Cholinergic Receptor-Mediated Responses in the Arteriolar and Venous Vascular Beds of the Human Forearm

In arterioles, acetylcholine (ACh) is a well known vasodilator. However, in veins a wide variation in responses to ACh has been reported. In the present study the effects of the cholinergic agonists acetylcholine and methacholine (MCh) were determined simultaneously both in arterial and venous vasculature in the forearm vascular bed of healthy volunteers by means of venous occlusion plethysmography. The vasodilator sodium nitroprusside (SNP) served as an endothelium-independent control agent. The vascular beds were preconstricted by the selective α1-adrenoceptor agonist methoxamine. Atropine, a non-selective muscarinic receptor antagonist, was used to antagonize the dilator effect of MCh. Overall we observed a weaker relaxant effect of ACh, MCh and SNP in the veins compared with their dilator responses in the arteries. ACh, which is highly sensitive to the hydrolytic inactivation by choline esterases, failed to induce a significant vasodilation in the venous vascular bed. Atropine blocked the dilator effects of MCh, indicating the involvement of muscarinic receptors. In arteries, MCh did not induce a significantly stronger vasodilatation than SNP on a molar basis. However, in veins, MCh had a weaker relaxant effect (p < 0.05).