What are the FOUR MAIN STAGES of haemostasis?
1. Vascular spasm & constriction
2. Platelet adhesion, activation & aggregation - von-Willebrand factor, release of COX & seratonin, formation of platelet plug or temporary haemostatic plug
3. Coagulation - triggering of various factors and final common pathway involving prothrombin to thrombin, fibrinogen to fibrin & formation of clot (clotting cascade)
4. Fibrinolysis - tissue plasminogen activator to form plasmin & lysis of clot
In haemostasis, what happens in the first stage of vasoconstriction & what does it achieve?
Smooth-muscle contraction due to local myogenic spasm (initiated by muscle cell itself due to direct damage to vascular wall), pain/sensory nerve reflexes & release of vasoconstrictor TxA2 (thromboxane A2) by platelet enzymes.
It reduces blood flow to ruptured vessel.
What happens in the second stage of haemostasis-- platelet adhesion, activation & aggregation?
What is achieved at this stage?
Adhesion: platelets stick to exposed collagen & von Willebrand factor (leaked into tissue from plasma)
Activation: platelets swell, grow pseudopods, contract & release ADP & vasoconstrictor 5-HT (5-hydroxytryptamine, aka seratonin) ➔ platelet membrane phospholipids generate arachidonic-acid that serve as substrate for cyclooxygenase enzyme (COX) ➔ COX synthesizes TxA2 ➔ ADP + TxA2 activate nearby platelets
Aggregation: GP IIb/IIIa receptors on adjacent platelets bind soluble fibrinogen to form links (surface of negative charge facilitates assembly of fibrin clots via later coagulation)
Platelet plug aka temporary haemostatic plug is achieved.
* raising cAMP or cGMP can block all events
What happens in the third stage of haemostasis -- coagulation -- and what is achieved?
Two pathways, extrinsic & intrinsic, which share the same Common Final Pathway that ends with the formation of a fibrin clot.
In the extrinsic pathway, which is what occurs in vivo: Damage to vascular wall & surrounding tissues ➔ release of tissue factor (TF) composed esp of phospholipids & lipoprotein that acts as proteolytic enzyme ➔ activation of Factor X to Xa ➔ Formation of prothrombin activator + Ca++ ➔ conversion of prothrombin ➔ thrombin ➔ fibrinogen (soluble) ➔ fibrin (insoluble) ➔ clot
In intrinsic pathway, in vitro (no tissue factor released):
Trauma to blood or exposure of blood to collagen of traumatised blood-vessel wall (eg., in vivo, on glass) ➔ various clotting factors activated (VII, XI, XII, IX, VIII)➔ activation of Factor X to Xa ➔ Formation of prothrombin activator + Ca++ ➔ conversion of prothrombin ➔ thrombin ➔ fibrinogen (soluble) ➔ fibrin (insoluble) ➔ clot
Why is calcium necessary for clotting?
Ca++ is necessary for conversion of prothrombin to thrombin.
Ca++ can be precipitated out to prevent clotting.
Why is Vitamin K necessary for clotting?
Vit. K is necessary for liver formation of 5 key clotting factors:
1. Prothrombin (in final common pathway)
2. Clotting Factor VII
3. Clotting Factor IX
4. Clotting Factor X (in final common pathway)
5. Protein C
Vit. K deficiency can be caused by liver disease.
What happens in the fourth and final stage of haemostasis, lysis of the clot?
Injured tissues & vascular endothelium slowly release tPA (tissue plasminogen activator) ➔ converts plasminogen to plasmin ➔ digestion of fibrin fibres & fibrinogen, Factor V, VIII & XII & prothrombin ➔ lysis of clot
Name three antiplatelet drugs and explain how they work.
1. Acetyl salicylic acid (aspirin):
- Irreversible inhibitor of cyclooxygenase (COX)
- modifies platelet adhesion & activation, preventing thrombosis
- Without COX, thromboxane (TxA2) will not be synthesised & thus platelets can’t clump & positive feedback to produce more platelets will be blocked.
2. Prostacyclin (epoprostenol, PGI2):
- a vasodilator, prostacyclin elevates platelet cAMP, which is needed to ACTIVATE platelets, thus it inhibits platelet aggregation
- entire platelet & activation pathway is blocked with ↑cAMP inside platelets
- vasoactive mediator that prevents platelets sticking to vessel wall
- elevates platelet cGMP, thus inhibiting platelet aggregation, similar to prostacyclin
Why do some anticoagulant drugs target plasma calcium?
Calcium is critical in the conversion of clotting factor prothrombin to thrombin in the clotting cascade. Thrombin is needed to convert soluble fibrinogen to insoluble fibrin, which forms the fibrin clot.
What are some drugs that aim to reduce available Calcium to prevent clotting in vitro? Ie., Anticoagulant drugs that target plasma calcium outside of the body.
1. EDTA - Diaminoethane tetra-acetic acid - binds Ca++
2. Sodium citrate - precipitates Ca++
3. Sodium oxalate - precipitates Ca++
4. Acid citrate dextrose - used in blood-storage for transfusion
What are three injectable anticoagulants that work in vivo to prevent clotting?
- Large sulphated muco-polysaccharide that occurs naturally in mast cells & endothelial cells
- Often added to isotonic salt solutions used to flush intravenous lines
- Binds to antithrombin (ATIII) & INCREASES ATIII’s rate of inactivation of some clotting factors
- Binds thrombin & therefore also has antiplatelet action
- Inhibits blood clot formation in vivo & in vitro
2. Low molecular weight heparin-like molecules
- Activates/enhances inhibitory action of antithrombin III (ATIII) on factor Xa, but NOT on thrombin
- Less antiplatelet activity than with Heparin
- Better pharmacokinetics than heparin
- Anticoagulant substance from medicinal leech
- Synthesized by recombinant DNA
- Synthetic derivatives are available (eg., Hirugen)
- Causes little or no bleeding at clinically effective anti-thrombotic doses
- Binds directly to thrombin (at fibrinogen binding site)
- Acts INDEPENDENTLY of ATIII to inhibit coagulation
What's an important anti-coagulant drug that is taken orally and reduces clotting by acting as a Vit. K antagonist?
Why is Vit. K so important in clotting?
This is a derivative of coumarin, which structurally resembles Vit. K. As a Vit. K antagonist, it prevents Vit. K synthesis in the body and is used as a rat poison (the rat eats warfarin/coumarin, and it gets a bruise or a cut & bleeds to death as it can't clot).
Vit. K is important in clotting because it is necessary for liver formation of prothrombin, Factors VII, IX & X & protein C
- delays onset of anti-coagulant action, depending on the half-life of clotting factors
- has delayed effect because it takes time for liver to synthesize new clotting proteins
What are some drugs that are considered anticoagulant because they target the clot-lysis stage of haemostasis?
- From Streptococcal enzymes
- Activates plasminogen increase production of plasmin & cause generalised clot lysis
- Extracted from human urine
- Activates plasminogen
3. tPA (tissue plasminogen activator):
- Acts primarily on fibrin-bound plasminogen in the clot (clot- selective) ie., immobilised plasminogen
- Low affinity for circulating plasminogen
- Can lead to bleeding from widespread fibrinolysis activation.