Na+ currents are required for efficient excitation-contraction coupling in rabbit ventricular myocytes: a possible contribution of neuronal Na+ channels

A Possible Role of Tetrodotoxin-Sensitive Na+ Channels for Oxidation-Induced Late Na+ Currents in Cardiomyocytes

An accumulation of reactive oxygen species (ROS) in cardiomyocytes can induce pro-arrhythmogenic late Na+ currents by removing the inactivation of voltage-gated Na+ channels including the tetrodotoxin (TTX)-resistant cardiac α-subunit Nav1.5 as well as TTX-sensitive α-subunits like Nav1.2 and Nav1.3. Here, we explored oxidant-induced late Na+ currents in mouse cardiomyocytes and human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) as well as in HEK 293 cells expressing Nav1.2, Nav1.3, or Nav1.5. Na+ currents in mouse cardiomyocytes and hiPSC-CMs treated with the oxidant chloramine T (ChT) developed a moderate reduction in peak current amplitudes accompanied by large late Na+ currents. While ChT induced a strong reduction in peak current amplitudes but only small persistent currents on Nav1.5, both Nav1.2 and Nav1.3 produced increased peak current amplitudes and large persistent currents following oxidation. TTX (300 nM) blocked ChT-induced late Na+ currents significantly stronger as compared to peak Na+ currents in both mouse cardiomyocytes and hiPSC-CMs. Similar differences between Nav1.2, Nav1.3, and Nav1.5 regarding ROS sensitivity were also evident when oxidation was induced with UVA-light (380 nm) or the cysteine-selective oxidant nitroxyl (HNO). To conclude, our data on TTX-sensitive Na+ channels expressed in cardiomyocytes may be relevant for the generation of late Na+ currents following oxidative stress.

The Cardiac Na+ -Ca2+ Exchanger: From Structure to Function

Ca²⁺ homeostasis is essential for cell function and survival. As such, the cytosolic Ca²⁺ concentration is tightly controlled by a wide number of specialized Ca²⁺ handling proteins. One among them is the Na⁺ -Ca²⁺ exchanger (NCX), a ubiquitous plasma membrane transporter that exploits the electrochemical gradient of Na⁺ to drive Ca²⁺ out of the cell, against its concentration gradient. In this critical role, this secondary transporter guides vital physiological processes such as Ca²⁺ homeostasis, muscle contraction, bone formation, and memory to name a few. Herein, we review the progress made in recent years about the structure of the mammalian NCX and how it relates to function. Particular emphasis will be given to the mammalian cardiac isoform, NCX1.1, due to the extensive studies conducted on this protein. Given the degree of conservation among the eukaryotic exchangers, the information highlighted herein will provide a foundation for our understanding of this transporter family. We will discuss gene structure, alternative splicing, topology, regulatory mechanisms, and NCX's functional role on cardiac physiology. Throughout this article, we will attempt to highlight important milestones in the field and controversial topics where future studies are required. © 2021 American Physiological Society. Compr Physiol 12:1-37, 2021.

Activation of reverse Na+-Ca2+ exchanger by skeletal Na+ channel isoform increases excitation-contraction coupling efficiency in rabbit cardiomyocytes

Our prior work has shown that Na⁺ current (I_Na) affects sarcoplasmic reticular (SR) Ca²⁺ release by activating early reverse of the Na⁺-Ca²⁺ exchanger (NCX). The resulting Ca²⁺ entry primes the dyadic cleft, which appears to increase Ca²⁺ channel coupling fidelity. It has been shown that the skeletal isoform of the voltage-gated Na⁺ channel (Na_v1.4) is the main tetrodotoxin (TTX)-sensitive Na_v isoform expressed in adult rabbit ventricular cardiomyocytes. Here, I tested the hypothesis that it is also the principal isoform involved in the priming mechanism. Action potentials (APs) were evoked in isolated rabbit ventricular cells loaded with fluo-4, and simultaneously recorded Ca²⁺ transients before and after the application of either relatively low doses of TTX (100 nM), the specific Na_v1.4 inhibitor μ-Conotoxin GIIIB or the specific Na_v1.1 inhibitor ICA 121430. Although APs changes after the application of each drug reflected the relative abundance of each isoform, the effects of TTX and GIIIB on SR Ca²⁺ release (measured as the transient maximum upstroke velocity) were no different. Furthermore, this reduction in SR Ca²⁺ release was comparable with the value that we obtained previously when total I_Na was inactivated with a ramp applied under voltage clamp. Finally, SR Ca²⁺ release was unaltered by the same ramp in the presence of TTX or GIIB. In contrast, application of ICA had no effect of SR Ca²⁺ release. These results suggest that Na_v1.4 is the main Na_v isoform involved in regulating the efficiency of excitation-contraction coupling in rabbit cardiomyocytes by priming the junction via activation of reverse-mode NCX.NEW & NOTEWORTHY A number of studies suggest that the Na⁺-Ca²⁺ exchanger (NCX) activated by Na⁺ currents is involved in the process of excitation-contraction (EC) coupling in cardiac ventricular myocytes. Although insufficient to trigger sarcoplasmic Ca²⁺ release alone, the Ca²⁺ entering through reverse NCX during an action potential can prime the dyadic cleft and increase the Ca²⁺ current coupling fidelity. Using specific Na⁺ inhibitors in this study, we show that in rabbit ventricular cells the skeletal Na⁺ channel isoform (Na_v1.4) is the main isoform responsible for this priming. Our study provides insights into a mechanism that may have an increased relevance where EC coupling is remodeled.

Na+ microdomains and sparks: Role in cardiac excitation-contraction coupling and arrhythmias in ankyrin-B deficiency

Cardiac sodium (Na⁺) potassium ATPase (NaK) pumps, neuronal sodium channels (I_Na), and sodium calcium (Ca²⁺) exchangers (NCX1) may co-localize to form a Na⁺ microdomain. It remains controversial as to whether neuronal I_Na contributes to local Na⁺ accumulation, resulting in reversal of nearby NCX1 and influx of Ca²⁺ into the cell. Therefore, there has been great interest in the possible roles of a Na⁺ microdomain in cardiac Ca²⁺-induced Ca²⁺ release (CICR). In addition, the important role of co-localization of NaK and NCX1 in regulating localized Na⁺ and Ca²⁺ levels and CICR in ankyrin-B deficient (ankyrin-B^+/-) cardiomyocytes has been examined in many recent studies. Altered Na⁺ dynamics may contribute to the appearance of arrhythmias, but the mechanisms underlying this relationship remain unclear. In order to investigate this, we present a mechanistic canine cardiomyocyte model which reproduces independent local dyadic junctional SR (JSR) Ca²⁺ release events underlying cell-wide excitation-contraction coupling, as well as a three-dimensional super-resolution model of the Ca²⁺ spark that describes local Na⁺ dynamics as governed by NaK pumps, neuronal I_Na, and NCX1. The model predicts the existence of Na⁺ sparks, which are generated by NCX1 and exhibit significantly slower dynamics as compared to Ca²⁺ sparks. Moreover, whole-cell simulations indicate that neuronal I_Na in the cardiac dyad plays a key role during the systolic phase. Rapid inward neuronal I_Na can elevate dyadic [Na⁺] to 35-40 mM, which drives reverse-mode NCX1 transport, and therefore promotes Ca²⁺ entry into the dyad, enhancing the trigger for JSR Ca²⁺ release. The specific role of decreased co-localization of NaK and NCX1 in ankyrin-B^+/- cardiomyocytes was examined. Model results demonstrate that a reduction in the local NCX1- and NaK-mediated regulation of dyadic [Ca²⁺] and [Na⁺] results in an increase in Ca²⁺ spark activity during isoproterenol stimulation, which in turn stochastically activates NCX1 in the dyad. This alteration in NCX1/NaK co-localization interrupts the balance between NCX1 and NaK currents in a way that leads to enhanced depolarizing inward current during the action potential plateau, which ultimately leads to a higher probability of L-type Ca²⁺ channel reopening and arrhythmogenic early-afterdepolarizations.

Cardiac Arrhythmias as Manifestations of Nanopathies: An Emerging View

A nanodomain is a collection of proteins localized within a specialized, nanoscale structural environment, which can serve as the functional unit of macroscopic physiologic processes. We are beginning to recognize the key roles of cardiomyocyte nanodomains in essential processes of cardiac physiology such as electrical impulse propagation and excitation–contraction coupling (ECC). There is growing appreciation of nanodomain dysfunction, i.e., nanopathy, as a mechanistic driver of life-threatening arrhythmias in a variety of pathologies. Here, we offer an overview of current research on the role of nanodomains in cardiac physiology with particular emphasis on: (1) sodium channel-rich nanodomains within the intercalated disk that participate in cell-to-cell electrical coupling and (2) dyadic nanodomains located along transverse tubules that participate in ECC. The beat to beat function of cardiomyocytes involves three phases: the action potential, the calcium transient, and mechanical contraction/relaxation. In all these phases, cell-wide function results from the aggregation of the stochastic function of individual proteins. While it has long been known that proteins that exist in close proximity influence each other’s function, it is increasingly appreciated that there exist nanoscale structures that act as functional units of cardiac biophysical phenomena. Termed nanodomains, these structures are collections of proteins, localized within specialized nanoscale structural environments. The nano-environments enable the generation of localized electrical and/or chemical gradients, thereby conferring unique functional properties to these units. Thus, the function of a nanodomain is determined by its protein constituents as well as their local structural environment, adding an additional layer of complexity to cardiac biology and biophysics. However, with the emergence of experimental techniques that allow direct investigation of structure and function at the nanoscale, our understanding of cardiac physiology and pathophysiology at these scales is rapidly advancing. Here, we will discuss the structure and functions of multiple cardiomyocyte nanodomains, and novel strategies that target them for the treatment of cardiac arrhythmias.

Pharmacological characterization of crotamine effects on mice hind limb paralysis employing both ex vivo and in vivo assays: Insights into the involvement of voltage-gated ion channels in the crotamine action on skeletal muscles

The high medical importance of Crotalus snakes is unquestionable, as this genus is the second in frequency of ophidian accidents in many countries, including Brazil. With a relative less complex composition compared to other genera venoms, as those from the Bothrops genus, the Crotalus genus venom from South America is composed basically by the neurotoxin crotoxin (a phospholipase A2), the thrombin-like gyroxin (a serinoprotease), a very potent aggregating protein convulxin, and a myotoxic polypeptide named crotamine. Interestingly not all Crotalus snakes express crotamine, which was first described in early 50s due to its ability to immobilize animal hind limbs, contributing therefore to the physical immobilization of preys and representing an important advantage for the envenoming efficacy, and consequently, for the feeding and survival of these snakes in nature. Representing about 10–25% of the dry weight of the crude venom of crotamine-positive rattlesnakes, the polypeptide crotamine is also suggested to be of importance for antivenom therapy, although the contribution of this toxin to the main symptoms of envenoming process remains far unknown until now. Herein, we concomitantly performed in vitro and in vivo assays to show for the first time the dose-dependent response of crotamine-triggered hind limbs paralysis syndrome, up to now believed to be observable only at high (sub-lethal) concentrations of crotamine. In addition, ex vivo assay performed with isolated skeletal muscles allowed us to suggest here that compounds active on voltage-sensitive sodium and/or potassium ion channels could both affect the positive inotropic effect elicited by crotamine in isolated diaphragm, besides also affecting the hind limbs paralysis syndrome imposed by crotamine in vivo. By identifying the potential molecular targets of this toxin, our data may contribute to open new roads for translational studies aiming to improve the snakebite envenoming treatment in human. Interestingly, we also demonstrate that the intraplantal or intraperitoneal (ip) injections of crotamine in mice do not promote pain. Therefore, this work may also suggest the profitable utility of non-toxic analogs of crotamine as a potential tool for targeting voltage-gated ion channels in skeletal muscles, aiming its potential use in the therapy of neuromuscular dysfunctions and envenoming therapy.

Representing more than 10% of the dry weight of the crude venom of crotamine-positive rattlesnakes, crotamine may act as toxin mainly by imposing the physical immobilization of preys. Its presence was described to be important for antivenom therapy, although the knowledge on the effective contribution of crotamine to the main symptoms of envenoming process remains elusive and limited. Herein, we show for the first time the dose-dependent response for the hind limbs paralysis syndrome promoted by crotamine. We also report herein that compounds active on voltage-sensitive sodium and/or potassium ion channels can affect the positive inotropic effect elicited by crotamine in vitro in isolated diaphragm and also in the hind limbs paralysis syndrome triggered by crotamine in vivo. This potential targeting of voltage-sensitive sodium and/or potassium ion channels suggested here for crotamine may contribute to open new roads for translational studies aiming to improve the snakebite envenoming treatment in human. More importantly, nociceptive threshold evaluation demonstrated that crotamine does not trigger pain, and therefore, we also suggest crotamine as a potential tool for targeting voltage-gated ion channels present in skeletal muscles, with potential to be used as a lead compound to develop drugs for neuromuscular dysfunctions therapy.

Ryanodine receptor dysfunction in human disorders

Regulation of intracellular calcium (Ca²⁺) is critical in all cell types. The ryanodine receptor (RyR), an intracellular Ca²⁺ release channel located on the sarco/endoplasmic reticulum (SR/ER), releases Ca²⁺ from intracellular stores to activate critical functions including muscle contraction and neurotransmitter release. Dysfunctional RyR-mediated Ca²⁺ handling has been implicated in the pathogenesis of inherited and non-inherited conditions including heart failure, cardiac arrhythmias, skeletal myopathies, diabetes, and neurodegenerative diseases. Here we have reviewed the evidence linking human disorders to RyR dysfunction and describe novel approaches to RyR-targeted therapeutics.

A privileged role for neuronal Na+ channels in regulating ventricular [Ca2+] and arrhythmias

Jones et al. provide commentary on the intricate crosstalk between ion transporters that goes awry in long QT arrhythmia.

Tetrodotoxin-sensitive Navs contribute to early and delayed afterdepolarizations in long QT arrhythmia models

Neuronal Na⁺ channels contribute to catecholaminergic polymorphic ventricular tachycardia in the heart, but their role in other types of arrhythmias is unknown. Koleske et al. show that they contribute to early and delayed afterdepolarizations common to long QT, catecholaminergic polymorphic ventricular tachycardia, and overlap phenotypes.

Recent evidence suggests that neuronal Na⁺ channels (nNa_vs) contribute to catecholamine-promoted delayed afterdepolarizations (DADs) and catecholaminergic polymorphic ventricular tachycardia (CPVT). The newly identified overlap between CPVT and long QT (LQT) phenotypes has stoked interest in the cross-talk between aberrant Na⁺ and Ca²⁺ handling and its contribution to early afterdepolarizations (EADs) and DADs. Here, we used Ca²⁺ imaging and electrophysiology to investigate the role of Na⁺ and Ca²⁺ handling in DADs and EADs in wild-type and cardiac calsequestrin (CASQ2)-null mice. In experiments, repolarization was impaired using 4-aminopyridine (4AP), whereas the L-type Ca²⁺ and late Na⁺ currents were augmented using Bay K 8644 (BayK) and anemone toxin II (ATX-II), respectively. The combination of 4AP and isoproterenol prolonged action potential duration (APD) and promoted aberrant Ca²⁺ release, EADs, and DADs in wild-type cardiomyocytes. Similarly, BayK in the absence of isoproterenol induced the same effects in CASQ2-null cardiomyocytes. In vivo, it prolonged the QT interval and, upon catecholamine challenge, precipitated wide QRS polymorphic ventricular tachycardia that resembled human torsades de pointes. Treatment with ATX-II produced similar effects at both the cellular level and in vivo. Importantly, nNa_v inhibition with riluzole or 4,9-anhydro-tetrodotoxin reduced the incidence of ATX-II–, BayK-, or 4AP-induced EADs, DADs, aberrant Ca²⁺ release, and VT despite only modestly mitigating APD prolongation. These data reveal the contribution of nNa_Vs to triggered arrhythmias in murine models of LQT and CPVT-LQT overlap phenotypes. We also demonstrate the antiarrhythmic impact of nNa_V inhibition, independent of action potential and QT interval duration, and provide a basis for a mechanistically driven antiarrhythmic strategy.

Sensitized signalling between L-type Ca2+ channels and ryanodine receptors in the absence or inhibition of FKBP12.6 in cardiomyocytes

Aims

The heart contraction is controlled by the Ca2+-induced Ca2+ release (CICR) between L-type Ca2+ channels and ryanodine receptors (RyRs). The FK506-binding protein FKBP12.6 binds to RyR subunits, but its role in stabilizing RyR function has been debated for long. Recent reports of high-resolution RyR structure show that the HD2 domain that binds to the SPRY2 domain of neighbouring subunit in FKBP-bound RyR1 is detached and invisible in FKBP-null RyR2. The present study was to test the consequence of FKBP12.6 absence on the in situ activation of RyR2.

Methods and results

Using whole-cell patch-clamp combined with confocal imaging, we applied a near threshold depolarization to activate a very small fraction of LCCs, which in turn activated RyR Ca2+ sparks stochastically. FKBP12.6-knockout and FK506/rapamycin treatments increased spark frequency and LCC-RyR coupling fidelity without altering LCC open probability. Neither FK506 nor rapamycin further altered LCC-RyR coupling fidelity in FKBP12.6-knockout cells. In loose-seal patch-clamp experiments, the LCC-RyR signalling kinetics, indexed by the delay for a LCC sparklet to trigger a RyR spark, was accelerated after FKBP12.6 knockout and FK506/rapamycin treatments. These results demonstrated that RyRs became more sensitive to Ca2+ triggers without FKBP12.6. Isoproterenol (1 μM) further accelerated the LCC-RyR signalling in FKBP12.6-knockout cells. The synergistic sensitization of RyRs by catecholaminergic signalling and FKBP12.6 dysfunction destabilized the CICR system, leading to chaotic Ca2+ waves and ventricular arrhythmias.

Conclusion

FKBP12.6 keeps the RyRs from over-sensitization, stabilizes the potentially regenerative CICR system, and thus may suppress the life-threatening arrhythmogenesis.

Neuronal sodium channels: emerging components of the nano-machinery of cardiac calcium cycling

Excitation-contraction coupling is the bridge between cardiac electrical activation and mechanical contraction. It is driven by the influx of Ca²⁺ across the sarcolemma triggering Ca²⁺ release from the sarcoplasmic reticulum (SR) - a process termed Ca²⁺ -induced Ca²⁺ release (CICR) - followed by re-sequestration of Ca²⁺ into the SR. The Na⁺ /Ca²⁺ exchanger inextricably couples the cycling of Ca²⁺ and Na⁺ in cardiac myocytes. Thus, influx of Na⁺ via voltage-gated Na⁺ channels (Na_V ) has emerged as an important regulator of CICR both in health and in disease. Recent insights into the subcellular distribution of cardiac and neuronal Na_V isoforms and their ultrastructural milieu have important implications for the roles of these channels in mediating Ca²⁺ -driven arrhythmias. This review will discuss functional insights into the role of neuronal Na_V isoforms vis-à-vis cardiac Na_V s in triggering such arrhythmias and their potential as therapeutic targets in the context of the aforementioned structural observations.

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.

Cardiac arrhythmia in a mouse model of sodium channel SCN8A epileptic encephalopathy

Patients with early infantile epileptic encephalopathy (EIEE) are at increased risk for sudden unexpected death in epilepsy (SUDEP). De novo mutations of the sodium channel gene SCN8A, encoding the sodium channel Na_v1.6, result in EIEE13 (OMIM 614558), which has a 10% risk of SUDEP. Here, we investigated the cardiac phenotype of a mouse model expressing the gain of function EIEE13 patient mutation p.Asn1768Asp in Scn8a (Na_v1.6-N1768D). We tested Scn8a^N1768D/+ mice for alterations in cardiac excitability. We observed prolongation of the early stages of action potential (AP) repolarization in mutant myocytes vs. controls. Scn8a^N1768D/+ myocytes were hyperexcitable, with a lowered threshold for AP firing, increased incidence of delayed afterdepolarizations, increased calcium transient duration, increased incidence of diastolic calcium release, and ectopic contractility. Calcium transient duration and diastolic calcium release in the mutant myocytes were tetrodotoxin-sensitive. A selective inhibitor of reverse mode Na/Ca exchange blocked the increased incidence of diastolic calcium release in mutant cells. Scn8a^N1768D/+ mice exhibited bradycardia compared with controls. This difference in heart rate dissipated after administration of norepinephrine, and there were no differences in heart rate in denervated ex vivo hearts, implicating parasympathetic hyperexcitability in the Scn8a^N1768D/+ animals. When challenged with norepinephrine and caffeine to simulate a catecholaminergic surge, Scn8a^N1768D/+ mice showed ventricular arrhythmias. Two of three mutant mice under continuous ECG telemetry recording experienced death, with severe bradycardia preceding asystole. Thus, in addition to central neuron hyperexcitability, Scn8a^N1768D/+ mice have cardiac myoycte and parasympathetic neuron hyperexcitability. Simultaneous dysfunction in these systems may contribute to SUDEP associated with mutations of Scn8a.

Modeling Na+-Ca2+ exchange in the heart: Allosteric activation, spatial localization, sparks and excitation-contraction coupling

The cardiac sodium (Na⁺)/calcium (Ca²⁺) exchanger (NCX1) is an electrogenic membrane transporter that regulates Ca²⁺ homeostasis in cardiomyocytes, serving mainly to extrude Ca²⁺ during diastole. The direction of Ca²⁺ transport reverses at membrane potentials near that of the action potential plateau, generating an influx of Ca²⁺ into the cell. Therefore, there has been great interest in the possible roles of NCX1 in cardiac Ca²⁺-induced Ca²⁺ release (CICR). Interest has been reinvigorated by a recent super-resolution optical imaging study suggesting that ~18% of NCX1 co-localize with ryanodine receptor (RyR2) clusters, and ~30% of additional NCX1 are localized to within ~120nm of the nearest RyR2. NCX1 may therefore occupy a privileged position in which to modulate CICR. To examine this question, we have developed a mechanistic biophysically-detailed model of NCX1 that describes both NCX1 transport kinetics and Ca²⁺-dependent allosteric regulation. This NCX1 model was incorporated into a previously developed super-resolution model of the Ca²⁺ spark as well as a computational model of the cardiac ventricular myocyte that includes a detailed description of CICR with stochastic gating of L-type Ca²⁺ channels and RyR2s, and that accounts for local Ca²⁺ gradients near the dyad via inclusion of a peri-dyadic (PD) compartment. Both models predict that increasing the fraction of NCX1 in the dyad and PD decreases spark frequency, fidelity, and diastolic Ca²⁺ levels. Spark amplitude and duration are less sensitive to NCX1 spatial redistribution. On the other hand, NCX1 plays an important role in promoting Ca²⁺ entry into the dyad, and hence contributing to the trigger for RyR2 release at depolarized membrane potentials and in the presence of elevated local Na⁺ concentration. Whole-cell simulation of NCX1 tail currents are consistent with the finding that a relatively high fraction of NCX1 (~45%) resides in the dyadic and PD spaces, with a dyad-to-PD ratio of roughly 1:2. Allosteric Ca²⁺ activation of NCX1 helps to "functionally localize" exchanger activity to the dyad and PD by reducing exchanger activity in the cytosol thereby protecting the cell from excessive loss of Ca²⁺ during diastole.

Neuronal Na+ Channels Are Integral Components of Pro-arrhythmic Na+/Ca2+ Signaling Nanodomain That Promotes Cardiac Arrhythmias During β-adrenergic Stimulation

BACKGROUND

Cardiac arrhythmias are a leading cause of death in the US. Vast majority of these arrhythmias including catecholaminergic polymorphic ventricular tachycardia (CPVT) are associated with increased levels of circulating catecholamines and involve abnormal impulse formation secondary to aberrant Ca²⁺ and Na⁺ handling. However, the mechanistic link between β-AR stimulation and the subcellular/molecular arrhythmogenic trigger(s) remains elusive.

METHODS AND RESULTS

We performed functional and structural studies to assess Ca²⁺ and Na⁺ signaling in ventricular myocyte as well as surface electrocardiograms in mouse models of cardiac calsequestrin (CASQ2)-associated CPVT. We demonstrate that a subpopulation of Na⁺ channels (neuronal Na⁺ channels; nNa_v) that colocalize with RyR2 and Na⁺/Ca²⁺ exchanger (NCX) are a part of the β-AR-mediated arrhythmogenic process. Specifically, augmented Na⁺ entry via nNa_v in the settings of genetic defects within the RyR2 complex and enhanced sarcoplasmic reticulum (SR) Ca²⁺-ATPase (SERCA)-mediated SR Ca²⁺ refill is both an essential and a necessary factor for the arrhythmogenesis. Furthermore, we show that augmentation of Na⁺ entry involves β-AR-mediated activation of CAMKII subsequently leading to nNa_v augmentation. Importantly, selective pharmacological inhibition as well as silencing of Na_v1.6 inhibit myocyte arrhythmic potential and prevent arrhythmias in vivo.

CONCLUSION

These data suggest that the arrhythmogenic alteration in Na⁺/Ca²⁺ handling evidenced ruing β-AR stimulation results, at least in part, from enhanced Na⁺ influx through nNa_v. Therefore, selective inhibition of these channels and Na_v1.6 in particular can serve as a potential antiarrhythmic therapy.

Na+/Ca2+ exchange and Na+/K+-ATPase in the heart

This paper is the third in a series of reviews published in this issue resulting from the University of California Davis Cardiovascular Symposium 2014: Systems approach to understanding cardiac excitation–contraction coupling and arrhythmias: Na⁺ channel and Na⁺ transport. The goal of the symposium was to bring together experts in the field to discuss points of consensus and controversy on the topic of sodium in the heart. The present review focuses on cardiac Na⁺/Ca²⁺ exchange (NCX) and Na⁺/K⁺-ATPase (NKA). While the relevance of Ca²⁺ homeostasis in cardiac function has been extensively investigated, the role of Na⁺ regulation in shaping heart function is often overlooked. Small changes in the cytoplasmic Na⁺ content have multiple effects on the heart by influencing intracellular Ca²⁺ and pH levels thereby modulating heart contractility. Therefore it is essential for heart cells to maintain Na⁺ homeostasis. Among the proteins that accomplish this task are the Na⁺/Ca²⁺ exchanger (NCX) and the Na⁺/K⁺ pump (NKA). By transporting three Na⁺ ions into the cytoplasm in exchange for one Ca²⁺ moved out, NCX is one of the main Na⁺ influx mechanisms in cardiomyocytes. Acting in the opposite direction, NKA moves Na⁺ ions from the cytoplasm to the extracellular space against their gradient by utilizing the energy released from ATP hydrolysis. A fine balance between these two processes controls the net amount of intracellular Na⁺ and aberrations in either of these two systems can have a large impact on cardiac contractility. Due to the relevant role of these two proteins in Na⁺ homeostasis, the emphasis of this review is on recent developments regarding the cardiac Na⁺/Ca²⁺ exchanger (NCX1) and Na⁺/K⁺ pump and the controversies that still persist in the field.

Modeling calcium regulation of contraction, energetics, signaling, and transcription in the cardiac myocyte

Calcium (Ca(2+)) plays many important regulatory roles in cardiac muscle cells. In the initial phase of the action potential, influx of Ca(2+) through sarcolemmal voltage-gated L-type Ca(2+) channels (LCCs) acts as a feed-forward signal that triggers a large release of Ca(2+) from the junctional sarcoplasmic reticulum (SR). This Ca(2+) drives heart muscle contraction and pumping of blood in a process known as excitation-contraction coupling (ECC). Triggered and released Ca(2+) also feed back to inactivate LCCs, attenuating the triggered Ca(2+) signal once release has been achieved. The process of ECC consumes large amounts of ATP. It is now clear that in a process known as excitation-energetics coupling, Ca(2+) signals exert beat-to-beat regulation of mitochondrial ATP production that closely couples energy production with demand. This occurs through transport of Ca(2+) into mitochondria, where it regulates enzymes of the tricarboxylic acid cycle. In excitation-signaling coupling, Ca(2+) activates a number of signaling pathways in a feed-forward manner. Through effects on their target proteins, these interconnected pathways regulate Ca(2+) signals in complex ways to control electrical excitability and contractility of heart muscle. In a process known as excitation-transcription coupling, Ca(2+) acting primarily through signal transduction pathways also regulates the process of gene transcription. Because of these diverse and complex roles, experimentally based mechanistic computational models are proving to be very useful for understanding Ca(2+) signaling in the cardiac myocyte.

Distinguishing between overdrive excited and suppressed ventricular beats in guinea pig ventricular myocardium

Rapid ventricular pacing rates induces two types of beats following pacing cessation: recovery cycle length (RCL) prolongation (overdrive suppression) and RCL shortening (overdrive excitation). The goals of this study were to compare common experimental protocols for studying triggered activity in whole-heart preparations and differentiate between recovery beats using a new methodology. Post-pacing recovery beat cycle length (RCL) and QRS were normalized to pre-paced R-R and QRS intervals and analyzed using a K-means clustering algorithm. Control hearts only produced suppressed beats: RCL ratio increased with rapid pacing (25 ± 4.0%, n = 10) without changing QRS duration. Rapid pacing during hypercalcemia + hypothermia (5.5 mM and 34°C) produced significantly earlier excited beats (53 ± 14%, n = 5) with wider QRS durations (58 ± 6.3%, n = 5) than suppressed beats. Digoxin + hypothermia (0.75 μM) produced the most excited beats with significantly earlier RCL (44 ± 3.2%, n = 6) and wider QRS (60 ± 3.1%, n = 6) ratios relative to suppressed beats. Increasing pacing further shortened RCL (30 ± 7.8%, n = 6). In a prospective study, TTX (100 nM) increased RCL ratio (15 ± 6.0%, n = 10) without changing the QRS duration of excited beats. The algorithm was compared to a cross-correlation analysis with 93% sensitivity and 94% specificity. This ECG based algorithm distinguishes between triggered and automatic activity.

Neuronal Na+ channel blockade suppresses arrhythmogenic diastolic Ca2+ release

AIMS

Sudden death resulting from cardiac arrhythmias is the most common consequence of cardiac disease. Certain arrhythmias caused by abnormal impulse formation including catecholaminergic polymorphic ventricular tachycardia (CPVT) are associated with delayed afterdepolarizations resulting from diastolic Ca2+ release (DCR) from the sarcoplasmic reticulum (SR). Despite high response of CPVT to agents directly affecting Ca2+ cycling, the incidence of refractory cases is still significant. Surprisingly, these patients often respond to treatment with Na+ channel blockers. However, the relationship between Na+ influx and disturbances in Ca2+ handling immediately preceding arrhythmias in CPVT remains poorly understood and is the object of this study.

METHODS AND RESULTS

We performed optical Ca2+ and membrane potential imaging in ventricular myocytes and intact cardiac muscles as well as surface ECGs on a CPVT mouse model with a mutation in cardiac calsequestrin. We demonstrate that a subpopulation of Na+ channels (neuronal Na+ channels; nNav) colocalize with ryanodine receptor Ca2+ release channels (RyR2). Disruption of the crosstalk between nNav and RyR2 by nNav blockade with riluzole reduced and also desynchronized DCR in isolated cardiomyocytes and in intact cardiac tissue. Such desynchronization of DCR on cellular and tissue level translated into decreased arrhythmias in CPVT mice.

CONCLUSIONS

Thus, our study offers the first evidence that nNav contribute to arrhythmogenic DCR, thereby providing a conceptual basis for mechanism-based antiarrhythmic therapy.

Junctional cleft [Ca²⁺]i measurements using novel cleft-targeted Ca²⁺ sensors

RATIONALE

Intracellular Ca(2+) concentration ([Ca(2+)]i) is regulated and signals differently in various subcellular microdomains, which greatly enhances its second messenger versatility. In the heart, sarcoplasmic reticulum Ca(2+) release and signaling are controlled by local [Ca(2+)]i in the junctional cleft ([Ca(2+)]Cleft), the small space between sarcolemma and junctional sarcoplasmic reticulum. However, methods to measure [Ca(2+)]Cleft directly are needed.

OBJECTIVE

To construct novel sensors that allow direct measurement of [Ca(2+)]Cleft.

METHODS AND RESULTS

We constructed cleft-targeted [Ca(2+)] sensors by fusing Ca(2+)-sensor GCaMP2.2 and a new lower Ca(2+)-affinity variant GCaMP2.2Low to FKBP12.6, which binds with high affinity and selectivity to ryanodine receptors. The fluorescence pattern, affinity for ryanodine receptors, and competition by untagged FKBP12.6 demonstrated that FKBP12.6-tagged sensors are positioned to measure local [Ca(2+)]Cleft in adult rat myocytes. Using GCaMP2.2Low-FKBP12.6, we showed that [Ca(2+)]Cleft reaches higher levels with faster kinetics than global [Ca(2+)]i during excitation-contraction coupling. Diastolic sarcoplasmic reticulum Ca(2+) leak or sarcolemmal Ca(2+) entry may raise local [Ca(2+)]Cleft above bulk cytosolic [Ca(2+)]i ([Ca(2+)]Bulk), an effect that may contribute to triggered arrhythmias and even transcriptional regulation. We measured this diastolic standing [Ca(2+)]Cleft-[Ca(2+)]Bulk gradient with GCaMP2.2-FKBP12.6 versus GCaMP2.2, using [Ca(2+)] measured without gradients as a reference point. This diastolic difference ([Ca(2+)]Cleft=194 nmol/L versus [Ca(2+)]Bulk=100 nmol/L) is dictated mainly by the sarcoplasmic reticulum Ca(2+) leak rather than sarcolemmal Ca(2+) flux.

CONCLUSIONS

We have developed junctional cleft-targeted sensors to measure [Ca(2+)]Cleft versus [Ca(2+)]Bulk and demonstrated dynamic differences during electric excitation and a standing diastolic [Ca(2+)]i gradient, which could influence local Ca(2+)-dependent signaling within the junctional cleft.

Decreased cardiac excitability secondary to reduction of sodium current may be a significant contributor to reduced contractility in a rat model of sepsis

Introduction

Multisystem organ failure remains a poorly understood complication of sepsis. During sepsis, reduced excitability contributes to organ failure of skeletal muscle, nerves and the spinal cord. The goal of this study was to determine whether reduced excitability might also contribute to cardiac failure during sepsis.

Methods

Wistar rats were made septic by cecal ligation and puncture. One day later, action potentials were recorded from beating left ventricular papillary muscle ex vivo by impaling myocytes with sharp microelectrodes.

Results

In cardiac papillary muscle from septic rats, action potential amplitude and rate of rise were reduced, while threshold was elevated. These changes in action potential properties suggest sepsis selectively reduces sodium current. To determine the effects of selective reduction in sodium current, we applied tetrodotoxin to papillary muscle from healthy rats and found reduction in action potential amplitude and rate of rise, as well as elevation of threshold. The changes were similar to those triggered by sepsis. Blocking calcium current using nifedipine did not mimic action potential changes induced by sepsis. Contractility of healthy papillary muscle was reduced to 40% of normal following partial block of sodium current by tetrodotoxin, close to the low contractility of septic papillary muscle, which was 30% of normal.

Conclusions

Our data suggest cardiac excitability is reduced during sepsis in rats. The reduction in excitability appears to be primarily due to reduction of sodium current. The reduction in sodium current may be sufficient to explain most of the reduction in cardiac contractility during sepsis.

Localization of sodium channel subtypes in mouse ventricular myocytes using quantitative immunocytochemistry

Voltage-gated sodium channels are responsible for the rising phase of the action potential in cardiac muscle. Previously, both TTX-sensitive neuronal sodium channels (NaV1.1, NaV1.2, NaV1.3, NaV1.4 and NaV1.6) and the TTX-resistant cardiac sodium channel (NaV1.5) have been detected in cardiac myocytes, but relative levels of protein expression of the isoforms were not determined. Using a quantitative approach, we analyzed z-series of confocal microscopy images from individual mouse myocytes stained with either anti-NaV1.1, anti-NaV1.2, anti-NaV1.3, anti-NaV1.4, anti-NaV1.5, or anti-NaV1.6 antibodies and calculated the relative intensity of staining for these sodium channel isoforms. Our results indicate that the TTX-sensitive channels represented approximately 23% of the total channels, whereas the TTX-resistant NaV1.5 channel represented 77% of the total channel staining in mouse ventricular myocytes. These ratios are consistent with previous electrophysiological studies in mouse ventricular myocytes. NaV1.5 was located at the cell surface, with high density at the intercalated disc, but was absent from the transverse (t)-tubular system, suggesting that these channels support surface conduction and inter-myocyte transmission. Low-level cell surface staining of NaV1.4 and NaV1.6 channels suggest a minor role in surface excitation and conduction. Conversely, NaV1.1 and NaV1.3 channels are localized to the t-tubules and are likely to support t-tubular transmission of the action potential to the myocyte interior. This quantitative immunocytochemical approach for assessing sodium channel density and localization provides a more precise view of the relative importance and possible roles of these individual sodium channel protein isoforms in mouse ventricular myocytes and may be applicable to other species and cardiac tissue types.

Cardiac sodium transport and excitation-contraction coupling

The excitation-contraction coupling (EC-coupling) links membrane depolarization with contraction in cardiomyocytes. Ca(2+) induced opening of ryanodine receptors (RyRs) leads to Ca(2+) induced Ca(2+) release (CICR) from the sarcoplasmic reticulum (SR) into the dyadic cleft between the t-tubules and SR. Ca(2+) is removed from the cytosol by the SR Ca(2+) ATPase (SERCA2) and the Na,Ca-exchanger (NCX). The NCX connects cardiac Ca(2+) and Na(+)-transport, leading to Na(+)-dependent regulation of EC-coupling by several mechanisms of which some still lack firm experimental evidence. Firstly, NCX might contribute to CICR during an action potential (AP) as Na(+)-accumulation at the intracellular site together with depolarization will trigger reverse mode exchange bringing Ca(2+) into the dyadic cleft. The controversial issue is the nature of the compartment in which Na(+) accumulates. It seems not to be the bulk cytosol, but is it part of a widespread subsarcolemmal space, a localized microdomain ("fuzzy space"), or as we propose, a more localized "spot" to which only a few membrane proteins have shared access (nanodomains)? Also, there seems to be spots where the Na,K-pump (NKA) will cause local Na(+) depletion. Secondly, Na(+) determines the rate of cytosolic Ca(2+) removal and SR Ca(2+) load by regulating the SERCA2/NCX-balance during the decay of the Ca(2+) transient. The aim of this review is to describe available data and current concepts of Na(+)-mediated regulation of cardiac EC-coupling, with special focus on subcellular microdomains and the potential roles of Na(+) transport proteins in regulating CICR and Ca(2+) extrusion in cardiomyocytes. We propose that voltage gated Na(+) channels, NCX and the NKA α2-isoform all regulate cardiac EC-coupling through control of the "Na(+) concentration in specific subcellular nanodomains in cardiomyocytes. This article is part of a Special Issue entitled "Na(+) Regulation in Cardiac Myocytes."

Na/Ca exchange and contraction of the heart

Sodium-calcium exchange (NCX) is the major calcium (Ca) efflux mechanism of ventricular cardiomyocytes. Consequently the exchanger plays a critical role in the regulation of cellular Ca content and hence contractility. Reductions in Ca efflux by the exchanger, such as those produced by elevated intracellular sodium (Na) in response to cardiac glycosides, raise sarcoplasmic reticulum (SR) Ca stores. The result is an increased Ca transient and cardiac contractility. Enhanced Ca efflux activity by the exchanger, for example during heart failure, may reduce diadic cleft Ca and excitation-contraction (EC) coupling gain. This aggravates the impaired contractility associated with SR Ca ATPase dysfunction and reduced SR Ca load in failing heart muscle. Recent data from our laboratories indicate that NCX can also impact the efficiency of EC coupling and contractility independent of SR Ca load through diadic cleft priming with Ca during the upstroke of the action potential. This article is part of a Special Issue entitled "Na(+) Regulation in Cardiac Myocytes".

Na⁺ transport in the normal and failing heart - remember the balance

In the heart, intracellular Na(+) concentration ([Na(+)]i) is a key modulator of Ca(2+) cycling, contractility and cardiac myocyte metabolism. Several Na(+) transporters are electrogenic, thus they both contribute to shaping the cardiac action potential and at the same time are affected by it. [Na(+)]i is controlled by the balance between Na(+) influx through various pathways, including the Na(+)/Ca(2+) exchanger and Na(+) channels, and Na(+) extrusion via the Na(+)/K(+)-ATPase. [Na(+)]i is elevated in HF due to a combination of increased entry through Na(+) channels and/or Na(+)/H(+) exchanger and reduced activity of the Na(+)/K(+)-ATPase. Here we review the major Na(+) transport pathways in cardiac myocytes and how they participate in regulating [Na(+)]i in normal and failing hearts. This article is part of a Special Issue entitled "Na(+) Regulation in Cardiac Myocytes."

Alterations in T-tubule and dyad structure in heart disease: challenges and opportunities for computational analyses

Compelling recent experimental results make clear that sub-cellular structures are altered in ventricular myocytes during the development of heart failure, in both human samples and diverse experimental models. These alterations can include, but are not limited to, changes in the clusters of sarcoplasmic reticulum (SR) Ca(2+)-release channels, ryanodine receptors, and changes in the average distance between the cell membrane and ryanodine receptor clusters. In this review, we discuss the potential consequences of these structural alterations on the triggering of SR Ca(2+) release during excitation-contraction coupling. In particular, we describe how mathematical models of local SR Ca(2+) release can be used to predict functional changes resulting from diverse modifications that occur in disease states. We review recent studies that have used simulations to understand the consequences of sub-cellular structural changes, and we discuss modifications that will allow for future modelling studies to address unresolved questions. We conclude with a discussion of improvements in both experimental and mathematical modelling techniques that will be required to provide a stronger quantitative understanding of the functional consequences of changes in sub-cellular structure in heart disease.

Cardiac sodium-calcium exchange and efficient excitation-contraction coupling: implications for heart disease

Cardiovascular disease is a leading cause of death worldwide, with ischemic heart disease alone accounting for >12% of all deaths, more than HIV/AIDS, tuberculosis, lung, and breast cancer combined. Heart disease has been the leading cause of death in the United States for the past 85 years and is a major cause of disability and health-care expenditures. The cardiac conditions most likely to result in death include heart failure and arrhythmias, both a consequence of ischemic coronary disease and myocardial infarction, though chronic hypertension and valvular diseases are also important causes of heart failure. Sodium-calcium exchange (NCX) is the dominant calcium (Ca2+) efflux mechanism in cardiac cells. Using ventricular-specific NCX knockout mice, we have found that NCX is also an essential regulator of cardiac contractility independent of sarcoplasmic reticulum Ca2+ load. During the upstroke of the action potential, sodium (Na+) ions enter the diadic cleft space between the sarcolemma and the sarcoplasmic reticulum. The rise in cleft Na+, in conjunction with depolarization, causes NCX to transiently reverse. Ca2+ entry by this mechanism then "primes" the diadic cleft so that subsequent Ca2+ entry through Ca2+ channels can more efficiently trigger Ca2+ release from the sarcoplasmic reticulum. In NCX knockout mice, this mechanism is inoperative (Na+ current has no effect on the Ca2+ transient), and excitation-contraction coupling relies upon the elevated diadic cleft Ca2+ that arises from the slow extrusion of cytoplasmic Ca2+ by the ATP-dependent sarcolemmal Ca2+ pump. Thus, our data support the conclusion that NCX is an important regulator of cardiac contractility. These findings suggest that manipulation of NCX may be beneficial in the treatment of heart failure.

Inhibition of Na+ channels ameliorates arrhythmias in a drug-induced model of Andersen-Tawil syndrome

BACKGROUND

Andersen-Tawil syndrome (ATS1)-associated ventricular tachycardias (VTs) are initiated by frequent, hypokalemia-exacerbated, premature ventricular activity (PVA). We previously demonstrated that a guinea pig model of drug-induced ATS1 (DI-ATS1) evidenced increased arrhythmias from regions with high Na(+)/Ca(2+)-exchange expression.

OBJECTIVE

Therefore, we hypothesize that reduced cytosolic Na(+) entry through either cardiac isoform of or tetrodotoxin (TTX)-sensitive Na(+) channels during DI-ATS1 can ameliorate arrhythmia burden.

METHODS

DI-ATS1 was induced with 10 μM BaCl(2) and 2 mM extracellular K(+). Ca(2+) transients and conduction velocity (CV) were optically mapped with indo-1 and di-4-ANEPPS, respectively, from Langendorff-perfused guinea pig ventricles.

RESULTS

Nonselective Na(+) channel blockade with 1 μM flecainide reduced amplitude (Ca(A)), slowed left ventricular CV, reduced tissue excitability, and abolished the incidence of VT while decreasing the incidence of PVA relative to DI-ATS1. Selective, TTX-sensitive Na(+) channel blockade with TTX (100 nM) during DI-ATS1 decreased Ca(A) and decreased the inducibility of VTs and PVA relative to DI-ATS1 without slowing CV. Ranolazine altered Ca(A), left ventricular CV, tissue excitability, and reduced inducibility of VT and PVA in a concentration-dependent manner. None of the aforementioned interventions altered diastolic Ca(2+) levels or Ca(2+) transient decay time constant.

CONCLUSIONS

These data suggest that cytosolic Na(+) entry and its modulation of Ca(2+) handling are necessary for arrhythmogenesis. During the loss of inward-rectifier K(+) current function, not only Na(+)/Ca(2+)-exchange dominance but Na(+) flux may determine arrhythmia burden. Therefore, selective inhibition of TTX-sensitive Na(+) channels may offer a potential therapeutic target to alleviate arrhythmias during states of Ca(2+) overload secondary to loss of inward-rectifier K(+) current function without compromising the excitability reserve.

Modeling effects of L-type ca(2+) current and na(+)-ca(2+) exchanger on ca(2+) trigger flux in rabbit myocytes with realistic T-tubule geometries

The transverse tubular system of rabbit ventricular myocytes consists of cell membrane invaginations (t-tubules) that are essential for efficient cardiac excitation-contraction coupling. In this study, we investigate how t-tubule micro-anatomy, L-type Ca²⁺ channel (LCC) clustering, and allosteric activation of Na⁺/Ca²⁺ exchanger by L-type Ca²⁺ current affects intracellular Ca²⁺ dynamics. Our model includes a realistic 3D geometry of a single t-tubule and its surrounding half-sarcomeres for rabbit ventricular myocytes. The effects of spatially distributed membrane ion-transporters (LCC, Na⁺/Ca²⁺ exchanger, sarcolemmal Ca²⁺ pump, and sarcolemmal Ca²⁺ leak), and stationary and mobile Ca²⁺ buffers (troponin C, ATP, calmodulin, and Fluo-3) are also considered. We used a coupled reaction-diffusion system to describe the spatio-temporal concentration profiles of free and buffered intracellular Ca²⁺. We obtained parameters from voltage-clamp protocols of L-type Ca²⁺ current and line-scan recordings of Ca²⁺ concentration profiles in rabbit cells, in which the sarcoplasmic reticulum is disabled. Our model results agree with experimental measurements of global Ca²⁺ transient in myocytes loaded with 50 μM Fluo-3. We found that local Ca²⁺ concentrations within the cytosol and sub-sarcolemma, as well as the local trigger fluxes of Ca²⁺ crossing the cell membrane, are sensitive to details of t-tubule micro-structure and membrane Ca²⁺ flux distribution. The model additionally predicts that local Ca²⁺ trigger fluxes are at least threefold to eightfold higher than the whole-cell Ca²⁺ trigger flux. We found also that the activation of allosteric Ca²⁺-binding sites on the Na⁺/Ca²⁺ exchanger could provide a mechanism for regulating global and local Ca²⁺ trigger fluxes in vivo. Our studies indicate that improved structural and functional models could improve our understanding of the contributions of L-type and Na⁺/Ca²⁺ exchanger fluxes to intracellular Ca²⁺ dynamics.

Na+/Ca2+ exchanger-1 protects against systolic failure in the Akitains2 model of diabetic cardiomyopathy via a CXCR4/NF-κB pathway

Diabetic cardiomyopathy is characterized, in part, by calcium handling imbalances associated with ventricular dysfunction. The cardiac Na(+)/Ca(2+) exchanger 1 (NCX1) has been implicated as a compensatory mechanism in response to reduced contractility in the heart; however, its role in diabetic cardiomyopathy remains unknown. We aimed to fully characterize the Akita(ins2) murine model of type 1 diabetes through assessing cardiac function and NCX1 regulation. The CXCL12/CXCR4 chemokine axis is well described in its cardioprotective effects via progenitor cell recruitment postacute myocardial infarction; however, it also functions in regulating calcium dependent processes in the cardiac myocyte. We therefore investigated the potential impact of CXCR4 in diabetic cardiomyopathy. Cardiac performance in the Akita(ins2) mouse was monitored using echocardiography and in vivo pressure-volume analysis. The Akita(ins2) mouse is protected against ventricular systolic failure evident at both 5 and 12 mo of age. However, the preserved contractility was associated with a decreased sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA2a)/phospholamban ratio and increased NCX1 content. Direct myocardial injection of adenovirus encoding anti-sense NCX1 significantly decreased NCX1 expression and induced systolic failure in the Akita(ins2) mouse. CXCL12 and CXCR4 were both upregulated in the Akita(ins2) heart, along with an increase in IκB-α and NF-κB p65 phosphorylation. We demonstrated that CXCR4 activation upregulates NCX1 expression through a NF-κB-dependent signaling pathway in the cardiac myocyte. In conclusion, the Akita(ins2) type 1 diabetic model is protected against systolic failure due to increased NCX1 expression. In addition, our studies reveal a novel role of CXCR4 in the diabetic heart by regulating NCX1 expression via a NF-κB-dependent mechanism.

Dynamic local changes in sarcoplasmic reticulum calcium: physiological and pathophysiological roles

Evidence obtained in recent years indicates that, in cardiac myocytes, release of Ca(2+) from the sarcoplasmic reticulum (SR) is regulated by changes in the concentration of Ca(2+) within the SR. In this review, we summarize recent advances in our understanding of this regulatory role, with a particular emphasis on dynamic and local changes in SR [Ca(2+)]. We focus on five important questions that are to some extent unresolved and controversial. These questions concern: (1) the importance of SR [Ca(2+)] depletion in the termination of Ca(2+) release; (2) the quantitative extent of depletion during local release events such as Ca(2+) sparks; (3) the influence of SR [Ca(2+)] refilling on release refractoriness and the propensity for pathological Ca(2+) release; (4) dynamic changes in SR [Ca(2+)] during propagating Ca(2+) waves; and (5) the speed of Ca(2+) diffusion within the SR. With each issue, we discuss data supporting alternative viewpoints, and we identify fundamental questions that are being actively investigated. We conclude with a discussion of experimental and computational advances that will help to resolve controversies. This article is part of a special issue entitled "Local Signaling in Myocytes."

The interaction of Ca2+ with sarcomeric proteins: role in function and dysfunction of the heart

The hallmarks of the normal heartbeat are both rapid onset of contraction and rapid relaxation as well as an inotropic response to both increased end-diastolic volume and increased heart rate. At the microscopic level, Ca(2+) plays a crucial role in normal cardiac contraction. This paper reviews the cycle of Ca(2+) fluxes during the normal heartbeat, which underlie the coupling between excitation and contraction and permit a highly synchronized action of cardiac sarcomeres. Length dependence of the response of the regulatory sarcomeric proteins mediates the Frank-Starling Law of the heart. However, Ca(2+) transport may go astray in heart disease such as in congestive heart failure, and both jeopardize systole and diastole and triggering arrhythmias. The interaction between weak and strong segments in nonuniform cardiac muscle allows partial preservation of force of contraction but may further lead to mechanoelectric feedback or reverse excitation-contraction coupling mediating an early diastolic Ca(2+) transient caused by the rapid force decrease during the relaxation phase. These rapid force changes in nonuniform muscle may cause arrhythmogenic Ca(2+) waves to propagate by the activation of neighboring sarcoplasmic reticulum by diffusing Ca(2+) ions.

A null mutation of the neuronal sodium channel NaV1.6 disrupts action potential propagation and excitation-contraction coupling in the mouse heart

Evidence supports the expression of brain-type sodium channels in the heart. Their functional role, however, remains controversial. We used global Na(V)1.6-null mice to test the hypothesis that Na(V)1.6 contributes to the maintenance of propagation in the myocardium and to excitation-contraction (EC) coupling. We demonstrated expression of transcripts encoding full-length Na(V)1.6 in isolated ventricular myocytes and confirmed the striated pattern of Na(V)1.6 fluorescence in myocytes. On the ECG, the PR and QRS intervals were prolonged in the null mice, and the Ca(2+) transients were longer in the null cells. Under patch clamping, at holding potential (HP) = -120 mV, the peak I(Na) was similar in both phenotypes. However, at HP = -70 mV, the peak I(Na) was smaller in the nulls. In optical mapping, at 4 mM [K(+)](o), 17 null hearts showed slight (7%) reduction of ventricular conduction velocity (CV) compared to 16 wild-type hearts. At 12 mM [K(+)](o), CV was 25% slower in a subset of 9 null vs. 9 wild-type hearts. These results highlight the importance of neuronal sodium channels in the heart, whereby Na(V)1.6 participates in EC coupling, and represents an intrinsic depolarizing reserve that contributes to excitation.

Colocalization of voltage-gated Na+ channels with the Na+/Ca2+ exchanger in rabbit cardiomyocytes during development

Reverse-mode activity of the Na+/Ca2+ exchanger (NCX) has been previously shown to play a prominent role in excitation-contraction coupling in the neonatal rabbit heart, where we have proposed that a restricted subsarcolemmal domain allows a Na+ current to cause an elevation in the Na+ concentration sufficiently large to bring Ca2+ into the myocyte through reverse-mode NCX. In the present study, we tested the hypothesis that there is an overlapping expression and distribution of voltage-gated Na+ (Nav) channel isoforms and the NCX in the neonatal heart. For this purpose, Western blot analysis, immunocytochemistry, confocal microscopy, and image analyses were used. Here, we report the robust expression of skeletal Nav1.4 and cardiac Nav1.5 in neonatal myocytes. Both isoforms colocalized with the NCX, and Nav1.5-NCX colocalization was not statistically different from Nav1.4-NCX colocalization in the neonatal group. Western blot analysis also showed that Nav1.4 expression decreased by sixfold in the adult ( P < 0.01) and Nav1.1 expression decreased by ninefold ( P < 0.01), whereas Nav1.5 expression did not change. Although Nav1.4 underwent large changes in expression levels, the Nav1.4-NCX colocalization relationship did not change with age. In contrast, Nav1.5-NCX colocalization decreased ∼50% with development. Distance analysis indicated that the decrease in Nav1.5-NCX colocalization occurs due to a statistically significant increase in separation distances between Nav1.5 and NCX objects. Taken together, the robust expression of both Nav1.4 and Nav1.5 isoforms and their colocalization with the NCX in the neonatal heart provides structural support for Na+ current-induced Ca2+ entry through reverse-mode NCX. In contrast, this mechanism is likely less efficient in the adult heart because the expression of Nav1.4 and NCX is lower and the separation distance between Nav1.5 and NCX is larger.