After 3 h the venous segments containing the thrombi were tied off and split open longitudinally, and the remaining thrombi were removed. The radioactivity of the circulating fibrinogen was estimated from the mean of the blood samples collected at hourly intervals throughout the infusion. The ratio of the radioactivity of the thrombus to that of the circulating fibrinogen was used to quantify the amount of radioactive fibrinogen accreted on the thrombus (expressed in micrograms).
In preliminary experiments, mI-labeled human albumin was infused into rabbits with jugular vein thrombi. The radioactivity that accumulated on these thrombi was assessed to evaluate the role of nonspecific absorption of plasma proteins on thrombi. In radioactive albumin experiments, no radioactivity was detected after washing the thrombi.
Fibrinolysis model: The fibrinolytic activity of bolus doses of rt-PA was assessed in different rabbits by lysis of standard-size preformed ia5I-fibrinogen-labeled thrombi produced as described for the thrombus growth inhibition experiments. To produce radioactive thrombi, 5 til of ia5I-fibrinogen (10 |i.Ci) was added to citrated rabbit blood used to produce the thrombi. Urea solubility and uniform labelling with l2SI-fibrinogen were as described previously.