The molecular physiology of the cardiac transient outward potassium current (I(to)) in normal and diseased myocardium

Regression analysis for constraining free parameters in electrophysiological models of cardiac cells

A major challenge in computational biology is constraining free parameters in mathematical models. Adjusting a parameter to make a given model output more realistic sometimes has unexpected and undesirable effects on other model behaviors. Here, we extend a regression-based method for parameter sensitivity analysis and show that a straightforward procedure can uniquely define most ionic conductances in a well-known model of the human ventricular myocyte. The model's parameter sensitivity was analyzed by randomizing ionic conductances, running repeated simulations to measure physiological outputs, then collecting the randomized parameters and simulation results as "input" and "output" matrices, respectively. Multivariable regression derived a matrix whose elements indicate how changes in conductances influence model outputs. We show here that if the number of linearly-independent outputs equals the number of inputs, the regression matrix can be inverted. This is significant, because it implies that the inverted matrix can specify the ionic conductances that are required to generate a particular combination of model outputs. Applying this idea to the myocyte model tested, we found that most ionic conductances could be specified with precision (R(2) > 0.77 for 12 out of 16 parameters). We also applied this method to a test case of changes in electrophysiology caused by heart failure and found that changes in most parameters could be well predicted. We complemented our findings using a Bayesian approach to demonstrate that model parameters cannot be specified using limited outputs, but they can be successfully constrained if multiple outputs are considered. Our results place on a solid mathematical footing the intuition-based procedure simultaneously matching a model's output to several data sets. More generally, this method shows promise as a tool to define model parameters, in electrophysiology and in other biological fields.

Phase-2 reentry in cardiac tissue: role of the slow calcium pulse

Phase-2 re-entry is thought to underlie many causes of idiopathic ventricular arrhythmias as, for instance, those occurring in Brugada syndrome. In this paper, we study under which circumstances a region of depolarized tissue can re-excite adjacent regions that exhibit shorter action potential duration (APD), eventually inducing reentry. For this purpose, we use a simplified ionic model that reproduces well the ventricular action potential. With the help of this model, we analyze the conditions that lead to very short action potentials (APs), as well as possible mechanisms for re-excitation in a cable. We then study the induction of re-entrant waves (spiral waves) in simulations of AP propagation in the heart ventricles. We show that re-excitation takes place via a slow pulse produced by calcium current that propagates into the region of short APs until it encounters excitable tissue. We calculate analytically the speed of the slow pulse, and also give an estimate of the minimal tissue size necessary for allowing reexcitation to take place.

Changes in the calcium current among different transmural regions contributes to action potential heterogeneity in rat heart

To clarify the transmural heterogeneity of action potential (AP) time course, we examined the regulation of L-type Ca(2+) current (I(Ca,L)) by voltage and Ca(2+)-dependent mechanisms. Currents were recorded using patch clamp of single rat subepicardial (EPI) and subendocardial (ENDO) of left ventricular, right ventricular (RV) and septal (SEP) cardiomyocytes. Voltage clamp commands were derived from ENDO and EPI APs or rectangular voltage pulses. During rectangular pulses, peak I(Ca,L) was significantly greater in EPI than in other cells. The inactivation of I(Ca,L) by Ca(2+)-dependent mechanisms (suppressed by ryanodine and BAPTA) was present in all cells but greater in extent in ENDO and SEP cells. Activation and inactivation curves for all regions show subtle differences that are Ca(2+) sensitive, with Ca(2+) inactivation shifting the activation variables negative by approximately 7 mV and inactivation variables positive by 2-7 mV (EPI being least, RV greatest). In AP-clamps, the peak I(Ca,L) was significantly smaller in ENDO than in EPI cells, while the integrated current was significantly larger in ENDO than in EPI cells. The results are discussed with regard to the interplay of AP time course and net Ca(2+) influx.

Electrophysiological remodeling in heart failure

Heart failure affects nearly 6 million Americans, with a half-million new cases emerging each year. Whereas up to 50% of heart failure patients die of arrhythmia, the diverse mechanisms underlying heart failure-associated arrhythmia are poorly understood. As a consequence, effectiveness of antiarrhythmic pharmacotherapy remains elusive. Here, we review recent advances in our understanding of heart failure-associated molecular events impacting the electrical function of the myocardium. We approach this from an anatomical standpoint, summarizing recent insights gleaned from pre-clinical models and discussing their relevance to human heart failure.

KChIP2 attenuates cardiac hypertrophy through regulation of Ito and intracellular calcium signaling

Recent evidence shows that the auxiliary subunit KChIP2, which assembles with pore-forming Kv4-subunits, represents a new potential regulator of the cardiac calcium-independent transient outward potassium current (I(to)) density. In hypertrophy and heart failure, KChIP2 expression has been found to be significantly decreased. Our aim was to examine the role of KChIP2 in cardiac hypertrophy and the effect of restoring its expression on electrical remodeling and cardiac mechanical function using a combination of molecular, biochemical and gene targeting approaches. KChIP2 overexpression through gene transfer of Ad.KChIP2 in neonatal cardiomyocytes resulted in a significant increase in I(to)-channel forming Kv4.2 and Kv4.3 protein levels. In vivo gene transfer of KChIP2 in aortic banded adult rats showed that, compared to sham-operated or Ad.beta-gal-transduced hearts, KChIP2 significantly attenuated the developed left ventricular hypertrophy, robustly increased I(to) densities, shortened action potential duration, and significantly altered myocyte mechanics by shortening contraction amplitudes and maximal rates of contraction and relaxation velocities and decreasing Ca(2+) transients. Interestingly, blocking I(to) with 4-aminopyridine in KChIP2-overexpressing adult cardiomyocytes significantly increased the Ca(2+) transients to control levels. One-day-old rat pups intracardially transduced with KChIP2 for two months then subjected to aortic banding for 6-8 weeks (to induce hypertrophy) showed similar echocardiographic, electrical and mechanical remodeling parameters. In addition, in cultured adult cardiomyocytes, KChIP2 overexpression increased the expression of Ca(2+)-ATPase (SERCA2a) and sodium calcium exchanger but had no effect on ryanodine receptor 2 or phospholamban expression. In neonatal myocytes, KChIP2 notably reversed Ang II-induced hypertrophic changes in protein synthesis and MAP-kinase activation. It also significantly decreased calcineurin expression, NFATc1 expression and nuclear translocation and its downstream target, MCiP1.4. Altogether, these data show that KChIP2 can attenuate cardiac hypertrophy possibly through modulation of intracellular calcium concentration and calcineurin/NFAT pathway.

hERG-related drug toxicity and models for predicting hERG liability and QT prolongation

BACKGROUND

hERG K(+) channels have been recognized as a primary antitarget in safety pharmacology. Their blockade, caused by several drugs with different therapeutic indications, may lead to QT prolongation and, eventually, to potentially fatal arrhythmia, namely torsade de pointes. Therefore, a number of preclinical models have been developed to predict hERG liability early in the drug development process.

OBJECTIVE

The aim of this review is to outline the present state of the art on drug-induced hERG blockade, providing insights on the predictive value of in vitro and in silico models for hERG liability.

METHODS

On the basis of latest reports, high-throughput preclinical models have been discussed outlining advantages and limitations.

CONCLUSION

Although no single model has an absolute value, an integrated risk assessment is recommended to predict the pro-arrhythmic risk of a given drug. This prediction requires expertise from different areas and should encompass emerging issues such as interference with hERG trafficking and QT shortening.

Experimental and theoretical ventricular electrograms and their relation to electrophysiological gradients in the adult rat heart

The electrical activity of adult mouse and rat hearts has been analyzed extensively, often as a prerequisite for genetic engineering studies or for the development of rodent models of human diseases. Some aspects of the initiation and conduction of the cardiac action potential in rodents closely resemble those in large mammals. However, rodents have a much higher heart rate and their ventricular action potential is triangular and very short. As a consequence, an interpretation of the electrocardiogram in the mouse and rat remains difficult and controversial. In this study, optical mapping techniques have been applied to an in vitro left ventricular adult rat preparation to obtain patterns of conduction and action potential duration measurements from the epicardial surface. This information has been combined with previously published mathematical models of the rat ventricular myocyte to develop a bidomain model for action potential propagation and electrogram formation in the rat left ventricle. Important insights into the basis for the repolarization waveform in the ventricular electrogram of the adult rat have been obtained. Notably, our model demonstrated that the biphasic shape of the rat ventricular repolarization wave can be explained in terms of the transmural and apex-to-base gradients in action potential duration that exist in the rat left ventricle.

Molecular determinants of cardiac transient outward potassium current (I(to)) expression and regulation

Rapidly activating and inactivating cardiac transient outward K(+) currents, I(to), are expressed in most mammalian cardiomyocytes, and contribute importantly to the early phase of action potential repolarization and to plateau potentials. The rapidly recovering (I(t)(o,f)) and slowly recovering (I(t)(o,s)) components are differentially expressed in the myocardium, contributing to regional heterogeneities in action potential waveforms. Consistent with the marked differences in biophysical properties, distinct pore-forming (alpha) subunits underlie the two I(t)(o) components: Kv4.3/Kv4.2 subunits encode I(t)(o,f), whereas Kv1.4 encodes I(t)(o,s), channels. It has also become increasingly clear that cardiac I(t)(o) channels function as components of macromolecular protein complexes, comprising (four) Kvalpha subunits and a variety of accessory subunits and regulatory proteins that influence channel expression, biophysical properties and interactions with the actin cytoskeleton, and contribute to the generation of normal cardiac rhythms. Derangements in the expression or the regulation of I(t)(o) channels in inherited or acquired cardiac diseases would be expected to increase the risk of potentially life-threatening cardiac arrhythmias. Indeed, a recently identified Brugada syndrome mutation in KCNE3 (MiRP2) has been suggested to result in increased I(t)(o,f) densities. Continued focus in this area seems certain to provide new and fundamentally important insights into the molecular determinants of functional I(t)(o) channels and into the molecular mechanisms involved in the dynamic regulation of I(t)(o) channel functioning in the normal and diseased myocardium.

Ethnic differences in ST height in the multiethnic study of atherosclerosis

BACKGROUND

ST elevation in precordial leads has been associated with genetic syndromes of arrhythmias and sudden death. ST height data in different ethnic groups are limited.

METHODS

ST height was determined in 4612 African-American, Chinese, Hispanic, and non-Hispanic white men and women aged 45-84 years in the Multiethnic Study of Atherosclerosis (MESA). For leads I, II, and V(1) to V(6,) ST height, measured at the J point and 60 ms after the J point, adjusted for covariates were compared between non-Hispanic white and other ethnic groups using analysis of covariance (ANCOVA).

RESULTS

Among men, ST height was significantly different across all ethnic groups at both time points for all leads (P < 0.01), except at the J point for limb lead II (P = 0.2). Among women, differences were also significant at the J point and 60 ms past the J point (P < 0.01). ST height was lowest for non-Hispanic whites in all leads and at both time points. At the J point, Chinese had the highest ST height for leads V(1) and V(2), whereas African Americans had the greatest ST height for leads I and V(3) to V(6). At 60 ms past the J point, Chinese men had the greatest ST height for lead I and V(1) to V(6;) and Chinese women had greatest ST height for leads V(1) to V(3).

CONCLUSIONS

There were significant differences in ST height among ethnic groups in all ECG leads. The physiological mechanisms and clinical significance of these differences and the possible association with arrhythmias require further study.

Divergent regulation of cardiac KCND3 potassium channel expression by the thyroid hormone receptors alpha1 and beta1

The cardiac transient outward current I(to) is regulated by thyroid hormone (T3). However, it remains unclear whether T3 directly modulates underlying gene transcription and which thyroid receptor (TR) isoform might be responsible for gene transactivation. To clarify this situation, we analysed the role of T3 and its receptors alpha1 (TRalpha1) and beta1 (TRbeta1) in regulation of KCNA4, KCND2, KCND3 and KCNIP2 transcription in rat cardiomyocytes. Initial results demonstrated a T3-mediated increase of I(to) current density. T3 stimulation enhanced KCND2 and KCND3 expression and decreased KCNA4 transcription, while KCNIP2 remained unaffected. To dissect the role of TRalpha1 and TRbeta1 in T3-dependent I(to) modulation, TRalpha1 and TRbeta1 were overexpressed in cardiomyocytes by adenovirus-mediated gene transfer. TRalpha1 increased I(to), while TRbeta1 significantly reduced I(to) in size, which was associated with TRalpha1-mediated increase and TRbeta1-mediated reduction of KCND2/3 transcription. To further evaluate a possible direct interaction of TRalpha1 and TRbeta1 with the KCND3 promoter, TR expression vectors were cotransfected with a construct containing 2335 bp of the KCND3 5'-flanking sequence linked to a luciferase reporter into ventricular myocytes. While the TRalpha1 aporeceptor enhanced KCND3 transcription, the TRbeta1 aporeceptor suppressed KCND3 expression, with both effects exhibiting ligand-dependent amplification upon T3 stimulation. Deletion of the KCND3 5'-flanking region localized the suppressible promoter sequence for TRbeta1 to within -293 bp and the activating promoter sequence for TRalpha1 to within -2335 to -1654 bp of the transcription start site. Disruption of putative TR binding sites by mutagenesis abolished the TRalpha1- (G-1651T) and TRbeta1- (G-73T) mediated effects, indicating that TRalpha1 and TRbeta1 response elements map to different regions of the KCND3 promoter. Thus, I(to) is modulated by diverse T3-dependent regulation of underlying gene transcription. TRalpha1 and TRbeta1 exhibit distinct effects on KCND3 transactivation with TRalpha1 enhancing and TRbeta1 suppressing KCND3 transcription.

Properties and ionic mechanisms of action potential adaptation, restitution, and accommodation in canine epicardium

Computational models of cardiac myocytes are important tools for understanding ionic mechanisms of arrhythmia. This work presents a new model of the canine epicardial myocyte that reproduces a wide range of experimentally observed rate-dependent behaviors in cardiac cell and tissue, including action potential (AP) duration (APD) adaptation, restitution, and accommodation. Model behavior depends on updated formulations for the 4-aminopyridine-sensitive transient outward current (I(to1)), the slow component of the delayed rectifier K(+) current (I(Ks)), the L-type Ca(2+) channel current (I(Ca,L)), and the Na(+)-K(+) pump current (I(NaK)) fit to data from canine ventricular myocytes. We found that I(to1) plays a limited role in potentiating peak I(Ca,L) and sarcoplasmic reticulum Ca(2+) release for propagated APs but modulates the time course of APD restitution. I(Ks) plays an important role in APD shortening at short diastolic intervals, despite a limited role in AP repolarization at longer cycle lengths. In addition, we found that I(Ca,L) plays a critical role in APD accommodation and rate dependence of APD restitution. Ca(2+) entry via I(Ca,L) at fast rate drives increased Na(+)-Ca(2+) exchanger Ca(2+) extrusion and Na(+) entry, which in turn increases Na(+) extrusion via outward I(NaK). APD accommodation results from this increased outward I(NaK). Our simulation results provide valuable insight into the mechanistic basis of rate-dependent phenomena important for determining the heart's response to rapid and irregular pacing rates (e.g., arrhythmia). Accurate simulation of rate-dependent phenomena and increased understanding of their mechanistic basis will lead to more realistic multicellular simulations of arrhythmia and identification of molecular therapeutic targets.

RNA silencing: small RNA-mediated posttranscriptional regulation of mRNA and the implications for heart electropathophysiology

Gene silencing refers to the "switching off" of genes within the cell: it can occur at transcriptional and posttranscriptional levels, controlling, respectively, how much mRNA is transcribed from each gene and how much protein is translated from this mRNA. Knowledge of its governing mechanisms is fundamental to our understanding of physiology; moreover, where there is a relevance for pathology, new diagnostic and therapeutic tools may be developed. Recently, families of noncoding RNA (ncRNA)-RNA that does not encode for a protein end-product--have been discovered that function as regulators of gene silencing. This has revolutionized biology by challenging the credence in the centrality of proteins as the regulators of biological processes, and is changing the way we study pathophysiology. In fact, a subfamily of small ncRNAs, called microRNA (miRNA), is now known as one of the most abundant class of regulatory molecules, and over one-third of human genes-including a growing number of key genes of the heart-may be targeted by one or more of the hundreds of miRNAs that exist. Here, we focus on how these small ncRNAs control translation, on the extraordinary consequences this class of regulator is currently known to have on aspects of cardiac excitability, and on the exciting therapeutic potential they have in this field.

Novel transient outward and ultra-rapid delayed rectifier current antagonist, AVE0118, protects against ventricular fibrillation induced by myocardial ischemia

AVE0118 is a novel drug that blocks the transient outward current (Ito), the ultra rapid component of the delayed rectifier current (IKur), and the acetylcholine dependent potassium channel (IKach). The latter 2 channels are more abundant in atrial tissue. It is possible that AVE0118 could reduce regional differences in repolarization and thereby prevent malignant arrhythmias provoked by ischemia. To test this hypothesis, ventricular fibrillation was induced by a 2-minute occlusion of the left circumflex coronary artery during the last min of exercise in dogs with healed myocardial infarctions (n = 9). On a subsequent day, this exercise plus ischemia test was repeated after pretreatment with AVE0118 (1.0 mg/kg, IV). AVE0118 did not change QTc (Van de Water's correction) interval [245 +/- 6.0 ms (control) versus 242 +/- 2.3 ms (AVE)] and attenuated the dispersion of repolarization as measured by the duration of the descending portion of the T wave (Tpeak - Tend) induced by ischemia [ischemic changes: +11.1 +/- 2.4 ms (no drug) versus +2.2 +/- 3.7 ms (AVE)]. AVE0118 also significantly reduced the incidence of ventricular fibrillation, protecting 7 of 9 animals. Thus, AVE0118 abolished ischemically induced repolarization abnormalities and prevented malignant arrhythmias induced by ischemia without altering QTc interval.

Regulation of Kv4 channel expression in failing rat heart by the thioredoxin system

Redox imbalance elicited by oxidative stress contributes to pathogenic remodeling of ion channels that underlies arrhythmogenesis and contractile dysfunction in the failing heart. This study examined whether the expression of K(+) channels in the remodeled ventricle is controlled by the thioredoxin system, a principal oxidoreductase network regulating redox-sensitive proteins. Ventricular dysfunction was induced in rats by coronary artery ligation, and experiments were conducted 6-8 wk postinfarction. Biochemical assays of tissue extracts from infarcted hearts showed that thioredoxin reductase activity was decreased by 32% from sham-operated controls (P < 0.05), whereas thioredoxin activity was 51% higher postinfarction (P < 0.05). These differences in activities paralleled changes in protein abundance as determined by Western blot analysis. However, whereas real-time PCR showed thioredoxin reductase mRNA levels to be significantly decreased postinfarction, thioredoxin mRNA was not altered. In voltage-clamp studies of myocytes from infarcted hearts, the characteristic downregulation of transient-outward K(+) current density was reversed by exogenous pyruvate (5 mmol/l), and this effect was blocked by the specific inhibitors of the thioredoxin system: auranofin or 13-cis-retinoic acid. Real-time PCR and Western blot analyses of myocyte suspensions from infarcted hearts showed that pyruvate increased mRNA and protein abundance of Kv4.2 and Kv4.3 channel alpha-subunits as well as the accessory protein KChIP2 when compared with time-matched, untreated cells (P < 0.05). The pyruvate-induced increase in Kv4.x expression was blocked by auranofin, but the upregulation of KChIP2 expression was not affected. These data suggest that the expression of Kv4.x channels is redox-regulated by the thioredoxin system, which may be a novel therapeutic target to reverse or limit electrical remodeling of the failing heart.

The hERG K+ channel: target and antitarget strategies in drug development

The human ether-à-go-go related gene (hERG) K+ channel is of great interest for both basic researchers and clinicians because its blockade by drugs can lead to QT prolongation, which is a risk factor for torsades de pointes, a potentially life-threatening arrhythmia. A growing list of agents with "QT liability" have been withdrawn from the market or restricted in their use, whereas others did not even receive regulatory approval for this reason. Thus, hERG K+ channels have become a primary antitarget (i.e. an unwanted target) in drug development because their blockade causes potentially serious side effects. On the other hand, the recent identification and functional characterization of hERG K+ channels not only in the heart, but also in several other tissues (e.g. neurons, smooth muscle and cancer cells) may have far reaching implications for drug development for a possible exploitation of hERG as a target, especially in oncology and cardiology.

Modification of K+ channel-drug interactions by ancillary subunits

Reconciling ion channel alpha-subunit expression with native ionic currents and their pharmacological sensitivity in target organs has proved difficult. In native tissue, many K(+) channel alpha-subunits co-assemble with ancillary subunits, which can profoundly affect physiological parameters including gating kinetics and pharmacological interactions. In this review, we examine the link between voltage-gated potassium ion channel pharmacology and the biophysics of ancillary subunits. We propose that ancillary subunits can modify the interaction between pore blockers and ion channels by three distinct mechanisms: changes in (1) binding site accessibility; (2) orientation of pore-lining residues; (3) the ability of the channel to undergo post-binding conformational changes. Each of these subunit-induced changes has implications for gating, drug affinity and use dependence of their respective channel complexes. A single subunit may modulate its associated alpha-subunit by more than one of these mechanisms. Voltage-gated potassium channels are the site of action of many therapeutic drugs. In addition, potassium channels interact with drugs whose primary target is another channel, e.g. the calcium channel blocker nifedipine, the sodium channel blocker quinidine, etc. Even when K(+) channel block is the intended mode of action, block of related channels in non-target organs, e.g. the heart, can result in major and potentially lethal side-effects. Understanding factors that determine specificity, use dependence and other properties of K(+) channel drug binding are therefore of vital clinical importance. Ancillary subunits play a key role in determining these properties in native tissue, and so understanding channel-subunit interactions is vital to understanding clinical pharmacology.

Kv4.3 is not required for the generation of functional Ito,f channels in adult mouse ventricles

Accumulated evidence suggests that the heteromeric assembly of Kv4.2 and Kv4.3 alpha-subunits underlies the fast transient Kv current (I(to,f)) in rodent ventricles. Recent studies, however, demonstrated that the targeted deletion of Kv4.2 results in the complete elimination of I(to,f) in adult mouse ventricles, revealing an essential role for the Kv4.2 alpha-subunit in the generation of mouse ventricular I(to,f) channels. The present study was undertaken to investigate directly the functional role of Kv4.3 by examining the effects of the targeted disruption of the KCND3 (Kv4.3) locus. Mice lacking Kv4.3 (Kv4.3-/-) appear indistinguishable from wild-type control animals, and no structural or functional abnormalities were evident in Kv4.3-/- hearts. Voltage-clamp recordings revealed that functional I(to,f) channels are expressed in Kv4.3-/- ventricular myocytes, and that mean I(to,f) densities are similar to those recorded from wild-type cells. In addition, I(to,f) properties (inactivation rates, voltage dependences of inactivation and rates of recovery from inactivation) in Kv4.3-/- and wild-type mouse ventricular myocytes were indistinguishable. Quantitative RT-PCR and Western blot analyses did not reveal any measurable changes in the expression of Kv4.2 or the Kv channel interacting protein (KChIP2) in Kv4.3-/- ventricles. Taken together, the results presented here suggest that, in contrast with Kv4.2, Kv4.3 is not required for the generation of functional mouse ventricular I(to,f) channels.

Interaction of syntaxin 1A with the N-terminus of Kv4.2 modulates channel surface expression and gating

Kv4.2 channels are responsible in the heart for the Ca2+-independent transient outward currents and are important in regulating myocardial excitability and Ca2+ homeostasis. We have identified previously the expression of syntaxin 1A (STX1A) on the cardiac ventricular myocyte plasma membranes, and its modulation of cardiac ATP-sensitive K+ channels. We speculated that STX1A interacts with other cardiac ion channels, thus we examined the interaction of STX1A with Kv4.2 channels. Co-immunoprecipitation and GST pulldown assays demonstrated a direct interaction of STX1A with the Kv4.2 N-terminus. We next investigated the functional alterations of Kv4.2 gating by STX1A in Xenopus oocytes. Coexpression of Kv4.2 with STX1A (1) resulted in a reduction of Kv4.2 current amplitude; (2) caused a depolarizing shift of the steady-state inactivation curve; (3) enhanced the rate of current decay; and (4) accelerated the rate of recovery from inactivation. Additional coexpression of botulinum neurotoxin C, which cleaves STX1A, reversed the effects of STX1A on Kv4.2. STX1A inhibited partially the gating changes by KChIP2, suggesting a competitive interaction of these proteins for an overlapping binding region on the N-terminus of Kv4.2. Indeed, the N-terminal truncation mutants of Kv4.2 (Kv4.2Delta2-40 and Kv4.2Delta7-11), which form part of the KChIP2 binding site, displayed reduced sensitivity to STX1A modulation. Our study suggests that STX1A directly modulates Kv4.2 current amplitude and gating through its interaction with an overlapping region of the KChIP binding motif domain on the Kv4.2 N-terminus.

Simulation of Ca-calmodulin-dependent protein kinase II on rabbit ventricular myocyte ion currents and action potentials

Ca-calmodulin-dependent protein kinase II (CaMKII) was recently shown to alter Na(+) channel gating and recapitulate a human Na(+) channel genetic mutation that causes an unusual combined arrhythmogenic phenotype in patients: simultaneous long QT syndrome and Brugada syndrome. CaMKII is upregulated in heart failure where arrhythmias are common, and CaMKII inhibition can reduce arrhythmias. Thus, CaMKII-dependent channel modulation may contribute to acquired arrhythmic disease. We developed a Markovian Na(+) channel model including CaMKII-dependent changes, and incorporated it into a comprehensive myocyte action potential (AP) model with Na(+) and Ca(2+) transport. CaMKII shifts Na(+) current (I(Na)) availability to more negative voltage, enhances intermediate inactivation, and slows recovery from inactivation (all loss-of-function effects), but also enhances late noninactivating I(Na) (gain of function). At slow heart rates, with long diastolic time for I(Na) recovery, late I(Na) is the predominant effect, leading to AP prolongation (long QT syndrome). At fast heart rates, where recovery time is limited and APs are shorter, there is little effect on AP duration, but reduced availability decreases I(Na), AP upstroke velocity, and conduction (Brugada syndrome). CaMKII also increases cardiac Ca(2+) and K(+) currents (I(Ca) and I(to)), complicating CaMKII-dependent AP changes. Incorporating I(Ca) and I(to) effects individually prolongs and shortens AP duration. Combining I(Na), I(Ca), and I(to) effects results in shortening of AP duration with CaMKII. With transmural heterogeneity of I(to) and I(to) downregulation in heart failure, CaMKII may accentuate dispersion of repolarization. This provides a useful initial framework to consider pathways by which CaMKII may contribute to arrhythmogenesis.

Remodeling of Outward K+Currents in Pressure-Overload Heart Failure

OBJECTIVES

Outward K+ currents are critical determinants of action potential repolarization and the site of action of a number of electrophysiologically active drugs. Further, expression and processing of the channels underlying these currents is altered in heart disease. Here, we investigated the native transmural gradient of outward K+ currents in murine left ventricle (LV) and delineated disease-related remodeling of these currents in heart failure (HF).

METHODS

Pressure-overload heart failure was induced in mice by thoracic aortic constriction. Outward K+ currents were recorded using the whole-cell patch clamp technique in acutely dissociated ventricular myocytes.

RESULTS

Unambiguous gradients of outward K+ current density and Kv4.2 protein abundance were observed across the wall of the LV, with significantly larger current density and protein levels in subepicardial (SEP) myocytes, compared with subendocardial (SEN) myocytes. Voltage dependences of current activation and inactivation were similar in SEP and SEN myocytes. In failing LV, however, outward K+ current density was significantly decreased in SEP but not in SEN cells leading to elimination of the native transmural gradient. In failing LV, the voltage dependences of K+ current activation and inactivation were not altered. However, current inactivation (decay) was significantly accelerated and recovery from inactivation was significantly slowed. Consistent with this, Western blot analysis revealed a decrease in KChIP2 protein abundance in failing LV.

CONCLUSIONS

This is the first report of HF-related remodeling of outward K+ currents in murine LV. Similar to humans, disease-related remodeling occurs differentially across the murine ventricular wall, leading to loss of the native gradient of repolarization. Together with slowed recovery from inactivation, these alterations likely promote abnormal impulse conduction, a major proarrhythmic mechanism.

Angiotensin II-induced sudden arrhythmic death and electrical remodeling

Rats harboring the human renin and angiotensinogen genes (dTGR) feature angiotensin (ANG) II/hypertension-induced cardiac damage and die suddenly between wk 7 and 8. We observed by electrocardiogram (ECG) telemetry that ventricular tachycardia (VT) is a common terminal event in these animals. Our aim was to investigate electrical remodeling. We used ECG telemetry, noninvasive cardiac magnetic field mapping (CMFM) at wk 5 and 7, and performed in vivo programmed electrical stimulation at wk 7. We also investigated whether or not losartan (Los; 30 mg x kg(-1) x day(-1)) would prevent electrical remodeling. Cardiac hypertrophy and systolic blood pressure progressively increased in dTGR compared with Sprague-Dawley (SD) controls. Already by wk 5, untreated dTGR showed increased perivascular and interstitial fibrosis, connective tissue growth factor expression, and monocyte infiltration compared with SD rats, differences that progressed through time. Left-ventricular mRNA expression of potassium channel subunit Kv4.3 and gap-junction protein connexin 43 were significantly reduced in dTGR compared with Los-treated dTGR and SD. CMFM showed that depolarization and repolarization were prolonged and inhomogeneous. Los ameliorated all disturbances. VT could be induced in 88% of dTGR but only in 33% of Los-treated dTGR and could not be induced in SD. Untreated dTGR show electrical remodeling and probably die from VT. Los treatment reduces myocardial remodeling and predisposition to arrhythmias. ANG II target organ damage induces VT.

Modulation of the transient outward K+ current by inhibition of endothelin-A receptors in normal and hypertrophied rat hearts

Inhibition of endothelin-A (ET(A)) receptors has been shown to reduce ventricular electrical abnormalities associated with cardiac failure. In this study, we investigate the effect of ET(A)-receptor inhibition on the development of regional alterations of the transient outward K(+) current (I (to)) in the setting of pressure-induced left ventricular (LV) hypertrophy. Cardiac hypertrophy was induced in female Sprague-Dawley rats by stenosis of the ascending aorta (AS) for 7 days. Treatment with the selective ET(A)-receptor antagonist darusentan (LU135252, 35 mg [kg body weight](-1) day(-1)) was started 1 day before the surgery. AS induced a 46% increase in the relative LV weight (p < 0.001) and caused a significant reduction in I (to) (at +40 mV) in epicardial myocytes (19.5 +/- 1.2 pA pF(-1), n = 32 vs 23.2 +/- 1.2 pA pF(-1), n = 35, p < 0.05). Darusentan further reduced I (to) in AS (15.4 +/- 1.3 pA pF(-1), n = 37, p < 0.05) and sham-operated animals (19.8 +/- 1.6 pA pF(-1), n = 48, ns.). The effects of AS and darusentan on I (to) were significant and independent as tested by two-way analysis of variance. I (to) was not affected in endocardial myocytes. These results indicate that endothelin-1 may exert a tonic effect on the magnitude of I (to) in the epicardial region of the left ventricle but that ET(A)-receptor activation is not necessary for the development of electrical alterations associated with pressure-induced hypertrophy.

Differences in arrhythmogenicity between the canine right ventricular outflow tract and anteroinferior right ventricle in a model of Brugada syndrome

BACKGROUND

The Brugada syndrome is characterized by ST-segment elevation on the ECG, especially in the right precordial leads sensitive to the right ventricular outflow tract (RVOT).

OBJECTIVES

The purpose of this study was to evaluate the hypothesis that right ventricular electrophysiologic heterogeneity caused arrhythmogenicity in the Brugada syndrome.

METHODS

Action potentials (APs) were mapped on the epicardium of 14 RVOT preparations and on the transmural surfaces of 15 pairs of RVOT and right ventricular anteroinferior (RVAI) preparations isolated from canine hearts. Brugada ECG and arrhythmias were induced with pilsicainide (2.5-12.5 micromol/L), pinacidil (1.25-12.5 micromol/L), and terfenadine (2.0 micromol/L).

RESULTS

Low doses of drugs elevated the J-ST segment and induced APs with both short and long action potential durations (APDs) in contiguous RVOT epicardial regions. In addition, APs in the RVOT had a larger phase 1 notch and longer APD than in RVAI. The longest APDs were in the epicardium in RVOT but in the endocardium in RVAI regions. High doses of drugs eliminated the phase 2 dome of the AP and abbreviated APDs in the epicardium but not in endocardium and reduced the epicardial heterogeneity of APs but increased the transmural gradient of APD in 14 (93%) of the RVOT preparations. In contrast, abbreviations of epicardial APDs occurred in only 4 (27%) of the RVAI preparations. Ventricular tachycardia occurred more frequently in the RVOT (47%) than in paired RVAI preparations (7%). Blocking the transient outward current reduced the heterogeneity of APs and eliminated arrhythmogenicity in all preparations.

CONCLUSION

Compared with the RVAI region, the RVOT has greater electrophysiologic heterogeneity that contributes to arrhythmogenicity in this model of Brugada syndrome.

Antiarrhythmic effect of atorvastatin on autoimmune myocarditis is mediated by improving myocardial repolarization

3-Hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitors, or statins, are known to inhibit cholesterol biosynthesis and prevent inflammation and oxidative stress. To explore the effects of atorvastatin on inflammatory progression and major cardiac electrophysiological changes in myocarditis, we used an animal model of experimental autoimmune myocarditis (EAM). In this model, BALB/c mice were treated with atorvastatin and we evaluated the levels of inflammation markers and currents of ionic channels that contribute to the duration of action potential (APD) of ventricular myocytes. We demonstrated that atorvastatin treatment attenuated inflammatory infiltration and suppressed the increase in TNF-alpha and IFN-gamma levels in EAM mouse hearts. In the whole-cell patch-clamp experiment, ventricular cardiomyocyte APD was prolonged in EAM group, and atorvastatin blocked this change. We further found that atorvastatin attenuated the significant decrease in outward potassium currents in EAM myocytes. Our results suggested that atorvastatin may ameliorate EAM progression by reducing inflammatory cytokine level. Atorvastatin exerted the antiarrhythmic effects by selectively affecting cardiomyocyte ion channel activity and therefore improves myocardial repolarization.

Spatial distributions of Kv4 channels and KChip2 isoforms in the murine heart based on laser capture microdissection

OBJECTIVE

Regional differences in repolarizing K(+) current densities and expression levels of their molecular components are important for coordinating the pattern of electrical excitation and repolarization of the heart. The small size of hearts from mice may obscure these interventricular and/or transmural expression differences of K(+) channels. We have examined this possibility in adult mouse ventricle using a technology that provides very high spatial resolution of tissue collection.

METHODS

Conventional manual dissection and laser capture microdissection (LCM) were utilized to dissect tissue from distinct ventricular regions. RNA was isolated from epicardial, mid-myocardial and endocardial layers of both the right and left ventricles. Real-time RT-PCR was used to quantify the transcript expression in these different regions.

RESULTS

LCM revealed significant interventricular and transmural gradients for both Kv4.2 and the alpha-subunit of KChIP2. The expression profile of a second K(+) channel transcript, Kir2.1, which is responsible for the inwardly rectifying K(+) current I(k1), showed no interventricular or transmural gradients and therefore served as a negative control.

CONCLUSIONS

Our findings are in contrast to previous reports of a relatively uniform left ventricular transmural pattern of expression of Kv4.2, Kv4.3 and KChIP2 in adult mouse heart, which appear to be different than that in larger mammals. Specifically, our results demonstrate significant epi- to endocardial differences in the patterns of expression of both Kv4.2 and KChIP2.

Calcineurin increases cardiac transient outward K+ currents via transcriptional up-regulation of Kv4.2 channel subunits

Fast transient outward potassium currents (I(to,f)) are critical determinants of regional heterogeneity of cardiomyocyte repolarization as well as cardiomyocyte contractility. Additionally, I(to,f) densities are markedly down-regulated in cardiac hypertrophy and heart disease, conditions associated with activation of the serine/threonine phosphatase calcineurin (Cn). In this study, we investigated the regulation of I(to,f) expression by Cn in cultured neonatal rat ventricular myocytes (NRVMs) with and without alpha(1)-adrenoreceptor stimulation with phenylephrine (PE). Overexpression of constitutively active Cn in NRVMs induced hypertrophy and caused profound increases in I(to,f) density as well as Kv4.2 mRNA and protein expression and promoter activity, without affecting Kv4.3 or KChIP2 levels. The effects of Cn on hypertrophy, I(to,f), and Kv4.2 transcription were associated with NFAT activation and were abrogated by NFAT inhibition. Despite activating Cn and inducing hypertrophy in NRVMs, PE resulted in profound down-regulation of I(to,f) densities as well as Kv4.2, Kv4.3, and KChIP2 expression. Although hypertrophy and NFAT activation were inhibited by the Cn inhibitory peptide CAIN, I(to,f) and Kv4.2 expression were further reduced by CAIN, whereas Cn overexpression eliminated PE-induced reductions in I(to,f) and Kv4.2 expression without affecting Kv4.3 or KChIP2 levels. We conclude that Cn increases cardiac I(to,f) densities by positively regulating Kv4.2 gene transcription. Consistent with this conclusion, we found that I(to,f) was increased in myocytes isolated from young mice overexpressing Cn prior to the development of heart disease. This positive regulation of Kv4.2 transcription by Cn activation is expected to minimize the reductions in I(to,f) and Kv4.2 expression observed in hypertrophic cardiomyocytes.

Differential modulation of Kv4.2 and Kv4.3 channels by calmodulin-dependent protein kinase II in rat cardiac myocytes

In this work we have combined biochemical and electrophysiological approaches to explore the modulation of rat ventricular transient outward K+ current ( Ito) by calmodulin kinase II (CaMKII). Intracellular application of CaMKII inhibitors KN93, calmidazolium, and autocamtide-2-related inhibitory peptide II (ARIP-II) accelerated the inactivation of Ito, even at low [Ca2+]. In the same conditions, CaMKII coimmunoprecipitated with Kv4.3 channels, suggesting that phosphorylation of Kv4.3 channels modulate inactivation of Ito. Because channels underlying Ito are heteromultimers of Kv4.2 and Kv4.3, we have explored the effect of CaMKII on human embryonic kidney (HEK) cells transfected with either of those Kvα-subunits. Whereas Kv4.3 inactivated faster upon inhibition of CaMKII, Kv4.2 inactivation was insensitive to CaMKII inhibitors. However, Kv4.2 inactivation became slower when high Ca2+ was used in the pipette or when intracellular [Ca2+] ([Ca2+]i) was transiently increased. This effect was inhibited by KN93, and Western blot analysis demonstrated Ca2+-dependent phosphorylation of Kv4.2 channels. On the contrary, CaMKII coimmunoprecipitated with Kv4.3 channels without a previous Ca2+ increase, and the association was inhibited by KN93. These results suggest that both channels underlying Ito are substrates of CaMKII, although with different sensitivities; Kv4.2 remain unphosphorylated unless [Ca2+]i increases, whereas Kv4.3 are phosphorylated at rest. In addition to the functional impact that phosphorylation of Kv4 channels could cause on the shape of action potential, association of CaMKII with Kv4.3 provides a new role of Kv4.3 subunits as molecular scaffolds for concentrating CaMKII in the membrane, allowing Ca2+-dependent modulation by this enzyme of the associated Kv4.2 channels.

Properties of a Time-Dependent Potassium Current in Pig Atrium: Evidence for a Role of Kv1.5 in Repolarization

Cardiac electrical activity is modulated by potassium currents. Pigs have been used for antiarrhythmic drug testing, but only sparse data exist regarding porcine atrial ionic electrophysiology. Here, we used electrophysiological, molecular, and pharmacological tools to characterize a prominent porcine outward K(+) current (I(K,PO)) in atrial cardiomyocytes isolated from adult pigs. I(K,PO) activated rapidly (time to peak at +60 mV; 2.1 +/- 0.2 ms), inactivated slowly (tau(f) = 45 +/- 10; tau(s) = 215 +/- 28 ms), and showed very slow recovery (tau(f) = 1.54 +/- 0.73 s; tau(s) = 7.91 +/- 1.78 s; n = 9; 36 degrees C). Activation and inactivation were voltage-dependent, and current properties were consistent with predominant K(+) conductance. Neurotoxins (heteropodatoxin, hongatoxin, and blood depressing substance) that block K(v)4.x, K(v)1.1, -1.2, -1.3, and -3.4 in a highly selective manner as well as H(2)O(2) and tetraethylammonium, did not affect the current. Drugs with K(v)1.5-blocking properties (flecainide, perhexiline, and the novel atrial-selective antiarrhythmic 2'-{2-(4-methoxyphenyl)-acetylamino-methyl}-biphenyl-2-carboxylic acid (2-pyridin-3-yl-ethyl)-amide; AVE0118) inhibited I(K,PO) (IC(50) of 132 +/- 47, 17 +/- 10, and 1.25 +/- 0.62 microM, respectively). 4-Aminopyridine suppressed the current and accelerated its decay, reducing charge carriage with an IC(50) of 39 +/- 15 microM. Porcine-specific K(v) channel subunit sequences were cloned to permit real-time quantitative reverse transcription-polymerase chain reaction on RNA extracted from isolated cardiomyocytes, which showed much greater abundance of K(v)1.5 mRNA compared with K(v)1.4, K(v)4.2, and K(v)4.3. Action potential recordings showed that I(K,PO) inhibition with 0.1 mM 4-AP delayed repolarization (e.g., action potential duration at -50 mV increased from 45 +/- 9 to 69 +/- 5 ms at 3 Hz; P < 0.05). In conclusion, porcine atrium displays a current that is involved in repolarization, inactivates more slowly than classic transient outward current, is associated with strong K(v)1.5 expression, and shows a pharmacological profile typical of K(v)1.5-dependent currents.

Effect of simulated Ito on guinea pig and canine ventricular action potential morphology

The transient outward current ( Ito) is a major repolarizing current in the heart. Marked reduction of Ito density occurs in heart failure and is accompanied by significant action potential duration (APD) prolongation. To understand the species-dependent role of Ito in regulating the ventricular action potential morphology and duration, we introduced simulated Ito conductance in guinea pig and canine endocardial ventricular myocytes using the dynamic clamp technique and perforated patch-clamp recordings. The effects of simulated Ito in both types of cells were complex and biphasic, separated by a clear density threshold of ∼40 pA/pF. Below this threshold, simulated Ito resulted in a distinct phase 1 notch and had little effect on or moderately prolonged the APD. Ito above the threshold resulted in all-or-none repolarization and precipitously reduced the APD. Qualitatively, these results agreed with our previous studies in canine ventricular cells using whole cell recordings. We conclude that 1) contrary to previous gene transfer studies involving the Kv4.3 current, the response of guinea pig ventricular myocytes to a fully inactivating Ito is similar to that of canine ventricular cells and 2) in animals such as dogs that have a broad cardiac action potential, Ito does not play a major role in setting the APD.

Deletion of peroxisome proliferator-activated receptor-α induces an alteration of cardiac functions

The peroxisome proliferator-activated receptor-α (PPARα) plays a major role in the control of cardiac energy metabolism. The role of PPARα on cardiac functions was evaluated by using PPARα knockout (PPARα −/−) mice. Hemodynamic parameters by sphygmomanometric measurements show that deletion of PPARα did not affect systolic blood pressure and heart rate. Echocardiographic measurements demonstrated reduced systolic performance as shown by the decrease of left ventricular fractional shortening in PPARα −/− mice. Telemetric electrocardiography revealed neither atrio- nor intraventricular conduction defects in PPARα −/− mice. Also, heart rate, P-wave duration and amplitude, and QT interval were not affected. However, the amplitude of T wave from PPARα −/− mice was lower compared with wild-type (PPARα +/+) mice. When the myocardial function was measured by ex vivo Langendorff's heart preparation, basal and β-adrenergic agonist-induced developed forces were significantly reduced in PPARα-null mice. In addition, Western blot analysis shows that the protein expression of β1-adrenergic receptor is reduced in hearts from PPARα −/− mice. Histological analysis showed that hearts from PPARα −/− but not PPARα +/+ mice displayed myocardial fibrosis. These results suggest that PPARα-null mice have an alteration of cardiac contractile performance under basal and under stimulation of β1-adrenergic receptors. These effects are associated with myocardial fibrosis. The data shed light on the role of PPARα in maintaining cardiac functions.

H-89 inhibits transient outward and inward rectifier potassium currents in isolated rat ventricular myocytes

1. Voltage clamp was used to investigate the effects of N-[2-p-bromo-cinnamylamino)ethyl]-5-isoquinolinesulfonamide (H-89), a potent inhibitor of PKA, on transient outward K(+) current (I(to)) and inward rectifying K(+) current (I(K1)) in rat cardiac muscle. 2. Initial experiments, performed using descending voltage ramps, showed that H-89 inhibited both the outward and inward ramp currents in a concentration-dependent manner at concentrations between 5 and 60 micromol l(-1). A similar degree of inhibition was observed when I(to) and I(K1) were recorded using square wave depolarising and hyperpolarising voltage steps, respectively. 3. The IC(50) was 35.8 micromol l(-1) for I(to) and 27.8 micromol l(-1) for I(K1) compared to 5.4 micromol l(-1) for L-type Ca(2+) current (I(Ca)). The Hill coefficients for I(to), I(K1) and I(Ca) were -1.97, -1.60 and -1.21, respectively. In addition to inhibiting I(to) amplitude, H-89 also accelerated the time to peak and the rate of voltage-dependent inactivation so that the time course of I(to) was abbreviated. 4. Paired-pulse protocols were performed to study the effects of H-89 on steady-state activation and inactivation as well as recovery from voltage-dependent inactivation. H-89 produced a concentration-dependent rightward shift in voltage-dependent activation but had no significant effect on steady-state inactivation. Recovery from voltage-dependent inactivation was delayed, although this was only visible at the highest concentration (60 micromol l(-1)) used. In experiments investigating the effects of elevated cyclic AMP, the beta-adrenergic agonist isoprenaline and the phosphatase inhibitor calyculin A had no major effects on I(to) or I(K1). 6. Data suggest that the effects of H-89 on K(+) currents are more complex than simple inhibition of PKA-mediated phosphorylation.

Germain D, MacLennan DH, Backx PH. Cardiac-specific elevations in thyroid hormone enhance contractility and prevent pressure overload-induced cardiac dysfunction

Thyroid hormone (TH) is critical for cardiac development and heart function. In heart disease, TH metabolism is abnormal, and many biochemical and functional alterations mirror hypothyroidism. Although TH therapy has been advocated for treating heart disease, a clear benefit of TH has yet to be established, possibly because of peripheral actions of TH. To assess the potential efficacy of TH in treating heart disease, type 2 deiodinase (D2), which converts the prohormone thyroxine to active triiodothyronine (T3), was expressed transiently in mouse hearts by using the tetracycline transactivator system. Increased cardiac D2 activity led to elevated cardiac T3 levels and to enhanced myocardial contractility, accompanied by increased Ca(2+) transients and sarcoplasmic reticulum (SR) Ca(2+) uptake. These phenotypic changes were associated with up-regulation of sarco(endo)plasmic reticulum calcium ATPase (SERCA) 2a expression as well as decreased Na(+)/Ca(2+) exchanger, beta-myosin heavy chain, and sarcolipin (SLN) expression. In pressure overload, targeted increases in D2 activity could not block hypertrophy but could completely prevent impaired contractility and SR Ca(2+) cycling as well as altered expression patterns of SERCA2a, SLN, and other markers of pathological hypertrophy. Our results establish that elevated D2 activity in the heart increases T3 levels and enhances cardiac contractile function while preventing deterioration of cardiac function and altered gene expression after pressure overload.

Transient outward potassium current and Ca2+ homeostasis in the heart: beyond the action potential

The present review deals with Ca2+-independent, K+-carried transient outward current (Ito), an important determinant of the early repolarization phase of the myocardial action potential. The density of total Ito and of its fast and slow components (I(to,f) and I(to,s), respectively), as well as the expression of their molecular correlates (pore-forming protein isoforms Kv4.3/4.2 and Kv1.4, respectively), vary during postnatal development and aging across species and regions of the heart. Changes in Ito may also occur in disease conditions, which may affect the profile of cardiac repolarization and vulnerability to arrhythmias, and also influence excitation-contraction coupling. Decreased Ito density, observed in immature and aging myocardium, as well as during several types of cardiomyopathy and heart failure, may be associated with action potential prolongation, which favors Ca2+ influx during membrane depolarization and limits voltage-dependent Ca2+ efflux via the Na+/Ca2+ exchanger. Both effects contribute to increasing sarcoplasmic reticulum (SR) Ca2+ content (the main source of contraction-activating Ca2+ in mammalian myocardium), which, in addition to the increased Ca2+ influx, should enhance the amount of Ca2+ released by the SR during systole. This change usually takes place under conditions in which SR function is depressed, and may be adaptive since it provides partial compensation for SR deficiency, although possibly at the cost of asynchronous SR Ca2+ release and greater propensity to triggered arrhythmias. Thus, Ito modulation appears to be an additional mechanism by which excitation-contraction coupling in myocardial cells is indirectly regulated.

KChIP2b modulates the affinity and use-dependent block of Kv4.3 by nifedipine

Rapidly activating Kv4 voltage-gated ion channels are found in heart, brain, and diverse other tissues including colon and uterus. Kv4.3 can co-assemble with KChIP ancillary subunits, which modify kinetic behavior. We examined the affinity and use dependence of nifedipine block on Kv4.3 and its modulation by KChIP2b. Nifedipine (150 microM) reduced peak Kv4.3 current approximately 50%, but Kv4.3/KChIP2b current only approximately 27%. Nifedipine produced a very rapid component of open channel block in both Kv4.3 and Kv4.3/KChIP2b. However, recovery from the blocked/inactivated state was strongly sensitive to KChIP2b. Kv4.3 Thalf,recovery was slowed significantly by nifedipine (120.0+/-12.4 ms vs. 213.1+/-18.2 ms), whereas KChIP2b eliminated nifedipine's effect on recovery: Kv4.3/KChIP2b Thalf,recovery was 45.3+/-7.2 ms (control) and 47.8+/-8.2 ms (nifedipine). Consequently, Kv4.3 exhibited use-dependent nifedipine block in response to a series of depolarizing pulses which was abolished by KChIP2b. KChIPs alter drug affinity and use dependence of Kv4.3.

The homeodomain transcription factor Irx5 establishes the mouse cardiac ventricular repolarization gradient

Rhythmic cardiac contractions depend on the organized propagation of depolarizing and repolarizing wavefronts. Repolarization is spatially heterogeneous and depends largely on gradients of potassium currents. Gradient disruption in heart disease may underlie susceptibility to fatal arrhythmias, but it is not known how this gradient is established. We show that, in mice lacking the homeodomain transcription factor Irx5, the cardiac repolarization gradient is abolished due to increased Kv4.2 potassium-channel expression in endocardial myocardium, resulting in a selective increase of the major cardiac repolarization current, I(to,f), and increased susceptibility to arrhythmias. Myocardial Irx5 is expressed in a gradient opposite that of Kv4.2, and Irx5 represses Kv4.2 expression by recruiting mBop, a cardiac transcriptional repressor. Thus, an Irx5 repressor gradient negatively regulates potassium-channel-gene expression in the heart, forming an inverse I(to,f) gradient that ensures coordinated cardiac repolarization while also preventing arrhythmias.

Excitation-contraction coupling in Na+-Ca2+ exchanger knockout mice: reduced transsarcolemmal Ca2+ flux

Cardiac-specific Na+-Ca2+ exchanger (NCX) knockout (KO) mice surprisingly survive into adulthood without compensatory changes in protein expression levels. To determine how cardiac function is maintained in the absence of NCX, we investigated membrane currents, intracellular Ca2+, and action potentials (APs) in whole cell patch-clamped myocytes from wild-type (WT) and NCX knockout mice. There was no difference in resting Ca2+ or sarcoplasmic reticular Ca2+ load between KO and WT. During prolonged caffeine exposure, the decrease of the Ca2+ transient was drastically slowed in KO versus WT myocytes, indicating that no alternative Ca2+-extrusion mechanism is upregulated to compensate for the absence of NCX. Peak L-type Ca2+ current (ICa) was reduced by 62% in KO myocytes compared with WT. Nevertheless, the corresponding Ca2+ transients were similar, implying an increase in the gain of excitation-contraction coupling in KO cells. APs recorded from KO cells repolarized more rapidly than in WT. In WT myocytes, applying a KO AP waveform voltage clamp reduced Ca2+ influx via ICa by 59% compared with WT AP waveform clamps. Again, the corresponding Ca2+ transients remained similar. Our findings indicate that NCX KO myocytes limit Ca2+ influx to &20% of that in WT by reducing ICa and by abbreviating the AP. Contractility is maintained by an increase in the gain of excitation-contraction coupling resulting from both a more rapid repolarization of the AP and an as yet unidentified AP-independent mechanism.

Transient outward potassium current, 'Ito', phenotypes in the mammalian left ventricle: underlying molecular, cellular and biophysical mechanisms

At least two functionally distinct transient outward K(+) current (I(to)) phenotypes can exist across the free wall of the left ventricle (LV). Based upon their voltage-dependent kinetics of recovery from inactivation, these two phenotypes are designated 'I(to,fast)' (recovery time constants on the order of tens of milliseconds) and 'I(to,slow)' (recovery time constants on the order of thousands of milliseconds). Depending upon species, either I(to,fast), I(to,slow) or both current phenotypes may be expressed in the LV free wall. The expression gradients of these two I(to) phenotypes across the LV free wall are typically heterogeneous and, depending upon species, may consist of functional phenotypic gradients of both I(to,fast) and I(to,slow) and/or density gradients of either phenotype. We review the present evidence (molecular, biophysical, electrophysiological and pharmacological) for Kv4.2/4.3 alpha subunits underlying LV I(to,fast) and Kv1.4 alpha subunits underlying LV I(to,slow) and speculate upon the potential roles of each of these currents in determining frequency-dependent action potential characteristics of LV subepicardial versus subendocardial myocytes in different species. We also review the possible functional implications of (i) ancillary subunits that regulate Kv1.4 and Kv4.2/4.3 (Kvbeta subunits, DPPs), (ii) KChIP2 isoforms, (iii) spider toxin-mediated block of Kv4.2/4.3 (Heteropoda toxins, phrixotoxins), and (iv) potential mechanisms of modulation of I(to,fast) and I(to,slow) by cellular redox state, [Ca(2)(+)](i) and kinase-mediated phosphorylation. I(to) phenotypic activation and state-dependent gating models and molecular structure-function relationships are also discussed.

Dual effects of mesenteric lymph isolated from rats with burn injury on contractile function in rat ventricular myocytes

Gut-derived factors in intestinal lymph have been shown to trigger myocardial contractile dysfunction. However, the underlying cellular mechanisms remain unclear. We examined the effects of physiologically relevant concentrations of mesenteric lymph collected from rats with 40% burn injury (burn lymph) on excitation-contraction coupling in rat ventricular myocytes. Burn lymph (0.1-5%), but not control mesenteric lymph from sham-burn animals, induced dual positive and negative inotropic effects depending on the concentrations used. At lower concentrations (<0.5%), burn lymph increased the amplitude of myocyte contraction (1.6 +/- 0.3-fold; n = 12). At higher concentrations (>0.5%), burn lymph initially enhanced myocyte contraction, which was followed by a block of contraction. These effects were partially reversible on washout. The initial positive inotropic effect was associated with a prolongation of action potential duration (measured at 90% repolarization, 2.5 +/- 0.6-fold; n = 10), leading to significant increases in the net Ca2+ influx (1.7 +/- 0.1-fold; n = 8). There were no significant changes in the resting membrane potential. The negative inotropic effect was accompanied by a decrease in the action potential plateau (overshoot decrease by 69 +/- 10%; n = 4) and membrane depolarization. Voltage-clamp experiments revealed that the positive inotropic effects of burn lymph were due to an inhibition of the transient outward K+ currents that prolong action potential duration, and the inhibitory effects were due to a concentration-dependent inhibition of Ca2+ currents that lead to a reduction of action potential plateau. These burn lymph-induced changes in cardiac myocyte Ca2+ handling can contribute to burn-induced contractile dysfunction and ultimately to heart failure.

Sulfur dioxide derivative modulation of potassium channels in rat ventricular myocytes

The effects of sulfur dioxide (SO2) derivatives (bisulfite and sulfite, 1:3 M/M) on voltage-dependent potassium current in isolated adult rat ventricular myocyte were investigated using the whole cell patch-clamp technique. SO2 derivatives (10 microM) increased transient outward potassium current (I(to)) and inward rectifier potassium current (I(K1)), but did not affect the steady-state outward potassium current (I(ss)). SO2 derivatives significantly shifted the steady-state activation curve of I(to) toward the more negative potential at the V(h) point, but shifted the inactivation curve to more positive potential. SO2 derivatives markedly shifted the curve of time-dependent recovery of I(to) from the steady-state inactivation to the left, and accelerated the recovery of I(to) from inactivation. In addition, SO2 derivatives also significantly change the inactivation time constants of I(to) with increasing fast time constant and decreasing slow time constant. These results indicated a possible correlation between the change of properties of potassium channel and SO2 inhalation toxicity, which might cause cardiac myocyte injury through increasing extracellular potassium via voltage-gated potassium channels.

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.

Blocking Characteristics of hKv1.5 and hKv4.3/hKChIP2.2 After Administration of the Novel Antiarrhythmic Compound AZD7009

AZD7009 is a novel antiarrhythmic compound in early clinical development for management of atrial fibrillation. Electrophysiological studies in animals have shown high antiarrhythmic efficacy, predominant action on atrial electrophysiology, and low proarrhythmic activity. AZD7009 has previously been shown to inhibit hERG and hNav1.5 currents. The main objective of the present study was to characterize the effects of AZD7009 on hKv1.5 and hKv4.3/hKChIP2.2 currents to get a deeper understanding of the ion channel-blocking properties of the compound. hKv1.5 and hKv4.3/hKChIP2.2 currents were expressed in CHO cells. Currents were measured using the whole-cell configuration of the voltage-clamp technique. AZD7009 inhibited hKv1.5 and hKv4.3/hKChIP2.2 currents with equal potency: the IC50 for hKv1.5 block was 27.0 +/- 1.6 muM (n = 6), and the IC50 for hKv4.3/hKChIP2.2 block was 23.7 +/- 4.4 muM (n = 5). Block of the hKv4.3/hKChIP2.2 current was frequency dependent with larger block at higher frequency, whereas block of the hKv1.5 current was slightly decreased at higher frequency. In conclusion, AZD7009 inhibits both the hKv1.5 and the hKv4.3/hKChIP2.2 currents. These effects likely contribute to the effects described in animals in vivo.

Modulation of potassium currents by angiotensin and oxidative stress in cardiac cells from the diabetic rat

Diabetes induces oxidative stress and leads to attenuation of cardiac K+ currents. We investigated the role of superoxide ions and angiotensin II (ANG II) in generating and linking oxidative stress to the modulation of K+ currents under diabetic conditions. K+ currents were measured using patch-clamp methods in ventricular myocytes from streptozotocin (STZ)-induced diabetic rats. Superoxide ion levels, indicating oxidative stress, were measured by fluorescent labelling with dihydroethidium (DHE). ANG II content was measured using enzyme-linked immunosorbent asssay (ELISA). The results showed DHE fluorescence to be significantly higher in cells from diabetic males, compared to controls. Relief of stress by the NADPH oxidase inhibitor apocynin or by superoxide dismutase (SOD) but not by catalase reversed the attenuation of K+ currents and reduced DHE fluorescence. In cells from diabetic females, neither apocynin nor SOD augmented K+ currents, ANG II was not elevated and DHE fluorescence was significantly weaker than in cells from males. Reduced glutathione (GSH) also augmented K+ currents in cells from diabetic males but not females. In ovariectomized diabetic females K+ currents were augmented by GSH and apocynin. Current augmentation and the attenuation of DHE fluorescence by apocynin were significantly blunted by excess ANG II (300 nm). Diabetic male rats pretreated with the angiotensin-converting enzyme (ACE) inhibitor quinapril were hyperglycaemic, but their cellular ANG II levels and DHE fluorescence were significantly decreased. In cells from these rats, K+ currents were insensitive to apocynin. In conclusion, diabetes-related oxidative stress attenuates K+ currents through ANG II-generated increased superoxide ion levels. When ANG II levels are lower, as in diabetic females or following ACE inhibition in males, oxidative stress is reduced, with blunted alterations in K+ currents.

Mutations in the genes KCND2 and KCND3 encoding the ion channels Kv4.2 and Kv4.3, conducting the cardiac fast transient outward current (ITO,f), are not a frequent cause of long QT syndrome

BACKGROUND

Long QT syndrome (LQTS) is a hereditary cardiac arrhythmogenic disorder characterized by prolongation of the QT interval in the electrocardiogram, torsades de pointes arrhythmia, and syncopes and sudden death. LQTS is caused by mutations in ion channel genes. However, only in half of the families is it possible to identify mutations in one of the seven known LQTS genes, why further genetic heterogeneity is expected. The genes KCND2 and KCND3, encoding the alpha-subunits of the voltage-gated potassium channels Kv4.2 and Kv4.3 conducting the fast transient outward current (I(TO,f)) of the cardiac action potential (AP) in the myocardium, have been associated with prolongation of AP duration and QT prolongation in murine models.

METHODS

KCND2 and KCND3 were examined for mutations using single-strand conformation polymorphism (SSCP) analysis in 43 unrelated LQTS patients, where mutations in the coding regions of known LQTS genes had been excluded.

RESULTS

Seven single nucleotide polymorphismsm (SNPs) were found, two exonic SNPs in KCND2 and three exonic and two intronic in KCND3. None of the five exonic SNPs had coding effect. All seven SNPs are considered normal variants.

CONCLUSION

The data suggest that mutations in KCND2 and KCND3 are not a frequent cause of long QT syndrome.

Identification of the alternative promoters of the KChIP4 subfamily

The subfamily of voltage-dependent potassium (Kv) channel interacting protein 4 (KChIP4) is made up of the auxiliary interacting protein of voltage-dependent potassium channels. In this study, the structure of four splicing variants of the human KChIP4 gene was analyzed. Three of the four isoforms of the KChIP4 gene, KChIP4.1, KChIP4.2 and KChIP4.4, were amplified from mouse and human fetal brain tissues by reverse transcription-polymerase chain reaction and then identified. Based on the bioinformatics analysis of the genomic sequences of the gene, we cloned and characterized two promoter fragments from the KChIP4 gene. One was a 325 bp fragment upstream of the 5' end of the KChIP4.1 mRNA sequence and the other was an 818 bp fragment located immediately at the 5' end of the KChIP4.4 variant. Both of them can initiate the transcription of the reporter gene in HT1080 cells and Sprague-Dawley (SD) rat fetal brain neurons, and they contain CG islands, except typical TATA boxes and CAAT boxes. This shows that the KChIP4 gene expression is regulated by an alternative promoter.

Molecular physiology and modulation of somatodendritic A-type potassium channels

The somatodendritic subthreshold A-type K+ current (ISA) in nerve cells is a critical component of the ensemble of voltage-gated ionic currents that determine somatodendritic signal integration. The underlying K+ channel belongs to the Shal subfamily of voltage-gated K+ channels. Most Shal channels across the animal kingdom share a high degree of structural conservation, operate in the subthreshold range of membrane potentials, and exhibit relatively fast inactivation and recovery from inactivation. Mammalian Shal K+ channels (Kv4) undergo preferential closed-state inactivation with features that are generally inconsistent with the classical mechanisms of inactivation typical of Shaker K+ channels. Here, we review (1) the physiological and genetic properties of ISA, 2 the molecular mechanisms of Kv4 inactivation and its remodeling by a family of soluble calcium-binding proteins (KChIPs) and a membrane-bound dipeptidase-like protein (DPPX), and (3) the modulation of Kv4 channels by protein phosphorylation.