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1 Noll Physiological Research Center, The Pennsylvania State University, University Park, Pennsylvania 16802; and 2 Institute for Exercise and Environmental Medicine, University of Texas Southwestern Medical Center and Presbyterian Hospital of Dallas, Dallas, Texas 75231
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 
6° head-down bed rest.
Both lower body negative pressure (LBNP; 15 and 30 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 ± 2.1 to
21.2 ± 1.5 min, respectively (means ± SE, P = 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
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