The physical and chemical properties of the airway-surface liquid (ASL) of the lung are believed to be perturbed in cystic fibrosis, but precisely how they differ from conditions in the healthy lung, and why such a disturbance should render the lung sensitive to infections are hotly disputed. High salt levels in the ASL might allow for bacterial growth by blocking the antibiotic activity of endogenous defense peptides; alternatively, a dehydrated and relatively viscous ASL, even at isotonic saline, might provide a hospitable climate for bacteria. Unfortunately, these models are not easily tested; fluid flow across the delicate pulmonary epithelium could be readily disturbed by any probes introduced to sample the ASL, raising serious doubts about the accuracy of in vivo measurements.
Jayraman and colleagues have now provided a set of tools that may resolve this difficulty. Their approach employs fluorescent indicators that can be introduced into the ASL and observed in several settings, including within the living trachea. Using confocal fluorescence microscopy Jayraman et al. measure the depth and the concentrations of various ions of the ASL layer produced by normal human and mouse pulmonary tissue, and they show that there is no significant difference in ASL salinity between normal and CFTR-/- mice. The authors note that this noninvasive approach may be of use in the study of other lung diseases where the properties of the ASL has not been adequately studied.