Ion and liquid transport across the bronchiolar epithelium

The proper homeostasis of the airway surface liquid (ASL) depends on transepithelial ion and fluid transport and is critically important for lung defence, and more specifically for mucociliary transport. In cystic fibrosis (CF), abnormal ion and fluid transport lead to depleted ASL volume resulting in mucus plugs and recurrent lung infections. Like bronchi, human bronchioles exhibit amiloride-sensitive Na+ absorption and cyclic-AMP and Ca2+-activated Cl− secretion. However, cyclic-AMP-stimulated Cl− and fluid secretion appears to be quantitatively more important in bronchioles than in bronchi. In CF bronchioles, like in CF bronchi, the ASL height is reduced because of an abnormally persistent Na+ absorption, combined with a lacking CFTR-dependent Cl− secretion. The precocity and severity of the bronchiolar disease in CF could be attributed in part to the more important role of CFTR-dependent Cl− secretion and fluid secretion, and the lack of compensatory ATP-driven Cl− secretion and fluid secretion, in bronchioles compared to bronchi.