This is mediated by active ion transport, for instance by the cystic fibrosis transmembrane conductance regulator protein or calcium-activated chloride channels such as
anoctamin-1 (ANO-1/TMEM16A).[38],[39] Chloride secretion and reabsorption of sodium by the epithelial sodium channel have been shown to regulate ASL volume.[40] This has important consequences for mucociliary clearance as it determines the hydration state of the mucus gel, as well as the height of the periciliary layer which is an important determinant of ciliary movement.[41] Moreover, transport of bicarbonate regulates the pH of the ASL,[42] which may affect the activity of pH-sensitive AMPs and mucus viscosity.[43],[44]{Figure 3}