Different Responses to Acetylcholine in the Presence of Nitric Oxide Inhibitor in Rat Aortae and Mesenteric Arteries

This study compared the relaxation induced by acetylcholine (ACh) in aortic and mesenteric arterial rings from Sprague-Dawley (SD) rats in the presence and absence of inhibitors of the known endothelium-derived relaxing factors. 2. ACh-induced relaxations were completely blocked by methylene blue and N omega-nitro-L-arginine (LNNA) in aortae, whereas these were only partially attenuated by methylene blue and LNNA in mesenteric arteries. 3. This methylene blue-resistant relaxation of ACh was partly attenuated by potassium channel blockers (tetraethylammonium and barium) but not affected by LNNA, indomethacin and calcium-free solution. 4. These results suggest that there may be another endothelial relaxing factor which is not nitric oxide (NO), prostanoids or endothelium-derived hyperpolarizing factor (EDHF) in mesenteric arteries but not in aortae. This unknown factor seems to be extracellular calcium ([Ca2+]0)-independent.