Regulation of muscle sympathetic nerve activity after bed rest deconditioning

Cardiovascular deconditioning reduces orthostatic tolerance. To determine w hether changes in autonomic function might produce this effect, we develope d stimulus-response curves relating limb vascular resistance, muscle sympat hetic nerve activity (MSNA), and pulmonary capillary wedge pressure (PCWP) with seven subjects before and after 18 days of -6 degrees head-down bed re st. Both lower body negative pressure (LBNP; -15 and -30 mmHg) and rapid sa line infusion (15 and 30 ml/ kg body wt) were used to produce a wide variat ion in PCWP. Orthostatic tolerance was assessed with graded LBNP to presync ope. 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 unch anged after bed rest, though at any stage of the LBNP protocol PCWP was low er, and MSNA was greater. Thus bed rest deconditioning produced hypovolemia , causing a shift in operating point on the stimulus-response curve. The re lationship between limb vascular resistance and MSNA was not significantly altered after bed rest. We conclude that bed rest deconditioning does not a lter reflex control of MSNA, but may produce orthostatic intolerance throug h a combination of hypovolemia and cardiac atrophy.