**7. NADPH oxidases inhibitors**

pacing, and markedly reduces the promotion and maintenance of AF. Also, probucol signifi‐ cantly reduces atrial oxidative stress and increases total antioxidant capacity [55]. A further study from this group suggested that probucol attenuates structural remodeling and preventes atrial apoptosis, decreasing the left atrial MDA content in paced dogs [56]. Very recently, we investigated the effects of probucol on atrial structural and electrical remodeling in alloxaninduced diabetic rabbits [57]. After treatment for 8 weeks, the diabetic rabbits on probucol exhibited alleviation of oxidative stress displayed as decreased plasma MDA and increased plasma SOD levels compared with diabetic controls, while probucol significantly reduced left atrial interstitial fibrosis and AF inducibility [57]. Therefore, it seems reasonable to speculate that the antioxidant effects of probucol may favorably affect atrial autonomic and structural

Succinobucol (AGI-1067), a derivative of probucol, is a metabolically stable compound that has greater intracellular antioxidant efficacy in vitro than probucol without causing a signifi‐ cant prolongation of the QT interval [58]. Surprisingly, the ARISE study [59] showed that use of succinobucol was associated with increased incidence of new-onset AF in patients with an acute coronary syndrome. Our previous meta-analysis suggested that increased CRP levels are associated with greater risk of immediate and short-term AF recurrence following electrical cardioversion [60, 61]. In this context, although succinobucol have potent antioxidant effects, its unfavourable influence on CRP levels may be a possible potential explanation for this undesirable effect [62, 63]. Undoubtedly, further studies are needed to elucidate the precise role of probucol and succinobucol in atrial remodeling and their clinical impact on AF.

Nitric oxide is an important endothelium-derived relaxing factor that plays a pivotal role in the maintenance of vascular tone. NO is synthesized from L-arginine through the effects of endothelial NO synthase (eNOS) with the critical cofactor tetrahydrobrobiopterin (BH4). BH4 depletion induces NOS uncoupling which shifts the enzymatic activity from NO production

oxidative stress and inflammation and also impairs NO dependent vaso-relaxation. Endothe‐ lial dysfunction with decreased NO production has been implicated on the development of

It has been indicated that L-Arginine supplementation, as a NO precursor, increases plasma nitrite levels, decreases MDA release and attenuates ROS mediated myocardial injury [67]. In a canine tachypacing model of heart failure, Nishijima et al. [68] found increased inducible NOS in the left atrium which was associated with BH4 depletion, NOS2 uncoupling, and increased superoxide anion production. These biochemical changes were associated with atrial electrophysiological changes with increased AF inducibility. BH4 supplementation reduced atrial oxidative stress and inducibility of atrial fibrillation. Thus, modulation of NOS activity may be an interesting therapeutic approach to prevent AF [69]. At the clinical level, a pilot randomized placebo-controlled study examined the potential role of sodium nitroprusside

) production [64]. Endothelial dysfunction (ED) promotes

remodeling.

34 Atrial Fibrillation - Mechanisms and Treatment

**6. Nitric oxide donors or precursors**

towards superoxide anion (O2 <sup>−</sup>

atrial fibrillation [65, 66].

NADPH oxidases (NOXs) have been investigated as a key enzymatic source of ROS and seem to play an important role in the pathogenesis of hypertension, atherosclerosis, and heart failure [72-74]. NOXs are multi-subunit transmembrane enzymes that utilize NADPH as an electron donor to reduce oxygen to superoxide anion and hydrogen peroxide. NOX2 and NOX4 are the most abundant NOX subtypes in cardiomyocytes.

Recent evidence indicates that NOX-derived ROS plays a pivotal role in the development and maintenance of AF. Dudley et al. [75] showed reduced NO and increased superoxide produc‐ tion production in the left atrial appendage (LAA), which was related to increased NADPH oxidase activity in an experimental model of atrial tachy-pacing. Of note, the NADPH oxidase inhibitor apocynin reduced the superoxide production by 91%. In addition, Kim et al. [76] investigated the sources of superoxide production from the right atrial appendage (RAA) of patients undergoing cardiac surgery. They indicated that the membrane-bound subunit gp91phox (NOX2) containing NADPH oxidase was the main source of atrial superoxide production in human atrial myocytes during sinus rhythm and AF. Also, NADPH-stimulated superoxide release from RAA homogenates was significantly increased in patients with AF. In a subsequent study, they measured atrial NADPH oxidase activity in RAA samples from 170 consecutive patients undergoing coronary artery bypass surgery. The multivariate analysis showed that atrial NADPH oxidase activity was the strongest independent risk factor for the development of POAF [77]. Remarkably, recent clinical evidence [78] suggests that the behaviour of NADPH oxidase is related to the type of AF. Cangemi et al. demonstrated that NOX2 was upregulated in patients with paroxysmal/persistent AF compared with those with permanent AF and controls [78]. Also, NOX4-derived hydrogen peroxide production is markedly increased in the LAA tissues of AF patients. Moreover, treatment of HL-1 atrial cells with angiotensin II, resulted in upregulation of NOX4 and H2O2 production [79]. Bearing in mind the potent NADPH oxidase inhibitors such as NOX2 inhibitors and apocynin [80] may be served as potential candidate for the novel preventive agents on AF.
