Endothelium-dependent relaxation of human epicardial coronary arteries: frequent lack of effect of acetylcholine

Strip preparations of human epicardial coronary arteries (free of atherosclerosis) relaxed in an endothelium-dependent fashion to substance P and Ca2+-ionophore A23187. Acetylcholine generally caused contraction in the same strips. Glyceryl trinitrate and isoproterenol induced relaxation irrespective of the presence or absence of endothelium. Nordihydroguaiaretic acid abolished the relaxation produced by substance P and A23187. Mepacrine only blocked substance P relaxation. Haemoglobin and methylene blue inhibited substance P and A23187 relaxations but also reduced the response to glyceryl trinitrate. These inhibitor experiments indicate that human coronary arteries are relaxed by the endothelium-derived relaxing factor.