Muscle sympathetic nerve activity and plasma norepinephrine during 6 degrees head-down bed rest

Alterations in autonomic function are evident during spaceflight. These conditions are also simulated by ground-based experimental models of a microgravity environment such as 6 degrees head-down bed rest (BR). They include a reduction in baroreflex function, a decrease in vagal tone and a loss of circulatory blood volume during weightlessness. However, it is not clear whether vasomotor sympathetic outflow, which controls peripheral vascular tone neurally, and plasma norepinephrine concentration will change or not during weightlessness. The purpose of the present study is to examine changes in vasomotor sympathetic nerve activity during six days of 6 degrees head-down BR. Vasomotor sympathetic nervous activity was evaluated by the direct recording of muscle sympathetic nerve activity (MSNA) by a use of microneurographic technique. MSNA on the sixth day during BR was the same as that in a horizontally supine position before BR (before BR; 15.4 +/- 1.6 bursts/min, during BR; 16.7 +/- 3.5 bursts/min), though plasma norepinephrine concentrations during BR were reduced by 25% compared with those before BR (before BR; 263 +/- 33 pg/ml, during BR; 193 +/- 79 pg/ml, p < 0.05). This dissociation cannot be determined precisely, but it suggests the possibility of peripheral changes in terminal sympathetic nerve endings, etc.