Effects of lower body positive pressure on muscle sympathetic nerve activity response to head-up tilt

The present study was performed to test the hypothesis that application of lower body positive pressure (LBPP) during orthostasis would reduce the bar oreflex-mediated enhancement in sympathetic activity in humans. Eight healt hy young men were exposed to a 70 degrees head-up tilt (HUT) on application of 30 mmHg LBPP. Muscle sympathetic nerve activity (MSNA) was microneurogr aphically recorded from the tibial nerve, along with hemodynamic variables. We found that in the supine position with LBPP, MSNA remained unchanged (1 3.4 +/- 3.3 vs. 11.8 +/- 2.3 bursts/min, without vs. with LBPP; P > 0.05), mean arterial pressure was elevated, but arterial pulse pressure and heart rate did not alter. At 70 degrees HUT with LBPP, the enhanced MSNA response was reduced (33.8 +/- 5.0 vs. 22.5 +/- 2.2 bursts/min, without vs. with LB PP; P < 0.05), mean arterial pressure was higher, the decreased pulse press ure was restored, and the increased heart rate was attenuated. We conclude that the baroreflex-mediated enhancement in sympathetic activity during HUT was reduced by LBPP. Application of LBPP in HUT induced an obvious cephala d fluid shift as well as a restoration of arterial pulse pressure, which re duced the inhibition of the baroreceptors. However, the activation of the i ntramuscular mechanoreflexes produced by 30 mmHg LBPP might counteract the effects of baroreflexes.