New approaches to regulation of fluid secretion in airways.

Physiologic, morphologic, biochemical, and immunologic methods have been used to study cholinergic and adrenergic regulation of airway submucosal gland secretion. Using a newly developed micropipette method, we can now measure the rate of fluid flow from each individual gland duct and we can harvest these secretions for specific chemical analysis. Stimulation of both the cholinergic and adrenergic systems affect gland secretions. The vagus nerves are the pathways for many reflexes affecting secretion: laryngeal, gastric and carotid body chemoreceptor stimulation accelerate the fluid production, and lung inflation (Hering-Breuer reflex) inhibits fluid secretion. Stimulation by cholinergic, muscarinic agonists depletes both mucous and serous cells, so vagal reflexes probably produce mixed secretions. In contrast, alpha-adrenergic agonists deplete serous cell contents, release high concentrations of lysozyme and produce large volumes of fluid with a low protein concentration, while beta-adrenergic stimulation may selectively deplete mucous cells, releases little lysozyme and produces scanty fluid with a high protein concentration. The significance of this cholinergic and adrenergic modulation on cough and ciliary clearance are important subjects for future study.