Background: The luminal surface of respiratory mucous membranes is lined with an epithelial lining fluid (ELF) layer. Previous attempts to determine ELF volumes in airways have used dyes or freely diffusible molecules such as urea, yet have not led to a universally accepted method. The nasal mucous membrane provides an accessible area to examine whether urea is an appropriate marker of respiratory ELF volume.
Methods and results: Collection of undiluted nasal secretions after either glandular stimulation or induction of vascular permeability confirmed that plasma urea and nasal urea concentrations are equivalent. Baseline ELF volume was calculated as 800 microliters/nostril. The calculated molar concentrations of urea in ELF did not vary with either methacholine or histamine challenge.
Conclusions: These data indicate the plasma, interstitial, glandular, and ELF urea concentrations are equivalent and, therefore, that urea is a useful marker of ELF volume in the nasal mucosa.