Basic science of platelets and coagulation factors Flashcards
Write out the intrinsic clotting pathway.
1) exposure to - charged surface
2) X11 to X11a
3) XI to XIa
4) IX to IXa
5) IXa binds to VIIIa
What happens when damage disrupts endothelium?
circulating vMF binds exposed collagen, which alters its tertiary structure to expose platelet binding sites (and also bind VIII)
T or F. vMF is a large multimer of homo-dimers that must be trimmed to an optimal length
T.
How do platelets bind to vMF?
GP1b
Binding of platelets to vMF causes what to happen?
activation and release of alpha and dense granules
What are the contents of the alpha granules?
- TxA2
- fibrinogen
- factor VIII and V
What are the contents of the dense granules?
- calcium
- magnesium
- histamine
- ADP
What role does Ca2+ play in clotting?
Calcium ions cause fatty acids to precipitate by binding their anionic head regions; this forces the hydrophobic tail regions to the outside of the complex, where they are free to interact with other similar complexes to form large insoluble complexes.
Factor VII, and some of the other coag factors, responds to the presence of calcium according to the same principles. Exposure of more hydrophobic residues result in the protein becoming membrane bound.
What reaction facilitates factor binding to Ca2+?
-gamma carboxylation of AA residues via vitamin K
What factors require gamma carboxylation (and thus vitamin K)?
II, VII, IX, and X
Once endothelial damage occurs and TF is exposed, what protease activates VII?
Unknown. What is clear is that after binding of TF, VII is much, much more susceptible to activation by any of several proteases in the coag cascade, including VIIa itself.
What does the TF/VIIa complex do?
activates X, which in turn activates a small amount of factor V and complexes with it
What does the small amount of Xa/Va do?
convert II to IIa (prothrombin to thrombin) - which does not, at this point, cleave a lot of fibrinogen
Why doesn’t this thrombin cleave a lot of fibrinogen?
Because its activity is localized to the surface of damaged endothelial cells.
Thrombin activation in an environment loaded with other coag factors – in the middle of a bunch of activated platelets – will be needed to get that job done.
What else does thrombin do at this point?
Thrombin activates the intrinsic pathway by generating an activated IXa/VIIIa complex (sometimes called “tenase”).
The factor VIII is released from activated platelets.
What does the activated IXa/VIIa do?
It diffuses from the site on which it is generated (damaged endothelial cells) to the site at which more of its substrate (factor X) is located – on the surface of immobilized platelets to activate more Xa.
There is some ambiguity in the literature about whether it does this primarily while complexed with VIIIa, or whether it acquires factor VIIIa after diffusing over to the platelet side.
What happens what a lot of thrombin is built up?
- gradually more and more fibrinogen is produced to fibrin
- it converts XIII to XIIIa to facilitate cross-linking multiple fibrin-platelet-fibrin chains
- it facilitates yet more conversion of V to Va
this is called on propagation phase
What happens what a lot of thrombin is built up?
gradually more and more fibrinogen is produced to fibrin
Where is antithrombin made?
liver
What does antithrombin attach to on cell surfaces?
a complex polysaacharide called heparin
What does antithrombin do?
inactivates thrombin (IIa) and Xa
What cofactor does protein C use?
protein S
T or F. Protein C and S are both gamma carboxylated
T. Thus, their function is both vitamin K dependent
What is the function of Protein C?
inactivate factor V and VIII
What activates plasmin?
t-PA, a protease on the surface of UNDAMAGED endothelial cells
What does prothrombin time measure?
how long it takes plasma to form fibrin
How does the prothrombin time assay work?
The “prothrombin” mixture is mixed with plasma. We collect blood for this in the presence of citrate (blue top), which chelates any Ca++ in plasma (i.e. its an anticoagulant). Prothrombin includes enough Ca++ to saturate the citrate and start this portion of the coag cascade.
Prothrombin time assays are particularly sensitive to what?
The effects of drugs that inhibit gamma carboxylation of coag factors and cofactors, i.e. to vitamin K antagonists like warfarin.
Most textbooks describe this as an assay of the extrinsic and common pathways; the latter term refers to the portion of the coagulation cascade involving factors Va, Xa, and Iia. But bear in mind that in vivo, activation of factor VII only generates a small amount of fibrin. We are exaggerating that part of the pathway in this test in order to generate an easily measurable readout
T or F. VII is the vitamin-K dependent factor with the fastest normal turnover rate.
T.
The activated partial thromboplastin (aPTT) assay is particularly sensitive to reductions in?
the level of IIa, VIIIa, and IXa.
What drug can target VMF disease and how does it work?
We can augment vMF secretion with a drug called DDAVP. This will both increase circulating vWF and increase circulating factor VIII
Defect in Gp1b leads to what?
Bernard-Soulier syndrome
What drug blocks ADP release from dense granules of activated platelets?
Clopidogrel
Defect in the binding of GPIIb/IIIa leads to what disease?
Glanzman-Thrombasthenia
What drugs can treat GT?
abciximab,
eptifibatide
tirofiban
What are some direct factor Xa inhibitors?
apixaban,
rivaroxaban
What are some direct thrombin inhibitors?
dabigatran
hirudin
What does warfarin do?
blocks vitamin-K dependent gamma carboxylation.
First factor to show the effect is VII because it turns over fastest.
Heparin is an anticoagulant. What happens to PT when it is given?
the PT will become abnormal when the warfarin starts to be effective
What test is affected by factory VIII or IX deficiency, PT or aPTT?
aPTT only
What test is affected by heparin, PT or aPTT?
the aTTP test is particularly dependent on activated thrombin, so it’s very sensitive to the effects of heparin
What are the subtypes of vMF disease?
I- poor expression of vMF gene (recessive)
IIa/c- increased degradation of appropriate sized vMF
IIb- increased binding of appropriate sized vMF to plts
IIN- reduced binding to VIIIa (normally stabilizes it)
IIM- reduced binding of appropriate sized vMF to plts
III-nonsense/frameshift in vMF gene
What is INR?
way of standardizing prothrombin time
time with warfarin/ normal)^(coefficient
