Murphy, Joseph G.

A model of proliferative human restenosis was developed in domestic pigs by using deep injury to the coronary arterial media. Metal wire coils were delivered percutaneously to the coronary arteries of 11 pigs with an oversized, high-pressure (14 atm) balloon and were left in place for times ranging from 28 to 70 days. During placement, the balloon expanded the coils and delivered them securely within the arterial lumen. Light microscopic examination of the vessels confirmed fracture of the internal elastic lamina by the coil. An extensive proliferative response occurred in 10 of the 11 pigs and was associated with a luminal area narrowing of at least 50% in all but one pig. The histopathologic features of the proliferative response were identical to those observed in human cases of restenosis after angioplasty. Immunohistochemical studies confirmed the prominence of smooth muscle cells in the proliferative tissue. A similar response was obtained in two of five porcine coronary arteries in which balloon inflation only was performed, without coil implant. This model is practical and inexpensive and closely mimics the proliferative portion of human restenosis both grossly and microscopically. Thus, it may be useful for understanding human restenosis and for testing therapies aimed at preventing restenosis after balloon angioplasty or other coronary interventional procedures.

Addition of exogenous plasminogen activator inhibitor-1 (PAI-1) to fibrin clots inhibits fibrinolysis in vivo. However, it is unknown whether the localized concentrations of active PAI-1 necessary to produce this antifibrinolytic effect can be recruited to acute arterial thrombi by endogenous mechanisms. We measured PAI-1 activity and antigen in porcine coronary artery thrombi that formed in response to acute vascular injury. Mean PAI-1 activity in thrombi (n=5) was 36±5.1 μg/mL, which is >2000 times its concentration in normal porcine plasma. The presence of markedly elevated concentrations of active PAI-1 in thrombi was confirmed by an immunoactivity assay and by demonstrating formation of sodium dodecyl sulfate–stable complexes after addition of 125 I-urokinase to thrombus extracts. Comparative analysis of PAI-1 antigen by Western blotting and urokinase inhibition assay suggested that approximately one third of thrombus-associated PAI-1 was active. Histological examination of coronary thrombi revealed that they consisted predominantly 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. Washed porcine platelets prepared from peripheral blood contained sufficient PAI-1 antigen and activity to account for the concentrations observed in coronary artery thrombi. However, the specific activity of human platelet PAI-1 was lower than that of porcine platelet PAI-1 (2% versus 50% active, respectively), and human platelets inhibited in vitro fibrinolysis to a lesser extent than did porcine platelets. These results indicate that active PAI-1 accumulates in porcine coronary artery thrombi in concentrations markedly higher than those present in plasma and that PAI-1 may be an important determinant of the known resistance of platelet-rich thrombi to lysis by tissue-type plasminogen activator. These studies also underscore the importance of considering possible species differences in protein function when comparing animal models of thrombosis to acute coronary thrombosis in humans.