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This Article Full Text Full Text (PDF) All Versions of this Article: 297/1/H53    most recent 00260.2009v1 Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Google Scholar Articles by Kamiya, A. Articles by Mano, T. PubMed PubMed Citation Articles by Kamiya, A. Articles by Mano, T.

Slow head-up tilt causes lower activation of muscle sympathetic nerve activity: loading speed dependence of orthostatic sympathetic activation in humans

¹Department of Cardiovascular Dynamics, National Cardiovascular Center Research Institute, Suita; ²Research Institute of Environmental Medicine, Nagoya University, Nagoya; ³Department of Physiology, Aichi Medical University, Aichi; and ⁴Gifu University of Medical Science, Gifu, Japan

Submitted 17 March 2009 ; accepted in final form 13 May 2009

Many earlier human studies have reported that increasing the tilt angle of head-up tilt (HUT) results in greater muscle sympathetic nerve activity (MSNA) response, indicating the amplitude dependence of sympathetic activation in response to orthostatic stress. However, little is known about whether and how the inclining speed of HUT influences the MSNA response to HUT, independent of the magnitude of HUT. Twelve healthy subjects participated in passive 30° HUT tests at inclining speeds of 1° (control), 0.1° (slow), and 0.0167° (very slow) per second. We recorded MSNA (tibial nerve) by microneurography and assessed nonstationary time-dependent changes of R-R interval variability using a complex demodulation technique. MSNA averaged over every 10° tilt angle increased during inclination from 0° to 30°, with smaller increases in the slow and very slow tests than in the control test. Although a 3-min MSNA overshoot after reaching 30° HUT was observed in the control test, no overshoot was detected in the slow and very slow tests. In contrast with MSNA, increases in heart rate during the inclination and after reaching 30° were similar in these tests, probably because when compared with the control test, greater increases in plasma epinephrine counteracted smaller autonomic responses in the very slow test. These results indicate that slower HUT results in lower activation of MSNA, suggesting that HUT-induced sympathetic activation depends partially on the speed of inclination during HUT in humans.

autonomic nervous system; baroreflex; heart rate variability; microneurography