the contralateral resting leg by microneurography. In both pre- and post-bed-rest exercise tests, exercise intensities were set at 30 and 70% of the maximum voluntary force measured before bed rest. Bed rest attenuated the increase in MSNA in response to fatiguing plantar flexion by 70% at both exercise intensities (both P < 0.05 vs. before bed rest) and reduced the maximal voluntary force of plantar flexion by 15%. In contrast, bed rest did not alter the increase in MSNA response to fatiguing handgrip and had no effects on the maximal voluntary force of handgrip. Although PEMI sustained MSNA activation before bed rest in all trials, bed rest entirely eliminated the PEMI-induced increase in MSNA in leg exercises but partially attenuated it in forearm exercises. These results do not support our hypothesis but indicate that bed rest causes a reduction in isometric exercise-induced sympathetic activation in (probably atrophied) antigravity leg muscles.

autonomic nervous system; deconditioning; inactivity; muscle atrophy; microneurography; sympathetic nerve activity

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