Airway mucus.

Airway surface liquid (ASL), a mixture of periciliary fluid and submucosal gland secretions, was collected from the ferret isolated trachea in vitro. The trachea was closed, without possibility of evaporation. The collected ASL was hyperosmolar (310-350 mosmol.kg-1) compared with Krebs-Henseleit solution (280 mosmol.kg-1). Compared with surrounding Krebs-Henseleit solution, the ASL had higher sodium and chloride contents, and considerably higher potassium and calcium contents. The ASL was acid (pH about 7.00) compared with Krebs-Henseleit solution (pH 7.40). Applying methacholine and salbutamol to the preparation significantly changed most of the electrolyte concentrations, and reduced pH. The pH was not significantly changed by bubbling the surrounding buffer with 0-20% CO2, with corresponding buffer changes in pH of 6.95-8.05. Adding labelled albumin to the external buffer resulted in lumenal concentrations that, in the presence of salbutamol, were higher than outside. This and other evidence suggested that albumin could be actively secreted into the lumen, a process enhanced by salbutamol. Thus ASL is hyperosmolar, of different electrolyte composition from interstitial fluid, and of low pH which is homeostatically regulated. The epithelium can actively secrete albumin into the lumen.