The mechanism of in vitro clot lysis induced by vascular plasminogen activator

The contribution of vascular plasminogen activator (v-PA) to the lysis of whole blood and plasma clots was investigated. v-PA released into the circulation after infusion of deamino-D-arginine vasopressin (DDAVP) was shown to bind quantitatively to plasma clots. Its apparent molecular weight, determined by the SDS-PAGE fibrin-agarose underlay method, was approximately 68,000 daltons, and its activity was quenched by antibodies against human tissue plasminogen activator (t-PA). Clots prepared from post-DDAVP plasma or post-DDAVP whole blood, rich in v-PA, did not lyse when incubated in imidazole buffer or normal plasma, as determined by the release of 125I from radiolabeled clots. However, clots made of v-PA-poor plasma or whole blood, incubated in v-PA-rich plasma, underwent substantial lysis. The concentration of PA in clots incubated in v-PA-rich plasma progressively increased in relation to the initial concentration of v-PA in the surrounding plasma. The results suggest that, at low concentrations of circulating v-PA, a hemostatic plug will lyse at a very low rate. However, when the v-PA concentration in the clot environment is increased, v-PA will accumulate progressively onto fibrin and induce thrombolysis.