Physical and functional properties of airway secretions in cystic fibrosis--therapeutic approaches.

The airway secretions which line the respiratory tract form a biphasic layer composed of an aqueous 'sol' layer and a more superficial 'gel' layer. In the sol layer, also described as the 'periciliary' layer or 'airway surface fluid', the cilia beat and relax. The lubricant sol layer enables the gel mucus present at the tips of the cilia to be transported by the ciliary beating of the ciliated cells. Due to difficulties with sampling, little is known about the physical and biochemical properties of the sol layer. The gel layer is composed of high molecular weight glycoproteins (mucins) linked with proteins and lipids. They form a gel network with a high water content (95%) and rheologic and physical properties (viscoelasticity, adhesivity) adapted in normal conditions to protect the airway mucosa, particularly through mucociliary transport. The adhesive properties of mucus, which are influenced by its lipid composition and degree of hydration, are very important in controlling the efficacy of mucus transport through ciliary activity and coughing. An intermediate viscosity and elasticity is required for optimal mucociliary transport. In obstructive airway diseases, either of genetic origin, such as cystic fibrosis (CF), or acquired (acute or chronic bronchitis), and particularly during inflammatory and infectious episodes, mucus dehydration is associated with an increase in secreted or transudated molecules and with marked augmentation of DNA content. These abnormalities contribute to the increased viscosity and adhesivity of the airway secretions and are responsible for their abnormally low transport rate by ciliary activity and for inefficient cough clearance. In view of these alterations in the physical and functional properties of CF airway secretions, pharmacologic approaches should aim to rehydrate the mucus and to restore normal mucociliary or cough transport by stimulating chloride ion secretion (i.e. using UTP or ATP associated with amiloride in order to block sodium ion and water reabsorption). During acute episodes of infection, recombinant human DNase (rhDNase) may rapidly prevent mucus stasis by improving its rheologic properties. Lubrication of the mucus at the sol phase interface by 'surfactant' therapy may also represent a very promising therapeutic perspective to reduce the hyperviscosity and hyperadhesivity of airway secretions.