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Cystic fibrosis (CF) is one of the most common fatal inherited diseases, most prevalent among Caucasians. CF is caused by a mutation in the gene coding for the cystic fibrosis transmembrane conductance regulator (CFTR), which helps to create sweat, digestive juices, and airway surface liquid (ASL). The airways are covered with a thin layer of fluid, the airway surface liquid, in which the cilia bathe. Defective or absent CFTR leads to a defective water and ion transport in the epithelial cells, which results in viscous mucus, defective ciliary clearance, bacterial inflammation and tissue damage. The volume and composition of ASL are important in the pathogenesis of cystic fibrosis and it is therefore relevant to determine its composition. However, there are a number of difficulties in determining the ionic composition of the ASL due to its small volume. Literature data vary from very hypotonic to markedly hypertonic. These controversial data inspired the development of a simple method for determining the elemental composition of the ASL in different animal species and humans. Two techniques were developed to determine the composition of ASL, from which indirect information on chloride transport in airway epithelium can be obtained. In the first method, tissue is removed from the animals under anesthesia, frozen and analyzed in the frozen-hydrated state. In the second method, the ASL is collected with small dextran (Sephadex) beads; the dried beads are then analyzed by X-ray microanalysis. The Sephadex-bead method appears more accurate compared to the frozen-hydrated samples. Both methods were applied to collect tracheal and/or nasal fluid in pigs, normal and transgenic cystic fibrosis mice, the fluid covering the apical surface of normal bronchial cells (16HBE14o-) and a cystic fibrosis human bronchial cell line (CFBE41o-), and finally nasal fluid in healthy and diseased subjects. The ionic composition of the ASL was isotonic both in pigs and healthy human subjects. CF patients had much higher levels of Na and Cl ions than healthy subjects. The ASL under control conditions was hypotonic in mice and cell cultures, whereas the concentrations of Na and Cl ions in the species with the ΔF508 mutation or absent CFTR were significantly higher than in the corresponding controls. It was also demonstrated that the ionic composition of the ASL can be influenced by pharmacological treatment. The study confirms earlier findings that CFTR also is involved in bicarbonate transport. Mist tent therapy has been tested in the study of a treatment for CF patients, in order to hydrate the viscous mucus. But the effect of mist tent therapy on ion concentrations in the ASL appeared to be short-lived, although no patients became chronically colonized with pseudomonads while on nocturnal mist tent therapy.