Effect of 100% O2 on passage of uncharged dextrans from blood to lung lymph

Since charge as well as size may influence the passage of plasma proteins from blood to lung lymph, we used uncharged dextrans as tracers to study the effects of hyperoxic lung injury on the molecular sieving properties of the pulmonary microcirculation in unanesthetized sheep. Polydisperse [3H]dextran was infused intravenously into five sheep before and after the animals breathed 100% O2 until lymph flow increased threefold (66-84 h). Lymph-to-plasma concentration ratios (L/P) were determined for [3H]dextran fractions of graded molecular sizes (1.6-8.4 nm effective radius) from samples obtained during the infusions. Before hyperoxia the blood-lymph barrier was highly restrictive to transport of [3H]dextrans above 5.0 nm in radius; steady-state L/P for these molecules averaged 0.03 or less. After the sheep breathed 100% O2, [3H]dextrans as large as 8.4 nm radius appeared in the lymph. Posthyperoxia, the L/P were significantly increased relative to prehyperoxia base-line values for every [3H]dextran fraction larger than 2.0 nm radius (P less than 0.05). In contrast, neither the L/P for albumin or total protein changed significantly. At autopsy, electron microscopy showed widespread damage to the endothelium of the alveolar capillaries with infrequent gaps between endothelial cells. In two control sheep, inhalation of compressed air for 96 h had no effect on lymph flow or L/P for the [3H]dextrans. We conclude that O2 poisoning reduced the selective sieving of uncharged dextrans across the blood-lymph barrier of the lungs and allowed larger dextrans to enter the lymph. These larger molecules may have leaked from the pulmonary microcirculation via disruptions in the continuity of the endothelial lining.