Thrombin platelets treated with

Inhibition of thrombin- but not ADP-induced aggregation was observed in rat platelets treated with aqueous extracts of horsetail, with an IC50 value of 6.75 mg/ml (Mekhfi et al. 2004). 

Thrombin attacks synthetic esters, namely tosyl l-arginine methyl ester (TAMe), and when thrombin is treated with DIPT, the loss of esterase activity is proportional to the loss of clotting activity. Studies with synthetic esters have suggested that, as with trypsin, the reaction between thrombin and its substrates occurs in three steps an enzyme-substrate complex is formed the acyl portion of the ester is transferred to the enzyme, which becomes acylated while the alcoholic portion of the ester is released and the enzyme is deacylated and the peptide is transferred to water. The natural substrate of thrombin is fibrinogen. In addition to converting fibrinogen to fibrin, thrombin releases the contractile protein from platelets, activates fibrinase, and may participate in the conversion of prothrombin to thrombin. 

Platelets treated with l-p-chlorostyryl-3Hnethyl-3-carbomethoxy-6,7-dimethoxy-3,4 dihydroisoquinoline (SSDHI) lose their ability to spread, adhere, aggregate and retract. They cannot undergo the morphological and biochemical cluuiges of viscous metamorphosis produced by thrombin ... 

Fibrinolytic agents have prothrombotic properties as well. The plasmin generated by thrombolysis leads to the production of thrombin, which is a potent platelet activator and converts fibrinogen to fibrin. Indeed, studies have shown early reocclusion in as many as 17% of the patients treated with lAT and 34% of the patients treated with IV rt-PA. Therefore, a strong rationale exists for the adjuvant use of antithrombotic agents. 

Activated rat platelets secreted PS-PLAi as well as SPLA2-IIA, both of which are stored in a-granules [20]. The PS-PLAi was purified from culture medium of thrombin-activated platelets prepared from 1000 rats [17]. In rats, PS-PLAj expressed not only in platelets but also in heart and lung [22]. The enhanced expression was detected in many tissues when rats were treated with bacterial lipo-poly-saccharide in vivo (unpublished observation). The expression of PS-PI.A, differed between species. For example, the mRNA encoding the enzyme was poorly expressed in human platelets but highly expressed in rat platelets. 

While arachidonic acid is selectively released in thrombin-stimulated platelets, small amounts of palmitic, stearic and oleic acids are also released (Figure 1.1). The most likely explanation of this is that they are released by the action of lysophospholipase on the l-acyl-2-lyso-PC or l-acyl-2-lyso-PE formed after selective phospholipase A2 hydrolysis of l-acyl-2-arachidonyl-PC or l-acyl-2-arachidonyl-PE. It has been reported that palmitate, stearate and oleate occupy more than 90% of the 1-position of platelet PC. Furthermore, it has been shown that thrombin treatment of platelets prelabelled with [ " Clstearic or [ " Qpalmitic acids induces the formation of both radioactive lysophosphatidylcholine and free fatty acidsThis subsequent hydrolysis by lysophospholipase also explains why the amounts of lysophosphatidylcholine that accumulate in thrombin-treated platelets are considerably lower than the amounts of arachidonic acid which are released. 

Inhibition of platelet aggregation induced by arachidonic acid, U46619, collagen, and ADP (with IC50 values of 51, 84, 191, and 640 pg/ml, respectively) was observed in guinea pig platelet-rich plasma treated with lavandin essential oil. Lavandin oil also destabilized clot retraction (IC50 of 149 pg/ml) induced by thrombin on rat platelet-rich plasma (Ballabeni et al. 2004). 

The following points should be considered in all patients receiving heparin Platelet counts should be performed frequently thrombocytopenia appearing in a time frame consistent with an immune response to heparin should be considered suspicious for HIT and any new thrombus occurring in a patient receiving heparin therapy should raise suspicion of HIT. Patients who develop HIT are treated by discontinuance of heparin and administration of a direct thrombin inhibitor or fondaparinux (see below). 

Morimitsu et al. (2000) have shown that after incubation with MS-ITC (5 or 10 min), the treated platelets were washed two times with phosphate-buffered saline, and MS-ITC potently inhibited platelet aggregation induced by thrombin. Then, MS-ITC could not show any effects on arachidonic acid cascade (phospholipase A prostaglandin synthase, and thromboxane synthase) in human platelets. 

Based on the encouraging results obtained in these simulated flow studies, knitted polyester patches were subjected to the same modification with rHir (42). Segments with covalently boimd rHir and control (BSA-treated) polyester patches were implanted in the thoracic aortas of canines and exposed to non-heparinized aortic blood flow for two hours. The materials were explanted and the loss of rHir was found to be 20%. The remaining antithrombin activity was assessed explanted patches were able to inhibit 7 NIHU of thrombin. Gross and microscopic examination of the explanted patches showed that the controls had a thick surface acellular layer (pseudointima) composed of fibrin rich thrombus, while the rHir modified patches had no gross thrombus, and a thin pseudointima of platelets and plasma proteins (Figure 4). 

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