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Modulation of control of muscle sympathetic nerve activity during orthostatic stress in humans

¹Institute of Health and Sport Sciences, University of Tsukuba, Tsukuba City, Ibaraki 305-8574; ²Applied Physiology Laboratory, Toyota Technological Institute, Nagoya 468-8511; and ³Faculty of Human Development, Kobe University, Kobe 657-8501, Japan

Submitted 4 March 2004 ; accepted in final form 24 June 2004

We tested the hypothesis that orthostatic stress would modulate the arterial baroreflex (ABR)-mediated beat-by-beat control of muscle sympathetic nerve activity (MSNA) in humans. In 12 healthy subjects, ABR control of MSNA (burst incidence, burst strength, and total activity) was evaluated by analysis of the relation between beat-by-beat spontaneous variations in diastolic blood pressure (DAP) and MSNA during supine rest (CON) and at two levels of lower body negative pressure (LBNP: –15 and –35 mmHg). At –15 mmHg LBNP, the relation between burst incidence (bursts per 100 heartbeats) and DAP showed an upward shift from that observed during CON, but the further shift seen at –35 mmHg LBNP was only marginal. The relation between burst strength and DAP was shifted upward at –15 mmHg LBNP (vs. CON) and further shifted upward at –35 mmHg LBNP. At –15 mmHg LBNP, the relation between total activity and DAP was shifted upward from that obtained during CON and further shifted upward at –35 mmHg LBNP. These results suggest that ABR control of MSNA is modulated during orthostatic stress and that the modulation is different between a mild (nonhypotensive) and a moderate (hypotensive) level of orthostatic stress.

arterial baroreflex; sympathetic nervous system; lower body negative pressure

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