Quantitative assessment of subcutaneous fibrinolysis in the rat

Method to measure in vivo blood fibrinolytic activity with a (125)I-fibrin coated aorta loop validated with agents which affect blood fibrinolytic activity

A functional animal model to measure in vivo the blood fibrinolytic activity and pharmacological-induced changes thereof are described. A (125)I-fibrin coated plastic loop is inserted in the rat aorta; the rate of label disappearance (sigmoid curve) is directly registered outside the animal with a gamma scintillation probe. The time needed to let disappear 50% of the removable-labeled fibrin is used as measure for the blood fibrinolytic activity. The direct advantage of this model is the absence of a blood or plasma clot: a thin labeled fibrin layer attached to the inner wall of the loop is in direct contact with the blood and is therefore sensitive to increased or decreased blood fibrinolytic activity. The total experiment needs about 60 min. Experiments with nontreated rats showed that, after an initial lag phase of about 10 min, the labeled fibrin started to disappear from the loop. A sigmoid pattern was obtained showing that about 20-30% of the coated-labeled fibrin is resistant to removal. Registration of the total curve of a nontreated (control or placebo) rat required about 30-40 min. The clinically used thrombolytics (intravenously administered) urokinase and t-PA showed a dose-dependent fibrinolytic activity resulting in increased removal of the bound (125)I-fibrin. Streptokinase was not active, which is in agreement with literature. Tranexamic acid, dexamethasone and endotoxin (inhibitors of fibrinolysis) showed dose-dependent inhibition of removal of the coated fibrin. Retinoic acid was tested as compound, which may enhance the blood fibrinolytic activity; retinoic acid was not found to be significantly active in this model. The disappearance of labeled fibrin is not sensitive to inhibitors of coagulation or platelet aggregation. This technically simple and fast model can thus be used to measure in vivo quantitatively the effects of pharmacological active compounds, which increase or decrease the blood fibrinolytic activity.

Role of endogenous platelet-derived growth factor in arterial smooth muscle cell migration after balloon catheter injury

The process of intimal thickening after de-endothelializing injury to the rat carotid artery is dependent on the migration of smooth muscle cells from the media. Recent reports have suggested that platelet-derived growth factor may be an important mediator of migration after injury. We have addressed this issue by directly determining smooth muscle cell migration in injured arteries of animals depleted of platelets and after administration of an antibody that blocks platelet-derived growth factor. Because there is a reported association between plasminogen activator synthesis and smooth muscle cell migration, we assayed the activity levels of plasminogen activators after arterial injury and also assessed the effect of a plasmin inhibitor on migration. The data suggest that platelet-derived growth factor, released by platelets at sites of arterial injury, is an endogenous mediator of smooth muscle cell migration; that plasmin generation, catalyzed by tissue-type plasminogen activator, is necessary for migration; and that one way in which platelet-derived growth factor may act is by stimulation of the synthesis of tissue-type plasminogen activator by smooth muscle cells.

The role of plasminogen activation in smooth muscle cell migration after arterial injury

The migration of smooth muscle cells from the media to the intima that occurs after balloon catheter injury to the rat common carotid artery has been quantified by an electron microscopic surveying technique. These vessels have also been assayed for plasminogen-activator activity, which was found to rise sharply 4 days after balloon injury. At this time point smooth muscle cells begin to migrate in appreciable numbers. In order to investigate whether there is a causal relationship between plasminogen-activator activity and smooth muscle cell migration, animals were dosed with tranexamic acid. This synthetic inhibitor of plasmin activity reduced smooth muscle cell migration by 73% (p < 0.05), indicating that plasmin activity is necessary for migration after balloon injury. Lisinopril, an inhibitor of angiotensin-converting enzyme, inhibited smooth muscle cell migration after balloon injury by 78% (p < 0.01) but did not influence plasminogen-activator activity. Taken together, these results show that plasmin is a necessary but not sufficient component in the pathway that leads to smooth muscle cell migration after balloon catheter injury in the rat.