Blunted Sympathetic Vasoconstriction in Contracting Skeletal Muscle of Healthy Humans: is Nitric Oxide Obligatory?
Article first published online: 16 JUL 2004
The Journal of Physiology
Volume 553, Issue 1, pages 281–292, November 2003
How to Cite
Dinenno, F. A. and Joyner, M. J. (2003), Blunted Sympathetic Vasoconstriction in Contracting Skeletal Muscle of Healthy Humans: is Nitric Oxide Obligatory?. The Journal of Physiology, 553: 281–292. doi: 10.1113/jphysiol.2003.049940
- Issue published online: 16 JUL 2004
- Article first published online: 16 JUL 2004
- (Received 24 June 2003; accepted after revision 28 August 2003; first published online 29 August 2003)
We tested the hypothesis that nitric oxide (NO) is responsible for blunting sympathetic α-adrenergic vasoconstriction in the active muscles of humans (functional sympatholysis). We measured forearm blood flow (Doppler ultrasound) and calculated the reductions in forearm vascular conductance (FVC) in response to α-adrenergic receptor stimulation during rhythmic handgrip exercise and during a control non-exercise vasodilator condition (intra-arterial adenosine), before and after local NO synthase (NOS) inhibition in healthy men. The forearm vasoconstrictor responses to endogenous noradrenaline release (intra-arterial tyramine) were significantly blunted during moderate exercise compared with adenosine, and these vasoconstrictor responses were not restored by NOS inhibition with NG-monomethyl-l-arginine (l-NMMA; n= 6) or NG-nitro-l-arginine methyl ester (l-NAME; n= 8). Similarly, l-NAME did not restore the vasoconstrictor responses to tyramine in contracting muscle during heavy rhythmic handgrip exercise (n= 4). In four additional subjects, we also found that the vasoconstrictor responses evoked by tyramine during exercise or adenosine were repeatable in the absence of NOS inhibition (i.e. time control). Finally, in five subjects the forearm vasoconstrictor responses to direct α1-adrenergic (phenylephrine) and α2-adrenergic (clonidine) receptor stimulation were blunted during moderate exercise compared with adenosine; these responses were also unaffected by l-NAME. Taken together, our results demonstrate that NO is not obligatory for functional sympatholysis in contracting skeletal muscles of healthy men.