REGULATION OF DEPTH AND COMPOSITION OF AIRWAY SURFACE LIQUID

The depth and composition of human airway surface liquid (ASL) may dep end on secretion from airway glands, ion transport across the surface epithelium, goblet cell discharge, transepithelial gradients in hydros tatic pressure, and surface tension, Published values for the frequenc y of airway glands and for the secretory rates of individual glands su ggest that total gland secretion in human trachea can amount to simila r to 60 mu L.cm(-2).h(-1). Volume absorption directly measured across cultures of surface epithelium from human trachea is similar to 5 mu L .cm(-2).h(-1). These flows should alter the depth of ASL at +10 and -1 mu m.min(-1). We have looked for I changes in ASL depth of this magni tude using low-temperature scanning electron microscopy (LT-SEM) of ra pidly frozen specimens of bovine trachea, Stimulation of gland secreti on with methacholine led to an initial increase in depth of similar to 25 mu m.min(-1) followed by a decline at similar to 15 mu m.min(-1). Whereas the initial increase in depth was probably due to transient gl and secretion, the later decline reflected active absorption of liquid across the surface epithelium, Finally, we present preliminary data s helving that LT-SEM can be combined with X-ray microanalysis to determ ine the elemental composition of ASL.