Hemostasis - Platelet Function Flashcards
platelet morphology and function
resting platelets are discoid, anucleate cells 2-3 uM in diameter
form by budding or retraction from megakaryocyte proplatelet processes in bone marrow myeloid sinuses and other tissues
microtubule coil helps maintain platelet discoid shape
upon activation, platelet changes shape from discoid to round and then with many projections
canalicular sytstem
provides easy release of granule contentss and excess platelet membrane during shape change
phases of platelet activation
adhesion - binding to subendothelial matrix
activation - change from quiescent state to promote hemostasis
secretion - release of granule contents
aggregation - binding to other platelets through active GPIIbIIIa
process of injury that activates platelets
i) Collagen exposure
ii) Exposure of von Willebrand factor that binds platelet receptors
iii) Thrombin generation
iv) Decreased endothelial production of CD39 ecto-ADPase
v) Endothelin release (vasoconstriction)
agonists that cause platelet activation
ADP, thromboxane A2, collagen, thrombin, epinephrine
i) Endothelium-dependent platelet inhibitors such as Prostacyclin (PGI2)
stimulates platelet cyclic AMP and Nitric Oxide (Endothelium-Derived Relaxing Factor (EDRF), which stimulates cyclic GMP
consequence of platelet activation
a. Platelet shape change
b. Thromboxane A2 generation and release
c. Release of granule contents – recruits and activates additional platelets
d. Inside-out signaling presenting the active fibrinogen binding form of GPIIbIIIa receptors on the platelet surface.
e. Exposure of an anionic phospholipid surface on the outer leaflet of the plasma membrane which promotes the assembly of factor tenase and prothrombinase complexes.
Thromboxane A2
produced by activated platelets is a potent agonist for platelet activation and aggregation.
Aspirin
acetylates cyclooxygenase and irreversibly inhibits its activity immediately upon exposure within the portal circulation
affects function for the life of the platelet – return to normal requires 7-10 days when a significant number of new platelets appear in the circulation
dense granule contents
ADP
ATP
Serotonin
Pyrophosphate
Calcium
alpha granule contents
Platelet Factor-4
beta-Thromboglobulin
Platelet-Derived Growth Factor
Fibrinogen
Factor V
vWF
High Molecular Weight Kininogen
GP IIb/IIIa receptor
mediates platelet aggregation when activated
fibrinogen
the most important multivalent adhesive glycoprotein that can bridge two activated platelets (on a molar basis) but other ligands such as von Willebrand factor can also serve this function
platelet function tests
Peripheral smear – count, size, granules
Bleeding time
Point of care instruments:
a. Platelet function analyzer PFA-100® - a screening test for platelet function
b. Accumetrics VerifyNow® - for therapeutic drug monitoring
c. Thromboelastography – aspirin and thienopyridine drug monitoring
d. Other assays
Clot retraction
platelet aggregation studies
- *Optical nephelometry** – uses platelet rich plasma
- gold standard assay, but cumbersome for routine screening and clinical monitoring
Electrical impedance - whole blood
agonists: ADP, epinephrine, collagen, arachidonic acid, Ristocetin
clinical manifestations of platelet dysfunction
mucocutaneous bleeding, petechiae, purpura, epistaxis, gingival bleeding, menorrhagia
Bernard-Soulier Syndrome
GP Ib/IX (vWF Receptor) defect
Glanzmann’s Thrombasthenia
GP IIb/IIIa Complex defect
delta-Storage Pool Disease
Dense Body Deficiency
alpha-Storage Pool Disease (Gray Platelet Syndrome)
Alpha Granule Deficiency
Scott Syndrome
Decreased platelet surface acidic phospholipid expression
Quebec Syndrome
Multimerin defect; alpha-granule and procoagulant activity defect
Stimulus-response coupling defects
Heterogeneous; defects of cyclooxygenase, G-couple proteins or calcium response
acquired disorders of platelet function.
Uremia – metabolic byproducts inhibit platelet function in Myeloproliferative Disorders
Post-Cardiac Bypass - artificial surfaces activate and cause “spent platelets”
Dysproteinemia – plasma immunoglobulin paraprotein receptor interference
Antiplatelet antibodies – interfere with platelet receptors
Liver disease with fibrin degradation product (fdp) interferes with binding of fibrinogen with platelet receptor
Drugs
ecto-ADPase (CD39)
normally expressed by endothelial cells, which removes ADP and ATP nucleotides from the local fluid environment
platelet products in atherogenesis
CD40 ligand
interleukin-1beta
P-selectin
platelet factor 4
ROS
F2-isoprostanes
tissue factor
chemokines, cytokines
matrix metalloproteinases
CD40 ligand in atherogenesis
endothelial cell expression of chemokines, tissue factor, cell adhesion molecules, and ROS
interleukin-1beta in atherogenesis
endothelial cell activation, cell adhesion, chemokine production
P-selectin in atherogenesis
adhesion to endothelial cells and monocytes
platelet factor 4 in atherogenesis
macrophage differentiation, endothelial cell E-selectin
ROS in atherogenesis
scavenges nitric oxide, prevents thrombus disaggregation, lipid peroxidation, F2-isoprostanes
F2-isoprostanes in atherogenesis
activates platelets, modulates cell adhesion
tissue factor in atherogenesis
promotes thrombosis
chemokines and cytokines in atherogenesis
leukocyte recruitment and inflammation
matrix metalloproteinases in atherogenesis
plaque destabilization
NSAIDs
inhibits cyclooxygenase reversibly, platelet function returns after drug levels fall
use of NSAIDs can interfere with therapeutic antiplatelet aspirin effect
Thienopyridines: clopidigrel (Plavix®)
P2Y12 ADP receptor blocking agent which may be used in conjunction with aspirin in patients with stroke and coronary artery disease
the drug must be metabolized to the active form; some persons resistant
Ticlopidine (Ticlid®)
ADP receptor blocking agent use associated with increased incidence of Thrombotic Thrombocytopenic Purpura (TTP) in 1:2000-4000 cases
Prasugrel, Ticagrelor
efficacious new ADP receptor blocking agents
may be used in patients who have resistance to clopidogrel
Abciximab (ReoPro®)
humanized murine monoclonal Fab monoclonal antibody that binds to and inhibits GP IIb/IIIa function
used in cardiovascular catheter interventions
also consider: Eptifibatide, tirofiban
Dipyridamole (Persantine®)
inhibits adenosine uptake, acts on PLA2 and causes increased platelet cAMP, which leads to inhibited function
GP IIb/IIIa
fibrinogen/vWF (aggregation)
GP Ib-V-IX
vWF (adhesion)
GP Ia-IIa
collagen (adhesion)
GP VI
collagen (activation)
isoprostane
bioactive molecule that is formed due to inflammation and increases platelet activation
