The role of the renin-angiotensin-aldosterone system in diabetes and its vascular complications.

Clinical trials have demonstrated the benefit of blood pressure (BP) reduction in reducing the risk of cardiovascular and renal complications in patients with diabetes mellitus. Incorporation of agents that inhibit the renin-angiotensin-aldosterone system (RAAS) into antihypertensive regimens has been shown to provide reductions in renal and cardiovascular events that are mediated by both BP-dependent and -independent mechanisms. Recent studies exploring these potential mechanisms have demonstrated a direct role of angiotensin II (ATII) in the pathology of the vasculature and other sites of end-organ injury. In animal models of diabetes, inhibition of the RAAS with angiotensin-converting enzyme (ACE) inhibition or angiotensin type 1 (AT(1)) receptor blockade has been shown to prevent atherosclerosis, an effect that was independent of BP reduction. In addition to its direct effects on the vasculature, ATII also has direct detrimental effects on end organs, including the kidney and the heart, which lead to the development of proteinuria and left ventricular hypertrophy (LVH), respectively. Left ventricular hypertrophy has been shown to be predictive of cardiovascular and renal events, and the benefits of RAAS inhibition with angiotensin receptor blocker therapy are accompanied by a reduction in LVH. In addition to preventing the cardiovascular and renal complications of diabetes, the RAAS blockade has also been shown, in several large randomized clinical trials, to inhibit new onset of diabetes. Recent studies have revealed that many tissues, including pancreatic islets and adipose tissue, have a local RAAS. In the diabetic rat model (Zucker diabetic fatty rats), pancreatic islets exhibit an increased intraislet expression of ACE and AT(1) as well as increased intraislet fibrosis, apoptosis, and oxidative stress. The local RAAS also appears to play a role in the function of the adipocyte. Angiotensin II inhibits adipocyte differentiation, potentially decreasing the storage capacity of adipose tissue. The reduced capacity of adipose tissue to store fatty acids may cause their accumulation in other tissues, leading to insulin resistance and development of diabetes. Collectively, these studies demonstrate that ATII has direct effects on multiple tissues, and inhibition of ATII action in these tissues may be responsible for many of the clinical benefits observed with RAAS inhibition.