Mucosal trypsin, a protease-activated receptor (PAR) stimulant, may have an endogenous bronchoprotective role on airway smooth muscle. To test this possibility the effects of lumenal trypsin on airway tone in segments of pig bronchus were tested.Bronchial segments from pigs were mounted in an organ chamber containing Kreb's solution. Contractions were assessed from isovolumetric lumen pressure induced by acetylcholine (ACh) or carbachol added to the adventitia.Trypsin, added to the airway lumen (300 mu g center dot mL(-1)) had no immediate effect on smooth muscle tone but suppressed ACh-induced contractions after 60 min, for at least 3 h. Synthetic activating peptides (AP) for PAR(1), PAR(2) or PAR(3) were without effect, but PAR(4) AP caused rapid, weak suppression of contractions. Lumenal thrombin was without effect and did not prevent the effects of trypsin. Effects of trypsin were reduced by H-omega-nitro-L-arginine methyl ester but not indomethacin. Trypsin, thrombin and PAR(4) AP released prostaglandin E2. Adventitially, trypsin, thrombin and PAR(4) AP (but not PAR(2) AP) relaxed carbachol-toned airways after <3 min.The findings of this study show that trypsin causes delayed and persistent bronchoprotection by interacting with airway cells accessible from the lumen. The signalling mechanism may involve nitric oxide synthase but not prostanoids or protease-activated receptors.