Characterization of substance P-induced bronchoconstriction in the guinea-pig: a comparison with acetylcholine.

Substance P (0.5-8.0 micrograms/kg, i.v.) induced bronchoconstriction in anaesthetized, mechanically-ventilated guinea-pigs, comprising increases in airways resistance and decreases in dynamic compliance. These bronchoconstrictor responses were unaffected by bilateral vagotomy, pretreatment with pheniramine (2 mg/kg, i.v.) or by pretreatment with atropine (100 micrograms/kg, i.v.). Acetylcholine-induced (4-32 micrograms/kg, i.v.) bronchoconstriction was prevented by atropine pretreatment, whereas bilateral vagotomy inhibited responses to acetylcholine. Ganglionic blockade using hexamethonium (20 mg/kg, i.v.) potentiated both substance P and acetylcholine on airways resistance and dynamic compliance. Indomethacin (1 or 5 mg/kg, i.v.) did not affect substance P-induced bronchoconstriction, whereas the higher dose enhanced acetylcholine-induced increases in airways resistance. In addition, aspirin pretreatment (20 mg/kg, i.v.) did not alter the bronchoconstrictor potency for either substance P or acetylcholine. On the other hand, the combined cyclo-oxygenase/lipoxygenase inhibitors eicosatetraynoic acid (ETYA, 20 mg/kg, i.v.) and BW755C (20 mg/kg, i.v.) potentiated both acetylcholine and substance P on airways resistance and dynamic compliance. The results suggest that substance P-induced bronchoconstriction may be modulated by the sympathetic nervous system and does not appear to be influenced by vagal or histaminergic mechanisms. The failure of indomethacin or aspirin to affect substance P-induced bronchoconstriction, together with the enhancing effects of ETYA and BW755C pretreatments, provide evidence consistent with the existence of a bronchodilator mechanism which may be inhibited by compounds inhibiting lipoxygenase enzymes.