Na/Ca Exchange: Regulator of Intracellular Calcium and Source of Arrhythmias in the Heart

Calmodulin kinase II inhibition suppresses atrioventricular conduction by regulating intracellular Ca2+ homeostasis

BACKGROUND

Ca2+/calmodulin-dependent protein kinase II (CaMKII) inhibition decelerates atrioventricular node (AVN) conduction, providing a potential treatment of tachycardia. However, the effectiveness of CaMKII inhibition on tachycardia and its underlying mechanism remains unclear.

OBJECTIVE

We aimed to assess the effectiveness of CaMKII inhibition in reducing ventricular rates during atrial fibrillation and to elucidate the underlying mechanism in affecting AVN electrophysiology.

METHODS

Cardiac CaMKII inhibition (AC3-I) mice were used. Transesophageal atrial pacing was performed to evaluate AVN conduction function and to induce atrial fibrillation. Patch-clamp techniques were employed to record action potentials and ionic currents in AVN cells. Intracellular Ca2+ transients and sarcomere length measurements were obtained with the IonOptix system. Masson trichrome stain was used to evaluate fibrosis in the AVN region. Western blotting and immunofluorescence techniques were employed to detect connexin expression and localization.

RESULTS

CaMKII inhibition decreased the ventricular rate during atrial fibrillation and isoproterenol-induced tachycardia. Esophageal electrocardiogram results from AC3-I mice showed longer AVN conduction than in wild-type mice. AN- and N-type AVN cells from AC3-I mice exhibited slower action potential frequencies and diastolic depolarization rates than those of wild-type mice. The study revealed that CaMKII inhibition reduced AVN cell sarcoplasmic reticulum (SR) Ca2+ content, Ca2+ release rate from the SR during diastole, Ca2+ transient amplitude, and SR Ca2+ uptake rate. In addition, CaMKII inhibition prolonged the sarcomere diastole duration and enhanced the sensitivity of sarcomeres to Ca2+.

CONCLUSION

CaMKII inhibition effectively decreases the ventricular rate during atrial fibrillation and tachycardia by slowing down AVN conduction through suppressing Ca2+ overload in AVN cells.

CaMKII and reactive oxygen species contribute to early reperfusion arrhythmias, but oxidation of CaMKIIδ at methionines 281/282 is not a determining factor

BACKGROUND

Available evidence suggest that Ca²⁺/calmodulin-dependent protein kinase type IIδ (CaMKIIδ) and reactive oxygen species (ROS) are important in early ischemia-reperfusion arrhythmias (IRA). Since ROS can activate CaMKIIδ by oxidation of two methionines at positions 281/282, oxidized-CaMKIIδ (Ox-CaMKIIδ) has been proposed to be important for IRA. However, direct evidence for this is missing.

METHODS

We exposed Langendorff-perfused hearts and ventricular cardiomyocytes from C57BL/6 mice to global and simulated ischemia, respectively, and recorded arrhythmic events during early reperfusion. Hearts were collected for immunoblotting of key phosphoproteins. We evaluated the effects of beta-adrenoceptor stimulation, inhibition of CaMKII, and reduced ROS levels with isoprenaline, KN93/AIP and N-acetylcysteine (NAC), respectively. We further tested the importance of Ox-CaMKIIδ by using hearts and cardiomyocytes from mice with CaMKIIδ resistant to oxidation of methionines 281 and 282 (MMVV).

RESULTS

Hearts treated with KN93, AIP or NAC had lower incidence of early IRA, and NAC-treated cardiomyocytes had lower incidence of arrhythmogenic events. However, hearts from MMVV mice had a similar incidence of early IRA to wild type mice (WT), and MMVV and WT cardiomyocytes had a similar frequency of Ca²⁺ waves and Ca²⁺ sparks. Immunoblotting confirmed high levels of oxidation in early reperfusion, but revealed no significant differences in the phosphorylation levels of Ca²⁺-handling proteins in MMVV and WT hearts.

CONCLUSIONS

Although CaMKII and ROS both contribute to early IRA, hearts from mice with CaMKII resistant to oxidation at methionines 281/282 were not protected from such arrhythmias, suggesting that oxidation at these sites is not a determining factor.

Antiarrhythmic and Inotropic Effects of Selective Na+/Ca2+ Exchanger Inhibition: What Can We Learn from the Pharmacological Studies?

Life-long stable heart function requires a critical balance of intracellular Ca²⁺. Several ion channels and pumps cooperate in a complex machinery that controls the influx, release, and efflux of Ca²⁺. Probably one of the most interesting and most complex players of this crosstalk is the Na⁺/Ca²⁺ exchanger, which represents the main Ca²⁺ efflux mechanism; however, under some circumstances, it can also bring Ca²⁺ into the cell. Therefore, the inhibition of the Na⁺/Ca²⁺ exchanger has emerged as one of the most promising possible pharmacological targets to increase Ca²⁺ levels, to decrease arrhythmogenic depolarizations, and to reduce excessive Ca²⁺ influx. In line with this, as a response to increasing demand, several more or less selective Na⁺/Ca²⁺ exchanger inhibitor compounds have been developed. In the past 20 years, several results have been published regarding the effect of Na⁺/Ca²⁺ exchanger inhibition under various circumstances, e.g., species, inhibitor compounds, and experimental conditions; however, the results are often controversial. Does selective Na⁺/Ca²⁺ exchanger inhibition have any future in clinical pharmacological practice? In this review, the experimental results of Na⁺/Ca²⁺ exchanger inhibition are summarized focusing on the data obtained by novel highly selective inhibitors.

Therapeutic Approaches of Ryanodine Receptor-Associated Heart Diseases

Cardiac diseases are the leading causes of death, with a growing number of cases worldwide, posing a challenge for both healthcare and research. Therefore, the most relevant aim of cardiac research is to unravel the molecular pathomechanisms and identify new therapeutic targets. Cardiac ryanodine receptor (RyR2), the Ca²⁺ release channel of the sarcoplasmic reticulum, is believed to be a good therapeutic target in a group of certain heart diseases, collectively called cardiac ryanopathies. Ryanopathies are associated with the impaired function of the RyR, leading to heart diseases such as congestive heart failure (CHF), catecholaminergic polymorphic ventricular tachycardia (CPVT), arrhythmogenic right ventricular dysplasia type 2 (ARVD2), and calcium release deficiency syndrome (CRDS). The aim of the current review is to provide a short insight into the pathological mechanisms of ryanopathies and discuss the pharmacological approaches targeting RyR2.

The Oxidative Balance Orchestrates the Main Keystones of the Functional Activity of Cardiomyocytes

This review is aimed at providing an overview of the key hallmarks of cardiomyocytes in physiological and pathological conditions. The main feature of cardiac tissue is the force generation through contraction. This process requires a conspicuous energy demand and therefore an active metabolism. The cardiac tissue is rich of mitochondria, the powerhouses in cells. These organelles, producing ATP, are also the main sources of ROS whose altered handling can cause their accumulation and therefore triggers detrimental effects on mitochondria themselves and other cell components thus leading to apoptosis and cardiac diseases. This review highlights the metabolic aspects of cardiomyocytes and wanders through the main systems of these cells: (a) the unique structural organization (such as different protein complexes represented by contractile, regulatory, and structural proteins); (b) the homeostasis of intracellular Ca²⁺ that represents a crucial ion for cardiac functions and E-C coupling; and (c) the balance of Zn²⁺, an ion with a crucial impact on the cardiovascular system. Although each system seems to be independent and finely controlled, the contractile proteins, intracellular Ca²⁺ homeostasis, and intracellular Zn²⁺ signals are strongly linked to each other by the intracellular ROS management in a fascinating way to form a "functional tetrad" which ensures the proper functioning of the myocardium. Nevertheless, if ROS balance is not properly handled, one or more of these components could be altered resulting in deleterious effects leading to an unbalance of this "tetrad" and promoting cardiovascular diseases. In conclusion, this "functional tetrad" is proposed as a complex network that communicates continuously in the cardiomyocytes and can drive the switch from physiological to pathological conditions in the heart.

Alternating morphology ventricular tachycardia associated with stable coronary artery disease: A case report

Bidirectional ventricular tachycardia (BVT) is part of the spectrum of polymorphic ventricular tachycardia (VT). This is a case report of a 59-year-old male with stable coronary artery disease and baseline normal left ventricular ejection fraction (LVEF) who was followed for high burden symptomatic monomorphic premature ventricular complexes (PVC). He was admitted for syncope. The ECG showed recurrent non-sustained alternating morphology and double cycle length VT runs associated with deterioration in LVEF but without coronary artery disease progression. The patient had a successful catheter ablation of two distinct left ventricular PVC focus. This is a rare ECG pattern of alternating morphology VT similar to a bidirectional VT.

Mechanisms of artemether toxicity on single cardiomyocytes and protective effect of nanoencapsulation

BACKGROUND AND PURPOSE

The artemisinin derivative, artemether, has antimalarial activity with potential neurotoxic and cardiotoxic effects. Artemether in nanocapsules (NC-ATM) is more efficient than free artemether for reducing parasitaemia and increasing survival of Plasmodium berghei-infected mice. NCs also prevent prolongation of the QT interval of the ECG. Here, we assessed cellular cardiotoxicity of artemether and how this toxicity was prevented by nanoencapsulation.

EXPERIMENTAL APPROACH

Mice were treated with NC-ATM orally (120 mg·kg-1 twice daily) for 4 days. Other mice received free artemether, blank NCs, and vehicle for comparison. We measured single-cell contraction, intracellular Ca2+ transient using fluorescent Indo-1AM Ca2+ dye, and electrical activity using the patch-clamp technique in freshly isolated left ventricular myocytes. The acute effect of free artemether was also tested on cardiomyocytes of untreated animals.

KEY RESULTS

Artemether prolonged action potentials (AP) upon acute exposure (at 0.1, 1, and 10 μM) of cardiomyocytes from untreated mice or after in vivo treatment. This prolongation was unrelated to blockade of K+ currents, increased Ca2+ currents or promotion of a sustained Na+ current. AP lengthening was abolished by the NCX inhibitor SEA-0400. Artemether promoted irregular Ca2+ transients during pacing and spontaneous Ca2+ events during resting periods. NC-ATM prevented all effects. Blank NCs had no effects compared with vehicle.

CONCLUSION AND IMPLICATIONS

Artemether induced NCX-dependent AP lengthening (explaining QTc prolongation) and disrupted Ca2+ handling, both effects increasing pro-arrhythmogenic risks. NCs prevented these adverse effects, providing a safe alternative to the use of artemether alone, especially to treat malaria.

Regulation of NCX1 by palmitoylation

Palmitoylation (S-acylation) is the reversible conjugation of a fatty acid (usually C16 palmitate) to intracellular cysteine residues of proteins via a thioester linkage. Palmitoylation anchors intracellular regions of proteins to membranes because the palmitoylated cysteine is recruited to the lipid bilayer. NCX1 is palmitoylated at a single cysteine in its large regulatory intracellular loop. The presence of an amphipathic α-helix immediately adjacent to the NCX1 palmitoylation site is required for NCX1 palmitoylation. The NCX1 palmitoylation site is conserved through most metazoan phlya. Although palmitoylation does not regulate the normal forward or reverse ion transport modes of NCX1, NCX1 palmitoylation is required for its inactivation: sodium-dependent inactivation and inactivation by PIP2 depletion are significantly impaired for unpalmitoylatable NCX1. Here we review the role of palmitoylation in regulating NCX1 activity, and highlight future questions that must be addressed to fully understand the importance of this regulatory mechanism for sodium and calcium transport in cardiac muscle.

A thermodynamic description for physiological transmembrane transport

A general formulation for both passive and active transmembrane transport is derived from basic thermodynamical principles. The derivation takes into account the energy required for the motion of molecules across membranes and includes the possibility of modeling asymmetric flow. Transmembrane currents can then be described by the general model in the case of electrogenic flow. As it is desirable in new models, it is possible to derive other well-known expressions for transmembrane currents as particular cases of the general formulation. For instance, the conductance-based formulation for current turns out to be a linear approximation of the general formula for current. Also, under suitable assumptions, other formulas for current based on electrodiffusion, like the constant field approximation by Goldman, can be recovered from the general formulation. The applicability of the general formulations is illustrated first with fits to existing data, and after, with models of transmembrane potential dynamics for pacemaking cardiocytes and neurons. The general formulations presented here provide a common ground for the biophysical study of physiological phenomena that depend on transmembrane transport.

Fibroblast growth factor 23 dysregulates late sodium current and calcium homeostasis with enhanced arrhythmogenesis in pulmonary vein cardiomyocytes

Fibroblast growth factor 23 (FGF23), elevated in chronic renal failure, increases atrial arrhythmogenesis and dysregulates calcium homeostasis. Late sodium currents (I_Na-Late) critically induces ectopic activity of pulmoanry vein (the most important atrial fibrillation trigger). This study was to investigate whether FGF23 activates the I_Na-Late leading to calcium dysregulation and increases PV arrhythmogenesis. Patch clamp, western blot, and confocal microscopy were used to evaluate the electrical activities, calcium homeostasis, and mitochondrial reactive oxygen species (ROS) in PV cardiomyocytes with or without FGF23 (0.1 or 1 ng/mL) incubation for 4~6 h. Compared to the control, FGF23 (1 ng/mL, but not 0.1 ng/mL)-treated PV cardiomyocytes had a faster beating rate. FGF23 (1 ng/mL)-treated PV cardiomyocytes had larger I_Na-Late, calcium transients, and mitochondrial ROS than controls. However, ranolazine (an inhibitor of I_Na-Late) attenuated FGF23 (1 ng/mL)-increased beating rates, calcium transients and mitochondrial ROS. FGF23 (1 ng/mL)-treated PV cardiomyocytes exhibited larger phosphorylation of calcium/calmodulin-dependent protein kinase II (CaMKII). Chelerythrine chloride (an inhibitor of protein kinase C) decreased I_Na-Late in FGF23 (1 ng/mL)-treated PV cardiomyocytes. However, KN93 (a selective CaMKII blocker) decreased I_Na-Late in control and FGF23 (1 ng/mL)-treated PV cardiomyocytes to a similar extent. In conclusion, FGF23 increased PV arrhythmogenesis through sodium and calcium dysregulation by acting protein kinase C signaling.

Structural heterogeneity of the rat pulmonary vein myocardium: consequences on intracellular calcium dynamics and arrhythmogenic potential

Mechanisms underlying ectopic activity in the pulmonary vein (PV) which triggers paroxysmal atrial fibrillation are unknown. Although several studies have suggested that calcium signalling might be involved in these arrhythmias, little is known about calcium cycling in PV cardiomyocytes (CM). We found that individual PV CM showed a wide range of transverse tubular incidence and organization, going from their virtual absence, as described in atrial CM, to well transversally organised tubular systems, like in ventricular CM. These different types of CM were found in groups scattered throughout the tissue. The variability of the tubular system was associated with cell to cell heterogeneity of calcium channel (Ca_v1.2) localisation and, thereby, of Ca_v1.2-Ryanodine receptor coupling. This was responsible for multiple forms of PV CM calcium transient. Spontaneous calcium sparks and waves were not only more abundant in PV CM than in LA CM but also associated with a higher depolarising current. In conclusion, compared with either the atrium or the ventricle, PV myocardium presents marked structural and functional heterogeneity.

The Effect of a Novel Highly Selective Inhibitor of the Sodium/Calcium Exchanger (NCX) on Cardiac Arrhythmias in In Vitro and In Vivo Experiments

Background

In this study the effects of a new, highly selective sodium-calcium exchanger (NCX) inhibitor, ORM-10962 were investigated on cardiac NCX current, Ca²⁺ transients, cell shortening and in experimental arrhythmias. The level of selectivity of the novel inhibitor on several major transmembrane ion currents (L-type Ca²⁺ current, major repolarizing K⁺ currents, late Na⁺ current, Na⁺/K⁺ pump current) was also determined.

Methods

Ion currents in single dog ventricular cells (cardiac myocytes; CM), and action potentials in dog cardiac multicellular preparations were recorded utilizing the whole-cell patch clamp and standard microelectrode techniques, respectively. Ca²⁺ transients and cell shortening were measured in fluorescent dye loaded isolated dog myocytes. Antiarrhythmic effects of ORM-10962 were studied in anesthetized ouabain (10 μg/kg/min i.v.) pretreated guinea pigs and in ischemia-reperfusion models (I/R) of anesthetized coronary artery occluded rats and Langendorff perfused guinea pigs hearts.

Results

ORM-10962 significantly reduced the inward/outward NCX currents with estimated EC50 values of 55/67 nM, respectively. The compound, even at a high concentration of 1 μM, did not modify significantly the magnitude of I_CaL in CMs, neither had any apparent influence on the inward rectifier, transient outward, the rapid and slow components of the delayed rectifier potassium currents, the late and peak sodium and Na⁺/K⁺ pump currents. NCX inhibition exerted moderate positive inotropic effect under normal condition, negative inotropy when reverse, and further positive inotropic effect when forward mode was facilitated. In dog Purkinje fibres 1 μM ORM-10962 decreased the amplitude of digoxin induced delayed afterdepolarizations (DADs). Pre-treatment with 0.3 mg/kg ORM-10962 (i.v.) 10 min before starting ouabain infusion significantly delayed the development and recurrence of ventricular extrasystoles (by about 50%) or ventricular tachycardia (by about 30%) in anesthetized guinea pigs. On the contrary, ORM-10962 pre-treatment had no apparent influence on the time of onset or the severity of I/R induced arrhythmias in anesthetized rats and in Langendorff perfused guinea-pig hearts.

Conclusions

The present study provides strong evidence for a high efficacy and selectivity of the NCX-inhibitory effect of ORM-10962. Selective NCX inhibition can exert positive as well as negative inotropic effect depending on the actual operation mode of NCX. Selective NCX blockade may contribute to the prevention of DAD based arrhythmogenesis, in vivo, however, its effect on I/R induced arrhythmias is still uncertain.

Calcium handling precedes cardiac differentiation to initiate the first heartbeat

The mammalian heartbeat is thought to begin just prior to the linear heart tube stage of development. How the initial contractions are established and the downstream consequences of the earliest contractile function on cardiac differentiation and morphogenesis have not been described. Using high-resolution live imaging of mouse embryos, we observed randomly distributed spontaneous asynchronous Ca²⁺-oscillations (SACOs) in the forming cardiac crescent (stage E7.75) prior to overt beating. Nascent contraction initiated at around E8.0 and was associated with sarcomeric assembly and rapid Ca²⁺ transients, underpinned by sequential expression of the Na⁺-Ca²⁺ exchanger (NCX1) and L-type Ca²⁺ channel (LTCC). Pharmacological inhibition of NCX1 and LTCC revealed rapid development of Ca²⁺ handling in the early heart and an essential early role for NCX1 in establishing SACOs through to the initiation of beating. NCX1 blockade impacted on CaMKII signalling to down-regulate cardiac gene expression, leading to impaired differentiation and failed crescent maturation.

DOI:http://dx.doi.org/10.7554/eLife.17113.001

The heart is the first organ to form and to begin working in an embryo during pregnancy. It must begin pumping early to supply oxygen and nutrients to the developing embryo. Coordinated contractions of specialised muscle cells in the heart, called cardiomyocytes, generate the force needed to pump blood. The flow of calcium ions into and out of the cardiomyocytes triggers these heartbeats. In addition to triggering heart contractions, calcium ions also act as a messenger that drives changes in which genes are active in the cardiomyocytes and how these cells behave.

Scientists commonly think of the first heartbeat as occurring after a tube-like structure forms in the embryo that will eventually develop into the heart. However, it is not yet clear how the first heartbeat starts or how the initial heartbeats affect further heart development.

Tyser, Miranda et al. now show that the first heartbeat actually occurs much earlier in embryonic development than widely appreciated. In the experiments, videos of live mouse embryos showed that prior to the first heartbeat the flow of calcium ions between different cardiomyocytes is not synchronised. However, as the heart grows these calcium flows become coordinated leading to the first heartbeat. The heartbeats also become faster as the heart grows. Using drugs to block the movement of calcium ions, Tyser, Miranda et al. also show that a protein called NCX1 is required to trigger the calcium flows prior to the first heartbeat. Moreover, the experiments revealed that these early heartbeats help drive the growth of cardiomyocytes and shape the developing heart.

Together, the experiments show that the first heartbeats are essential for normal heart development. Future studies are needed to determine what controls the speed of the first heartbeats, and what organises the calcium flows that trigger the first heartbeat. Such studies may help scientists better understand birth defects of the heart, and may suggest strategies to rebuild hearts that have been damaged by a heart attack or other injury.

DOI:http://dx.doi.org/10.7554/eLife.17113.002

The investigation of the cellular electrophysiological and antiarrhythmic effects of a novel selective sodium-calcium exchanger inhibitor, GYKB-6635, in canine and guinea-pig hearts

The sodium-calcium exchanger (NCX) is considered as the major transmembrane transport mechanism that controls Ca²⁺ homeostasis. Its contribution to the cardiac repolarization has not yet been directly studied due to lack of specific inhibitors, so that an urgent need for more selective compounds. In this study, the electrophysiological effects of GYKB-6635, a novel NCX inhibitor, on the NCX, L-type calcium, and main repolarizing potassium currents as well as action potential (AP) parameters were investigated. Ion currents and AP recordings were investigated by applying the whole-cell patch clamp and standard microelectrode techniques in canine heart at 37 °C. Effects of GYKB-6635 were studied in ouabain-induced arrhythmias in isolated guinea-pig hearts. At a concentration of 1 μmol/L, GYKB significantly reduced both the inward and outward NCX currents (57% and 58%, respectively). Even at a high concentration (10 μmol/L), GYKB-6635 did not change the I_CaL, the maximum rate of depolarization (dV/dt_max), the main repolarizing K⁺ currents, and the main AP parameters. GYKB-6635 pre-treatment significantly delayed the time to the development of ventricular fibrillation (by about 18%). It is concluded that GYKB-6635 is a potent and highly selective inhibitor of the cardiac NCX and, in addition, it is suggested to also contribute to the prevention of DAD-based arrhythmias.

Palmitoylation of the Na/Ca exchanger cytoplasmic loop controls its inactivation and internalization during stress signaling

The electrogenic Na/Ca exchanger (NCX) mediates bidirectional Ca movements that are highly sensitive to changes of Na gradients in many cells. NCX1 is implicated in the pathogenesis of heart failure and a number of cardiac arrhythmias. We measured NCX1 palmitoylation using resin-assisted capture, the subcellular location of yellow fluorescent protein–NCX1 fusion proteins, and NCX1 currents using whole-cell voltage clamping. Rat NCX1 is substantially palmitoylated in all tissues examined. Cysteine 739 in the NCX1 large intracellular loop is necessary and sufficient for NCX1 palmitoylation. Palmitoylation of NCX1 occurs in the Golgi and anchors the NCX1 large regulatory intracellular loop to membranes. Surprisingly, palmitoylation does not influence trafficking or localization of NCX1 to surface membranes, nor does it strongly affect the normal forward or reverse transport modes of NCX1. However, exchangers that cannot be palmitoylated do not inactivate normally (leading to substantial activity in conditions when wild-type exchangers are inactive) and do not promote cargo-dependent endocytosis that internalizes 50% of the cell surface following strong G-protein activation or large Ca transients. The palmitoylated cysteine in NCX1 is found in all vertebrate and some invertebrate NCX homologs. Thus, NCX palmitoylation ubiquitously modulates Ca homeostasis and membrane domain function in cells that express NCX proteins.—Reilly, L., Howie, J., Wypijewski, K., Ashford, M. L. J., Hilgemann, D. W., Fuller, W. Palmitoylation of the Na/Ca exchanger cytoplasmic loop controls its inactivation and internalization during stress signaling.

TRPC3 contributes to regulation of cardiac contractility and arrhythmogenesis by dynamic interaction with NCX1

Aim

TRPC3 is a non-selective cation channel, which forms a Ca²⁺ entry pathway involved in cardiac remodelling. Our aim was to analyse acute electrophysiological and contractile consequences of TRPC3 activation in the heart.

Methods and results

We used a murine model of cardiac TRPC3 overexpression and a novel TRPC3 agonist, GSK1702934A, to uncover (patho)physiological functions of TRPC3. GSK1702934A induced a transient, non-selective conductance and prolonged action potentials in TRPC3-overexpressing myocytes but lacked significant electrophysiological effects in wild-type myocytes. GSK1702934A transiently enhanced contractility and evoked arrhythmias in isolated Langendorff hearts from TRPC3-overexpressing but not wild-type mice. Interestingly, pro-arrhythmic effects outlasted TRPC3 current activation, were prevented by enhanced intracellular Ca²⁺ buffering, and suppressed by the NCX inhibitor 3′,4′-dichlorobenzamil hydrochloride. GSK1702934A substantially promoted NCX currents in TRPC3-overexpressing myocytes. The TRPC3-dependent electrophysiologic, pro-arrhythmic, and inotropic actions of GSK1702934A were mimicked by angiotensin II (AngII). Immunocytochemistry demonstrated colocalization of TRPC3 with NCX1 and disruption of local interaction upon channel activation by either GSK1702934A or AngII.

Conclusion

Cardiac TRPC3 mediates Ca²⁺ and Na⁺ entry in proximity of NCX1, thereby elevating cellular Ca²⁺ levels and contractility. Excessive activation of TRPC3 is associated with transient cellular Ca²⁺ overload, spatial uncoupling between TRPC3 and NCX1, and arrhythmogenesis. We propose TRPC3-NCX micro/nanodomain communication as determinant of cardiac contractility and susceptibility to arrhythmogenic stimuli.

Leucine-rich repeat kinase 2-sensitive Na+/Ca2+ exchanger activity in dendritic cells

Gene variants of the leucine-rich repeat kinase 2 (LRRK2) are associated with susceptibility to Parkinson's disease (PD). Besides brain and periphery, LRRK2 is expressed in various immune cells including dendritic cells (DCs), antigen-presenting cells linking innate and adaptive immunity. However, the function of LRRK2 in the immune system is still incompletely understood. Here, Ca(2+)-signaling was analyzed in DCs isolated from gene-targeted mice lacking lrrk2 (Lrrk2(-/-)) and their wild-type littermates (Lrrk2(+/+)). According to Western blotting, Lrrk2 was expressed in Lrrk2(+/+) DCs but not in Lrrk2(-/-)DCs. Cytosolic Ca(2+) levels ([Ca(2+)]i) were determined utilizing Fura-2 fluorescence and whole cell currents to decipher electrogenic transport. The increase of [Ca(2+)]i following inhibition of sarcoendoplasmatic Ca(2+)-ATPase with thapsigargin (1 µM) in the absence of extracellular Ca(2+) (Ca(2+)-release) and the increase of [Ca(2+)]i following subsequent readdition of extracellular Ca(2+) (SOCE) were both significantly larger in Lrrk2(-/-) than in Lrrk2(+/+) DCs. The augmented increase of [Ca(2+)]i could have been due to impaired Ca(2+) extrusion by K(+)-independent (NCX) and/or K(+)-dependent (NCKX) Na(+)/Ca(2+)-exchanger activity, which was thus determined from the increase of [Ca(2+)]i, (Δ[Ca(2+)]i), and current following abrupt replacement of Na(+) containing (130 mM) and Ca(2+) free (0 mM) extracellular perfusate by Na(+) free (0 mM) and Ca(2+) containing (2 mM) extracellular perfusate. As a result, both slope and peak of Δ[Ca(2+)]i as well as Na(+)/Ca(2+) exchanger-induced current were significantly lower in Lrrk2(-/-) than in Lrrk2(+/+) DCs. A 6 or 24 hour treatment with the LRRK2 inhibitor GSK2578215A (1 µM) significantly decreased NCX1 and NCKX1 transcript levels, significantly blunted Na(+)/Ca(2+)-exchanger activity, and significantly augmented the increase of [Ca(2+)]i following Ca(2+)-release and SOCE. In conclusion, the present observations disclose a completely novel functional significance of LRRK2, i.e., the up-regulation of Na(+)/Ca(2+) exchanger transcription and activity leading to attenuation of Ca(2+)-signals in DCs.

Functional cardiac imaging by random access microscopy

Advances in the development of voltage sensitive dyes and Ca²⁺ sensors in combination with innovative microscopy techniques allowed researchers to perform functional measurements with an unprecedented spatial and temporal resolution. At the moment, one of the shortcomings of available technologies is their incapability of imaging multiple fast phenomena while controlling the biological determinants involved. In the near future, ultrafast deflectors can be used to rapidly scan laser beams across the sample, performing optical measurements of action potential and Ca²⁺ release from multiple sites within cardiac cells and tissues. The same scanning modality could also be used to control local Ca²⁺ release and membrane electrical activity by activation of caged compounds and light-gated ion channels. With this approach, local Ca²⁺ or voltage perturbations could be induced, simulating arrhythmogenic events, and their impact on physiological cell activity could be explored. The development of this optical methodology will provide fundamental insights in cardiac disease, boosting new therapeutic strategies, and, more generally, it will represent a new approach for the investigation of the physiology of excitable cells.

Effects of proarrhythmic drugs on relaxation time and beating pattern in rat engineered heart tissue

The assessment of proarrhythmic risks of drugs remains challenging. To evaluate the suitability of rat engineered heart tissue (EHT) for detecting proarrhythmic effects. We monitored drug effects on spontaneous contractile activity and, in selected cases, on action potentials (sharp microelectrode) and Ca²⁺ transients (Fura-2) and contraction under electrical pacing. The I_to-blocker inhibitor 4-aminopyridine increased action potential duration and T2 and caused aftercontractions, which were abolished by inhibitors of ryanodine receptors (RyR2; JTV-519) or sodium calcium exchanger (NCX; SEA0400). 77 Drugs were then tested at 1-10-100× free therapeutic plasma concentrations (FTPC): Inhibitors of I_Kr, I_Ks, I_to, antiarrhythmics (8), drugs withdrawn from market for torsades des pointes arrhythmias (TdP, 5), drugs with measurable (7) or isolated TdP incidence (13), drugs considered safe (14), 28 new chemical entities (NCE). Inhibitors of I_Kr or I_Ks had no effect alone, but substantially prolonged relaxation time (T2) when combined at high concentration. 15/33 drugs associated with TdP and 6/14 drugs considered non-torsadogenic (cibenzoline, diltiazem, ebastine, ketoconazole, moxifloxacin, and phenytoin) induced concentration-dependent T2 prolongations (10-100× FTPC). Bepridil, desipramine, imipramine, thioridazine, and erythromycin induced irregular beating. Three NCE prolonged T2, one reduced force. Drugs inhibiting repolarization prolong relaxation in rat EHTs and cause aftercontractions involving RyR2 and NCX. Insensitivity to I_Kr inhibitors makes rat EHTs unsuitable as general proarrhythmia screen, but favors detection of effects on I_to, I_Ks + I_to or I_Ks + I_Kr. Screening a large panel of drugs suggests that effects on these currents, in addition to I_Kr, are more common than anticipated.

ORM-10103, a novel specific inhibitor of the Na+/Ca2+ exchanger, decreases early and delayed afterdepolarizations in the canine heart

BACKGROUND AND PURPOSE

At present there are no small molecule inhibitors that show strong selectivity for the Na(+) /Ca(2+) exchanger (NCX). Hence, we studied the electrophysiological effects of acute administration of ORM-10103, a new NCX inhibitor, on the NCX and L-type Ca(2+) currents and on the formation of early and delayed afterdepolarizations.

EXPERIMENTAL APPROACH

Ion currents were recorded by using a voltage clamp technique in canine single ventricular cells, and action potentials were obtained from canine and guinea pig ventricular preparations with the use of microelectrodes.

KEY RESULTS

ORM-10103 significantly reduced both the inward and outward NCX currents. Even at a high concentration (10 μM), ORM-10103 did not significantly change the L-type Ca(2+) current or the maximum rate of depolarization (dV/dtmax ), indicative of the fast inward Na(+) current. At 10 μM ORM-10103 did not affect the amplitude or the dV/dtmax of the slow response action potentials recorded from guinea pig papillary muscles, which suggests it had no effect on the L-type Ca(2+) current. ORM-10103 did not influence the Na(+) /K(+) pump or the main K(+) currents of canine ventricular myocytes, except the rapid delayed rectifier K(+) current, which was slightly diminished by the drug at 3 μM. The amplitudes of pharmacologically- induced early and delayed afterdepolarizations were significantly decreased by ORM-10103 (3 and 10 μM) in a concentration-dependent manner.

CONCLUSIONS AND IMPLICATIONS

ORM-10103 is a selective inhibitor of the NCX current and can abolish triggered arrhythmias. Hence, it has the potential to be used to prevent arrhythmogenic events.

[Ryanodine receptor, calcium leak and arrhythmias]

The participation of the ionic Ca(2+) release channel/ryanodine receptor in cardiac excitation-contraction coupling is well known since the late '80s, when various seminal papers communicated its purification for the first time and its identity with the "foot" structures located at the terminal cisternae of the sarcoplasmic reticulum. In addition to its main role as the Ca(2+) channel responsible for the transient Ca(2+) increase that activates the contractile machinery of the cardiomyocytes, the ryanodine receptor releases Ca(2+) during the relaxation phase of the cardiac cycle, giving rise to a diastolic Ca(2+) leak. In normal physiological conditions, diastolic Ca(2+) leak regulates the proper level of luminal Ca(2+), but in pathological conditions it participates in the generation of both, acquired and hereditary arrhythmias. Very recently, several groups have focused their efforts into the development of pharmacological tools to control the altered diastolic Ca(2+) leak via ryanodine receptors. In this review, we focus our interest on describing the participation of cardiac ryanodine receptor in the diastolic Ca(2+) leak under physiological or pathological conditions and also on the therapeutic approaches to control its undesired exacerbated activity during diastole.

Cardiac potassium channel subtypes: new roles in repolarization and arrhythmia

About 10 distinct potassium channels in the heart are involved in shaping the action potential. Some of the K+ channels are primarily responsible for early repolarization, whereas others drive late repolarization and still others are open throughout the cardiac cycle. Three main K+ channels drive the late repolarization of the ventricle with some redundancy, and in atria this repolarization reserve is supplemented by the fairly atrial-specific KV1.5, Kir3, KCa, and K2P channels. The role of the latter two subtypes in atria is currently being clarified, and several findings indicate that they could constitute targets for new pharmacological treatment of atrial fibrillation. The interplay between the different K+ channel subtypes in both atria and ventricle is dynamic, and a significant up- and downregulation occurs in disease states such as atrial fibrillation or heart failure. The underlying posttranscriptional and posttranslational remodeling of the individual K+ channels changes their activity and significance relative to each other, and they must be viewed together to understand their role in keeping a stable heart rhythm, also under menacing conditions like attacks of reentry arrhythmia.

Effect of hypoxic paracrine media on calcium-regulatory proteins in infarcted rat myocardium

BACKGROUND AND OBJECTIVES

An increase in intracellular calcium concentration due to loss of Ca(2+) homeostasis triggers arrhythmia or cardiac cell death in the heart. Paracrine factors released from stem cells have beneficial cardioprotective effects. However, the mechanism of modulation of Ca(2+) homeostasis by paracrine factors in ischemic myocardium remains unclear.

MATERIALS AND METHODS

We isolated rat bone marrow-derived mesenchymal stem cells (MSCs), and prepared paracrine media (PM) from MSCs under hypoxic or normoxic conditions (hypoxic PM and normoxic PM). We induced rat myocardial infarction by left anterior descending ligation for 1 hour, and treated PM into the border region of infarcted myocardium (n=6/group) to identify the alteration in calcium-regulated proteins. We isolated and stained the heart tissue with specific calcium-related antibodies after 11 days.

RESULTS

The hypoxic PM treatment increased Ca(2+)-related proteins such as L-type Ca(2+) channel, sarcoplasmic reticulum Ca(2+) ATPase, Na(+)/K(+) ATPase, and calmodulin, whereas the normoxic PM treatment increased those proteins only slightly. The sodium-calcium exchanger was significantly reduced by hypoxic PM treatment, compared to moderate suppression by the normoxic PM treatment.

CONCLUSION

Our results suggest that hypoxic PM was significantly associated with the positive regulation of Ca(2+) homeostasis in infarcted myocardium.

Ryanodine receptors as leak channels

Ryanodine receptors are Ca(2+) release channels of internal stores. This review focuses on those situations and conditions that transform RyRs from a finely regulated ion channel to an unregulated Ca(2+) leak channel and the pathological consequences of this alteration. In skeletal muscle, mutations in either CaV1.1 channel or RyR1 results in a leaky behavior of the latter. In heart cells, RyR2 functions normally as a Ca(2+) leak channel during diastole within certain limits, the enhancement of this activity leads to arrhythmogenic situations that are tackled with different pharmacological strategies. In smooth muscle, RyRs are involved more in reducing excitability than in stimulating contraction so the leak activity of RyRs in the form of Ca(2+) sparks, locally activates Ca(2+)-dependent potassium channels to reduce excitability. In neurons the enhanced activity of RyRs is associated with the development of different neurodegenerative disorders such as Alzheimer and Huntington diseases. It appears then that the activity of RyRs as leak channels can have both physiological and pathological consequences depending on the cell type and the metabolic condition.

Altered regulation of cytosolic Ca²⁺ concentration in dendritic cells from klotho hypomorphic mice

The function of dendritic cells (DCs), antigen-presenting cells regulating naïve T-cells, is regulated by cytosolic Ca²⁺ concentration ([Ca²⁺]i). [Ca²⁺]i is increased by store-operated Ca²⁺ entry and decreased by K⁺-independent (NCX) and K⁺-dependent (NCKX) Na⁺/Ca²⁺ exchangers. NCKX exchangers are stimulated by immunosuppressive 1,25-dihydroxyvitamin D3 [1,25(OH)₂D₃], the biologically active form of vitamin D. Formation of 1,25(OH)₂D₃ is inhibited by the antiaging protein Klotho. Thus 1,25(OH)₂D₃ plasma levels are excessive in Klotho-deficient mice (klothohm). The present study explored whether Klotho deficiency modifies [Ca²⁺]i regulation in DCs. DCs were isolated from the bone marrow of klothohm mice and wild-type mice (klotho+/+) and cultured for 7-9 days with granulocyte-macrophage colony-stimulating factor. According to major histocompatibility complex II (MHC II) and CD86 expression, differentiation and lipopolysaccharide (LPS)-induced maturation were similar in klothohm DCs and klotho+/+ DCs. However, NCKX1 membrane abundance and NCX/NCKX-activity were significantly enhanced in klothohm DCs. The [Ca²⁺]i increase upon acute application of LPS (1 μg/ml) was significantly lower in klothohm DCs than in klotho+/+ DCs, a difference reversed by the NCKX blocker 3',4'-dichlorobenzamyl (DBZ; 10 μM). CCL21-dependent migration was significantly less in klothohm DCs than in klotho+/+ DCs but could be restored by DBZ. NCKX activity was enhanced by pretreatment of klotho+/+ DC precursors with 1,25(OH)₂D₃ the first 2 days after isolation from bone marrow. Feeding klothohm mice a vitamin D-deficient diet decreased NCKX activity, augmented LPS-induced increase of [Ca²⁺]i, and enhanced migration of klothohm DCs, thus dissipating the differences between klothohm DCs and klotho+/+ DCs. In conclusion, Klotho deficiency upregulates NCKX1 membrane abundance and Na⁺/Ca²⁺-exchange activity, thus blunting the increase of [Ca²⁺]i following LPS exposure and CCL21-mediated migration. The effects are in large part due to excessive 1,25(OH)₂D₃ formation.

Cardiac Na+-Ca2+ exchanger: dynamics of Ca2+-dependent activation and deactivation in intact myocytes

Cardiac Na(+)-Ca(2+) exchange (NCX) activity is regulated by [Ca(2+)]i. The physiological role and dynamics of this process in intact cardiomyocytes are largely unknown. We examined NCX Ca(2+) activation in intact rabbit and mouse cardiomyocytes at 37°C. Sarcoplasmic reticulum (SR) function was blocked, and cells were bathed in 2 mm Ca(2+). We probed Ca(2+) activation without voltage clamp by applying Na(+)-free (0 Na(+)) solution for 5 s bouts, repeated each 10 s, which should evoke [Ca(2+)]i transients due to Ca(2+) influx via NCX. In rested rabbit myocytes, Ca(2+) influx was undetectable even after 0 Na(+) applications were repeated for 2-5 min or more, suggesting that NCX was inactive. After external electric field stimulation pulses were applied, to admit Ca(2+) via L-type Ca(2+) channels, 0 Na(+) bouts activated Ca(2+) influx efficaciously, indicating that NCX had become active. Calcium activation increased with more field pulses, reaching a maximum typically after 15-20 pulses (1 Hz). At rest, NCX deactivated with a time constant typically of 20-40 s. An increase in [Na(+)]i, either in rabbit cardiomyocytes as a result of inhibition of Na(+)-K(+) pumping, or in mouse cardiomyocytes where normal [Na(+)]i is higher vs. rabbit, sensitized NCX to self-activation by 0 Na(+) bouts. In experiments with the SR functional but initially empty, the activation time course was slowed. It is possible that the SR initially accumulated Ca(2+) that would otherwise cause activation. We modelled Ca(2+) activation as a fourth-order highly co-operative process ([Ca]i required for half-activation K0.5act = 375 nm), with dynamics severalfold slower than the cardiac cycle. We incorporated this NCX model into an established ventricular myocyte model, which allowed us to predict responses to twitch stimulation in physiological conditions with the SR intact. Model NCX fractional activation increased from 0.1 to 1.0 as the frequency was increased from 0.2 to 2 Hz. By adjusting Ca(2+) activation on a multibeat time scale, NCX might better maintain a stable long-term Ca(2+) balance while contributing to the ability of myocytes to produce Ca(2+) transients over a wide range of intensity.

Cardiac ion channels and mechanisms for protection against atrial fibrillation

Atrial fibrillation (AF) is recognised as the most common sustained cardiac arrhythmia in clinical practice. Ongoing drug development is aiming at obtaining atrial specific effects in order to prevent pro-arrhythmic, devastating ventricular effects. In principle, this is possible due to a different ion channel composition in the atria and ventricles. The present text will review the aetiology of arrhythmias with focus on AF and include a description of cardiac ion channels. Channels that constitute potentially atria-selective targets will be described in details. Specific focus is addressed to the recent discovery that Ca(2+)-activated small conductance K(+) channels (SK channels) are important for the repolarisation of atrial action potentials. Finally, an overview of current pharmacological treatment of AF is included.

Atrial arrhythmia, triggering events and conduction abnormalities in isolated murine RyR2-P2328S hearts

AIM

RyR2 mutations are associated with catecholaminergic polymorphic tachycardia, a condition characterized by ventricular and atrial arrhythmias. The present experiments investigate the atrial electrophysiology of homozygotic murine RyR2-P2328S (RyR2(S/S)) hearts for ectopic triggering events and for conduction abnormalities that might provide a re-entrant substrate.

METHODS

Electrocardiograph recordings were made from regularly stimulated RyR2(S/S) and wild type (WT) hearts, perfused using a novel modified Langendorff preparation. This permitted the simultaneous use of either floating intracellular microelectrodes to measure action potential (AP) parameters, or a multielectrode array to measure epicardial conduction velocity (CV).

RESULTS

RyR2(S/S) showed frequent sustained tachyarrhythmias, delayed afterdepolarizations and ectopic APs, increased interatrial conduction delays, reduced epicardial CVs and reduced maximum rates of AP depolarization ((dV/dt)(max)), despite similar effective refractory periods, AP durations and AP amplitudes. Effective interatrial CVs and (dV/dt)(max) values of APs following ectopic (S2) stimulation were lower than those of APs following regular stimulation and decreased with shortening S1S2 intervals. However, although RyR2(S/S) atria showed arrhythmias over a wider range of S1S2 intervals, the interatrial CV and (dV/dt)(max) of S2 APs provoking such arrhythmias were similar in RyR2(S/S) and WT.

CONCLUSIONS

These results suggest that abnormal intracellular Ca(2+) homoeostasis produces both arrhythmic triggers and a slow-conducting arrhythmic substrate in RyR2(S/S) atria. A similar mechanism might also contribute to arrhythmogenesis in other conditions, associated with diastolic Ca(2+) release, such as atrial fibrillation.

CaMKIIδC slows [Ca]i decline in cardiac myocytes by promoting Ca sparks

Acute activation of calcium/calmodulin-dependent protein kinase (CaMKII) in permeabilized phospholamban knockout (PLN-KO) mouse myocytes phosphorylates ryanodine receptors (RyRs) and activates spontaneous local sarcoplasmic reticulum (SR) Ca release events (Ca sparks) even at constant SR Ca load. To assess how CaMKII regulates SR Ca release in intact myocytes (independent of SR Ca content changes or PLN effects), we compared Ca sparks in PLN-KO versus mice, which also have transgenic cardiac overexpression of CaMKIIδC in the PLN-KO background (KO/TG). Compared with PLN-KO mice, these KO/TG cardiomyocytes exhibited 1), increased twitch Ca transient and fractional release (both by ∼35%), but unaltered SR Ca load; 2), increased resting Ca spark frequency (300%) despite a lower diastolic [Ca]i, which also slowed twitch [Ca]i decline (suggesting CaMKII-dependent RyR Ca sensitization); 3), elevated Ca spark amplitude and rate of Ca release (which might indicate that more RyR channels participate in a single spark); 4), prolonged Ca spark rise time (which implies that CaMKII either delays RyR closure or prolongs the time when openings can occur); 5), more frequent repetitive sparks at single release sites. Analysis of repetitive sparks from individual Ca release sites indicates that CaMKII enhanced RyR Ca sensitivity, but did not change the time course of SR Ca refilling. These results demonstrate that there are dramatic CaMKII-mediated effects on RyR Ca release that occur via regulation of both RyR activation and termination processes.

Enhanced Ca²⁺ entry and Na+/Ca²⁺ exchanger activity in dendritic cells from AMP-activated protein kinase-deficient mice

In dendritic cells (DCs), chemotactic chemokines, such as CXCL12, rapidly increase cytosolic Ca(2+)concentrations ([Ca(2+)](i)) by triggering Ca(2+) release from intracellular stores followed by store-operated Ca(2+) (SOC) entry. Increase of [Ca(2+)](i) is blunted and terminated by Ca(2+) extrusion, accomplished by K(+)-independent Na(+)/Ca(2+) exchangers (NCXs) and K(+)-dependent Na(+)/Ca(2+) exchangers (NCKXs). Increased [Ca(2+)](i) activates energy-sensing AMP-activated protein kinase (AMPK), which suppresses proinflammatory responses of DCs and macrophages. The present study explored whether AMPK participates in the regulation of DC [Ca(2+)](i) and migration. DCs were isolated from AMPKα1-deficient (ampk(-/-)) mice and, as control, from their wild-type (ampk(+/+)) littermates. AMPKα1, Orai1-2, STIM1-2, and mitochondrial calcium uniporter protein expression was determined by Western blotting, [Ca(2+)](i) by Fura-2 fluorescence, SOC entry by inhibition of endosomal Ca(2+) ATPase with thapsigargin (1 μM), Na(+)/Ca(2+) exchanger activity from increase of [Ca(2+)](i), and respective whole-cell current in patch clamp following removal of extracellular Na(+). Migration was quantified utilizing transwell chambers. AMPKα1 protein is expressed in ampk(+/+) DCs but not in ampk(-/-) DCs. CXCL12 (300 ng/ml)-induced increase of [Ca(2+)](i), SOC entry, Orai 1 protein abundance, NCX, and NCKX were all significantly higher in ampk(-/-) DCs than in ampk(+/+) DCs. NCX and NCKX currents were similarly increased in ampk(-/-) DCs. Moreover, CXCL12 (50 ng/ml)-induced DC migration was enhanced in ampk(-/-) DCs. AMPK thus inhibits SOC entry, Na(+)/Ca(2+) exchangers, and migration of DCs.

Efficacy and potency of class I antiarrhythmic drugs for suppression of Ca2+ waves in permeabilized myocytes lacking calsequestrin

Ca(2+) waves can trigger ventricular arrhythmias such as catecholaminergic-polymorphic ventricular tachycardia (CPVT). Drugs that prevent Ca(2+) waves may have antiarrhythmic properties. Here, we use permeabilized ventricular myocytes from a CPVT mouse model lacking calsequestrin (casq2) to screen all clinically available class I antiarrhythmic drugs and selected other antiarrhythmic agents for activity against Ca(2+) waves. Casq2-/- myocytes were imaged in line-scan mode and the following Ca(2+) wave parameters analyzed: wave incidence, amplitude, frequency, and propagation speed. IC(50) (potency) and maximum inhibition (efficacy) were calculated for each drug. Drugs fell into 3 distinct categories. Category 1 drugs (flecainide and R-propafenone) suppressed wave parameters with the highest potency (IC(50)<10 μM) and efficacy (>50% maximum wave inhibition). Category 2 drugs (encainide, quinidine, lidocaine, and verapamil) had intermediate potency (IC(50) 20-40 μM) and efficacy (20-40% maximum wave inhibition). Category 3 drugs (procainamide, disopyramide, mexiletine, cibenzoline, and ranolazine) had no significant effects on Ca(2+) waves at the highest concentration tested (100 μM). Propafenone was stereoselective, with R-propafenone suppressing waves more potently than S-propafenone (IC(50): R-propafenone 2 ± 0.2 μM vs. S-propafenone 54 ± 18 μM). Both flecainide and R-propafenone decreased Ca(2+) spark mass and converted propagated Ca(2+) waves into non-propagated wavelets and frequent sparks, suggesting that reduction in spark mass, not spark frequency, was responsible for wave suppression. Among all class I antiarrhythmic drugs, flecainide and R-propafenone inhibit Ca(2+) waves with the highest potency and efficacy. Permeabilized casq2-/- myocytes are a simple in-vitro assay for finding drugs with activity against Ca(2+) waves. This article is part of a Special Issue entitled 'Possible Editorial'.

Inhibition of cardiac Ca2+ release channels (RyR2) determines efficacy of class I antiarrhythmic drugs in catecholaminergic polymorphic ventricular tachycardia

BACKGROUND

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is caused by mutations in the cardiac ryanodine receptor (RyR2) or calsequestrin (Casq2) and can be difficult to treat. The class Ic antiarrhythmic drug flecainide blocks RyR2 channels and prevents CPVT in mice and humans. It is not known whether other class I antiarrhythmic drugs also block RyR2 channels and to what extent RyR2 channel inhibition contributes to antiarrhythmic efficacy in CPVT.

METHODS AND RESULTS

We first measured the effect of all class I antiarrhythmic drugs marketed in the United States (quinidine, procainamide, disopyramide, lidocaine, mexiletine, flecainide, and propafenone) on single RyR2 channels incorporated into lipid bilayers. Only flecainide and propafenone inhibited RyR2 channels, with the S-enantiomer of propafenone having a significantly lower potency than R-propafenone or flecainide. In Casq2(-/-) myocytes, the propafenone enantiomers and flecainide significantly reduced arrhythmogenic Ca(2+) waves at clinically relevant concentrations, whereas Na(+) channel inhibitors without RyR2 blocking properties did not. In Casq2(-/-) mice, 5 mg/kg R-propafenone or 20 mg/kg S-propafenone prevented exercise-induced CPVT, whereas procainamide (20 mg/kg) or lidocaine (20 mg/kg) were ineffective (n=5 to 9 mice, P<0.05). QRS duration was not significantly different, indicating a similar degree of Na(+) channel inhibition. Clinically, propafenone (900 mg/d) prevented ICD shocks in a 22-year-old CPVT patient who had been refractory to maximal standard drug therapy and bilateral stellate ganglionectomy.

CONCLUSIONS

RyR2 cardiac Ca(2+) release channel inhibition appears to determine efficacy of class I drugs for the prevention of CPVT in Casq2(-/-) mice. Propafenone may be an alternative to flecainide for CPVT patients symptomatic on β-blockers.

Repolarization of the cardiac action potential. Does an increase in repolarization capacity constitute a new anti-arrhythmic principle?

The cardiac action potential can be divided into five distinct phases designated phases 0-4. The exact shape of the action potential comes about primarily as an orchestrated function of ion channels. The present review will give an overview of ion channels involved in generating the cardiac action potential with special emphasis on potassium channels involved in phase 3 repolarization. In humans, these channels are primarily K(v)11.1 (hERG1), K(v)7.1 (KCNQ1) and K(ir)2.1 (KCNJ2) being the responsible alpha-subunits for conducting I(Kr), I(Ks) and I(K1). An account will be given about molecular components, biophysical properties, regulation, interaction with other proteins and involvement in diseases. Both loss and gain of function of these currents are associated with different arrhythmogenic diseases. The second part of this review will therefore elucidate arrhythmias and subsequently focus on newly developed chemical entities having the ability to increase the activity of I(Kr), I(Ks) and I(K1). An evaluation will be given addressing the possibility that this novel class of compounds have the ability to constitute a new anti-arrhythmic principle. Experimental evidence from in vitro, ex vivo and in vivo settings will be included. Furthermore, conceptual differences between the short QT syndrome and I(Kr) activation will be accounted for.

Key role of a PtdIns-4,5P2 micro domain in ionic regulation of the mammalian heart Na+/Ca2+ exchanger

Phosphatidylinositol biphosphate (PtdIns-4,5P(2)) plays a key role in the regulation of the mammalian heart Na(+)/Ca(2+) exchanger (NCX1) by protecting the intracellular Ca(2+) regulatory site against H(+)(i) and (H(+)(i)+Na(+)(i)) synergic inhibition. MgATP and MgATP-gamma-S up-regulation of NCX1 takes place via the production of this phosphoinositide. In microsomes containing PtdIns-4,5P(2) incubated in the absence of MgATP and at normal [Na(+)](i), alkalinization increases the affinity for Ca(2+)(i) to the values seen in the presence of the nucleotide at normal pH; under this condition, addition of MgATP does not increase the affinity for Ca(2+)(i) any further. On the other hand, prevention of Na(+)(i) inhibition by alkalinization in the absence of MgATP does not take place when the microsomes are depleted of PtdIns-4,5P(2). Experiments on NCX1-PtdIns-4,5P(2) cross-coimmunoprecipitation show that the relevant PtdIns-4,5P(2) is not the overall membrane component but specifically that tightly attached to NCX1. Consequently, the highest affinity of the Ca(2+)(i) regulatory site is seen in the deprotonated and PtdIns-4,5P(2)-bound NCX1. Confirming these results, a PtdIns-5-kinase also cross-coimmunoprecipitates with NCX1 without losing its functional competence. These observations indicate, for the first time, the existence of a PtdIns-5-kinase in the NCX1 microdomain.

Ryanodine receptor-mediated arrhythmias and sudden cardiac death

The cardiac ryanodine receptor-Ca²⁺ release channel (RyR2) is an essential sarcoplasmic reticulum (SR) transmembrane protein that plays a central role in excitation–contraction coupling (ECC) in cardiomyocytes. Aberrant spontaneous, diastolic Ca²⁺ leak from the SR due to dysfunctional RyR2 contributes to the formation of delayed after-depolarisations, which are thought to underlie the fatal arrhythmia that occurs in both heart failure (HF) and in catecholaminergic polymorphic ventricular tachycardia (CPVT). CPVT is an inherited disorder associated with mutations in either the RyR2 or a SR luminal protein, calsequestrin. RyR2 shows normal function at rest in CPVT but the RyR2 dysfunction is unmasked by physical exercise or emotional stress, suggesting abnormal RyR2 activation as an underlying mechanism. Several potential mechanisms have been advanced to explain the dysfunctional RyR2 observed in HF and CPVT, including enhanced RyR2 phosphorylation status, altered RyR2 regulation at luminal/cytoplasmic sites and perturbed RyR2 intra/inter-molecular interactions. This review considers RyR2 dysfunction in the context of the structural and functional modulation of the channel, and potential therapeutic strategies to stabilise RyR2 function in cardiac pathology.

Characterization of cytotoxicity induced by arsenic trioxide (a potent anti-APL drug) in rat cardiac myocytes

Arsenic, a known environmental toxicant, is ubiquitously present in the environment. Arsenic trioxide (ATO), an anti-acute promyelocytic leukemia (APL) drug, is associated with cardiac toxicity. It is reported to induce cardiac arrhythmia via altering various ion channels involved in the repolarization phase of cardiac action potential. The exact molecular mechanism of cardiovascular adverse effect due to ATO exposure has not been fully elucidated except for alteration on ion channels. To evaluate the cytotoxic effect of ATO on cardiac myocytes, primary culture of myocytes was treated with different doses (30, 60 and 90 microM) of ATO for various periods (24, 48 and 72 h). Cardiac toxicity was assessed by monitoring cell viability, mitochondrial and deoxyribonucleic acid (DNA) integrity, reactive oxygen species (ROS) generation, calcium overload and apoptosis. ATO exposure caused alteration in mitochondrial integrity, generation of ROS, calcium overload and apoptosis in cardiac cells in dose- and duration-dependent manner. There was no DNA fragmentation. Hence our results show that ATO causes apoptosis in cardiomyocytes by generation of ROS and the induction of calcium overload.

Na+/Ca2+ exchangers: three mammalian gene families control Ca2+ transport

Mammalian Na+/Ca2+ exchangers are members of three branches of a much larger family of transport proteins [the CaCA (Ca2+/cation antiporter) superfamily] whose main role is to provide control of Ca2+ flux across the plasma membranes or intracellular compartments. Since cytosolic levels of Ca2+ are much lower than those found extracellularly or in sequestered stores, the major function of Na+/Ca2+ exchangers is to extrude Ca2+ from the cytoplasm. The exchangers are, however, fully reversible and thus, under special conditions of subcellular localization and compartmentalized ion gradients, Na+/Ca2+ exchangers may allow Ca2+ entry and may play more specialized roles in Ca2+ movement between compartments. The NCX (Na+/Ca2+ exchanger) [SLC (solute carrier) 8] branch of Na+/Ca2+ exchangers comprises three members: NCX1 has been most extensively studied, and is broadly expressed with particular abundance in heart, brain and kidney, NCX2 is expressed in brain, and NCX3 is expressed in brain and skeletal muscle. The NCX proteins subserve a variety of roles, depending upon the site of expression. These include cardiac excitation-contraction coupling, neuronal signalling and Ca2+ reabsorption in the kidney. The NCKX (Na2+/Ca2+-K+ exchanger) (SLC24) branch of Na+/Ca2+ exchangers transport K+ and Ca2+ in exchange for Na+, and comprises five members: NCKX1 is expressed in retinal rod photoreceptors, NCKX2 is expressed in cone photoreceptors and in neurons throughout the brain, NCKX3 and NCKX4 are abundant in brain, but have a broader tissue distribution, and NCKX5 is expressed in skin, retinal epithelium and brain. The NCKX proteins probably play a particularly prominent role in regulating Ca2+ flux in environments which experience wide and frequent fluctuations in Na+ concentration. Until recently, the range of functions that NCKX proteins play was generally underappreciated. This situation is now changing rapidly as evidence emerges for roles including photoreceptor adaptation, synaptic plasticity and skin pigmentation. The CCX (Ca2+/cation exchanger) branch has only one mammalian member, NCKX6 or NCLX (Na+/Ca2+-Li+ exchanger), whose physiological function remains unclear, despite a broad pattern of expression.