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Effect of mechanical stimulation, substance P and vasoactive intestinal polypeptide on the electrical and mechanical activities of circular smooth muscles from pig coronary arteries contracted with acetylcholine: role of endothelium.

Mechanical stimulation, substance P and vasoactive intestinal polypeptide (VIP) were found to relax the transversal strip of anterior descending branches of pig coronary arteries precontracted by acetylcholine. The effects of mechanical stimulation and substance P required the presence of intact endothelium, while VIP did not. The effect of VIP did not appear to be mediated by catecholamines. Simultaneous measurements of intracellular membrane potential and tension developed by coronary smooth muscle precontracted with Ach showed that the smooth muscle relaxation by substance P is accompanied by membrane hyperpolarization. In contrast VIP relaxed the same tissue without affecting the membrane potential. In a cascade experiment, the fluid perfused intraluminally in intact segments of coronary arteries was dropped over a de-endothelialized strip which relaxed in response to substance P and mechanical stimulation. This indicates that substance P and mechanical stimulation act by releasing from the endothelium a humoral factor that produces arterial smooth muscle relaxation.