The role of blood coagulation in capillary permeability Vitalmicroscopic contributions.

It is our concept that the blood coagulation and fibrinolytic systems contribute to capillary permeability. By means of fluorescent intravital microscopy we measured in situ the permeability of proteins and fluorescent tracers through the walls of various kinds of microvessels (exposed rat mesentery). Fluorescent dyes applied intravenously pass across the walls of all kinds of microcirculatory vessels into the perivascular tissue in animals having a state of hypocoagulemia in a shorter time than in normals. Fluorescent-tagged fibrinogen accumulates at the vascular wall, especially at the inner lining of venules, thus marking free receptor sites for fibrinogen at these places. Blocking the fibrinolytic activity of the blood augments the fibrinogen deposition. Pretreatment by heparin does not prevent it, which speaks against the fibrinogen-fibrin coagulation during the accumulation phenomenon. In contrast to this behavior of fibrinogen, albumin, gammaglobulin, antithrombin III, plasminogen never accumulate at the vessel wall but pass through the vascular wall, mostly at venules. The speed and amount of the transvascular passage of these proteins depends on their molecular weight. Fibronectin shares the same receptor sites at the inner lining of microcirculatory vessels as fibrinogen. It seems possible, therefore, that both proteins, fibrinogen and fibronectin, interact with each other at the endo-endothelial cell border. A low molecular weight polypeptide fraction of factor VIII accumulates at the vascular wall similar to fibrinogen. At the same time it decreases the capillary permeability for serum proteins.