Poly‐l‐arginine‐mediated release of acetylcholine from parasympathetic nerves in rat and guinea‐pig airways

1 The synthetic cationic polypeptide, poly-l-arginine (0.03–1 mg ml−1) induced concentration-dependent contraction of guinea-pig and rat isolated trachea. In guinea-pig isolated trachea, this response was attenuated in the presence of the muscarinic cholinoceptor antagonist, atropine (0.1 μm) and augmented by the acetylcholinesterase inhibitor, ecothiophate (0.1 μm). The neuronal sodium channel blocker, tetrodotoxin (3 μm) failed to alter the contractile response to poly-l-arginine and acetylcholine. 2 The contractile response to poly-l-arginine in rat isolated trachea was inhibited in the presence of atropine (0.1 μm) and the 5-hydroxytryptamine (5-HT) receptor antagonist, methysergide (1 μm). Treatment of rat tracheal preparations with capsaicin (100 μm) or tetrodotoxin (3 μm) failed to alter the contractile response to poly-l-arginine. In contrast, ecothiophate (0.1 μm) augmented the contractile response to poly-l-arginine in rat isolated trachea. 3 Electrical field stimulation (5 Hz, 2 min) of epithelium-denuded guinea-pig tracheal preparations preloaded with [3H]-choline resulted in a contractile response and the simultaneous efflux of radioactivity into the superfusate. Both these responses were abolished in the presence of tetrodotoxin (1.5 μm). Poly-l-arginine (1 mg ml−1) also increased the efflux of total radioactivity from epithelium-denuded guinea-pig isolated tracheal preparations preloaded with [3H]-choline, but this response was tetrodotoxin-insensitive. The negatively charged polyanion, heparin (1 mg ml−1) failed to increase significantly the efflux of radioactivity from epithelium-denuded preparations. 4 In conclusion, the synthetic cationic polypeptide, poly-l-arginine, caused contraction of guinea-pig isolated tracheal preparations via the release of acetylcholine from parasympathetic nerves. Similarly, poly-l-arginine-induced contraction of rat isolated trachea is secondary to the release of acetylcholine from parasympathetic nerves and/or the release of mast cell-derived 5-HT.