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Am J Physiol Heart Circ Physiol (July 3, 2003). doi:10.1152/ajpheart.01076.2002
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Submitted on December 13, 2002
Accepted on June 30, 2003

Nitric oxide synthase inhibition does not affect regulation of muscle sympathetic nerve activity during head-up tilt

Jian Cui1, Rong Zhang2, Thad E Wilson1, Sarah Witkowski1, Craig G Crandall2, and Benjamin D Levine2*

1 Institute for Exercise and Environmental Medicine, Presbyterian Hospital of Dallas, Dallas, Texas, USA
2 Institute for Exercise and Environmental Medicine, Presbyterian Hospital of Dallas, Dallas, Texas, USA; Department of Internal Medicine, University of Texas Southwestern Medical Center, Dallas, Texas, USA

* To whom correspondence should be addressed. E-mail: BenjiaminLevine{at}texashealth.org.

To test the hypothesis that systemic inhibition of nitric oxide (NO) synthase does not alter the regulation of sympathetic outflow during head-up tilt in humans, in eight healthy subjects NO synthase was blocked by intravenous infusion of NG-monomethyl-L-arginine (L-NMMA). Blood pressure, heart rate, cardiac output, total peripheral resistance (TPR) and muscle sympathetic nerve activity (MSNA) were recorded in the supine position and during 60 ° head-up tilt. In the supine position, infusion of L-NMMA increased blood pressure via increased TPR, and inhibited MSNA. However, the increase in MSNA evoked by head-up tilt during L-NMMA infusion ({Delta}burst rate: 24 ± 4 bursts/min; {Delta} total activity: 209 ± 36 units/min) was similar to that during head-up tilt without L-NMMA ({Delta} burst rate: 23 ± 4 bursts/min; {Delta} total activity: 251 ± 52 units/min, n=6, all P>0.05). Moreover, changes in TPR and heart rate during head-up tilt were virtually identical between the two conditions. These results suggest that systemic inhibition of NO synthase with L-NMMA does not affect regulation of sympathetic outflow and vascular resistance during head-up tilt in humans.




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