hemostasis and coagulation Flashcards
endothelial cells
reversal of anti-thrombic properties
- anti-platelet effects
- anti-coagulant effect
- Fibrinolytic effect
platelets
formation of primary, secondary platelet plugs
coagulation cascade purpose
formation of fibrin
in resting state, intact endothelium….
inhibit binding and coagulation factor activation
anti-thrombic properties of endothelial cells
anti-coagulation effects
-membrane associated heparin-like molecules-> interact with anti-thrombin III to inactivate thrombin
thrombomodulin
tissue factor pathway inhibitor
thrombomodulin
binds thrombin to activated protein C
- inactivates Va and VIIIa
- requires protein S
antiplatelet effects of endothelial cells
- coverage of underlying ECM
- production of vasodilators: prostacyclin, NO
- adenosine disphosphate: degrades ADP
fibrinolytic effect of endothelial cells
synthesizes tissue-type plasminogen activator
-promotes fibrinolysis
what initiates blood coagulation?
- exposure of ECM (collagen)
- release of tissue factor
- activation of endothelium by bacterial products and cytokines
- stasis
platelet plus formation results from
activation of platelets and overcoming the anti-thrombotic effect of the endothelium
pro-thrombotic properties of endothelial cells
- production of vWF-> binding of platelets to ECM
- formation of tissue factor: bacterial endotoxin, cytokine (IL-1, TNF)
- secretion of plasminogen activator inhibitors
what promotes platelet aggregation and coagulation?
endothelial injury or activation
formation of platelet plug
- platelet adhesion
- platelet activation and secretion
- platelet aggregation
- platelet plug stabilization by fibrin
platelet adhesion
- binding to exposed collagen (ECM and basement membrane) by vWF bridging
- vWF
vWF
synthesized by endothelial cells (stored in Weilbel-Palade bodies)
-bridge binding collagen to platelet via gp1b receptor
Bernard-Soulier Syndrome
defects vWF or gp1b result in decreased platelet adhesion
platelet activation and secretion
- Similar to other inflammatory cells: requires Ca, synthesis of thromboxane from a.a. release
- Alpha granules
- express P-selectin on their membrane
- contain fibrinogen, fibronectin, factors V and VIII, platelet factor 4 (a heparin-binding chemokine), beta-thrombomglobulin, PGDF, and TGF-beta - Dense bodies
- ADP, ATP
- ionized calcium
- histamine, serotonin
- epinephrine
activation of dense bodies causes
leads to expression of phospholipid complexes required for clotting factor assembly
stimulators of platelet aggregation
ADP
TxA2
Thrombin
ADP
highly potent mediator of platelet aggregation
TxA2
- production by platelets
- also causes vasoconstriction
thrombin
- formed by activation of coagulation cascade
- binds to thrombin receptors on platelets
platelet aggregration
- autocatylitic release of ADP and thromboxane A2 to build a platelet aggregate
- platelets bind to each other through bidirectional binding of fibrinogen to GpIIb-IIIa
- initial aggregation forms primary hemostatic plug (reversible)
- FURTHER activation results from contraction platelets to from secondary hemostatic plug (irreversible)
what stabilizes platelet plug?
fibrin
platelet stabilzation
- fibrin binds to gp IIb/IIIa
- cleavage of fibrinogen by thrombin (end result of coagulation cascade)
- entrapment of RBC’s and leukocytes that contribute to local inflammatory reaction
- generation of fibrin split products by the action of plasmin results in further chemotaxis of neutrophils
summary of platelet aggregation
- platelets adhere at sites of endothelial injury and become activated
- activated platelets secrete granule contents (ADP, fibrinogen, Ca) and synthesize thromboxane
- after formation of primary hemostatic plug, platelets contract to form secondary hemostatic plug (aggregation)
- stabilization and anchoring of platelet plug by formation of fibrin
how are most coagulation factors found?
circulating in an inactivated serine protease
coagulation cascade requires
complexing with phospholipids and Ca (surface of activated platelets)
extrinsic system
formation of activated factor VII
- activation of VII by tissue factor (tissue or endothelial cell injury)
- probably most important in vivo
intrinsic system
formation of activated factor IX
- activation of Hageman factor XIII
- sequential activation of XI and IX
- activation of factor X by IXa requires factor VIIIa as a co-factor
- hereditary absence of VIII or IX results in hemophilia
- factor IX can also be activated by VIIa
common pathway of pro-thrombin activation
X/V/II
- activation of X by VIIa or IXa
- activation of II (prothrombin) by Xa and activated Co-factor V
what cross-links fibrin
factor XIII
inhibits of cascade
thrombomodulin/factor C
TFPI
anti-thrombotic
two different ways to activate factor X
- Factor VIIa
- Factor IXa+VIIIa
prothrombin to thrombin
Factor Xa in presence of co-factor Va
Xa/Va= IIa (10/5=2)
thrombin (IIa)
thrombin
- Cleaves fibrinogen to form fibrin
- Activates co-factors VIII and V
- Activates factor XIII that cross-links fibrin
- Binds to thrombomodulin to activate protein C-> thus inhibiting further thrombin synthesis
how does thrombin act on cells?
via protease-activated receptors (PAR’s) that are linked to G protein (7-spanning receptor family)
- clips the ends of the receptors to result in conformation change of the receptor
- induced local inflammatory cell effects
local inflammatory cell effects from thrombin
- aggregation of platelets
- expression of endothelial cell adhesion molecules
- secretion of NO, PGI2, tPA, and PDGF by endothelial cells
- activation of neutrophil, monocytes
fibrinolytic cascade
activation of plasminogen by plasminogen activator to form plasmin
- PAI-1 inhibits this process
- plasmin degradation of fibrin
- allows for eventual dissolution of clot
what does plasmin do?
degrades fibrin to fibrin degradation products
- chemotaxis
- measure of ongoing activation of coagulation system-> DIC
natural anti-coagulants: anti-thrombins
- anti-thrombin III inhibits: thrombin, serine proteases (IXa, Xa, XIa, XIIa)
- activated by binding heparin-like molecules on endothelial cells
natural anti-coagulants
anti-thrombin protein C and S tissue factor pathway inhibitor plasminogen/plasmin system plasminogen activators
protein C and S
- vitamin K dependent
- inactivate Va and VIIIa (co-factors)
- mechanism of action of thrombomdulin: protein C inactivation of thrombin, requires protein S
tissue factor pathway inhibitor
- secreted by endothelium
- inactivates factor Xa and tissue factor-> VIIa
plasminogen/plasmin system
plasmin (active form)
- fibrinolysis
- inhibits fibrin polymerization
plasminogen activators
plasminogen-> plasmin
- urokinase-like plasminogen activators (u-PA): activates plasminogen in fluid phase
- tissue-type plasminogen activators (t-PA): produced by endothelium, activated when bound to fibrin
therapeutic anti-coagulants
- platelet antagonists
- coagulation factors
- fibrinolytic enzymes
platelet antagonists
- Aspirin
- Clopidogrel (Plavix)
- GpIIb-IIIa blockers
aspirin
inhibition of cyclo-oxygenase and thromboxane synthesis
- irreversible inactivation of platelets: requires several days to replace platelets
- effective only in low dose: high dose inhibits prostacyclin
Clopidogrel (Plavix)
inhibits binding of ADP to low-affinity platelet receptor and subsequent activation (conformational change) to GpIIb/IIIa
GpIIb/IIIa blockers
-platelet integrin that binds fibrinogen to link platelets together
-used to prevent secondary thrombosis
-individual agents:
Eptifibatide (integrilin)-GpIIb/IIa antagonist
Tirofiban (aggrastat)-GpIIb/III antagonist
Abciximab (ReoPro)-mAb that binds to GpIIb/IIIa receptor
coagulation factors
heparin
fondaparinux
coumadin, warfain
heparin
activation of antithrombin III
- blocks both thrombin and X-activation
- low molecular weight heparin works better than unfractionaled heparin
fondaparinux
selective factor X inhibitor
coumadin, warfain
prevents carboxylation of factors II, VII, IX, X
fibrinolytic enzymes
streptokinase urokinase t-PA third generation agents -Alteplase (activase) -Reteplase (rPA, retavase): Activates fibrin-bound plasminogen -Tenecteplase (TNK-t-PA, TNKase): more resistant to PAI-1 -Lanoteplase (n-PA) (novel PA)
