Differential permeability of endothelial and epithelial barriers to albumin flux

We measured the flux of albumin between the vascular space and the pulmonary interstitial and luminal lining fluids in 20 adult sheep with chronic lung lymph fistulas. We sampled the bronchoalveolar lining layer by episodic fiberbronchoscopic lavage. A total of 62 alveolar lavages were performed at times ranging between 30 min and 60 h after intra-arterial injection of 100 microCi of 125I-labeled albumin. Samples of lymph and plasma were obtained simultaneously with lavage fluid, and the radioactivity and albumin content of all samples were measured and expressed as specific activity (counts/min . g albumin). We found that alveolar lavage fluid collected by our technique is not significantly contaminated by plasma or interstitial fluid proteins. Proteins present in alveolar lavage fluid and also present in plasma reach the alveolar space by a normal diffusive process, and not as a result of epithelial membrane damage occurring at the time of lavage. Lung epithelial permeability to albumin in small, but finite (4.3--5.8 x 10(-10) cm/s). Virtually all (greater than 92%) of resistance to albumin flux across the alveolocapillary membrane lies in the epithelial barrier. Increases in permeability of the respiratory epithelium, even minor, would have a marked effect on water and solute balance in the lung. Epithelial injury will potentiate pulmonary edema formation even in the presence of normal pulmonary microvascular pressure, plasma oncotic pressure, and endothelial permeability.