The road from atherosclerosis to myocardial infarction : Studies in an experimental model

Abstract: Atherosclerosis is a slow progressing disease with continuous lipid deposition and inflammation in the arterial wall. Stenotic arteries may cause angina pectoris but it is the sudden formation of an occluding thrombosis on the atherosclerotic plaque that leads to myocardial infarction. In a majority of cases such complications derive from plaque rupture or plaque erosion. Several steps in the series of events leading from silent atherosclerosis to myocardial infarction have remained unclear, in part due the lack of suitable animal models. This thesis has aimed to develop a mouse model in which myocardial infarction is triggered in order to study the physiological and molecular mechanisms that are important in the development of myocardial infarction. Isoflurane anesthetized apoE -/- x LDLR -/- and C57BL/6J mice were exposed to systemic hypoxia by reducing the inhaled oxygen concentration to 10% for 10 minutes. Mental stress was induced in conscious mice by blowing pressurized air into the cage. Physiological parameters were recorded every 30 minutes for 2-6 days by implanted transmitters. To investigate whether thrombosis was involved a thrombin inhibitor was administered in a bolus dose before the stress and after the hypoxic stress by continuous administration by osmotic mini-pumps. Controls were given PBS. Infarction development was determined by troponin T analysis and histology. The plaques and hearts were isolated at different time points after hypoxia and analysed by transcriptional profiling and quantitative real time RT-PCR. On selected genes involved in plaque vulnerability, protein expression and activity was assessed by immunohistochemistry, western blot, zymography. The results show that myocardial infarction can be induced by hypoxic stress in a two-phase pathway: an initial phase comprising a transient ischemic response which triggers a delayed second phase of ischemia and myocardial infarction. Previous results show that the first phase is endothelin dependent and in our present studies we show that the first ischemic phase triggers a delayed phase which involves thrombin. Furthermore our data show that hypoxia induces expression of inflammatory, proteolytic and pro-coagulant genes that may promote a vulnerable and pro-thrombotic plaque environment and we hypothesise that these factors may lead to endothelial or plaque damage and an activation of haemostasis. This model should be a useful tool to investigate plaque activation, thrombus formation and infarction development.