Relaxant action of VIP on cat pulmonary artery: comparison with acetylcholine, isoproterenol, and PGE1

Vasoactive intestinal peptide (VIP), a potent systemic vasodilator, was recently identified in nerve fibers within the pulmonary arterial wall of cats and other mammals. We have investigated the smooth muscle relaxant action of VIP on isolated strips of kitten pulmonary artery that were placed in a 5-ml bath and precontracted with prostaglandin endoperoxide (PGH2) analogue. The action of VIP was compared with that of acetylcholine, isoproterenol, and prostaglandin E1 (PGE1). The effectiveness of VIP as a relaxant of pulmonary artery, measured as maximal relaxation and expressed in weight equivalents, was 29.9 +/- 2.0 X 10(-2) g in the presence of adrenergic and cholinergic blockers, or 28.8 +/- 4.5 X 10(-2) g in the absence of blockers. This relaxation was similar to that of acetylcholine (30.4 +/- 4.0 X 10(-2) g) and isoproterenol (26.4 +/- 2.2 X 10(-2) g), but smaller than that of PGE1 (41.1 +/- 3.7 X 10(-2) g). Judging by the EC50 or EC10 (effective concentration needed to develop 50 or 10%, respectively, of maximal relaxation), VIP was at least 40 times as potent as acetylcholine, 2,770 times as potent as isoproterenol, and 120 times as potent as PGE1, on a molar basis. The effect of VIP was unaltered in the presence of atropine, propranolol, or phenoxybenzamine in the incubation medium. The findings demonstrate the potent relaxant action of VIP in isolated pulmonary arterial strips and the independence of this action from cholinergic and adrenergic receptors. The results support the possibility that VIP may play an important role in regulating pulmonary vascular tone in kittens.