Evaporation-induced changes in airway surface liquid on an isolated guinea pig trachea

An isolated preparation of the guinea pig trachea was developed. The trachea was exposed serosally to Krebs-Henseleit solution and mucosally to tidal airflow, designed to mimic conditions in vivo. The preparation establishes stable layers of airway surface liquid (ASL). Typical depth, transepithelial potential differences, and sodium activity are 200 microns, -3 mV, and 125 mM, respectively (approximate sodium concn 170 mM). When exposed to air with a vapor pressure deficit (VPD), evaporation of water occurs from ASL, ASL depth decreases, and the concentration of sodium ions in ASL increases. The water content of air passing over the trachea also increases. This measurement is compared with measurements of the change in volume of ASL, based on depth changes, to yield estimates of net water transport (NWT). Measurements of changes in the sodium content of ASL allow for the calculation of net sodium transport by the trachea. Evaporation rate, changes in the volume of ASL, NWT, and net sodium transport are all influenced by VPD. The results suggest that evaporation from ASL increases its sodium concentration (and osmotic pressure) and increases osmotically driven NWT to replace water lost. Evaporation-induced increases in the sodium concentration appear to be limited by enhanced sodium uptake at high VPD.