The image above shows a blood vessel in the longitudinal section. Number one shows how platelets begin to converge, slow down and roll along the vessel wall near the damaged area. Platelets begin to attach to the collagen in the damaged area and this activates them. Activated platelets experience a physical shape change and are able to attract other platelets to attach to them and to any other exposed collagen. This is called a platelet plug. The image below shows a close up of the platelet plug and more detail involved in the chemical messengers that facilitate a successful platelet plug.
When blood vessels suffer damage, a protein secreted by endothelial cells known as von Willebrand factor (vWf) binds to exposed collagen within the vessel wall. Circulating platelets in turn bind to the collagen bound molecules of vWf and anchor themselves to the damaged area. This interaction causes a platelet activation which increases the likelihood that more platelets will accumulate.
Following platelet adhesion, the anchored platelets begin to secrete a variety of chemical compounds from granules called "alpha" and "dense" granules. Alpha granules release additional vWf and platelet derived growth factors (PDGF). vWF assists with further platelet adherence and activation. PDGF facilitates a variety of functions that assist in the long-term wound healing of tissue damage. Dense granules release adenosine diphosphate (ADP). ADP and thromboxane (also called TXA2 and released from the platelet cytoplasm) bind to surface receptors on additional circulating platelets and promote further activation. These newly activated platelets, in turn, activate additional platelets, establishing a chemical cascade.
Platelet activation also results in the expression of membrane receptors called fibrinogen receptors. These receptors bind a plasma protein known as fibrinogen (the process to make fibrinogen will be explained in 2.2.3). Like vWf, fibrinogen serves as a type of linking molecule. However, whereas vWf links collagen to platelets, fibrinogen links multiple platelets together. Generally, enough platelets link up that they span across and "plug" the opening in a damaged vessel. Platelets are rich in the proteins actin and myosin which allows for contraction which forms a more compact platelet plug. This is called Platelet Aggregation.
Unless you have a clotting disorder (like hemophilia), your blood will spontaneously form a clot in order to close an open wound, but what is it about the wound that induces clot formation? Equally important, how does your body prevent clot formation when there is no wound? In order to understand hemostasis (the “stopping” of blood flow), it is also important to understand the chemical signals that block hemostasis under normal conditions (because spontaneous clot formation is very painful and can be lethal). Spontaneous clot formation is primarily prevented by the active inhibition of platelets, and the most important cell type to regulate platelets is the endothelial cell.
Healthy, intact endothelial cells block spontaneous platelet activation using three primary mechanisms. However, all three mechanisms share a common goal: reduce Ca++ levels within the platelet. Acting as a second messenger, increased Ca++ within platelets will lead to the exocytosis of platelet granules. Granule release is synonymous with platelet activation. Thus, by reducing Ca++, platelets are maintained in an inactive state.
The first approach used by endothelial cells to inhibit platelet activation is the production and secretion of nitric oxide (NO). Secreted NO enters platelets where it activates the enzyme guanylate cyclase, which then stimulates the formation of cGMP (cyclic guanosine mono-phosphate). cGMP then blocks the function of phospholipase C. In the absence of NO, phospholipase C cleaves the membrane phospholipid PIP2, which leads to the production of inositol triphosphate (IP3). IP3 then binds to and opens Ca++ channels on the surface of the endoplasmic reticulum, causing Ca++ to flood the cell. Remember – intracellular Ca++ causes exocytosis (granule release). Thus, by blocking the release of Ca++ from the ER, NO from intact endothelial cells inhibits platelet granule release.
The second mechanism used by endothelial cells to inhibit platelet activation is the production and secretion of a signaling molecule called prostacyclin. Prostacyclin is the ligand for a G-protein coupled receptor (GPCR) on the surface of the platelet. Activation of this GPCR by prostacyclin leads to the production of cAMP. cAMP then activates a Ca++ pump on the surface of the platelet, which actively pumps Ca++ out of the platelet. Thus, increased Ca++ efflux reduces Ca++ within the platelets, which inhibits granule release.
The third mechanism involves endothelial cells and the enzyme ADPase found on the surfaces of the endothelial cell membranes. This ADPase is an enzyme that metabolizes ADP to AMP (adenosine mono-phosphate). ADP is a potent platelet activator and works by binding to ADP receptors on the surface of the platelet. Activation of this receptor (P2Y12) directly inhibits the effects of prostacyclin (described in the previous paragraph). As such, P2Y12 activation causes Ca++ to build up inside the cell, resulting in granule release. The endothelial ADPase inhibits P2Y12 activation by eliminating its ligand, ADP. Interestingly, you have likely seen TV commercials for the drug Plavix (clopidogrel bisulfate). Plavix is often prescribed to patients following a heart attack or stroke to help prevent clot formation. Plavix is a P2Y12 antagonist.
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