**3. Renin angiotensin system (RAS)**

The RAS is a central element of the physiological and pathological responses of the cardiovascular system. Its primary effector hormone, Ang II, not only intercedes immediate physiological effects of vasoconstriction and blood pressure regulation, but is also implicated in inflammation, endothelial dysfunction, hypertension and heart failure (Opie & Sack, 2001). Many of the cellular effects of Ang II appear to be mediated by ROS generated by NAD(P)H oxidase (Koumallos et al., 2011). Two subtypes of Ang II receptors have been defined on the basis of their differential pharmacological and biochemical properties: Ang II type 1 receptors (AT1), which are involved in most of the well-known physiological effects of Ang II, and Ang II type 2 receptors (AT2), which have a less well-defined role but appear capable of counterbalancing some of the effects of AT1 stimulation. AT1 transactivates growth pathways and mediates major Ang II effects such as vasoconstriction, increased cardiac contractility, renal tubular sodium reabsorption, cell proliferation, vascular and cardiac hypertrophy, inflammatory responses, and oxidative stress. AT2 is believed to induce essentially opposite effects, including vasodilation, antigrowth and antihypertrophic effects, and to play a significant role in blood pressure (BP) regulation (Oudit & Penninger, 2011; Horiuchi et al., 1999; Matsubara, 1998; Siragy, 2000; Carey et al., 2001).

The discovery of Ang (1–7), an endogenous peptide which opposes the pressor, proliferative, profibrotic, and prothrombotic actions mediated by Ang II has contributed to the realization that the RAS is composed of two opposing arms: the pressor arm constituted by the enzyme angiotensin-converting enzyme (ACE), Ang II as the product, and the AT1 receptor as the main protein mediating the biological actions of Ang II; the second arm is composed of the monocarboxypeptidase ACE2, Ang (1–7) produced through hydrolysis of Ang II, and the Mas receptor as the protein conveying the vasodilator, antiproliferative, antifibrotic, and antithrombotic effects of Ang (1–7) (Petty et al., 2009; Ferrario, 2011).

Experimental Autoimmune Myocarditis: Role of Renin Angiotensin System 313

dose dependent manner. Treatment with oral ARB improved both systolic and diastolic functions, increased neurohormonal parameter, such as plasma Ang II, and ameliorated myocardial remodeling and its marker molecules (Sukumaran et al., 2010, 2011a, 2011b;

EAM rats also suffer from various stresses including reactive oxygen species (ROS) mediated oxidative stress. There are several evidences for the adverse cardiac effects triggered by redox cycling of ROS, generated in part by an NADPH oxidase dependent pathway. Reports also add the role of Ang II in triggering the oxidative stress in which increase in the levels of NADPH oxidase subunits like gp91phox, NOX4, p22phox, p40phox, p47phox, p67phox, rac1 and 3-Nitrotyrosine in rat EAM. ARBs can block the myocardial oxidative stress in EAM evidenced by the decreased levels of these markers (Sukumaran et

Central venous pressure (CVP) and left ventricular end diastolic pressure (LVEDP) were significantly higher and mean blood pressure (MBP), LVP and +dP/dt were significantly lower in EAM rats indicating systolic and diastolic dysfunction. CVP and LVEDP were significantly decreased in the ARB treated EAM rats. Myocardial contractility parameters including intraventricular pressure change were improved in EAM rats treated with ARB. Echocardiographic analysis also showed the improvement of cardiac remodeling with ARB treatment evidenced by decreased LVDd and LVDs and increased fractional shortening and ejection fraction (Sukumaran et al., 2010; 2011a; 2011b; Shirai et al., 2005; Tsutsui et al., 2007).

Inappropriate apoptosis contributes to the pathogenesis of a number of cardiac diseases and is recognized as an important factor in cardiovascular remodeling. AT1 mediated cardiomyocyte apoptosis is due to the pathologic involvement of RAS where ARB can block the actions of Ang II on AT1 thereby preventing the cellular apoptosis in the myocardium. There were several reports indicating the myocardial apoptosis in the EAM rats and ARBs can effectively prevent it due to their action on RAS. For instance, ARB can block the endoplamic reticulum stress and caspase12 activation in the EAM rats thus prevents cardiomyocyte apoptosis (Singh et al., 2008; Tsutsui et al., 2007; Ye et al., 2010; Matsusaka et

AT1 antagonists are reported to suppress cytokine production and the transcription of cytokine genes in vitro and in vivo (Matsubara, 1998; Siragy, 2000; Carey et al., 2001). ARBs can decrease the expression of IFN-gamma (interferon-gamma), FasL (Fas ligand), iNOS (inducible nitric oxide synthase) and PFP (pore-forming protein) in myocardial tissue, indicating suppression of the activation of infiltrating killer lymphocytes (Seko, 2006). ARB administration downregulates Th1 cytokines (IFN-gamma and IL-2) while upregulating Th2 cytokines (IL-4 and IL-10). Thus, studies of RAS antagonists in inflammatory diseases suggested that Ang II was involved in immune and inflammatory responses and ARBs are useful candidates in preventing the inflammation associated with those disorders. In our lab

Shirai et al., 2005).

**4.1 ARB and oxidative stress** 

al., 2011b; Seko, 2006; Singh et al., 2008).

**4.3 ARB and cardiomyocyte apoptosis** 

**4.4 ARB and inflammation in EAM** 

al., 2006) (Figure 1).

**4.2 ARB and hemodynamics** 

Hypertrophy of cardiac myocytes is an adaptive response in the damaged heart. Initially, hypertrophy acts as a compensatory mechanism to preserve cardiac function, but when sustained, it becomes a major risk factor for congestive heart failure and sudden cardiac death. Until recently, most in vitro and in vivo studies of the roles of AT1 and AT2 indicated that AT1 mediates the growth promoting, fibrotic, and hypertrophic effects of Ang II on cardiovascular tissues and that AT2 exerts counterbalancing suppressant effects (Gao & Zucker, 2011). Evidence has been provided that the circulating and local RAS promote the development of myocardial fibrosis in hypertensive heart disease and chronic heart failure where both Ang II and aldosterone stimulate collagen synthesis in a dose-dependent manner while Ang II additionally suppresses the activity of matrix metalloproteinase 1, the key enzyme of interstitial collagen degradation, that synergistically leads to progressive collagen accumulation within the myocardial interstitium (Lijnen & Petrov, 2003). Therefore, the physiological role of RAS on the development of myocardial fibrosis could be established.

In addition to its role in the regulation of arterial pressure, Ang II is known to mediate effects on cell growth and apoptosis and to have pro-oxidative and proinflammatory effects. Apoptosis can be induced in cardiomyocytes by a variety of factors and pathways, a number of findings suggest that the effectors of the RAS can be critically involved in cardiomyocyte apoptosis (Fabris et al., 2011; Guleria et al., 2011; Yamada et al., 1996).

Peroxisome proliferator activated receptors (PPARs), members of the superfamily of ligand regulated transcription factors, are expressed in the cardiovascular system and control diverse vascular functions by mediating appropriate changes to gene expression. PPARα and PPARγ modulate the RAS by transcriptional control of renin, angiotensinogen, ACE and AT1 (Takeyama et al., 2000; Lansang et al., 2006).
