Molecular physiology of cardiac repolarization

Mechanisms underlying adaptation of action potential duration by pacing rate in rat myocytes

Heart rate is an essential determinant of cardiac performance. In rat ventricular myocytes, a sudden increase in rate yields to a prolongation of the action potential duration (APD). The mechanism underlying this prolongation is controversial: it has been proposed that the longer APD is due to either: (1) a decrease in K+ currents only or (2) an increase in Ca2+ current only. The aim of this study was to quantitatively investigate the contribution of Ca2+ and K+ currents in the adaptation of APD to pacing rate. Simulation using a mathematical model of ventricular rat cardiac cell model [Pandit, S.V., Clark, R.B., Giles, W.R., Demir, S.S., 2001. A mathematical model of action potential heterogeneity in adult rat left ventricular myocytes. Biophys. J. 81, 3029-3051] predicted a role in the prolongation of APD for K+ currents only. In patch clamp experiments, increasing the pacing rate leads to a significant increase in APD in both control and detubulated myocytes, although it was more marked in control than detubulated myocytes. Supporting the model prediction, we observed that increasing stimulation frequency leads to a decrease in K+ currents in voltage clamped rat ventricular myocytes (square and action potential waveforms), and to a similar extent in both cell types. We have also observed that frequency-dependent facilitation of Ca2+ current occurred in control cells but not in detubulated cells (square and action potential waveforms). From these experiments, we calculated that the relative contribution of Ca2+ and K+ currents to the longer APD following an increase in pacing rate is approximately 65% and approximately 35%, respectively. Therefore, in contrast to the model prediction, Ca2+ current has a significant role in the adaptation of APD to pacing rate. Finally, we have introduced a simplistic modification to the Pandit's model to account for the frequency-dependent facilitation of Ca2+ current.

Late sodium current in failing heart: friend or foe?

Most cardiac Na+ channels open transiently upon membrane depolarization and then are quickly inactivated. However, some channels remain active, carrying the so-called persistent or late Na+ current (INaL) throughout the action potential (AP) plateau. Experimental data and the results of numerical modeling accumulated over the past decade show the emerging importance of this late current component for the function of both normal and failing myocardium. INaL is produced by special gating modes of the cardiac-specific Na+ channel isoform. Heart failure (HF) slows channel gating and increases INaL, but HF-specific Na+ channel isoform underlying these changes has not been found. Na+ channels represent a multi-protein complex and its activity is determined not only by the pore-forming alpha subunit but also by its auxiliary beta subunits, cytoskeleton, calmodulin, regulatory kinases and phosphatases, and trafficking proteins. Disruption of the integrity of this protein complex may lead to alterations of INaL in pathological conditions. Increased INaL and the corresponding Na+ flux in failing myocardium contribute to abnormal repolarization and an increased cell Ca2+ load. Interventions designed to correct INaL rescue normal repolarization and improve Ca2+ handling and contractility of the failing cardiomyocytes. This review considers (1) quantitative integration of INaL into the established electrophysiological and Ca2+ regulatory mechanisms in normal and failing cardiomyocytes and (2) a new therapeutic strategy utilizing a selective inhibition of INaL to target both arrhythmias and impaired contractility in HF.

A tale of switched functions: from cyclooxygenase inhibition to M-channel modulation in new diphenylamine derivatives

Cyclooxygenase (COX) enzymes are molecular targets of nonsteroidal anti-inflammatory drugs (NSAIDs), the most used medication worldwide. However, the COX enzymes are not the sole molecular targets of NSAIDs. Recently, we showed that two NSAIDs, diclofenac and meclofenamate, also act as openers of Kv7.2/3 K(+) channels underlying the neuronal M-current. Here we designed new derivatives of diphenylamine carboxylate to dissociate the M-channel opener property from COX inhibition. The carboxylate moiety was derivatized into amides or esters and linked to various alkyl and ether chains. Powerful M-channel openers were generated, provided that the diphenylamine moiety and a terminal hydroxyl group are preserved. In transfected CHO cells, they activated recombinant Kv7.2/3 K(+) channels, causing a hyperpolarizing shift of current activation as measured by whole-cell patch-clamp recording. In sensory dorsal root ganglion and hippocampal neurons, the openers hyperpolarized the membrane potential and robustly depressed evoked spike discharges. They also decreased hippocampal glutamate and GABA release by reducing the frequency of spontaneous excitatory and inhibitory post-synaptic currents. In vivo, the openers exhibited anti-convulsant activity, as measured in mice by the maximal electroshock seizure model. Conversion of the carboxylate function into amide abolished COX inhibition but preserved M-channel modulation. Remarkably, the very same template let us generating potent M-channel blockers. Our results reveal a new and crucial determinant of NSAID-mediated COX inhibition. They also provide a structural framework for designing novel M-channel modulators, including openers and blockers.

Cardiac dynamics during daily torpor in the Djungarian hamster (Phodopus sungorus)

Djungarian or Siberian hamsters (Phodopus sungorus) acclimated to short photoperiod display episodes of spontaneous daily torpor with metabolic rate depressed by approximately 70% and body temperature (T(b)) reduced by approximately 20 degrees C. To study the cardiovascular adjustment to daily torpor in Phodopus, electrocardiogram (ECG) and T(b) were continuously recorded by telemetry during entrance into torpor, in deep torpor, and during arousal from torpor. Minimum T(b) during torpor bouts was approximately 21 degrees C, and heart rate, approximately 349 beats/min at euthermy, displayed marked sinus bradyarrhythmia at approximately 70 beats/min. Arousal was typically completed within approximately 40 min, followed by a sustained post-torpor inactivity tachycardia ( approximately 540 beats/min). The absence of episodes of conduction block, tachyarrhythmia, or other forms of ectopy throughout the torpor cycle demonstrates a remarkable resistance to arrhythmogenesis. The ECG morphology lacks a distinct isoelectric interval following the QRS complex, and the ST segment resembles the ECG pattern in mice, with a prominent fast transient outward K(+) current (I(to,f)) determining the early phase of ventricular repolarization. During low-temperature torpor, the amplitudes of the QRS complex substantially increased, suggesting that in the euthermic state the terminal portion of ventricular depolarization is fused with the beginning of repolarization, low T(b) acting to decorrelate the superposition between depolarization and repolarization by delaying the repolarization onset. Atrioventricular and ventricular conduction times were prolonged as function of T(b). In contrast, the QT vs. T(b) relationship showed marked hysteresis indicating the operation of nonlinear control mechanisms whereby the rapid QT shortening during arousal results from additional mechanisms (probably sympathetic stimulation) other than temperature alone.

Adrenergic regulation of a key cardiac potassium channel can contribute to atrial fibrillation: evidence from an I Ks transgenic mouse

Inherited gain-of-function mutations of genes coding for subunits of the heart slow potassium (I Ks) channel can cause familial atrial fibrillation (AF). Here we consider a potentially more prevalent mechanism and hypothesize that beta-adrenergic receptor (beta-AR)-mediated regulation of the I Ks channel, a natural gain-of-function pathway, can also lead to AF. Using a transgenic I Ks channel mouse model, we studied the role of the channel and its regulation by beta-AR stimulation on atrial arrhythmias. In vivo administration of isoprenaline (isoproterenol) predisposes I Ks channel transgenic mice but not wild-type (WT) littermates that lack I Ks to prolonged atrial arrhythmias. Patch-clamp analysis demonstrated expression and isoprenaline-mediated regulation of I Ks in atrial myocytes from transgenic but not WT littermates. Furthermore, computational modelling revealed that beta-AR stimulation-dependent accumulation of open I Ks channels accounts for the pro-arrhythmic substrate. Our results provide evidence that beta-AR-regulated I Ks channels can play a role in AF and imply that specific I Ks deregulation, perhaps through disruption of the I Ks macromolecular complex necessary for beta-AR-mediated I Ks channel regulation, may be a novel therapeutic strategy for treating this most common arrhythmia.

Toward physiological conditions for cell analyses: forces of heart muscle cells suspended between elastic micropillars

Almost each mammalian cell permanently applies forces to its environment. These forces are essential for many vital processes such as tissue formation or cell movement. In turn, the environmental conditions of cells strongly affect force production. Here we report on the development of an array of elastomeric micropillars as cellular environment. Within these micropillar arrays, we cultivated rat heart muscle cells (cardiac myocytes). For lattice constants between 20 and 30 mum, cells strongly preferred spanning between the elastic micropillars over adhering to the underlying flat substrate. In addition, the architectures of the cytoskeleton and of protein complexes formed for adhesion were strongly dependent on the environment of the cell. On flat parts of the substrates, we observed prominent stress fibers and focal adhesion sites. In contrast, cells suspended between micropillars exhibited well organized myofibers and costameric adhesions at the locations of Z-bands. These observations argue for close-to-nature environmental conditions within micropillar arrays. Resting as well as contraction forces of myocytes resulted in measurable pillar bending. Using an approximate theoretical treatment of elastically founded micropillars, we calculated average cell forces of 140 nN in the relaxed and 400 nN in the contracted state.

Electromechanical characterization of cinnamophilin, a natural thromboxane A2 receptor antagonist with anti-arrhythmic activity, in guinea-pig heart

BACKGROUND AND PURPOSE

Cinnamophilin, a thromboxane A(2) receptor antagonist, has been identified as a prominent anti-arrhythmic agent in rat heart. This study aimed to determine its electromechanical and anti-arrhythmic effects in guinea-pig hearts.

EXPERIMENTAL APPROACH

Microelectrodes were used to study action potentials in ventricular papillary muscles. Fluo-3 fluorimetric ratio and whole-cell voltage-clamp techniques were used to record calcium transients and membrane currents in single ventricular myocytes, respectively. Intracardiac electrocardiograms were obtained and the anti-arrhythmic efficacy was determined from isolated perfused hearts.

KEY RESULTS

In papillary muscles, cinnamophilin decreased the maximal rate of upstroke (V(max)) and duration of action potential, and reduced the contractile force. In single ventricular myocytes, cinnamophilin reduced Ca(2+) transient amplitude. Cinnamophilin decreased the L-type Ca(2+) current (I(Ca,L))(IC(50)=7.5 microM) with use-dependency, induced a negative shift of the voltage-dependent inactivation and retarded recovery from inactivation. Cinnamophilin also decreased the Na(+) current (I(Na)) (IC(50)=2.7 microM) and to a lesser extent, the delayed outward (I(K)), inward rectifier (I(K1)), and ATP-sensitive (I(K,ATP)) K(+) currents. In isolated perfused hearts, cinnamophilin prolonged the AV nodal conduction interval and Wenckebach cycle length and the refractory periods of the AV node, His-Purkinje system and ventricle, while shortening the ventricular repolarization time. Additionally, cinnamophilin reduced the occurrence of reperfusion-induced ventricular fibrillation.

CONCLUSIONS AND IMPLICATIONS

These results suggest that the promising anti-arrhythmic effect and the changes in the electromechanical function induced by cinnamophilin in guinea-pig heart can be chiefly accounted for by inhibition of I(Ca,L) and I(Na).

Effects of azimilide on the muscarinic acetylcholine receptor-operated K+ current and experimental atrial fibrillation in guinea-pig hearts

Effects of azimilide, a class III antiarrhythmic drug, on the acetylcholine (ACh) receptor-operated K+ current (I K.ACh) and the delayed rectifier K+ current (IK) were examined in guinea-pig atrial cells using patch-clamp techniques. Effects of azimilide on experimental atrial fibrillation (AF) were also examined in isolated guinea-pig hearts. In single atrial myocytes, azimilide inhibited both the rapid (IKr) and slow component of IK (IKs). Azimilide inhibited the I K.ACh induced by carbachol (CCh, 1 microM), adenosine (10 microM), and intracellular loading of GTPgammaS (100 microM) in a concentration-dependent manner. The IC50 values of azimilide for inhibiting the CCh-, adenosine-, and GTPgammaS-induced I K.ACh were 1.25, 29.1, and 20.9 microM, respectively, suggesting that azimilide inhibits I K.ACh mainly by blocking the muscarinic receptors. Azimilide concentration-dependently (0.3 - 10 microM) prolonged the action potential duration (APD) in the absence and presence of muscarinic stimulation. In isolated hearts, perfusion of CCh shortened the duration of the monophasic action potential (MAP) and effective refractory period (ERP) of the left atrium and lowered the atrial fibrillation threshold (AFT). Addition of azimilide inhibited the induction of AF by prolonging the duration of MAP and ERP. The I K.ACh inhibition by azimilide may at least in part contribute to the effectiveness to prevent parasympathetic-type AF.

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.

What Cardiologists Should Know About Calcium Ion Channels and Their Regulation by Reactive Oxygen Species

Ion channels underlie the electrical activity of cells. Calcium channels have a unique functional role, because not only do they participate in this activity, they form the means by which electrical signals are converted to responses within the cell. Calcium channels play an integral role in excitation in the heart and shaping the cardiac action potential. In addition, calcium influx through calcium channels is responsible for initiating contraction. Abnormalities in calcium homeostasis underlie cardiac arrhythmia, contractile dysfunction and cardiac remodelling. Reactive oxygen species participate in the development of pathology by altering the redox state of regulatory proteins. There is now good evidence that reactive oxygen species regulate the function of calcium channels. In this mini-review, the evidence for regulation of calcium channels by reactive oxygen species and implications with respect to pathology are presented. Calcium channels may represent a target for intervention during hypoxic trigger of arrhythmia or chronic pathological remodelling.

Atrial fibrillation--a new cardiac channelopathy

Atrial fibrillation (AF) is a common complication of a wide range of cardiac and systemic diseases and is regarded generally as a sporadic, acquired disorder. Familial clustering of AF has been reported but definitive links of genetic factors with AF pathogenesis have been lacking. Genome-wide linkage studies and the discovery of mutations in families with AF have provided compelling evidence that genetic factors can have a role in the development of AF. Although relatively few disease genes have been identified, current data indicate that inherited defects in cardiac ion channel genes can predispose to AF by altering ion channel activation and atrial conduction properties. Mutations in the reported disease genes account for only a minority of all familial AF cases and further gene discovery studies are required. Characterisation of the genetic variants that cause AF in families provides a framework for elucidation of key disease pathways that underlie the more commonly-occurring complex forms of AF. A better understanding of the molecular and electrophysiological defects that promote AF in families and in the general population will facilitate new approaches to the diagnosis, prevention and treatment of AF.

An inactivation gate in the selectivity filter of KCNQ1 potassium channels

Inactivation is an inherent property of most voltage-gated K(+) channels. While fast N-type inactivation has been analyzed in biophysical and structural details, the mechanisms underlying slow inactivation are yet poorly understood. Here, we characterized a slow inactivation mechanism in various KCNQ1 pore mutants, including L273F, which hinders entry of external Ba(2+) to its deep site in the pore and traps it by slowing its egress. Kinetic studies, molecular modeling, and dynamics simulations suggest that this slow inactivation involves conformational changes that converge to the outer carbonyl ring of the selectivity filter, where the backbone becomes less flexible. This mechanism involves acceleration of inactivation kinetics and enhancement of Ba(2+) trapping at elevated external K(+) concentrations. Hence, KCNQ1 slow inactivation considerably differs from C-type inactivation where vacation of K(+) from the filter was invoked. We suggest that trapping of K(+) at s(1) due to filter rigidity and hindrance of the dehydration-resolvation transition underlie the slow inactivation of KCNQ1 pore mutants.

The plateau outward current in canine ventricle, sensitive to 4-aminopyridine, is a constitutive contributor to ventricular repolarization

BACKGROUND AND PURPOSE

I(Kur) (Ultra-rapid delayed rectifier current) has microM sensitivity to 4-aminopyridine (4-AP) and is an important modulator of the plateau amplitude and action potential duration in canine atria. Kv1.5 encodes I(Kur) and is present in both atria and ventricles in canines and humans. We hypothesized that a similar plateau outward current with microM sensitivity to 4-AP is present in canine ventricle.

EXPERIMENTAL APPROACH

We used established voltage clamp protocols and used 4-AP (50 and 100 microM) to measure a plateau outward current in normal canine myocytes isolated from the left ventricular mid-myocardium.

KEY RESULTS

Action potential recordings in the presence of 4-AP showed significant prolongation of action potential duration at 50 and 90% repolarization at 0.5 and 1 Hz (P<0.05), while no prolongation occurred at 2 Hz. Voltage clamp experiments revealed a rapidly activating current, similar to current characteristics of canine atrial I(Kur), in approximately 70% of left ventricular myocytes. The IC(50) of 4-AP for this current was 24.2 microM. The concentration of 4-AP used in our experiments resulted in selective blockade of an outward current that was not I(to) or I(Kr). Beta-adrenergic stimulation with isoprenaline significantly increased the 4-AP sensitive outward current density (P<0.05), suggesting a role for this current during increased sympathetic stimulation. In silico incorporation into a canine ventricular cell model revealed selective AP prolongation after current blockade.

CONCLUSIONS AND IMPLICATIONS

Our results support the existence of a canine ventricular plateau outward current sensitive to micromolar 4-AP and its constitutive role in ventricular repolarization.

Sodium channel Scn1b null mice exhibit prolonged QT and RR intervals

In neurons, voltage-gated sodium channel beta subunits regulate the expression levels, subcellular localization, and electrophysiological properties of sodium channel alpha subunits. However, the contribution of beta subunits to sodium channel function in heart is poorly understood. We examined the role of beta1 in cardiac excitability using Scn1b null mice. Compared to wildtype mice, electrocardiograms recorded from Scn1b null mice displayed longer RR intervals and extended QT(c) intervals, both before and after autonomic block. In acutely dissociated ventricular myocytes, loss of beta1 expression resulted in a approximately 1.6-fold increase in both peak and persistent sodium current while channel gating and kinetics were unaffected. Na(v)1.5 expression increased in null myocytes approximately 1.3-fold. Action potential recordings in acutely dissociated ventricular myocytes showed slowed repolarization, supporting the extended QT(c) interval. Immunostaining of individual myocytes or ventricular sections revealed no discernable alterations in the localization of sodium channel alpha or beta subunits, ankyrin(B), ankyrin(G), N-cadherin, or connexin-43. Together, these results suggest that beta1 is critical for normal cardiac excitability and loss of beta1 may be associated with a long QT phenotype.

Cardiac repolarization is prolonged in CD4C/HIV transgenic mice

Pharmacological agents used to treat patients with AIDS have been associated with QT prolongation and result in delayed repolarization. New evidence suggests that delayed repolarization can occur independently of pharmacological therapy. However, the effect of HIV on ventricular repolarization has not been investigated. Therefore, the objective of this study was to characterize cardiac repolarization in a mouse model of human HIV disease. All experiments were conducted on HIV transgenic mice (CD4C/HIV). These mice express the human HIV gene nef in cells of immune system and develop a severe AIDS-like disease that is similar to that observed in humans. ECG was recorded in conscious free moving mice and patch-clamp techniques were used to record action potentials and K+ current densities in single ventricular myocytes. Results showed that the QT interval and action potential duration were significantly prolonged in CD4C/HIV mice compared to wild-type littermates. This delay in repolarization was associated with a significant reduction in outward K+ currents. Echocardiography showed that cardiac structure and function were similar in CD4C/HIV and littermate control mice. This suggests that the changes in ventricular repolarization were not the result of heart failure or cardiac hypertrophy. Overall, this study shows that repolarization was delayed in CD4C/HIV mice and that this phenotype occurred in the absence of any pharmacological intervention. Thus, it appears that HIV may be responsible for the delayed ventricular repolarization phenotype observed in CD4C/HIV mice.

G-protein coupled receptor signaling in myocardium: not for the faint of heart

Catecholamines, endothelin-1 and angiotensin II are among a diverse group of diffusible extracellular signals that regulate pump function of the heart by binding to G-protein coupled receptors (GPCR). When the body demands a temporary boost of power output or if temporary budgeting of resources is required, these signals can adjust heart rate and contractile strength to maintain continuous perfusion of all vascular beds with nutrient- and oxygen-rich blood. Given adequate time in the face of prolonged challenges, activation of GPCRs can also promote "remodeling of the heart" by increasing cell size, organ size, and chamber dimensions, or by varying tissue composition and altering the expression of protein isoforms controlling excitability and contractility. A common feature of heart disease is the state of chronic activation of GPCR signaling systems. Paradoxically, whereas acute activation is beneficial, chronic activation often contributes to further deterioration of cardiac performance. A better understanding of how chronic GPCR activation contributes to the development of heart disease is needed so that it can be translated into better prevention and therapeutic strategies in the clinic.

ERRgamma directs and maintains the transition to oxidative metabolism in the postnatal heart

At birth, the heart undergoes a critical metabolic switch from a predominant dependence on carbohydrates during fetal life to a greater dependence on postnatal oxidative metabolism. This remains the principle metabolic state throughout life, although pathologic conditions such as heart failure and cardiac hypertrophy reactivate components of the fetal genetic program to increase carbohydrate utilization. Disruption of the ERRgamma gene (Esrrg), which is expressed at high levels in the fetal and postnatal mouse heart, blocks this switch, resulting in lactatemia, electrocardiographic abnormalities, and death during the first week of life. Genomic ChIP-on-chip and expression analysis identifies ERRgamma as both a direct and an indirect regulator of a nuclear-encoded mitochondrial genetic network that coordinates the postnatal metabolic transition. These findings reveal an unexpected and essential molecular genetic component of the oxidative metabolic gene program in the heart and highlight ERRgamma in the study of cardiac hypertrophy and failure.

Role of spatial dispersion of repolarization in inherited and acquired sudden cardiac death syndromes

This review examines the role of spatial electrical heterogeneity within the ventricular myocardium on the function of the heart in health and disease. The cellular basis for transmural dispersion of repolarization (TDR) is reviewed, and the hypothesis that amplification of spatial dispersion of repolarization underlies the development of life-threatening ventricular arrhythmias associated with inherited ion channelopathies is evaluated. The role of TDR in long QT, short QT, and Brugada syndromes, as well as catecholaminergic polymorphic ventricular tachycardia (VT), is critically examined. In long QT syndrome, amplification of TDR is often secondary to preferential prolongation of the action potential duration (APD) of M cells; in Brugada syndrome, however, it is thought to be due to selective abbreviation of the APD of the right ventricular epicardium. Preferential abbreviation of APD of the endocardium or epicardium appears to be responsible for the amplification of TDR in short QT syndrome. In catecholaminergic polymorphic VT, reversal of the direction of activation of the ventricular wall is responsible for the increase in TDR. In conclusion, long QT, short QT, Brugada, and catecholaminergic polymorphic VT syndromes are pathologies with very different phenotypes and etiologies, but they share a common final pathway in causing sudden cardiac death.

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.

Heterogeneity of ventricular fibrillation dominant frequency during global ischemia in isolated rabbit hearts

INTRODUCTION

Ventricular fibrillation (VF) studies show that ECG-dominant frequency (DF) decreases as ischemia develops. This study investigates the contribution of the principle ischemic metabolic components to this decline.

METHODS AND RESULTS

Rabbit hearts were Langendorff-perfused at 40 mL/min with Tyrode's solution and loaded with RH237. Epicardial optical action potentials were recorded with a photodiode array (256 sites, 15 x 15 mm). After 60 seconds of VF (induced by burst pacing), global ischemia was produced by low flow (6 mL/min), or the solution changed to impose hypoxia (95% N2/5% CO2), low pH(o) (6.7, 80% O2/20% CO2), or raised [K+](o) (8 mM). DF of the optical signals was determined at each site. Conduction velocity (CV), action potential duration (APD90), effective refractory period (ERP), activation threshold, dV/dt(max), and membrane potential were measured in separate experiments during ventricular pacing. During VF, ischemia decreased DF in the left ventricle (LV) (to [58 +/- 6]%, P < 0.001), but not the right (RV) ([93 +/- 5]%). Raised [K+](o) reproduced this DF pattern (LV: [67 +/- 12]%, P < 0.001; RV: [95 +/- 9]%). LV DF remained elevated in hypoxia or low pH(o). During ventricular pacing, ischemia decreased CV in LV but not RV. Raised [K+](o) did not change CV in either ventricle. Ischemia and raised [K+](o) shortened APD90 without altering ERP. LV activation threshold increased in both ischemia and raised [K+](o) and was associated with diastolic depolarization and decreased dV/dt(max).

CONCLUSIONS

These results suggest that during VF, decreased ECG DF in global ischemia is largely due to elevated [K+](o) affecting the activation thresholds in the LV rather than RV.

Quantitative mRNA analysis of serotonin 5-HT4 receptor isoforms, calcium handling proteins and ion channels in human atrial fibrillation

Serotonin 5-HT(4) receptors are present in human atrial myocytes and have been proposed to contribute to the generation of atrial fibrillation (AF). Here, we quantified 5-HT(4) receptors as well as other key genes involved in cardiac rhythm and contraction in right atrial appendages of patients with chronic AF (CAF) and acute AF (AAF). Right atrial appendages were obtained from eleven patients in sinus rhythm (SR), five with AAF and six with CAF (>12 months). TaqMan real time quantitative RT-PCR was performed on total RNA. Results were normalised to the average of three housekeeping genes, cyclophilin, GADPH and RL-19. The rank order of expression of h5-HT(4) receptors variants was (b)>(a)>(g)>(c) in the group of patients in SR. In AAF, we found a strong decrease in h5-HT(4(b)), h5-HT(4(c),) and h5-HT(4(g)) transcripts. In CAF patients, the mRNA expression level of the h5-HT(4(b)) isoform significantly increased two fold versus SR. A similar increase was reported for beta(1)-adrenergic receptor, connexin 43 and the L-type Ca(2+) channel CaCNA1C subunit. Interestingly, CAF was associated with a strong increase in the expression of Na(+)/Ca(2+) exchanger and the voltage-dependent Na(+) channel SCN5A subunit. Our results indicate that h5-HT(4(b)) is the dominant cardiac isoform of human 5-HT(4) receptors and its expression is increased in CAF. These data support the involvement of 5-HT(4) receptors in atrial arrhythmia.

Effects of monocarboxylic acid-derived Cl−channel blockers on depolarization-activated potassium currents in rat ventricular myocytes

The effects of monocarboxylic acid-derived Cl(-) channel blockers on cardiac depolarization-activated K(+) currents were investigated. Membrane currents in rat ventricular myocytes were recorded using the whole-cell configuration of the patch-clamp technique. 5-Nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB) and niflumic acid (NFA) induced an outward current at 0 mV. Both NPPB and NFA failed to induce any current when used intracellularly or after K(+) in the bath and pipette solutions was replaced by equimolar Cs(+). Voltage pulse protocols revealed that NPPB and NFA enhanced the steady-state K(+) current but inhibited the transient outward K(+) current. Genistein, a tyrosine kinase (PTK) inhibitor, inhibited NPPB- and NFA-induced outward current. Another PTK inhibitor, lavendustin A, produced a comparable effect. In contrast, the inactive analogue of genistein, daidzein, was ineffective. Orthovanadate, a tyrosine phosphatase inhibitor, markedly slowed the deactivation of the outward current induced by NPPB and NFA. The protein kinase A (PKA) inhibitor H-89 inhibited NPPB-induced outward current at 0 mV. In contrast, the protein kinase C (PKC) inhibitor H-7 was without significant effect on the action of NPPB. Pretreatment of the myocytes with genistein or H-89 prevented the enhancing effect of NPPB. Increasing intracellular Cl(-) from 22 to 132 mm slightly reduced NPPB-induced outward current at 0 mV. These results demonstrate that the monocarboxylic acid-derived Cl(-) channel blockers NPPB and NFA enhance cardiac steady-state K(+) current, and suggest that the enhancing effect of the Cl(-) channel blockers is mediated by stimulation of PKA and PTK signalling pathways.

Redox control of calcium channels: from mechanisms to therapeutic opportunities

Calcium plays an integral role in cellular function. It is a well-recognized second messenger necessary for signaling cellular responses, but in excessive amounts can be deleterious to function, causing cell death. The main route by which calcium enters the cytoplasm is either from the extracellular compartment or internal addistores via calcium channels. There is good evidence that calcium channels can respond to pharmacological compounds that reduce or oxidize thiol groups on the channel protein. In addition, reactive oxygen species such as hydrogen peroxide and superoxide that can mediate oxidative pathology also mediate changes in channel function via alterations of thiol groups. This review looks at the structure and function of calcium channels, the evidence that changes in cellular redox state mediate changes in channel function, and the role of redox modification of channels in disease processes. Understanding how redox modification of the channel protein alters channel structure and function is providing leads for the design of therapeutic interventions that target oxidative stress responses.

Arrhythmogenic Ion-Channel Remodeling in the Heart: Heart Failure, Myocardial Infarction, and Atrial Fibrillation

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

Vernakalant (RSD1235): a novel, atrial-selective antifibrillatory agent

This review summarizes the mechanistic properties and the recent experience in the development of a new antiarrhythmic agent, RSD1235 (recently named vernakalant), for the acute conversion of atrial fibrillation to sinus rhythm. Atrial fibrillation is the most common sustained cardiac arrhythmia that is observed in clinical practice and is associated with increased morbidity and mortality, resulting from stroke and exacerbation of heart failure. At present, there is a lack of pharmacologic agents that are able to safely and effectively convert the arrhythmia back to sinus rhythm. Vernakalant has the electrophysiologic properties of a multiple ion channel blocker, developed using a novel approach to target potassium channels that are selectively present in human atria rather than ventricles, and using a rate-dependent blocking strategy for its additional sodium channel block. This paper reviews the mechanism of action of this drug, its performance in preclinical models of efficacy and human disease, and its actions on patients in the completed and published preregistration clinical trials for vernakalant. Overall, vernakalant converted 51.5% of patients who had < 7 days duration of atrial fibrillation and it did this without significantly more cardiovascular adverse events than placebo. Therefore, it must be considered as an important new agent for the treatment of this growing health problem.

Effects of atorvastatin and simvastatin on atrial plateau currents

Recent evidence has shown that the inhibitors of the 3-hydroxy-3-methylglutaryl coenzyme A reductase (statins) might exert antiarrhythmic effects both in experimental models and in humans. In this study we analyzed the effects of atorvastatin and simvastatin acid (SVA) on the currents responsible for the duration of the plateau of human atrial action potentials: hKv1.5, Kv4.3, and L-type Ca(2+) (I(Ca,L)). hKv1.5 and Kv4.3 currents were recorded in transfected Ltk(-) and Chinese hamster ovary cells, respectively, and I(Ca,L) in mouse ventricular myocytes, using whole-cell patch-clamp. Atorvastatin and SVA produced a concentration-dependent block of hKv1.5 channels (IC(50)=4.5+/-1.7 microM and 5.7+/-0.03 microM, respectively) and shifted the midpoint of the activation and inactivation curves to more negative potentials. Importantly, atorvastatin- and SVA-induced block was added to that produced by quinidine, a drug that blocks hKv1.5 channels by binding to their pore cavity. Atorvastatin and SVA blocked Kv4.3 channels in a concentration-dependent manner (IC(50)=13.9+/-3.6 nM and 7.0+/-0.8 microM, respectively). Both drugs accelerated the inactivation kinetics and shifted the inactivation curve to more negative potentials. SVA (10 nM), but not atorvastatin, also blocked I(Ca,L) producing a frequency-dependent block that, at 2 Hz, reached a 50.2+/-1.5%. As a consequence of these effects, at nanomolar concentrations, atorvastatin lengthened, whereas SVA shortened, the duration of mouse atrial action potentials. The results suggest that atorvastatin and SVA alter Kv1.5 and Kv4.3 channel activity following a complex mechanism that does not imply the binding of the drug to the channel pore.

S-acylation regulates Kv1.5 channel surface expression

The number of ion channels expressed on the cell surface shapes the complex electrical response of excitable cells. An imbalance in the ratio of inward and outward conducting channels is unfavorable and often detrimental. For example, over- or underexpression of voltage-gated K(+) (Kv) channels can be cytotoxic and in some cases lead to disease. In this study, we demonstrated a novel role for S-acylation in Kv1.5 cell surface expression. In transfected fibroblasts, biochemical evidence showed that Kv1.5 is posttranslationally modified on both the NH(2) and COOH termini via hydroxylamine-sensitive thioester bonds. Pharmacological inhibition of S-acylation, but not myristoylation, significantly decreased Kv1.5 expression and resulted in accumulation of channel protein in intracellular compartments and targeting for degradation. Channel protein degradation was rescued by treatment with proteasome inhibitors. Time course experiments revealed that S-acylation occurred in the biosynthetic pathway of nascent channel protein and showed that newly synthesized Kv1.5 protein, but not protein expressed on the cell surface, is sensitive to inhibitors of thioacylation. Sensitivity to inhibitors of S-acylation was governed by COOH-terminal, but not NH(2)-terminal, cysteines. Surprisingly, although intracellular cysteines were required for S-acylation, mutation of these residues resulted in an increase in Kv1.5 cell surface channel expression, suggesting that screening of free cysteines by fatty acylation is an important regulatory step in the quality control pathway. Together, these results show that S-acylation can regulate steady-state expression of Kv1.5.

Aryl sulfonamido indane inhibitors of the Kv1.5 ion channel

A collection of aryl sulfonamido indanes based on the lead compound 1 was synthesized and evaluated for Kv1.5 inhibitory activity. Kv1.5 inhibitors have the potential to be atrium-selective agents for treatment of atrial fibrillation. (1R,2R)-1 has an IC(50) of 0.033microM against Kv1.5 and is selective against other cardiac ion channels, including hERG.

A mathematical model of electrotonic interactions between ventricular myocytes and fibroblasts

Functional intercellular coupling has been demonstrated among networks of cardiac fibroblasts, as well as between fibroblasts and atrial or ventricular myocytes. In this study, the consequences of these interactions were examined by implementing the ten Tusscher model of the human ventricular action potential, and coupling it to our electrophysiological models for mammalian ventricular fibroblasts. Our simulations reveal significant electrophysiological consequences of coupling between 1 and 4 fibroblasts to a single ventricular myocyte. These include alterations in plateau height and/or action potential duration (APD) and changes in underlying ionic currents. Two series of simulations were carried out. First, fibroblasts were modeled as a spherical cell with a capacitance of 6.3 pF and an ohmic membrane resistance of 10.7 G Omega. When these "passive" fibroblasts were coupled to a myocyte, they caused slight prolongation of APD with no changes in the plateau, threshold for firing, or rate of initial depolarization. In contrast, when the same myocyte-fibroblast complexes were modeled after addition of the time- and voltage-gated K(+) currents that are expressed in fibroblasts, much more pronounced effects were observed: the plateau height of the action potential was reduced and the APD shortened significantly. In addition, each fibroblast exhibited significant electrotonic depolarizations in response to each myocyte action potential and the resting potential of the fibroblasts closely approximated the resting potential of the coupled ventricular myocyte.

Defining the Cellular Phenotype of “Ankyrin-B Syndrome” Variants

Background— Mutations in the ankyrin-B gene ( ANK2 ) cause type 4 long-QT syndrome and have been described in kindreds with other arrhythmias. The frequency of ANK2 variants in large populations and molecular mechanisms underlying the variability in the clinical phenotypes are not established. More importantly, there is no cellular explanation for the range of severity of cardiac phenotypes associated with specific ANK2 variants.

Methods and Results— We performed a comprehensive screen of ANK2 in populations (control, congenital arrhythmia, drug-induced long-QT syndrome) of different ethnicities to discover unidentified ANK2 variants. We identified 7 novel nonsynonymous ANK2 variants; 4 displayed abnormal activity in cardiomyocytes. Including the 4 new variants, 9 human ANK2 loss-of-function variants have been identified. However, the clinical phenotypes associated with these variants vary strikingly, from no obvious phenotype to manifest long-QT syndrome and sudden death, suggesting that mutants confer a spectrum of cellular phenotypes. We then characterized the relative severity of loss-of-function properties of all 9 nonsynonymous ANK2 variants identified to date in primary cardiomyocytes and identified a range of in vitro phenotypes, including wild-type, simple loss-of-function, and severe loss-of-function activity, seen with the variants causing severe human phenotypes.

Conclusions— We present the first description of differences in cellular phenotypes conferred by specific ANK2 variants. We propose that the various degrees of ankyrin-B loss of function contribute to the range of severity of cardiac dysfunction. These data identify ANK2 variants as modulators of human arrhythmias, provide the first insight into the clinical spectrum of “ankyrin-B syndrome,” and reinforce the role of ankyrin-B–dependent protein interactions in regulating cardiac electrogenesis.

SUMO modification regulates inactivation of the voltage-gated potassium channel Kv1.5

The voltage-gated potassium (Kv) channel Kv1.5 mediates the I(Kur) repolarizing current in human atrial myocytes and regulates vascular tone in multiple peripheral vascular beds. Understanding the complex regulation of Kv1.5 function is of substantial interest because it represents a promising pharmacological target for the treatment of atrial fibrillation and hypoxic pulmonary hypertension. Herein we demonstrate that posttranslational modification of Kv1.5 by small ubiquitin-like modifier (SUMO) proteins modulates Kv1.5 function. We have identified two membrane-proximal and highly conserved cytoplasmic sequences in Kv1.5 that conform to established SUMO modification sites in transcription factors. We find that Kv1.5 interacts specifically with the SUMO-conjugating enzyme Ubc9 and is a target for modification by SUMO-1, -2, and -3 in vivo. In addition, purified recombinant Kv1.5 serves as a substrate in a minimal in vitro reconstituted SUMOylation reaction. The SUMO-specific proteases SENP2 and Ulp1 efficiently deconjugate SUMO from Kv1.5 in vivo and in vitro, and disruption of the two identified target motifs results in a loss of the major SUMO-conjugated forms of Kv1.5. In whole-cell patch-clamp electrophysiological studies, loss of Kv1.5 SUMOylation, by either disruption of the conjugation sites or expression of the SUMO protease SENP2, leads to a selective approximately 15-mV hyperpolarizing shift in the voltage dependence of steady-state inactivation. Reversible control of voltage-sensitive channels through SUMOylation constitutes a unique and likely widespread mechanism for adaptive tuning of the electrical excitability of cells.

Electrical remodeling in a canine model of ischemic cardiomyopathy

The nature of electrical remodeling in a canine model of ischemic cardiomyopathy (ICM; induced by repetitive intracoronary microembolizations) that exhibits spontaneous ventricular tachycardia is not entirely clear. We used the patch-clamp technique to record action potentials and ionic currents of left ventricular myocytes isolated from the region affected by microembolizations. We also used the immunoblot technique to examine channel subunit expression in adjacent affected tissue. Ventricular myocytes and tissue isolated from the corresponding region of normal hearts served as control. ICM myocytes had prolonged action potential duration (APD) and more pronounced APD dispersion. Slow delayed rectifier current ( IKs) was reduced at voltages positive to 0 mV, along with a negative shift in its voltage dependence of activation. Immunoblots showed that there was no change in KCNQ1.1 ( IKs pore-forming or α-subunit), but KCNE1 ( IKs auxiliary or β-subunit) was reduced, and KCNQ1.2 (a truncated KCNQ1 splice variant with a dominant-negative effect on IKs) was increased. Transient outward current ( Ito) was reduced, along with an acceleration of the slow phase of recovery from inactivation. Immunoblots showed that there was no change in Kv4.3 (α-subunit of fast-recovering Ito component), but KChIP2 (β-subunit of fast-recovering component) and Kv1.4 (α-subunit of slow-recovering component) were reduced. Inward rectifier current was reduced. L-type Ca current was unaltered. The immunoblot data provide mechanistic insights into the observed changes in current amplitude and gating kinetics of IKs and Ito. We suggest that these changes, along with the decrease in inward rectifier current, contribute to APD prolongation in ICM hearts.

Ionic mechanisms underlying region-specific remodeling of rabbit atrial action potentials caused by intermittent burst stimulation

BACKGROUND

Pulmonary veins (PVs) and the coronary sinus (CS) play pivotal roles in triggering some episodes of atrial fibrillation. In isolated rabbit right or left atrial preparations, a 3-hour intermittent burst pacing protocol shortens action potential duration (APD) in CS and PV, but not in sinus node (SN) and left Bachmann bundle (BB) regions.

OBJECTIVE

The purpose of this study was to use patch clamp techniques to study the rapidly inactivating (I(to)) and sustained (I(sus)) K(+) currents as well as Ca(2+) currents (I(Ca)) in cells dispersed from intermittent burst pacing and sham PV, BB, CS, and SN regions to determine whether changes in these currents contributed to APD shortening.

METHODS

Real-time polymerase chain reaction was performed for transient outward K(+) and Ca(2+) channel subunit mRNAs to determine if intermittent burst pacing affected expression levels.

RESULTS

I(to) densities were unaffected by intermittent burst pacing in PV and Bachmann bundle cells. mRNA levels of K(V)4.3, K(V)4.2, K(V)1.4, and KChIP2 subunits of I(to) in both regions were stable. In CS cells, I(to) densities in intermittent burst pacing were greater than in sham (P <.05), but there were no parallel mRNA changes. I(Ca) density of PV cells was reduced from 14.27 +/- 2.08 pA/pF (at -5 mV) in sham to 7.52 +/- 1.65 pA/pF in intermittent burst pacing PV cells (P <.05) due to a significant shift in voltage dependence of activation. These results were seen in the absence of mRNA changes in alpha(1C) and alpha(1D) Ca(2+) channel subunits. In contrast, intermittent burst pacing had no effect on Ca(2+) current densities and kinetics of CS cells, but decreased alpha(1)C and alpha(1)D mRNA levels.

CONCLUSION

There is region-specific remodeling of I(to) and I(Ca) by intermittent burst pacing protocols in rabbit atrium. Increased I(to) in CS cells could account for the APD shortening observed with intermittent burst pacing, whereas an intermittent burst pacing-induced shift in voltage dependence of activation may contribute to APD shortening in PV cells.

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.

Altered Na+ channels promote pause-induced spontaneous diastolic activity in long QT syndrome type 3 myocytes

Long QT syndrome (LQTS) type 3 (LQT3), typified by the DeltaKPQ mutation (LQT3 mutation in which amino acid residues 1505 to 1507 [KPQ] are deleted), is caused by increased sodium entry during the action potential plateau resulting from mutation-altered inactivation of the Na(v)1.5 channel. Although rare, LQT3 is the most lethal of common LQTS variants. Here we tested the hypothesis that cellular electrical dysfunction, caused not only by action potential prolongation but also by mutation-altered Na(+) entry, distinguishes LQT3 from other LQTS variants and may contribute to its distinct lethality. We compared cellular electrical activity in myocytes isolated from mice heterozygous for the DeltaKPQ mutation (DeltaKPQ) and myocytes from wild-type littermates. Current-clamp pause protocols induced rate-dependent spontaneous diastolic activity (delayed after depolarizations) in 6 of 7 DeltaKPQ, but no wild-type, myocytes (n=11) tested. Voltage-clamp pause protocols that independently control depolarization duration and interpulse interval identified a distinct contribution of both depolarization duration and mutant Na(+) channel activity to the generation of Ca(i)(2+)-dependent diastolic transient inward current. This was found at rates and depolarization durations relevant both to the mouse model and to LQT3 patients. Flecainide, which preferentially inhibits mutation-altered late Na(+) current and is used to treat LQT3 patients, suppresses transient inward current formation in voltage-clamped DeltaKPQ myocytes. Our results demonstrate a marked contribution of mutation-altered Na(+) entry to the incidence of pause-dependent spontaneous diastolic activity in DeltaKPQ myocytes and suggest that altered Na(+) entry may contribute to the elevated lethality of LQT3 versus other LQTS variants.

Heart Hypertrophy During Pregnancy: A Better Functioning Heart?

During pregnancy, healthy women develop ventricular hypertrophy and diastolic dysfunction as a result of volume overload as well as increased stretch and force demand. Pregnancy also induces electrocardiogram disturbances such as longer QT-interval dispersion. Surprisingly, it was not until recently that the underlying molecular mechanisms or the role of sex hormones was addressed in this critical female reproductive stage. Recent work with the use of mouse and rat models show that the molecular signature of pregnancy-related hypertrophy differs from that of a pathologic form in that classic gene markers (e.g., myosin heavy chains [alpha and beta], atrial natriuretic peptide, phospholamban, and sarcoplasmic reticulum Ca(2+)-ATPase) remain unchanged. However, both types of hypertrophies have the commonality of a reduced expression of the Kv4.3 channel, a membrane protein that can prevent cardiac hypertrophy when overexpressed. Increased estrogen in late pregnancy may be a mechanism to induce Kv4.3 protein downregulation and increased activity of the stretch-activated c-Src kinase. Cellular/molecular mechanisms used to make a pregnant woman's heart work more efficiently and recover to normal cardiac function postpartum are beginning to emerge as cardioprotective natriuretic peptides- and NO-cGMP cascades get upregulated postpartum. This exciting initial work calls for more research in this underexplored area that should set the basis for better treatment of women during pregnancy.

Potassium channel remodeling in cardiac hypertrophy

Cardiac hypertrophy is an adaptive process against increased work loads; however, hypertrophy also presents substrates for lethal ventricular arrhythmias, resulting in sudden arrhythmic deaths that account for about one third of deaths in cardiac hypertrophy. To maintain physiological cardiac function in the face of increased work loads, hypertrophied cardiomyocytes undergo K(+) channel remodeling that provides a prolongation in action potential duration and an increase in Ca(2+) entry. Increased Ca(2+) entry, in turn, activates signaling mechanisms including a calcineruin/NFAT pathway to permit remodeling of the K(+) channels. This results in a positive feedback loop between the K(+) channel remodeling and altered Ca(2+) handling; this loop may represent a potential therapeutic target against sudden arrhythmic deaths in cardiac hypertrophy. The purposes of this review are to: (1) discuss types of K(+) channels and their mRNA that undergo remodeling in cardiac hypertrophy; (2) report on recent research on molecular mechanisms of K(+) channel remodeling; and (3) address physiological events underlying new therapeutic modalities to ameliorate arrhythmias and sudden death in cardiac hypertrophy.

[Molecular pharmacological studies on potassium channels and their regulatory molecules]

K+ channels play important roles in the control of a large variety of physiological functions such as muscle contraction, neurotransmitter release, hormone secretion, and cell proliferation. Over 100 cloned K+ channel pore-forming alpha and accessory beta subunits have been identified so far. Here, we introduce a series of molecular pharmacological and physiological studies on some types of voltage-dependent K+ channels and Ca2+-activated K+ channels. We examined molecular cloning and functional characterization of novel, fast-inactivating, A-type K+ channel alpha (Kv4.3L) and beta (KChIP2S) subunits predominantly expressed in mammalian heart and found the sites in Kv4 channels for 1) the regulation of voltage dependency and 2) the CaMKII phosphorylation in the C-terminal cytoplasmic domain. Moreover, we found that delayed rectifier-type K+ channels (ERG1 and KCNQ) contribute to the resting membrane conductance in vascular and gastrointestinal smooth muscles. The large-conductance Ca2+-activated K+ (BK) channel is ubiquitously expressed and contributes to diverse physiological processes. Recent reports have shown that a BK-like channel (mitoKCa) is expressed in cardiac mitochondria, suggesting that BK channel openers protect mammalian hearts against ischemic injury. Our studies revealed that BKbeta1 interacts with cytochrome c oxidase I (Cco1) in cardiac mitochondria, and that the activation of BK channels by 17beta-estradiol results in a significant increase in the survival rate of ventricular myocytes. These findings suggest that BKbeta1 may play an important role in the regulation of cell respiration in cardiac myocytes and be a target for the modulation by female gonadal hormones.

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.

Mechanism of shortened action potential duration in Na+-Ca2+ exchanger knockout mice

In cardiac-specific Na(+)-Ca(2+) exchanger (NCX) knockout (KO) mice, the ventricular action potential (AP) is shortened. The shortening of the AP, as well as a decrease of the L-type Ca(2+) current (I(Ca)), provides a critical mechanism for the maintenance of Ca(2+) homeostasis and contractility in the absence of NCX (Pott C, Philipson KD, Goldhaber JI. Excitation-contraction coupling in Na(+)-Ca(2+) exchanger knockout mice: reduced transsarcolemmal Ca(2+) flux. Circ Res 97: 1288-1295, 2005). To investigate the mechanism that underlies the accelerated AP repolarization, we recorded the transient outward current (I(to)) in patch-clamped myocytes isolated from wild-type (WT) and NCX KO mice. Peak I(to) was increased by 78% and decay kinetics were slowed in KO vs. WT. Consistent with increased I(to), ECGs from KO mice exhibited shortened QT intervals. Expression of the I(to)-generating K(+) channel subunit Kv4.2 and the K(+) channel interacting protein was increased in KO. We used a computer model of the murine AP (Bondarenko VE, Szigeti GP, Bett GC, Kim SJ, and Rasmusson RL. Computer model of action potential of mouse ventricular myocytes. Am J Physiol Heart Circ Physiol 287: 1378-1403, 2004) to determine the relative contributions of increased I(to), reduced I(Ca), and reduced NCX current (I(NCX)) on the shape and kinetics of the AP. Reduction of I(Ca) and elimination of I(NCX) had relatively small effects on the duration of the AP in the computer model. In contrast, AP repolarization was substantially accelerated when I(to) was increased in the computer model. Thus, the increase in I(to), and not the reduction of I(Ca) or I(NCX), is likely to be the major mechanism of AP shortening in KO myocytes. The upregulation of I(to) may comprise an important regulatory mechanism to limit Ca(2+) influx via a reduction of AP duration, thus preventing Ca(2+) overload in situations of reduced myocyte Ca(2+) extrusion capacity.

Endothelin-1 receptor blockade prevented the electrophysiological dysfunction in cardiac myocytes of streptozotocin-induced diabetic rats

Diabetes mellitus is complicated with the development of cardiac contractile dysfunction and electrical instability, which contributes to high morbidity and mortality in diabetic patients. This study examined the possible roles of enhanced endothelin-1 (ET-1) on diabetes-induced alterations in ventricular myocyte electrophysiology. Type 1 diabetic rats were induced by single dose injection of streptozotocin (STZ) and treated with or without ET-1 receptor antagonist bosentan for 8 wk before myocyte isolation. Action potential, outward K+ currents, and inward Ca2+ currents in ventricular myocytes were recorded using whole-cell patch clamp technique. STZ-injected rats exhibited hyperglycemia, reduced body weight gain, and elevated plasma ET-1 concentration, indicative of diabetes induction. Ventricular myocytes isolated from diabetic rats exhibited prolonged action potential and reduced all three types of outward K+ currents. Resting membrane potential, height of action potential, and L-type Ca2+ current were not altered in diabetic myocytes. In vivo chronic treatment of diabetic rats with bosentan significantly augmented K+ currents and reversed action potential prolongation in ventricular myocytes. On the other hand, bosentan treatment had no detectable effect on the electrophysiological properties in control myocytes. In addition, bosentan had no effect on Ltype Ca2+ currents in both control and diabetic myocytes. Our data suggest that altered electrophysiological properties in ventricular myocytes were largely resulted from augmented ET-1 system in diabetic animals.

Computational biology in the study of cardiac ion channels and cell electrophysiology

The cardiac cell is a complex biological system where various processes interact to generate electrical excitation (the action potential, AP) and contraction. During AP generation, membrane ion channels interact nonlinearly with dynamically changing ionic concentrations and varying transmembrane voltage, and are subject to regulatory processes. In recent years, a large body of knowledge has accumulated on the molecular structure of cardiac ion channels, their function, and their modification by genetic mutations that are associated with cardiac arrhythmias and sudden death. However, ion channels are typically studied in isolation (in expression systems or isolated membrane patches), away from the physiological environment of the cell where they interact to generate the AP. A major challenge remains the integration of ion-channel properties into the functioning, complex and highly interactive cell system, with the objective to relate molecular-level processes and their modification by disease to whole-cell function and clinical phenotype. In this article we describe how computational biology can be used to achieve such integration. We explain how mathematical (Markov) models of ion-channel kinetics are incorporated into integrated models of cardiac cells to compute the AP. We provide examples of mathematical (computer) simulations of physiological and pathological phenomena, including AP adaptation to changes in heart rate, genetic mutations in SCN5A and HERG genes that are associated with fatal cardiac arrhythmias, and effects of the CaMKII regulatory pathway and beta-adrenergic cascade on the cell electrophysiological function.

Contributions of sustained INa and IKv43 to transmural heterogeneity of early repolarization and arrhythmogenesis in canine left ventricular myocytes

The roles of sustained components of I(Na) and I(Kv43) in shaping the action potentials (AP) of myocytes isolated from the canine left ventricle (LV) have not been studied in detail. Here we investigate the hypothesis that these two currents can contribute substantially to heterogeneity of early repolarization and arrhythmic risk. Quantitative data from voltage-clamp and expression profiling experiments were used to complete meaningful modifications to an existing "local control" model of canine midmyocardial myocyte excitation-contraction coupling for epicardial and endocardial cells. We include 1) heterogeneous I(Kv43), I(Ks), and I(SERCA) density; 2) modulation of I(Kv43) by Kv channel interacting protein type 2 (KChIP2) channel subunits; 3) a possible Ca(2+)-dependent open-state inactivation of I(Kv43); and 4) a sustained component of the inward Na(+) current, I(NaL). The resulting simulations illustrate ways in which KChIP2- and Ca(2+)-dependent control of I(Kv43) can result in a sustained outward current that can neutralize I(NaL) in a rate- and myocyte subtype-dependent manner. Both these currents appear to play significant roles in modulating AP duration and rate dependence in midmyocardial myocytes. Furthermore, an increased ratio of I(Kv43) to I(NaL) is capable of protecting epicardial myocytes from the early afterdepolarizations resulting from the SCN5A-I1768V mutation-induced increase in I(NaL). Experimentally observed transmural differences in Ca(2+) handling, including greater sarcoplasmic reticulum Ca(2+) content and faster Ca(2+) transient decay rates on the epicardium, were recapitulated in our simulations. By design, these models allow upward integration into organ models or may be used as a basis for further investigations into cellular heterogeneities.

Cellular basis for electrocardiographic and arrhythmic manifestations of Andersen-Tawil syndrome (LQT7)

BACKGROUND

Andersen-Tawil syndrome, a skeletal muscle syndrome associated with periodic paralysis and long QT intervals on the ECG, has been linked to defects in KCNJ2, the gene encoding for the inward rectifier potassium channel (I(K1).)

OBJECTIVES

The purpose of this study was to examine the cellular mechanisms underlying the ECG and arrhythmic manifestations of Andersen-Tawil syndrome.

METHODS

To investigate the effects of KCNJ2 loss-of-function mutations responsible for Andersen-Tawil syndrome, we used barium chloride (BaCl(2)) to inhibit I(K1) in arterially perfused wedge preparation. Transmembrane action potentials (APs) were simultaneously recorded from endocardial, midmyocardial, and epicardial cells, together with a transmural ECG.

RESULTS

BaCl(2) (1 to 30 microM) produced a concentration-dependent prolongation of the QT interval, secondary to a homogeneous prolongation of AP duration of the three cell types. QT interval was prolonged without an increase in transmural dispersion of repolarization (TDR). Low extracellular potassium (2.0 mM), isoproterenol (20-50 nM), and an abrupt increase in temperature (36 degrees C-39 degrees C) in the presence of 10 microM BaCl(2) did not significantly increase TDR but increased ectopic extrasystolic activity. Early afterdepolarizations were not observed under any condition. Spontaneous torsades de pointes arrhythmias were never observed, nor could they be induced with programmed electrical stimulation under any of the conditions studied.

CONCLUSION

Our results provide an understanding of why QT prolongation associated with Andersen-Tawil syndrome is relatively benign in the clinic and provide further support for the hypothesis that the increase in TDR, rather than QT interval, is responsible for development of torsades de pointes.

Cardiac Hypertrophy Diminished the Effects of Isoproterenol on Delayed Rectifier Potassium Current in Rat Heart

We investigated the association between sympathetic nerve activity and delayed rectifier potassium current (I(K)) in hypertrophic rat hearts. Left ventricular hypertrophy was induced by a 50% constriction of suprarenal abdominal aorta for 6 weeks. The effects of isoproterenol on action potential duration (APD), I(K), and L-type calcium current (I(Ca)) were investigated using the whole-cell patch clamp technique. In hypertrophic rats, I(K) was decreased by 28.2%, resulting in significant APD(90) (90% repolarization) prolongation (sham: 55 +/- 3.9, hypertrophy: 98 +/- 11 ms, P = 0.01). Isoproterenol (100 nM)-stimulated I(K) was increased by 54.9% +/- 0.10% in sham-operated rats, but not in hypertrophic rats. On the other hand, isoproterenol increased I(Ca) in both sham-operated (77.7% +/- 7.6%) and hypertrophic rats (69.6% +/- 9.7%). Consequently, isoproterenol prolonged further APD in hypertrophic rats (98 +/- 11 vs. 145 +/- 8.6 ms, P < 0.01), but not in sham-operated rats (55 +/- 3.9 vs. 61 +/- 5.6 ms, n.s.). Forskolin (1 microM, an adenylyl cyclase stimulator) did not enhance I(K) in hypertrophic rats, but IBMX (100 microM, a nonselective phosphodiesterase inhibitor) enhanced the current (30.2 +/- 0.05%), as much as in sham-operated rats. We concluded that in hypertrophic hearts, I(K) was not increased by isoproterenol because of the enhanced activity of phosphodiesterase, which leads to excessive APD prolongation.

Targeted deletion of Kv4.2 eliminates I(to,f) and results in electrical and molecular remodeling, with no evidence of ventricular hypertrophy or myocardial dysfunction

Previous studies have demonstrated a role for voltage-gated K+ (Kv) channel alpha subunits of the Kv4 subfamily in the generation of rapidly inactivating/recovering cardiac transient outward K+ current, I(to,f), channels. Biochemical studies suggest that mouse ventricular I(to,f) channels reflect the heteromeric assembly of Kv4.2 and Kv4.3 with the accessory subunits, KChIP2 and Kvbeta1, and that Kv4.2 is the primary determinant of regional differences in (mouse ventricular) I(to,f) densities. Interestingly, the phenotypic consequences of manipulating I(to,f) expression in different mouse models are distinct. In the experiments here, the effects of the targeted deletion of Kv4.2 (Kv4.2(-/-)) were examined. Unexpectedly, voltage-clamp recordings from Kv4.2(-/-) ventricular myocytes revealed that I(to,f) is eliminated. In addition, the slow transient outward K+ current, I(to,s), and the Kv1.4 protein (which encodes I(to,s)) are upregulated in Kv4.2(-/-) ventricles. Although Kv4.3 mRNA/protein expression is not measurably affected, KChIP2 expression is markedly reduced in Kv4.2(-/-) ventricles. Similar to Kv4.3, expression of Kvbeta1, as well as Kv1.5 and Kv2.1, is similar in wild-type and Kv4.2(-/-) ventricles. In addition, and in marked contrast to previous findings in mice expressing a truncated Kv4.2 transgene, the elimination I(to,f) in Kv4.2(-/-) mice does not result in ventricular hypertrophy. Taken together, these findings demonstrate not only an essential role for Kv4.2 in the generation of mouse ventricular I(to,f) channels but also that the loss of I(to,f) per se does not have overt pathophysiological consequences.