The Influence of Mechanical Forces on the Renin Angiotensin System in Cultured Human Vascular Smooth Muscle Cells

In cardiovascular disease, the deleterious effects of Angiotensin II (Ang II) on the vasculature are well recognised. This is not necessarily due to elevated levels of circulating Ang II. An alternative hypothesis implicates the role of vascular cell signalling in disease. Therefore this thesis examined the role of the Ang II type I (AT1) receptor in a cell culture model of hypertension: cultured human vascular smooth muscle (VSM) cells were exposed to cyclical mechanical strain regimes (Flexcell®) designed to mimic the forces generated by hypertension in vivo. Early experimentation demonstrated cyclical mechanical strain and Ang II induction of VSM cell gene expression and synthesis of extracellular matrix proteins. Importantly, AT1 receptor antagonism inhibited the strain-induced fibrogenic activity of VSM cells suggesting a synergistic relationship between the renin-angiotensin system and mechanical strain. The response of VSM cell AT1 receptor to mechanical strain was analysed by three-colour flow cytometry. After accounting for non-specific binding, two distinct populations of human VSM cells were identified based on their level of AT1 receptor expression. In the population of VSM cells with a high expression of AT1 receptor, cyclical mechanical strain resulted in an increase in the expression of the AT1 receptors. This thesis has revealed for the first time, conclusive evidence that mechanical strain up-regulates cell-surface expression of AT1 receptors on human VSM cells. This may highlight a mechanism whereby mechanical strain may lead to sensitisation of human VSM cells to Ang II in the early stages of vascular disease.