The Fate of Soluble Fibrin Monomer in Relation to Intravascular Fibrin Formation and Degradation in Rabbits

¹²⁵I-labeled fibrinogen or fibrin monomer (FM) were infused into rabbits, and the radioactivity in the blood, certain organs, and urine was followed. In the FM rabbits, a progressive, relatively rapid loss of blood radioactivity occurred which was accompanied by radioactive deposits in the organs and the excretion of radioactivity in the urine. Extraction of radioactive material from homogenized organ tissues of FM-infused rabbits showed that most of it had the characteristics of stabilized fibrin. Administration of EACA did not change any of the measurements in the fibrinogen animals but increased the rate and extent of fibrin deposition in the FM animals. In animals made leukopenic with HN₂, fibrin deposition was inhibited. The findings indicate that a major pathway of FM clearance from the blood involves fibrin formation and deposition with subsequent degradation and excretion. The reticuloendothelial system appeared to play a major role in FM clearance, since most of the fibrin deposits were found in the liver, and the highest concentration was in the spleen. A nonenzymatic mechanism of fibrin formation from soluble FM involving leukocytes is postulated.