Mechanisms of liquid flux across pulmonary alveolar epithelial cell monolayers

Active transport of sodium by pulmonary alveolar epithelial cells (AEC) is believed to be an important component of edema clearance in the normal and injured lung. Data supporting this premese have come from measurements of sodium movement across AEC monolayers or from perfused lung model systems. However, direct measurement of fluid flux across AEC monolayers has not been reported. In the present work, AEC were studied with an experimental system for the measurement of fluid flux (Jv) across functionally intact cell monolayers. Primary adult rat type II alveolar epithelial cells were cultured on 0.8 μm nuleopore filters previously coated with gelatin and fibroneclin. Intact monolayers were verified by high electrical resistance (> 1000 Ω) at 4-5 d of primary culture. At the same time interval, transmission electron microscopy revealed cells with type I cell-like morphology throughout the monolayer. These were characterized by both adherens and tight junctional attachments. Fluid flux across the monolayers was measured volumetrically over a period of 2 h in the presence of HEPES-buffered DMEM containing 3% fatty acid-free hevine serum albumin. Flux (Jv) was inhibited 39% by 1 X 10^{- 4} M ouabain (P < 0.01) and 27% by 5 X 10^-4 M amiloride P < 0.05). These data support the concept that AEC Na⁺/K⁺-ATPase and Na⁺ transport systems are important determinants of AEC transepithelial fluid movement in vitro.