Regional upregulation of Kv2.1-encoded current, IK,slow2, in Kv1DN mice is abolished by crossbreeding with Kv2DN mice

A compartmentalized mathematical model of the β1- and β2-adrenergic signaling systems in ventricular myocytes from mouse in heart failure

Mouse models of heart failure are extensively used to research human cardiovascular diseases. In particular, one of the most common is the mouse model of heart failure resulting from transverse aortic constriction (TAC). Despite this, there are no comprehensive compartmentalized mathematical models that describe the complex behavior of the action potential, [Ca²⁺]_i transients, and their regulation by β₁- and β₂-adrenergic signaling systems in failing mouse myocytes. In this paper, we develop a novel compartmentalized mathematical model of failing mouse ventricular myocytes after TAC procedure. The model describes well the cell geometry, action potentials, [Ca²⁺]_i transients, and β₁- and β₂-adrenergic signaling in the failing cells. Simulation results obtained with the failing cell model are compared with those from the normal ventricular myocytes. Exploration of the model reveals the sarcoplasmic reticulum Ca²⁺ load mechanisms in failing ventricular myocytes. We also show a larger susceptibility of the failing myocytes to early and delayed afterdepolarizations and to a proarrhythmic behavior of Ca²⁺ dynamics upon stimulation with isoproterenol. The mechanisms of the proarrhythmic behavior suppression are investigated and sensitivity analysis is performed. The developed model can explain the existing experimental data on failing mouse ventricular myocytes and make experimentally testable predictions of a failing myocyte's behavior.

Modulation of cardiac voltage-activated K+ currents by glypican 1 heparan sulfate proteoglycan

BACKGROUND

Glypican 1 (Gpc1) is a heparan sulfate proteoglycan attached to the cell membrane via a glycosylphosphatidylinositol anchor, where it holds glycosaminoglycans nearby. We have recently shown that Gpc1 knockout (Gpc1-/-) mice feature decreased systemic blood pressure. To date, none has been reported regarding the role of Gpc1 on the electrical properties of the heart and specifically, in regard to a functional interaction between Gpc1 and voltage-gated K+ channels.

METHODS

We used echocardiography and in vivo (electrocardiographic recordings) and in vitro (patch clamping) electrophysiology to study mechanical and electric properties of mice hearts. We used RT-PCR to probe K+ channels' gene transcription in heart tissue.

RESULTS

Gpc1-/- hearts featured increased cardiac stroke volume and preserved ejection fraction. Gpc1-/- electrocardiograms showed longer QT intervals, abnormalities in the ST segment, and delayed T waves, corroborated by longer action potentials in isolated ventricular cardiomyocytes. In voltage-clamp, these cells showed decreased Ito and IK voltage-activated K+ current densities. Moreover, IK showed activation at less negative voltages, but a higher level of inactivation at a given membrane potential. Kcnh2 and Kcnq1 voltage-gated K+ channels subunits' transcripts were remarkably more abundant in heart tissues from Gpc1-/- mice, suggesting that Gpc1 may interfere in the steps between transcription and translation in these cases.

CONCLUSION

Our data reveals an unprecedented connection between Gpc1 and voltage-gated K+ channels expressed in the heart and this knowledge contributes to the understanding of the role of this HSPG in cardiac function which may play a role in the development of cardiovascular disease.

Progesterone Changes the Pregnancy-Induced Adaptation of Cardiomyocyte Kv2.1 Channels via MicroRNA-29b

The cardiovascular system adaptation occurs during pregnancy to ensure adequate maternal circulation. Progesterone (P4) is widely used in hormone therapy to support pregnancy, but little is known about its effects on maternal cardiac function. In this study, we investigated the cardiac repolarization and ion channel expression in pregnant subjects and mice models and studied the effects of P4 administrations on these pregnancy-mediated adaptations. P4 administrations shortened the prolongation of QTC intervals and action potential duration (APD) that occurred during pregnancy, which was mainly attributable to the reduction in the voltage-gated potassium (Kv) current under basal conditions. In vitro studies indicated that P4 regulated the Kv2.1 channel in a bidirectional manner. At a low dose (1 μM), P4 induced upregulation of Kv2.1 through P4 receptor, while at a higher dose (5 μM), P4 downregulated Kv2.1 by targeting microRNA-29b (miR-29b). Our data showed that P4 modulated maternal cardiac repolarization by regulating Kv2.1 channel activity during pregnancy. Kv2.1, as well as miR-29b, might be used as potential therapeutic targets for adaptations of the maternal cardiovascular system or evaluation of progesterone medication during pregnancy.

A compartmentalized mathematical model of mouse atrial myocytes

Various experimental mouse models are extensively used to research human diseases, including atrial fibrillation, the most common cardiac rhythm disorder. Despite this, there are no comprehensive mathematical models that describe the complex behavior of the action potential and [Ca²⁺]_i transients in mouse atrial myocytes. Here, we develop a novel compartmentalized mathematical model of mouse atrial myocytes that combines the action potential, [Ca²⁺]_i dynamics, and β-adrenergic signaling cascade for a subpopulation of right atrial myocytes with developed transverse-axial tubule system. The model consists of three compartments related to β-adrenergic signaling (caveolae, extracaveolae, and cytosol) and employs local control of Ca²⁺ release. It also simulates ionic mechanisms of action potential generation and describes atrial-specific Ca²⁺ handling as well as frequency dependences of the action potential and [Ca²⁺]_i transients. The model showed that the T-type Ca²⁺ current significantly affects the later stage of the action potential, with little effect on [Ca²⁺]_i transients. The block of the small-conductance Ca²⁺-activated K⁺ current leads to a prolongation of the action potential at high intracellular Ca²⁺. Simulation results obtained from the atrial model cells were compared with those from ventricular myocytes. The developed model represents a useful tool to study complex electrical properties in the mouse atria and could be applied to enhance the understanding of atrial physiology and arrhythmogenesis.NEW & NOTEWORTHY A new compartmentalized mathematical model of mouse right atrial myocytes was developed. The model simulated action potential and Ca²⁺ dynamics at baseline and after stimulation of the β-adrenergic signaling system. Simulations showed that the T-type Ca²⁺ current markedly prolonged the later stage of atrial action potential repolarization, with a minor effect on [Ca²⁺]_i transients. The small-conductance Ca²⁺-activated K⁺ current block resulted in prolongation of the action potential only at the relatively high intracellular Ca²⁺.

Deletion in mice of X-linked, Brugada syndrome- and atrial fibrillation-associated Kcne5 augments ventricular KV currents and predisposes to ventricular arrhythmia

KCNE5 is an X-linked gene encoding KCNE5, an ancillary subunit to voltage-gated potassium (K_V) channels. Human KCNE5 mutations are associated with atrial fibrillation (AF)- and Brugada syndrome (BrS)-induced cardiac arrhythmias that can arise from increased potassium current in cardiomyocytes. Seeking to establish underlying molecular mechanisms, we created and studied Kcne5 knockout ( Kcne5^-/0) mice. Intracardiac ECG revealed that Kcne5 deletion caused ventricular premature beats, increased susceptibility to induction of polymorphic ventricular tachycardia (60 vs. 24% in Kcne5^+/0 mice), and 10% shorter ventricular refractory period. Kcne5 deletion increased mean ventricular myocyte K_V current density in the apex and also in the subpopulation of septal myocytes that lack fast transient outward current ( I_to,f). The current increases arose from an apex-specific increase in slow transient outward current-1 ( I_Kslow,1) (conducted by K_V1.5) and I_to,f (conducted by K_V4) and an increase in I_Kslow,2 (conducted by K_V2.1) in both apex and septum. Kcne5 protein localized to the intercalated discs in ventricular myocytes, where K_V2.1 was also detected in both Kcne5^-/0 and Kcne5^+/0 mice. In HL-1 cardiac cells and human embryonic kidney cells, KCNE5 and K_V2.1 colocalized at the cell surface, but predominantly in intracellular vesicles, suggesting that Kcne5 deletion increases I_K,slow2 by reducing K_V2.1 intracellular sequestration. The human AF-associated mutation KCNE5-L65F negative shifted the voltage dependence of K_V2.1-KCNE5 channels, increasing their maximum current density >2-fold, whereas BrS-associated KCNE5 mutations produced more subtle negative shifts in K_V2.1 voltage dependence. The findings represent the first reported native role for Kcne5 and the first demonstrated Kcne regulation of K_V2.1 in mouse heart. Increased K_V current is a manifestation of KCNE5 disruption that is most likely common to both mouse and human hearts, providing a plausible mechanistic basis for human KCNE5-linked AF and BrS.-David, J.-P., Lisewski, U., Crump, S. M., Jepps, T. A., Bocksteins, E., Wilck, N., Lossie, J., Roepke, T. K., Schmitt, N., Abbott, G. W. Deletion in mice of X-linked, Brugada syndrome- and atrial fibrillation-associated Kcne5 augments ventricular K_V currents and predisposes to ventricular arrhythmia.

A Heart for Diversity: Simulating Variability in Cardiac Arrhythmia Research

In cardiac electrophysiology, there exist many sources of inter- and intra-personal variability. These include variability in conditions and environment, and genotypic and molecular diversity, including differences in expression and behavior of ion channels and transporters, which lead to phenotypic diversity (e.g., variable integrated responses at the cell, tissue, and organ levels). These variabilities play an important role in progression of heart disease and arrhythmia syndromes and outcomes of therapeutic interventions. Yet, the traditional in silico framework for investigating cardiac arrhythmias is built upon a parameter/property-averaging approach that typically overlooks the physiological diversity. Inspired by work done in genetics and neuroscience, new modeling frameworks of cardiac electrophysiology have been recently developed that take advantage of modern computational capabilities and approaches, and account for the variance in the biological data they are intended to illuminate. In this review, we outline the recent advances in statistical and computational techniques that take into account physiological variability, and move beyond the traditional cardiac model-building scheme that involves averaging over samples from many individuals in the construction of a highly tuned composite model. We discuss how these advanced methods have harnessed the power of big (simulated) data to study the mechanisms of cardiac arrhythmias, with a special emphasis on atrial fibrillation, and improve the assessment of proarrhythmic risk and drug response. The challenges of using in silico approaches with variability are also addressed and future directions are proposed.

Fish, the better model in human heart research? Zebrafish Heart aggregates as a 3D spontaneously cardiomyogenic in vitro model system

The zebrafish (ZF) has become an essential model for biomedical, pharmacological and eco-toxicological heart research. Despite the anatomical differences between fish and human hearts, similarities in cellular structure and conservation of genes as well as pathways across vertebrates have led to an increase in the popularity of ZF as a model for human cardiac research. ZF research benefits from an entirely sequenced genome, which allows us to establish and study cardiovascular mutants to better understand cardiovascular diseases. In this review, we will discuss the importance of in vitro model systems for cardiac research and summarise results of in vitro 3D heart-like cell aggregates, consisting of myocardial tissue formed spontaneously from enzymatically digested whole embryonic ZF larvae (Zebrafish Heart Aggregate - ZFHA). We will give an overview of the similarities and differences of ZF versus human hearts and highlight why ZF complement established mammalian models (i.e. murine and large animal models) for cardiac research. At this stage, the ZFHA model system is being refined into a high-throughput (more ZFHA generated than larvae prepared) and stable in vitro test system to accomplish the same longevity of previously successful salmonid models. ZFHA have potential for the use of high-throughput-screenings of different factors like small molecules, nucleic acids, proteins and lipids which is difficult to achieve in the zebrafish in vivo screening models with lethal mutations as well as to explore ion channel disorders and to find appropriate drugs for safety screening.

Calcium Oscillation Frequency-Sensitive Gene Regulation and Homeostatic Compensation in Pancreatic β-Cells

Pancreatic islet [Formula: see text]-cells are electrically excitable cells that secrete insulin in an oscillatory fashion when the blood glucose concentration is at a stimulatory level. Insulin oscillations are the result of cytosolic [Formula: see text] oscillations that accompany bursting electrical activity of [Formula: see text]-cells and are physiologically important. ATP-sensitive [Formula: see text] channels (K(ATP) channels) play the key role in setting the overall activity of the cell and in driving bursting, by coupling cell metabolism to the membrane potential. In humans, when there is a defect in K(ATP) channel function, [Formula: see text]-cells fail to respond appropriately to changes in the blood glucose level, and electrical and [Formula: see text] oscillations are lost. However, mice compensate for K(ATP) channel defects in islet [Formula: see text]-cells by employing alternative mechanisms to maintain electrical and [Formula: see text] oscillations. In a recent study, we showed that in mice islets in which K(ATP) channels are genetically knocked out another [Formula: see text] current, provided by inward-rectifying [Formula: see text] channels, is increased. With mathematical modeling, we demonstrated that a sufficient upregulation in these channels can account for the paradoxical electrical bursting and [Formula: see text] oscillations observed in these [Formula: see text]-cells. However, the question of determining the correct level of upregulation that is necessary for this compensation remained unanswered, and this question motivates the current study. [Formula: see text] is a well-known regulator of gene expression, and several examples have been shown of genes that are sensitive to the frequency of the [Formula: see text] signal. In this mathematical modeling study, we demonstrate that a [Formula: see text] oscillation frequency-sensitive gene transcription network can adjust the gene expression level of a compensating [Formula: see text] channel so as to rescue electrical bursting and [Formula: see text] oscillations in a model [Formula: see text]-cell in which the key K(ATP) current is removed. This is done without the prescription of a target [Formula: see text] level, but evolves naturally as a consequence of the feedback between the [Formula: see text]-dependent enzymes and the cell's electrical activity. More generally, the study indicates how [Formula: see text] can provide the link between gene expression and cellular electrical activity that promotes wild-type behavior in a cell following gene knockout.

Molecular Basis of Functional Myocardial Potassium Channel Diversity

Multiple types of voltage-gated K(+) and non-voltage-gated K(+) currents have been distinguished in mammalian cardiac myocytes based on differences in time-dependent and voltage-dependent properties and pharmacologic sensitivities. Many of the genes encoding voltage-gated K(+) (Kv) and non-voltage-gated K(+) (Kir and K2P) channel pore-forming and accessory subunits are expressed in the heart, and a variety of approaches have been, and continue to be, used to define the molecular determinants of native cardiac K(+) channels and to explore the molecular mechanisms controlling the diversity, regulation, and remodeling of these channels in the normal and diseased myocardium.

Cardiac expression of the CREM repressor isoform CREM-IbΔC-X in mice leads to arrhythmogenic alterations in ventricular cardiomyocytes

Chronic β-adrenergic stimulation is regarded as a pivotal step in the progression of heart failure which is associated with a high risk for arrhythmia. The cAMP-dependent transcription factors cAMP-responsive element binding protein (CREB) and cAMP-responsive element modulator (CREM) mediate transcriptional regulation in response to β-adrenergic stimulation and CREM repressor isoforms are induced after stimulation of the β-adrenoceptor. Here, we investigate whether CREM repressors contribute to the arrhythmogenic remodeling in the heart by analyzing arrhythmogenic alterations in ventricular cardiomyocytes (VCMs) from mice with transgenic expression of the CREM repressor isoform CREM-IbΔC-X (TG). Patch clamp analyses, calcium imaging, immunoblotting and real-time quantitative RT-PCR were conducted to study proarrhythmic alterations in TG VCMs vs. wild-type controls. The percentage of VCMs displaying spontaneous supra-threshold transient-like Ca(2+) releases was increased in TG accompanied by an enhanced transduction rate of sub-threshold Ca(2+) waves into these supra-threshold events. As a likely cause we discovered enhanced NCX-mediated Ca(2+) transport and NCX1 protein level in TG. An increase in I NCX and decrease in I to and its accessory channel subunit KChIP2 was associated with action potential prolongation and an increased proportion of TG VCMs showing early afterdepolarizations. Finally, ventricular extrasystoles were augmented in TG mice underlining the in vivo relevance of our findings. Transgenic expression of CREM-IbΔC-X in mouse VCMs leads to distinct arrhythmogenic alterations. Since CREM repressors are inducible by chronic β-adrenergic stimulation our results suggest that the inhibition of CRE-dependent transcription contributes to the formation of an arrhythmogenic substrate in chronic heart disease.

SUMOylation and Potassium Channels: Links to Epilepsy and Sudden Death

Neuronal potassium ion channels play an essential role in the generation of the action potential and excitability of neurons. The dysfunction of ion channel subunits can cause channelopathies, which are associated in some cases with sudden unexplained death in epilepsy SUDEP. The physiological roles of neuronal ion channels have been largely determined, but little is known about the molecular mechanisms underlying neurological channelopathies, especially the determinants of the channels' regulation. SUMO (small ubiquitin-like modifier) proteins covalently conjugate lysine residues in a large number of target proteins and modify their functions. SUMO modification (SUMOylation) has emerged as an important regulatory mechanism for protein stability, function, subcellular localization, and protein-protein interactions. Since SUMO was discovered almost 20 years ago, the biological contribution of SUMOylation has not fully understood. It is until recently that the physiological impacts of SUMOylation on the regulation of neuronal potassium ion channels have been investigated. It is well established that SUMOylation controls many aspects of nuclear function, but it is now clear that it is also a key determinant in the function of potassium channels, and SUMOylation has also been implicated in a wide range of channelopathies, including epilepsy and sudden death.

Kv4.3-Encoded Fast Transient Outward Current Is Presented in Kv4.2 Knockout Mouse Cardiomyocytes

Gradients of the fast transient outward K⁺ current (I_to,f) contribute to heterogeneity of ventricular repolarization in a number of species. Cardiac I_to,f levels and gradients change notably with heart disease. Human cardiac I_to,f appears to be encoded by the Kv4.3 pore-forming α-subunit plus the auxiliary KChIP2 β-subunit while mouse cardiac I_to,f requires Kv4.2 and Kv4.3 α-subunits plus KChIP2. Regional differences in cardiac I_to,f are associated with expression differences in Kv4.2 and KChIP2. Although I_to,f was reported to be absent in mouse ventricular cardiomyocytes lacking the Kv4.2 gene (Kv4.2-/-) when short depolarizing voltage pulses were used to activate voltage-gated K⁺ currents, in the present study, we showed that the use of long depolarization steps revealed a heteropodatoxin-sensitive I_to,f (at ~40% of the wild-type levels). Immunohistological studies further demonstrated membrane expression of Kv4.3 in Kv4.2-/- cardiomyocytes. Transmural I_to,f gradients across the left ventricular wall were reduced by ~3.5-fold in Kv4.2-/- heart, compared to wild-type. The I_to,f gradient in Kv4.2-/- hearts was associated with gradients in KChIP2 mRNA expression while in wild-type there was also a gradient in Kv4.2 expression. In conclusion, we found that Kv4.3-based I_to,f exists in the absence of Kv4.2, although with a reduced transmural gradient. Kv4.2-/- mice may be a useful animal model for studying Kv4.3-based I_to,f as observed in humans.

miRNA in the regulation of ion channel/transporter expression

Ion channels and transporters are expressed in every living cell, where they participate in controlling a plethora of biological processes and physiological functions, such as excitation of cells in response to stimulation, electrical activities of cells, excitation-contraction coupling, cellular osmolarity, and even cell growth and death. Alterations of ion channels/transporters can have profound impacts on the cellular physiology associated with these proteins. Expression of ion channels/transporters is tightly regulated and expression deregulation can trigger abnormal processes, leading to pathogenesis, the channelopathies. While transcription factors play a critical role in controlling the transcriptome of ion channels/transporters at the transcriptional level by acting on the 5'-flanking region of the genes, microribonucleic acids (miRNAs), a newly discovered class of regulators in the gene network, are also crucial for expression regulation at the posttranscriptional level through binding to the 3'untranslated region of the genes. These small noncoding RNAs fine tune expression of genes involved in a wide variety of cellular processes. Recent studies revealed the role of miRNAs in regulating expression of ion channels/transporters and the associated physiological functions. miRNAs can target ion channel genes to alter cardiac excitability (conduction, repolarization, and automaticity) and affect arrhythmogenic potential of heart. They can modulate circadian rhythm, pain threshold, neuroadaptation to alcohol, brain edema, etc., through targeting ion channel genes in the neuronal systems. miRNAs can also control cell growth and tumorigenesis by acting on the relevant ion channel genes. Future studies are expected to rapidly increase to unravel a new repertoire of ion channels/transporters for miRNA regulation.

Cardiac electrophysiology in mice: a matter of size

Over the last decade, mouse models have become a popular instrument for studying cardiac arrhythmias. This review assesses in which respects a mouse heart is a miniature human heart, a suitable model for studying mechanisms of cardiac arrhythmias in humans and in which respects human and murine hearts differ. Section I considers the issue of scaling of mammalian cardiac (electro) physiology to body mass. Then, we summarize differences between mice and humans in cardiac activation (section II) and the currents underlying the action potential in the murine working myocardium (section III). Changes in cardiac electrophysiology in mouse models of heart disease are briefly outlined in section IV, while section V discusses technical considerations pertaining to recording cardiac electrical activity in mice. Finally, section VI offers general considerations on the influence of cardiac size on the mechanisms of tachy-arrhythmias.

Exploiting mathematical models to illuminate electrophysiological variability between individuals

Across individuals within a population, several levels of variability are observed, from the differential expression of ion channels at the molecular level, to the various action potential morphologies observed at the cellular level, to divergent responses to drugs at the organismal level. However, the limited ability of experiments to probe complex interactions between components has hitherto hindered our understanding of the factors that cause a range of behaviours within a population. Variability is a challenging issue that is encountered in all physiological disciplines, but recent work suggests that novel methods for analysing mathematical models can assist in illuminating its causes. In this review, we discuss mathematical modelling studies in cardiac electrophysiology and neuroscience that have enhanced our understanding of variability in a number of key areas. Specifically, we discuss parameter sensitivity analysis techniques that may be applied to generate quantitative predictions based on considering behaviours within a population of models, thereby providing novel insight into variability. Our discussion focuses on four issues that have benefited from the utilization of these methods: (1) the comparison of different electrophysiological models of cardiac myocytes, (2) the determination of the individual contributions of different molecular changes in complex disease phenotypes, (3) the identification of the factors responsible for the variable response to drugs, and (4) the constraining of free parameters in electrophysiological models of heart cells. Together, the studies that we discuss suggest that rigorous analyses of mathematical models can generate quantitative predictions regarding how molecular-level variations contribute to functional differences between experimental samples. These strategies may be applicable not just in cardiac electrophysiology, but in a wide range of disciplines.

CREB critically regulates action potential shape and duration in the adult mouse ventricle

The cAMP response element binding protein (CREB) belongs to the CREB/cAMP response element binding modulator/activating transcription factor 1 family of cAMP-dependent transcription factors mediating a regulation of gene transcription in response to cAMP. Chronic stimulation of β-adrenergic receptors and the cAMP-dependent signal transduction pathway by elevated plasma catecholamines play a central role in the pathogenesis of heart failure. Ion channel remodeling, particularly a decreased transient outward current (I(to)), and subsequent action potential (AP) prolongation are hallmarks of the failing heart. Here, we studied the role of CREB for ion channel regulation in mice with a cardiomyocyte-specific knockout of CREB (CREB KO). APs of CREB KO cardiomyocytes were prolonged with increased AP duration at 50 and 70% repolarization and accompanied by a by 51% reduction of I(to) peak amplitude as detected in voltage-clamp measurements. We observed a 29% reduction of Kcnd2/Kv4.2 mRNA in CREB KO cardiomyocytes mice while the other I(to)-related channel subunits Kv4.3 and KChIP2 were not different between groups. Accordingly, Kv4.2 protein was reduced by 37% in CREB KO. However, we were not able to detect a direct regulation of Kv4.2 by CREB. The I(to)-dependent AP prolongation went along with an increase of I(Na) and a decrease of I(Ca,L) associated with an upregulation of Scn8a/Nav1.6 and downregulation of Cacna1c/Cav1.2 mRNA in CREB KO cardiomyocytes. Our results from mice with cardiomyocyte-specific inactivation of CREB definitively indicate that CREB critically regulates the AP shape and duration in the mouse ventricle, which might have an impact on ion channel remodeling in situations of altered cAMP-dependent signaling like heart failure.

Ubiquitylation and SUMOylation of cardiac ion channels

Cardiac ion channels play an essential role in the generation of the action potential of cardiomyocytes. Over the past 15 years, a new field of research called channelopathies has emerged; it regroups all diseases caused by ion channel dysfunction. Investigators have largely determined the physiological roles of cardiac ion channels, but little is known about the molecular determinants of their regulation. Two posttranslational mechanisms that are crucial in determining the fate of proteins are the ubiquitylation and the SUMOylation pathways, which lead to the degradation and/or regulation of modified proteins. Recently, several groups have investigated the physiological impacts of these mechanisms on the regulation of different classes of cardiac ion channels. The objective of this review was to summarize and briefly discuss these results.

Pore mutants of HERG and KvLQT1 downregulate the reciprocal currents in stable cell lines

We previously reported a transgenic rabbit model of long QT syndrome based on overexpression of pore mutants of repolarizing K(+) channels KvLQT1 (LQT1) and HERG (LQT2).The transgenes in these rabbits eliminated the slow and fast components of the delayed rectifier K(+) current (I(Ks) and I(Kr), respectively), as expected. Interestingly, the expressed pore mutants of HERG and KvLQT1 downregulated the remaining reciprocal repolarizing currents, I(Ks) and I(Kr), without affecting the steady-state levels of the native polypeptides. Here, we sought to further explore the functional interactions between HERG and KvLQT1 in heterologous expression systems. Stable Chinese hamster ovary (CHO) cell lines expressing KvLQT1-minK or HERG were transiently transfected with expression vectors coding for mutant or wild-type HERG or KvLQT1. Transiently expressed pore mutant or wild-type KvLQT1 downregulated I(Kr) in HERG stable CHO cell lines by 70% and 44%, respectively. Immunostaining revealed a severalfold lower surface expression of HERG, which could account for the reduction in I(Kr) upon KvLQT1 expression. Deletion of the KvLQT1 NH(2)-terminus did not abolish the downregulation, suggesting that the interactions between the two channels are mediated through their COOH-termini. Similarly, transiently expressed HERG reduced I(Ks) in KvLQT1-minK stable cells. Coimmunoprecipitations indicated a direct interaction between HERG and KvLQT1, and surface plasmon resonance analysis demonstrated a specific, physical association between the COOH-termini of KvLQT1 and HERG. Here, we present an in vitro model system consistent with the in vivo reciprocal downregulation of repolarizing currents seen in transgenic rabbit models, illustrating the importance of the transfection method when studying heterologous ion channel expression and trafficking. Moreover, our data suggest that interactions between KvLQT1 and HERG are mediated through COOH-termini.

Nitroxyl enhances myocyte Ca2+ transients by exclusively targeting SR Ca2+-cycling

Nitroxyl (HNO), the 1-electron reduction product of nitric oxide, improves myocardial contraction in normal and failing hearts. Here we test whether the HNO donor Angeli's salt (AS) will change myocyte action potential (AP) waveform by altering the L-type Ca2+ current (ICa) and contrast the contractile effects of HNO with that of the hydroxyl radical (.OH) and nitrite (NO2-), two potential breakdown products of AS. We confirmed the positive effect of AS/HNO on basal cardiomyocyte function, as opposed to the detrimental effect of .OH and the negligible effect of NO2-. Upon examination of the myocyte AP, we observed no change in resting membrane potential or AP duration to 20 per cent repolarization with AS/HNO, whereas AP duration to 90 per cent repolarization was slightly prolonged. However, perfusion with AS/HNO did not elicit a change in basal ICa, but did hasten ICa inactivation. Upon further examination of the SR, the AS/HNO-induced increase in cardiomyocyte Ca2+ transients was abolished with inhibition of SR Ca2+-cycling. Therefore, the HNO-induced increase in Ca2+ transients results exclusively from changes in SR Ca2+-cycling, and not from ICa.

How multiple conductances determine electrophysiological properties in a multicompartment model

Most neurons have large numbers of voltage- and time-dependent currents that contribute to their electrical firing patterns. Because these currents are nonlinear, it can be difficult to determine the role each current plays in determining how a neuron fires. The lateral pyloric (LP) neuron of the stomatogastric ganglion of decapod crustaceans has been studied extensively biophysically. We constructed approximately 600,000 versions of a four-compartment model of the LP neuron and distributed 11 different currents into the compartments. From these, we selected approximately 1300 models that match well the electrophysiological properties of the biological neuron. Interestingly, correlations that were seen in the expression of channel mRNA in biological studies were not found across the approximately 1300 admissible LP neuron models, suggesting that the electrical phenotype does not require these correlations. We used cubic fits of the function from maximal conductances to a series of electrophysiological properties to ask which conductances predominantly influence input conductance, resting membrane potential, resting spike rate, phasing of activity in response to rhythmic inhibition, and several other properties. In all cases, multiple conductances contribute to the measured property, and the combinations of currents that strongly influence each property differ. These methods can be used to understand how multiple currents in any candidate neuron interact to determine the cell's electrophysiological behavior.

In vivo effect of a dominant negative Kv4.2 loss-of-function mutation eliminating I(to,f) on atrial refractoriness and atrial fibrillation in mice

BACKGROUND

Gain-of-function K(+) channel mutations cause familial atrial fibrillation (AF) by shortening of the atrial action potential duration (APD). APD-prolonging K(+) channel blockers are an effective therapeutic option in AF. In vitro, the dominant negative Kv4.2W362F mutation (Kv4DN) eliminates I(to,f) in murine atrial myocytes and markedly prolongs the APD, so whether this loss-of-function of I(to,f) alters the atrial effective refractory period (AERP) in vivo and/or affects AF-inducibility was investigated in the present study.

METHODS AND RESULTS

Transvenous electrophysiological studies were performed in vivo in Kv4DN and wild-type littermate control (LMC) mice. Intriguingly, no difference was found between Kv4DN and LMC for the AERP in vivo either at baseline or after carbachol. Consequently, AF-inducibility at baseline (Kv4DN: 10/16 vs LMC: 7/13) and after carbachol (Kv4DN: 9/16 vs LMC: 6/13) did not differ between groups. However, AF-inducibility was associated with a significantly shorter AERP (inducible 51.1 +/-1.4 vs non-inducible 58.4 +/-1.6; P<0.01) irrespective of genotype.

CONCLUSIONS

The loss-of-function of I(to,f) prolongs the APD in mouse atrial myocytes in vitro, but this effect on single cells does not translate into measurable AERP prolongation in vivo and hence does not exert an anti-arrhythmic effect. However, the susceptibility of mice to AF in vivo is determined by the individual AERP, irrespective of genotype.

Feedback Remodeling of Cardiac Potassium Current Expression

Background— Inhibition of individual K + currents causes functionally based compensatory increases in other K + currents that minimize changes in action potential duration, a phenomenon known as repolarization reserve. The possibility that sustained K + channel inhibition may induce remodeling of ion current expression has not been tested. Accordingly, we assessed the effects of sustained inhibition of one K + current on various other cardiac ionic currents.

Methods and Results— Adult canine left ventricular cardiomyocytes were incubated in primary culture and paced at a physiological rate (1 Hz) for 24 hours in the presence or absence of the highly selective rapid delayed-rectifier K + current (I Kr ) blocker dofetilide (5 nmol/L). Sustained dofetilide exposure led to shortened action potential duration and increased repolarization reserve (manifested as a reduced action potential duration–prolonging response to I Kr blockade). These repolarization changes were accompanied by increased slow delayed-rectifier (I Ks ) density, whereas I Kr , transient-outward (I to ), inward-rectifier (I K1 ), L-type Ca 2+ (I CaL ), and late Na + current remained unchanged. The mRNA expression corresponding to KvLQT1 and minK (real-time polymerase chain reaction) was unchanged, but their protein expression (Western blot) was increased, suggesting posttranscriptional regulation. To analyze possible mechanisms, we quantified the muscle-specific microRNA subtypes miR -133a and miR -133b, which can posttranscriptionally regulate and repress KvLQT1 protein expression without affecting mRNA expression. The expression levels of miR -133a and miR -133b were significantly decreased in cells cultured in dofetilide compared with control, possibly accounting for KvLQT1 protein upregulation.

Conclusions— Sustained reductions in I Kr may lead to compensatory upregulation of I Ks through posttranscriptional upregulation of underlying subunits, likely mediated (at least partly) by microRNA changes. These results suggest that feedback control of ion channel expression may influence repolarization reserve.

Mechanisms of cardiac arrhythmias and sudden death in transgenic rabbits with long QT syndrome

Long QT syndrome (LQTS) is a heritable disease associated with ECG QT interval prolongation, ventricular tachycardia, and sudden cardiac death in young patients. Among genotyped individuals, mutations in genes encoding repolarizing K+ channels (LQT1:KCNQ1; LQT2:KCNH2) are present in approximately 90% of affected individuals. Expression of pore mutants of the human genes KCNQ1 (KvLQT1-Y315S) and KCNH2 (HERG-G628S) in the rabbit heart produced transgenic rabbits with a long QT phenotype. Prolongations of QT intervals and action potential durations were due to the elimination of IKs and IKr currents in cardiomyocytes. LQT2 rabbits showed a high incidence of spontaneous sudden cardiac death (>50% at 1 year) due to polymorphic ventricular tachycardia. Optical mapping revealed increased spatial dispersion of repolarization underlying the arrhythmias. Both transgenes caused downregulation of the remaining complementary IKr and IKs without affecting the steady state levels of the native polypeptides. Thus, the elimination of 1 repolarizing current was associated with downregulation of the reciprocal repolarizing current rather than with the compensatory upregulation observed previously in LQTS mouse models. This suggests that mutant KvLQT1 and HERG interacted with the reciprocal wild-type alpha subunits of rabbit ERG and KvLQT1, respectively. These results have implications for understanding the nature and heterogeneity of cardiac arrhythmias and sudden cardiac death.

Reactive oxygen species-induced activation of p90 ribosomal S6 kinase prolongs cardiac repolarization through inhibiting outward K+ channel activity

p90 ribosomal S6 kinase (p90RSK) is activated in cardiomyopathies caused by conditions such as ischemia/reperfusion injury and diabetes mellitus in which prolongation of cardiac repolarization and frequent arrhythmias are common. Molecular mechanisms underlying the electric remodeling in cardiac diseases are largely unknown. In the present study, we determined the role of p90RSK activation in the modulation of voltage-gated K+ channel activity determining cardiac repolarization. Mice with increased cardiac p90RSK activity due to transgenic expression of p90RSK (p90RSK-Tg) had prolongation of QT intervals and of ventricular myocyte action potential durations. Fast transient outward K+ current (I(to,f)), slow delayed outward K+ current (I(K,slow)), and steady-state K+ current (I(SS)) were significantly decreased in p90RSK-Tg mouse ventricular myocytes. mRNA levels of Kv4.3, Kv4.2, Kv1.5, Kv2.1, and KChIP2 from ventricles between p90RSK-Tg and nontransgenic littermate control mice were similar, as assessed by quantitative reverse transcriptase-polymerase chain reaction, indicating that p90RSK regulates voltage-gated K+ channels through posttranslational modification. Kv4.3- and Kv1.5- rather than Kv4.2- and Kv2.1-encoded channels in HEK 293 cells were inhibited by p90RSK. In vitro phosphorylation analysis showed that Kv4.3 was phosphorylated by p90RSK at 2 conserved sites, Ser516 and Ser550. p90RSK expression significantly inhibited Kv4.3- and Kv4.3 and KChIP2-encoded channel activities in HEK 293 cells, whereas p90RSK's effects were blocked by amino acid mutation(s) at phosphorylation site(s) in Kv4.3. Hydrogen peroxide, a mediator of induced cardiac p90RSK activation in ischemia/reperfusion injury and diabetes mellitus, had effects similar to those of p90RSK on Kv4.3- or Kv4.3- and KChIP2-encoded channels. Fluoromethylketone, a specific p90RSK inhibitor, abolished hydrogen peroxide effects. These findings indicate that p90RSK activation is critical for reactive oxygen species-mediated inhibition of voltage-gated K+ channel activity and leads to prolongation of cardiac repolarization.

Cardiac-specific overexpression of the human type 1 angiotensin II receptor causes delayed repolarization

AIMS

Mice with cardiac-specific overexpression of human angiotensin II type 1 receptor (AT1R) undergo cardiac remodelling and die prematurely of sudden death. Since excessive QT prolongation is a major risk factor for ventricular arrhythmias and sudden death, we hypothesize that chronic stimulation of AT1R might contribute to sudden death by promoting delayed repolarization and ventricular arrhythmias.

METHODS

In the present study, a detailed analysis of ventricular repolarization parameters was undertaken in AT1R mice.

RESULTS

Measurement of K+ currents in ventricular myocytes isolated from 6-8 months AT1R male mice revealed a significant reduction of the Ca2+-independent transient outward (I(to)), the ultra-rapid delayed rectifier (I Kur)), and the inward rectifier (I K1) K+ currents compared with littermate controls (CTL). The expression of the underlying K+ channels was also decreased in AT1R ventricles. Moreover, reactivation of I(to) was slower in AT1R mice. Consistent with these findings, AT1R mice presented a longer action potential duration (APD90, CTL: 19.0 +/- 1.8 ms; AT1R: 39.1 +/- 4.7 ms, P = 0.0001) and QTc interval (CTL: 53.6 +/- 1.5 ms, AT1R: 64.2 +/- 1.4 ms, P = 0.0005). In addition, spontaneous ventricular arrhythmias were reported in the AT1R mice. Importantly, the increased incidence of arrhythmia and the repolarization defects also occurred in much younger AT1R mice that do not present signs of hypertrophy, confirming that these arrhythmogenic changes are not secondary to cardiac remodelling.

CONCLUSION

These results strongly suggest that chronic stimulation of AT1R directly leads to an increased incidence of cardiac arrhythmia associated with delayed repolarization.

Single-channel properties of I K,slow1 and I K,slow2 in mouse ventricular myocytes

I K,slow1 and I K,slow2 are two important voltage-gated potassium (K+) currents expressed in mouse ventricular myocytes. However, their properties at the single-channel level have not been characterized. In this paper, we report two new single K+ channels, mK1 and mK2, in myocytes isolated from mouse ventricles and their possible correlation with the macroscopic currents I K,slow1 and I K,slow2. The conductance of mK1 and mK2 was 24 and 17 pS, respectively. Ensemble-averaged current demonstrated an inactivation time constant of 400 to 500 ms for mK1 compared with 1,300 to 2,000 ms for mK2. The mK1 channel was more sensitive than the MK2 channel to the K channel blocker 4-AP. In myocytes isolated from Kv1DN mice with functional knock out of the Kv1.5 channel, mK1 was not detectable but mK2 was present. Our data suggest that the newly characterized K+ channels, mK1 and mK2, likely correspond to the macroscopic currents of I K,slow1 and I K,slow2, respectively.

Localization and mobility of the delayed-rectifer K+ channel Kv2.1 in adult cardiomyocytes

The delayed-rectifier voltage-gated K(+) channel (Kv) 2.1 underlies the cardiac slow K(+) current in the rodent heart and is particularly interesting in that both its function and localization are regulated by many stimuli in neuronal systems. However, standard immunolocalization approaches do not detect cardiac Kv2.1; therefore, little is known regarding its localization in the heart. In the present study, we used recombinant adenovirus to determine the subcellular localization and lateral mobility of green fluorescent protein (GFP)-Kv2.1 and yellow fluorescent protein-Kv1.4 in atrial and ventricular myocytes. In atrial myocytes, Kv2.1 formed large clusters on the cell surface similar to those observed in hippocampal neurons, whereas Kv1.4 was evenly distributed over both the peripheral sarcolemma and the transverse tubules. However, fluorescence recovery after photobleach (FRAP) experiments indicate that atrial Kv2.1 was immobile, whereas Kv1.4 was mobile (tau = 252 +/- 42 s). In ventricular myocytes, Kv2.1 did not form clusters and was localized primarily in the transverse-axial tubules and sarcolemma. In contrast, Kv1.4 was found only in transverse tubules and sarcolemma. FRAP studies revealed that Kv2.1 has a higher mobility in ventricular myocytes (tau = 479 +/- 178 s), although its mobility is slower than Kv1.4 (tau(1) = 18.9 +/- 2.3 s; tau(2) = 305 +/- 55 s). We also observed the movement of small, intracellular transport vesicles containing GFP-Kv2.1 within ventricular myocytes. These data are the first evidence of Kv2.1 localization in living myocytes and indicate that Kv2.1 may have distinct physiological roles in atrial and ventricular myocytes.

Simulations of propagated mouse ventricular action potentials: effects of molecular heterogeneity

The molecular heterogeneity of repolarizing currents produces significant spatial heterogeneity and/or dispersion of repolarization in many mammalian cardiac tissues. Transgenic mice are prominent experimental models for the study of the molecular basis of repolarization and arrhythmias. However, it is debated whether the small mouse heart can sustain physiologically relevant heterogeneity of repolarization. We used a comprehensive model of the mouse action potential (AP) to predict how small a region of the cardiac tissue can maintain spatial gradients of repolarization due to differential expression of channels. Our simulations of a one-dimensional multicellular ring or cable predict that substantial gradients in repolarization and intracellular Ca(2+) concentration transients can be maintained through heterogeneity of expression of K(+) channels in distances of approximately 10 cells that are sufficient to block propagation. The abruptness of expression gradients and the site of stimulation can cause Ca(2+) transient oscillations and affect the stability of Ca(2+) dynamics and AP propagation. Two different mechanisms of instability of AP propagation in one-dimensional cable occur at fast pacing rates. Transitions from periodic activity to alternans or to irregular behavior were observed. Abrupt gradients of channel expression can cause alternans at slower pacing rates than gradual changes. Our simulations demonstrate the importance of incorporating realistic Ca(2+) dynamics and current densities into models of propagated AP. They also emphasize that microscopic aspects of tissue organization are important for predicting large-scale propagation phenomena. Finally, our results predict that the mouse heart should be able to sustain substantial molecularly based heterogeneity of repolarization.

Molecular physiology of cardiac repolarization

The heart is a rhythmic electromechanical pump, the functioning of which depends on action potential generation and propagation, followed by relaxation and a period of refractoriness until the next impulse is generated. Myocardial action potentials reflect the sequential activation and inactivation of inward (Na(+) and Ca(2+)) and outward (K(+)) current carrying ion channels. In different regions of the heart, action potential waveforms are distinct, owing to differences in Na(+), Ca(2+), and K(+) channel expression, and these differences contribute to the normal, unidirectional propagation of activity and to the generation of normal cardiac rhythms. Changes in channel functioning, resulting from inherited or acquired disease, affect action potential repolarization and can lead to the generation of life-threatening arrhythmias. There is, therefore, considerable interest in understanding the mechanisms that control cardiac repolarization and rhythm generation. Electrophysiological studies have detailed the properties of the Na(+), Ca(2+), and K(+) currents that generate cardiac action potentials, and molecular cloning has revealed a large number of pore forming (alpha) and accessory (beta, delta, and gamma) subunits thought to contribute to the formation of these channels. Considerable progress has been made in defining the functional roles of the various channels and in identifying the alpha-subunits encoding these channels. Much less is known, however, about the functioning of channel accessory subunits and/or posttranslational processing of the channel proteins. It has also become clear that cardiac ion channels function as components of macromolecular complexes, comprising the alpha-subunits, one or more accessory subunit, and a variety of other regulatory proteins. In addition, these macromolecular channel protein complexes appear to interact with the actin cytoskeleton and/or the extracellular matrix, suggesting important functional links between channel complexes, as well as between cardiac structure and electrical functioning. Important areas of future research will be the identification of (all of) the molecular components of functional cardiac ion channels and delineation of the molecular mechanisms involved in regulating the expression and the functioning of these channels in the normal and the diseased myocardium.

Molecular mechanisms of regulation of fast-inactivating voltage-dependent transient outward K+ current in mouse heart by cell volume changes

The K(v)4.2/4.3 channels are the primary subunits that contribute to the fast-inactivating, voltage-dependent transient outward K(+) current (I(to,fast)) in the heart. I(to,fast) is the critical determinant of the early repolarization of the cardiac action potential and plays an important role in the adaptive remodelling of cardiac myocytes, which usually causes cell volume changes, during myocardial ischaemia, hypertrophy and heart failure. It is not known, however, whether I(to,fast) is regulated by cell volume changes. In this study we investigated the molecular mechanism for cell volume regulation of I(to,fast) in native mouse left ventricular myocytes. Hyposmotic cell swelling caused a marked increase in densities of the peak I(to,fast) and a significant shortening in phase 1 repolarization of the action potential duration. The voltage-dependent gating properties of I(to,fast) were, however, not altered by changes in cell volume. In the presence of either protein kinase C (PKC) activator (12,13-dibutyrate) or phosphatase inhibitors (calyculin A and okadaic acid), hyposmotic cell swelling failed to further up-regulate I(to,fast). When expressed in NIH/3T3 cells, both K(v)4.2 and K(v)4.3 channels were also strongly regulated by cell volume in the same voltage-independent but PKC- and phosphatase-dependent manner as seen in I(to,fast) in the native cardiac myocytes. We conclude that K(v)4.2/4.3 channels in the heart are regulated by cell volume through a phosphorylation/dephosphorylation pathway mediated by PKC and serine/threonine phosphatase(s). These findings suggest a novel role of K(v)4.2/4.3 channels in the adaptive electrical and structural remodelling of cardiac myocytes in response to myocardial hypertrophy, ischaemia and reperfusion.

Effects of hyperthyroidism on delayed rectifier K+ currents in left and right murine atria

Hyperthyroidism has been associated with atrial fibrillation (AF); however, hyperthyroidism-induced ion channel changes that may predispose to AF have not been fully elucidated. To understand the electrophysiological changes that occur in left and right atria with hyperthyroidism, the patch-clamp technique was used to compare action potential duration (APD) and whole cell currents in myocytes from left and right atria from both control and hyperthyroid mice. Additionally, RNase protection assays and immunoblotting were performed to evaluate the mRNA and protein expression levels of K(+) channel alpha-subunits in left and right atria. The results showed that 1) in control mice, the APD was shorter and the ultra-rapid delayed rectifier K(+) conductance (I(Kur)) and the sustained delayed rectifier K(+) conductance (I(ss)) were larger in the left than in the right atrium; also, mRNA and protein expression levels of Kv1.5 and Kv2.1 were higher in the left atrium; 2) in hyperthyroid mice, the APD was shortened and I(Kur) and I(ss) were increased in both left and right atrial myocytes, and the protein expression levels of Kv1.5 and Kv2.1 were increased significantly in both atria; and 3) the influence of hyperthyroidism on APD and delayed rectifier K(+) currents was more prominent in right than in left atrium, which minimized the interatrial APD difference. In conclusion, hyperthyroidism resulted in more significant APD shortening and greater delayed rectifier K(+) current increases in the right vs. the left atrium, which can contribute to the propensity for atrial arrhythmia in hyperthyroid heart.

Characterization of chromanol 293B-induced block of the delayed-rectifier K+ current in heart-derived H9c2 cells

The effects of chromanol 293B on ion currents in rat embryonic heart-derived H9c2 cells were investigated in this study. Chromanol 293B suppressed the amplitude of delayed rectified K+ current (I(K)) in a concentration-dependent manner. The IC50 value for chromanol 293B-induced inhibition of I(K) was 8 microM. The I(K) present in these cells, the electrical properties of which resembled those for the Kv2.1-related K+ current, was sensitive to inhibition by quinidine or dendrotoxin, yet not by pandinotoxin-Kalpha, E-4031 or apamin. Chromanol 293B reduced the activation time constant of I(K) and the effective gating charge of this channel. However, little or no modification in the steady-state inactivation of I(K) in response to long-lasting conditioning pulses could be demonstrated in the presence of chromanol 293B. These results clearly demonstrate that chromanol 293B can effectively interact with the K+ channel functionally expressed in H9c2 myoblasts. The chromanol 293B-induced inhibition of these channels could primarily be attributed to open channel block.

Accessory Kvbeta1 subunits differentially modulate the functional expression of voltage-gated K+ channels in mouse ventricular myocytes

Voltage-gated K+ (Kv) channel accessory (beta) subunits associate with pore-forming Kv alpha subunits and modify the properties and/or cell surface expression of Kv channels in heterologous expression systems. There is very little presently known, however, about the functional role(s) of Kv beta subunits in the generation of native cardiac Kv channels. Exploiting mice with a targeted disruption of the Kvbeta1 gene (Kvbeta1-/-), the studies here were undertaken to explore directly the role of Kvbeta1 in the generation of ventricular Kv currents. Action potential waveforms and peak Kv current densities are indistinguishable in myocytes isolated from the left ventricular apex (LVA) of Kvbeta1-/- and wild-type (WT) animals. Analysis of Kv current waveforms, however, revealed that mean+/-SEM I(to,f) density is significantly (P< or =0.01) lower in Kvbeta1-/- (21.0+/-0.9 pA/pF; n=68), than in WT (25.3+/-1.4 pA/pF; n=42), LVA myocytes, and that mean+/-SEM I(K,slow) density is significantly (P< or =0.01) higher in Kvbeta1-/- (19.1+/-0.9 pA/pF; n=68), compared with WT (15.9+/-0.7 pA/pF; n=42), LVA cells. Pharmacological studies demonstrated that the TEA-sensitive component of I(K,slow), I(K,slow2,) is selectively increased in Kvbeta1-/- LVA myocytes. In parallel with the alterations in I(to,f) and I(K,slow2) densities, Kv4.3 expression is decreased and Kv2.1 expression is increased in Kvbeta1-/- ventricles. Taken together, these results demonstrate that Kvbeta1 differentially regulates the functional cell surface expression of myocardial I(to,f) and I(K,slow2) channels.

Transgenic upregulation ofIK1in the mouse heart leads to multiple abnormalities of cardiac excitability

To assess the functional significance of upregulation of the cardiac current ( IK1), we have produced and characterized the first transgenic (TG) mouse model of IK1upregulation. To increase IK1density, a pore-forming subunit of the Kir2.1 (green fluorescent protein-tagged) channel was expressed in the heart under control of the α-myosin heavy chain promoter. Two lines of TG animals were established with a high level of TG expression in all major parts of the heart: line 1 mice were characterized by 14% heart hypertrophy and a normal life span; line 2 mice displayed an increased mortality rate, and in mice ≤1 mo old, heart weight-to-body weight ratio was increased by >100%. In adult ventricular myocytes expressing the Kir2.1-GFP subunit, IK1conductance at the reversal potential was increased ∼9- and ∼10-fold in lines 1 and 2, respectively. Expression of the Kir2.1 transgene in line 2 ventricular myocytes was heterogeneous when assayed by single-cell analysis of GFP fluorescence. Surface ECG recordings in line 2 mice revealed numerous abnormalities of excitability, including slowed heart rate, premature ventricular contractions, atrioventricular block, and atrial fibrillation. Line 1 mice displayed a less severe phenotype. In both TG lines, action potential duration at 90% repolarization and monophasic action potential at 75–90% repolarization were significantly reduced, leading to neuronlike action potentials, and the slow phase of the T wave was abolished, leading to a short Q-T interval. This study provides a new TG model of IK1upregulation, confirms the significant role of IK1in cardiac excitability, and is consistent with adverse effects of IK1upregulation on cardiac electrical activity.

Functional properties of K+ currents in adult mouse ventricular myocytes

Although the K+ currents expressed in hearts of adult mice have been studied extensively, detailed information concerning their relative sizes and biophysical properties in ventricle and atrium is lacking. Here we describe and validate pharmacological and biophysical methods that can be used to isolate the three main time- and voltage-dependent outward K+ currents which modulate action potential repolarization. A Ca2+ -independent transient outward K+ current, Ito, can be separated from total outward current using an 'inactivating prepulse'. The rapidly activating, slowly inactivating delayed rectifier K+ current, IKur, can be isolated using submillimolar concentrations of 4-aminopyridine (4-AP). The remaining K+ current, Iss, can be obtained by combining these two procedures: (i) inactivating Ito and (ii) eliminating IKur by application of low concentration of 4-AP. Iss activates relatively slowly and shows very little inactivation, even during depolarizations lasting several seconds. Our findings also show that the rate of reactivation of Ito is more than 20-fold faster than that of IKur. These results demonstrate that the outward K+ currents in mouse ventricles can be separated based on their distinct time and voltage dependence, and different sensitivities to 4-AP. Data obtained at both 22 and 32 degrees C demonstrate that although the duration of the inactivating prepulse has to be adapted for the recording temperature, this approach for separation of K+ current components is also valid at more physiological temperatures. To demonstrate that these methods also allow separation of these K+ currents in other cell types, we have applied this same approach to myocytes from mouse atria. Molecular approaches have been used to compare the expression levels of different K+ channels in mouse atrium and ventricle. These findings provide new insights into the functional roles of IKur, Ito and Iss during action potential repolarization.

Heterogeneous expression of repolarizing, voltage-gated K+ currents in adult mouse ventricles

Previous studies have documented the expression of four kinetically distinct voltage-gated K(+) (Kv) currents, I(to,f), I(to,s), I(K,slow) and I(ss), in mouse ventricular myocytes and demonstrated that I(to,f) and I(to,s) are differentially expressed in the left ventricular apex and the interventricular septum. The experiments here were undertaken to test the hypothesis that there are further regional differences in the expression of Kv currents or the Kv subunits (Kv4.2, Kv4.3, KChIP2, Kv1.5, Kv2.1) encoding these currents in adult male and female (C57BL6) mouse ventricles. Whole-cell voltage-clamp recordings revealed that mean (+/-s.e.m.) peak outward K(+) current and I(to,f) densities are significantly (P < 0.001) higher in cells isolated from the right (RV) than the left (LV) ventricles. Within the LV, peak outward K(+) current and I(to,f) densities are significantly (P < 0.05) higher in cells from the apex than the base. In addition, I(to,f) and I(K,slow) densities are lower in cells isolated from the endocardial (Endo) than the epicardial (Epi) surface of the LV wall. Importantly, similar to LV apex cells, I(to,s) is not detected in RV, LV base, LV Epi or LV Endo myocytes. No measurable differences in K(+) current densities or properties are evident in RV or LV cells from adult male and female mice, although I(to,f), I(to,s), I(K,slow) and I(ss) densities are significantly (P < 0.01) higher, and action potential durations at 50% (APD(50)) are significantly (P < 0.05) shorter in male septum cells. Western blot analysis revealed that the expression levels of Kv4.2, Kv4.3, KChIP2, Kv1.5 and Kv2.1 are similar in male and female ventricles. In addition, consistent with the similarities in repolarizing Kv current densities, no measurable differences in ECG parameters, including corrected QT (QT(c)) intervals, are detected in telemetric recordings from adult male and female (C57BL6) mice.

Computer model of action potential of mouse ventricular myocytes

We have developed a mathematical model of the mouse ventricular myocyte action potential (AP) from voltage-clamp data of the underlying currents and Ca2+ transients. Wherever possible, we used Markov models to represent the molecular structure and function of ion channels. The model includes detailed intracellular Ca2+ dynamics, with simulations of localized events such as sarcoplasmic Ca2+ release into a small intracellular volume bounded by the sarcolemma and sarcoplasmic reticulum. Transporter-mediated Ca2+ fluxes from the bulk cytosol are closely matched to the experimentally reported values and predict stimulation rate-dependent changes in Ca2+ transients. Our model reproduces the properties of cardiac myocytes from two different regions of the heart: the apex and the septum. The septum has a relatively prolonged AP, which reflects a relatively small contribution from the rapid transient outward K+ current in the septum. The attribution of putative molecular bases for several of the component currents enables our mouse model to be used to simulate the behavior of genetically modified transgenic mice.

Regulation of Ion Channel Expression

A potentially important mechanism controlling ion channel expression is homeostatic regulation, which can act to maintain a stable electrophysiological phenotype in cardiac myocytes as well as to provide plasticity in response to genetic, pathological, or pharmacological insults. The capabilities and limitations of the homeostatic regulatory mechanisms that contribute to the control of cardiac ion channel expression are the primary topic of this review.

Dynamic remodeling of K+ and Ca2+ currents in cells that survived in the epicardial border zone of canine healed infarcted heart

Action potentials (APs) of the epicardial border zone (EBZ) cells from the day 5 infarcted heart continue to be altered by day 14 postocclusion, namely, they shortened. However, by 2 mo, EBZ APs appear "normal," yet conduction of wave fronts remains abnormal. We hypothesize that the changes in transmembrane APs are due to a change in the distribution of ion channels in either density or function. Thus we focused on the changes in Ca2+ and K+ currents in cells isolated from the 14-day (IZ14d) and 2-mo (IZ2m) EBZ and compared them with those occurring in cells from the same hearts but remote (Rem) from the EBZ. Whole cell voltage-clamp techniques were used to measure and compare Ca2+ and K+ currents in cells from the different groups. Ca2+ current densities remain reduced in cells of the 14-day and 2-mo infarcted heart and the kinetic changes previously identified in the 5-day heart begin to, but do not recover to, cells from noninfarcted epicardium (NZ) values. Importantly, I(Ca,L) in both the EBZ and Rem regions still show a slowed recovery from inactivation. Furthermore, during the remodeling process, there is an increased expression of T-type Ca2+ currents, but only regionally, and only within a specific time window postmyocardial infarction (MI). Regional heterogeneity in beta-adrenergic responsiveness of I(Ca,L) exists between EBZ and remote cells of the 14-day hearts, but this regional heterogeneity is gone in the healed infarcted heart. In IZ14d, the transient outward K+ current (Ito) begins to reemerge and is accompanied by an upregulated tetraethylammonium-sensitive outward current. By 2-mo postocclusion, Ito and sustained outward K+ current have completed the reverse remodeling process. During the healing process post-MI, canine epicardial cells downregulate the fast Ito but compensate by upregulating a K+ current that in normal cells is minimally functional. For recovering I(Ca,L) of the 14-day and 2-mo EBZ cells, voltage-dependent processes appear to be reset, such that I(Ca,L) "window" current occurs at hyperpolarized potentials. Thus dynamic changes in both Ca2+ and K+ currents contribute to the altered AP observed in 14-day fibers and may account for return of APs of 2 mo EBZ fibers.

Attenuation of I(K,slow1) and I(K,slow2) in Kv1/Kv2DN mice prolongs APD and QT intervals but does not suppress spontaneous or inducible arrhythmias

Overexpression of a truncated Kv1.1 or Kv2.1 channel polypeptide in the heart (Kv1DN or Kv2DN) resulted in mice with a prolonged action potential duration (APD) due to marked attenuation of rapidly activating, slowly inactivating K+ current (I(K,slow1)) or slowly inactivating outward K(+) current (I(K,slow2)) in ventricular myocytes. ECG monitoring, optical mapping, and programmed electrical stimulation of Kv1DN mice revealed spontaneous and inducible reentrant ventricular tachycardia due to spatial dispersion of repolarization and refractoriness. Recently, we demonstrated upregulation of I(K,slow2) in apical cardiomyocytes derived from Kv1DN mice. We therefore hypothesized that the selective upregulation of Kv2.1-encoded currents underlies the apex-to-base dispersion of repolarization and the reentrant arrhythmias. To test this hypothesis, the Kv1DN line was crossbred with the Kv2DN line to produce Kv1/Kv2DN lines. Whole cell voltage-clamp recordings from left ventricular cells of Kv1/Kv2DN confirmed that the 4-aminopyridine- and tetraethylammonium-sensitive components of IK,slow were eliminated, resulting in marked APD prolongation compared with wild-type, Kv1DN, and Kv2DN cells. Telemetric ECG recordings revealed prolongation of the corrected QT in Kv1/Kv2DN compared with Kv1DN and Kv2DN mice. However, attenuation of Kv2.1-encoded currents in Kv1DN mice did not suppress the arrhythmias. Thus, the elimination of I(K,slow2) prolongs APD and the QT intervals, but does not have an antiarrhythmic effect.

Single-channel recordings of a rapid delayed rectifier current in adult mouse ventricular myocytes: basic properties and effects of divalent cations

The rapidly delayed rectifier current (I(Kr)) has been described in ventricular myocytes isolated from many species, as well as from neonatal mice. However, whether I(Kr) is present in the adult mouse heart remains controversial. We used cell-attached patch-clamp recording in symmetrical K(+) solutions to assess the presence and behaviour of single I(Kr) channels in adult mouse cardiomyocytes (mI(Kr)). Of 314 patches, 158 (50.1%) demonstrated mI(Kr) currents as compared with 131 (42.3%) for the I(K1) channel. Single mI(Kr) channel activity was rarely observed at potentials positive to -10 mV. The slope conductance at negative potentials was 12 pS. Upon repolarization, ensemble-averaged mI(Kr) showed slow deactivation with a biexponential time course. A selective I(Kr) blocker, E-4031 (1 microm), completely blocked mI(Kr) channel activity. Extracellular Ca(2+) and Mg(2+) at physiological concentrations shifted the activation by approximately 30 mV, accelerated deactivation kinetics, prolonged long-closed time, and reduced open probability without affecting single-channel conductance, suggesting a direct channel-blocking effect in addition to well-recognized voltage shifts. HERG subunits expressed in Chinese hamster ovary cells produced channels with properties similar to those of mI(Kr), except for the more-negative activation of the HERG channels. Despite the abundant expression of mI(Kr), single-channel events were rarely observed during action-potential clamp and 5 microm E-4031 had no detectable effect on the action potential parameters, confirming that mI(Kr) plays at best a minor role in repolarization of adult mouse cardiomyocytes, probably because the modulatory effects of divalent cations prevent significant mI(Kr) opening under physiological conditions.

Selective elimination of I(K,slow1) in mouse ventricular myocytes expressing a dominant negative Kv1.5alpha subunit

Although previous studies have revealed a role for the voltage-gated K+ channel alpha-subunit Kv1.5 (KCNA5) in the generation of the 4-aminopyridine (4-AP)-sensitive component of delayed rectification in mouse ventricles (IK,slow1), the phenotypic consequences of manipulating IK,slow1 expression in vivo in different (mouse) models are distinct. In these experiments, point mutations were introduced in the pore region of Kv1.5 to change the tryptophan (W) at position 461 to phenylalanine (F) to produce a nonconducting subunit, Kv1.5W461F, that is shown to function as a Kv1 subfamily-specific dominant negative (Kv1.5DN). With the use of the alpha-myosin heavy chain promoter to direct cardiac-specific expression, three lines of Kv1.5DN-expressing (C57BL6) transgenic mice were generated and characterized. Electrophysiological recordings from Kv1.5-DN-expressing left ventricular myocytes revealed that the micromolar 4-AP sensitive IK,slow1 is selectively eliminated. The attenuation of IK,slow1 is accompanied by increased ventricular action potential durations and marked QT prolongation. In contrast to previous findings in mice expressing a truncated (DN) Kv1.1 transgene; however, no electrical remodeling is evident in Kv1.5DN-expressing ventricular myocytes, and the (Kv1.5DN-induced) elimination of IK,slow1 does not result in spontaneous ventricular arrhythmias.

Chronic atrial fibrillation does not further decrease outward currents. It increases them

Rapid atrial pacing causes electrical remodeling that leads to atrial fibrillation (AF). AF can further remodel atrial electrophysiology to maintain AF. Our previous studies showed that there was a marked difference in the duration of AF in dogs that have been atrial paced at 400 beats/min for 6 wk. We hypothesized that this difference is based on the changes in the degree of electrical remodeling caused by rapid atrial pacing versus that by AF. Right atrial cells were isolated from control dogs (Con, N = 28), from dogs with chronic AF (cAF dogs, N = 13, episodes lasting at least 6 days), or from dogs with nonsustained or brief episodes of AF (nAF dogs, N = 10, episodes lasting minutes to hours). Both transient outward (Ito) and sustained outward K+ current (Isus) densities/functions were determined using whole cell voltage-clamp techniques. In nAF cells, Ito density was reduced by 69% at +40 mV: from 7.1 +/- 0.5 pA/pF (Con, n = 59) to 2.2 +/- 0.2 pA/pF (nAF, n = 24) (P < 0.05). The voltage dependence of inactivation of Ito was shifted positively and decay kinetics were changed; however, recovery from inactivation was not altered in nAF cells. In contrast, Ito density in cAF cells was both significantly different from Con cells and larger than that in nAF cells [at +40 mV, 3.5 +/- 0.3 pA/pF (cAF, n = 29), P < 0.05]. In cAF cells, recovery from inactivation and decay of Ito were both slow; yet, voltage dependence inactivation of Ito approached that of Con cells. Furthermore, "recovered" Ito of cAF cells was more sensitive to tetraethylammonium than currents of Con and nAF cells. Isus densities of nAF and cAF cells did not differ. Both nAF and cAF cells have reduced Ito versus Con cells, but Ito remodeling of nAF cells differed from that of cAF cells. Ito in cAF dogs was likely remodeled by AF per se, whereas that in nAF dogs was likely the consequence of the rapid rate in the absence of sustained AF.

Long-term restitution of 4-aminopyridine-sensitive currents in Kv1DN ventricular myocytes using adeno-associated virus-mediated delivery of Kv1.5

Overexpression of a dominant-negative truncated Kv1.1 (Kv1DN) polypeptide in the mouse heart resulted in marked attenuation of a 4-aminopyridine (4-AP)-sensitive current, I(K,slow1). We used recombinant adeno-associated virus (rAAV) as a vector for direct delivery of Kv1.5 into the mouse myocardium in order to normalize the action potential duration (APD) 6 months after injection. The injection of rAAV-Kv1.5 reconstituted the 4-AP-sensitive outward potassium currents, shortened the APD, and eliminated spontaneous early afterdepolarizations. Immunoblots detected the FL-Kv1.5 polypeptides only in rAAV-Kv1.5-infected hearts. These data demonstrate long-term expression of 4-AP-sensitive potassium currents in ventricular myocytes by gene transfer using rAAV vector encodes Kv1.5.