HIGHLY WATER-PERMEABLE TYPE-I ALVEOLAR EPITHELIAL-CELLS CONFER HIGH WATER PERMEABILITY BETWEEN THE AIRSPACE AND VASCULATURE IN RAT LUNG

Water permeability measured between the airspace and vasculature in in tact sheep a;nd mouse lungs is high. More than 95% of the internal sur face area of the lung is lined by alveolar epithelial type I cells. Th e purpose of this study was to test whether osmotic water permeability (P-f) in type I alveolar epithelial cells is high enough to account f or the high P-f of the intact lung, P-f measured between the airspace and vasculature in the perfused fluid-filled rat lung by the pleural s urface fluorescence method was high (0.019 +/- 0.004 cm/s st 12 degree s C) and weakly temperature-dependent (activation energy 3.7 kcal/mol) . To resolve the contributions of type I and type II alveolar epitheli al cells to lung water permeability, P-f was measured by stopped-flow light scattering in suspensions of purified type I or type II cells ob tained 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 (t(1/2)) of 60-80 ms at 10 degrees C, giving a plasma membrane P-f of 0.06-0.08 cm/s, P-f in typ e I cells was independent of osmotic gradient size and was weakly temp erature-dependent (activation energy 3.4 kcal/mol). In contrast, t(1/2 ) for type II cells in suspension was much slower, approximate to 1 s; P-f for type II cells was 0.013 cm/s. Vesicles derived from type I ce lls also had a very high P-f of 0.06-0.08 cm/s at 10 degrees C that wa s inhibited 95% by HgCl2. The P-f in type I cells is the highest measu red for any mammalian cell membrane and would account for the high wat er permeability of the lung.