HEMOSTASIS AND THROMBOGENESIS: PLATELET, ENDOTHELIAL AND CLOTTING FACTOR INTERACTIONS

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HEMODYNAMIC DISORDERS HEMOSTASIS THROMBOGENESIS
PLATELET, ENDOTHELIAL AND CLOTTING FACTOR
INTERACTIONS

• Arthur S. Schneider, M.D.

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NORMAL HEMOSTASIS

• platelets
• endothelium
• plasma clotting factors

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NOTE CLUMPS OF PLATELETS
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PLATELET FUNCTIONS

• maintenance of vascular integrity
• platelet plug formation
• platelet phospholipid complex expression
• leads to promotion of coagulation cascade
• promotion of endothelial repair

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PLATELET ULTRASTRUCTURE
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OCS open canalicular system, ?.Galpha
granules, GLYglycogen granules, ECexternal
glycoprotein coat, CM cell membrane, DBdense
bodies, M mitochondria, DTS dense tubular
system, LY lysosomes
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PLATELET ADHESION TO SUBENDOTHELIUM
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PLATELET ADHESION TO SUBENDOTHELIUM
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PLATELET PLUG FORMATION
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PLATELET INHIBITORS AND PHARMACOLOGIC AGENTS

• cyclooxygenase inhibitors (aspirin)
• ADP receptor antagonists (ticlopidine and
  clopidogrel)

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PLATELET INHIBITORS AND PHARMACOLOGIC AGENTS

• others
• phosphodiesterase inhibitors (dipyridamole)
• increases concentrion of cyclic AMP and cyclic
  GMP --gt potentiation of effect of prostacyclin
• glycoprotein (GP) IIb/IIIa receptor antagonists
  (eptifibatide, tirofiban and abciximab)

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ENDOTHELIAL FUNCTIONS

• metabolic transfers
• barrier functions
• synthesis of mediators of interactions between
  endothelium and blood components
• thromboresistance
• mediation of vascular repair

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ENDOTHELIAL THROMBORESISTANCE

• heparin-like molecules
• PGI2 and nitric oxide (NO) synthesis
• plasminogen activation
• ADP degradation
• thrombin inactivation
• thrombomodulin production
• protein S synthesis

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HEPARIN-LIKE MOLECULES

• act at interface of endothelial cells and plasma
• activate antithrombin III
• activated AT III cleaves thrombin, factors Xa and
  IXa

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THROMBOMODULIN

• endothelial surface protein
• receptor for thrombin
• binds thrombin and converts it to an activator of
  protein C
• activated protein C (APC) cleaves factors Va and
  VIIIa
• protein S is a cofactor for APC

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OTHER ACTIONS OF APC

• contributes to significant reduction of mortality
  in patients with severe sepsis
• attenuates various deleterious events induced by
  lipopolysaccharide (LPS)
• cannot be solely explained by its antithrombotic
  activity
• has associated anti-inflammatory and
  profibrinolytic effects
• also inhibits tumor necrosis factor-alpha
  (TNF-alpha) production

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EVENTS INITIATED BY ENDOTHELIAL DISRUPTION

• vasoconstriction
• neurogenic
• endothelin
• platelet adhesion and aggregation
• release of TxA2
• initiation of coagulation
• fibrinolysis

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Classic representation
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Newer representation
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NEWEST REPRESENTATION

• three distinct phases
• initiation
• propagation
• termination

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INITIATION

• chemical or mechanical damage to tissue
• exposes tissue factor (TF) to the plasma
  circulation
• induces TF expression by monocytes,and activated
  endothelium

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INITIATION

• TF interacts with a small amount of preexistant
  factor VIIa in the plasma, to form extrinsic
  tenase (TF/VIIa/IX/X) on a phospholipid surface
• rapid inactivation of the extrinsic tenase by
  tissue factor pathway inhibitor (TFPI).
• extrinsic tenase generates small amounts of
  factors IXa and Xa

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INITIATION

• Xa cleaves prothrombin (II) to form small amounts
  of thrombin (IIa)
• initial small amount of thrombin activates
  cofactor proteins, factors V and VIII to Va and
  VIIIa
• initial small amount of thrombin also activates
  platelets

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PROPAGATION

• cofactor activation (especially VIIIa) ? assembly
  on platelet phospholipid surface of intrinsic
  tenase (VIIIa/IXa,X) ? rapid Xa activation

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PROPAGATION

• again on a phospholipid surface, the active
  complex prothrombinase forms (Va/Xa/II/Ca) ?
  accelerated thrombin formation (IIa)
• thrombin fibrinogen ? fibrin (several steps
  required)
• removal of fibrinopeptide A and B
• cross-linking

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TERMINATION

• formation of thrombin/thrombomodulin complex ?
  activated protein C (APC) ? inactivation of Va
  and VIIIa
• decay of extrinsic tenase through spontaneous
  activity loss and by TFPI
• inactivation of Xa and thrombin (IIa) by
  antithrombin III

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HIGHLY SIMPLIFIED VERSION OF NEWEST REPRESENTATION
Extrinsic tenase
Intrinsic tenase
Prothrombinase
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PROTHROMBIN TIME (PT)

• measure of extrinsic pathway of coagulation
• measures Factors VII, X, V, II, and fibrinogen
• mixture of thromboplastin (tissue factor),
  plasma, calcium ions
• clot forms in about 12 seconds

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PARTIAL THROMBOPLASTIN TIME (PTT)

• often modified to activated partial
  thromboplastin time (APTT)
• measure of intrinsic pathway of coagulation
• measures Factors XII, XI, IX, VIII, X, V, II, and
  fibrinogen
• mixture of phospholipid platelet substitute
  (partial thromboplastin), plasma, calcium ions
• clot forms in about 25 seconds

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DISORDERS OF HEMOSTASIS

• antithrombotic (hemorrhagic) disorders
• classic hemophilia, Christmas disease, von
  Willebrands disease, disseminated intravascular
  coagulation with consumption coagulopathy, etc.

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DISORDERS OF HEMOSTASIS

• prothrombotic (hypercoaguable) states
• hereditary thrombophilia
• lupus anticoagulant
• HIT syndrome (heparin induced thrombocytopenia
  and thrombosis syndrome)
• disseminated intravascular coagulation (DIC)

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HEREDITARY THROMBOPHILIA

• familial
• most often presents in adolescents or young women
  
• venous thrombosis
• recurrent thromboembolism

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HEREDITARY THROMBOPHILIA

• factor V Leiden
• mutation in factor V (ARG506GLN, 1691G-A, G1691A)
  (arginine to glutamine)
• causes resistance to activated protein C (APC)
• prothrombin 20210A transition
• downstream mutation (prothrombin G20210A)
• increased prothrombin activity
• methylene tetrahydrofolate reductase mutation
• MTHFR C677T

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HEREDITARY THROMBOPHILIA

• antithrombin III deficiency
• protein C deficiency
• protein S deficiency

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Amino acid nomenclature

• Alanine Ala A
• Aginine Arg R
• Asparagine Asn N
• Aspartic acid Asp D
• Cysteine Cys C
• Glutamine Gln Q
• Glutamic acid Glu E
• Glycine Gly G
• Histidine His H
• Isoleucine Ile I
• Leucine Leu L

• Lysine Lys K
• Methionine Met M
• Phenylalanine Phe F
• Proline Pro P
• Serine Ser S
• Threonine Thr T
• Tryptophan Trp W
• Tyrosine Tyr Y
• Valine Val V
• Unspecified Xaa X

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GENES IMPLICATED IN HEREDITARY THROMBOPHILIA

• factor V Leiden and other factor V variants
• prothrombin 20210G-A polymorphism
• hyperhomocysteinemia and homocystinuria
• cystathionine synthetase deficiency
• MTHFR gene (677C-T polymorphism)

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GENES IMPLICATED IN HEREDITARY THROMBOPHILIA

• antithrombin 3
• protein C
• protein S
• numerous others
• histidine-rich glycoprotein
• plasminogen and plasminogen activator inhibitor
• fibrinogen-alpha, -beta, and -gamma
• heparin cofactor II
• thrombomodulin

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LUPUS ANTICOAGULANT

• prolonged partial thromboplastin time
• antiphospholipid antibodies interfere with PTT
  test
• increased venous and arterial thrombosis
• sometimes associated with autoimmune diseases
  such as SLE

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HIT SYNDROME

• heparin-induced thrombocytopenia (and
  thrombosis)
• related to therapy with high molecular weight
  heparin (avoided by using low molecular weight
  heparin
• caused by antibodies to the complex of heparin
  and platelet factor 4 (PF4)

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HIT SYNDROME

• clinical picture is heparin induced
  thrombocytopenia followed by thrombotic
  complications

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DISSEMINATED INTRAVASCULAR COAGULATION (DIC)

• obstetric complications
• retained dead fetus
• premature separation of placenta (abruptio
  placentae)
• amniotic fluid embolism
• gram negative sepsis
• trauma lung surgery, hemolytic transfusion
  reactions
• malignancy lung, pancreas, other

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DISSEMINATED INTRAVASCULAR COAGULATION (DIC)

• widespread deposition of fibrin in
  microvasculature
• consumption coagulopathy depletion of platelets
  and coagulation factors
• factor VIII, factor V, factor II, and fibrinogen

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DISSEMINATED INTRAVASCULAR THROMBOSIS (DIC)

• prolongation of prothrombin time (V, II,
  fibrinogen)
• prolongation of partial thromboplastin time
  (VIII, V, II, fibrinogen)
• prolongation of thrombin time (fibrinogen)
• presence of fibrin-fibrinogen degradation (split)
  products (FDP)

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MICROTHROMBI IN GLOMERULUS IN DIC
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MICROTHROMBI IN GLOMERULUS IN DIC
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Thank you for your attention.