Modulation of arterial stiffness by angiotensin receptors and nitric oxide in the insulin resistance syndrome

The insulin resistance syndrome [INSR] is associated with increased cardiovascular risk and affects up to 25% of the Australian population. The mechanism underlying the relationship between the INSR and increased cardiovascular risk is controversial. We postulated that perturbations in the renin-angiotensin system [RAS] and endothelium-derived NO may be implicated in the development of early vascular changes in the INSR.Repeated measurements of arterial stiffness [using digital photoplethysmography] and haemodynamic parameters in response to vasoactive medications were used to demonstrate the functional expression of angiotensin II [Ang II] receptors and NO synthase [NOS].Ang II acts via two main receptor sub-types: the Ang II type 1 [AT1] and Ang II type 2 [AT2] receptors. The AT1 receptor is central to the development of arterial stiffness and endothelial dysfunction. The role of AT2 receptors in humans is controversial but is postulated to counter-act AT1 receptor mediated effects in diseased vascular beds. We demonstrated increased AT1 and AT2 receptor-mediated effects in small to medium-sized arteries of subjects with early INSR [Chapter 6]. In addition, functional expression of AT2 receptors in adult insulin resistant humans [Chapter 5], but not in healthy volunteers [Chapter 4] was demonstrated. AT1 receptor blockade in subjects with early INSR resulted in improvements in vascular function, with a consequent functional down-regulation of AT2 receptors [Chapter 7].Functional NOS expression was demonstrated to be increased in subjects with early INSR compared with healthy controls [Chapter 6]. This was postulated to be a homeostatic response to counteract early vascular changes in subjects with early INSR. AT1 receptor blockade in these subjects reduced functional NOS expression [Chapter 8]. In conclusion, patients with early INSR represent a model of early disease where early intervention may be able to reverse the process incited by the initial exposure to multiple cardiovascular risk factors. Early vascular changes in these individuals are mediated at least in part, by increased AT1 receptor activity and/or expression, and may be detected by changes in arterial stiffness indices and non-invasive vascular reactivity studies. There is a compensatory increase in AT2 receptor and NOS expression/activity to counter-act these vascular changes