Platelet Function and Hemostasis DSA 8/22

Question 1

Four steps of normal Hemostasis

Answer 1

1. Arteriolar vasoconstriction: 2. Platelet adherence+activation: primary hemostasis 3. Tissue Factor synthesized: secondary hemostasis 4. Permanent plug formation

Question 2

1. Arteriolar Vasoconstriction

Answer 2

mediated by neurogenic mechanisms and local endothelial factors like endothelin

Question 3

2. Platelet Adherence and activation- “primary hemostasis”

Answer 3

Platelet adherence+activation: platelets change from rounded shape to plates and aggregation occurs to form the hemostatic plug (primary hemostasis). Platelets do this by binding to (Von Willerbrand Faactor) vWF on the exposed ECM.

Question 4

3. Tissue factor synthesis –> “secondary hemostasis”

Answer 4

Factor III and Thromboplastin are synthesized by endothelial cells –> act with Factor VIII–> results in coagulation cascade–> thrombin generation–> thrombin cleaves fibrinogen into insoluble fibrin –> additional platelets recruited –> creates fibrin meshwork = “secondary hemostasis” –> results in the consolidation of the initial platelet plug.

Question 5

4. Permanent Plug

Answer 5

Polymerized fibrin and platelet aggregates form a solid permanent plug to prevent further hemorrhage. - At this point, counter regulatory mechanisms (tissue plasminogen activator and t-Pa) are activated to limit the hemostatic plug to the site of injury - thrombomodulin is also released (which normally serves to block the coagulation cascade)

Question 6

Antithrombotic properties of endothelial cells

Answer 6

1. Antiplatelet Effects 2. anticoagulant effects 3. fibrinolytic effects

Question 7

1. Antiplatelet effects of endothelial cells

Answer 7

intact endothelium prevents platelets from engaging the highly thrombogenic subendothelial ECM. Nonactivated platelets don’t adhere, and if they are activated then prostacyclin (PGI2) and NO impede platelet adhesion.

Question 8

2. anticoagulant effects of endothelial cells

Answer 8

mediated by: - heparin like molecules: act indirectly and greatly enhance the inactivation of thrombin through antithrombin III - thrombomodulin: binds to thrombin and converts it to anti-coagulant - tissue factor pathway inhibitor (TFPI)- a cell surface protein that inhibits tissue factors VIIIa and Xa

Question 9

3. fibrinolytic effects of endothelial cells

Answer 9

endothelial cells synthesize tissue-type plasminogen activator (t-PA), a protease that cleaves plasminogen to form plasmin; plasmin cleaves fibrin to degrade thrombi.

Question 10

Prothrombotic Properties of endothelial cells

Answer 10

1. platelet effects 2. procoagulant effects 3. antifibrinolytic effects

Question 11

1. Platelet effects of endothelial cells

Answer 11

endothelial injury allows platelets to contact the underlying ECM. adherence occurs through interactions with von Willebrand factor (vWF) which allows for platelets to bind to matrix elements

Question 12

2. Procoagulant effects of EC’s

Answer 12

In response to cytokines (TNF and IL-1), or bacterial endotoxin, EC’s synthesize tissue factor- the major activator of the extrinsic clotting cascade. Also activate Coagulation IXa and Xa

Question 13

3. Antifibrinolytic effects of EC’s

Answer 13

EC’s secrete inhibitor of plasminogen activator (PAIs) which limit fibrinolysis and favor thrombosis

Question 14

What happens when platelets encounter adhesive glycoprotein vWF?

Answer 14

1. adhesion and shape change 2. secretion (release reaction) 3. aggregation

Question 15

1. Platelet Adhesion to ECM

Answer 15

mediated by interaction with vWF which acts as a bridge between platelet surface receptors and collagen fibers. (normally prevented by negative surface charge on platelets and EC cells repelling each other) - vWF-GpIb associations are needed to overscome high shear of forces of flowing blood - genetic deficiencies in vWF (vonWillebrand disease) or its receptor (Bernard-Soulier syndrome) results in bleeding disorders

Question 16

2. Activation and Secretion (release reaction) of granules

Answer 16

• activation is through G protein signal transduction cascade mediated via activation of phospholipase C with Ca2+ influx -secretion of both granule types from platelets occurs after adhesion: - aGranules: have P-selectin on membranes, contain fibrinogen, fibronectin, factors V and VIII, plately factor 4 and PDGF and TGFb -BGranules: contain ADP and ATP, Ca2+, histamine, serotonin, ephinephrine. - under activation of thrombin, platelet turns to disc with extensions called filopodia (pseudopods)

Question 17

3. Platelet Aggregation

Answer 17

Platelet aggregation follows adhesion and granule release. 1. Thromboxane A2- platelet-derived stimulsu that amplifies aggregation and leads to primary plug 2. Thrombin is generated which stabilizes the platelet plug and results in irreversibly fused mass of platelets = secondary plug 3. noncleaved fibrinogen also plays an important role in platelet aggregation

Question 18

Platelet endothelial cell interactions

Answer 18

• PGI2 (prostacyclin) inhibits platelet aggregation, potent vasodilator - TxA2 (prostaglandin) activates platelet aggregation, vasoconstrictor - aspirin is used for people who are at risk for coronary thrombosis because it permanently blocks TxA2 synthesis.

Question 19

Thrombin

Answer 19

Prothrombin (coagulation factor II) is proteolytically cleaved to form thrombin in the coagulation cascade, which ultimately results in the reduction of blood loss. Thrombin in turn acts as a serine protease that converts soluble fibrinogen into insoluble strands of fibrin, as well as catalyzing many other coagulation-related reactions.

Question 20

Factor XI

Answer 20

Factor XI or plasma thromboplastin antecedent is the zymogen form of factor XIa, one of the enzymes of the coagulation cascade.

Question 21

Thrombopoeitin

Answer 21

• TPO is a glycoprotein hormone prduced by liver and kidney that regulates platelet production by stimulating the production and differentiation of megakaryocytes. It is the major cytokine involved in regulation of platelet production. - TPO receptors on platelets regulate a feedback mechanism which senses thrombocytopenia (resulting in stimulation of bone marrrow megakaryocyte production) or thrombocytosis (turning off megakaryocyte production)

Question 22

Megakaryocytes

Answer 22

under the influence of thrombopoietin (TPO) and interleukin-3 (IL3). originate from common myeloid progenitors. They are broken down to thrombocytes

Question 23

Components of Platelets:

Answer 23

1. external surface glycoprotein receptors 2. tubulin MT’s form inner skeleton 3. actin + myosin contractile proteins in interior 4. alpha granules: store vWF, platelet factor 4, TGF-B1, PDGF, factors V and VIII, fibrinogen 5. dense core granules: store ADP, serotonin histamine, epinephrine, Ca2+ (important in platelet aggregation)

Question 24

Platelets in Peripheral Blood: Circulating and Sequestered

Answer 24

1. circulating: in the central laminar flow- measured when platelet count is performed 2. sequestered: (non-activated platelets)- located in vascular beds of pulmonary venous and splenic sinusoids- function as a reserve and are activated by strenuous exercise, epinephrine and corticosteroids

Question 25

Clots vs Platelet Plugs

Answer 25

• blood clot = composed of platelets and fibrin with entrapped erythrocytes, leukocytes and serum. release of Ca2+ from activated platelets promotes clot formation. - platelet plugs: clot shrinks to a plug via the interaction of actin and myosin in the platelets - serum is expelled and thrombin and fibrinogen play part in platelet plug formation

Question 26

Thrombin and fibrinogen

Answer 26

activates factor XI–> XIA on activated platelet surface. also facilitates interaction between extrinsic and intrinsic coagulation pathways (via activation of factor IX)

Question 27

Hemostasis

Answer 27

maintenance of intact vessel surface with blood in a fluid state to ensure tissue perfusion: - balance between clot formation and anticoagulation/fibrinolysis - after plug formation, fibrinolysis occurs and clot is dissolved

Question 28

Von Willerbrand Disease

Answer 28

vWF serves as a carrier protein for factor VIII and as an adhesive surface for platelts that tethers them to the exposed collagen at sites of vascular injury. It is normally released upon platelet activation by the alpha granules. Defects in structure, function, or release of this factor results in VonWIllerbrand disease with bleeding tendencies due to platelet disfunction