title: Plasmin inhibition of thrombin-induced platelet aggregation.
abstract: The effects of plasmin treatment upon washed human platelets were studied in an attempt to elucidate the mechanisms underlying thrombin-induced platelet aggregation. At calcium concentrations of 10-20 muM, PLASMIN (0.2 CTA U/ml) inhibited thrombin-induced aggregation almost completely, but did not diminish the thrombin-induced release of adenine nucleotides, 5-hydroxytryptamine, or calcium. Increasing the calcium concentration partially antagonized plasmin's inhibition of aggregation. Studies utilizing calcium chelators and the Kunitz soybean trypsin inhibitor (SBTI) as a plasmin inhibitor indicated that in order to achieve maximal block of aggregation, plasmin must act upon a substrate made fully available only after an initial thrombin-platelet interaction has taken place. Moreover, the time course of this inhibition parallels the time course of the thrombin-induced release reaction. Plasmin inhibition of aggregation could not be mimicked by exposing the platelets to proteolytic digests of fibrinogen at concentrations as high as 17% total platelet protein. Nor could inhibitory activity be recovered from supernatants of plasmin-treated platelets, upon centrifugation and treatment with SBTI. With the use of a "cold initiation" technique, the release by thrombin of 46.7 plus or minus 6.7 (mean plus or minus SEM) mu-g of fibrinogen immunological equivalents per mg platelet protein could be demonstrated. Platelets in which thrombin-induced aggregation was abolished by plasmin treatment (and the plasmin subsequently inactivated by STBI) aggregated normally upon addition of as little as 10 mu-g human plasma fibrinogen per mg platelet protein. It is concluded that plasmin inhibition of aggregation most likely results from its attack upon a protein that is released or becomes fully available subsequent to interaction of thrombin with a platelet receptor mediating release. The results of this study are consistent with a cofactor role for fibrinogen in the aggregation of human platelets by thrombin.
mesh_terms: D000227:Adenine Nucleotides / Q000378:metabolism; D001241:Aspirin / Q000494:pharmacology; D001792:Blood Platelets / Q000378:metabolism / Q000648:ultrastructure; D002118:Calcium / Q000378:metabolism / Q000494:pharmacology; D002614:Chelating Agents; D003080:Cold Temperature; D005340:Fibrinogen / Q000378:metabolism; D005341:Fibrinolysin / Q000494:pharmacology*; D006801:Humans; D006868:Hydrolysis; D007770:L-Lactate Dehydrogenase / Q000378:metabolism; D008239:Lysine / Q000494:pharmacology; D010973:Platelet Adhesiveness / Q000187:drug effects*; D010974:Platelet Aggregation / Q000187:drug effects*; D012701:Serotonin / Q000378:metabolism; D013917:Thrombin / Q000037:antagonists & inhibitors* / Q000494:pharmacology; D013997:Time Factors; D014361:Trypsin Inhibitors / Q000494:pharmacology
chemical_list: D000227:Adenine Nucleotides; D002614:Chelating Agents; D014361:Trypsin Inhibitors; D012701:Serotonin; D005340:Fibrinogen; D007770:L-Lactate Dehydrogenase; D013917:Thrombin; D005341:Fibrinolysin; D008239:Lysine; D001241:Aspirin; D002118:Calcium