Substance P fails to mimic vagally mediated nonadrenergic bronchodilation

The ability of substance P to mimic vagally mediated nonadrenergic bronchodilation was assessed in vivo in anesthetized, paralyzed, artificially ventilated cats. Infusion of the peptide at a rate of 10 micrograms kg-1 min-1 for 10 minutes did not attenuate the increase in pulmonary resistance evoked by efferent vagal stimulation. Similarly, when administered as an aerosol (1-15 breaths of a 100 micrograms ml-1 aqueous solution) or by bolus intravenous injection, substance P failed to reverse the increase in pulmonary resistance maintained by a continuous intravenous infusion of 5-hydroxytryptamine. These results indicate that substance P is unlikely to be the neurotransmitter responsible for mediating nonadrenergic inhibitory responses in feline airways.