Ion transport and water flow in the mammalian lung

The coupling of bulk water flow to active ion transport has been described in various epithelia; evidence presented here suggests that this is also a feature of the mammalian lung. Measurements of the ionic composition of lung liquid and its rate of formation in the fetal lamb in vivo have made it possible to estimate the net flux of each ion and, with water tracer measurements of ion one-way fluxes, to calculate flux ratios. When these are compared with the ratios predicted by the Ussing flux ratio equation it is clear that the secretion of lung liquid is linked to active transport of Cl- from plasma; sodium moves passively. In addition there is an apparent uphill transfer of HCO2- out of lung liquid. In an in vitro preparation of adult canine trachea Cl- is actively transported towards the lumen and is associated with a small net flux of Na+ in the opposite direction. Addition of acetylcholine increases the net Cl- flux towards the lumen but reverses the orientation of the net Na+ flux. Changes such as these may be important determinants of bulk liquid flow in vivo as well as in vitro.