-anuclear, discoid

-contain RNA, mitochondria, lysosomes

-dense granules (ADP, ATP, 5HT, histamine, Ca2+)

-alpha granules (fibrinogen, Von Willebrand factor, coagulation factors V and XIII)

-sources of phospholipids, cofactor for coagulation factors (II, VII, IX, X)

1. TxA2 is generated by activated platelets -> COX converts arachidonic acid to TxA2

2. TxA2 binds to the TxA2 receptor (GPCR)  which then activates phospholipase C

3. PLC hydrolizes PIP2 to yield IP3 and DAG

4. IP3 increases calcium release into the cytosol

5. DAG activates PKC, which, along with increased Calcium, activates PLA2

6. PLA2 activates GPIIb-IIIa on the membrane

7. fibrinogen binds to GPIIb-IIIa which causes platelet aggregation

-platelet inhibition involves an increase in cAMP or cGMP

-platelets are inhibited by:  PGI2 (from endothelium), PGD2 (from platelets), adenosine (released by hypoxic cells), NO

-low doses of ASA selectively inhibit the synthesis of TXA2 without having as much effect on prostacyclin

-ASA irreversibly inhibits COX, the enzyme that catalyzes the early step in TXA2 synthesis

-ASA inhibits platelet aggregation for the life of the platelet (5-7d)

-therapeutic doses of ASA (650-975 mg) can cause gastric irritation and GI bleeds

-primary action:  PDE3 inhibitor -> increases cAMP

-also:  increases extracellular adenosine by inhibiting cellular reuptake, and by inhibiting its breakdown  -> Adenosine binds to GPCRs which also increases cAMP

-will reduce activation and expression of cell surface GPIIb/IIIa receptors

-prodrugs that require the conversion to the active thiol metabolites

-inhibit platelet aggregation by irreversible blockade of the ADP receptors (GPCRs) on    platelet membrane

-activates P2Y12 (Gi) and P2Y1 (Gq) receptors on platelet membrane

P2Y12 (ADP) receptor

1. binding of ADP to the GPCR activates Gi protein

2. inhibition of adenylyl cyclase decreases synthesis of cAMP, which decreases PKA activity 

3. since PKA inhibits platelet activation, decreasing the activity of PKA causes platelet activation

P2Y1 receptor, thrombin receptor

1. ADP activates Gq by binding to P2Y1; thrombin activates Gq by binding to thrombin receptor

2. Gq activation activates PLC which leads to platelet activation

glycoprotein IIb/IIIa Antagonist

-monoclonal antibody

-binds irreversibly to the GPIIb/IIIa receptors and blocks the binding of fibrinogen

-can reduce platelet aggregation by more than 90%

-administered via infusion

glycoprotein IIb/IIIa antagonists

-competitive reversible inhibitors of fibrinogen binding to GP IIb/IIIa receptors

-used primarily in patients with acute coronary syndromes (unstable angina and MI)

1. when small vessel is injured, vasospasm reduces blood flow and facilitates platelet aggregation and coagulation

2. platelets adhere to extravascular collagen and are activated to release mediators that cause platelet aggregation and form a platelet plug

3. exposure of the blood to tissue clotting factors also activates coagulation

4. after repair, the clot is removed by fibrinolysis -> plasminogen converted to plasmin and degrades fibrin to products

Coagulation Cascade

intrinsic pathway

-largely in vitro pathway known as the contact activation pathway

-when blood comes into contact with kaolin, glass, or other surfaces, factor XII, prekallikrein, and HMW kininogen will interact with these surfaces and form small amounts of factor XIIa

-kallikrein is formed from prekallikrein and converts XII to XIIa.  

-XIIa along with HMWK activates XI to XIa. 

-XIa and VIIa (extrinsic) activate IX to IXa

-thrombin activates VIII to VIIIa

-Together VIIIa and IXa activate factor X to Xa

simplified:  

Prekallikrein -> Kallikrein -> XIIa

XIIa + HMWK -> XIa

XIa + VIIa + Ca²⁺ (IV) -> IXa

IIa (thrombin) -> VIIIa

IXa + VIIIa + Ca²⁺ (IV) -> Xa

coagulation cascade

extrinsic pathway

-happens more commonly in vivo; also called the tissue factor (factor III) pathway

-factor VII is activated to VIIa by thrombin (IIa), Vila, and factor Xa

-small amounts of factor VIIa bind to subendothelial tissue factor (factor III) following vascular injury.  

-tissue factor (factor III) accellerates the activation of factor X by VIIa, phospholipids, and Ca²⁺ (factor II) by 30,000

Simplified:

Thrombin (IIa), Vila, or Xa -> VIIa

Tissue injury -> tissue factor (III)

tissue factor (III) -- catalyzes -->

VIIa or Ca²⁺ (IV) -> Xa

Coagulation Cascade

Common Pathway

-starts with production of factor Xa

Thrombin (IIa) activates factor V to Va

factor Xa + factor Va + Ca²⁺ (IV) activates prothrombin (II) to thrombin (IIa)

Thrombin is the final product

Thrombin actions on coagulation:

-Thrombin (IIa) + Ca (IV) activates fibrin (I) to fibrinogen (Ia)

-Fibrinogen (I) converts to a fibrin (Ia) polymer

-Thrombin (IIa) activates XIII to XIIIa

-XIIIa causes fibrin polymers to crosslink -> coagulation/platelet aggregation

Calcium and phospholipids- enhance activation of clotting factors

Vitamin K- essential for post-translational modification factors, II, VII, IX, X

Protein C- degrades Va and VIIIa

Antithrombin- a serine protease inhibitor that inhibits the activity of II(thrombin), VIIa, IXa, Xa, XIa, XIIa

Antithrombin is consitutively active and its binding to heparin increases its activity by a thousand-fold

oral anticoagulant

-blocks vitamin-K dependent biosynthesis of VII, IX, X, and II

-blocks epoxide reductase and, therefore, blocks the reduction of oxidized vitamin K preventing the post-translational carboxylation of these factors

Interactions:

-broad spectrum antibacterial agents reduce vitamin K levels and have additive effect

-NSAIDs displace warfarin from its binding site on plasma proteins -> increase its effect

-amiodarone and cimetidine inhibit P450 metabolism of warfarin

-phenytoin, phenobarbital, and alcohol increase metabolism of warfarin

Prothrombin Time - is a measure of the extrinsic pathway of coagulation

INR = (PTobserved/PTcontrol)^ISI

INR- international normalized ratio

ISI-international sensitivity index

activated partial thromboplastin time

-measures the intrinsic pathway of coagulation

-used to monitor heparin tx

Heparin and related agents

-inactivates clotting factors by potentiating the activity of an endogenous anticoagulant antithrombin III

-binds to pentasaccharide site on AT-III

-inhibits II (thrombin), VIIa, IXa, Xa, XIa, XIIa

-unfractionated- also inactivates thrombin

-LMWH and fondaparinux - little or no effect on thrombin
-Heparin can be neutralized by protamine

-monitoring with aPTT - clotting time of 1.8 - 2.5 is the normal mean

bivalirudin

direct thrombin inhibitor

-does not require antithrombin III as a cofactor

-used to prevent thrombosis in unstable angina and acute myocardial infarction

-when clots are formed, the fibrinolytic system is activated

-plasminogen is activated to plasmin -> breaks down fibrin and fibrinogen into degradation products

Alteplase

Activase

recombinant of naturally occuring t-PA 

-enzyme that converts plasminogen to plasmin

-used for acute MI, PE, DVT, Peripheral arterial thromboembolisms

Reteplase

Retevase

recombinant t-PA that contains 335 of the 527 aa's of naturally occuring t-PA

-converts plasminogen to plasmin

-used for acute MI, PE, DVT, Peripheral arterial thromboembolisms

-first combines with plasminogen to form an activator complex that then converts plasminogen to plasmin

-can cause hypersensitivity reaction

-used for acute MI, PE, DVT, Peripheral arterial thromboembolisms

aminocaproic acid

Amicar

antifibrinolytic agent

-inhibits fibrinolysis by competitively blocking plasminogen activation to plasmin and by blocking binding of plasminogen to fibrin

tranexamic acid

Cyklokapron

antifibrinolytic agent

-competitive inhibitor of activation of plasminogen to plasmin