Effect of platelet-activating factor (PAF) and synthetic analogues on fibrinolysis in rats

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.

Characterization of platelet-activating factor synthesis in glomerular endothelial cell lines

Platelet-activating factor synthesis in two transformed lines of glomerular endothelial cells was characterized and contrasted with platelet-activating factor production in macrovascular-derived endothelial cells as well as with glomerular cells of mesenchymal origin. Platelet-activating factor synthesis was assessed in intact cells and in cell-free preparations. Glomerular endothelial cells constitutively synthesize bio-active alkyl-PAF, and this basal activity can be chronically augmented by various inflammatory and thrombotic agents. In contrast, thrombin-mediated platelet-activating factor formation in bovine pulmonary aortic endothelial cells as well as in glomerular mesangial cells is acute and transient. The potential role of anti-inflammatory prostanoids to function as negative feedback modulators of thrombin- or endothelin-mediated platelet-activating factor synthesis was also investigated, as the synthesis of platelet-activating factor is often associated with the formation of these prostanoids. Indomethacin augmented receptor-mediated platelet-activating factor synthesis while prostanoids of the E and I series reduced agonist-stimulated PAF synthesis. In summary, the unique capacity of glomerular endothelial cells to respond to inflammatory stimuli with sustained platelet-activating factor synthesis is a clear indication of this cell's pivotal role in augmenting the inflammatory response in the limited environment of the glomerulus.