RAPID COMMUNICATION Platelets Inhibit Fibrinolysis In Vitro by Both Plasminogen Activator Inhibitor-l-Dependent and -Independent Mechanisms

Platelet-rich thrombi are resistant to lysis by tissue-type plasminogen activator (t-PA). Although platelet a-granules contain plasminogen activator inhibitor-l (PAI-l), a fastacting inhibitor of t-PA, the contribution of PAL1 to the antifibrinolytic effect of platelets has remained a subject of controversy. We recently reported a patient with a homozygous mutation within the PAL1 gene that results in complete loss of PAL1 expression. Platelets from this individual constitute a unique r agent with which to probe the role of platelet PAL1 in the regulation of fibrinolysis. The effects of PAI-1 -deficient platelets were compared with those of normal platelets in an in vitro clot lysis assay. Although the incorporation of PAI-1-deficient platelets into clots resulted in a moderate inhibition of t-PA-mediated fibrino-