Propranolol enhances acetylcholine-induced relaxation in the various arterial segments of rabbit

The effect of propranolol on acetylcholine-induced relaxation and vasodilatation was studied in the isolated rabbit aortic strips, aortic rings and whole perfused carotid, femoral arteries precontracted by phenylephrine. Acetylcholine produced a concentration-dependent relaxation and vasodilatation in all investigated endothelium-intact arterial segments. The relaxing and vasodilator effects of acetylcholine were completely prevented after the removal of endothelium and prior addition of methylene blue to the incubation medium in endothelium intact segments. Addition of propranolol to the medium significantly enhanced the relaxing activity of acetylcholine. Acetylsalicylic acid failed to inhibit the potentiating effect of propranolol on acetylcholine-induced relaxation. From these results it was concluded that the potentiation by propranolol of the vascular relaxing effect of acetylcholine is probably mediated through the increased release of EDRF from vascular endothelium. The possible mechanisms of this effect are discussed.