Mechanism of substance P-induced liquid secretion across bronchial epithelium

The present study was undertaken to identify and determine the mechanism of noncholinergic pathways for the induction of liquid secretion across airway epithelium. Excised porcine bronchi secreted substantial and significant quantities of liquid when exposed to acetylcholine, substance P, or forskolin but not to isoproterenol, norepinephrine, or phenylephrine. Bumetanide, an inhibitor of Na+-K+-2Cl- cotransport, reduced the liquid secretion response to substance P by 69%. Approximately two-thirds of bumetanide-insensitive liquid secretion was blocked by dimethylamiloride (DMA), a Na+/H+ exchange inhibitor. Substance P responses were preserved in airways after surface epithelium removal, suggesting that secreted liquid originated from submucosal glands. The anion channel blockers diphenylamine-2-carboxylate (DPC) and 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) inhibited >90% of substance P-induced liquid secretion, whereas DIDS had no effect. DMA, DPC, and NPPB had greater inhibitory effects on net HCO3- secretion than on liquid secretion. Although preserved relative to liquid secretion, net HCO3- secretion was reduced by 39% in the presence of bumetanide. We conclude that substance P induces liquid secretion from bronchial submucosal glands of pigs through active transport of Cl- and HCO3-. The pattern of responses to secretion agonists and antagonists suggests that the cystic fibrosis transmembrane conductance regulator mediates this process.