The formation of atherosclerotic plaques is linked to a change in vascular function, with evidence of endothelial dysfunction and the proliferation of the underlying vascular smooth muscle cells (VSMCs). Prior to plaque development, risk factors are present that are capable of altering vascular function and promoting disease progression. These risk factors include hypercholesterolaemia, obesity and inflammation. The specific mechanisms of these risk factors in the early stages of atherosclerotic disease development have yet to be fully explored and are likely to be closely interwoven. The aim of this thesis was to assess the effects of these atherosclerotic risk factors, with a primary focus on the direct action of hypercholesterolaemia, on vascular function prior to the development of overt atherosclerosis. After acute ex vivo cholesterol depletion and enrichment, a range of contractile and relaxant stimuli were applied to thoracic aortic rings of wildtype C57BL/6 mice. Cholesterol depletion significantly reduced contractility to phenylephrine (p<0.05) and serotonin (p<0.01). Acute cholesterol enrichment had no effect on vascular contractility, however, acetylcholine stimulated endothelial-dependent relaxation was significantly reduced (p<0.05).Feeding with either a standard chow or a high fat ‘western’ diet was undertaken for eight weeks in both ApoE-/- and C57BL/6 mice. The extent of atherosclerotic disease development was measured through en face lipid staining and histological analysis of aortae. Atherosclerosis was present in the aortic root and intercostal branches of chow and high fat fed ApoE-/- mice but not in diet-matched C57BL/6 mice. No atherosclerotic lesions were observed in the thoracic aortae. In addition, to allow the possibility for direct associations to be made between the associated risk factors of hypercholesterolaemia, obesity and inflammation, and vascular function, a phenotypic assessment of these characteristics was conducted. Wire myography was employed to assess the vascular function of thoracic aortic rings from chow and high fat diet fed ApoE-/- mice and their age and diet matched wildtype C57BL/6 controls. It was found that contractility to both phenylephrine and serotonin was significantly increased in chow fed ApoE-/- mice (both p<0.05). Further investigation into the mechanism, using intracellular calcium imaging and the indo-1 dye, concluded that VSMC store-operated calcium entry was not altered. The exact mechanism behind this increase in contractility is therefore still unknown and there was no clear relationship to the atherosclerotic risk factors assessed. In addition to altered contractility, endothelial-dependent relaxation was shown to be significantly enhanced in high fat fed C57BL/6 (p<0.01) and ApoE-/- mice (p<0.001). Enhanced endothelial-dependent relaxations were transient and sensitive to specific inhibition of cyclooxygenase-2 but not nitric oxide synthase. These changes were hypercholesterolaemia-independent but correlated with signs of obesity and inflammation. In summary, this investigation has demonstrated that vascular function was altered in the murine thoracic aorta prior to overt atherosclerotic plaque development. The implications for these observations relate to the possibility of masked early signs of vascular dysfunction and also the induction of compensatory mechanisms which may have amplified effects over a longer time course; possibly promoting the development and advancement of atherosclerotic disease.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:607404 |
| Date | January 2013 |
| Creators | Cobb, Christopher John |
| Contributors | Austin, Clare; Eisner, David; Holt, Cathy |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://www.research.manchester.ac.uk/portal/en/theses/vascular-function-prior-to-the-development-of-overt-atherosclerosis(dd14fd62-3cae-4932-a497-e55a092f504d).html |
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