['The release of nitric oxide is controlled by cholinergic and adrenergic receptors. Recent observations suggest that activation of alpha-adrenoceptors can inhibit the release of nitric oxide. The aim of the present study was to examine the effect of alpha1- and alpha2-adrenoceptor activation on nitric oxide-mediated vasodilation.', 'In a first set of experiments, the endothelium-dependent vasodilators acetylcholine (ACh), 5-hydroxytryptamine (5HT), and bradykinin (BK), and the nitric oxide donor sodium nitroprusside (SNP) were administered in a random order in the brachial artery together with saline, or the nonselective alpha-adrenoceptor agonists norepinephrine or clonidine, or the nitric oxide synthase inhibitor NG-monomethyl-L-arginine (L-NMMA). The infusions of saline, norepinephrine, clonidine, and L-NMMA started 10 minutes before the infusions of ACh, BK, 5HT, and SNP. In a second set of experiments, cumulative doses of ACh, BK, and 5HT were infused, in a random order, intra-arterially together with saline or the selective alpha1-adrenoceptor agonist methoxamine. The infusions of saline and methoxamine started 5 minutes before the infusions of ACh, BK, and 5HT. Forearm blood flow was measured using computerized venous occlusion plethysmography.', 'ACh, 5HT, BK, and SNP induced a significant increase in forearm blood flow (p < 0.05 for all). These vasodilator responses were significantly attenuated by norepinephrine, clonidine, and L-NMMA (p <0.05 for all), except for SNP. In the second set of experiments, all three endothelium-dependent vasodilators induced a dose-dependent vasodilation, which was significantly inhibited by methoxamine (p <0.05).'] Is endothelium-dependent NO-mediated vasodilation in humans attenuated by peripheral alpa1-adrenoceptor activation?