Water Permeability Adjusts Resorption in Lung Epithelia to Increased Apical Surface Liquid Volumes

The apical surface liquid (ASL) layer covers the airways and forms a first line of defense against pathogens. Maintenance of ASL volume by airway epithelia is essential for maintaining lung function. The proteolytic activation of epithelial Na⁺ channels is believed to be the dominating mechanism to cope with increases in ASL volumes. Alternative mechanisms, in particular increases in epithelial osmotic water permeability (P_osm), have so far been regarded as rather less important. However, most studies mainly addressed immediate effects upon apical volume expansion (AVE) and increases in ASL. This study addresses the response of lung epithelia to long-term AVE. NCI-H441 cells and primary human tracheal epithelial cells, both cultivated in air-liquid interface conditions, were used as models for the lung epithelium. AVE was established by adding isotonic solution to the apical surface of differentiated lung epithelia, and time course of ASL volume restoration was assessed by the deuterium oxide dilution method. Concomitant ion transport was investigated in Ussing chambers. We identified a low resorptive state immediately after AVE, which coincided with proteolytic ion transport activation within 10-15 minutes after AVE. The main clearance of excess ASL occurred during a delayed (hours after AVE) high resorptive state, which did not correlate with ion transport activation. Instead, high resorptive state onset coincided with an increase in P_osm, which depended on aquaporin up-regulation. In summary, our data demonstrate that, aside from ion transport activation, modulation of P_osm is a major mechanism to compensate for long-term AVE in lung epithelia.