Role of platelets in maintenance of pulmonary vascular permeability to protein

The authors examined the role of platelets in maintenance of pulmonary vascular integrity by inducing thrombocytopenia in sheep using antiplatelet serum (APS). A causal relationship between thrombocytopenia and increase in pulmonary vascular permeability was established by platelet repletion using platelet-rich plasma (PRP). Sheep were chronically instrumented and lung lymph fistulas prepared to monitor pulmonary lymph flow (Qlym). A balloon catheter was positioned in the left atrium to assess pulmonary vascular permeability to protein after raising the left atrial pressure (Pla). Thrombocytopenia was maintained for 3 days by daily intramuscular APS injections. In studies using cultured bovine pulmonary artery endothelial monolayers, transendothelia permeability of 125I-labeled albumin was reduced 50 and 95%, respectively, when 2.5 x 107 or 5 x 107 platelets were added onto endothelial monolayers. However, addition of 5 x 106 platelets or 5 x 107 red blood cells did not reduce endothelial monolayer albumin permeability. Results indicate that platelets are required for the maintenance of pulmonary vascular permeability. Reduction in permeability appears to involve an interaction of platelets with the endothelium