ESTIMATION OF NASAL EPITHELIAL LINING FLUID USING UREA AS A MARKER

Background: The luminal surface of respiratory mucous membranes is lin ed with an epithelial lining fluid (ELF) layer. Previous attempts to d etermine ELF volumes in airways have used dyes or freely diffusible mo lecules such as urea, yet have not led to a universally accepted metho d. The nasal mucous membrane provides an accessible area to examine wh ether urea is an appropriate marker of respiratory ELF volume. Methods and Results: Collection of undiluted nasal secretions after either gl andular stimulation or induction of vascular permeability confirmed th at plasma urea and nasal urea concentrations are equivalent. Baseline ELF volume was calculated as 800 mul/nostril. The calculated molar con centrations of urea in ELF did not vary with either methacholine or hi stamine challenge. Conclusions: These data indicate the plasma, inters titial, glandular and ELF urea concentrations are equivalent and, ther efore, that urea is a useful marker of ELF volume in the nasal mucosa.