Effects of sympathetic stimulation and applied catecholamines on mechanical and electrical responses to stimulation of the vagus nerve in guinea-pig isolated trachea.

Mechanical and electrical responses to stimulation of the vagus nerve were studied in the isolated, innervated trachea of the guinea-pig. In approximately half the preparations tested, the amplitudes of mechanical constrictor responses to stimulation of the vagus were reduced substantially during a period of sympathetic stimulation. Vagal responses were unaltered in the remainder. In single trachealis cells, stimulation of the vagus nerve or sympathetic stellate ganglion elicited depolarization and hyperpolarization, respectively. Vagally-mediated depolarization was decreased, unchanged or increased in amplitude after a period of sympathetic stimulation. Isoprenaline almost abolished mechanical responses induced by stimulation of the vagus, and this effect was blocked by propranolol. Noradrenaline attenuated markedly vagal mechanical responses also, and this effect was blocked by a combination of propranolol and phentolamine. Both noradrenaline and isoprenaline hyperpolarized single trachealis cells and greatly reduced the amplitude of vagally-mediated depolarization. Neither sympathetic stimulation nor applied catecholamines altered mechanical responses to applied acetylcholine, strongly suggesting that their effects on vagal responses are predominantly presynaptic.