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

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.