The tissue plasminogen activator and urokinase response in vivo during natural resolution of venous thrombus.

PURPOSE: The aim of this study was to measure the distribution of endogenous plasminogen activators during thrombolysis with an endothelial-conserving model of laminated thrombosis. METHODS: Thrombi were raised in the inferior vena cava of rats with thrombin and flow reduction. The thrombi, adjacent vein wall, and distant veins (the superior vena cava) were removed at intervals from 1 hour to 21 days from formation and then cryohomogenized and assayed with specific bioimmunoassays for tissue-type (t-PA) and urokinase-type plasminogen activators (u-PA). RESULTS: The measured t-PA activity of the vein wall around the thrombus was reduced compared with the control inferior vena cava at 4 days. Both the u-PA and t-PA content of the thrombus increased progressively during thrombolysis. The t-PA activity increased significantly in the distant vein walls in the animals with thrombi. Immunocytochemistry and in situ hybridization localized the t-PA to a mononuclear cell infiltrate and showed up-regulation of mRNA for rat t-PA in these monocytes. CONCLUSIONS: The local plasminogen activator response was predominantly within the thrombus itself. Increased t-PA activity was additionally found in distant veins but was reduced in the vessel wall adjacent to the thrombus. This is the first report to show that u-PA activity is increased within organizing thrombus in vivo and that most of the t-PA activity is localized to a monocyte infiltrate.