HIGH-CONCENTRATIONS OF ACTIVE PLASMINOGEN-ACTIVATOR INHIBITOR-1 IN PORCINE CORONARY-ARTERY THROMBI

Addition of exogenous plasminogen activator inhibitor-1 (PAI-1) to fib rin clots inhibits fibrinolysis in vivo. However, it is unknown whethe r the localized concentrations of active PAI-1 necessary to produce th is antifibrinolytic effect can be recruited to acute arterial thrombi by endogenous mechanisms. We measured PAI-1 activity and antigen in po rcine coronary artery thrombi that formed in response to acute vascula r injury. Mean PAI-1 activity in thrombi (n=5) was 36+/-5.1 mu g/mL, w hich is >2000 times its concentration in normal porcine plasma. The pr esence of markedly elevated concentrations of active PAI-1 in thrombi was confirmed by an immunoactivity assay and by demonstrating formatio n of sodium dodecyl sulfate-stable complexes after addition of I-125-u rokinase to thrombus extracts. Comparative analysis of PAI-1 antigen b y Western blotting and urokinase inhibition assay suggested that appro ximately one third of thrombus-associated PAI-1 was active. Histologic al examination of coronary thrombi revealed that they consisted predom inantly of dense aggregates of platelets with interspersed islands of fibrin, which closely resemble the histological appearance of thrombi in patients with myocardial infarction and unstable angina pectoris. W ashed porcine platelets prepared from peripheral blood contained suffi cient PAI-1 antigen and activity to account for the concentrations obs erved in coronary artery thrombi. However, the specific activity of hu man platelet PAI-1 was lower than that of porcine platelet PAI-1 (2% v ersus 50% active, respectively), and human platelets inhibited in vitr o fibrinolysis to a lesser extent than did porcine platelets. These re sults indicate that active PAI-1 accumulates in porcine coronary arter y thrombi in concentrations markedly higher than those present in plas ma and that PAI-I may be an important determinant of the known resista nce of platelet-rich thrombi to lysis by tissue-type plasminogen activ ator. These studies also underscore the importance of considering poss ible species differences in protein function when comparing animal mod els of thrombosis to acute coronary thrombosis in humans.