RAS-Mediated Adaptive Mechanisms in Cardiovascular Tissues: Confounding Factors of RAS Blockade Therapy and Alternative Approaches

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Abstract
Since the classic experiments by Tigerstedt and Bergman that established the role of renin in hypertension a century ago, aggressive efforts have been launched to effectively block the renin-angiotensin system (RAS). Blockade of RAS is advocated at multiple levels by direct renin inhibitor, angiotensin-converting enzyme inhibitor and/or angiotensin II type 1 receptor blocker, or aldosterone inhibitor (spironolactone), and has now become part of the standard of care to control hypertension and related metabolic diseases including diabetes. However, recent lessons learned from randomized clinical trials question the wisdom of blocking RAS at multiple levels. In this context, it is highly pertinent that components of RAS are evolutionarily conserved, and novel physiological/adaptive/protective roles for renin and angiotensin-converting enzyme are currently emerging. Angiotensin II, the classical RAS effector peptide responsible for hypertension, hypertrophy, fluid retention and fibrosis, manifests its cardiovascular protective effect when it activates the angiotensin II type 2 receptor. Additionally, angiotensin-converting enzyme 2 and the angiotensin II metabolite Ang-(1–7) that acts through the Mas proto-oncogene constitute the cardiovascular and renal protective branch of RAS. It is conceivable that modulating this vasodilative/anti-inflammatory branch of RAS by activation of the RAS components that constitute this branch may offer a safer long-term treatment strategy to balance RAS activity and achieve homeostasis compared to chronic multilevel RAS inhibition.

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