Differential bronchial and pulmonary vascular responses to vagal stimulation in the pig

The pulmonary and bronchial vascular responses and changes in bronchial tone upon vagal stimulation (240 impulses at 2 Hz or 10 Hz) were studied in anaesthetized pigs paralyzed with pancuronium. The acetylcholine-evoked vasodilatation in the tracheobronchial circulation had the same magnitude when using pancuronium or succinylcholine as skeletal muscle relaxants. Atropine-sensitive bradycardia, hypotension and bronchoconstriction were observed upon vagal stimulation. A vasoconstrictor response in the pulmonary vascular bed and clear-cut vasodilatation in the bronchial circulation supplied by the bronchial artery also occurred upon vagal stimulation. The vagally-evoked increase in pulmonary vascular resistance was markedly reduced after atropine while the bronchial vasodilatation was unchanged. This suggests that the vagally-induced increase in bronchial blood flow was not secondary to changes in the pulmonary circulation. Furthermore, the pulmonary vasoconstrictor response caused by vagal stimulation under control conditions is probably explained by reflex sympathetic activation due to the fall in systemic blood pressure. These data indicate selective vagal non-cholinergic influence of blood flow in the bronchial vascular bed compared to the pulmonary circulation.