Home About us Contact
|
Home About us Contact | ||
|
|
||
Fibrin Clot Formation (fibrin + clot_formation)
Selected AbstractsClinical approach to the patient with unexpected bleedingINTERNATIONAL JOURNAL OF LABORATORY HEMATOLOGY, Issue 2000J. M. Teitel Bleeding can be considered unexpected if it is disproportionate to the intensity of the haemostatic stress in a patient with no known haemorrhagic disorder or if it occurs in a patient in whom a bleeding disorder has been characterized but is adequately treated. A thorough history usually allows the clinician to predict reasonably accurately whether the patient is likely to have a systemic haemostatic defect (and if so whether it is congenital or acquired), or whether the bleeding likely has a purely anatomical basis. The nature of bleeding is instructive with respect to preliminary categorization. Thus, mucocutaneous bleeding suggests defects of primary haemostasis (disordered platelet,vascular interactions). Bleeding into deeper structures is more suggestive of coagulation defects leading to impaired fibrin clot formation, and delayed bleeding after primary haemostasis is characteristic of hyperfibrinolysis. Localized bleeding suggests an anatomical cause, although an underlying haemostatic defect may coexist. Where bleeding is so acutely threatening as to require urgent intervention, diagnosis and treatment must proceed simultaneously. In the case of minor haemorrhage (not threatening to life or limb) it may be preferable to defer therapy while the nature of the bleeding disorder is methodically investigated. Initial laboratory evaluation is guided by the preliminary clinical impression. The amount of blood loss can be inferred from the haematocrit or haemoglobin concentration, and the platelet count will quickly identify cases in which thrombocytopenia is the likely cause of bleeding. In the latter instance, examination of the red cell morphology, leucocyte differential, and mean platelet volume may allow the aetiological mechanism to be presumptively identified as hypoproliferative or consumptive. With regard to coagulation testing, the activated PTT, prothrombin time, and thrombin time usually constitute an adequate battery of screening tests, unless the clinical picture is sufficiently distinctive to indicate the immediate need for more focused testing. In any event, sufficient blood should be taken to allow more detailed studies to be done based on the results of these screening tests. These results will direct the need for further assays, such as specific clotting factor activity levels, von Willebrand factor assays, tests for coagulation inhibitors, platelet function assays, and markers of primary or secondary fibrinolytic activity. [source] 4-Thio-deoxyuridylate-modified thrombin aptamer and its inhibitory effect on fibrin clot formation, platelet aggregation and thrombus growth on subendothelial matrixJOURNAL OF THROMBOSIS AND HAEMOSTASIS, Issue 10 2008S. MENDELBOUM RAVIV Summary.,Background:,The consensus thrombin aptamer C15-mer is a single-stranded DNA of 15 nucleotides [d(GGTTGGTGTGGTTGG)] that was identified by the selection of thrombin-binding molecules from a large combinatorial library of oligonucleotides. It is capable of inhibiting thrombin at nanomolar concentrations through binding to a specific region within thrombin exosite 1. As has been shown in our earlier studies, the 4-thio-deoxyuridylate (s4dU)-containing oligonucleotides have high affinity for a number of proteins, due to the reduced hydrophilic character of the modified oligonucleotide. Methods:,Three different analogs of the original thrombin-inhibiting sequence, in which some of the thymidylate residues were replaced by 4-thio-deoxyuridylates, were synthesized. The inhibitory effect of modified aptamers was tested on thrombin-catalyzed fibrin clot formation and fibrinopeptide A release from fibrinogen, thrombin-induced platelet aggregation/secretion, and the formation of thrombus on coverslips coated with human collagen type III, thrombin-treated fibrinogen or subendothelial matrix of human microvascular endothelial cells. Results:,As compared with the C15-mer, the analog with the sequence GG(s4dU)TGG(s4dU)G(s4dU)GGT(s4dU)GG (UC15-mer) showed a 2-fold increased inhibition of thrombin-catalyzed fibrin clot formation, fibrinopeptide A release, platelet aggregation and secretion in human plasma and thrombus formation on thrombin-treated fibrinogen surfaces under flow conditions. Concerning the inhibition of thrombin-induced fibrin formation from purified fibrinogen and activation of washed platelets, UC15-mer was 3-fold and twelve-fold more effective than C15-mer, respectively. Conclusion:,The replacement of four thymidylate residues in C15-mer by 4-thio-deoxyuridylate resulted in a new thrombin aptamer with increased anticoagulant and antithrombotic properties. [source] B:b interactions are essential for polymerization of variant fibrinogens with impaired holes ,a',JOURNAL OF THROMBOSIS AND HAEMOSTASIS, Issue 12 2007N. OKUMURA Summary. Background:,Fibrin polymerization is mediated by interactions between knobs ,A' and ,B' exposed by thrombin cleavage, and holes ,a' and ,b' always present in fibrinogen. The role of A:a interactions is well established, but the roles of knob:hole interactions A:b, B:b or B:a remain ambiguous.Objectives:,To determine whether A:b or B:b interactions have a role in thrombin-catalyzed polymerization, we examined a series of fibrinogen variants with substitutions altering holes ,a': ,364Ala, ,364His or ,364Val.Methods:,We examined thrombin- and reptilase-catalyzed fibrinopeptide release by high-performance liquid chromatography, fibrin clot formation by turbidity, fibrin clot structure by scanning electron microscopy (SEM) and factor (F) XIIIa-catalyzed crosslinking by sodium dodecylsulfate polyacrylamide gel electrophoresis.Results:,Thrombin-catalyzed fibrinopeptide A release was normal, but fibrinopeptide B release was delayed for all variants. The variant fibrinogens all showed markedly impaired thrombin-catalyzed polymerization; polymerization of ,364Val and ,364His were more delayed than ,364Ala. There was absolutely no polymerization of any variant with reptilase, which exposed only knobs ,A'. SEM showed that the variant clots formed after 24 h had uniform, ordered fibers that were thicker than normal. Polymerization of the variant fibrinogens was inhibited dose-dependently by the addition of either Gly-Pro-Arg-Pro (GPRP) or Gly-His-Arg-Pro (GHRP), peptides that specifically block holes ,a' and ,b', respectively. FXIIIa-catalyzed crosslinking between ,-chains was markedly delayed for all the variants.Conclusion:,These results demonstrate that B:b interactions are critical for polymerization of variant fibrinogens with impaired holes ,a'. Based on these data, we propose a model wherein B:b interactions participate in protofibril formation. [source] | ||||||||||