Regulation of muscle sympathetic nerve activity after bed rest deconditioning

James A. Pawelczyk, Julie H. Zuckerman, C. Gunnar Blomqvist, Benjamin D. Levine
2001 American Journal of Physiology. Heart and Circulatory Physiology  
Regulation of muscle sympathetic nerve activity after bed rest deconditioning. Am J Physiol Heart Circ Physiol 280: H2230-H2239, 2001.-Cardiovascular deconditioning reduces orthostatic tolerance. To determine whether changes in autonomic function might produce this effect, we developed stimulus-response curves relating limb vascular resistance, muscle sympathetic nerve activity (MSNA), and pulmonary capillary wedge pressure (PCWP) with seven subjects before and after 18 days of Ϫ6°head-down bed
more » ... of Ϫ6°head-down bed rest. Both lower body negative pressure (LBNP; Ϫ15 and Ϫ30 mmHg) and rapid saline infusion (15 and 30 ml/kg body wt) were used to produce a wide variation in PCWP. Orthostatic tolerance was assessed with graded LBNP to presyncope. Bed rest reduced LBNP tolerance from 23.9 Ϯ 2.1 to 21.2 Ϯ 1.5 min, respectively (means Ϯ SE, P ϭ 0.02). The MSNA-PCWP relationship was unchanged after bed rest, though at any stage of the LBNP protocol PCWP was lower, and MSNA was greater. Thus bed rest deconditioning produced hypovolemia, causing a shift in operating point on the stimulus-response curve. The relationship between limb vascular resistance and MSNA was not significantly altered after bed rest. We conclude that bed rest deconditioning does not alter reflex control of MSNA, but may produce orthostatic intolerance through a combination of hypovolemia and cardiac atrophy. vasoconstriction; adrenergic nervous system; sympathetic reflex Address for reprint requests and other correspondence: J. A. Pawelczyk,
doi:10.1152/ajpheart.2001.280.5.h2230 pmid:11299226 fatcat:gzhp42ihsjelbjj34iv4iehaqu