Substance P-induced contraction of rabbit airways: mechanism of action

The mechanism by which substance P induces contraction of airway smooth muscle has been the subject of numerous reports. It has been suggested that in rabbit airways the action of substance P is indirect, via the release of endogenous acetylcholine, whereas this is not so in other species. The present detailed study investigated whether substance P-induced contraction in rabbit isolated bronchus and trachea is due to the release of endogenous acetylcholine or in bronchus is due to histamine release and whether substance P is metabolized by the enzymes enkephalinase and acetylcholinesterase. Isometric contraction to cumulative addition of substance P was measured in the presence of 10(-6) and 10(-4) M atropine, 10(-6) M pyrilamine, 10(-5) M phosphoramidon, or 3 x 10(-7) M neostigmine. Neither atropine nor pyrilamine had any effect on the substance P responses. Phosphoramidon, however, produced a 12-fold shift to the left in the response curve with a decrease in the 50% effective concentration from 7.0 x 10(-8) to 6.1 x 10(-9) M (n = 4 control and 5 treated; P less than 0.05). In contrast, neostigmine at a concentration that produced a sixfold shift to the left in the acetylcholine response curve had no effect on substance P responses. We conclude that, in rabbit airways in vitro, substance P-induced contraction is not mediated by release of endogenous acetylcholine or histamine. In addition, endogenous enkephalinase but not acetylcholinesterase may be involved in the degradation of substance P. Our results show that, in contrast to previous studies in rabbits, the mechanism of action of substance P may resemble that described in humans.