Rhythmic and effective cardiac contraction depends on appropriately timed generation and spread of cardiac electrical activity. The basic cellular unit of such activity is the action potential, which is shaped by specialized proteins (channels and transporters) that control the movement of ions across cardiac cell membranes in a highly regulated fashion. Cardiac disease modifies the operation of ion channels and transporters in a way that promotes the occurrence of cardiac rhythm disturbances, a process called "arrhythmogenic remodeling." Arrhythmogenic remodeling involves alterations in ion channel and transporter expression, regulation and association with important protein partners, and has important pathophysiological implications that contribute in major ways to cardiac morbidity and mortality. We review the changes in ion channel and transporter properties associated with three important clinical and experimental paradigms: congestive heart failure, myocardial infarction, and atrial fibrillation. We pay particular attention to K+, Na+, and Ca2+ channels; Ca2+ transporters; connexins; and hyperpolarization-activated nonselective cation channels and discuss the mechanisms through which changes in ion handling processes lead to cardiac arrhythmias. We highlight areas of future investigation, as well as important opportunities for improved therapeutic approaches that are being opened by an improved understanding of the mechanisms of arrhythmogenic remodeling.

The various cardiac regions have specific action potential properties appropriate to their electrical specialization, resulting from a specific pattern of ion-channel functional expression. The present study addressed regionally defined differential ion-channel expression in the non-diseased human heart with a genomic approach. High-throughput real-time RT-PCR was used to quantify the expression patterns of 79 ion-channel subunit transcripts and related genes in atria, ventricular epicardium and endocardium, and Purkinje fibres isolated from 15 non-diseased human donor hearts. Two-way non-directed hierarchical clustering separated atria, Purkinje fibre and ventricular compartments, but did not show specific patterns for epicardium versus endocardium, nor left-versus right-sided chambers. Genes that characterized the atria (versus ventricles) included Cx40, Kv1.5 and Kir3.1 as expected, but also Cav1.3, Cav3.1, Cavα2δ2, Navβ1, TWIK1, TASK1 and HCN4. Only Kir2.1, RyR2, phospholamban and Kv1.4 showed higher expression in the ventricles. The Purkinje fibre expression-portrait (versus ventricle) included stronger expression of Cx40, Kv4.3, Kir3.1, TWIK1, HCN4, ClC6 and CALM1, along with weaker expression of mRNA encoding Cx43, Kir2.1, KChIP2, the pumps/exchangers Na + ,K + -ATPase, NCX1, SERCA2, and the Ca 2+ -handling proteins RYR2 and CASQ2. Transcripts that were more strongly expressed in epicardium (versus endocardium) included Cav1.2, KChIP2, SERCA2, CALM3 and calcineurin-α. Nav1.5 and Navβ1 were more strongly expressed in the endocardium. For selected genes, RT-PCR data were confirmed at the protein level. This is the first report of the global portrait of regional ion-channel subunit-gene expression in the non-diseased human heart. Our data point to significant regionally determined ion-channel expression differences, with potentially important implications for understanding regional electrophysiology, arrhythmia mechanisms, and responses to ion-channel blocking drugs. Concordance with previous functional studies suggests that regional regulation of cardiac ion-current expression may be primarily transcriptional.

Background-Valvular heart disease (VHD), which often leads to atrial fibrillation (AF), and AF both cause ion-channel remodeling. We evaluated the ion-channel gene expression profile of VHD patients, in permanent AF (AF-VHD) or in sinus rhythm (SR-VHD), in comparison with patients without AF or VHD, respectively. Methods and Results-We used microarrays containing probes for human ion-channel and Ca 2ϩ -regulator genes to quantify mRNA expression in atrial tissues from 7 SR-VHD patients and 11 AF-VHD patients relative to 11 control patients in SR without structural heart disease (SR-CAD). From the data set, we selected for detailed analysis 59 transcripts expressed in the human heart. SR-VHD patients differentially expressed 24/59 ion-channel and Ca 2ϩ -regulator transcripts. There was significant overlap between VHD groups, with 66% of genes altered in SR-VHD patients being similarly modified in AF-VHD. Statistical differences between the AF-and SR-VHD groups identified the specific molecular portrait of AF, which involved 12 genes that were further confirmed by real-time reverse transcription-polymerase chain reaction. For example, phospholamban, the ␤-subunit MinK (KCNE1) and MIRP2 (KCNE3), and the 2-pore potassium channel TWIK-1 were upregulated in AF-VHD compared with SR-VHD, whereas the T-type calcium-channel Cav3.1 and the transient-outward potassium channel Kv4.3 were downregulated. Two-way hierarchical clustering separated SR-VHD from AF-VHD patients. AF-related changes in L-type Ca 2ϩ -current and inward-rectifier current were confirmed at protein and functional levels. Finally, for 13 selected genes, SR restoration reversed ion-channel remodeling. Conclusions-VHD

No abstract

Background— We have previously linked hereditary progressive cardiac conduction defect (hereditary Lenègre’s disease) to a loss-of-function mutation in the gene encoding the main cardiac Na + channel, SCN5A . In the present study, we investigated heterozygous Scn5a -knockout mice ( Scn5a +/− mice) as a model for hereditary Lenègre’s disease. Methods and Results— In Scn5a +/− mice, surface ECG recordings showed age-related lengthening of the P-wave and PR- and QRS-interval duration, coinciding with previous observations in patients with Lenègre’s disease. Old but not young Scn5a +/− mice showed extensive fibrosis of their ventricular myocardium, a feature not seen in wild-type animals. In old Scn5a +/− mice, fibrosis was accompanied by heterogeneous expression of connexin 43 and upregulation of hypertrophic markers, including β-MHC and skeletal α-actin. Global connexin 43 expression as assessed with Western blots was similar to wild-type mice. Decreased connexin 40 expression was seen in the atria. Using pangenomic microarrays and real-time PCR, we identified in Scn5a +/− mice an age-related upregulation of genes encoding Atf3 and Egr1 transcription factors. Echocardiography and hemodynamic investigations demonstrated conserved cardiac function with aging and lack of ventricular hypertrophy. Conclusions— We conclude that Scn5a +/− mice convincingly recapitulate the Lenègre’s disease phenotype, including progressive impairment with aging of atrial and ventricular conduction associated with myocardial rearrangements and fibrosis. Our work provides the first demonstration that a monogenic ion channel defect can progressively lead to myocardial structural anomalies.

Abstract-Although electrophysiological remodeling occurs in various myocardial diseases, the underlying molecular mechanisms are poorly understood. cDNA microarrays containing probes for a large population of mouse genes encoding ion channel subunits ("IonChips") were developed and exploited to investigate remodeling of ion channel transcripts associated with altered thyroid status in adult mouse ventricle. Functional consequences of hypo-and hyperthyroidism were evaluated with patch-clamp and ECG recordings. Hypothyroidism decreased heart rate and prolonged QTc duration. Opposite changes were observed in hyperthyroidism. Microarray analysis revealed that hypothyroidism induces significant reductions in KCNA5, KCNB1, KCND2, and KCNK2 transcripts, whereas KCNQ1 and KCNE1 expression is increased. In hyperthyroidism, in contrast, KCNA5 and KCNB1 expression is increased and KCNQ1 and KCNE1 expression is decreased. Real-time RT-PCR validated these results. Consistent with microarray analysis, Western blot experiments confirmed those modifications at the protein level. Patch-clamp recordings revealed significant reductions in I to,f and I K,slow densities, and increased I Ks density in hypothyroid myocytes. In addition to effects on K ϩ channel transcripts, transcripts for the pacemaker channel HCN2 were decreased and those encoding the ␣1C Ca 2ϩ channel (CaCNA1C) were increased in hypothyroid animals. The expression of Na ϩ , Cl Ϫ , and inwardly rectifying K ϩ channel subunits, in contrast, were unaffected by thyroid hormone status. Taken together, these data demonstrate that thyroid hormone levels selectively and differentially regulate transcript expression for at least nine ion channel ␣-and ␤-subunits. Our results also document the potential of cDNA microarray analysis for the simultaneous examination of ion channel transcript expression levels in the diseased/remodeled myocardium. Key Words: ion channel Ⅲ cDNA microarray Ⅲ repolarization Ⅲ ionic remodeling E lectrical remodeling refers to changes in cardiac electrophysiological function caused by heart disease. Various pathologies are associated with electrophysiological remodeling including cardiac hypertrophy and failure, 1 chronic arrhythmia (eg, atrial fibrillation), 2 ischemic injury, 3 or altered thyroid status, 4 and several lines of evidence suggest that the underlying molecular mechanisms are complex and may well be model-dependent. In addition, most studies in the remodeled myocardium have focused on examination of individual ionic currents and/or expression levels of the channel subunits generating these currents.The application of genomic techniques, however, holds the promise of allowing the expression levels of thousands of genes to be examined simultaneously. 5 We have developed cDNA microarrays ("IonChips") containing probes for a large subset of genes encoding ion channel subunit proteins. With the objective to validate this novel tool, we have investigated cardiac ion channel remodeling associated with altered thyroid hormone status in the mo...

Abstract-Gene-expression changes in atrial fibrillation patients reflect both underlying heart-disease substrates and changes because of atrial fibrillation-induced atrial-tachycardia remodeling. These are difficult to separate in clinical investigations. This study assessed time-dependent mRNA expression-changes in canine models of atrial-tachycardia remodeling and congestive heart failure. Five experimental groups (5 dogs/group) were submitted to atrial (ATP, 400 bpm ϫ24 hours, 1 or 6 weeks) or ventricular (VTP, 240 bpm ϫ24 hours or 2 weeks) tachypacing. The expression of Ϸ21,700 transcripts was analyzed by microarray in isolated left-atrial cardiomyocytes and (for 18 genes) by real-time RT-PCR. Protein-expression changes were assessed by Western blot. In VTP, a large number of significant mRNA-expression changes occurred after both 24 hours (2209) and 2 weeks (2720). In ATP, fewer changes occurred at 24 hours (242) and fewer still (87) at 1 week, with no statistically-significant alterations at 6 weeks. Expression changes in VTP varied over time in complex ways. Extracellular matrix-related transcripts were strongly upregulated by VTP consistent with its pathophysiology, with 8 collagen-genes upregulated Ͼ10-fold, fibrillin-1 8-fold and MMP2 4.5-fold at 2 weeks (time of fibrosis) but unchanged at 24 hours. Other extracellular matrix genes (eg, fibronectin, lysine oxidase-like 2) increased at both time-points (Ϸ10, Ϸ5-fold respectively). In ATP, mRNA-changes almost exclusively represented downregulation and were quantitatively smaller. This study shows that VTP-induced congestive heart failure and ATP produce qualitatively different temporally-evolving patterns of gene-expression change, and that specific transcriptomal responses associated with atrial fibrillation versus underlying heart disease substrates must be considered in assessing gene-expression changes in man. Key Words: arrhythmia Ⅲ remodeling Ⅲ genomic A trial fibrillation (AF) is the most common sustained cardiac rhythm disorder, and with the aging of the population both the prevalence and economic impact of AF are increasing progressively. 1 Although the mechanistic basis of AF remains incompletely understood, active research promises to provide new insights that may lead to improved therapeutic options. 2,3 A variety of animal models have been used to assess AF pathophysiology under controlled conditions. Atrial tachyarrhythmias, including AF itself, alter atrial electrophysiology in ways that promote AF vulnerability. [4][5][6] Experimentally-induced congestive heart failure (CHF) also creates a substrate for AF maintenance, but by quite different mechanisms. 7 The atrial-tachycardia remodeling paradigm shows prominent changes in ion-channel function that lead to action-potential abbreviation and the promotion of atrial reentry. 8,9 CHF-induced ionic-current changes do not promote reentry but may favor ectopic-impulse formation, 10 and CHF-induced fibrosis promotes reentry by interfering with intraatrial conduction. 7 The molecular basis of AF ...

Abstract-Transient outward Kϩ current (I to ) downregulation following sustained tachycardia in vivo is usually attributed to tachycardiomyopathy. This study assessed potential direct rate regulation of cardiac I to and underlying mechanisms. Cultured adult canine left ventricular cardiomyocytes (37°C) were paced continuously at 1 or 3 Hz for 24 hours. I to was recorded with whole-cell patch clamp. The 3-Hz pacing reduced I to by 44% (PϽ0.01). Kv4.3 mRNA and protein expression were significantly reduced (by Ϸ30% and Ϸ40%, respectively) in 3-Hz paced cells relative to 1-Hz cells, but KChIP2 expression was unchanged. Prevention of Ca 2ϩ loading with nimodipine or calmodulin inhibition with W-7, A-7, or W-13 eliminated 3-Hz pacing-induced I to downregulation, whereas downregulation was preserved in the presence of valsartan. Inhibition of Ca 2ϩ /calmodulin-dependent protein kinase (CaMK)II with KN93, or calcineurin with cyclosporin A, also prevented I to downregulation. CaMKII-mediated phospholamban phosphorylation at threonine 17 was increased in 3-Hz paced cells, compatible with enhanced CaMKII activity, with functional significance suggested by acceleration of the Ca 2ϩ i transient decay time constant (Indo 1-acetoxymethyl ester microfluorescence). Total phospholamban expression was unchanged, as was expression of Na ϩ /Ca 2ϩ exchange and sarcoplasmic reticulum Ca 2ϩ -ATPase proteins. Nuclear localization of the calcineurin-regulated nuclear factor of activated T cells (NFAT)c3 was increased in 3-Hz paced cells compared to 1-Hz (immunohistochemistry, immunoblot). INCA-6 inhibition of NFAT prevented I to reduction in 3-Hz paced cells. Calcineurin activity increased after 6 hours of 3-Hz pacing. CaMKII inhibition prevented calcineurin activation and NFATc3 nuclear translocation with 3-Hz pacing. We conclude that tachycardia downregulates I to expression, with the Ca 2ϩ /calmodulin-dependent CaMKII and calcineurin/NFAT systems playing key Ca 2ϩ -sensing and signal-transducing roles in rate-dependent I to control. Key Words: potassium channels Ⅲ calcium Ⅲ calmodulin Ⅲ remodeling Ⅲ arrhythmias S udden cardiac death caused by ventricular tachycardia or fibrillation is an important contributor to mortality in congestive heart failure (CHF) patients. 1 Rapid heart-rhythms can impair cardiac function and patients with "tachycardiomyopathy" are at risk of sudden cardiac death. 2 Chronic ventricular tachypacing in experimental animals produces a dilated cardiomyopathy that mimics clinical tachycardiomyopathy and is often used as an experimental model to study CHF-related cardiac remodeling. 3 Changes in cardiac ion channel transport are important components of this remodeling, and extensive evidence suggests that these ion transport changes are crucial contributors to the pathogenesis of CHFrelated ventricular tachyarrhythmias and sudden death. 3,4 Among the most ubiquitous changes are alterations in the transient outward K ϩ current (I to ), 3 which play potentially important roles in repolarization abnormalities, 5,6 c...