Role of absorptive and secretory processes in hydration of the airway surface.

In the conducting airways, epithelial electrolyte transport processes play an important role in controlling the quantity and the composition of the airway surface solution layer. Hydration of the airway surface appears to promote effective mucociliary clearance and to influence the rheology of mucins, and together these factors provide a major mechanism for airway defense. The cellular processes governing absorption and secretion of salt are similar, if not identical, to those that have been described in a variety of other absorptive and secretory epithelial cells where they may be studied in isolation. A better molecular understanding of the channels that control apical membrane transport of Na and Cl, the rate-determining steps in salt absorption and secretion, should enhance our basic understanding of the factors that control the quantity and composition of airway fluids. This may elucidate the mechanisms for the reciprocal control of apical Na and Cl channel activities and provide an explanation for the balance between these oppositely directed salt transport processes.