Inward rectifiers in the heart: an update on I(K1)

The network of cardiac KIR2.1: its function, cellular regulation, electrical signaling, diseases and new drug avenues

The functioning of the human heart relies on complex electrical and communication systems that coordinate cardiac contractions and sustain rhythmicity. One of the key players contributing to this intricate system is the KIR2.1 potassium ion channel, which is encoded by the KCNJ2 gene. KIR2.1 channels exhibit abundant expression in both ventricular myocytes and Purkinje fibers, exerting an important role in maintaining the balance of intracellular potassium ion levels within the heart. And by stabilizing the resting membrane potential and contributing to action potential repolarization, these channels have an important role in cardiac excitability also. Either gain- or loss-of-function mutations, but also acquired impairments of their function, are implicated in the pathogenesis of diverse types of cardiac arrhythmias. In this review, we aim to elucidate the system functions of KIR2.1 channels related to cellular electrical signaling, communication, and their contributions to cardiovascular disease. Based on this knowledge, we will discuss existing and new pharmacological avenues to modulate their function.

Overexpression of KCNJ2 enhances maturation of human-induced pluripotent stem cell-derived cardiomyocytes

BACKGROUND

Although human-induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) are a promising cell resource for cardiovascular research, these cells exhibit an immature phenotype that hampers their potential applications. The inwardly rectifying potassium channel K_ir2.1, encoded by the KCNJ2 gene, has been thought as an important target for promoting electrical maturation of iPSC-CMs. However, a comprehensive characterization of morphological and functional changes in iPSC-CMs overexpressing KCNJ2 (KCNJ2 OE) is still lacking.

METHODS

iPSC-CMs were generated using a 2D in vitro monolayer differentiation protocol. Human KCNJ2 construct with green fluorescent protein (GFP) tag was created and overexpressed in iPSC-CMs via lentiviral transduction. The mixture of iPSC-CMs and mesenchymal cells was cocultured with decellularized natural heart matrix for generation of 3D human engineered heart tissues (EHTs).

RESULTS

We showed that mRNA expression level of KCNJ2 in iPSC-CMs was dramatically lower than that in human left ventricular tissues. KCNJ2 OE iPSC-CMs yielded significantly increased protein expression of K_ir2.1 and current density of K_ir2.1-encoded I_K1. The larger I_K1 linked to a quiescent phenotype that required pacing to elicit action potentials in KCNJ2 OE iPSC-CMs, which can be reversed by I_K1 blocker BaCl₂. KCNJ2 OE also led to significantly hyperpolarized maximal diastolic potential (MDP), shortened action potential duration (APD) and increased maximal upstroke velocity. The enhanced electrophysiological maturation in KCNJ2 OE iPSC-CMs was accompanied by improvements in Ca²⁺ signaling, mitochondrial energy metabolism and transcriptomic profile. Notably, KCNJ2 OE iPSC-CMs exhibited enlarged cell size and more elongated and stretched shape, indicating a morphological phenotype toward structural maturation. Drug testing using hERG blocker E-4031 revealed that a more stable MDP in KCNJ2 OE iPSC-CMs allowed for obtaining significant drug response of APD prolongation in a concentration-dependent manner. Moreover, KCNJ2 OE iPSC-CMs formed more mature human EHTs with better tissue structure and cell junction.

CONCLUSIONS

Overexpression of KCNJ2 can robustly enhance maturation of iPSC-CMs in electrophysiology, Ca²⁺ signaling, metabolism, transcriptomic profile, cardiomyocyte structure and tissue engineering, thus providing more accurate cellular model for elucidating cellular and molecular mechanisms of cardiovascular diseases, screening drug-induced cardiotoxicity, and developing personalized and precision cardiovascular medicine.

Mitochondrial Dysfunction in Cardiac Arrhythmias

Electrophysiological and structural disruptions in cardiac arrhythmias are closely related to mitochondrial dysfunction. Mitochondria are an organelle generating ATP, thereby satisfying the energy demand of the incessant electrical activity in the heart. In arrhythmias, the homeostatic supply-demand relationship is impaired, which is often accompanied by progressive mitochondrial dysfunction leading to reduced ATP production and elevated reactive oxidative species generation. Furthermore, ion homeostasis, membrane excitability, and cardiac structure can be disrupted through pathological changes in gap junctions and inflammatory signaling, which results in impaired cardiac electrical homeostasis. Herein, we review the electrical and molecular mechanisms of cardiac arrhythmias, with a particular focus on mitochondrial dysfunction in ionic regulation and gap junction action. We provide an update on inherited and acquired mitochondrial dysfunction to explore the pathophysiology of different types of arrhythmias. In addition, we highlight the role of mitochondria in bradyarrhythmia, including sinus node dysfunction and atrioventricular node dysfunction. Finally, we discuss how confounding factors, such as aging, gut microbiome, cardiac reperfusion injury, and electrical stimulation, modulate mitochondrial function and cause tachyarrhythmia.

Adenosine and Adenosine Receptors: Advances in Atrial Fibrillation

Atrial fibrillation (AF) is the most common arrhythmia in the world. Because the key to developing innovative therapies that limit the onset and the progression of AF is to fully understand the underlying molecular mechanisms of AF, the aim of the present narrative review is to report the most recent advances in the potential role of the adenosinergic system in the pathophysiology of AF. After a comprehensive approach describing adenosinergic system signaling and the mechanisms of the initiation and maintenance of AF, we address the interactions of the adenosinergic system's signaling with AF. Indeed, adenosine release can activate four G-coupled membrane receptors, named A₁, A_2A, A_2B and A₃. Activation of the A_2A receptors can promote the occurrence of delayed depolarization, while activation of the A₁ receptors can shorten the action potential's duration and induce the resting membrane's potential hyperpolarization, which promote pulmonary vein firing, stabilize the AF rotors and allow for functional reentry. Moreover, the A_2B receptors have been associated with atrial fibrosis homeostasis. Finally, the adenosinergic system can modulate the autonomous nervous system and is associated with AF risk factors. A question remains regarding adenosine release and the adenosine receptors' activation and whether this would be a cause or consequence of AF.

Molecular stratification of arrhythmogenic mechanisms in the Andersen Tawil Syndrome

Andersen Tawil Syndrome (ATS) is a rare inheritable disease associated with loss-of-function mutations in KCNJ2, the gene coding the strong inward rectifier potassium channel Kir2.1, which forms an essential membrane protein controlling cardiac excitability. ATS is usually marked by a triad of periodic paralysis, life-threatening cardiac arrhythmias and dysmorphic features, but its expression is variable and not all patients with a phenotype linked to ATS have a known genetic alteration. The mechanisms underlying this arrhythmogenic syndrome are poorly understood. Knowing such mechanisms would be essential to distinguish ATS from other channelopathies with overlapping phenotypes and to develop individualized therapies. For example, the recently suggested role of Kir2.1 as a countercurrent to sarcoplasmic calcium reuptake might explain the arrhythmogenic mechanisms of ATS and its overlap with catecholaminergic polymorphic ventricular tachycardia (CPVT). Here we summarize current knowledge on the mechanisms of arrhythmias leading to sudden cardiac death in ATS. We first provide an overview of the syndrome and its pathophysiology, from the patient´s bedside to the protein, and discuss the role of essential regulators and interactors that could play a role in cases of ATS. The review highlights novel ideas related to some post-translational channel interactions with partner proteins that might help define the molecular bases of the arrhythmia phenotype. We then propose a new all-embracing classification of the currently known ATS loss-of-function mutations according to their position in the Kir2.1 channel structure and their functional implications. We also discuss specific ATS pathogenic variants, their clinical manifestations and treatment stratification. The goal is to provide a deeper mechanistic understanding of the syndrome toward the development of novel targets and personalized treatment strategies.

Kir2.1 channel regulates macrophage polarization via Ca2+/CaMK II/ERK/NF-κB signaling pathway

Macrophage polarization plays a key role in inflammatory response. Various ion channels expressed in macrophages has been documented, but very little is known about their roles in macrophage polarization. We find that knockdown or blockade of Kir2.1 channel significantly inhibits M1 polarization, but promotes M2 polarization. LPS induced M1 polarization is also remarkably suppressed in high extracellular K+ solutions (70 mM K+), and this inhibition is partially abolished by adding Ca2+ in the culture medium. Calcium imaging shows that Ca2+ influx is dependent on the hyperpolarized membrane potential generated by Kir2.1 channel. The upregulation of p-CaMK II, p-ERK1/2, and p-NF-κB proteins in RAW264.7 macrophages stimulated with LPS are significantly reversed by blocking Kir2.1 channel or culturing the cells with 70 mM K+ medium. Furthermore, in vivo study shows that mice treated with Kir2.1 channel blocker are protected from LPS-induced peritonitis. In summary, our data reveal the essential role of Kir2.1 channel in regulating macrophage polarization via Ca2+ / CaMK II / ERK1/2 / NF-κB pathway.

Methadone Blockade of Cardiac Inward Rectifier K+ Current Augments Membrane Instability and Amplifies U Waves on Surface ECGs: A Translational Study

Background Methadone is associated with a disproportionate risk of sudden death and ventricular tachyarrhythmia despite only modest inhibition of delayed rectifier K⁺ current (I_Kr)_, the principal mechanism of drug-associated arrhythmia. Congenital defects of inward rectifier K⁺ current (I_K1) have been linked to increased U-wave amplitude on ECG and fatal arrhythmia. We hypothesized that methadone may also be a potent inhibitor of I_K1, contributing to delayed repolarization and manifesting on surface ECGs as augmented U-wave integrals. Methods and Results Using a whole-cell voltage clamp, methadone inhibited both recombinant and native I_K1 with a half-maximal inhibitory concentration IC50) of 1.5 μmol/L, similar to that observed for I_Kr block (half-maximal inhibitory concentration of 2.9 μmol/L). Methadone modestly increased the action potential duration at 90% repolarization and slowed terminal repolarization at low concentrations. At higher concentrations, action potential duration at 90% repolarization lengthening was abolished, but its effect on terminal repolarization rose steadily and correlated with increased fluctuations of diastolic membrane potential. In parallel, patient ECGs were analyzed before and after methadone initiation, with 68% of patients having a markedly increased U-wave integral compared with premethadone (lead V3; mean +38%±15%, P=0.016), along with increased QT and T_Peak to T_End intervals, likely reflective of I_Kr block. Conclusions Methadone is a potent I_K1 inhibitor that causes augmentation of U waves on surface ECG. We propose that increased membrane instability resulting from I_K1 block may better explain methadone's arrhythmia risk beyond I_Kr inhibition alone. Drug-induced augmentation of U waves may represent evidence of blockade of multiple repolarizing ion channels, and evaluation of the effect of that agent on I_K1 may be warranted.

Ionic currents underlying different patterns of electrical activity in working cardiac myocytes of mammals and non-mammalian vertebrates

The orderly contraction of the vertebrate heart is determined by generation and propagation of cardiac action potentials (APs). APs are generated by the integrated activity of time- and voltage-dependent ionic channels which carry inward Na⁺ and Ca²⁺ currents, and outward K⁺ currents. This review compares atrial and ventricular APs and underlying ion currents between different taxa of vertebrates. We have collected literature data and attempted to find common electrophysiological features for two or more vertebrate groups, show differences between taxa and cardiac chambers, and indicate gaps in the existing data. Although electrical excitability of the heart in all vertebrates is based on the same superfamily of channels, there is a vast variability of AP waveforms between atrial and ventricular myocytes, between different species of the same vertebrate class and between endothermic and ectothermic animals. The wide variability of AP shapes is related to species-specific differences in animal size, heart rate, stage of ontogenetic development, excitation-contraction coupling, temperature and oxygen availability. Some of the differences between taxa are related to evolutionary development of genomes, which appear e.g. in the expression of different Na⁺ and K⁺ channel orthologues in cardiomyocytes of vertebrates. There is a wonderful variability of AP shapes and underlying ion currents with which electrical excitability of vertebrate heart can be generated depending on the intrinsic and extrinsic conditions of animal body. This multitude of ionic mechanisms provides excellent material for studying how the function of the vertebrate heart can adapt or acclimate to prevailing physiological and environmental conditions.

Proteomic Analysis of the Functional Inward Rectifier Potassium Channel (Kir) 2.1 Reveals Several Novel Phosphorylation Sites

Membrane proteins represent a large family of proteins that perform vital physiological roles and represent key drug targets. Despite their importance, bioanalytical methods aiming to comprehensively characterize the post-translational modification (PTM) of membrane proteins remain challenging compared to other classes of proteins in part because of their inherent low expression and hydrophobicity. The inward rectifier potassium channel (Kir) 2.1, an integral membrane protein, is critical for the maintenance of the resting membrane potential and phase-3 repolarization of the cardiac action potential in the heart. The importance of this channel to cardiac physiology is highlighted by the recognition of several sudden arrhythmic death syndromes, Andersen-Tawil and short QT syndromes, which are associated with loss or gain of function mutations in Kir2.1, often triggered by changes in the β-adrenergic tone. Therefore, understanding the PTMs of this channel (particularly β-adrenergic tone-driven phosphorylation) is important for arrhythmia prevention. Here, we developed a proteomic method, integrating both top-down (intact protein) and bottom-up (after enzymatic digestion) proteomic analyses, to characterize the PTMs of recombinant wild-type and mutant Kir2.1, successfully mapping five novel sites of phosphorylation and confirming a sixth site. Our study provides a framework for future work to assess the role of PTMs in regulating Kir2.1 functions.

Effect of atrial artificial electrical stimulation on depolarization and repolarization and hemodynamics of the heart ventricle in rainbow trout Oncorhynchus mykiss

The spatial-temporal organization of the activation, repolarization and hemodynamics of the heart ventricle in rainbow trout, Oncorhynchus mykiss, adapted to a temperature of 5-7 °C, were studied from the normal sinus rhythm (21.6 ± 4.9 bpm) to the highest possible heart rhythm (HR) (60 bpm), during which deterioration of the contractile activity of the myocardium occurred. Regardless of the HR, the main pattern of excitation of the heart ventricle was the movement of the depolarization wave from the dorsal areas of the base in the base-apical and ventral directions with the capture of the entire thickness of the walls, with a slight difference in the time of activation of the subendocardium compared to the subepicardium. The increase in HR above the sinus rhythm caused significant shortening of local repolarization durations in all areas and layers (endocardial, intramural and subepicardial) of the heart ventricle. Changes in local durations of repolarization led to an increase in the heterogeneity of repolarization of the ventricular myocardium; as a result, a deterioration of its contractility was observed. In relation to the sinus rhythm, the maximal systolic pressure in the heart ventricle decreased, the diastolic and end-diastolic pressure increased, and the maximum rates of pressure rise and fall decreased. In rainbow trout adapted to a temperature of 5-7 °C at sinus rhythm, the pumping function of the heart was probably within the upper limit of the physiological norm, and a further increase in the heart rate led to a decline in myocardial contractility.

Diabetes-induced changes in cardiac voltage-gated ion channels

Diabetes mellitus affects the heart through various mechanisms such as microvascular defects, metabolic abnormalities, autonomic dysfunction and incompatible immune response. Furthermore, it can also cause functional and structural changes in the myocardium by a disease known as diabetic cardiomyopathy (DCM) in the absence of coronary artery disease. As DCM progresses it causes electrical remodeling of the heart, left ventricular dysfunction and heart failure. Electrophysiological changes in the diabetic heart contribute significantly to the incidence of arrhythmias and sudden cardiac death in diabetes mellitus patients. In recent studies, significant changes in repolarizing K⁺ currents, Na⁺ currents and L-type Ca²⁺ currents along with impaired Ca²⁺ homeostasis and defective contractile function have been identified in the diabetic heart. In addition, insulin levels and other trophic factors change significantly to maintain the ionic channel expression in diabetic patients. There are many diagnostic tools and management options for DCM, but it is difficult to detect its development and to effectively prevent its progress. In this review, diabetes-associated alterations in voltage-sensitive cardiac ion channels are comprehensively assessed to understand their potential role in the pathophysiology and pathogenesis of DCM.

The Agonist of Inward Rectifier Potassium Channel (IK1) Attenuates Rat Reperfusion Arrhythmias Linked to CaMKII Signaling

Inward rectifier potassium channels (IK1, Kir) are known to play critical roles in arrhythmogenesis. Thus, how IK1 agonist affects reperfusion arrhythmias needs to be clarified, and its underlying mechanisms should be determined. Reperfusion arrhythmias were modeled by coronary ligation (ischemia, 15 minutes) and release (reperfusion, 15 minutes). Zacopride (1.5-50 μg/kg in vivo, or 0.1-10 μmol/Lex vivo) was applied in the settings of pretreatment (3 minutes before coronary ligation) and posttreatment (5 minutes after coronary ligation). Hypoxia (45 minutes) /reoxygenation (30 minutes) model was established in cultured H9c2 (2-1) cardiomyocytes. Zacopride or KN93 was applied before hypoxia (pretreatment). In the setting of pre- or posttreatment, zacopride at 15 μg/kg in vivo or 1 μmol/Lin vitro exhibited superlative protections on reperfusion arrhythmias or intracellular calcium overload. Western blot data from ex vivo hearts or H9c2 (2-1) cardiomyocytes showed that I/R (H/R) induced the inhibition of Kir2.1 (the dominant subunit of IK1 channel in ventricle), phosphorylation and oxidation of CaMKII, downregulation of SERCA2, phosphorylation of phospholamban (at Thr17), and activation of caspase-3. Zacopride treatment (1 μmol/L) was noted to strikingly restore the expression of Kir2.1 and SERCA2 and decrease the activity of CaMKII, phospholamban, and caspase-3. These effects were largely eliminated by co-application of IK1 blocker BaCl2. CaMKII inhibitor KN93 attenuated calcium overload and p-PLB (Thr17) in an IK1-independent manner. IK1-depedent inhibition of CaMKII activity is found to be a key cardiac salvage signaling under Ca2+ dyshomeostasis and reactive oxygen species (ROS) stress. IK1 might be a novel target for pharmacological conditioning of reperfusion arrhythmia, especially for the application after unpredictable ischemia.

Exposure to Hydraulic Fracturing Flowback Water Impairs Mahi-Mahi (Coryphaena hippurus) Cardiomyocyte Contractile Function and Swimming Performance

Publicly available toxicological studies on wastewaters associated with unconventional oil and gas (UOG) activities in offshore regions are nonexistent. The current study investigated the impact of hydraulic fracturing-generated flowback water (HF-FW) on whole organism swimming performance/respiration and cardiomyocyte contractility dynamics in mahi-mahi (Coryphaena hippurus-hereafter referred to as "mahi"), an organism which inhabits marine ecosystems where offshore hydraulic fracturing activity is intensifying. Following exposure to 2.75% HF-FW for 24 h, mahi displayed significantly reduced critical swimming speeds (U_crit) and aerobic scopes (reductions of ∼40 and 61%, respectively) compared to control fish. Additionally, cardiomyocyte exposures to the same HF-FW sample at 2% dilutions reduced a multitude of mahi sarcomere contraction properties at various stimulation frequencies compared to all other treatment groups, including an approximate 40% decrease in sarcomere contraction size and a nearly 50% reduction in sarcomere relaxation velocity compared to controls. An approximate 8-fold change in expression of the cardiac contractile regulatory gene cmlc2 was also seen in ventricles from 2.75% HF-FW-exposed mahi. These results collectively identify cardiac function as a target for HF-FW toxicity and provide some of the first published data on UOG toxicity in a marine species.

Genetic Loss of IK1 Causes Adrenergic-Induced Phase 3 Early Afterdepolariz ations and Polymorphic and Bidirectional Ventricular Tachycardia

BACKGROUND

Arrhythmia syndromes associated with KCNJ2 mutations have been described clinically; however, little is known of the underlying arrhythmia mechanism. We create the first patient inspired KCNJ2 transgenic mouse and study effects of this mutation on cardiac function, IK1, and Ca2+ handling, to determine the underlying cellular arrhythmic pathogenesis.

METHODS

A cardiac-specific KCNJ2-R67Q mouse was generated and bred for heterozygosity (R67Q+/-). Echocardiography was performed at rest, under anesthesia. In vivo ECG recording and whole heart optical mapping of intact hearts was performed before and after adrenergic stimulation in wild-type (WT) littermate controls and R67Q+/- mice. IK1 measurements, action potential characterization, and intracellular Ca2+ imaging from isolated ventricular myocytes at baseline and after adrenergic stimulation were performed in WT and R67Q+/- mice.

RESULTS

R67Q+/- mice (n=17) showed normal cardiac function, structure, and baseline electrical activity compared with WT (n=10). Following epinephrine and caffeine, only the R67Q+/- mice had bidirectional ventricular tachycardia, ventricular tachycardia, frequent ventricular ectopy, and/or bigeminy and optical mapping demonstrated high prevalence of spontaneous and sustained ventricular arrhythmia. Both R67Q+/- (n=8) and WT myocytes (n=9) demonstrated typical n-shaped IK1IV relationship; however, following isoproterenol, max outward IK1 increased by ≈20% in WT but decreased by ≈24% in R67Q+/- (P<0.01). R67Q+/- myocytes (n=5) demonstrated prolonged action potential duration at 90% repolarization and after 10 nmol/L isoproterenol compared with WT (n=7; P<0.05). Ca2+ transient amplitude, 50% decay rate, and sarcoplasmic reticulum Ca2+ content were not different between WT (n=18) and R67Q+/- (n=16) myocytes. R67Q+/- myocytes (n=10) under adrenergic stimulation showed frequent spontaneous development of early afterdepolarizations that occurred at phase 3 of action potential repolarization.

CONCLUSIONS

KCNJ2 mutation R67Q+/- causes adrenergic-dependent loss of IK1 during terminal repolarization and vulnerability to phase 3 early afterdepolarizations. This model clarifies a heretofore unknown arrhythmia mechanism and extends our understanding of treatment implications for patients with KCNJ2 mutation.

Cell-Adhesion Properties of β-Subunits in the Regulation of Cardiomyocyte Sodium Channels

Voltage-gated sodium (Nav) channels drive the rising phase of the action potential, essential for electrical signalling in nerves and muscles. The Nav channel α-subunit contains the ion-selective pore. In the cardiomyocyte, Nav1.5 is the main Nav channel α-subunit isoform, with a smaller expression of neuronal Nav channels. Four distinct regulatory β-subunits (β1–4) bind to the Nav channel α-subunits. Previous work has emphasised the β-subunits as direct Nav channel gating modulators. However, there is now increasing appreciation of additional roles played by these subunits. In this review, we focus on β-subunits as homophilic and heterophilic cell-adhesion molecules and the implications for cardiomyocyte function. Based on recent cryogenic electron microscopy (cryo-EM) data, we suggest that the β-subunits interact with Nav1.5 in a different way from their binding to other Nav channel isoforms. We believe this feature may facilitate trans-cell-adhesion between β1-associated Nav1.5 subunits on the intercalated disc and promote ephaptic conduction between cardiomyocytes.

A novel role of cardiac inwardly rectifying potassium channels explaining autonomic cardiovascular dysfunctions in a cuprizone-induced mouse model of multiple sclerosis

Multiple sclerosis (MS) is a chronic inflammatory demyelinating and neurodegenerative disease of the central nervous system (CNS), believed to have an autoimmune etiology. MS patients showed an increased cardiovascular (CV) risk probably related to an impairment in the autonomic control of CV functions, but the underlying molecular mechanisms are not completely elucidated. Inwardly-rectifying potassium (Kir) channels play a key role in cardiac excitability by contributing to the repolarization phase of action potential and were recently identified as target of the autoantibody response in MS patients. Therefore, we investigated the role of cardiac Kir channels in the CV dysfunctions occurring in MS. Cardiac functions were evaluated by electrocardiographic recordings (ECG) in cuprizone-fed C57BL/6 mice, a classic demyelination animal model. Gene expression profiling of cardiac Kir2.2, Kir4.1 and Kir6.2 channels was performed using real-time PCR in mice. Cuprizone-induced mouse model was confirmed by immunohistochemistry analysis showing demyelination in the corpus callosum. ECG recordings from mice showed a significant decreased duration of the P wave and RR interval as well as an increase of the heart rate in cuprizone-treated mice as compared with the controls. Significant increased relative expression levels of Kcnj11 and Kcnj12, encoding for Kir6.2 and Kir2.2 channels respectively, were observed in mouse heart tissue, whereas no differences were found in mRNA levels of Kir4.1 channel as compared with controls. For the first time, these findings provided valuable insights into the potential role of Kir channels in cardiac problems associated with MS.

Polyaromatic hydrocarbons in pollution: a heart-breaking matter

Air pollution is associated with detrimental effects on human health, including decreased cardiovascular function. However, the causative mechanisms behind these effects have yet to be fully elucidated. Here we review the current epidemiological, clinical and experimental evidence linking pollution with cardiovascular dysfunction. Our focus is on particulate matter (PM) and the associated low molecular weight polycyclic aromatic hydrocarbons (PAHs) as key mediators of cardiotoxicity. We begin by reviewing the growing epidemiological evidence linking air pollution to cardiovascular dysfunction in humans. We next address the pollution-based cardiotoxic mechanisms first identified in fish following the release of large quantities of PAHs into the marine environment from point oil spills (e.g. Deepwater Horizon). We finish by discussing the current state of mechanistic knowledge linking PM and PAH exposure to mammalian cardiovascular patho-physiologies such as atherosclerosis, cardiac hypertrophy, arrhythmias, contractile dysfunction and the underlying alterations in gene regulation. Our aim is to show conservation of toxicant pathways and cellular targets across vertebrate hearts to allow a broad framework of the global problem of cardiotoxic pollution to be established. AhR; Aryl hydrocarbon receptor. Dark lines indicate topics discussed in this review. Grey lines indicate topics reviewed elsewhere.

Multivariate analysis of TU wave complex on electrocardiogram in Andersen-Tawil syndrome with KCNJ2 mutations

BACKGROUND

The exact differences between the TU wave complex of ATS1 and that of healthy individuals remain to be investigated. We sought to characterize the TU wave complex of Andersen-Tawil syndrome type 1 (ATS1) using high frequency electrocardiogram (ECG) data.

METHODS

Electrocardiograms were recorded as time series data with a 2 kHz frequency ECG amplifier in 13 patients with ATS1 (positive for KCNJ2 mutation, ATS1 group) and age-matched healthy individuals (control group). Conventional ECG parameters were measured, and principal component analysis (PCA) and independent component analysis (ICA) were applied to the TU wave complex.

RESULTS

Time from T peak (Tp) to U peak (Up), time from bottom (B) to Up, and time from B to U end (BUe, U duration) (0.232 ± 0.018 vs. 0.165 ± 0.017, p < .0001), where B is the lowest point between T and U waves, were all longer in the ATS1 group than the control group. Multivariate logistic regression analysis revealed that BUe could completely differentiate the two groups. PCA ratios in the ATS1 group were significantly larger than the control group (26.5 ± 12.3 vs. 10.4 ± 6.2, p = .0005). ICA revealed 1 or 2 U-wave-specific independent components (ICs) that exclusively comprise the U wave in ATS1, whereas U waves in the control group were composed of some ICs that also comprised T waves.

CONCLUSIONS

U-wave-related temporal parameters, particularly BUe, and the existence of U-wave-specific ICs, extracted in the ICA, are useful for differentiation of U waves in ATS1 from those in healthy individuals.

Apelin shorten QT interval by inhibiting Kir2.1/IK1 via a PI3K way in acute myocardial infarction

QT interval prolongation and depolarization of resting membrane potential (RMP) were found in acute myocardial infarction (MI) which is involved in the arrhythmogenic mechanism and raising the risk to initiate torsade de pointes. However, clinical anti-arrhythmic agents that primarily act on QT interval and RMP are not currently available. Our objective was to determine whether Apelin, an endogenous peptide ligand of receptor APJ, affects QT interval and RMP and underlying mechanisms. To test this viewpoint, mice were subjected to MI by ligating the left main coronary artery and Apelin was applied through tail vein at 5 min prior coronary occlusion in tested group. Compared to MI group, pretreatment of Apelin (15 μg/kg) shortened QTc and QT interval induced by MI, significantly elevated RMP and shortened action potential duration (APD) by increased I_K1 currents recorded using whole-cell patch technique from cardiomyocytes underwent MI. In cultured neonatal mouse cardiomyocytes, Apelin (1 μmol/L) restored hypoxia-induced Kir2.1 down-regulation, which was abolished by IP3K inhibitor LY-294002. Additionally, Apelin elicited a time-dependent increase in phosphorylation of Akt leading to increase in PI3-kinase activity. These results showed that Apelin enhanced I_K1/Kir2.1 currents via IP3K pathway as by rescue ischemia- and hypoxia-induced RMP depolarization and prolongation of QT interval, which may prevent or cure acute ischemic-mediated arrhythmias. This study brings new information to anti-arrhythmic theories and provides a potential target for the clinical management of acute ischemia-related arrhythmias.

Genetic, Epigenetic and Transcription Factors in Atrial Fibrillation

Atrial fibrillation (AF) is one of the most common arrhythmia that occurs in patients with cardiovascular diseases. Congenital forms of AF are quite rare. Many studies have shown that genetic, epigenetic and transcription factors may play an important role in the development and the progression of AF. In our review, studies have been conducted on the identification of mutations in ionic and non-ionic channels, possibly associated with AF. These mutations were found only in isolated groups of patients with AF, and in general, monogenic forms of AF are a rare subtype of the disease. Genomic association studies have helped to identify potential links between single nucleotide polymorphisms and AF. The risk of AF in the general population is likely to be determined by the interaction between environmental factors and many alleles. In recent years, the emergence of a genome-wide associative studies has significantly expanded the understanding of the genetic basis for the inheritance of AF and has led to the emergence of new evidence of the important role of genetic factors in the development of AF, in the risk stratification of AF and the recurrence of AF. Epigenetic factors are also important in AF. Epigenetic therapy aimed at treating a disease through exposure to epigenome is currently under development. A newly emerged area of ablatogenomics includes the use of genetic profiles that allow assessing the likelihood of recurrence of AF after catheter ablation. The results of genetic studies in AF show that, in addition to their role in the appearance of congenital heart pathologies, transcription factors play an important role in the pathogenesis of AF.

Inward Rectifier Potassium Channels (Kir2.x) and Caveolin-3 Domain-Specific Interaction: Implications for Purkinje Cell-Dependent Ventricular Arrhythmias

BACKGROUND

In human cardiac ventricle, I_K1 is mainly comprised Kir2.1, but Kir2.2 and Kir2.3 heterotetramers occur and modulate I_K1. Long-QT syndrome-9-associated CAV3 mutations cause decreased Kir2.1 current density, but Kir2.x heterotetramers have not been studied. Here, we determine the effect of long-QT syndrome-9-CAV3 mutation F97C on Kir2.x homo- and heterotetramers and model-associated arrhythmia mechanisms.

METHODS AND RESULTS

Super-resolution microscopy, co-immunoprecipitation, cellular electrophysiology, on-cell Western blotting, and simulation of Purkinje and ventricular myocyte mathematical models were used. Kir2.x isoforms have unique subcellular colocalization in human cardiomyocytes and coimmunoprecipitate with Cav3. F97C-Cav3 decreased peak inward Kir2.2 current density by 50% (-120 mV; P=0.019) and peak outward by 75% (-40 mV; P<0.05) but did not affect Kir2.3 current density. FRET (Förster resonance energy transfer) efficiency for Kir2.2 with Cav3 is high, and on-cell Western blotting demonstrates decreased Kir2.2 membrane expression with F97C-Cav3. Cav3-F97C reduced peak inward and outward current density of Kir2.2/Kir2.1 or Kir2.2/Kir2.3 heterotetramers (P<0.05). Only Cav3 scaffolding and membrane domains co-immunoprecipitation with Kir2.1 and Kir2.2 and Kir2.x-N-terminal Cav3 binding motifs are required for interaction. Mathematical Purkinje, but not ventricular, myocyte model incorporating simulated current reductions, predicts spontaneous delayed after-depolarization-mediated triggered activity.

CONCLUSIONS

Kir2.x isoforms have a unique intracellular pattern of distribution in association with specific Cav3 domains and that critically depends on interaction with N-terminal Kir2.x Cav3-binding motifs. Long-QT syndrome-9-CAV3 mutation differentially regulates current density and cell surface expression of Kir2.x homomeric and heteromeric channels. Mathematical Purkinje cell model incorporating experimental findings suggests delayed after-depolarization-type triggered activity as a possible arrhythmia mechanism.

[Electrhopysiological Effect of the Polyamine Spermine in Normoxic and Ischemic Ventricular Myocardium]

Cytoplasmic polyamines (PA) are involved in control of many cellular functions and are well known as regulators of so called inward-rectifier potassium ion channels. Nevertheless, functional significance of extracellular PA in the heart is poorly elucidated. Aim of this study was to study effects of endogenous PA spermine in the ventricular myocardium. Effects of the extracellular spermine were investigated in isolated multicellular preparations of rabbit and rat ventricular myocardium. Langendorff-perfused  isolated rat and rabbit hearts were also used. Action potential (APs) duration and pattern of excitation in ventricular myocardium were estimated using standard microelectrode technique and optical mapping. Functional refractory periods were assessed in Langendorff perfused hearts with the help of programmedelectrical stimulation of the ventricle. In this study extracellular PA spermine (0.1-5 mM) induced shortening of the APs in multicellular preparations of rat ventricular myocardium registered using sharp microelectrode technique. However, spermine caused only weak effect in preparations of ventricular myocardium from rabbit heart: highest tested concentration of spermine (5 mM) induced 4.7 % APs shortening. Similarly, 0.1-1 mM of spermine was unable to alter substantially ventricular effective refractory periods in isolated perfused rabbit hearts. In two animal species tested (rat and rabbit) 0.1-1 mM of spermine failed to affect conduction velocity and activation pattern in ventricles of isolated Langendorff-perfused hearts under normoxia. However, in the rat no-flow model of ischemia-reperfusion extracellular spermine improved conduction of excitation in ventricles. Our results allow suggesting that extracellular spermine can prevent ischemia-induced proarrhythmic changes in ventricular myocardium probably due to reduction of calcium accumulation, but this effect is significant only when PA is applied in millimolar concentrations. Also, potential anti-ischemic effect of the PA may be species specific.

Electrophysiological and Pharmacological Characterization of Human Inwardly Rectifying Kir2.1 Channels on an Automated Patch-Clamp Platform

Inwardly rectifying I_K1 potassium currents of the heart control the resting membrane potential of ventricular cardiomyocytes during diastole and contribute to their repolarization after each action potential. Mutations in the gene encoding K_ir2.1 channels, which primarily conduct ventricular I_K1, are associated with inheritable forms of arrhythmias and sudden cardiac death. Therefore, potential iatrogenic inhibition of K_ir2.1-mediated I_K1 currents is a cardiosafety concern during new drug discovery and development. K_ir2.1 channels are part of the panel of cardiac ion channels currently considered for refined early compound risk assessment within the Comprehensive in vitro Proarrhythmia Assay initiative. In this study, we have validated a cell-based assay allowing functional quantification of K_ir2.1 inhibitors using whole-cell recordings of Chinese hamster ovary cells stably expressing human K_ir2.1 channels. We reproduced key electrophysiological and pharmacological features known for native I_K1, including current enhancement by external potassium and voltage- and concentration-dependent blockade by external barium. Furthermore, the K_ir inhibitors ML133, PA-6, and chloroquine, as well as the multichannel inhibitors chloroethylclonidine, chlorpromazine, SKF-96365, and the class III antiarrhythmic agent terikalant demonstrated slowly developing inhibitory activity in the low micromolar range. The robustness of this assay authorizes medium throughput screening for cardiosafety purposes and could help to enrich the currently limited K_ir2.1 pharmacology.

Influence of Continuous Training on Atrial Myocytes IK1 and IKAch and on Induction of Atrial Fibrillation in a Rabbit Model

BACKGROUND

Elucidation of mechanisms underlying continuous training-related atrial fibrillation (AF) may inform formulation of novel therapeutic approaches and training method selection. This study was aimed at assessing mechanisms underlying continuous training-induced AF in an animal model.

METHODS

Healthy New Zealand rabbits were divided into three groups (n=8 each), namely, control (C), and moderate intensity (M), and high intensity (H) continuous training according to treadmill speed. Atrial size andintrinsic and resting heart rates were measured by transthoracic echocardiography before, and 8 and 12 weeks after training. Using a Langendorff perfusion system, AF was induced by S1S2 stimulation and the induction rate was recorded. Atrial IK1 and IKAch ion current densities were recorded using whole-cell patch-clamp technique in isolated atrial myocytes. Changes in atrial Kir2.1, Kir2.2, Kir3.1, and Kir3.4 mRNA expression were assessed by reverse transcriptase-coupled polymerase chain reaction.

RESULTS

After 8 and 12 weeks, Groups M and H vs. Group C had greater (all P < 0.05) atrial anteroposterior diameter; greater incidence of AF (60% and 90% vs. 45%, respectively; P < 0.05, also between Groups H and M); and greater atrial IKAch current density. In Group H, Kir2.1 and Kir2.2 mRNA expression in the left and right atria was increased (P < 0.05, vs. Groups C and M) as was left atrial Kir3.1 and Kir3.4 mRNA expression (P < 0.05, vs. Group C).

CONCLUSION

In a rabbit model, continuous training enlarges atrial diameter leading to atrial structural and electrical remodeling and increased AF incidence.

Voltage-Dependent Sarcolemmal Ion Channel Abnormalities in the Dystrophin-Deficient Heart

Mutations in the gene encoding for the intracellular protein dystrophin cause severe forms of muscular dystrophy. These so-called dystrophinopathies are characterized by skeletal muscle weakness and degeneration. Dystrophin deficiency also gives rise to considerable complications in the heart, including cardiomyopathy development and arrhythmias. The current understanding of the pathomechanisms in the dystrophic heart is limited, but there is growing evidence that dysfunctional voltage-dependent ion channels in dystrophin-deficient cardiomyocytes play a significant role. Herein, we summarize the current knowledge about abnormalities in voltage-dependent sarcolemmal ion channel properties in the dystrophic heart, and discuss the potentially underlying mechanisms, as well as their pathophysiological relevance.

Association of Kir genes with blood pressure responses to dietary sodium intervention: the GenSalt study

Blood pressure (BP) responses to dietary sodium intervention vary among individuals. The inwardly rectifying potassium channel (Kir) is a potassium-selective ion channel that allows potassium ions to move more easily into rather than out of the cell. We aimed to investigate the associations of Kir genes with BP responses to dietary sodium intervention. A 7-day low-sodium intervention followed by a 7-day high-sodium intervention was conducted among 1906 participants. BP measurements were obtained at baseline and during each dietary intervention. Both single-marker and gene-based analyses were performed to explore the associations between Kir gene variants and BP responses to dietary sodium interventions. The genetic risk score (GRS) was used to assess the cumulative effect of the variants on the BP response to the sodium interventions. During the low-sodium intervention, markers rs858009, rs2835904, and rs860795 in KCNJ6 were significantly associated with the systolic BP (SBP) response (P = 8.82 × 10^-6, 3.32 × 10^-6, and 2.39 × 10^-4, respectively), whereas rs858009 and rs2835904 were significantly correlated with the mean arterial pressure (MAP) response (P = 1.64 × 10^-4 and 2.72 × 10^-4, respectively). Marker rs2836023 showed a significant association with the SBP response (P = 5.72 × 10^-5) to the high-sodium intervention. The GRS stratified by quartile grouping or as a continuous variable was associated with the BP response to the sodium interventions. Gene-based analyses consistently revealed that KCNJ6 was significantly associated with the BP response to the sodium interventions. These findings suggest that KCNJ6 may contribute to variation of BP responses to dietary sodium interventions. Future studies are warranted to confirm these findings and to identify functional variants for salt sensitivity.

Characterization of a Human Induced Pluripotent Stem Cell-Derived Cardiomyocyte Model for the Study of Variant Pathogenicity: Validation of a KCNJ2 Mutation

BACKGROUND

Long-QT syndrome is a potentially fatal condition for which 30% of patients are without a genetically confirmed diagnosis. Rapid identification of causal mutations is thus a priority to avoid at-risk situations that can lead to fatal cardiac events. Massively parallel sequencing technologies are useful for the identification of sequence variants; however, electrophysiological testing of newly identified variants is crucial to demonstrate causality. Long-QT syndrome could, therefore, benefit from having a standardized platform for functional characterization of candidate variants in the physiological context of human cardiomyocytes.

METHODS AND RESULTS

Using a variant in Kir2.1 (Gly52Val) revealed by whole-exome sequencing in a patient presenting with symptoms of long-QT syndrome as a proof of principle, we demonstrated that commercially available human induced pluripotent stem cell-derived cardiomyocytes are a powerful model for screening variants involved in genetic cardiac diseases. Immunohistochemistry experiments and whole-cell current recordings in human embryonic kidney cells expressing the wild-type or the mutant Kir2.1 demonstrated that Kir2.1-52V alters channel cellular trafficking and fails to form a functional channel. Using human induced pluripotent stem cell-derived cardiomyocytes, we not only confirmed these results but also further demonstrated that Kir2.1-52V is associated with a dramatic prolongation of action potential duration with evidence of arrhythmic activity, parameters which could not have been studied using human embryonic kidney cells.

CONCLUSIONS

Our study confirms the pathogenicity of Kir2.1-52V in 1 patient with long-QT syndrome and also supports the use of isogenic human induced pluripotent stem cell-derived cardiomyocytes as a physiologically relevant model for the screening of variants of unknown function.

Possible association of CAG repeat polymorphism in KCNN3 encoding the potassium channel SK3 with oxaliplatin-induced neurotoxicity

INTRODUCTION

Data suggest a role of the potassium channel SK3 (KCNN3 gene) in oxaliplatin-induced neurotoxicity (OIN). Length variations in the polymorphic CAG repeat of the KCNN3 gene may be associated with the risk of OIN.

MATERIALS AND METHODS

We performed patch-clamp experiments on HEK293 cell lines, expressing SK3 channel isoforms with short (11) or long (24) CAG repetitions, to measure intracellular calcium concentrations to test the effects of oxaliplatin on current density. A retrospective study was carried out on patients with colorectal cancer who had received oxaliplatin-based chemotherapy. DNA for KCNN3 genotyping was extracted from leukocytes. The region containing the CAG repeats was amplified by PCR and the products separated by capillary electrophoresis for length analysis. The patients were divided into three groups depending on whether they carried two short alleles, one short allele and one long allele, or two long alleles. The primary endpoint was the onset of grade 2 or 3 neuropathy to oxaliplatin.

RESULTS

There was no difference in current density, but oxaliplatin induced a differential effect on apamin-sensitive current density between the two isoforms expressed in the HEK cell lines. There was a significant reduction of store-operated calcium entry into cells expressing the short and more active isoform only after high concentration of oxaliplatin exposition. Eighty-six patients were included in the clinical study. There was no significant association between OIN and KCNN3 polymorphism for the three groups.

CONCLUSION

We observed a slight association between OIN and CAG repeat polymorphisms of the KCNN3 gene in a preclinical model, but not a clinical study.

Genetic variations involved in sudden cardiac death and their associations and interactions

Although the mechanism of sudden cardiac death (SCD) in heart failure is not completely known, genetic variations are known to play key roles in this process. Increasing numbers of mutations and variants are being discovered through genome-wide association studies. The genetic variations involved in the mechanisms of SCD have aroused widespread concern. Comprehensive understanding of the genetic variations involved in SCD may help prevent it. To this end, we briefly reviewed the genetic variations involved in SCD and their associations and interactions, and observed that cardiac ion channels are the core molecules involved in this process. Genetic variations involved in cardiac structure, cardiogenesis and development, cell division and differentiation, and DNA replication and transcription are all speculated to be loci involved in SCD. Additionally, the systems involved in neurohumoral regulation as well as substance and energy metabolism are also potentially responsible for susceptibility to SCD. They form an elaborate network and mutually interact with each other to govern the fate of SCD-susceptible individuals.

Molecular and functional characterization of inwardly rectifying K+ currents in murine proximal colon

KEY POINTS

Interstitial cells of Cajal (ICC) from murine colonic muscles express genes encoding inwardly rectifying K+ channels. Transcripts of Kcnj2 (Kir2.1), Kcnj4 (Kir2.3), Kcnj14 (Kir2.4), Kcnj5 (Kir3.4), Kcnj8 (Kir 6.1) and Kcnj11 (Kir6.2) were found in colonic ICC. A conductance with properties consistent with Kir2 channels was observed in ICC but not in smooth muscle cells (SMC). Despite expression of gene transcripts, G-protein gated K+ channel (Kir3) and KATP (Kir6) currents were not resolved in ICC. KATP is a conductance prominent in SMC. Kir2 antagonist caused depolarization of freshly dispersed ICC and colonic smooth muscles, suggesting that this conductance is active under resting conditions in colonic muscles. The conclusion of the present study is that ICC express the Ba2+ -sensitive, inwardly rectifying K+ conductance in colonic muscles. This conductance is most probably a result of heterotetramers of Kir2 gene products, with this regulating resting potentials and the excitability of colonic muscles.

ABSTRACT

Membrane potentials of gastrointestinal muscles are important because voltage-dependent Ca2+ channels in smooth muscle cells (SMC) provide the Ca2+ that triggers contraction. Regulation of membrane potential is complicated because SMC are electrically coupled to interstitial cells of Cajal (ICC) and PDGFRα+ cells. Activation of conductances in any of these cells affects the excitability of the syncytium. We explored the role of inward rectifier K+ conductances in colonic ICC that might contribute to regulation of membrane potential. ICC expressed Kcnj2 (Kir2.1), Kcnj4 (Kir2.3), Kcnj14 (Kir2.4), Kcnj5 (Kir3.4), Kcnj8 (Kir 6.1) and Kcnj11 (Kir6.2). Voltage clamp experiments showed activation of inward current when extracellular K+ ([K+ ]o ) was increased. The current was inwardly rectifying and inhibited by Ba2+ (10 μm) and ML-133 (10 μm). A similar current was not available in SMC. The current activated in ICC by elevated [K+ ]o was not affected by Tertiapin-Q. Gβγ, when dialysed into cells, failed to activate a unique, Tertiapin-Q-sensitive conductance. Freshly dispersed ICC showed no evidence of functional KATP . Pinacidil failed to activate current and the inward current activated by elevated [K+ ]o was insensitive to glibenclamide. Under current clamp, ML-133 caused the depolarization of isolated ICC and also that of cells impaled with microelectrodes in intact muscle strips. These findings show that ICC, when isolated freshly from colonic muscles, expressed a Ba2+ -sensitive, inwardly rectifying K+ conductance. This conductance is most probably a result of the expression of multiple Kir2 family paralogues, and the inwardly rectifying conductance contributes to the regulation of resting potentials and excitability of colonic muscles.

Kir2 inward rectification-controlled precise and dynamic balances between Kir2 and HCN currents initiate pacemaking activity

Spontaneous rhythmic action potential or pacemaking activity of pacemaker cells controls rhythmic signaling such as heartbeat. The mechanism underlying the origin of pacemaking activity is not well understood. In this study, we created human embryonic kidney (HEK) 293 cells that show pacemaking activity through heterologous expression of strong inward rectifier K⁺ subfamily 2 isoform 1 (Kir2.1) channels, hyperpolarization-activated cyclic nucleotide-gated isoform 2 (HCN2) nonselective cation channels, and voltage-gated Na⁺ subfamily 1 isoform 5 or Ca²⁺ subfamily 3 isoform 1 (Na_v1.5 or Ca_v3.1) channels. A range of relative levels of Kir2.1 and HCN2 currents dynamically counterbalance, generating spontaneous rhythmic oscillation of resting membrane potential between -64 and -34 mV and determining oscillation rates. Each oscillation cycle begins with an autodepolarization phase, which slowly proceeds to the threshold potential that activates Na_v1.5 or Ca_v3.1 channels and triggers action potential, causing engineered HEK293 cells to exhibit pacemaking activity at a rate of ≤67 beats/min. Engineered HEK293 cells with Kir2.1 and either HCN3 or HCN4 also show the oscillation. Engineered HEK293 cells expressing HCN2 and other Kir2 channels, which lack Kir2.1-like complete inward rectification, do not show the oscillation. Therefore, Kir2.1-like inward rectification-controlled precise and dynamic balances between Kir2 and HCN currents initiate spontaneous rhythmic action potential and form an origin of pacemaking activity; Kir2 and HCN channels play essential roles in pacemaking activity.-Chen, K., Zuo, D., Wang, S.-Y. Chen, H. Kir2 inward rectification-controlled precise and dynamic balances between Kir2 and HCN currents initiate pacemaking activity.

Kir2.1 channels set two levels of resting membrane potential with inward rectification

Strong inward rectifier K⁺ channels (Kir2.1) mediate background K⁺ currents primarily responsible for maintenance of resting membrane potential. Multiple types of cells exhibit two levels of resting membrane potential. Kir2.1 and K2P1 currents counterbalance, partially accounting for the phenomenon of human cardiomyocytes in subphysiological extracellular K⁺ concentrations or pathological hypokalemic conditions. The mechanism of how Kir2.1 channels contribute to the two levels of resting membrane potential in different types of cells is not well understood. Here we test the hypothesis that Kir2.1 channels set two levels of resting membrane potential with inward rectification. Under hypokalemic conditions, Kir2.1 currents counterbalance HCN2 or HCN4 cation currents in CHO cells that heterologously express both channels, generating N-shaped current-voltage relationships that cross the voltage axis three times and reconstituting two levels of resting membrane potential. Blockade of HCN channels eliminated the phenomenon in K2P1-deficient Kir2.1-expressing human cardiomyocytes derived from induced pluripotent stem cells or CHO cells expressing both Kir2.1 and HCN2 channels. Weakly inward rectifier Kir4.1 or inward rectification-deficient Kir2.1•E224G mutant channels do not set such two levels of resting membrane potential when co-expressed with HCN2 channels in CHO cells or when overexpressed in human cardiomyocytes derived from induced pluripotent stem cells. These findings demonstrate a common mechanism that Kir2.1 channels set two levels of resting membrane potential with inward rectification by balancing inward currents through different cation channels such as hyperpolarization-activated HCN channels or hypokalemia-induced K2P1 leak channels.

Andersen's syndrome mutants produce a knockdown of inwardly rectifying K+ channel in mouse skeletal muscle in vivo

Andersen's syndrome (AS) is a rare autosomal disorder that has been defined by the triad of periodic paralysis, cardiac arrhythmia, and developmental anomalies. AS has been directly linked to over 40 different autosomal dominant negative loss-of-function mutations in the KCNJ2 gene, encoding for the tetrameric strong inward rectifying K⁺ channel K_IR2.1. While K_IR2.1 channels have been suggested to contribute to setting the resting membrane potential (RMP) and to control the duration of the action potential (AP) in skeletal and cardiac muscle, the mechanism by which AS mutations produce such complex pathophysiological symptoms is poorly understood. Thus, we use an adenoviral transduction strategy to study in vivo subcellular distribution of wild-type (WT) and AS-associated mutant K_IR2.1 channels in mouse skeletal muscle. We determined that WT and D71V AS mutant K_IR2.1 channels are localized to the sarcolemma and the transverse tubules (T-tubules) of skeletal muscle fibers, while the ∆314-315 AS K_IR2.1 mutation prevents proper trafficking of the homo- or hetero-meric channel complexes. Whole-cell voltage-clamp recordings in individual skeletal muscle fibers confirmed the reduction of inwardly rectifying K⁺ current (I_K1) after transduction with ∆314-315 K_IR2.1 as compared to WT channels. Analysis of skeletal muscle function revealed reduced force generation during isometric contraction as well as reduced resistance to muscle fatigue in extensor digitorum longus muscles transduced with AS mutant K_IR2.1. Together, these results suggest that K_IR2.1 channels may be involved in the excitation-contraction coupling process required for proper skeletal muscle function. Our findings provide clues to mechanisms associated with periodic paralysis in AS.

Cardiac Potassium Channels: Physiological Insights for Targeted Therapy

The development of novel drugs specifically directed at the ion channels underlying particular features of cardiac action potential (AP) initiation, recovery, and refractoriness would contribute to an optimized approach to antiarrhythmic therapy that minimizes potential cardiac and extracardiac toxicity. Of these, K⁺ channels contribute numerous and diverse currents with specific actions on different phases in the time course of AP repolarization. These features and their site-specific distribution make particular K⁺ channel types attractive therapeutic targets for the development of pharmacological agents attempting antiarrhythmic therapy in conditions such as atrial fibrillation. However, progress in the development of such temporally and spatially selective antiarrhythmic drugs against particular ion channels has been relatively limited, particularly in view of our incomplete understanding of the complex physiological roles and interactions of the various ionic currents. This review summarizes the physiological properties of the main cardiac potassium channels and the way in which they modulate cardiac electrical activity and then critiques a number of available potential antiarrhythmic drugs directed at them.

Kir2.1 and K2P1 channels reconstitute two levels of resting membrane potential in cardiomyocytes

KEY POINTS

Outward and inward background currents across the cell membrane balance, determining resting membrane potential. Inward rectifier K⁺ channel subfamily 2 (Kir2) channels primarily maintain the resting membrane potential of cardiomyocytes. Human cardiomyocytes exhibit two levels of resting membrane potential at subphysiological extracellular K⁺ concentrations or pathological hypokalaemia, however, the underlying mechanism is unclear. In the present study, we show that human cardiomyocytes derived from induced pluripotent stem cells with enhanced expression of isoform 1 of Kir2 (Kir2.1) channels and mouse HL-1 cardiomyocytes with ectopic expression of two pore-domain K⁺ channel isoform 1 (K2P1) recapitulate two levels of resting membrane potential, indicating the contributions of Kir2.1 and K2P1 channels to the phenomenon. In Chinese hamster ovary cells that express the channels, Kir2.1 currents non-linearly counterbalance hypokalaemia-induced K2P1 leak cation currents, reconstituting two levels of resting membrane potential. These findings support the hypothesis that Kir2 currents non-linearly counterbalance inward background cation currents, such as K2P1 currents, accounting for two levels of resting membrane potential in human cardiomyocytes and demonstrating a novel mechanism that regulates excitability.

ABSTRACT

Inward rectifier K⁺ channel subfamily 2 (Kir2) channels primarily maintain the normal resting membrane potential of cardiomyocytes. At subphysiological extracellular K⁺ concentrations or pathological hypokalaemia, human cardiomyocytes show both hyperpolarized and depolarized resting membrane potentials; these depolarized potentials cause cardiac arrhythmia; however, the underlying mechanism is unknown. In the present study, we show that inward rectifier K⁺ channel subfamily 2 isoform 1 (Kir2.1) currents non-linearly counterbalance hypokalaemia-induced two pore-domain K⁺ channel isoform 1 (K2P1) leak cation currents, reconstituting two levels of resting membrane potential in cardiomyocytes. Under hypokalaemic conditions, both human cardiomyocytes derived from induced pluripotent stem cells with enhanced Kir2.1 expression and mouse HL-1 cardiomyocytes with ectopic expression of K2P1 channels recapitulate two levels of resting membrane potential. These cardiomyocytes display N-shaped current-voltage relationships that cross the voltage axis three times and the first and third zero-current potentials match the two levels of resting membrane potential. Inhibition of K2P1 expression eliminates the phenomenon, indicating contributions of Kir2.1 and K2P1 channels to two levels of resting membrane potential. Second, in Chinese hamster ovary cells that heterologously express the channels, Kir2.1 currents non-linearly counterbalance hypokalaemia-induced K2P1 leak cation currents, yielding the N-shaped current-voltage relationships, causing the resting membrane potential to spontaneously jump from hyperpolarization at the first zero-current potential to depolarization at the third zero-current potential, again recapitulating two levels of resting membrane potential. These findings reveal ionic mechanisms of the two levels of resting membrane potential, demonstrating a previously unknown mechanism for the regulation of excitability, and support the hypothesis that Kir2 currents non-linearly balance inward background cation currents, accounting for two levels of resting membrane potential of human cardiomyocytes.

The IK1/Kir2.1 channel agonist zacopride prevents and cures acute ischemic arrhythmias in the rat

Arrhythmogenesis in acute myocardial infarction (MI) is associated with depolarization of resting membraine potential (RMP) and decrease of inward rectifier potassium current (I_K1) in cardiomyocytes. However, clinical anti-arrhythmic agents that primarily act on RMP by enhancing the I_K1 channel are not currently available. We hypothesized that zacopride, a selective and moderate agonist of the I_K1/Kir2.1 channels, prevents and cures acute ischemic arrhythmias. To test this viewpoint, adult Sprague-Dawley (SD) rats were subjected to MI by ligating the left main coronary artery. The antiarrhythmic effects of zacopride (i.v. infusion) were observed in the settings of pre-treatment (zacopride given 3 min prior to coronary occlusion), post-treatment (zacopride given 3 min after coronary occlusion) and therapeutic treatment (zacopride given 30 s after the onset of the first sustained ventricular tachycardia (VT)/ventricular fibrillation (VF) post MI). In all the three treatment modes, zacopride (15 μg/kg) inhibited MI-induced ventricular tachyarrhythmias, as shown by significant decreases in the premature ventricular contraction (PVC) and the duration and incidence of VT or VF. In Langendorff perfused rat hearts, the antiarrhythmic effect of 1 μmol/L zacopride were reversed by 1 μmol/L BaCl₂, a blocker of I_K1 channel. Patch clamp results in freshly isolated rat ventricular myocytes indicated that zacopride activated the I_K1 channel and thereby reversed hypoxia-induced RMP depolarization and action potential duration (APD) prolongation. In addition, zacopride (1 μmol/L) suppressed hypoxia- or isoproterenol- induced delayed afterdepolarizations (DADs). In Kir2.x transfected Chinese hamster ovary (CHO) cells, zacopride activated the Kir2.1 homomeric channel but not the Kir2.2 or Kir2.3 channels. These results support our hypothesis that moderately enhancing I_K1/Kir2.1 currents as by zacopride rescues ischemia- and hypoxia- induced RMP depolarization, and thereby prevents and cures acute ischemic arrhythmias. This study brings a new viewpoint to antiarrhythmic theories and provides a promising target for the treatment of acute ischemic arrhythmias.

Cardiac voltage-gated ion channels in safety pharmacology: Review of the landscape leading to the CiPA initiative

Voltage gated ion channels are central in defining the fundamental properties of the ventricular cardiac action potential (AP), and are also involved in the development of drug-induced arrhythmias. Many drugs can inhibit cardiac ion currents, including the Na⁺ current (I_Na), L-type Ca²⁺ current (Ica-L), and K⁺ currents (I_to, I_K1, I_Ks, and I_Kr), and thereby affect AP properties in a manner that can trigger or sustain cardiac arrhythmias. Since publication of ICH E14 and S7B over a decade ago, there has been a focus on drug effects on QT prolongation clinically, and on the rapidly activating delayed rectifier current (I_Kr), nonclinically, for evaluation of proarrhythmic risk. This focus on QT interval prolongation and a single ionic current likely impacted negatively some drugs that lack proarrhythmic liability in humans. To rectify this issue, the Comprehensive in vitro proarrhythmia assay (CiPA) initiative has been proposed to integrate drug effects on multiple cardiac ionic currents with in silico modelling of human ventricular action potentials, and in vitro data obtained from human stem cell-derived ventricular cardiomyocytes to estimate proarrhythmic risk of new drugs with improved accuracy. In this review, we present the physiological functions and the molecular basis of major cardiac ion channels that contribute to the ventricle AP, and discuss the CiPA paradigm in drug development.

Role of suppression of the inward rectifier current in terminal action potential repolarization in the failing heart

BACKGROUND

The failing heart exhibits an increased arrhythmia susceptibility that is often attributed to action potential (AP) prolongation due to significant ion channel remodeling. The inwardly rectifying K⁺ current (I_K1) has been reported to be reduced, but its contribution to shaping the AP waveform and cell excitability in the failing heart remains unclear.

OBJECTIVE

The purpose of this study was to define the effect of I_K1 suppression on the cardiac AP and excitability in the normal and failing hearts.

METHODS

We used electrophysiological and pharmacological approaches to investigate I_K1 function in a swine tachy-pacing model of heart failure (HF).

RESULTS

Terminal repolarization of the AP (TRAP; the time constant of the exponential fit to terminal repolarization) was markedly prolonged in both myocytes and arterially perfused wedges from animals with HF. TRAP was increased by 54.1% in HF myocytes (P < .001) and 26.2% in HF wedges (P = .014). The increase in TRAP was recapitulated by the potent and specific I_K1 inhibitor, PA-6 (pentamidine analog 6), indicating that I_K1 is the primary determinant of the final phase of repolarization. Moreover, we find that I_K1 suppression reduced the ratio of effective refractory period to AP duration at 90% of repolarization, permitting re-excitation before full repolarization, reduction of AP upstroke velocity, and likely promotion of slow conduction.

CONCLUSION

Using an objective measure of terminal repolarization, we conclude that I_K1 is the major determinant of the terminal repolarization time course. Moreover, suppression of I_K1 prolongs repolarization and reduces postrepolarization refractoriness without marked effects on the overall AP duration. Collectively, these findings demonstrate how I_K1 suppression may contribute to arrhythmogenesis in the failing heart.

Inhibition of inwardly rectifying Kir2.x channels by the novel anti-cancer agent gambogic acid depends on both pore block and PIP2 interference

The caged xanthone gambogic acid (GA) is a novel anti-cancer agent which exhibits anti-proliferative, anti-inflammatory and cytotoxic effects in many types of cancer tissues. In a recent phase IIa study, GA exhibits a favourable safety profile. However, limited data are available concerning its interaction with cardiac ion channels. Heteromeric assembly of Kir2.x channels underlies the cardiac inwardly rectifying I_K1 current which is responsible for the stabilization of the diastolic resting membrane potential. Inhibition of the cardiac I_K1 current may lead to ventricular arrhythmia due to delayed afterdepolarizations. Compared to Kv2.1, hERG and Kir1.1, a slow, delayed inhibition of Kir2.1 channels by GA in a mammalian cell line was reported before but no data exist in literature concerning action of GA on homomeric Kir2.2 and Kir2.3 and heteromeric Kir2.x channels. Therefore, the aim of this study was to provide comparative data on the effect of GA on homomeric and heteromeric Kir2.x channels. Homomeric and heteromeric Kir2.x channels were heterologously expressed in Xenopus oocytes, and the two-microelectrode voltage-clamp technique was used to record Kir2.x currents. To investigate the mechanism of the channel inhibition by GA, alanine-mutated Kir2.x channels with modifications in the channels pore region or at phosphatidylinositol 4,5-bisphosphate (PIP₂)-binding sites were employed. GA caused a slow inhibition of homomeric and heteromeric Kir2.x channels at low micromolar concentrations (with IC₅₀ Kir2.1/2.2 < Kir2.2 < Kir2.2/2.3 < Kir2.3 < Kir2.1 < Kir2.1/2.3). The effect did not reach saturation within 60 min and was not reversible upon washout for 30 min. The inhibition showed no strong voltage dependence. We provide evidence for a combination of direct channel pore blockade and a PIP₂-dependent mechanism as a molecular basis for the observed effect. We conclude that Kir2.x channel inhibition by GA may be relevant in patients with pre-existing cardiac disorders such as chronic heart failure or certain rhythm disorders and recommend a close cardiac monitoring for those patients when treated with GA.

Relationship between QT and JT peak interval variability in prepubertal children

BACKGROUND

The QT variability index (QTVI) is a noninvasive index of repolarization lability that has been applied to subjects with cardiovascular disease. QTVI provides a ratio of normalized QT variability to normalized heart rate variability, and therefore includes an assessment of autonomic nervous activity. However, measurement of QT time is particularly difficult in children, who exhibit physiologically high heart rates compared with adults. In this study, we developed a set of standard values of J-point to Tpeak interval (JTp) for infants by age, and assessed the correlation of QTVI with the JTp variability index (JTpVI).

METHODS

Subjects included 623 infants and children (0-7 years of age) without heart disease and 57 healthy university students. All subjects were divided into three groups by age. QTVI and JTpVI were calculated based on an electrocardiogram, and age-specific standard values, a gender-specific classification, and a standard growth curve were constructed.

RESULTS

JTpVI markedly decreased in infancy and slowly decreased thereafter, reaching adult values by school age. There was also a strong correlation of JTpVI with QTVI (r = .856).

CONCLUSIONS

JTp can be used to evaluate the variability of the repolarization time in healthy infants, and may be useful for detection of early repolarization abnormalities.

Dual Mechanism for Inhibition of Inwardly Rectifying Kir2.x Channels by Quinidine Involving Direct Pore Block and PIP2-interference

Class IA antiarrhythmic drug quinidine was one of the first clinically used compounds to terminate atrial fibrillation and acts as multichannel inhibitor with well-documented inhibitory effects on several cardiac potassium channels. In the mammalian heart, heteromeric assembly of Kir2.1-2.3 channels underlies I_K1 current. Although a low-affinity block of quinidine on Kir2.1 has already been described, a comparative analysis of effects on other Kir2.x channels has not been performed to date. Therefore, we analyzed the effects of quinidine on wild-type and mutant Kir2.x channels in the Xenopus oocyte expression system. Quinidine exerted differential inhibitory effects on Kir2.x channels with the highest affinity toward Kir2.3 subunits. Onset of block was slow and solely reversible in Kir2.2 subunits. Quinidine inhibited Kir2.x currents in a voltage-independent manner. By means of comparative Ala-scanning mutagenesis, we further found that residues E224, F254, D259, and E299 are essential for quinidine block in Kir2.1 subunits. Analogously, quinidine mediated Kir2.3 inhibition by binding corresponding residues E216, D247, D251, and E291. In contrast, Kir2.2 current block merely involved corresponding residue D260. Using channel mutants with altered (phosphatidylinositol 4,5-bisphosphate PIP₂) affinities, we were able to demonstrate that high PIP₂ affinities (i.e., Kir2.3 I214L) correlate with low quinidine sensitivity. Inversely, mutant channels interacting only weakly with PIP₂ (i.e., Kir2.1 K182Q, and L221I) are prone to a higher inhibitory effect. Thus, we conclude that inhibition of Kir2.x channels by quinidine is mediated by joint modes of action involving direct cytoplasmic pore block and an impaired channel stabilization via interference with PIP₂.

Species-Dependent Mechanisms of Cardiac Arrhythmia: A Cellular Focus

Although ventricular arrhythmia remains a leading cause of morbidity and mortality, available antiarrhythmic drugs have limited efficacy. Disappointing progress in the development of novel, clinically relevant antiarrhythmic agents may partly be attributed to discrepancies between humans and animal models used in preclinical testing. However, such differences are at present difficult to predict, requiring improved understanding of arrhythmia mechanisms across species. To this end, we presently review interspecies similarities and differences in fundamental cardiomyocyte electrophysiology and current understanding of the mechanisms underlying the generation of afterdepolarizations and reentry. We specifically highlight patent shortcomings in small rodents to reproduce cellular and tissue-level arrhythmia substrate believed to be critical in human ventricle. Despite greater ease of translation from larger animal models, discrepancies remain and interpretation can be complicated by incomplete knowledge of human ventricular physiology due to low availability of explanted tissue. We therefore point to the benefits of mathematical modeling as a translational bridge to understanding and treating human arrhythmia.

Flecainide ameliorates arrhythmogenicity through NCX flux in Andersen-Tawil syndrome-iPS cell-derived cardiomyocytes

Andersen-Tawil syndrome (ATS) is a rare inherited channelopathy. The cardiac phenotype in ATS is typified by a prominent U wave and ventricular arrhythmia. An effective treatment for this disease remains to be established. We reprogrammed somatic cells from three ATS patients to generate induced pluripotent stem cells (iPSCs). Multi-electrode arrays (MEAs) were used to record extracellular electrograms of iPSC-derived cardiomyocytes, revealing strong arrhythmic events in the ATS-iPSC-derived cardiomyocytes. Ca²⁺ imaging of cells loaded with the Ca²⁺ indicator Fluo-4 enabled us to examine intracellular Ca²⁺ handling properties, and we found a significantly higher incidence of irregular Ca²⁺ release in the ATS-iPSC-derived cardiomyocytes than in control-iPSC-derived cardiomyocytes. Drug testing using ATS-iPSC-derived cardiomyocytes further revealed that antiarrhythmic agent, flecainide, but not the sodium channel blocker, pilsicainide, significantly suppressed these irregular Ca²⁺ release and arrhythmic events, suggesting that flecainide's effect in these cardiac cells was not via sodium channels blocking. A reverse-mode Na^+/Ca²⁺exchanger (NCX) inhibitor, KB-R7943, was also found to suppress the irregular Ca²⁺ release, and whole-cell voltage clamping of isolated guinea-pig cardiac ventricular myocytes confirmed that flecainide could directly affect the NCX current (I_NCX). ATS-iPSC-derived cardiomyocytes recapitulate abnormal electrophysiological phenotypes and flecainide suppresses the arrhythmic events through the modulation of I_NCX.

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iPS cells are generated from three patients with ATS.

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ATS-iPS cell-derived cardiomyocytes show abnormal electrophysiological phenotypes.

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Flecainide suppresses abnormal electrophysiological phenotypes in ATS-iPS cell-derived cardiomyocytes.

Molecular Pathophysiology of Congenital Long QT Syndrome

Ion channels represent the molecular entities that give rise to the cardiac action potential, the fundamental cellular electrical event in the heart. The concerted function of these channels leads to normal cyclical excitation and resultant contraction of cardiac muscle. Research into cardiac ion channel regulation and mutations that underlie disease pathogenesis has greatly enhanced our knowledge of the causes and clinical management of cardiac arrhythmia. Here we review the molecular determinants, pathogenesis, and pharmacology of congenital Long QT Syndrome. We examine mechanisms of dysfunction associated with three critical cardiac currents that comprise the majority of congenital Long QT Syndrome cases: 1) IKs, the slow delayed rectifier current; 2) IKr, the rapid delayed rectifier current; and 3) INa, the voltage-dependent sodium current. Less common subtypes of congenital Long QT Syndrome affect other cardiac ionic currents that contribute to the dynamic nature of cardiac electrophysiology. Through the study of mutations that cause congenital Long QT Syndrome, the scientific community has advanced understanding of ion channel structure-function relationships, physiology, and pharmacological response to clinically employed and experimental pharmacological agents. Our understanding of congenital Long QT Syndrome continues to evolve rapidly and with great benefits: genotype-driven clinical management of the disease has improved patient care as precision medicine becomes even more a reality.

Murine Electrophysiological Models of Cardiac Arrhythmogenesis

Cardiac arrhythmias can follow disruption of the normal cellular electrophysiological processes underlying excitable activity and their tissue propagation as coherent wavefronts from the primary sinoatrial node pacemaker, through the atria, conducting structures and ventricular myocardium. These physiological events are driven by interacting, voltage-dependent, processes of activation, inactivation, and recovery in the ion channels present in cardiomyocyte membranes. Generation and conduction of these events are further modulated by intracellular Ca2+ homeostasis, and metabolic and structural change. This review describes experimental studies on murine models for known clinical arrhythmic conditions in which these mechanisms were modified by genetic, physiological, or pharmacological manipulation. These exemplars yielded molecular, physiological, and structural phenotypes often directly translatable to their corresponding clinical conditions, which could be investigated at the molecular, cellular, tissue, organ, and whole animal levels. Arrhythmogenesis could be explored during normal pacing activity, regular stimulation, following imposed extra-stimuli, or during progressively incremented steady pacing frequencies. Arrhythmic substrate was identified with temporal and spatial functional heterogeneities predisposing to reentrant excitation phenomena. These could arise from abnormalities in cardiac pacing function, tissue electrical connectivity, and cellular excitation and recovery. Triggering events during or following recovery from action potential excitation could thereby lead to sustained arrhythmia. These surface membrane processes were modified by alterations in cellular Ca2+ homeostasis and energetics, as well as cellular and tissue structural change. Study of murine systems thus offers major insights into both our understanding of normal cardiac activity and its propagation, and their relationship to mechanisms generating clinical arrhythmias.

Effects of zacopride, a moderate IK1 channel agonist, on triggered arrhythmia and contractility in human ventricular myocardium

Ventricular tachycardia is the leading cause of sudden arrhythmic death in the U.S. Recently, the moderate I_K1 channel activator, zacopride, was shown to suppress triggered ventricular tachycardia in rats. Nonetheless, concerns were raised about the possibility of pro-arrhythmic activity after I_K1 channel stimulation based on the promising anti-arrhythmic strategy of I_K1 blockade in other animal models. Therefore, the goal of the current study was to investigate the ex-vivo effects of zacopride on triggered arrhythmia and contractility in ventricular human myocardium in order to validate data that was solely obtained from animal models. Application of 100nmol/L isoproterenol and 0.5mmol/L caffeine led to triggered arrhythmia in isolated cardiac muscles from non-failing and end-stage failing hearts. However, the occurrence of arrhythmia in muscles of non-failing hearts was markedly higher than those of end-stage failing hearts. Interestingly, zacopride eliminated the ex-vivo triggered arrhythmia in these muscles of non-failing and failing hearts in a concentration-dependent manner, with an effective IC₅₀ in the range of 28-40μmol/L. Conversely, in the absence of isoproterenol/caffeine, zacopride led to a negative inotropic effect in a concentration-dependent manner. Reduced cardiac contraction was clearly observed at high zacopride concentration of 200μmol/L, along with the occurrence of contractile alternans in muscles of non-failing and failing hearts. Zacopride shows promising antiarrhythmic effects against triggered arrhythmia in human ventricular myocardium. However, in the absence of Ca²⁺ overload/arrhythmia, zacopride, albeit at high concentrations, decreases the force of contraction and increases the likelihood of occurrence of contractile alternans, which may predispose the heart to contractile dysfunction and/or arrhythmia. Overall, our results represent a key step in translating this drug from the benchtop to the bedside in the research area.

Activation of the Ca2+-sensing receptors increases currents through inward rectifier K+ channels via activation of phosphatidylinositol 4-kinase

Inward rectifier K⁺ channels are important for maintaining normal electrical function in many cell types. The proper function of these channels requires the presence of membrane phosphoinositide 4,5-bisphosphate (PIP₂). Stimulation of the Ca²⁺-sensing receptor CaR, a pleiotropic G protein-coupled receptor, activates both G_q/11, which decreases PIP₂, and phosphatidylinositol 4-kinase (PI-4-K), which, conversely, increases PIP₂. How membrane PIP₂ levels are regulated by CaR activation and whether these changes modulate inward rectifier K⁺ are unknown. In this study, we found that activation of CaR by the allosteric agonist, NPSR568, increased inward rectifier K⁺ current (I _K1) in guinea pig ventricular myocytes and currents mediated by Kir2.1 channels exogenously expressed in HEK293T cells with a similar sensitivity. Moreover, using the fluorescent PIP₂ reporter tubby-R332H-cYFP to monitor PIP₂ levels, we found that CaR activation in HEK293T cells increased membrane PIP₂ concentrations. Pharmacological studies showed that both phospholipase C (PLC) and PI-4-K are activated by CaR stimulation with the latter played a dominant role in regulating membrane PIP₂ and, thus, Kir currents. These results provide the first direct evidence that CaR activation upregulates currents through inward rectifier K⁺ channels by accelerating PIP₂ synthesis. The regulation of I _K1 plays a critical role in the stability of the electrical properties of many excitable cells, including cardiac myocytes and neurons. Further, synthetic allosteric modulators that increase CaR activity have been used to treat hyperparathyroidism, and negative CaR modulators are of potential importance in the treatment of osteoporosis. Thus, our results provide further insight into the roles played by CaR in the cardiovascular system and are potentially valuable for heart disease treatment and drug safety.

Dynamics and Molecular Mechanisms of Ventricular Fibrillation in Structurally Normal Hearts

Ventricular fibrillation (VF) is the most severe cardiac rhythm disturbance and one of the most important immediate causes of sudden cardiac death. In the structurally normal heart, a small number of stable reentrant sources, perhaps 1 or 2, underlie the mechanism of VF, and the stabilization of the sources, their frequency, and the complexity of the turbulent waves they generate depend on the expression, spatial distribution, and intermolecular interactions of the 2 most important ion channels that control cardiac excitability: the inward rectifier potassium channel, Kir2.1, and the alpha subunit of the main cardiac sodium channel, NaV1.5.

Decreased inward rectifier potassium current IK1 in dystrophin-deficient ventricular cardiomyocytes

Kir2.x channels in ventricular cardiomyocytes (most prominently Kir2.1) account for the inward rectifier potassium current I_K1, which controls the resting membrane potential and the final phase of action potential repolarization. Recently it was hypothesized that the dystrophin-associated protein complex (DAPC) is important in the regulation of Kir2.x channels. To test this hypothesis, we investigated potential I_K1 abnormalities in dystrophin-deficient ventricular cardiomyocytes derived from the hearts of Duchenne muscular dystrophy mouse models. We found that I_K1 was substantially diminished in dystrophin-deficient cardiomyocytes when compared to wild type myocytes. This finding represents the first functional evidence for a significant role of the DAPC in the regulation of Kir2.x channels.