Mechanism of Acetylcholine-induced Endothelium-dependent Relaxation in the Rabbit Carotid Artery by M3-receptor Activation

Mechanism of Acetylcholine-induced Endothelium-dependent Relaxation in the Rabbit Carotid Artery by M3-receptor Activation

The present study were designed to characterize the action mechanisms of acetylcholine (ACh)- induced endothelium-dependent relaxation in arteries precontracted with high K<SUP>&#8290;</SUP> (70 mM). For this, we simultaneously measured both muscle tension and cytosolic free Ca<SUP>2&#8290;</SUP> concentration ([Ca<SUP>2&#8290;</SUP>]<SUB>i</SUB>), using fura-2, in endothelium-intact, rabbit carotid arterial strips. In the artery with endothelium, high K<SUP>&#8290;</SUP> increased both [Ca<SUP>2&#8290;</SUP>]<SUB>i</SUB> and muscle tension whereas ACh (10μM) significantly relaxed the muscle and increased [Ca<SUP>2&#8290;</SUP>]<SUB>i</SUB>. In the presence of N<SUP>G</SUP>-nitro-L-arginine (L-NAME, 0.1 mM), ACh increased [Ca<SUP>2&#8290;</SUP>]<SUB>i</SUB> without relaxing the muscle. In the artery without endothelium, high K<SUP>&#8290;</SUP> increased both [Ca<SUP>2&#8290;</SUP>]<SUB>i</SUB> and muscle tension although ACh was ineffective. 4-DAMP (10 nM) or atropine (0.1μM) abolished ACh-induced increase in [Ca<SUP>2&#8290;</SUP>]<SUB>i</SUB> and relaxation. The increase of [Ca<SUP>2&#8290;</SUP>]<SUB>i</SUB> and vasorelaxation by ACh was siginificantly reduced by either 3μM gadolinium, 10μM lanthanum, or by 10μM SKF 96365. These results suggest that in rabbit carotid artery, ACh-evoked relaxation of 70 mM K<SUP>&#8290;</SUP>-induced contractions appears to be mediated by the release of NO. ACh-evoked vasorelaxation is mediated via the M<SUB>3</SUB> subtype, and activation of the M<SUB>3</SUB> subtype is suggested to stimulate nonselective cation channels, leading to increase of [Ca<SUP>2&#8290;</SUP>]<SUB>i</SUB> in endothelial cells.