Secretion and ion transport in airways during inflammation

Water and secretions interact in airways to produce the sol and gel layers that allow for entrapment of foreign materials and subsequent clearance by ciliary movement and by cough. Active Cl ion transport produces fluid, and this process is activated by products of mast cells (leukotrienes), eosinophils (major basic protein), and by other inflammatory mediators (prostaglandins, bradykinin). Gland secretions produce the bulk of the volume of secretions. Airway irritation stimulates gland secretion reflexly via vagal muscarinic pathways. Recently, the sensory nerves have been discovered to release substance P and other neuropeptides when the airways are irritated. The stimulatory effects of neuropeptides on gland secretion (and on other inflammatory sites) are modulated by enkephalinase a membrane-bound enzyme that cleaves neuropeptides and thereby inactivates them. Up- or down-regulation of enkephalinase is predicted to change the degree of inflammatory response to neuropeptides. Finally, the cell surface of airway epithelial cells have been discovered to secrete large molecular weight glycoconjugates; these secreted products are increased markedly by a series of proteinases produced by inflammatory cells (neutrophils, mast cells) and by bacteria. Their exact physiologic roles are still unknown but they may contribute to the bulk and viscoelastic properties of airway secretions, and they may serve an important role in bacterial, viral and inflammatory cell adhesion.