The effect of substance P on the antigen-induced bronchoconstriction in guinea pigs

The effect of substance P (SP) on the antigen-induced bronchoconstriction in vitro and in vivo was investigated in guinea pigs. SP caused the contraction of isolated non-sensitized guinea pig tracheal muscle at concentrations between 10(-9) to 10(-7) g/ml. The elimination of epithelium in the tracheal muscle produced a slight enhancement of SP response as compared to control. Antigen-induced contraction of isolated sensitized guinea pig tracheal muscle was slightly enhanced by the pretreatment with SP at a concentration of 10(-9) g/ml. The SP-induced enhancement of antigen-induced contraction was significantly augmented by the elimination of epithelium of sensitized tracheal muscle. Moreover, the clear bronchoconstriction was observed by the infusion of SP (1 micrograms/min) intravenously, but not by the infusion of SP at the rates between 0.1 and 0.01 micrograms/min. Antigen-induced asthmatic respiratory disorder was accelerated temporarily by the infusion of subthreshold SP (0.1 microgram/min). The reactivity of bronchial smooth muscle measured by repeated injection of acetylcholine was significantly potentiated by the infusion of subthreshold SP (0.1 microgram/min). These results suggest that SP has dual actions in the contractile response of guinea pig airway smooth muscle. One is the direct contractile activity, and the other one is enhancing activity of antigen- and acetylcholine-induced bronchoconstriction.