Permeability of the airway mucosa in asthma.

Mediators and drugs in the lumen of the airways have to penetrate a complex barrier to reach airway smooth muscle: luminal mucus, epithelium, basement membrane, interstitial fluid and vascular beds. The mucus can bind and transport chemicals. The epithelium is relatively impermeable, but it is structurally damaged in asthma, and experimental damage can be induced by irritants, inflammatory mediators, hypertonicity and drying, and viral diseases. The interstitial fluid volume increases in mucosal oedema, as in asthma. Changes in blood flow, as in the hyperaemia of asthma, will affect the uptake of drugs and mediators and, therefore, their transport through the mucosa. All these variables are under physiological control, and can be profoundly altered in pathological conditions. Such changes may be an important factor in determining the airway responses to endogenous mediators, and to exogenous drugs for therapy or agents for testing bronchial hyperresponsiveness.