Ionic composition of rat airway surface liquid determined by X-ray microanalysis

The thin layer of liquid that lines the conducting airway epithelium, the airway surface liquid (ASL), is important for mucociliary clearance. Altered ionic composition and/ or volume of the ASL play a major role in the pathology of airway diseases such as cystic fibrosis. Since the ASL is a thin layer, it has been difficult to exactly determine its composition. The present paper describes two techniques that have been developed and used to study ASL composition: X-ray microanalysis of frozen hydrated rat trachea, and an ion-exchange (dextran) bead method, where dextran beads were placed on the airway epithelium to equilibrate with the ASL; the beads were then collected under silicone oil, dried and analyzed by X-ray microanalysis. The results from both frozen-hydrated specimens and from the dextran beads showed that ASL from rat trachea is hypotonic. Concentrations of Na, P, S, and K were higher in the frozen-hydrated ASL, in which mainly the mucus layer is analyzed, compared with the dextran bead method, in which mainly the periciliary liquid is sampled. Also the composition of rat nasal fluid was investigated by the dextran bead method. This fluid was somewhat hypertonic because of a high K concentration. The ionic composition of the nasal and tracheal fluid can be manipulated by cholinergic or alpha- or beta-adrenergic stimulation. Collecting ASL with dextran beads did not disturb the integrity of the airway epithelium. The ionic composition of the collected beads remained stable for several days during storage in silicone oil. It is concluded that X-ray microanalysis is a suitable method to determine the ionic composition of ASL.