Autonomic Innervation of Pulmonary Artery: Evidence for a Nonadrenergic Noncholinergic Inhibitory System

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Abstract
We have investigated the presence and significance of the nonadrenergic noncholinergic relaxant system in isolated strips of guinea pig and cat pulmonary artery (PA), using transmural electrical field stimulation (EFS). Changes in PA strip tension were measured isometrically, in response to EFS with 50-70 V at 20 Hz and pulse durations of 1 or 2 ms. We examined the influence on these responses of α- and β-adrenergic and cholinergic muscarinic receptor blockade, the neurotoxin tetrodotoxin, and the local anesthetic lidocaine. EFS induced an initial brief contraction followed by a slow prolonged relaxation. Phenoxybenzamine, 1.5 × 10-5 M, greatly reduced the contraction (74.1-87.6% in guinea pig 65.7-67.4% in cat) and enhanced the subsequent relaxation (23.9-35.5% in guinea pig 66.8-67.8% in cat). The relaxation was moderately reduced by propranolol, 7.7 × 10-6 M (30.5-35.0% in guinea pig, 6.0-21.2% in cat), and by atropine, 1.4 × 10-6 M (23.7-32.1% in guinea pig 24.9-40.6% in cat). The major portion of the relaxation, however, persisted in the presence of these blockers. The neurogenic origin of this relaxation was confirmed by its loss with either tetrodotoxin or lidocaine. We conclude that in guinea pig and cat lungs (1) neurogenic pulmonary vascular contraction is principally mediated by the α-adrenergk system, (2) the β-adrenergic and cholinergic components have a moderate relaxant influence, but (3) the dominant part of neurogenic relaxation is mediated by a nonadrenergic noncholinergic mechanism.