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Atrial fibrillation (AF) is the most common sustained arrhythmia encountered in humans and is a significant source of morbidity and mortality. Despite its prevalence, our mechanistic understanding is incomplete, the therapeutic options have limited efficacy, and are often fraught with risks. A better biological understanding of AF is needed to spearhead novel therapeutic avenues. Although “natural” AF is nearly nonexistent in most species, animal models have contributed significantly to our understanding of AF and some therapeutic options. However, the impediments of animal models are also apparent and stem largely from the differences in basic physiology as well as the complexities underlying human AF; these preclude the creation of a “perfect” animal model and have obviated the translation of animal findings. Herein, we review the vast array of AF models available, spanning the mouse heart (weighing 1/1000th of a human heart) to the horse heart (10× heavier than the human heart). We attempt to highlight the features of each model that bring value to our understanding of AF but also the shortcomings and pitfalls. Finally, we borrowed the concept of a SWOT analysis from the business community (which stands for strengths, weaknesses, opportunities, and threats) and applied this introspective type of analysis to animal models for AF. We identify unmet needs and stress that is in the context of rapidly advancing technologies, these present opportunities for the future use of animal models.

Rationale: Autoantibodies directed against the second extracellular loop of the cardiac β1-adrenergic receptor (β1-AR) are thought to contribute to the pathogenesis of dilated cardiomyopathy (DCM) and Chagas heart disease. Various approaches have been used to detect such autoantibodies; however, the reported prevalence varies largely, depending on the detection method used. Objective: We analyzed sera from 167 DCM patients (ejection fraction Methods and Results: The novel assay was designed according to the currently most reliable anti-TSH receptor antibody-ELISA used to diagnose Graves disease (“third-generation assay”) and also detects the target antibodies by competition with a specific monoclonal anti–β1-AR antibody (β1-AR MAb) directed against the functionally relevant β1-AR epitope. Anti–β1-AR antibodies were detected in ≈60% of DCM patients and in ≈8% of healthy volunteers using the same cutoff values. The prevalence of these antibodies was 17% in patients with hypertensive heart disease. Anti–β1-AR antibody titers (defined as inhibition of β1-AR MAb-binding) were no longer detected after depleting sera from IgG antibodies by protein G adsorption. In contrast, a previously used ELISA conducted with a linear 26-meric peptide derived from the second extracellular β1-AR loop yielded a high number of false-positive results precluding any specific identification of DCM patients. Conclusions: We established a simple and efficient screening assay detecting disease-relevant β1-AR autoantibodies in patient sera yielding a high reproducibility also in high throughput screening. The assay was validated according to “good laboratory practice” and can serve as a companion biodiagnostic assay for the development and evaluation of antibody-directed therapies in antibody-positive heart failure.

Atrial fibrillation is associated with increased expression of ventricular myosin isoforms in atrial myocardium, regarded as part of a dedifferentiation process. Whether reexpression of ventricular isoforms in atrial fibrillation is restricted to transcripts encoding for contractile proteins is unknown. Therefore, this study compares atrial mRNA expression in patients with permanent atrial fibrillation to atrial mRNA expression in patients with sinus rhythm and to ventricular gene expression using Affymetrix U133 arrays. In atrial myocardium, we identified 1434 genes deregulated in atrial fibrillation, the majority of which, including key elements of calcium-dependent signaling pathways, displayed downregulation. Functional classification based on Gene Ontology provided the specific gene sets of the interdependent processes of structural, contractile, and electrophysiological remodeling. In addition, we demonstrate for the first time a prominent upregulation of transcripts involved in metabolic activities, suggesting an adaptive response to increased metabolic demand in fibrillating atrial myocardium. Ventricular-predominant genes were 5 times more likely to be upregulated in atrial fibrillation (174 genes upregulated, 35 genes downregulated), whereas atrial-specific transcripts were predominantly downregulated (56 genes upregulated, 564 genes downregulated). Overall, in fibrillating atrial myocardium, functional classes of genes characteristic of ventricular myocardium were found to be upregulated (eg, metabolic processes), whereas functional classes predominantly expressed in atrial myocardium were downregulated (eg, signal transduction and cell communication). Therefore, dedifferentiation with adoption of a ventricular-like signature is a general feature of the fibrillating atrium.

Altered myocardial repolarization is one of the important substrates of ventricular tachycardia and fibrillation. The influence of rare gene variants on repolarization is evident in familial long QT syndrome. To investigate the influence of common gene variants on the QT interval we performed a linkage disequilibrium based SNP association study of four candidate genes. Using a two-step design we analyzed 174 SNPs from the KCNQ1, KCNH2, KCNE1, and KCNE2 genes in 689 individuals from the population-based KORA study and 14 SNPs with results suggestive of association in a confirmatory sample of 3277 individuals from the same survey. We detected association to a gene variant in intron 1 of the KCNQ1 gene (rs757092, +1.7 ms/allele, P =0.0002) and observed weaker association to a variant upstream of the KCNE1 gene (rs727957, +1.2 ms/allele, P =0.0051). In addition we detected association to two SNPs in the KCNH2 gene, the previously described K897T variant (rs1805123, −1.9 ms/allele, P =0.0006) and a gene variant that tags a different haplotype in the same block (rs3815459, +1.7 ms/allele, P =0.0004). The analysis of additive effects by an allelic score explained a 10.5 ms difference in corrected QT interval length between extreme score groups and 0.95% of trait variance ( P

Abstract Membrane current abnormalities have been described in human heart failure. To determine whether similar current changes are observed in a large animal model of heart failure, we studied dogs with pacing-induced cardiomyopathy. Myocytes isolated from the midmyocardium of 13 dogs with heart failure induced by 3 to 4 weeks of rapid ventricular pacing and from 16 nonpaced control dogs did not differ in cell surface area or resting membrane potential. Nevertheless, action potential duration (APD) was significantly prolonged in myocytes isolated from failing ventricles (APD at 90% repolarization, 1097±73 milliseconds [failing hearts, n=30] versus 842±56 milliseconds [control hearts, n=25]; P 2+ current and whole-cell Na + current did not differ in cells from failing and from control hearts, but significant differences in K + currents were observed. The density of the inward rectifier K + current (I K1 ) was reduced in cells from failing hearts at test potentials below −90 mV (at −150 mV, −19.1±2.2 pA/pF [failing hearts, n=18] versus −32.2±5.1 pA/pF [control hearts, n=15]; P K1 was also reduced in cells from failing hearts (at −60 mV, 1.7±0.2 pA/pF [failing hearts] versus 2.5±0.2 pA/pF [control hearts]; P 2+ -independent transient outward current (I to ) was dramatically reduced in myocytes isolated from failing hearts compared with nonfailing control hearts (at +80 mV, 7.0±0.9 pA/pF [failing hearts, n=20] versus 20.4±3.2 pA/pF [control hearts, n=15]; P to kinetics or single-channel conductance. A reduction in the number of functional I to channels was demonstrated by nonstationary fluctuation analysis (0.4±0.03 channels per square micrometer [failing hearts, n=5] versus 1.2±0.1 channels per square micrometer [control hearts, n=3]; P to by 4-aminopyridine in control myocytes decreased the notch amplitude and prolonged the APD. Current clamp–release experiments in which current was injected for 8 milliseconds to reproduce the notch sufficed to shorten the APD significantly in cells from failing hearts. These data support the hypothesis that downregulation of I to in pacing-induced heart failure is at least partially responsible for the action potential prolongation. Because the repolarization abnormalities mimic those in cells isolated from failing human ventricular myocardium, canine pacing-induced cardiomyopathy may provide insights into the development of repolarization abnormalities and the mechanisms of sudden death in patients with heart failure.

Physical and emotional stress is accompanied by release of stress hormones such as the glucocorticoid cortisol. This hormone upregulates the serum- and glucocorticoid-inducible kinase (SGK)1, which in turn stimulates I Ks , a slow delayed rectifier potassium current that mediates cardiac action potential repolarization. Mutations in I Ks channel α (KCNQ1, KvLQT1, Kv7.1) or β (KCNE1, IsK, minK) subunits cause long QT syndrome (LQTS), an inherited cardiac arrhythmia associated with increased risk of sudden death. Together with the GTPases RAB5 and RAB11, SGK1 facilitates membrane recycling of KCNQ1 channels. Here, we show altered SGK1-dependent regulation of LQTS-associated mutant I Ks channels. Whereas some mutant KCNQ1 channels had reduced basal activity but were still activated by SGK1, currents mediated by KCNQ1(Y111C) or KCNQ1(L114P) were paradoxically reduced by SGK1. Heteromeric channels coassembled of wild-type KCNQ1 and the LQTS-associated KCNE1(D76N) mutant were similarly downregulated by SGK1 because of a disrupted RAB11-dependent recycling. Mutagenesis experiments indicate that stimulation of I Ks channels by SGK1 depends on residues H73, N75, D76, and P77 in KCNE1. Identification of the I Ks recycling pathway and its modulation by stress-stimulated SGK1 provides novel mechanistic insight into potentially fatal cardiac arrhythmias triggered by physical or psychological stress.

Abstract —Pacing-induced heart failure in the dog recapitulates many of the electrophysiological and hemodynamic abnormalities of the human disease; however, the mechanisms underlying altered Ca 2+ handling have not been investigated in this model. We now show that left ventricular midmyocardial myocytes isolated from dogs subjected to 3 to 4 weeks of rapid pacing have prolonged action potentials and Ca 2+ transients with reduced peaks, but durations ≈3-fold longer than controls. To discriminate between action potential effects on Ca 2+ kinetics and direct changes in Ca 2+ regulatory processes, voltage-clamp steps were used to examine the time constant for cytosolic Ca 2+ removal (τ Ca ). τ Ca was prolonged by just 35% in myocytes from failing hearts after fixed voltage steps in physiological solutions (τ Ca control, 216±25 ms, n=17; τ Ca failing, 292±23 ms, n=22; P + /Ca 2+ exchange was eliminated (τ Ca control, 282±30 ms, n=13; τ Ca failing, 576±83 ms, n=11; P 2+ uptake and a greater dependence on Na + /Ca 2+ exchange for cytosolic Ca 2+ removal was confirmed by inhibiting SR Ca 2+ ATPase with cyclopiazonic acid, which slowed Ca 2+ removal more in control than in failing myocytes. β-Adrenergic stimulation of SR Ca 2+ uptake in cells from failing hearts sufficed only to accelerate τ Ca to the range of unstimulated controls. Protein levels of SERCA2a, phospholamban, and Na + /Ca 2+ exchanger revealed a pattern of changes qualitatively similar to the functional measurements; SERCA2a and phospholamban were both reduced in failing hearts by 28%, and Na + /Ca 2+ exchange protein was increased 104% relative to controls. Thus, SR Ca 2+ uptake is markedly downregulated in failing hearts, but this defect is partially compensated by enhanced Na + /Ca 2+ exchange. The alterations are similar to those reported in human heart failure, which reinforces the utility of the pacing-induced dog model as a surrogate for the human disease.

See related article, pages 1384–1393 In this issue Tellez et al1 give a detailed analysis of differentially expressed genes regulating excitation and conduction in the sinoatrial node (SAN) in rabbit. Quantitative PCR and in situ hybridization, as well as action potential recordings, enabled the investigators to assign specific patterns to central and peripheral regions of SAN, and to compare these with the corresponding properteis of atrial tissue. Cluster analysis revealed that the SAN transcript profile is significantly different from that of atrial muscle. More importantly, there are apparent isoform switches on moving from atrial muscle to the SAN center: RYR2 to RYR3, Nav1.5 to Nav1.1, Cav1.2 to Cav1.3 and Kv1.4 to Kv4.2. In this context the transcript profile of the SAN periphery represents an intermediate pattern between that of central SAN and atrial muscle. Differential expression of a variety of genes has been demonstrated in anatomically distinct regions of the heart, for example atrial …

See related article, pages 386–393 Reduction in density of the cardiac calcium-independent transient outward potassium current ( I to1) is one of the most consistent findings in electrophysiological remodeling observed in animal models of left ventricular hypertrophy and failure as well as in the human heart under pathological conditions.1 Initially attributed to downregulation of the mRNA of pore-forming Kv4 subunits,2 a more complex picture emerged over time as several auxiliary subunits were identified, offering new potential regulatory elements for controlling I to1 density. KChIP2 is a dazzling member of this group of beta-subunits which assembles with pore-forming Kv4 subunits in 4:4 complexes to produce native I to1 channels.3 Several different KChIP2 splice variants are expressed in the human heart and the major KChIP2 isoform (KChIP2c) has been found to boost I to1 current density and to speed up recovery from inactivation, thereby recapitulating several, albeit not all, features of native I to1 channels.4 Importantly, myocytes isolated from KChIP2 knock-out mice exhibited a complete, selective loss of I to1, whereas an ≈50% reduction was observed in cardiomyocytes from heterozygous animals, indicating that KChIP2 can quantitatively regulate I to1.5 In line with this finding, KChIP2 mRNA typically distributes across the myocardial wall in a fashion paralleling the gradient of the transient outward current with fast recovery from inactivation encoded by Kv4 channels, with largest concentrations in subepicardium (EPI) and smallest in subendocardium (ENDO) in human and canine hearts.6 This suggests that KChIP2 is the limiting factor in cell surface expression of I to1 channels, rather than the pore-forming Kv4 subunits which are expressed at equal levels …