Effect of the effluent from canine internal mammary artery on coronary artery endothelium-free ring. Relaxation of the coronary artery endothelium-free ring induced by the effluent released from canine internal mammary artery stimulated by intraluminal and extraluminal perfusion of acetylcholine (10-5 M) before (control) and after intraluminal and extraluminal pirenzepine (10-5 M). Values represent mean ± SEM; n = 6. Relaxation magnitude is expressed as % of initial tonus. * p < 0.05.