title: Highly water-permeable type I alveolar epithelial cells confer high water permeability between the airspace and vasculature in rat lung.
abstract: Water permeability measured between the airspace and vasculature in intact sheep and mouse lungs is high. More than 95% of the internal surface area of the lung is lined by alveolar epithelial type I cells. The purpose of this study was to test whether osmotic water permeability (Pf) in type I alveolar epithelial cells is high enough to account for the high Pf of the intact lung. Pf measured between the airspace and vasculature in the perfused fluid-filled rat lung by the pleural surface fluorescence method was high (0.019 +/- 0.004 cm/s at 12 degrees C) and weakly temperature-dependent (activation energy 3.7 kcal/mol). To resolve the contributions of type I and type II alveolar epithelial cells to lung water permeability, Pf was measured by stopped-flow light scattering in suspensions of purified type I or type II cells obtained by immunoaffinity procedures. In response to a sudden change in external solution osmolality from 300 to 600 mOsm, the volume of type I cells decreased rapidly with a half-time (t1/2) of 60-80 ms at 10 degrees C, giving a plasma membrane Pf of 0.06-0.08 cm/s. Pf in type I cells was independent of osmotic gradient size and was weakly temperature-dependent (activation energy 3.4 kcal/mol). In contrast, t1/2 for type II cells in suspension was much slower, approximately 1 s; Pf for type II cells was 0.013 cm/s. Vesicles derived from type I cells also had a very high Pf of 0.06-0.08 cm/s at 10 degrees C that was inhibited 95% by HgCl2. The Pf in type I cells is the highest measured for any mammalian cell membrane and would account for the high water permeability of the lung.
authors.affiliation: Department of Medicine, University of California, San Francisco, CA, 94143, USA. dobbs@itsa.ucsf.edu
authors.forename: L G
authors.initials: LG
authors.lastname: Dobbs
authors.forename: R
authors.initials: R
authors.lastname: Gonzalez
authors.forename: M A
authors.initials: MA
authors.lastname: Matthay
authors.forename: E P
authors.initials: EP
authors.lastname: Carter
authors.forename: L
authors.initials: L
authors.lastname: Allen
authors.forename: A S
authors.initials: AS
authors.lastname: Verkman
mesh_terms: D000818:Animals; D015824:Blood-Air Barrier / Q000502:physiology*; D002463:Cell Membrane Permeability / Q000502:physiology*; D002469:Cell Separation; D004847:Epithelial Cells / Q000145:classification / Q000502:physiology*; D017022:Flow Injection Analysis; D008027:Light; D008297:Male; D008954:Models, Biological; D009995:Osmosis / Q000502:physiology*; D010477:Perfusion; D010994:Pleura; D011650:Pulmonary Alveoli / Q000166:cytology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D012542:Scattering, Radiation