Regulation of salt and water transport across airway mucosa

Effective mucociliary clearance of secretions by airway mucosa requires efficient ciliary beating. The structure of airway secretions provides for this requirement by having a viscous mucous layer touched underneath and propelled by ciliary tips, while the rest of the cilium is surrounded by a serous fluid layer. The regulation of the latter layer is thought to be a function of mucosal epithelial cells capable of active ion transport. Mammalian medium-sized bronchi actively absorb sodium, whereas the tracheal mucosae of several mammals are capable of sodium absorption as well as chloride secretion. By generating local osmotic gradients, these ion transport processes may regulate the depth of the periciliary sol layer. These transport processes generate an electrical PD across the mucosa such that the luminal side is negatively charged in reference to the submucosal side (electrogenic transport). Transport of sodium and chloride across the plasma membrane is against a steep electrochemical gradient, and cellular energy resources are utilized for this purpose (active transport). Chloride transport is coupled to sodium transport; therefore, inhibition of the sodium pump (Na-K-ATPase) with ouabain leads to inhibition of sodium as well as chloride transport. Several neurohumoral agents have been found to stimulate chloride secretion, such as PGs, beta-adrenergic agonists, VIP, substance P, and bradykinin. Mechanisms of regulation of sodium transport by airway epithelia are not clearly understood. Available evidence suggests that elevation of cellular PGs, cAMP, and calcium enhances apical cell-membrane conductance to chloride ion, with an opposite effect on sodium conductance. Therefore, it seems reasonable to suggest that neurohumoral control mechanisms may switch from sodium and fluid absorption to chloride and fluid secretion, and vice versa. Several lines of evidence support this proposal. First, the lung of fetal lamb secretes chloride and fluid in utero; this activity ceases at birth, when the catecholamine level is increased, causing a decrease in chloride secretion. In contrast, adult sheep trachea absorbs sodium. Second, agents that stimulate chloride secretion in bovine trachea concomitantly reduce sodium absorption, and vice versa. Similar observations were noted in some instances in dog trachea. Third, whereas unstimulated ferret and cat tracheas only absorb sodium, they secrete chloride upon exposure to beta agonists.(ABSTRACT TRUNCATED AT 400 WORDS)