Effect of lung inflation on alveolar-airway barrier protein permeability in dog lung

To determine the leakiness to protein of the barrier that separates the air space and interstitial compartment of the lung, we measured perivascular interstitial fluid cuff protein concentration and volume in 10 isolated and 9 intact closed-chest dog lung lobes, which we degassed and inflated to 25, 50, 75, or 100% of capacity with 5% bovine serum albumin labeled with Evans blue dye. After 1 h we froze the lobes in liquid N2 and made color transparencies of 20 randomly selected frozen samples of each lobe. We measured Evans blue dye-albumin concentrations from absorption by cuff images of a 50-micron-diam red (lambda = 620 nm) microspot. We measured absolute cuff volume (ml/g dry lung) by point counting on the transparencies. Using specific Evans blue-albumin fluorescence we determined that the dye was protein bound in airways and cuffs. Cuff protein concentration averaged 37% of instillate concentration and did not vary with inflation volume or between isolated and intact lobes. Cuff volume was 3.4 ml/g dry lung at total lung capacity in both isolated and intact lobes. We conclude that at some point the barrier is permeable to albumin as well as liquid at all lung volumes in dogs and that the protein sieving properties of the barrier do not change with lung expansion over the range examined. The liquid storage capacity of the cuffs can increase as much as 20-fold between low and high lung volumes.