THE EFFECT OF PLASMIN ON PLATELET-FUNCTION

There is controversy in the literature regarding the effects of plasmi n on human platelets, We have studied the effects of plasmin on platel et glycoproteins, aggregation, shape change and secretion and found th em to be dependent on experimental conditions: (a) Plasmin's effects o n human platelets are only seen in gel-filtered platelets (GFP), presu mably because in platelet rich plasma plasmin is bound to alpha 2-anti plasmin; (b) in GFP to which fibrinogen has been added, platelet funct ion remains intact; and (c) in the absence of fibrinogen, the effect o f plasmin on GFP depends on whether stirring is performed or not, With stirring, platelets undergo shape change, secretion and aggregation i n response to added plasmin. Aggregation is much stronger when CaCl2 1 mM is added, Without stirring, preincubation of GFP with plasmin lead s to inhibition of platelet aggregation induced by subsequent platelet stimuli (thrombin, collagen, ristocetin or U46619), We have demonstra ted that plasmin is a true platelet activating agent, in the sense tha t it induces platelet shape change and secretion, Plasmin will induce aggregation when added to stirred GFP. This may be because stirring pr otects glycoprotein (GP) IIbIIIa bound fibrinogen from being degradate d by plasmin, When added to unstirred GFP, GP IIbIIIa bound fibrinogen may be readily accessible to degradation by plasmin, which may then b ehave like a platelet inhibitor.