Blunted sympathetic vasoconstriction in contracting skeletal muscle of healthy humans: is nitric oxide obligatory?

Abstract

We tested the hypothesis that nitric oxide (NO) is responsible for blunting sympathetic alpha-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 alpha-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 alpha 1-adrenergic (phenylephrine) and alpha 2-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.

Figure 1. Experimental trial

Each trial consisted of a 2-min rest (baseline) period. After this time period, subjects either began rhythmic forearm exercise or received intra-arterial adenosine to elevate resting forearm blood flow to similar levels observed during exercise (control non-exercise vasodilator). During minutes 5 and 6 (pre-vasoconstrictor), the dose of the α-adrenergic agonist was calculated on the basis of steady state hyperaemic forearm blood flow and forearm volume. Subsequently, the α-agonist (tyramine, phenylephrine or clonidine) was infused at minute 6 and lasted for 3 min. An average of the forearm blood flow and mean arterial blood pressure during the last 30 s of α-agonist infusion was used to calculate the vasoconstrictor effect during both hyperaemic conditions.

Figure 2. Effects of l-NMMA on forearm vasoconstrictor responses to tyramine

The vasoconstrictor responses to tyramine are significantly blunted during rhythmic handgrip exercise (open bars) compared with a control vasodilator condition (adenosine; filled bars). Intra-arterial administration of l-NMMA to inhibit nitric oxide synthase (NOS) does not augment the vasoconstrictor responses during adenosine or (more importantly) during handgrip exercise. * P < 0.05vs. adenosine for given tyramine dose within same l-NMMA condition.

Figure 3. Effects of l-NAME on forearm vasoconstrictor responses to tyramine

The vasoconstrictor responses to tyramine are significantly blunted during rhythmic handgrip exercise (open bars) compared with a control vasodilator condition (adenosine; filled bars). Intra-arterial administration of l-NAME to inhibit NOS does not augment the vasoconstrictor responses during adenosine or (more importantly) during handgrip exercise. * P < 0.05vs. adenosine for given tyramine dose within same l-NAME condition.

Figure 4. Effects of l-NAME on forearm vasoconstrictor responses to tyramine during heavy rhythmic handgrip exercise

The vasoconstrictor responses to tyramine are significantly blunted during heavy handgrip exercise (open bars) compared with a control vasodilator condition (adenosine; filled bars). NOS inhibition with l-NAME does not augment the vasoconstrictor responses during either adenosine or exercise. * P < 0.05vs. adenosine within same l-NAME condition.

Figure 5. Effects of intra-arterial l-NAME on forearm vasodilator responses to acetylcholine

Baseline forearm vascular conductance (FVC) is reduced ≈40 % and the increases in FVC to acetylcholine are significantly blunted after NOS inhibition with l-NAME. Taken together, these data are consistent with NOS inhibition in the human forearm.

Figure 6. Effects of l-NAME on forearm vasoconstrictor responses to phenlyephrine and clonidine

The vasoconstrictor responses to the direct α₁-agonist (phenylephrine; PE) and α₂-agonist (clonidine; Clon) are significantly blunted during exercise compared with adenosine. Local NOS inhibition with l-NAME does not augment the vasoconstrictor responses to either α-agonist during adenosine or rhythmic handgrip exercise. * P < 0.05vs. adenosine for specific α-agonist within same l-NAME condition.