Blood platelet plasminogen activator inhibitor: two different pools of endothelial cell type plasminogen activator inhibitor in human blood.

An assay for plasminogen activator inhibitor in human platelets is described. With this assay we find an average value of 6.8 X 10(-8) IU/platelet (S.D. = 3.0 X 10(-8); n = 20) in a healthy population. We characterized the PA-inhibitor from platelets and identified it as endothelial cell type plasminogen activator inhibitor, by its immunologic and functional properties. Besides the plasma pool of plasminogen activator inhibitor with a very high turnover rate, platelets constitute a second pool of plasminogen activator inhibitor in the circulation of the same order of magnitude. The two different pools of plasminogen activator inhibitor might have a different physiologic function.