Ionic mechanism of forskolin-induced liquid secretion by porcine bronchi

Abstract

cAMP-elevating agents such as forskolin and vasoactive intestinal peptide induce liquid secretion by tracheobronchial submucosal glands. This pathway is thought to be CFTR dependent and thus defective in cystic fibrosis; however, the ionic mechanism that drives this secretion process is incompletely understood. To better define this mechanism, we studied the effects of ion transport inhibitors on the forskolin-induced liquid secretion response (Jv) of porcine distal bronchi. The forskolin-induced Jv was driven by a combination of bumetanide-sensitive Cl− secretion and DIDS-sensitive HCO3− secretion. When Cl− secretion was inhibited with bumetanide, Na+/H+ exchange-dependent HCO3− secretion was apparently induced to compensate for the loss of Cl− secretion. The forskolin-induced Jv was significantly inhibited by the anion channel blockers 5-nitro-2-(3-phenylpropylamino)benzoic acid, diphenylamine-2-carboxylate, and glibenclamide. We conclude that the forskolin-induced Jv shares many characteristics of cholinergically induced secretion except for the presence of a DIDS-sensitive component. Although the identity of the DIDS-sensitive component is unclear, we speculate that it represents a basolateral membrane Na+-HCO3− cotransporter or an Na+-dependent anion exchanger, which could account for transepithelial HCO3− secretion.