Electrical remodeling in hearts from a calcium-dependent mouse model of hypertrophy and failure: complex nature of K+ current changes and action potential duration

Cell size induced bias of current density in hypertrophic cardiomyocytes

Alterations in ion channel expression and function known as "electrical remodeling" contribute to the development of hypertrophy and to the emergence of arrhythmias and sudden cardiac death. However, comparing current density values - an electrophysiological parameter commonly utilized to assess ion channel function - between normal and hypertrophied cells may be flawed when current amplitude does not scale with cell size. Even more, common routines to study equally sized cells or to discard measurements when large currents do not allow proper voltage-clamp control may introduce a selection bias and thereby confound direct comparison. To test a possible dependence of current density on cell size and shape, we employed whole-cell patch-clamp recording of voltage-gated sodium and calcium currents in Langendorff-isolated ventricular cardiomyocytes and Purkinje myocytes, as well as in cardiomyocytes derived from trans-aortic constriction operated mice. Here, we describe a distinct inverse relationship between voltage-gated sodium and calcium current densities and cell capacitance both in normal and hypertrophied cells. This inverse relationship was well fit by an exponential function and may be due to physiological adaptations that do not scale proportionally with cell size or may be explained by a selection bias. Our study emphasizes the need to consider cell size bias when comparing current densities in cardiomyocytes of different sizes, particularly in hypertrophic cells. Conventional comparisons based solely on mean current density may be inadequate for groups with unequal cell size or non-proportional current amplitude and cell size scaling.

IK1 Channel Agonist Zacopride Alleviates Cardiac Hypertrophy and Failure via Alterations in Calcium Dyshomeostasis and Electrical Remodeling in Rats

Intracellular Ca²⁺ overload, prolongation of the action potential duration (APD), and downregulation of inward rectifier potassium (I_K1) channel are hallmarks of electrical remodeling in cardiac hypertrophy and heart failure (HF). We hypothesized that enhancement of I_K1 currents is a compensation for I_K1 deficit and a novel modulation for cardiac Ca²⁺ homeostasis and pathological remodeling. In adult Sprague-Dawley (SD) rats in vivo, cardiac hypertrophy was induced by isoproterenol (Iso) injection (i.p., 3 mg/kg/d) for 3, 10, and 30 days. Neonatal rat ventricular myocytes (NRVMs) were isolated from 1 to 3 days SD rat pups and treated with 1 μmol/L Iso for 24 h in vitro. The effects of zacopride, a selective I_K1/Kir2.1 channel agonist, on cardiac remodeling/hypertrophy were observed in the settings of 15 μg/kg in vivo and 1 μmol/L in vitro. After exposing to Iso for 3 days and 10 days, rat hearts showed distinct concentric hypertrophy and fibrosis and enhanced pumping function (P < 0.01 or P < 0.05), then progressed to dilatation and dysfunction post 30 days. Compared with the age-matched control, cardiomyocytes exhibited higher cytosolic Ca²⁺ (P < 0.01 or P < 0.05) and lower SR Ca²⁺ content (P < 0.01 or P < 0.05) all through 3, 10, and 30 days of Iso infusion. The expressions of Kir2.1 and SERCA2 were downregulated, while p-CaMKII, p-RyR2, and cleaved caspase-3 were upregulated. Iso-induced electrophysiological abnormalities were also manifested with resting potential (RP) depolarization (P < 0.01), APD prolongation (P < 0.01) in adult cardiomyocytes, and calcium overload in cultured NRVMs (P < 0.01). Zacopride treatment effectively retarded myocardial hypertrophy and fibrosis, preserved the expression of Kir2.1 and some key players in Ca²⁺ homeostasis, normalized the RP (P < 0.05), and abbreviated APD (P < 0.01), thus lowered cytosolic [Ca^{2 +}]_i (P < 0.01 or P < 0.05). I_K1channel blocker BaCl₂ or chloroquine largely reversed the cardioprotection of zacopride. We conclude that cardiac electrical remodeling is concurrent with structural remodeling. By enhancing cardiac I_K1, zacopride prevents Iso-induced electrical remodeling around intracellular Ca²⁺ overload, thereby attenuates cardiac structural disorder and dysfunction. Early electrical interventions may provide protection on cardiac remodeling.

The electrophysiological development of cardiomyocytes

The generation of human cardiomyocytes (CMs) from human pluripotent stem cells (hPSCs) has become an important resource for modeling human cardiac disease and for drug screening, and also holds significant potential for cardiac regeneration. Many challenges remain to be overcome however, before innovation in this field can translate into a change in the morbidity and mortality associated with heart disease. Of particular importance for the future application of this technology is an improved understanding of the electrophysiologic characteristics of CMs, so that better protocols can be developed and optimized for generating hPSC-CMs. Many different cell culture protocols are currently utilized to generate CMs from hPSCs and all appear to yield relatively “developmentally” immature CMs with highly heterogeneous electrical properties. These hPSC-CMs are characterized by spontaneous beating at highly variable rates with a broad range of depolarization-repolarization patterns, suggestive of mixed populations containing atrial, ventricular and nodal cells. Many recent studies have attempted to introduce approaches to promote maturation and to create cells with specific functional properties. In this review, we summarize the studies in which the electrical properties of CMs derived from stem cells have been examined. In order to place this information in a useful context, we also review the electrical properties of CMs as they transition from the developing embryo to the adult human heart. The signal pathways involved in the regulation of ion channel expression during development are also briefly considered.

Evidence for redox sensing by a human cardiac calcium channel

Ion channels are critical to life and respond rapidly to stimuli to evoke physiological responses. Calcium influx into heart muscle occurs through the ion conducting α1C subunit (Ca_v1.2) of the L-type Ca²⁺ channel. Glutathionylation of Ca_v1.2 results in increased calcium influx and is evident in ischemic human heart. However controversy exists as to whether direct modification of Ca_v1.2 is responsible for altered function. We directly assessed the function of purified human Ca_v1.2 in proteoliposomes. Truncation of the C terminus and mutation of cysteines in the N terminal region and cytoplasmic loop III-IV linker did not alter the effects of thiol modifying agents on open probability of the channel. However mutation of cysteines in cytoplasmic loop I-II linker altered open probability and protein folding assessed by thermal shift assay. We find that C543 confers sensitivity of Ca_v1.2 to oxidative stress and is sufficient to modify channel function and posttranslational folding. Our data provide direct evidence for the calcium channel as a redox sensor that facilitates rapid physiological responses.

Persistent increases in Ca(2+) influx through Cav1.2 shortens action potential and causes Ca(2+) overload-induced afterdepolarizations and arrhythmias

Persistent elevation of Ca(2+) influx due to prolongation of the action potential (AP), chronic activation of the β-adrenergic system and molecular remodeling occurs in stressed and diseased hearts. Increases in Ca(2+) influx are usually linked to prolonged myocyte action potentials and arrhythmias. However, the contribution of chronic enhancement of Cav1.2 activity on cardiac electrical remodeling and arrhythmogenicity has not been completely defined and is the subject of this study. Chronically increased Cav1.2 activity was produced with a cardiac specific, inducible double transgenic (DTG) mouse system overexpressing the β2a subunit of Cav (Cavβ2a). DTG myocytes had increased L-type Ca(2+) current (ICa-L), myocyte shortening, and Ca(2+) transients. DTG mice had enhanced cardiac performance, but died suddenly and prematurely. Telemetric electrocardiograms revealed shortened QT intervals in DTG mice. The action potential duration (APD) was shortened in DTG myocytes due to significant increases of potassium currents and channel abundance. However, shortened AP in DTG myocytes did not fully limit excess Ca(2+) influx and increased the peak and tail ICa-L. Enhanced ICa promoted sarcoplasmic reticulum (SR) Ca(2+) overload, diastolic Ca(2+) sparks and waves, and increased NCX activity, causing increased occurrence of early and delayed afterdepolarizations (EADs and DADs) that may contribute to premature ventricular beats and ventricular tachycardia. AV blocks that could be related to fibrosis of the AV node were also observed. Our study suggests that increasing ICa-L does not necessarily result in AP prolongation but causes SR Ca(2+) overload and fibrosis of AV node and myocardium to induce cellular arrhythmogenicity, arrhythmias, and conduction abnormalities.

Role of the cytoskeleton in communication between L-type Ca(2+) channels and mitochondria

The L-type Ca(2+) channel is the main route for Ca(2+) entry into cardiac myocytes, which is essential for the maintenance of cardiac excitation and contraction. Alterations in L-type Ca(2+) channel activity and Ca(2+) homeostasis have been implicated in the development of cardiomyopathies. Cardiac excitation and contraction is fuelled by ATP, synthesized predominantly by the mitochondria via the Ca(2+)-dependent process oxidative phosphorylation. Mitochondrial reactive oxygen species (ROS) are by-products of oxidative phosphorylation and are associated with the development of cardiac pathology. The cytoskeleton plays a role in the communication of signals from the plasma membrane to intracellular organelles. There is good evidence that both L-type Ca(2+) channel activity and mitochondrial function can be modulated by changes in the cytoskeletal network. Activation of the L-type Ca(2+) channel can regulate mitochondrial function through cytoskeletal proteins as a result of transmission of movement from the β(2)-subunit of the channel that occurs during activation and inactivation of the channel. An association between cytoskeletal proteins and the mitochondrial voltage-dependent anion channel (VDAC) may play a role in this response. The L-type Ca(2+) channel is the initiator of contraction in cardiac muscle and the VDAC is responsible for regulating mitochondrial ATP/ADP trafficking. This article presents evidence that a functional coupling between L-type Ca(2+) channels and mitochondria may assist in meeting myocardial energy demand on a beat-to-beat basis.

Nanoparticle-mediated dual delivery of an antioxidant and a peptide against the L-Type Ca2+ channel enables simultaneous reduction of cardiac ischemia-reperfusion injury

Increased reactive oxygen species (ROS) production and elevated intracellular Ca(2+) following cardiac ischemia-reperfusion injury are key mediators of cell death and the development of cardiac hypertrophy. The L-type Ca(2+) channel is the main route for calcium influx in cardiac myocytes. Activation of the L-type Ca(2+) channel leads to a further increase in mitochondrial ROS production and metabolism. We have previously shown that the application of a peptide derived against the alpha-interacting domain of the L-type Ca(2+) channel (AID) decreases myocardial injury post reperfusion. Herein, we examine the efficacy of simultaneous delivery of the AID peptide in combination with the potent antioxidants curcumin or resveratrol using multifunctional poly(glycidyl methacrylate) (PGMA) nanoparticles. We highlight that drug loading and dissolution are important parameters that have to be taken into account when designing novel combinatorial therapies following cardiac ischemia-reperfusion injury. In the case of resveratrol low loading capacity and fast release rates hinder its applicability as an effective candidate for simultaneous therapy. However, in the case of curcumin, high loading capacity and sustained release rates enable its effective simultaneous delivery in combination with the AID peptide. Simultaneous delivery of the AID peptide with curcumin allowed for effective attenuation of the L-type Ca(2+) channel-activated increases in superoxide (assessed as changes in DHE fluorescence; Empty NP = 53.1 ± 7.6%; NP-C-AID = 7.32 ± 3.57%) and mitochondrial membrane potential (assessed as changes in JC-1 fluorescence; Empty NP = 19.8 ± 2.8%; NP-C-AID=13.05 ± 1.78%). We demonstrate in isolated rat hearts exposed to ischemia followed by reperfusion, that curcumin and the AID peptide in combination effectively reduce muscle damage, decrease oxidative stress and superoxide production in cardiac myocytes.

Exome sequencing and systems biology converge to identify novel mutations in the L-type calcium channel, CACNA1C, linked to autosomal dominant long QT syndrome

BACKGROUND

Long QT syndrome (LQTS) is the most common cardiac channelopathy with 15 elucidated LQTS-susceptibility genes. Approximately 20% of LQTS cases remain genetically elusive.

METHODS AND RESULTS

We combined whole-exome sequencing and bioinformatic/systems biology to identify the pathogenic substrate responsible for nonsyndromic, genotype-negative, autosomal dominant LQTS in a multigenerational pedigree, and we established the spectrum and prevalence of variants in the elucidated gene among a cohort of 102 unrelated patients with "genotype-negative/phenotype-positive" LQTS. Whole-exome sequencing was used on 3 members within a genotype-negative/phenotype-positive family. Genomic triangulation combined with bioinformatic tools and ranking algorithms led to the identification of a CACNA1C mutation. This mutation, Pro857Arg-CACNA1C, cosegregated with the disease within the pedigree, was ranked by 3 disease-network algorithms as the most probable LQTS-susceptibility gene and involves a conserved residue localizing to the proline, gltamic acid, serine, and threonine (PEST) domain in the II-III linker. Functional studies reveal that Pro857Arg-CACNA1C leads to a gain of function with increased ICa,L and increased surface membrane expression of the channel compared to wild type. Subsequent mutational analysis identified 3 additional variants within CACNA1C in our cohort of 102 unrelated cases of genotype-negative/phenotype-positive LQTS. Two of these variants also involve conserved residues within Cav1.2's PEST domain.

CONCLUSIONS

This study provides evidence that coupling whole-exome sequencing and bioinformatic/systems biology is an effective strategy for the identification of potential disease-causing genes/mutations. The identification of a functional CACNA1C mutation cosegregating with disease in a single pedigree suggests that CACNA1C perturbations may underlie autosomal dominant LQTS in the absence of Timothy syndrome.

Bone morphogenetic protein-4 contributes to the down-regulation of Kv4.3 K+ channels in pathological cardiac hypertrophy

Kv4.3 K(+) channels contributing to Ito are involved in the repolarization of cardiac action potential. Kv4.3 K(+) channels decrease in pathological cardiac hypertrophy, but the mechanism remains unclear. Our previous study found that the expression of bone morphogenetic protein 4 (BMP4) increased in pressure-overload and Ang II constant infusion induced cardiac hypertrophy. Since the downregulation of Kv4.3 K(+) channels and the upregulation of BMP4 simultaneously occur in pathological cardiac hypertrophy, we hypothesize that the up-regulated BMP4 would contribute to the downregulation of Kv4.3 K(+) channels in cardiac hypertrophy. We found that BMP4 treatment reduced Kv4.3 but not Kv4.2 and Kv1.4 K(+) channel protein expression, and BMP4-induced decrease of Kv4.3 K(+) channel protein expression was reversed by BMP4 inhibitor noggin and DMH1 in cultured cardiomyocytes in vitro. BMP4-induced decrease of Kv4.3 K(+) channel protein expression was also reversed by the NADPH oxidase inhibitor apocynin and the radical scavenger tempol. In in vivo transverse aortic constriction (TAC)-induced cardiac hypertrophy, constant infusion of DMH1 completely rescued TAC-induced down-regulation of Kv4.3 K(+) channel protein expression. We conclude that BMP4 contributes to the downregulation of Kv4.3 K(+) channels in pathological cardiac hypertrophy and the underlying mechanism might be through increasing ROS production.

Resolution of hyposmotic stress in isolated mouse ventricular myocytes causes sealing of t-tubules

It has recently been shown that various stress-inducing manipulations in isolated ventricular myocytes may lead to significant remodelling of t-tubules. Osmotic stress is one of the most common complications in various experimental and clinical settings. This study was therefore designed to determine the effects of a physiologically relevant type of osmotic stress, hyposmotic challenge, to the integrity of the t-tubular system in mouse ventricular myocytes using the following two approaches: (1) electrophysiological measurements of membrane capacitance and inward rectifier (IK1) tail currents originating from K(+) accumulation in t-tubules; and (2) confocal microscopy of fluorescent dextrans trapped in sealed t-tubules. Importantly, we found that removal of '0.6 Na' (60% NaCl) hyposmotic solution, but not its application to myocytes, led to a ∼27% reduction in membrane capacitance, a ∼2.5-fold reduction in the amplitude of the IK1 tail current and a ∼2-fold reduction in the so-called IK1 'inactivation' (due to depletion of t-tubular K(+)) at negative membrane potentials; all these data were consistent with significant detubulation. Confocal imaging experiments also demonstrated that extracellularly applied dextrans become trapped in sealed t-tubules only upon removal of hyposmotic solutions, i.e. during the shrinking phase, but not during the initial swelling period. In light of these data, relevant previous studies, including those on excitation-contraction coupling phenomena during hyposmotic stress, may need to be reinterpreted, and the experimental design of future experiments should take into account the novel findings.

Comparison of the effects of antiarrhythmic drugs flecainide and verapamil on fKv1.4ΔN channel currents in Xenopus oocytes

AIM

To study the effects of Na(+) channel blocker flecainide and L-type Ca(2+) channel antagonist verapamil on the voltage-gated fKv1.4ΔN channel, an N-terminal-deleted mutant of the ferret Kv1.4 K(+) channel.

METHODS

fKv1.4ΔN channels were stably expressed in Xenopus oocytes. The K(+) currents were recorded using a two-electrode voltage-clamp technique. The drugs were administered through superfusion.

RESULTS

fKv1.4ΔN currents displayed slow inactivation, with a half-inactivation potential of -41.74 mV and a slow recovery from inactivation (τ=1.90 s at -90 mV). Flecainide and verapamil blocked the currents with IC(50) values of 512.29 ± 56.92 and 260.71 ± 18.50 μmol/L, respectively. The blocking action of the drugs showed opposite voltage-dependence: it was enhanced with depolarization for flecainide, and was attenuated with depolarization for verapamil. Both the drugs exerted state-dependent blockade on fKv1.4ΔN currents, but verapamil showed a stronger use-dependent blockage compared with flecainide. Flecainide accelerated the C-type inactivation rate without affecting the recovery kinetics and the steady-state activation. Verapamil also accelerated the inactivation kinetics of the currents, but unlike flecainide, it affected both the recovery and the steady-state activation, causing slower recovery of fKv1.4ΔN channel and a depolarizing shift of the steady-state activation curve.

CONCLUSION

The results demonstrate that widely used antiarrhythmic drugs flecainide and verapamil substantially inhibit fKv1.4ΔN channels expressed in Xenopus oocytes by binding to the open state of the channels. Therefore, caution should be taken when these drugs are administered in combination with K(+) channel blockers to treat arrhythmia.

Ca(v)1.2 calcium channel is glutathionylated during oxidative stress in guinea pig and ischemic human heart

Glutathionylation as a posttranslational modification of proteins is becoming increasingly recognized, but its role in many diseases has not been demonstrated. Oxidative stress and alterations in calcium homeostasis are associated with the development of cardiac hypertrophy. Because the cardiac L-type Ca(2+) channel can be persistently activated after exposure to H(2)O(2), the aim of this study was to determine whether alterations in channel function were associated with glutathionylation of the α(1C) subunit (Ca(v)1.2) channel protein. Immunoblot analysis indicated that Ca(v)1.2 protein is significantly glutathionylated after exposure to H(2)O(2) and glutathione in vitro and after ischemia-reperfusion injury. L-type Ca(2+) channel macroscopic current and intracellular calcium were significantly increased in myocytes after exposure to oxidized glutathione and reversed by glutaredoxin. The increase in current correlated with an increase in open probability of the channel assessed as changes in single-channel activity after exposing the human long N-terminal Ca(v)1.2 to H(2)O(2) or oxidized glutathione. We also demonstrate that the Ca(v)1.2 channel is significantly glutathionylated in ischemic human heart. We conclude that oxidative stress is associated with an increase in glutathionylation of the Ca(v)1.2 channel protein. We suggest that the associated constitutive activity contributes to the development of pathology in ischemic heart disease.

Hypertrophic phenotype in cardiac cell assemblies solely by structural cues and ensuing self-organization

In vitro models of cardiac hypertrophy focus exclusively on applying "external" dynamic signals (electrical, mechanical, and chemical) to achieve a hypertrophic state. In contrast, here we set out to demonstrate the role of "self-organized" cellular architecture and activity in reprogramming cardiac cell/tissue function toward a hypertrophic phenotype. We report that in neonatal rat cardiomyocyte culture, subtle out-of-plane microtopographic cues alter cell attachment, increase biomechanical stresses, and induce not only structural remodeling, but also yield essential molecular and electrophysiological signatures of hypertrophy. Increased cell size and cell binucleation, molecular up-regulation of released atrial natriuretic peptide, altered expression of classic hypertrophy markers, ion channel remodeling, and corresponding changes in electrophysiological function indicate a state of hypertrophy on par with other in vitro and in vivo models. Clinically used antihypertrophic pharmacological treatments partially reversed hypertrophic behavior in this in vitro model. Partial least-squares regression analysis, combining gene expression and functional data, yielded clear separation of phenotypes (control: cells grown on flat surfaces; hypertrophic: cells grown on quasi-3-dimensional surfaces and treated). In summary, structural surface features can guide cardiac cell attachment, and the subsequent syncytial behavior can facilitate trophic signals, unexpectedly on par with externally applied mechanical, electrical, and chemical stimulation.

Evaluation of left ventricular hypertrophy in patients requiring permanent pacing

Objective: Hypertension may lead to left ventricular hypertrophy, fibrosis and degeneration of the conduction system. Our aim was to study the association of hypertrophy with certain arrhythmias such as complete atrioventricular block (AVB) and symptomatic sick sinus syndrome (SSS) that require implantation of permanent pacemaker.

Methods: We studied 130 patients that had been given a pacemaker because of complete AVB, 130 patients that had been given a pacemaker because of symptomatic SSS and 45 patients without cardiac conduction disturbances. In order to estimate left ventricular hypertrophy, indexes of relative wall thickness (RWT) and left ventricular mass (LVM) were evaluated by echocardiography.

Results: We observed significant association between complete AVB and abnormal values of echocardiographic indexes.

Conclusions: Our results lend further support to the hypothesis that complete AVB is associated with left ventricular hypertrophy. This hypothesis is enhanced by the observation that the majority of patients with complete AVB were hypertensive.

Early exercise training after myocardial infarction prevents contractile but not electrical remodelling or hypertrophy

AIMS

Exercise started early after myocardial infarction (MI) improves in vivo cardiac function and myofilament responsiveness to Ca(2+). We investigated whether this represents partial or complete reversal of cellular remodelling.

METHODS AND RESULTS

Mice with MI following left coronary ligation were given free access to a running wheel (MI(EXE), N = 22) or housed sedentary (MI(SED), N = 18) for 8 weeks and compared with sedentary sham-operated animals (SHAM, N = 11). Myocytes were enzymatically isolated from the non-infarcted left ventricle. Myocytes in MI were significantly longer and even more so with exercise (165 +/- 3 microm in MI(EXE) vs. 148 +/- 3 microm in MI(SED) and 136 +/- 2 microm in SHAM; P < 0.05, mean +/- SEM); cell width was not different. Contraction was measured during electrical field stimulation at 1, 2, and 4 Hz. Unloaded cell shortening was significantly reduced in MI(SED) (at 1 Hz, L/L(0)=4.4 +/- 0.3% vs. 6.7 +/- 0.4% in SHAM; P < 0.05, also at 2 and 4 Hz). Exercise restored cell shortening to SHAM values (MI(EXE), L/L(0)=6.4 +/- 0.5%). Membrane currents and [Ca(2+)](i) were measured via whole-cell patch clamping, with Fluo-3 as Ca(2+) indicator, all at 30 degrees C. Ca(2+) transient amplitude, I(CaL) and sarcoplasmic reticulum Ca(2+) content were not different between the three groups. Diastolic Ca(2+) levels at 4 Hz were significantly elevated in MI(SED) only, with a trend to increased spontaneous Ca(2+) release events (sparks). Action potential duration was increased and transient outward K(+) currents significantly reduced after MI; this was unaffected by exercise.

CONCLUSIONS

Early voluntary exercise training after MI restores cell contraction to normal values predominantly because of changes in the myofilament Ca(2+) response and has a beneficial effect on diastolic Ca(2+) handling. However, the beneficial effect is not a complete reversal of remodelling as hypertrophy and loss of repolarizing K(+) currents are not affected.

Ca/calmodulin kinase II differentially modulates potassium currents

BACKGROUND

Potassium currents contribute to action potential duration (APD) and arrhythmogenesis. In heart failure, Ca/calmodulin-dependent protein kinase II (CaMKII) is upregulated and can alter ion channel regulation and expression.

METHODS AND RESULTS

We examine the influence of overexpressing cytoplasmic CaMKIIdelta(C), both acutely in rabbit ventricular myocytes (24-hour adenoviral gene transfer) and chronically in CaMKIIdelta(C)-transgenic mice, on transient outward potassium current (I(to)), and inward rectifying current (I(K1)). Acute and chronic CaMKII overexpression increases I(to,slow) amplitude and expression of the underlying channel protein K(V)1.4. Chronic but not acute CaMKII overexpression causes downregulation of I(to,fast), as well as K(V)4.2 and KChIP2, suggesting that K(V)1.4 expression responds faster and oppositely to K(V)4.2 on CaMKII activation. These amplitude changes were not reversed by CaMKII inhibition, consistent with CaMKII-dependent regulation of channel expression and/or trafficking. CaMKII (acute and chronic) greatly accelerated recovery from inactivation for both I(to) components, but these effects were acutely reversed by AIP (CaMKII inhibitor), suggesting that CaMKII activity directly accelerates I(to) recovery. Expression levels of I(K1) and Kir2.1 mRNA were downregulated by CaMKII overexpression. CaMKII acutely increased I(K1), based on inhibition by AIP (in both models). CaMKII overexpression in mouse prolonged APD (consistent with reduced I(to,fast) and I(K1)), whereas CaMKII overexpression in rabbit shortened APD (consistent with enhanced I(K1) and I(to,slow) and faster I(to) recovery). Computational models allowed discrimination of contributions of different channel effects on APD.

CONCLUSIONS

CaMKII has both acute regulatory effects and chronic expression level effects on I(to) and I(K1) with complex consequences on APD.

The L-type Ca(2+) channel as a potential mediator of pathology during alterations in cellular redox state

The L-type Ca(2+) channel is the main route for calcium influx into cardiac myocytes and an important determinant of calcium homeostasis. There is now considerable evidence that the function of the L-type Ca(2+) channel is influenced by the cell's redox state. Reactive oxygen species such as hydrogen peroxide and superoxide can regulate biological function by directly altering the thiol redox state of proteins. Under conditions where cellular redox state varies, L-type Ca(2+) channel function and diastolic calcium levels can be significantly altered. This article will present the evidence for alterations in L-type Ca(2+) channel function by reactive oxygen species and the potential role for the channel in development of acute electrophysiological instability or chronic pathological remodelling under conditions of persistent oxidative stress.

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.

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.

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.

Sarcoplasmic reticulum adenosine triphosphatase overexpression in the L-type Ca2+ channel mouse results in cardiomyopathy and Ca2+ -induced arrhythmogenesis

BACKGROUND

Overexpression of the L-type voltage-dependent calcium channel alpha(1C)-subunit (L-VDCC OE) in transgenic mice results in adaptive hypertrophy followed by a maladaptive phase associated with a decrease in sarcoplasmic reticulum adenosine triphosphatase (SERCA)2a expression at 8 to 10 months of age. Overexpressing SERCA to manipulate calcium (Ca(2+)) cycling and prevent pathologic phenotypes in some models of heart failure has been proven to be a promising genetic strategy.

OBJECTIVE

In this study we investigated whether genetic manipulation that increases Ca(2+) uptake into the sarcoplasmic reticulum by overexpressing SERCA1a (skeletal muscle specific) into the L-VDCC OE background could restore or further deteriorate Ca(2+) cycling, contractile dysfunction, and electrical remodeling in the heart failure phenotype.

RESULTS

We found that the survival rate of L-VDCC OE/SERCA1a OE double transgenic mice decreased by 50%. L-VDCC OE/SERCA1a OE mice displayed an accelerated phenotype of severe dilation of both ventricles associated with deteriorated left ventricular function. Voltage clamp experiments revealed enhanced increased inward Ca(2+) current density and decreased the transient outward potassium current. Action potential duration in double transgenic ventricular myocytes was prolonged, and isoproterenol induced early after depolarization. These mice demonstrated a high incidence of spontaneous left ventricular arrhythmia. Expression of the proarrhythmic signaling protein Ca(2+)/calmodulin-dependent kinase II (CaMKII) was increased while connexin43 expression was decreased, defining an important putative mechanism in the electrophysiologic disturbances and mortality.

CONCLUSIONS

Despite previous reports of improved cardiac function in heart failure models after SERCA intervention, our results advocate the need to elucidate the involvement of augmented Ca(2+) cycling in arrhythmogenesis.

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.

Electrical remodeling of cardiac myocytes from mice with heart failure due to the overexpression of tumor necrosis factor-alpha

Mice that overexpress the inflammatory cytokine tumor necrosis factor-alpha in the heart (TNF mice) develop heart failure characterized by atrial and ventricular dilatation, decreased ejection fraction, atrial and ventricular arrhythmias, and increased mortality (males > females). Abnormalities in Ca2+ handling, prolonged action potential duration (APD), calcium alternans, and reentrant atrial and ventricular arrhythmias were previously observed with the use of optical mapping of perfused hearts from TNF mice. We therefore tested whether altered voltage-gated outward K+ and/or inward Ca2+ currents contribute to the altered action potential characteristics and the increased vulnerability to arrhythmias. Whole cell voltage-clamp recordings of K+ currents from left ventricular myocytes of TNF mice revealed an approximately 50% decrease in the rapidly activating, rapidly inactivating transient outward K+ current Ito and in the rapidly activating, slowly inactivating delayed rectifier current IK,slow1, an approximately 25% decrease in the rapidly activating, slowly inactivating delayed rectifier current IK,slow2, and no significant change in the steady-state current Iss compared with controls. Peak amplitudes and inactivation kinetics of the L-type Ca2+ current ICa,L were not altered. Western blot analyses revealed a reduction in the proteins underlying Kv4.2, Kv4.3, and Kv1.5. Thus decreased K+ channel expression is largely responsible for the prolonged APD in the TNF mice and may, along with abnormalities in Ca2+ handling, contribute to arrhythmias.

Molecular mechanisms underlying K+ current downregulation in canine tachycardia-induced heart failure

Heart failure (HF) is characterized by marked prolongation of action potential duration and reduction in cellular repolarization reserve. These changes are caused in large part by HF-induced K+ current downregulation. Molecular mechanisms underlying these changes remain unclear. We determined whether downregulation of K+ currents in a canine model of tachycardia-induced HF is caused by altered expression of underlying K+ channel α- and β-subunits encoding these currents. K+ channel subunit expression was quantified in normal and failing dogs at the mRNA and protein levels in epicardial (Epi), midmyocardial (Mid), and endocardial (Endo) layers of left ventricle. Analysis of mRNA and protein levels of candidate genes encoding the transient outward K+ current ( Ito) revealed marked reductions in canine cKv4.3 expression in HF in Epi (44% mRNA, 39% protein), Mid (52% mRNA, 34% protein), and Endo (49% mRNA, 73% protein) layers and a paradoxical enhancement (41% Epi, 97% Mid, 113% Endo) in cKv1.4 protein levels, without significant changes in Kv channel-interacting protein cKChIP2 expression. Expression of cKir2.1, the gene underlying inward rectifier K+ current ( IK1), was unaffected by HF at mRNA and protein levels despite significant reduction in IK1, whereas canine ether-à-go-go-related gene (cERG), which encodes the rapidly activating component of the delayed rectifier current ( IK), exhibited increased protein expression. HF was not accompanied by significant changes in cKvLQT1 or cMinK mRNA and protein levels. These data indicate that 1) downregulation of Ito in HF is associated with decreased cKv4.3 and not cKv1.4 or cKChIP2, and 2) alterations in both the rapidly activating and slowly activating components of IK as well as IK1 in nonischemic dilated cardiomyopathy are not caused by changes in either transcript or immunoreactive protein levels of relevant channel subunits, which suggests posttranslational modification of these currents by HF.

Molecular correlates of altered expression of potassium currents in failing rabbit myocardium

Action potential (AP) prolongation is a hallmark of failing myocardium. Functional downregulation of K currents is a prominent feature of cells isolated from failing ventricles. The detailed changes in K current expression differ depending on the species, the region of the heart, and the mechanism of induction of heart failure. We used complementary approaches to study K current downregulation in pacing tachycardia-induced heart failure in the rabbit. The AP duration (APD) at 90% repolarization was significantly longer in cells isolated from failing hearts compared with controls (539 +/- 162 failing vs. 394 +/- 114 control, P < 0.05). The major K currents in the rabbit heart, inward rectifier potassium current (I(K1)), transient outward (I(to)), and delayed rectifier current (I(K)) were functionally downregulated in cells isolated from failing ventricles. The mRNA levels of Kv4.2, Kv1.4, KChIP2, and Kir2.1 were significantly downregulated, whereas the Kv4.3, Erg, KvLQT1, and minK were unaltered in the failing ventricles compared with the control left ventricles. Significant downregulation in the long splice variant of Kv4.3, but not in the total Kv4.3, Kv4.2, and KChIP2 immunoreactive protein, was observed in cells isolated from the failing ventricle with no change in Kv1.4, KvLQT1, and in Kir2.1 immunoreactive protein levels. Multiple cellular and molecular mechanisms underlie the downregulation of K currents in the failing rabbit ventricle.

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

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

Structure and function of Kv4-family transient potassium channels

Shal-type (Kv4.x) K(+) channels are expressed in a variety of tissue, with particularly high levels in the brain and heart. These channels are the primary subunits that contribute to transient, voltage-dependent K(+) currents in the nervous system (A currents) and the heart (transient outward current). Recent studies have revealed an enormous degree of complexity in the regulation of these channels. In this review, we describe the surprisingly large number of ancillary subunits and scaffolding proteins that can interact with the primary subunits, resulting in alterations in channel trafficking and kinetic properties. Furthermore, we discuss posttranslational modification of Kv4.x channel function with an emphasis on the role of kinase modulation of these channels in regulating membrane properties. This concept is especially intriguing as Kv4.2 channels may integrate a variety of intracellular signaling cascades into a coordinated output that dynamically modulates membrane excitability. Finally, the pathophysiology that may arise from dysregulation of these channels is also reviewed.

Regulation of Ion Channel Expression

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