Factors influencing the thrombogenicity of injured rabbit aortae
The interaction of platelets with damaged vessel walls plays a role in the development of atherosclerosis and its thromboembolic complications. The platelets that adhere to injured vessels release a growth factor that promotes the migration of medical smooth muscle cells through the internal elastic lamina into the subendothelial connective tissue where they proliferate and form a neointima. If a vessel us exposed to repeated or continuous injury the process continues and ultimately a frank atherosclerotic plaque develops. A plaque contains not only smooth muscle cells, but large quantities of intracellular and extracellular lipid and cholesterol. These plaques can lead to disturbed flow, and the development of thrombi, particularly in regionis where abnormal flow patterns caused further endothelial cell injury or desquamation. The purpose of the work reported in this thesis was to examine the factors influencing the response of vessels to injury, since our understanding of these should contribute to our knowledge concerning the processes involved in atherosclerosis and its complications. Previous studies have examined the responsiveness of normal vessels of young animals to a single injury and may therefore have little relevance to injury of previously damaged or diseased vessels. The present experiments were therefore designed to produce conditions likely to be found in older vessels that have been exposed to previous injury. In these experiments rabbit aortae were de-endothelialized with a balloon catheter and a smooth muscle cell-rich neointima was allowed to develop over a 7 day period. The response of this surface to a second injury with a balloon catheter was examined and compared with the response of normal vessels to a single injury. The initial response to a second injury was very different from the response to a single injury. Whereas a monolayer of platelets accumulates after de-endothelialization, following injury of a smooth muscle cell-rich neointima, large platelet-fibrin thrombi-form on the injured surface. The platelet-fibrin thrombi are laregely oriented in the direction of blood flow. Despite the fact that coagulation is activated (the thrombin that is generated leads to fibrin formation) the number of platelets (measured using platelets prelabelled with ⁵¹Chromium) that accumulates on the injured aortic surface is similar to the number that accumulates on a de-endothelialized aorta, and the surface that is initially very reactive to circculating platelets rapidly loses its ability to attract fresh platelets. The reasons for the non-reactivity of normal endothelium and loss of reactivity of both de-endothelialized vessels and vessels subjected to a second injury were explored. Although it has been speculated that the PGI₂ produced by normal vessels of injured vessels could account for the lack of interaction of platelets with these surfaces, this did not seem to be so, since inhibition of PGI₂ production by treatment of vessels in vitro or in vivo with aspirin did not lead to platelet accumulation on undamaged vessels or enhance platelet accumulation on injured vessels. Similarily, products of the lipoxygenase pathway that have been implicated as responsible for the non-thrombogenicity of vessels did not appear to be responsible for the lack of accumulation of platelets on these surfaces, since inhibitors of lipoxygenase such as ETYA (15-hydroxy-5,8,11,13-eicososatetraenoic acid) or NDGA (nordihydroguiaretic acid) did not influence the number of platelets that accumulated on the vessels under the conditions tested. To determine if factors other than platelets could account for the lack of vessel wall reactivity, platelet accumulation in vivo on de-endothelialized vessels or on vessels with an injured neointima was prevented by treating animals with injections of dipyridamole or with constant infusions of PGI₂. It was observed that 6 to 8 hours of these treatments are required for the vessels to become non-reactivel; shorter durations of treatment do not allow the surface to develop its non-thrombogenic properties. Thus, the loss of reactivity of injured vessels do not require platelet adherence to the injured surface. In addition, it can be inferred that plasma factors or material from circulating red blood cells does not contribute to this effect. It may be that the loss of reactivity that develops over a 6 to 8 hour period may be attributable to a factor(s) elaborated by injured vessels and this is an avenue worthy of exploration. Several general conclusions can be reached based on the observations in this study. First, since injured vessels rapidly lose their reactivity to circulating platelets, repeated or continuous vessel injury is probably required for the development of severe arterial disease. Second, since activation of coagulation plays a major role in the response to injury of previously injured vessels, treatment with anticoagulants could be useful therapy in some forms of arterial thrombosis. Finally, since injured vessels lose their reactivity to platelets even when the initial interaction of platelets with injury site is prevented, it may prove useful to administer drugs that inhibit platelet adherence to damaged vessels for short periods only, for example, after coronary bypass surgery or transluminal angioplasty. This would allow "passivation" of the surface, prevent the effects on smooth muscle cell proliferation of growth factors released from adherent platelets, and reduce the vessel wall thickening that could compromise flow to the tissues.