Figure 2.

Schematic model for mucus secretion in bronchial epithelial cells (modified from Reference [132]). Na⁺–K⁺–Cl⁻ co-transporter (NKCC1) promotes the basolateral absorption of Cl⁻, which is then secreted into the lumen by CFTR and anoctamin 1 (ANO1), while pendrin exchanges Cl⁻ from the lumen with intracellular HCO₃⁻. This equilibrium is dysregulated by inflammatory events, with an increased HCO₃⁻ secretion leading to higher mucus formation. For graphic simplicity, all the transporters are shown in the same cell, although they may be expressed in different cells of the airway epithelium.