Differentiation of ion-associated and osmotically driven water transport in canine airways

We hypothesized that water transport associated with fluxes of Na+ and Cl- across airway epithelium coexists and is distinct from osmotically driven water transport. To investigate this, we anesthetized and mechanically ventilated dogs (n = 8) with warm humid air. The trachea of each dog was sequentially challenged with 250-mOsm and 950-mOsm mannitol aerosols given 30 min apart. Respiratory tract fluid output (RTFO) was collected at the posterior commissure at 6-min intervals. The percentages of mannitol in the RTFO were determined with fluorescent tracers and were subtracted from the RTFO to give airway secretory output (ASO). Unbound [Na+] and [Cl-] in the RTFO were measured. Following the 250-mOsm mannitol challenge, the ASO as well as its Na+ and Cl- contents increased. Following the 950-mOsm challenge, there was a further increase in ASO without any further increases in Na+ and Cl- contents. Increased mucociliary transport accounted for only part of the increase in ASO. These data are consistent with the hypothesis that net water transport into the airway lumen is the vectorial sum of the water fluxes associated with actively driven intracellular Na+ and Cl- transport and the water flux due to osmosis.