Vasoactive intestinal polypeptide relaxes pulmonary artery by an endothelium-independent mechanism.

Vasoactive intestinal polypeptide (VIP) is a potent endogenous vasodilator. VIP-induced vasodilation is independent of adrenergic or cholinergic receptors and, for the most part, of arachidonate metabolites, but its mechanism is still unknown. In view of the recently demonstrated essential role of endothelium in the relaxant effect of numerous vasodilators, we have investigated the importance of endothelium in the vascular relaxation induced by VIP. Vascular smooth muscle preparations were circular strips of the pulmonary artery of guinea pig, rat and rabbit, and rat thoracic aorta, previously contracted by a synthetic prostaglandin endoperoxide analog. VIP relaxed pulmonary artery of all species by an endothelium-independent mechanism, but relaxed rat thoracic aorta only in the presence of intact endothelium.