In the following section, the importance of platelets to arterial thrombosis is briefly highlighted since platelets have such a central role in haemostasis,299' 301 and throm
bosis plays such an important part in CHD. The thrombotic process results from an interaction between platelets, the vessel wall, the coagulation system and the fibrino lytic system. Normally after superficial endothelial injury only a monolayer of plate lets adheres to exposed subendothelium.302 However, thrombus formation can occur
following more severe damage following exposure of the components of the vessel wall which activate both platelets and the coagulation system.302 Localised haemos
tatic plug formation and arterial thrombosis are comparable processes with the same blood constituents responding to vascular injury.299 Platelet aggregation plays a vital
role in both physiologic and pathological haemostatic reactions.
1.7.1 Initial Platelet Responses Leading to Aggregation
Primary responses in haemostasis include localised vasoconstriction at the site of vessel injury and platelet adhesion to the exposed sub-endothelial basement mem brane and collagen. When platelets adhere to collagen or interact with thrombin there is a profound release reaction of platelet secretory products. Many of the compounds released are platelet aggregating agonists (adenosine diphosphate, thrombin, serotonin) and they can act synergistically to amplify the response of platelet activation to low concentrations of other aggregating agents.299 The effects
of platelets and their secretory and metabolic products on the vessel wall are well documented.239,244,303 Platelet dense-granules release ADP, adenosine triphosphate (ATP), calcium, serotonin, magnesium and other substances. The alpha-granules contain important compounds including platelet derived growth factor (PDGF), BTG, PF4, factor VIII related antigen, von Willebrand factor (VWF), albumin, thrombospondin, fibrinogen, fibronectin, antiplasmin, alpha-2-antitrypsin, alpha-2- macroglobulin, as well as chemotactic factors.299 Acid hydrolases are released from the lysosomal granules. Following the release reaction platelet aggregation occurs and with this, phospholipases C and A2 are activated. Phospholipase A2 cleaves arachidonic acid from platelet membrane phospholipid. A combination of phospholi pase C, diglyceride lipases and monoglyceride lipases free arachidonic acid from phosphatidyl inositol.299 Activated platelets release potent vasoactive agents such as serotonin, histamine and TXA2.304
Platelet adhesion is an integral step in the maintenance of vascular integrity and is mediated by a number of platelet surface receptors with a high affinity for the adhe sive glycoproteins in the sub-endothelium, such as fibronectin and collagen.301 It would appear that many of these ligands act on inactivated platelets which, in normal circumstances, are protected from these glycoproteins by the endothelium.301 Platelet secretory products, such as ADP and serotonin, and TXA2 manufactured following activation of the cyclooxygenase pathway, are formed locally further enhancing platelet activation.300
1.7.2 Platelet Receptor Function
In addition to the release and production of platelet agonists, platelet receptor expo sure is involved in aggregation. Glycoprotein (GP)IIb/IIIa receptors, when activated are exposed and bound by a number of glycoproteins, the most important being fibrinogen.301 Fibrinogen and other glycoproteins, such as von Willebrand factor and thrombospondin bind to the GPIIb/HIa receptor allowing stable platelet aggregates to form.305,306 The exposure of the GPIIb/IIIa receptor is the final common pathway fundamental to platelet aggregation.307 The exposure of this receptor is directly augmented by the released contents of platelet storage granules. Further, it is both directly and indirectly enhanced by the formation of TXA2.308 Of particular impor tance is the fact that all of the platelet agonists can lead to the exposure of the GPIIb/IIIa receptor and subsequently platelet aggregation independent of the cy clooxygenase pathway.301
1.7.3 Interaction with the Coagulation System
The adhesion and aggregation of platelets promotes coagulation at multiple stages of the intrinsic coagulation pathway resulting in the deposition of fibrin.309 This throm botic process can be enhanced when adjacent platelets are activated by thrombin which is formed locally after activation of the coagulation system. Factor Xa and prothrombin are activated following exposure to specific membrane binding sites for coagulation enzymes, zymogens and cofactors.310 The binding of activated clotting factors to platelet receptors results in the capacity of platelets to protect factor XIa and Xa from inactivation.310 Initiation of the coagulation cascade results in fibrin formation, reinforcing the platelet plug. Normally the fibrinolytic system is then
activated, digesting the haemostatic plug. New growth of endothelial cells of the vessel completes the repair process.311,312 Platelets may impair endogenous fibrinoly sis as they are a source of factor XIII activity which crosslinks fibrin, reducing its sensitivity to plasmin.313 However, platelets bind plasminogen directly, a process which is augmented after interaction with thrombin and platelet associated fibrino gen,314 thus localising plasminogen in the vicinity of platelet aggregates. Neverthe less, platelets also release a plasminogen activator inhibitor315 and platelet-rich thrombi appear to be resistant to thrombolysis,316 and the process of fibrinolysis influences platelet function, with numerous mechanisms being documented in vivo and in vitro for both inhibition and activation.301 Of clinical importance is the activa tion of platelets associated with pharmacological doses of thrombolytic agents.47'49 Thus, it is clear that activated platelets affect other important components of the haemostatic system. As stated, activated platelets demonstrate enhanced factor Xa activity and prothrombin activation accelerating the formation of thrombin.317 Fur ther, aggregation of platelets concentrates activated clotting factors in one localised site. Platelets influence the expression of the fibrinolytic system, having both profi brinolytic and antifibrinolytic effects.301
1.7.4 Summary - Platelets and Thrombus Formation
In summary, at least four separate but related pathways result in platelet activation and the formation of stable irreversible platelet aggregates which may subsequently progress to intra-arterial thrombus. Firstly, the adherence to collagen or interaction with thrombin and other platelet agonists induces the platelet release reaction and thence ag g reg atio n . This reaction can occur in d ep en d en t of TXA2 generation.300,301,312 However, in normal circumstances, stimulated platelets release arachidonic acid from phospholipids in the platelet cell membrane through the action of phospholipases C and A2,299,311 setting in progress the second major step in the sequence. Cyclooxygenase then converts arachidonic acid to the endoperoxides PGG2 and PGH2. Then TXA2, a powerful stimulator of platelet aggregation, is formed from thromboxane synthetase.299,300 Thirdly, the platelet agonists lead to the exposure of GPIIb-IIIa receptors allowing binding of fibrinogen to physically join the platelets to form aggregates. Finally, the formation of fibrin stabilises the platelet aggregates to form a thrombus. Further vessel contraction occurs under the influence of TXA2 and serotonin. Chemotactic factors released can attract leukocytes to the local thrombus. Subsequently clot retraction consolidates the platelet plug; the thrombus may be incorporated into the wall leading to wall thickening; parts may break away as plasmin lyses the fibrin and platelets deaggregate; or the thrombus may propagate further into the arterial lumen.299 This brief review highlights the importance of the interaction between platelets and fibrinogen in the formation o f platelet aggregates, and this interaction has important implications for the hypothesis
o f this Thesis.
1.8 SUMMARY
Atherosclerosis can be viewed as a common source epidemic.318 The determinants of common-source epidemics include susceptibility, pathogenesis and virulence of the
agent(s), and the length of the incubation period.318 Identification of the susceptible
individual has been of prime concern, hence the investigation of risk factors. Despite considerable efforts, still, little is known about the determinants of susceptibility to CAD.318
The content of the discussion in this introductory Chapter supports the idea that risk factors may be thought of in terms of being precipitators and/or potentiators of susceptibility to clinical CAD.202 Such a consideration has also arisen from the
observations of Cornfield and Detre. 319 The predisposition to thrombosis and/or
platelet aggregation within the coronary circulation could be considered a precipita tor and/or potentiator for the acute clinical syndromes. This predisposition could arise from differences in the pathology of the atherosclerotic plaque, functional abnormalities within the arterial endothelium adjacent to plaques, imbalances within the normal mechanisms of intra-arterial thrombin formation and fibrinolysis, local or systemic platelet hyperactivity, or a combination of any number of these possibili ties. More recently the existence of factors which are directly or indirectly thrombo- genic has appropriately received more attention.60
The results of angiographic studies support the concept that CHD progression invol ves two fundamental processes. The first consists of atherosclerotic lesions progress ing to plaque fissuring, platelet aggregation and subsequent healing, with or without minor progression. The second process occurs after plaque fissuring and platelet aggregation, with progression to thrombosis and partial occlusion of the coronary artery, and/or complete occlusion, and/or lysis, and/or thrombus incorporation into the plaque.4 In both circumstances the platelet may play a pivotal role.
At present there is unequivocal evidence that the activation of platelets and involve ment in thrombosis within coronary arteries occurs in the acute obstructive myocard ial ischaemic syndromes. Circulating activated platelets, however, are not a feature of stable chronic CHD, as far as can be currently measured. 45,298 Whether more reactive platelets are present in individuals who are susceptible to the development of CAD and/or CHD remains to be elucidated. Mural thrombosis over atherosclerotic plaques would appear to be an ongoing process without necessarily any clinical consequences at the time.238 Why an individual progresses to an occlusive arterial
event is unknown but probably results from numerous interacting components, one possibility being the presence of genetically derived and/or environmentally induced hyperactive platelets given the appropriate stimuli.
Heightened platelet reactivity may be important in platelet involvement in the early stages of atherosclerosis or the later clinical manifestations. If such platelet dysfunc tion does exist, it may require intermediary pathophysiological changes to become manifest. Such a proposal accommodates the concept of an interaction between the environmental "triggers" and the presently unaccounted for inheritance factor. Another possibility is that part, or all, of the familial predisposition to CAD is relat ed to an abnormality in platelet function. Therefore an evaluation of platelet function in young individuals at increased risk of the development of CAD and in those manifesting their susceptibility by the clinical manifestation of the disease is of value. The thesis that thrombosis is an active component in the development of CHD and CAD, and is unrelated to chance requires further clarification. This is particular-
ly important since an understanding of postulated thrombogenic factors, whether environmental and/or heritable, may have important implications for identifying susceptible individuals more accurately, thereby improving methods of prevention and enabling implementation of more definitive treatment.
1.9 THE QUESTION ASKED IN THIS THESIS