Plasmin inhibition increases MMP-9 activity and decreases vein wall stiffness during venous thrombosis resolution

Introduction. Deep venous thrombosis (DVT) resolution involves the plasmin and the matrix metalloproteinase (MMP) system. This study tested the hypothesis that pharmacological inhibition of the plasmin system would impair DVT resolution and worsen vein wall damage. Methods. A rat model of stasis DVT by inferior vena cava (IVC) ligation was performed with intravenous control saline or aprotinin (AP; 2.8 mg/kg at operation), and harvest of thrombosed IVC at 7 days. After laser Doppler imaging, DVT were separated and weighed, and vein wall stiffness was assessed by tensiometry. Thrombus and vein wall tissue analysis included total collagen by colorimetric assay, cytokines, chemokines, and D-dimer by ELISA, urokinase-plasminogen activator (uPA), and plasminogen activator inhibitor-1 (PAI-1) by immune-blotting, MMP-2 and -9 by zymography, and neutrophil (PMN) and monocyte (ED-1) leukocytes by immunohistochemistry. Results. DVT weights were 2-fold greater in the AP-treated rats (P < 0.05), but no significant differences in thrombus perfusion, collagen, or D-dimer levels were found. Vein wall stiffness was reduced 50% (P < 0.05), suggesting less biomechanical injury. The total vein wall MMP-9 was increased (P < 0.05) 5-fold in the AP group compared with controls, while MMP-2 was elevated but did not reach significance. No difference was found in vein wall tumor necrosis factor-alpha, tissue growth factor-beta, vein wall or thrombus monocytes, PMN, or uPA/PAI-1 ratio between groups. Discussion. AP inhibition of the plasmin system was associated with larger thrombi but less vein wall injury, but no difference in other measures of resolution, possibly because of increased vein wall MMP-9 activity. These data suggest an important redundant mechanism for DVT resolution.