**3. The renin-angiotensin-aldosterone system (RAAS) as a "novel" risk factor for AF**

Among many others, two factors contribute to the search of different therapeutic ap‐ proaches to AF specifically targeting substrate development and maintenance:[29] the recog‐ nition of novel risk factors for the development of this arrhythmia and the well-known limitations of the current antiarrhythmic drug therapy to maintain sinus rhythm, still having inadequate efficacy and potentially serious adverse effects.[30] In this setting, the inhibition of the renin-angiotensin-aldosterone system (RAAS) has been considered useful in both pri‐ mary and secondary prevention of AF, particularly in patients presenting left ventricular hypertrophy (LVH) or heart failure. The RAAS is a major endocrine/paracrine system in‐ volved in the regulation of the cardiovascular system.[31] Its key mediator is angiotensin II, an octapeptide that is cleaved from the liver-derived 485-aminoacid precursor angiotensino‐ gen through a process involving the enzymatic activities of renin and angiotensin convert‐ ing enzyme (ACE). Two main angiotensin II receptors exist, i.e angiotensin II type 1 (AT1) and type 2 (AT2). AT1-receptor mediated pathways lead to vasoconstriction, water retention, increased renal tubular sodium reabsorption, stimulation of cell growth and connective tis‐ sue deposition, and impaired endothelial function. AT2-receptor has opposing effects, inas‐ much as it mediates vasodilation, decreases renal tubular sodium reabsorption, inhibits cell growth and connective tissue deposition, and improves endothelial function. These two an‐ giotensin receptors have different expression patterns, AT1 being constitutively expressed in a wide range of tissues of the cardiovascular, renal, endocrine, and nervous system, and AT2 expression being activated during stress conditions.[32] It is becoming increasingly evident that all these mechanisms are involved in atrial remodeling and hence in AF development and maintenance. Moreover, among the other biologically active RAAS components that are involved in these processes, angiotensin-(1-7) [Ang-(1-7)] seems to be particularly impor‐ tant. In an experimental canine model of chronic atrial pacing, Ang-(1-7) has been shown to reduce AF vulnerability and atrial fibrosis,[33] influencing atrial tachycardia-induced atrial ionic remodeling. [34]

**4.2. The role of RAAS in structural remodeling**

firmed by others.[27,28]

by contrasting AF maintenance.[26,59,61-64]

**4.3. The role of RAAS in electrical remodeling**

Atrial fibrosis causes conduction heterogeneity, hence playing a key role in the development of a vulnerable *structural* substrate for AF, and the proinflammatory and profibrotic effects of angiotensin II have been extensively described.[46-48] Excessive fibrillar collagen deposi‐ tion, resulting from deregulated extracellular matrix metabolism, leads to atrial fibrosis, and it has been shown that angiotensin II has a direct effect in stimulating cardiac fibroblast pro‐ liferation and collagen synthesis, via AT1 receptor – mediated mechanisms involving a mito‐ gen-activated protein kinases (MAPKs) phosphorylation pathway. [49-51] The latter cascade is inhibited by AT2 receptor activation, that has an antiproliferative effects.[52] Moreover, cardiac fibroblast function is modulated by angiotensin II through mechanisms involving TGF-1, osteopontin (OPN), and endothelin-1 (ET-1). [49,53-55] Interestingly, Nakajima and coworkers showed that selective atrial fibrosis, conduction heterogeneity, and AF propensi‐ ty are enhanced in a TGFβ1 cardiac overexpression transgenic mice model,[56] as also con‐

Atrial Fibrillation and the Renin-Angiotensin-Aldosterone System

http://dx.doi.org/10.5772/53917

7

Beyond having both direct and indirect effects on collagen synthesis, angiotensin II inter‐ feres with collagen degradation by modulating interstitial matrix metalloproteinase (MMP) activity and tissue inhibitor of metalloproteinase (TIMP) concentrations,[52] and an atrial tissue imbalance between MMPs and TIMPs has been reported in both clinical and animal studies on AF. [52,57] Goette and coworkers showed increased atrial expression of ACE and increased activation of the angiotensin II-related intracellular signal transduction pathway in human atrial tissue derived from AF patients,[58] and atrial overexpression of angioten‐ sin II has also been shown in a canine model of ventricular tachycardia-induced CHF[26,59] In transgenic mice experiments with cardiac-restricted ACE overexpression, Xiao et al. have demonstrated that elevated atrial tissue angiotensin II concentrations stimulates atrial fibro‐ sis and hence an AF-promoting substrate.[60] In contrast, RAAS inhibition reduces tissue angiotensin II concentration, and attenuates atrial structural remodeling and fibrosis, there‐

Electrical remodeling has been hypothesized as a main mechanism by which, once estab‐ lished, "AF begets further AF" self-perpetuation.[12] In the clinical practice, this phe‐ nomenon is evident when considering that over time it becomes more and more difficult to keep in sinus rhythm a patient with AF. The concept of electrical remodeling has been originally proposed by Wijffels et al.[12] to explain the experimental observation that when AF is maintained artificially, the duration of burst pacing-induced paroxysms progressively increases until AF becomes sustained. This indicates that AF itself alters the atrial tissue electrical properties, thereby developing a functional substrate that pro‐ motes AF perpetuation and may involve alterations in ionic currents and in excitability cellular properties.[65] In their study, Wijffels et al. demonstrated that the increased pro‐ pensity to AF is associated with shortening of the atrial effective refractory period (AERP) in accordance with the multiple wavelet theory,[12] a mechanism that was sub‐

Among the compounds that may interfere RAAS four classes of drugs are particularly rele‐ vant in cardiovascular therapy: angiotensin receptor blockers (ARBs), ACE inhibitors (ACEIs), aldosterone antagonists and direct renin inhibitors. ARBs directly block AT1 recep‐ tor activation, ACEIs inhibit ACE-mediated production of angiotensin II, and the recently developed direct renin inhibitor aliskiren blocks RAAS further upstream.[32,35,36] Over the last decade, these drugs have been tested in the setting of AF treatment and prevention.

#### **4. The role of RAAS in the pathogenesis of AF**

#### **4.1. Atrial stretch and AF**

Atrial arrhythmias frequently occur under conditions associated with atrial dilatation and increased atrial pressure, causing atrial tissue stretch and modifying atrial refractoriness, and it has been shown in several animal as well as clinical models.[37-40] These factors in‐ crease susceptibility to AF, that is associated with shortening of the atrial effective refractory period (AERP), possibly by opening of stretch-activated ion channels. In the setting of arteri‐ al hypertension and congestive heart failure (CHF), angiotensin II has been associated with increased left atrial and left ventricular end-diastolic pressure,[41] and both ACEIs and ARBs have been shown to reduce left atrial pressure.[42-45] Therefore, one potential mecha‐ nism by which ACEIs and ARBs may reduce atrial susceptibility to AF is by reducing atrial stretch. Many other mechanisms appear to be involved in the antiarrhythmic properties of RAAS inhibition, and in an animal model of ventricular tachycardia-induced CHF it has been shown that ACE inhibition is more successful than hydralazine/isosorbide mononitrate association in reducing burst pacing-induced AF promotion, despite a similar reduction in left atrial pressure.[26] As described below, angiotensin II-mediated mechanisms contribute to both *structural* and *electrical remodeling* of the atrial tissue.

#### **4.2. The role of RAAS in structural remodeling**

gen through a process involving the enzymatic activities of renin and angiotensin convert‐ ing enzyme (ACE). Two main angiotensin II receptors exist, i.e angiotensin II type 1 (AT1) and type 2 (AT2). AT1-receptor mediated pathways lead to vasoconstriction, water retention, increased renal tubular sodium reabsorption, stimulation of cell growth and connective tis‐ sue deposition, and impaired endothelial function. AT2-receptor has opposing effects, inas‐ much as it mediates vasodilation, decreases renal tubular sodium reabsorption, inhibits cell growth and connective tissue deposition, and improves endothelial function. These two an‐ giotensin receptors have different expression patterns, AT1 being constitutively expressed in a wide range of tissues of the cardiovascular, renal, endocrine, and nervous system, and AT2 expression being activated during stress conditions.[32] It is becoming increasingly evident that all these mechanisms are involved in atrial remodeling and hence in AF development and maintenance. Moreover, among the other biologically active RAAS components that are involved in these processes, angiotensin-(1-7) [Ang-(1-7)] seems to be particularly impor‐ tant. In an experimental canine model of chronic atrial pacing, Ang-(1-7) has been shown to reduce AF vulnerability and atrial fibrosis,[33] influencing atrial tachycardia-induced atrial

Among the compounds that may interfere RAAS four classes of drugs are particularly rele‐ vant in cardiovascular therapy: angiotensin receptor blockers (ARBs), ACE inhibitors (ACEIs), aldosterone antagonists and direct renin inhibitors. ARBs directly block AT1 recep‐ tor activation, ACEIs inhibit ACE-mediated production of angiotensin II, and the recently developed direct renin inhibitor aliskiren blocks RAAS further upstream.[32,35,36] Over the last decade, these drugs have been tested in the setting of AF treatment and prevention.

Atrial arrhythmias frequently occur under conditions associated with atrial dilatation and increased atrial pressure, causing atrial tissue stretch and modifying atrial refractoriness, and it has been shown in several animal as well as clinical models.[37-40] These factors in‐ crease susceptibility to AF, that is associated with shortening of the atrial effective refractory period (AERP), possibly by opening of stretch-activated ion channels. In the setting of arteri‐ al hypertension and congestive heart failure (CHF), angiotensin II has been associated with increased left atrial and left ventricular end-diastolic pressure,[41] and both ACEIs and ARBs have been shown to reduce left atrial pressure.[42-45] Therefore, one potential mecha‐ nism by which ACEIs and ARBs may reduce atrial susceptibility to AF is by reducing atrial stretch. Many other mechanisms appear to be involved in the antiarrhythmic properties of RAAS inhibition, and in an animal model of ventricular tachycardia-induced CHF it has been shown that ACE inhibition is more successful than hydralazine/isosorbide mononitrate association in reducing burst pacing-induced AF promotion, despite a similar reduction in left atrial pressure.[26] As described below, angiotensin II-mediated mechanisms contribute

**4. The role of RAAS in the pathogenesis of AF**

to both *structural* and *electrical remodeling* of the atrial tissue.

ionic remodeling. [34]

6 Atrial Fibrillation - Mechanisms and Treatment

**4.1. Atrial stretch and AF**

Atrial fibrosis causes conduction heterogeneity, hence playing a key role in the development of a vulnerable *structural* substrate for AF, and the proinflammatory and profibrotic effects of angiotensin II have been extensively described.[46-48] Excessive fibrillar collagen deposi‐ tion, resulting from deregulated extracellular matrix metabolism, leads to atrial fibrosis, and it has been shown that angiotensin II has a direct effect in stimulating cardiac fibroblast pro‐ liferation and collagen synthesis, via AT1 receptor – mediated mechanisms involving a mito‐ gen-activated protein kinases (MAPKs) phosphorylation pathway. [49-51] The latter cascade is inhibited by AT2 receptor activation, that has an antiproliferative effects.[52] Moreover, cardiac fibroblast function is modulated by angiotensin II through mechanisms involving TGF-1, osteopontin (OPN), and endothelin-1 (ET-1). [49,53-55] Interestingly, Nakajima and coworkers showed that selective atrial fibrosis, conduction heterogeneity, and AF propensi‐ ty are enhanced in a TGFβ1 cardiac overexpression transgenic mice model,[56] as also con‐ firmed by others.[27,28]

Beyond having both direct and indirect effects on collagen synthesis, angiotensin II inter‐ feres with collagen degradation by modulating interstitial matrix metalloproteinase (MMP) activity and tissue inhibitor of metalloproteinase (TIMP) concentrations,[52] and an atrial tissue imbalance between MMPs and TIMPs has been reported in both clinical and animal studies on AF. [52,57] Goette and coworkers showed increased atrial expression of ACE and increased activation of the angiotensin II-related intracellular signal transduction pathway in human atrial tissue derived from AF patients,[58] and atrial overexpression of angioten‐ sin II has also been shown in a canine model of ventricular tachycardia-induced CHF[26,59] In transgenic mice experiments with cardiac-restricted ACE overexpression, Xiao et al. have demonstrated that elevated atrial tissue angiotensin II concentrations stimulates atrial fibro‐ sis and hence an AF-promoting substrate.[60] In contrast, RAAS inhibition reduces tissue angiotensin II concentration, and attenuates atrial structural remodeling and fibrosis, there‐ by contrasting AF maintenance.[26,59,61-64]

#### **4.3. The role of RAAS in electrical remodeling**

Electrical remodeling has been hypothesized as a main mechanism by which, once estab‐ lished, "AF begets further AF" self-perpetuation.[12] In the clinical practice, this phe‐ nomenon is evident when considering that over time it becomes more and more difficult to keep in sinus rhythm a patient with AF. The concept of electrical remodeling has been originally proposed by Wijffels et al.[12] to explain the experimental observation that when AF is maintained artificially, the duration of burst pacing-induced paroxysms progressively increases until AF becomes sustained. This indicates that AF itself alters the atrial tissue electrical properties, thereby developing a functional substrate that pro‐ motes AF perpetuation and may involve alterations in ionic currents and in excitability cellular properties.[65] In their study, Wijffels et al. demonstrated that the increased pro‐ pensity to AF is associated with shortening of the atrial effective refractory period (AERP) in accordance with the multiple wavelet theory,[12] a mechanism that was sub‐ sequently attributed to a reduction of action potential duration (APD) secondary to the progressive downregulation of the transient outward current (Ito) and of the L-typeCa2+ current (ICa*,*L).[66] As to the modulation of the ICa*,*L current, the role of angiotensin II is controversial, with studies reporting increase, decrease, or even no effect.[29,67] In contrast, angiotensin II has been demonstrated to downregulate Ito current,[68,67] inas‐ much as AT1 receptor stimulation leads to internalization of the Kv4.3 (i.e., the poreforming *α*-subunit underlying Ito), regulating its cell-surface expression.[68] As shown by Liu and coworkers, chronic Ang-(1-7) infusion prevented the decrease of Ito, ICa*,*L, and of Kv4.3 mRNA expression induced by chronic atrial pacing, [34] thereby contribu‐ ting to reduce AF vulnerability.[33] Subsequently, Nakashima et al. showed that ACEI or ARB treatment results in complete inhibition of the shortening of AERP, that is nor‐ mally induced by rapid atrial pacing.[69] A further mechanism by which the RAAS may exert a proarrhythmic effect is the modulation of gap junctions, that are low-resistance pathways for the propagation of impulses between cardiomyocytes formed by connexins (Cx).[70] Cx40 gene polymorphisms have been associated with the development of non familial AF,[71] and angiotensin II has been implicated in Cx43 downward remodeling. [72-74] Moreover, angiotensin II directly induces delayed after-depolarizations and accel‐ erates the automatic rhythm of isolated pulmonary vein cardiomyocytes.[75] These cells are considered an important source of ectopic beats and of atrial fibrillation bursts, rep‐ resenting the target of AF treatment with radio-frequency ablation.[76] Therefore these experimental results demonstrate that angiotensin II may play a role in the pathophysi‐ ology of atrial fibrillation also by modulating the pulmonary vein electrical activity via an electrophysiological effect that was shown to be AT1 receptor – mediated, being in‐ hibited by losartan, [75] and that is attenuated by heat-stress responses.[77] Recently, al‐ so the direct renin inhibitor aliskiren was shown to reduce the arrhythmogenic activity of pulmonary vein cardiomyocytes.[36] It has also been demonstrated that aldosterone promotes atrial fibrillation, causing a substrate for atrial arrhythmias characterized by at‐ rial fibrosis, myocyte hypertrophy, and conduction disturbances,[78] and the specific an‐ tagonist spironolactone has been shown to prevent aldosterone-induced increased duration of atrial fibrillation in a rat model.[79]

aldosterone synthase gene polymorphism might also be associated with atrial remodelling

Atrial Fibrillation and the Renin-Angiotensin-Aldosterone System

http://dx.doi.org/10.5772/53917

9

A possible relationship between the RAAS and the risk of developing AF was brought about by several clinical data, derived from patient series in different settings, that are

In heart failure, several observations indicate a possible effect of RAAS inhibition in re‐ ducing the incidence of new onset AF. In a retrospective analysis of the SOLVD trial, Vermes et al. showed that enalapril reduces the risk of AF development in patients with various degrees of heart failure.[87] Similarly, Maggioni et al. demonstrated that use of the ARB valsartan is associated with a reduction in the risk of AF in the Val-HeFT trial population.[88] Since the vast majority of these patients (92.5%) were already receiving an ACEI, a combination effect was hypothesized, and the benefit of combined treatment with both an ARB and an ACEI was also supported by the results of the CHARM trial with candesartan.[89] The latter study was composed by three component trials based on left ventricular ejection fraction (LVEF) and ACEI treatment. CHARM-Alternative tri‐ al enrolled patients with LVEF ≤40% not treated with ACEIs because of prior intoler‐ ance, CHARM-Added recruited patients with LVEF ≤40% already treated with an ACEI, and CHARM-Preserved included patients with LVEF >40%, independent of ACEI treat‐ ment. The incidence of new-onset AF was reduced in candesartan-treated patients, espe‐ cially (but not exclusively) in the CHARM-Alternative trial.[89] These data indicate additional benefits in AF prevention, on the top of the already known effects of

After an acute myocardial infarction, treatment with the ACEI trandolapril reduced the inci‐ dence AF in patients with impaired left ventricular function, irrespective of the effects on ejection fraction per se.[90] Similar results were reported by Pizzetti et al. with lisinopril in

The issue of the possible role of ACEI/ARB drug treatment in the primary prevention of AF in hypertensive patients derives from several conflicting observations. According to the CAPPP and the STOP-H2 trials, ACEIs were comparable to other antihypertensive

**5. Atrial fibrillation and the renin-angiotensin-aldosterone system**

in hypertensive patients.[86]

here summarized.

**5.1. Heart failure**

**5.2. Post-MI**

**5.3. Hypertension**

**(RAAS): Clinical observations**

ACEI/ARB treatment in patients with heart failure.

their analysis of the GISSI-3 trial.[91]

#### **4.4. RAAS gene polymorphisms and AF**

The ACE DD (deletion/deletion) genotype of the ACE gene has been shown to be a predis‐ posing factor for persistent AF,[80] and it was recently reported that the same genotype is associated with lowest rates of symptomatic response in patients with lone AF.[81] More‐ over, polymorphisms of the angiotensinogen gene have also been associated with nonfami‐ lial AF,[82] and it has been shown that significant interactions exist between angiotensinogen gene haplotypes and ACE I/D (insertion/deletion) polymorphism resulting in increased susceptibility to AF.[83,84] Also aldosterone synthase (CYP11B2) T-344C poly‐ morphism, which is associated with increased aldosterone activity, was shown to be an in‐ dependent predictor of AF in patients with HF.[85] According to Sun and coworkers, this aldosterone synthase gene polymorphism might also be associated with atrial remodelling in hypertensive patients.[86]
