As pointed out above, the upstroke or initial phase of the cardiac AP is controlled by the entry of I_Na through voltage-gated Na channels, specifically Na_v1.5, which is encoded by the gene SCNA5 (Lieve et al., 2017). The Na channel is composed of a single pore-forming α-subunit and its regulatory β-subunits (β1-4, encoded by SCN1B-4B) that are widely expressed in the heart (Valdivia et al., 2010). Whether and how I_Na is directly modulated in obesity is poorly understood; however, there is indirect evidence for the potential modulation of I_Na in obesity. For example, Lin et al. demonstrated that acute exposure (1-h), of rabbit atrial myocytes to leptin, a peptide hormone involved in regulation of food intake (Wildman et al., 2000) and elevated in obesity (Ravussin et al., 2014), increased peak I_Na density (Lin et al., 2013). Peak I_Na density is also increased by pro-inflammatory cytokines (Zhao et al., 2016), while late I_Na current is increased by FFAs (Lin et al., 2014), in line with altered functional expression of I_Na in obesity.

Changes in serum FFAs are associated with increased risk of cardiac arrhythmias (Aromolaran et al., 2016), and suggests an important role for a functional interplay between cardiac ion channels and FFAs in myocardial disease. The cardiac I_Na is a prime candidate for electrical disturbances caused by exposure of the heart to elevated levels of FFAs. Despite the role of I_Na in normal cardiac depolarization and therefore heart excitability (Luo and Rudy, 1991), there is still controversy about its contribution to metabolic disease associated arrhythmias. Previously, O'Connell et al., demonstrated that while short-term exposure of ovine left atrial myocytes to the saturated FFA, stearic acid (SA) abbreviated the APD, I_Na remained essentially unchanged (O'Connell et al., 2015). In dog ventricular myocytes elevated levels of FFAs increased I_Na density, altered its gating properties, and increased cardiac excitability through augmentation of intracellular Ca concentration (Biet et al., 2014). By contrast, hypercholesterolemic rabbits displayed significantly depressed ventricular peak I_Na, leftward shift in the inactivation potential and a slowed time course of recovery when compared to normolipidemic control myocytes (Wu et al., 1997). Considering the implications of altered I_Na electrical remodeling, a good understanding of the molecular mechanisms underlying Na channel gating and functional regulation in obese heart is critical for fundamental insights into the prevalent condition of LQTS and metabolic disease-related arrhythmias in patients.

Ca influx through high voltage-activated Ca channels is an important regulator of cellular excitation-contraction (E-C) coupling (Fu et al., 2014). In myocytes, E-C coupling is established through Ca entry, I_{Ca, L}, through Ca channels which in turn triggers Ca release from intracellular Ca stores (Brandenburg et al., 2016). The α or pore-forming subunit (Ca_v1.2) of the I_{Ca, L} channel is encoded by the CACNAC1 gene (Catterall et al., 2003; Fu et al., 2013; Qian et al., 2017). I_{Ca, L} is modulated by interactions with cytoplasmic regulatory subunits (Ca_vβ1-4, Ca_vα₂δ1-4, Ca_vγ1-8) that play a pivotal role in channel gating properties (Colecraft et al., 2002; Yang et al., 2011), subcellular localization, and surface expression of the α-subunits (Yang and Colecraft, 2013; Tetreault et al., 2016).

In the context of metabolic diseases, altered cholesterol and/or FFA content of membranes will be expected to also affect Ca channel function, possibly through decreased cytosolic Ca levels and impaired cardiac contractility. O'Connell et al. demonstrated that short-term exposure of ovine left atrial myocytes to SA caused a significant reduction of I_{Ca, L} density (O'Connell et al., 2015), in line with a contribution of FFA-mediated atrial I_{Ca, L} dysfunction in obesity. In ventricular myocytes isolated from New Zealand white rabbits fed a cholesterol-rich diet for 12 weeks, I_{Ca, L} density was only slightly increased and not significantly different from normal chow-fed controls (Luo et al., 2004). These observations suggest the possibility that atrial I_{Ca, L} may be more sensitive to alterations in FFAs in vitro compared to in vivo effects in animal models involving additive effects of multiple combinations of FFAs.

Further, distinct proinflammatory cytokines that are involved in obesity such as IL-1β, IL-6, and TNF-α have also been shown to alter I_{Ca, L}, density, although these studies have yielded varying results. IL-1β and IL-6 either decreased (El Khoury et al., 2014) or increased (Hagiwara et al., 2007), I_{Ca, L} amplitude in ventricular myocytes. In contrast, TNF-α decreased I_{Ca, L} density and the amplitude of Ca transients in rat ventricular myocytes (Duncan et al., 2010), which would be consistent with an abbreviated ventricular APD.

The effects of obesity on the functional expression of I_{Ca, L} has also been investigated in animal models with contrasting outcomes. For example, in rats fed a high-fat diet for 15 weeks, obesity reduced Ca influx, while gene expression of CACNAC1 was either decreased (Leopoldo et al., 2011) or unchanged (Lima-Leopoldo et al., 2008). However, in another study Leopoldo et al. found that mRNA expression of L-type Ca channel is increased at 30 weeks (Lima-Leopoldo et al., 2013). Ashrafi et al. also reported increased mRNA levels of ventricular I_{Ca, L} after 8 weeks in high fat diet fed rats; while Leopoldo et al. found no change in protein expression (Leopoldo et al., 2011). In a DIO Psammomys obesus Gerbil model, Sahraoui et al. found that mRNA and protein expression levels of CACNAC1 was decreased after 16 weeks (Sahraoui et al., 2016), consistent with a contribution of defective I_{Ca, L} channel gating/trafficking in obesity.

Obesity-related altered functional expression of intracellular Ca release channels, such as ryanodine receptors (RyRs) or inositol triphosphate receptors (IP₃R), involved in regulating the intracellular Ca concentration are also likely to contribute to the pathogenesis of arrhythmias (Table 2). In this context Dincer et al., previously reported a significantly increased phosphorylation of ventricular RyR type 2 (RyR2) in a dog model of metabolic syndrome while RyR2 mRNA and protein expression remained essentially unchanged (Dincer et al., 2006). Similar results were also observed in obese Gerbils (Sahraoui et al., 2016). However, in rabbits that were fed a cholesterol-rich diet for 12 weeks, mRNA levels of RyR were decreased (Luo et al., 2004). Similar results were also seen in db/db obese mouse myocytes (Pereira et al., 2006). In rats fed a high-fat diet for 8 weeks, obesity increased mRNA levels of ventricular expression of RyR2 (Ashrafi et al., 2016), while Leopoldo et al. showed increased expression in obese rats at 30 weeks and no change of expression at 15 and 45 weeks (Lima-Leopoldo et al., 2013). These contrasting data further emphasize the inconsistencies between studies and supports the notion that early altered transcript expression may not reflect the impact of long-term obesity.

Recently in a mouse model of abnormal cardiac lipid accumulation or cardiac lipid overload, mitochondrial oxidative stress was shown to promote increased sarcoplasmic reticulum (SR) Ca leak by oxidizing RyR2 (Joseph et al., 2016). Increased mRNA expression of the RyR2 has also been shown in a rat model of obesity (Lima-Leopoldo et al., 2008; Leopoldo et al., 2011), and further underscores an important role for RYRs in ventricular arrhythmias associated with metabolic disorders. In a db/db obese type 2 diabetic mouse model, cardiac abnormalities were associated with reduced SR Ca release and reduced expression of ventricular RyRs (Pereira et al., 2006). These outcomes support the notion that the molecular mechanisms and/or signaling pathways that underlie the contribution of RyR to arrhythmogenesis in models of metabolic disorders may differ depending on the pathology.

The role of IP₃/IP₃R in diet related obesity is poorly understood. However, in one study that utilized ventricular cardiomyocytes isolated from an ob/ob mouse model of obesity and type 2 diabetes, it was demonstrated that insulin increased IP₃ concentration while the expression of type 1 and type 2 IP₃R was unaltered compared to wild-type controls (Fauconnier et al., 2005). These results further emphasize the importance of studies that will assess the contribution of RyR/IP₃R signaling pathways to altered Ca regulation in obesity and metabolic disorders.

The Ca²⁺-ATPase pump (SERCA), its inhibitor phospholamban (PLB) (Hicks et al., 1979; Inui et al., 1986), and the Na-Ca exchanger (NCX) are important for, respectively, the SR Ca re-uptake and Ca extrusion, and have been investigated in obese animal models (Table 2). Ashrafi et al. reported increased mRNA levels of SERCA2a and NCX in high fat diet fed rats (Ashrafi et al., 2016). However, the expression of PLB was not investigated in these studies. In rats fed a high-fat diet for 15 weeks, Leopoldo et al. showed increased mRNA levels of SERCA2a and PLB (Lima-Leopoldo et al., 2008). Ashrafi et al. also reported increased mRNA levels of SERCA2a after 8 weeks in high fat diet fed rats (Ashrafi et al., 2016). While Leopoldo et al., found no change in protein expression after 8 weeks (Leopoldo et al., 2011), several other studies showed increased protein expression consistent with increased SERCA2a activity in ventricular myocytes (Xie et al., 2016). In a DIO Psammomys obesus Gerbil model, Sahraoui et al. found that mRNA and protein expression levels of SERCA2a decreased after 16 weeks (Sahraoui et al., 2016). Reduced expression of SERCA2a has also been described in a TG-PPAR-γ mouse model of cardiac lipid overload (Joseph et al., 2016) consistent with a pivotal role of Ca handling defects in obesity and metabolic disorders. In hypercholesterolemic rabbits, mRNA levels of ventricular SERCA2 are significantly lower at 12 weeks when compared to rabbits fed normal chow (Luo et al., 2004). However, the expression of SERCA is increased by FFAs in atrial myocytes (Lin et al., 2014), suggesting that the molecular mechanisms that underlie Ca regulation and predispose to arrhythmias in metabolic disorders may be time- and tissue-dependent.

There have also been contrasting data on the expression of NCX in obesity. mRNA levels of ventricular NCX are either increased at 12 weeks in hypercholesterolemic rabbits (Luo et al., 2004), and unchanged (Lima-Leopoldo et al., 2008), or decreased at 15 weeks (Lima-Leopoldo et al., 2013) but increased at 8 weeks (Ashrafi et al., 2016) and 30 weeks (Lima-Leopoldo et al., 2013) in a rat model of obesity. Furthermore, Ca efflux through NCX was also increased in myocytes isolated from a db/db model of obese type 2 diabetic mice (Pereira et al., 2006) in line with reduced SR Ca load and contractility in this model. In contrast, in a rat obese model the NCX current, (I_NCX) was not significantly different from control non-obese rats (Ricci et al., 2006). These results further support the inconsistencies between studies and provide strong evidence for additional studies in more relevant animal models.

Furthermore, there are also discrepancies associated with modulation of NCX function by individual obesity biomarkers. For example, in adult guinea pig ventricular myocytes, insulin was found to increase I_NCX in both freshly isolated and cultured myocytes (Villa-Abrille et al., 2008). In contrast, Lin et al., demonstrated that leptin treated atrial myocytes cells display reduced Ca transient and SR content largely due to reduced I_NCX. These observations further emphasize the modulation of the delicate balance of intracellular Ca homeostasis by obesity.

Taken together, the inconsistencies between the mRNA and protein expression and functional current data further demonstrate the importance of electrophysiological experiments in atrial and ventricular myocytes isolated from DIO models. Thus, further studies will be required to determine the pathophysiology of cardiac Ca channels, its auxiliary subunits, and Ca handling proteins in the settings of obesity and associated metabolic syndrome.

The transient outward K current, I_to is also an important contributor to cardiac AP waveform, and contributes prominently to the initial and early repolarization phase of atria (Workman et al., 2001; Virag et al., 2011) and ventricular AP (Rosati et al., 2001). In heart, I_to expression has been shown to be greater in the atria when compared to ventricular myocytes, which is likely to underlie the abbreviated atrial APD (Calloe et al., 2011). I_to is defined by two distinct components namely I_{to, fast} and I_{to, slow} (Patel and Campbell, 2005). I_{to, fast} is generated by a combination of K_v4.2 and K_v4.3 channels, which are encoded by KCND2 and KCND3 genes, respectively. The auxiliary and regulatory subunits, KChIP2 and dipeptidyl-aminopeptidase-like protein 6 (DPP6), when co-assembled with K_v4.3, modulates its trafficking and gating properties (Radicke et al., 2005), to generate currents that closely resemble I_{to, fast}.

The slow component of the transient outward current, referred to as I_{to, slow}, is conducted by the voltage gated K channel, K_v1.4, in the heart (Walsh et al., 2001; Akar et al., 2004; Patel and Campbell, 2005). The K_v1.4 subunit is encoded by the KCNA4 gene, and in contrast to I_{to, fast}, it is marked by a fast activation, slower inactivation and slower recovery from inactivation. A transgenic mouse model lacking I_{to, fast} and I_{to, slow} displayed reduced I_to density, action potential prolongation and ventricular tachycardia (Guo et al., 2000), consistent with a fundamental contribution of I_to to arrhythmias. Transgenic lipotoxic models, such as the MHC-FATP mouse, also displays reduced I_{to, fast} density (Marionneau et al., 2008).

The atrial specific I_Kur (or K_v1.5 encoded by KCNA5), is marked by a fast activation, outward rectification, and a relatively slow inactivation (Bhuyan and Seal, 2017). K_v1.5 channel subunits exist in macromolecular complexes with its auxiliary β-subunit (K_vβ1.2) which is crucial for its gating and trafficking properties and also its sensitivity to metabolic disorders (Tipparaju et al., 2012). The co-assembly of K_v1.5 with K_vβ1.2 subunits has also been shown to underlie I_Kur current in the human atrium (Christophersen et al., 2013). Pro-inflammatory cytokine studies have provided some evidence for a potential contribution of atrial K_v1.5 function in obesity. I_Kur density was shown to be decreased in myocytes isolated from mice treated with TNFα, although mRNA and protein expression levels of I_Kur channel subunits were not altered (Grandy and Fiset, 2009), consistent with TNFα-mediated gating and/or trafficking defects of I_Kur subunits.

The cardiac delayed rectifier K current, or I_K composed of I_Kr and I_Ks, is an important regulator of repolarization (Sanguinetti and Jurkiewicz, 1990). In the human heart, I_Kr exists as a tetramer composed of the human ether-á-go-go-related gene (or hERG), 1a and 1b pore-forming or α subunits (Puckerin et al., 2016). There have also been reports that I_Kr is generated by a combination of hERG and the MinK-related peptide 1 protein (Abbott et al., 1999) suggesting that the precise molecular composition of cardiac I_Kr is still a matter of debate. I_Ks is mediated by heteromeric channel complexes composed of pore-forming KCNQ1 (K_v7.1) subunits and the auxiliary regulatory KCNE1 subunits (Haitin et al., 2008). Channels that conduct I_Kr are fast activating (Sanguinetti and Jurkiewicz, 1991) and I_Ks channels are marked by slowly activating and inactivating kinetics (Aromolaran et al., 2014), and thus underlie their important contribution to repolarization in the late stages of the cardiac AP.

While there have been some studies on the electrophysiological effects of obesity-related molecular processes on some cardiac voltage-gated channels (O'Connell et al., 2015; Aromolaran et al., 2016), there is still a lack of studies that have assessed the functional properties of I_K, I_Kr, and I_Ks in obese animal models, most likely due to the lack of expression of these channels in commonly used rodent models (Killeen et al., 2008; Aromolaran et al., 2014).

Previously we have demonstrated that male and female guinea pigs showed significant weight gain and elevated levels of total cholesterol and triglycerides, typically associated with significant weight gain and/or obesity within 50 days on a high-fat diet (HFD) (Aromolaran et al., 2016). Therefore, perturbations including hyperlipidemia (Altarejos et al., 2005), that alter cholesterol levels is likely to have significant implications on the function of ion channels in the lipid bilayer. We also found that obese atrial myocytes displayed an abbreviated APD, and had a significantly larger I_K density compared to the low-fat diet controls. A similar picture was revealed with acute exposure of atrial myocytes to the saturated FFA, palmitic acid (PA), which also increased I_Kr and I_Ks densities in human embryonic kidney 293 (HEK293) cells, demonstrating that removal of PA from HFD is likely to prevent arrhythmic events in obese patients. Compared to PA, the unsaturated FFA, oleic acid (OA), prolonged atrial APD, depressed I_Kr density, and minimally increased I_Ks in HEK293 cells suggesting that increasing OA may prevent atrial arrhythmias in obese patients. The implication of these observations in ventricles is currently unknown; nevertheless, a previous report by Haim et al. have shown that palmitate reduced cardiac contractility, shortened APD, and increased the density of voltage-gated K channels in mouse ventricular myocytes (Haim et al., 2010). Therefore, further studies are required to confirm these in relevant small animal (such as the guinea pig), obese models and to elucidate the significance of these changes to targeted therapeutic interventions in obese patients that present with arrhythmic events.

In a DIO rat model mRNA expression of ERG subunit is significantly reduced (Ashrafi et al., 2016), suggesting that ERG/I_Kr functional expression is altered in obesity, and therefore may contribute to the LQT phenotype seen in clinically obese patients. Since I_Kr does not contribute prominently to repolarization in rat ventricular myocytes (Pond et al., 2000; Aromolaran et al., 2014), vigorous studies in relevant animal models of metabolic disorders utilizing protein and electrophysiological assays will be required to fully understand the role of I_Kr in DIO-related arrhythmias. In this context, Caillier et al. previously showed that guinea pigs with metabolic syndrome displayed prolonged APD in response to the drugs dofetilide and chromanol 293B which are known blockers of I_Kr and I_Ks respectively (Caillier et al., 2012). Furthermore, a recent report by Kannankeril et al. also demonstrated that obese patients display a higher susceptibility to drug-induced QT prolongation mediated by an I_Kr blocker Ibutilide (Kannankeril et al., 2011). These observations suggest that: (1) ventricular I_K is modulated in obesity and these effects are further exacerbated by drugs, which may lead to increased likelihood of fatal arrhythmias; (2) demonstrate the relevance of guinea pig as an model to investigate the pathogenesis of obesity-induced arrhythmias.

I_Kr is also reduced in diabetes (Zhang et al., 2011), which may contribute to lethal ventricular arrhythmias and sudden cardiac death in diabetic patients (Eranti et al., 2016). Hyperglycemia (Gateva et al., 2017), and diabetes (de Simone et al., 2017) are also highly associated with obesity. However, it is currently unknown whether the molecular changes of cardiac ion channels in diabetic patients without obesity recapitulate that in obesity. Therefore, identification of unique pathways that will distinguish between these disease mechanisms and the impact on ion channel expression is likely to have significant implications for novel therapies for arrhythmias associated with metabolic disorders.

Altered levels of proinflammatory cytokines are also associated with prolongation of QT interval in patients with inflammatory diseases (Adlan et al., 2015; Lazzerini et al., 2017), suggesting that cytokines may negatively regulate ion channel function, normal sinus rhythm, and predispose to arrhythmias. Obesity is also associated with elevated levels of inflammatory cytokines (Schmidt et al., 2015), which may also contribute to cardiomyopathies of obesity. Wang et al. previously demonstrated that TNF-α decreased hERG current density in HEK293 cells, significantly depressed I_Kr density and prolonged APD in canine ventricular myocytes, primarily due to changes in reactive oxygen species (ROS) (Wang et al., 2004). Obesity associated hyperglycemia has also been shown to reduced surface expression of hERG channel subunits and hERG1/I_Kr density which is rescued by insulin (Zhang et al., 2006), suggesting that ROS is an important regulator of hERG1/I_Kr functional expression. Recent reports have also demonstrated that the expression of the chaperone protein, heat shock protein 90 (Hsp90) plays a role in ERG channel trafficking defects seen in hyperglycemia (Shi et al., 2015), suggesting that in patients altered expression of ROS and Hsp90 may contribute to cardiomyopathies of obesity. Despite these results whether and how ROS influences Hsp90 function in metabolic disorders is poorly understood and therefore needs further investigation.

Similar to other reviews (Nerbonne and Kass, 2005; Schmitt et al., 2014; Chen-Izu et al., 2015; Grandi et al., 2017) we have emphasized the important role of ion channels and Ca handling proteins in the pathogenesis of arrhythmias but more importantly to obesity and metabolic syndrome. Due to the contribution of obesity and metabolic disorders to the increasing prevalence of arrhythmias, this review uniquely discusses the latest knowledge related to the molecular mechanisms of how obesity may contribute to the electrical remodeling that underlie arrhythmias such as LQTS and AF. This review also highlights the contribution of altered functional expression of I_K to the remodeling of electrical activity in male and female HFD-induced obese adult guinea pigs and establishes the emerging role of guinea pigs as an important pre-clinical model (Aromolaran et al., 2016). Finally, we have confirmed the urgent need for more translational studies in obese patients, which may reveal key ionic mechanisms that may be targeted for therapeutic interventions and help reduce arrhythmias.

For the purpose of this review article, we used pubmed central, embase, and google scholar databases to search for studies published in English language. Searches were not limited by date restrictions. Searches were free texts and included the following keywords: “high fat diet,” “ion channel,” “obesity,” “metabolic syndrome,” “LQTS,” “atrial fibrillation,” “dyslipidemia,” “cardiac calcium channel,” “cardiac potassium channel,” “cardiac sodium channel,” “Ca handling proteins,” “NCX,” “pro-inflammatory cytokines,” “insulin,” “leptin,” “hyperglycemia,” and “hyperuricemia.” Despite our thorough search, it is also possible that we may have missed relevant studies including non–English language studies. While we have comprehensively focused on the role of voltage gated ion channels and Ca handling proteins as the molecular basis of arrhythmias, there are other entities such as cardiac gap junctions and connexins that have been shown to be altered in obesity (Axelsen et al., 2015; Takahashi et al., 2016) and may also contribute to cardiomyopathies of metabolic disorders. However, the paucity of such studies makes it impossible to draw reliable conclusions and therefore were not discussed in this review.

Since this review describes altered electrical remodeling following short-term high-fat diet and progressive weight gain and/or obesity and not long-term obesity our analysis of the impact of electrical remodeling as an important contributor to cardiomyopathies should be interpreted with caution. Our interpretation is that early relative functional identity of distinct ionic channels and Ca handling proteins may change with severity of obesity. There has also been a paucity of human studies regarding the role of ion channels and Ca handling proteins in the development of obesity-induced arrhythmias; however we have highlighted relevant ones here. While the study is not a comprehensive review of the animal models of obesity or arrhythmias, we discuss the important animal models in the literature and highlight the ability of guinea pigs to be a more appropriate pre-clinical model.

Studies addressing the molecular and functional basis of arrhythmogenesis in relevant animal models of metabolic disorders are just emerging and significant gaps in knowledge remain, warranting further research investigations. Despite these advances, it is still not clear whether common mechanisms underlie altered regulation of ion channels and Ca handling proteins due to inherited mutations or acquired in obesity, diabetes, metabolic syndrome and predispose to fatal arrhythmias. Furthermore, whether weight loss and comorbid management can modulate ion channel expression and function in ways that will help to improve CVD outcomes is poorly understood. Recently, exercise training coupled with caloric restriction has been shown to prevent cardiac dysfunction in obese rats through modulation of Ca handling proteins (Paulino et al., 2010). However, these exciting results in animal studies have not translated into positive results in human clinical trials suggesting that there is considerable room for advances in targeted treatment approaches. One reason for the failure to translate to pre-clinical studies could be due to a paucity of metabolic studies utilizing human myocytes. Therefore, the effort to develop more realistic pre-clinical models that avoid the use of non-human tissues and which instead incorporates cardiomyocytes derived directly from human stem cells from obese patients is likely to provide crucial insight that will advance our knowledge of the association between metabolic disorders and arrhythmias. Although, to our knowledge, there have been no studies using human stem cells as a research paradigm for studying ion channel remodeling in metabolic diseases. Human stem cells, both embryonic stem cells and induced pluripotent, have been successfully used as models of arrhythmias (Priori et al., 2013; Moreau et al., 2017), suggesting that human stem cells could be a realistic pre-clinical human model for studies focused on arrhythmias underlain by metabolic disorders.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.