Attenuated sympathetic nerve responses after 24 hours of bed rest. Journal   American journal of physiology. Heart and circulatory physiology. Citation   Am J Physiol Heart Circ Physiol. 282(6):H2210-5 Publication date   2002 Jun Authors   Khan MH Kunselman AR Leuenberger UA Davidson WR Ray CA Gray KS Hogeman CS Sinoway LI Investigators   William R. Davidson, Jr. Allen R. Kunselman Urs A. Leuenberger Chester A. Ray Lawrence I. Sinoway Grant agencies   National Heart, Lung, and Blood Institute National Center for Research Resources National Institute on Aging Grants   NHLBI K24-HL-04011 NCRR M01-RR10732 NIA R01-AG-12227 NHLBI R01-HL-58503 NHLBI R01-HL-60800 MeSH headings   Bed Rest Lower Body Negative Pressure Sympathetic Nervous System MeSH qualifiers   physiology Abstract   Bed rest reduces orthostatic tolerance. Despite decades of study, the cause of this phenomenon remains unclear. In this report we examined hemodynamic and sympathetic nerve responses to graded lower body negative pressure (LBNP) before and after 24 h of bed rest. LBNP allows for baroreceptor disengagement in a graded fashion. We measured heart rate (HR), cardiac output (HR x stroke volume obtained by echo Doppler), and muscle sympathetic nerve activity (MSNA) during a progressive and graded LBNP paradigm. Negative pressure was increased by 10 mmHg every 3 min until presyncope or completion of -60 mmHg. After bed rest, LBNP tolerance was reduced in 11 of 13 subjects (P <.023), HR was greater (P <.002), cardiac output was unchanged, and the ability to augment MSNA at high levels of LBNP was reduced (rate of rise for 30- to 60-mmHg LBNP before bed rest 0.073 bursts x min(-1) x mmHg(-1); after bed rest 0.035 bursts x min(-1) x mmHg(-1); P < 0.016). These findings suggest that 24 h of bed rest reduces sympathetic nerve responses to LBNP. Medline ID   21998869