The effects of electrical stimulation of myelinated and non-myelinated vagal motor fibres on airway tone in the rabbit and the cat.

The bronchomotor effects of repetitive electrical stimulation of the peripheral end of the cervical vagus nerves were studied in anaesthetized and paralysed rabbits and cats. Stimulation of either myelinated or of all motor vagal fibres was obtained by varying the duration of electrical square pulses. In rabbits, selective stimulation of myelinated vagal fibres induced a bronchoconstriction, strongly potentiated by the recruitment of non-myelinated fibres. This potentiation was absent in cats. After ganglionic blockade with hexamethonium, the vagally mediated bronchoconstriction was abolished in rabbits, while a slight and transient effect persisted in cats. Propranolol did not modify the bronchoconstrictor response to vagal stimulation, which was abolished after further injection of atropine in both species. When propranolol plus atropine was administered and airway tone was increased by continuous i.v. injection of bronchoconstrictor agonists, the stimulation of vagal motor fibres was devoid of any bronchomotor effect in rabbits. However, in cats this resulted in a strong bronchodilation, which was doubled after recruitment of non-myelinated fibres. Thus, in the latter species preganglionic vagal motor fibres participate in the non-adrenergic-non-cholinergic vagal system. Amongst them, non-myelinated fibres play an important role.