Transepithelial water permeability in microperfused distal airways - Evidence for channel-mediated water transport

Water movement across the airway epithelium is important for regulation of the volume and composition of airspace fluid. A novel approach is reported here to measure osmotic and diffusional water permeability in intact airways. Small airways (100-200 mu m diameter, 1-2 mm length) from guinea pig lung were microdissected and perfused in vitro using concentric glass holding and perfusion pipettes. For measurement of osmotic water permeability (P-f), the airway lumen was perfused with PBS (300 mOsM) containing a membrane impermeable fluorophore, fluorescein sulfonate (FS), and the airway was bathed in solutions of specified osmolalities, P-f determination was based on the change in FS fluorescence at the distal end of the airway resulting from transepithelial water transport, P-f was 4-5 X 10(-3) cm/s at 23 degrees C and independent of lumen flow rate (10-100 nl/min) and the magnitude and direction of the osmotic gradient (bath osmolality 50-600 mOsM). Temperature dependence measurements gave an activation energy of 4.4 kcal/mol (15-37 degrees C). P-f was not altered by 0.3 mM HgCl2 or 50 mu M forskolin, but was increased to 31 X 10(-3) cm/s by 100 mu g/ml amphotericin B, indicating that osmosis is not limited by unstirred layers. Diffusional water permeability (P-d) was measured by H2O/(DO)-O-2 (deuterium oxide) exchange using the H2O/D2O-sensitive fluorescent probe aminonapthelane trisulfonic acid in the lumen. Measured P-d was 3-6 X 10(-6) cm/s at 23 degrees C, indicating significant restriction to water diffusion by unstirred layers. Antibody localization of water channels showed strong expression of the mercurial-insensitive water channel (AQP-4) at the basolateral membrane of airway epithelial cells. These results provide functional evidence that water movement across the distal airway epithelium is mediated by water channels.