[Na(+)] in the subsarcolemmal 'fuzzy' space and modulation of [Ca(2+)](i) and contraction in cardiac myocytes

Longitudinal diffusion barriers imposed by myofilaments and mitochondria in murine cardiac myocytes

Using optical and electrical methods, we document that diffusion in the cytoplasm of BL6 murine cardiomyocytes becomes restricted >20-fold as molecular weight increases from 30 to 2,000, roughly as expected for pores with porin channel dimensions. Bodipy-FL ATP diffuses >40-fold slower than in free water at 25°C. From several fluorophores analyzed, bound fluorophore fractions range from 0.1 for a 2 kD FITC-labeled polyethylene glycol to 0.93 for sulforhodamine. Unbound fluorophores diffuse at 0.5-8 × 10-7 cm2/s (5-80 μm2/s). Analysis of Na/K pump and veratridine-modified Na channel currents suggests that Na diffusion is nearly unrestricted at 35°C (time constant for equilibration with the pipette tip, ∼20 s). Using multiple strategies, we estimate that at 35°C, ATP diffuses four to eight times slower than in free water. To address whether restrictions are caused more by protein or membrane networks, we verified first that a protein gel, 10 g% gelatin, restricts diffusion with strong dependence on molecular weight. Solute diffusion in membrane-extracted cardiac myofilaments, confined laterally by suction into large-diameter pipette tips, is less restricted than in intact myocytes. Notably, myofilaments extracted similarly from skeletal (diaphragm) myocytes are less restrictive. Solute diffusion in myocytes with sarcolemma permeabilized by β-escin (80 µM) is similar to diffusion in intact myocytes. Restrictions are strain-dependent, being twofold greater in BL6 myocytes than in CD1/J6/129svJ myocytes. Furthermore, longitudinal diffusion is 2.5-fold more restricted in CD1/J6/129svJ myocytes lacking the mitochondrial porin, VDAC1, than in WT CD1/J6/129svJ myocytes. Thus, mitochondria networks restrict long-range diffusion while presumably optimizing nucleotide transfer between myofilaments and mitochondria. We project that diffusion restrictions imposed by both myofilaments and the outer mitochondrial membrane are important determinants of total free cytoplasmic AMP and ADP (∼10 μM). However, the capacity of diffusion to deliver ATP to myofilaments remains ∼100-fold greater than ATP consumption.

A detailed mathematical model of the human atrial cardiomyocyte: integration of electrophysiology and cardiomechanics

Mechano-electric regulations (MER) play an important role in the maintenance of cardiac performance. Mechano-calcium and mechano-electric feedback (MCF and MEF) pathways adjust the cardiomyocyte contractile force according to mechanical perturbations and affects electro-mechanical coupling. MER integrates all these regulations in one unit resulting in a complex phenomenon. Computational modelling is a useful tool to accelerate the mechanistic understanding of complex experimental phenomena. We have developed a novel model that integrates the MER loop for human atrial cardiomyocytes with proper consideration of feedforward and feedback pathways. The model couples a modified version of the action potential (AP) Koivumäki model with the contraction model by Quarteroni group. The model simulates iso-sarcometric and isometric twitches and the feedback effects on AP and Ca2+ -handling. The model showed a biphasic response of Ca2+ transient (CaT) peak to increasing pacing rates and highlights the possible mechanisms involved. The model has shown a shift of the threshold for AP and CaT alternans from 4.6 to 4 Hz under post-operative atrial fibrillation, induced by depressed SERCA activity. The alternans incidence was dependent on a chain of mechanisms including RyRs availability time, MCF coupling, CaMKII phosphorylation, and the stretch levels. As a result, the model predicted a 10% slowdown of conduction velocity for a 20% stretch, suggesting a role of stretch in creation of substrate formation for atrial fibrillation. Overall, we conclude that the developed model provides a physiological CaT followed by a physiological twitch. This model can open pathways for the future studies of human atrial electromechanics. KEY POINTS: With the availability of human atrial cellular data, interest in atrial-specific model integration has been enhanced. We have developed a detailed mathematical model of human atrial cardiomyocytes including the mechano-electric regulatory loop. The model has gone through calibration and evaluation phases against a wide collection of available human in-vitro data. The usefulness of the model for analysing clinical problems has been preliminaryly tested by simulating the increased incidence of Ca2+ transient and action potential alternans at high rates in post-operative atrial fibrillation condition. The model determines the possible role of mechano-electric feedback in alternans incidence, which can increase vulnerability to atrial arrhythmias by varying stretch levels. We found that our physiologically accurate description of Ca2+ handling can reproduce many experimental phenomena and can help to gain insights into the underlying pathophysiological mechanisms.

Pore-like diffusion barriers in murine cardiac myocytes

Using both optical and electrical methods, we document that solute diffusion in the cytoplasm of BL6 murine cardiac myocytes becomes restricted >30-fold as molecular weight increases from 30 to 2000, roughly as expected for pores with dimensions of cardiac porin channels. The Bodipy-FL ATP analogue diffuses ∼50-fold slower in BL6 cardiac cytoplasm than in free water. From several fluorophores analyzed, our estimates of bound fluorophore fractions range from 0.1 for a 2 kD FITC-labeled polyethylene glycol to 0.93 for sulforhodamine. We estimate that diffusion coefficients of unbound fluorophores range from 0.5 to 8 x 10 -7 cm 2 /s. Analysis of Na/K pump and veratridine-modified Na channel currents confirms that Na diffusion is nearly unrestricted (time constant for equilibration with the pipette tip, ∼20 s). Using three different approaches, we estimate that ATP diffuses 8 to 10-times slower in the cytoplasm of BL6 myocytes than in free water. To address whether restrictions are caused more by cytoplasmic protein or membrane networks, we verified first that a protein gel, 10 gram% gelatin, restricts solute diffusion with strong dependence on molecular weight. Solute diffusion in membrane-extracted cardiac myofilaments, confined laterally by suction into large-diameter pipette tips, is however less restricted than in intact myocytes. Notably, myofilaments from equivalently extracted skeletal (diaphragm) myocytes restrict diffusion less than cardiac myofilaments. Solute diffusion in myocytes with sarcolemma permeabilized by β-escin (80 µM) is similarly restricted as in intact myocytes. Diffusion restriction in cardiac myocytes is strain-dependent, being about two-fold greater in BL6 myocytes than in myocytes with a CD1/J6/129svJ background. Furthermore, diffusion is 2.5-fold more restricted in CD1/J6/129svJ myocytes lacking the mitochondrial porin, Vdac1, than in WT CD1/J6/129svJ myocytes. We conclude that both myofilaments and mitochondria networks restrict diffusion in cardiac myocytes. As a result, long-range solute diffusion may preferentially occur via passage through porin channels and intramembrane mitochondrial spaces, where diffusion is less restricted than in myofilament spaces.

Left ventricular dysfunction in heart failure with preserved ejection fraction-molecular mechanisms and impact on right ventricular function

The current classification of heart failure (HF) based on left ventricular (LV) ejection fraction (EF) identifies a large group of patients with preserved ejection fraction (HFpEF) with significant morbidity and mortality but without prognostic benefit from current HF therapy. Co-morbidities and conditions such as arterial hypertension, diabetes mellitus, chronic kidney disease, adiposity and aging shape the clinical phenotype and contribute to mortality. LV diastolic dysfunction and LV structural remodeling are hallmarks of HFpEF, and are linked to remodeling of the cardiomyocyte and extracellular matrix. Pulmonary hypertension (PH) and right ventricular dysfunction (RVD) are particularly common in HFpEF, and mortality is up to 10-fold higher in HFpEF patients with vs. without RV dysfunction. Here, we review alterations in cardiomyocyte function (i.e., ion homeostasis, sarcomere function and cellular metabolism) associated with diastolic dysfunction and summarize the main underlying cellular pathways. The contribution and interaction of systemic and regional upstream signaling such as chronic inflammation, neurohumoral activation, and NO-cGMP-related pathways are outlined in detail, and their diagnostic and therapeutic potential is discussed in the context of preclinical and clinical studies. In addition, we summarize prevalence and pathomechanisms of RV dysfunction in the context of HFpEF and discuss mechanisms connecting LV and RV dysfunction in HFpEF. Dissecting the molecular mechanisms of LV and RV dysfunction in HFpEF may provide a basis for an improved classification of HFpEF and for therapeutic approaches tailored to the molecular phenotype.

Co-localization and confinement of ecto-nucleotidases modulate extracellular adenosine nucleotide distributions

Nucleotides comprise small molecules that perform critical signaling roles in biological systems. Adenosine-based nucleotides, including adenosine tri-, di-, and mono-phosphate, are controlled through their rapid degradation by diphosphohydrolases and ecto-nucleotidases (NDAs). The interplay between nucleotide signaling and degradation is especially important in synapses formed between cells, which create signaling 'nanodomains'. Within these 'nanodomains', charged nucleotides interact with densely-packed membranes and biomolecules. While the contributions of electrostatic and steric interactions within such nanodomains are known to shape diffusion-limited reaction rates, less is understood about how these factors control the kinetics of nucleotidase activity. To quantify these factors, we utilized reaction-diffusion numerical simulations of 1) adenosine triphosphate (ATP) hydrolysis into adenosine monophosphate (AMP) and 2) AMP into adenosine (Ado) via two representative nucleotidases, CD39 and CD73. We evaluate these sequentially-coupled reactions in nanodomain geometries representative of extracellular synapses, within which we localize the nucleotidases. With this model, we find that 1) nucleotidase confinement reduces reaction rates relative to an open (bulk) system, 2) the rates of AMP and ADO formation are accelerated by restricting the diffusion of substrates away from the enzymes, and 3) nucleotidase co-localization and the presence of complementary (positive) charges to ATP enhance reaction rates, though the impact of these contributions on nucleotide pools depends on the degree to which the membrane competes for substrates. As a result, these contributions integratively control the relative concentrations and distributions of ATP and its metabolites within the junctional space. Altogether, our studies suggest that CD39 and CD73 nucleotidase activity within junctional spaces can exploit their confinement and favorable electrostatic interactions to finely control nucleotide signaling.

Hypokalemia Promotes Arrhythmia by Distinct Mechanisms in Atrial and Ventricular Myocytes

RATIONALE

Hypokalemia occurs in up to 20% of hospitalized patients and is associated with increased incidence of ventricular and atrial fibrillation. It is unclear whether these differing types of arrhythmia result from direct and perhaps distinct effects of hypokalemia on cardiomyocytes.

OBJECTIVE

To investigate proarrhythmic mechanisms of hypokalemia in ventricular and atrial myocytes.

METHODS AND RESULTS

Experiments were performed in isolated rat myocytes exposed to simulated hypokalemia conditions (reduction of extracellular [K⁺] from 5.0 to 2.7 mmol/L) and supported by mathematical modeling studies. Ventricular cells subjected to hypokalemia exhibited Ca²⁺ overload and increased generation of both spontaneous Ca²⁺ waves and delayed afterdepolarizations. However, similar Ca²⁺-dependent spontaneous activity during hypokalemia was only observed in a minority of atrial cells that were observed to contain t-tubules. This effect was attributed to close functional pairing of the Na⁺-K⁺ ATPase and Na⁺-Ca²⁺ exchanger proteins within these structures, as reduction in Na⁺ pump activity locally inhibited Ca²⁺ extrusion. Ventricular myocytes and tubulated atrial myocytes additionally exhibited early afterdepolarizations during hypokalemia, associated with Ca²⁺ overload. However, early afterdepolarizations also occurred in untubulated atrial cells, despite Ca²⁺ quiescence. These phase-3 early afterdepolarizations were rather linked to reactivation of nonequilibrium Na⁺ current, as they were rapidly blocked by tetrodotoxin. Na⁺ current-driven early afterdepolarizations in untubulated atrial cells were enabled by membrane hyperpolarization during hypokalemia and short action potential configurations. Brief action potentials were in turn maintained by ultra-rapid K⁺ current (I_Kur); a current which was found to be absent in tubulated atrial myocytes and ventricular myocytes.

CONCLUSIONS

Distinct mechanisms underlie hypokalemia-induced arrhythmia in the ventricle and atrium but also vary between atrial myocytes depending on subcellular structure and electrophysiology.

Sodium-activated potassium channels moderate excitability in vascular smooth muscle

KEY POINTS

We report that a sodium-activated potassium current, IKNa , has been inadvertently overlooked in both conduit and resistance arterial smooth muscle cells. IKNa is a major K+ resting conductance and is absent in cells of IKNa knockout (KO) mice. The phenotype of the IKNa KO is mild hypertension, although KO mice react more strongly than wild-type with raised blood pressure when challenged with vasoconstrictive agents. IKNa is negatively regulated by angiotensin II acting through Gαq protein-coupled receptors. In current clamp, KO arterial smooth muscle cells have easily evoked Ca2+ -dependent action potentials.

ABSTRACT

Although several potassium currents have been reported to play a role in arterial smooth muscle (ASM), we find that one of the largest contributors to membrane conductance in both conduit and resistance ASMs has been inadvertently overlooked. In the present study, we show that IKNa , a sodium-activated potassium current, contributes a major portion of macroscopic outward current in a critical physiological voltage range that determines intrinsic cell excitability; IKNa is the largest contributor to ASM cell resting conductance. A genetic knockout (KO) mouse strain lacking KNa channels (KCNT1 and KCNT2) shows only a modest hypertensive phenotype. However, acute administration of vasoconstrictive agents such as angiotensin II (Ang II) and phenylephrine results in an abnormally large increase in blood pressure in the KO animals. In wild-type animals Ang II acting through Gαq protein-coupled receptors down-regulates IKNa , which increases the excitability of the ASMs. The complete genetic removal of IKNa in KO mice makes the mutant animal more vulnerable to vasoconstrictive agents, thus producing a paroxysmal-hypertensive phenotype. This may result from the lowering of cell resting K+ conductance allowing the cells to depolarize more readily to a variety of excitable stimuli. Thus, the sodium-activated potassium current may serve to moderate blood pressure in instances of heightened stress. IKNa may represent a new therapeutic target for hypertension and stroke.

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.

Evidence for heterogeneous subsarcolemmal Na+ levels in rat ventricular myocytes

The intracellular Na⁺ concentration ([Na⁺]) regulates cardiac contractility. Previous studies have suggested that subsarcolemmal [Na⁺] is higher than cytosolic [Na⁺] in cardiac myocytes, but this concept remains controversial. Here, we used electrophysiological experiments and mathematical modeling to test whether there are subsarcolemmal pools with different [Na⁺] and dynamics compared with the bulk cytosol in rat ventricular myocytes. A Na⁺ dependency curve for Na⁺-K⁺-ATPase (NKA) current was recorded with symmetrical Na⁺ solutions, i.e., the same [Na⁺] in the superfusate and internal solution. This curve was used to estimate [Na⁺] sensed by NKA in other experiments. Three experimental observations suggested that [Na⁺] is higher near NKA than in the bulk cytosol: 1) when extracellular [Na⁺] was high, [Na⁺] sensed by NKA was ~6 mM higher than the internal solution in quiescent cells; 2) long trains of Na⁺ channel activation almost doubled this gradient; compared with an even intracellular distribution of Na⁺, the increase of [Na⁺] sensed by NKA was 10 times higher than expected, suggesting a local Na⁺ domain; and 3) accumulation of Na⁺ near NKA after trains of Na⁺ channel activation dissipated very slowly. Finally, mathematical models assuming heterogeneity of [Na⁺] between NKA and the Na⁺ channel better reproduced experimental data than the homogeneous model. In conclusion, our data suggest that NKA-sensed [Na⁺] is higher than [Na⁺] in the bulk cytosol and that there are differential Na⁺ pools in the subsarcolemmal space, which could be important for cardiac contractility and arrhythmogenesis. NEW & NOTEWORTHY Our data suggest that the Na⁺-K⁺-ATPase-sensed Na⁺ concentration is higher than the Na⁺ concentration in the bulk cytosol and that there are differential Na⁺ pools in the subsarcolemmal space, which could be important for cardiac contractility and arrhythmogenesis. Listen to this article's corresponding podcast at https://ajpheart.podbean.com/e/heterogeneous-sodium-in-ventricular-myocytes/ .

Application of Spectral Phasor analysis to sodium microenvironments in myoblast progenitor cells

Sodium ions (Na+) are key regulators of molecular events in many cellular processes, yet the dynamics of this ion remain poorly defined. Developing approaches to identify and characterise Na+ microenvironments will enable more detailed elucidation of the mechanisms of signal transduction. Here we report the application of Spectral Phasor analysis to the Na+ fluorophore, CoroNa Green, to identify and spatially map spectral emissions that report Na+ microenvironments. We use differentiating stem cells where Na+ fluxes were reported as an antecedent. Myoblast stem cells were induced to differentiate by serum starvation and then fixed at intervals between 0 and 40-minutes of differentiation prior to addition of CoroNa Green. The fluorescent intensity was insufficient to identify discrete Na+ microenvironments. However, using Spectral Phasor analysis we identified spectral shifts in CoroNa Green fluorescence which is related to the Na+ microenvironment. Further, spectral-heterogeneity appears to be contingent on the distance of Na+ from the nucleus in the early stages of differentiation. Spectral Phasor analysis of CoroNa Green in fixed stem cells demonstrates for the first time that CoroNa Green has unique spectral emissions depending on the nature of the Na+ environment in differentiating stem cells. Applying Spectral Phasor analysis to CoroNa Green in live stem cells is likely to further elucidate the role of Na+ microenvironments in the differentiation process.

How does pressure overload cause cardiac hypertrophy and dysfunction? High-ouabain affinity cardiac Na+ pumps are crucial

Left ventricular hypertrophy is frequently observed in hypertensive patients and is believed to be due to the pressure overload and cardiomyocyte stretch. Three recent reports on mice with genetically engineered Na⁺ pumps, however, have demonstrated that cardiac ouabain-sensitive α₂-Na⁺ pumps play a key role in the pathogenesis of transaortic constriction-induced hypertrophy. Hypertrophy was delayed/attenuated in mice with mutant, ouabain-resistant α₂-Na⁺ pumps and in mice with cardiac-selective knockout or transgenic overexpression of α₂-Na⁺ pumps. The latter, seemingly paradoxical, findings can be explained by comparing the numbers of available (ouabain-free) high-affinity (α₂) ouabain-binding sites in wild-type, knockout, and transgenic hearts. Conversely, hypertrophy was accelerated in α₂-ouabain-resistant (R) mice in which the normally ouabain-resistant α₁-Na⁺ pumps were mutated to an ouabain-sensitive (S) form (α₁^S/Sα₂^R/R or "SWAP" vs. wild-type or α₁^R/R α₂^S/S mice). Furthermore, transaortic constriction-induced hypertrophy in SWAP mice was prevented/reversed by immunoneutralizing circulating endogenous ouabain (EO). These findings show that EO and its receptor, ouabain-sensitive α₂, are critical factors in pressure overload-induced cardiac hypertrophy. This complements reports linking elevated plasma EO to hypertension, cardiac hypertrophy, and failure in humans and elucidates the underappreciated role of the EO-Na⁺ pump pathway in cardiovascular disease.

Cardiac striatin interacts with caveolin-3 and calmodulin in a calcium sensitive manner and regulates cardiomyocyte spontaneous contraction rate

Impaired cardiomyocyte contraction rate is detrimental to cardiac function and often lethal. Despite advancements in the field, there is a paucity of information regarding the coordination of molecules implicated in regulating the heart rate. Striatin (STRN) is a dynamic protein with binding domains to calmodulin (CaM) and caveolin (Cav), both of which are regulators of myocardial function. However, its role in cardiomyocyte contraction is not yet determined. Herein, we show that STRN is expressed in cardiomyocytes and is more abundant in atrial myocardium than in ventricles. Cardiac expression of STRN (protein and mRNA) was developmentally regulated with the highest expression being at neonatal stage (day one) and the lowest in adult rats (13 weeks). CaM pulldown assay indicated that the interaction of cardiac STRN with CaM and caveolin-3 (Cav-3) was calcium sensitive. Interestingly, the overexpression of STRN induced an increase (∼2-fold) in the rate of the spontaneous contraction of cultured cardiomyocytes, while the knockdown of STRN reduced their contraction rate (∼40%). The expression level of STRN was inversely proportional to the interaction of Cav-3 with the CaM/STRN complex. Collectively, our data delineate a novel role for STRN in regulating cardiomyocyte spontaneous contraction rate and the dynamics of the STRN/Cav-3/CaM complex.

The role of spatial organization of Ca2+ release sites in the generation of arrhythmogenic diastolic Ca2+ release in myocytes from failing hearts

In heart failure (HF), dysregulated cardiac ryanodine receptors (RyR2) contribute to the generation of diastolic Ca2+ waves (DCWs), thereby predisposing adrenergically stressed failing hearts to life-threatening arrhythmias. However, the specific cellular, subcellular, and molecular defects that account for cardiac arrhythmia in HF remain to be elucidated. Patch-clamp techniques and confocal Ca2+ imaging were applied to study spatially defined Ca2+ handling in ventricular myocytes isolated from normal (control) and failing canine hearts. Based on their activation time upon electrical stimulation, Ca2+ release sites were categorized as coupled, located in close proximity to the sarcolemmal Ca2+ channels, and uncoupled, the Ca2+ channel-free non-junctional Ca2+ release units. In control myocytes, stimulation of β-adrenergic receptors with isoproterenol (Iso) resulted in a preferential increase in Ca2+ spark rate at uncoupled sites. This site-specific effect of Iso was eliminated by the phosphatase inhibitor okadaic acid, which caused similar facilitation of Ca2+ sparks at coupled and uncoupled sites. Iso-challenged HF myocytes exhibited increased predisposition to DCWs compared to control myocytes. In addition, the overall frequency of Ca2+ sparks was increased in HF cells due to preferential stimulation of coupled sites. Furthermore, coupled sites exhibited accelerated recovery from functional refractoriness in HF myocytes compared to control myocytes. Spatially resolved subcellular Ca2+ mapping revealed that DCWs predominantly originated from coupled sites. Inhibition of CaMKII suppressed DCWs and prevented preferential stimulation of coupled sites in Iso-challenged HF myocytes. These results suggest that CaMKII- (and phosphatase)-dependent dysregulation of junctional Ca2+ release sites contributes to Ca2+-dependent arrhythmogenesis in HF.

No fuzzy space for intracellular Na+ in healthy ventricular myocytes

Sachse et al. highlight work that reveals a Na⁺-dependent inactivation mechanism in the Na⁺/K⁺ pump.

Na/K pump inactivation, subsarcolemmal Na measurements, and cytoplasmic ion turnover kinetics contradict restricted Na spaces in murine cardiac myocytes

The Na/K pump exports cytoplasmic Na ions while importing K ions, and its activity is thought to be affected by restricted intracellular Na diffusion in cardiac myocytes. Lu and Hilgemann find instead that the pump can enter an inactivated state and that inactivation can be relieved by cytoplasmic Na.

Decades ago, it was proposed that Na transport in cardiac myocytes is modulated by large changes in cytoplasmic Na concentration within restricted subsarcolemmal spaces. Here, we probe this hypothesis for Na/K pumps by generating constitutive transsarcolemmal Na flux with the Na channel opener veratridine in whole-cell patch-clamp recordings. Using 25 mM Na in the patch pipette, pump currents decay strongly during continuous activation by extracellular K (τ, ∼2 s). In contradiction to depletion hypotheses, the decay becomes stronger when pump currents are decreased by hyperpolarization. Na channel currents are nearly unchanged by pump activity in these conditions, and conversely, continuous Na currents up to 0.5 nA in magnitude have negligible effects on pump currents. These outcomes are even more pronounced using 50 mM Li as a cytoplasmic Na congener. Thus, the Na/K pump current decay reflects mostly an inactivation mechanism that immobilizes Na/K pump charge movements, not cytoplasmic Na depletion. When channel currents are increased beyond 1 nA, models with unrestricted subsarcolemmal diffusion accurately predict current decay (τ ∼15 s) and reversal potential shifts observed for Na, Li, and K currents through Na channels opened by veratridine, as well as for Na, K, Cs, Li, and Cl currents recorded in nystatin-permeabilized myocytes. Ion concentrations in the pipette tip (i.e., access conductance) track without appreciable delay the current changes caused by sarcolemmal ion flux. Importantly, cytoplasmic mixing volumes, calculated from current decay kinetics, increase and decrease as expected with osmolarity changes (τ >30 s). Na/K pump current run-down over 20 min reflects a failure of pumps to recover from inactivation. Simulations reveal that pump inactivation coupled with Na-activated recovery enhances the rapidity and effectivity of Na homeostasis in cardiac myocytes. In conclusion, an autoregulatory mechanism enhances cardiac Na/K pump activity when cytoplasmic Na rises and suppresses pump activity when cytoplasmic Na declines.

Changes in Intracellular Na+ following Enhancement of Late Na+ Current in Virtual Human Ventricular Myocytes

The slowly inactivating or late Na+ current, INa-L, can contribute to the initiation of both atrial and ventricular rhythm disturbances in the human heart. However, the cellular and molecular mechanisms that underlie these pro-arrhythmic influences are not fully understood. At present, the major working hypothesis is that the Na+ influx corresponding to INa-L significantly increases intracellular Na+, [Na+]i; and the resulting reduction in the electrochemical driving force for Na+ reduces and (may reverse) Na+/Ca2+ exchange. These changes increase intracellular Ca2+, [Ca2+]i; which may further enhance INa-L due to calmodulin-dependent phosphorylation of the Na+ channels. This paper is based on mathematical simulations using the O'Hara et al (2011) model of baseline or healthy human ventricular action potential waveforms(s) and its [Ca2+]i homeostasis mechanisms. Somewhat surprisingly, our results reveal only very small changes (≤ 1.5 mM) in [Na+]i even when INa-L is increased 5-fold and steady-state stimulation rate is approximately 2 times the normal human heart rate (i.e. 2 Hz). Previous work done using well-established models of the rabbit and human ventricular action potential in heart failure settings also reported little or no change in [Na+]i when INa-L was increased. Based on our simulations, the major short-term effect of markedly augmenting INa-L is a significant prolongation of the action potential and an associated increase in the likelihood of reactivation of the L-type Ca2+ current, ICa-L. Furthermore, this action potential prolongation does not contribute to [Na+]i increase.

Profound regulation of Na/K pump activity by transient elevations of cytoplasmic calcium in murine cardiac myocytes

Small changes of Na/K pump activity regulate internal Ca release in cardiac myocytes via Na/Ca exchange. We now show conversely that transient elevations of cytoplasmic Ca strongly regulate cardiac Na/K pumps. When cytoplasmic Na is submaximal, Na/K pump currents decay rapidly during extracellular K application and multiple results suggest that an inactivation mechanism is involved. Brief activation of Ca influx by reverse Na/Ca exchange enhances pump currents and attenuates current decay, while repeated Ca elevations suppress pump currents. Pump current enhancement reverses over 3 min, and results are similar in myocytes lacking the regulatory protein, phospholemman. Classical signaling mechanisms, including Ca-activated protein kinases and reactive oxygen, are evidently not involved. Electrogenic signals mediated by intramembrane movement of hydrophobic ions, such as hexyltriphenylphosphonium (C6TPP), increase and decrease in parallel with pump currents. Thus, transient Ca elevation and Na/K pump inactivation cause opposing sarcolemma changes that may affect diverse membrane processes.

Pivotal role of α2 Na+ pumps and their high affinity ouabain binding site in cardiovascular health and disease

Reduced smooth muscle (SM)-specific α2 Na⁺ pump expression elevates basal blood pressure (BP) and increases BP sensitivity to angiotensin II (Ang II) and dietary NaCl, whilst SM-α2 overexpression lowers basal BP and decreases Ang II/salt sensitivity. Prolonged ouabain infusion induces hypertension in rodents, and ouabain-resistant mutation of the α2 ouabain binding site (α2^R/R mice) confers resistance to several forms of hypertension. Pressure overload-induced heart hypertrophy and failure are attenuated in cardio-specific α2 knockout, cardio-specific α2 overexpression and α2^R/R mice. We propose a unifying hypothesis that reconciles these apparently disparate findings: brain mechanisms, activated by Ang II and high NaCl, regulate sympathetic drive and a novel neurohumoral pathway mediated by both brain and circulating endogenous ouabain (EO). Circulating EO modulates ouabain-sensitive α2 Na⁺ pump activity and Ca²⁺ transporter expression and, via Na⁺ /Ca²⁺ exchange, Ca²⁺ homeostasis. This regulates sensitivity to sympathetic activity, Ca²⁺ signalling and arterial and cardiac contraction.

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.

Induced NCX1 overexpression attenuates pressure overload-induced pathological cardiac remodelling

AIMS

Although increased Na(+)/Ca(2+) exchanger 1 (NCX1) expression is observed during heart failure (HF), the pathological role of NCX1 during the progression of HF remains unclear. We examined alterations of NCX1 expression and activity in hearts after transverse aortic constriction (TAC) surgery and explored whether NCX1 influences pressure overload-induced pathological cardiac remodelling.

METHODS AND RESULTS

We generated novel transgenic mice in which NCX1 expression is controlled by a cardiac-specific, doxycycline (DOX)-dependent promoter. In the absence of DOX, TAC surgery caused substantial chamber dilation with a gradual decrease in contractility by 16 weeks. Cardiomyocytes showed a decline in contractility with abnormal Ca(2+) handling during excitation-contraction (E-C) coupling. Reduced NCX1 activity was observed 8 weeks after TAC and was still apparent at 17 weeks. Induced NCX1 overexpression by DOX treatment starting 8 weeks after TAC returned NCX1 activity to pre-TAC levels and prevented chamber dilation with cardiac dysfunction. DOX treatment not only upregulated NCX1 expression in TAC-operated hearts but also returned L-type Ca(2+) channel and sarcoplasmic reticulum (SR) Ca(2+) ATPase expression levels to those in sham-operated hearts. In DOX-treated myocytes, contractility, T-tubule integrity, synchrony of Ca(2+) release from the SR, and Ca(2+) handling during E-C coupling was preserved 16 weeks after TAC surgery. In addition, DOX treatment attenuated the down-regulation of survival signalling and up-regulation of apoptosis signalling 16 weeks after TAC surgery.

CONCLUSION

Induced overexpression of NCX1 attenuated pressure overload-induced pathological cardiac remodelling. Thus, maintaining NCX1 activity may be a potential therapeutic strategy for preventing the progression of HF.

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.

Selective Na(+) /Ca(2+) exchanger inhibition prevents Ca(2+) overload-induced triggered arrhythmias

BACKGROUND AND PURPOSE

Augmented Na(+) /Ca(2+) exchanger (NCX) activity may play a crucial role in cardiac arrhythmogenesis; however, data regarding the anti-arrhythmic efficacy of NCX inhibition are debatable. Feasible explanations could be the unsatisfactory selectivity of NCX inhibitors and/or the dependence of the experimental model on the degree of Ca(2+) i overload. Hence, we used NCX inhibitors SEA0400 and the more selective ORM10103 to evaluate the efficacy of NCX inhibition against arrhythmogenic Ca(2+) i rise in conditions when [Ca(2+) ]i was augmented via activation of the late sodium current (INaL ) or inhibition of the Na(+) /K(+) pump.

EXPERIMENTAL APPROACH

Action potentials (APs) were recorded from canine papillary muscles and Purkinje fibres by microelectrodes. NCX current (INCX ) was determined in ventricular cardiomyocytes utilizing the whole-cell patch clamp technique. Ca(2+) i transients (CaTs) were monitored with a Ca(2+) -sensitive fluorescent dye, Fluo-4.

KEY RESULTS

Enhanced INaL increased the Ca(2+) load and AP duration (APD). SEA0400 and ORM10103 suppressed INCX and prevented/reversed the anemone toxin II (ATX-II)-induced [Ca(2+) ]i rise without influencing APD, CaT or cell shortening, or affecting the ATX-II-induced increased APD. ORM10103 significantly decreased the number of strophanthidin-induced spontaneous diastolic Ca(2+) release events; however, SEA0400 failed to restrict the veratridine-induced augmentation in Purkinje-ventricle APD dispersion.

CONCLUSIONS AND IMPLICATIONS

Selective NCX inhibition - presumably by blocking rev INCX (reverse mode NCX current) - is effective against arrhythmogenesis caused by [Na(+) ]i -induced [Ca(2+) ]i elevation, without influencing the AP waveform. Therefore, selective INCX inhibition, by significantly reducing the arrhythmogenic trigger activity caused by the perturbed Ca(2+) i handling, should be considered as a promising anti-arrhythmic therapeutic strategy.

Identification of caveolar resident proteins in ventricular myocytes using a quantitative proteomic approach: dynamic changes in caveolar composition following adrenoceptor activation

The lipid raft concept proposes that membrane environments enriched in cholesterol and sphingolipids cluster certain proteins and form platforms to integrate cell signaling. In cardiac muscle, caveolae concentrate signaling molecules and ion transporters, and play a vital role in adrenergic regulation of excitation–contraction coupling, and consequently cardiac contractility. Proteomic analysis of cardiac caveolae is hampered by the presence of contaminants that have sometimes, erroneously, been proposed to be resident in these domains. Here we present the first unbiased analysis of the proteome of cardiac caveolae, and investigate dynamic changes in their protein constituents following adrenoreceptor (AR) stimulation.

Rat ventricular myocytes were treated with methyl-β-cyclodextrin (MβCD) to deplete cholesterol and disrupt caveolae. Buoyant caveolin-enriched microdomains (BCEMs) were prepared from MβCD-treated and control cell lysates using a standard discontinuous sucrose gradient. BCEMs were harvested, pelleted, and resolubilized, then alkylated, digested, and labeled with iTRAQ reagents, and proteins identified by LC-MS/MS on a LTQ Orbitrap Velos Pro. Proteins were defined as BCEM resident if they were consistently depleted from the BCEM fraction following MβCD treatment. Selective activation of α-, β1-, and β2-AR prior to preparation of BCEMs was achieved by application of agonist/antagonist pairs for 10 min in populations of field-stimulated myocytes.

We typically identified 600–850 proteins per experiment, of which, 249 were defined as high-confidence BCEM residents. Functional annotation clustering indicates cardiac BCEMs are enriched in integrin signaling, guanine nucleotide binding, ion transport, and insulin signaling clusters. Proteins possessing a caveolin binding motif were poorly enriched in BCEMs, suggesting this is not the only mechanism that targets proteins to caveolae. With the notable exception of the cavin family, very few proteins show altered abundance in BCEMs following AR activation, suggesting signaling complexes are preformed in BCEMs to ensure a rapid and high fidelity response to adrenergic stimulation in cardiac muscle.

Scn1b deletion leads to increased tetrodotoxin-sensitive sodium current, altered intracellular calcium homeostasis and arrhythmias in murine hearts

KEY POINTS

Na(+) current (INa) results from the integrated function of a molecular aggregate (the voltage-gated Na(+) channel complex) that includes the β subunit family. Mutations or rare variants in Scn1b (encoding the β1 and β1B subunits) have been associated with various inherited arrhythmogenic syndromes, including Brugada syndrome and sudden unexpected death in patients with epilepsy. We used Scn1b null mice to understand better the relation between Scn1b expression, and cardiac electrical function. Loss of Scn1b caused, among other effects, increased amplitude of tetrodotoxin-sensitive INa, delayed after-depolarizations, triggered beats, delayed Ca(2+) transients, frequent spontaneous calcium release events and increased susceptibility to polymorphic ventricular arrhythmias. Most alterations in Ca(2+) homeostasis were prevented by 100 nM tetrodotoxin. We propose that life-threatening arrhythmias in patients with mutations in Scn1b, a gene classically defined as ancillary to the Na(+) channel α subunit, can be partly consequent to disrupted intracellular Ca(2+) homeostasis.

ABSTRACT

Na(+) current (INa) is determined not only by the properties of the pore-forming voltage-gated Na(+) channel (VGSC) α subunit, but also by the integrated function of a molecular aggregate (the VGSC complex) that includes the VGSC β subunit family. Mutations or rare variants in Scn1b (encoding the β1 and β1B subunits) have been associated with various inherited arrhythmogenic syndromes, including cases of Brugada syndrome and sudden unexpected death in patients with epilepsy. Here, we have used Scn1b null mouse models to understand better the relation between Scn1b expression, and cardiac electrical function. Using a combination of macropatch and scanning ion conductance microscopy we show that loss of Scn1b in juvenile null animals resulted in increased tetrodotoxin-sensitive INa but only in the cell midsection, even before full T-tubule formation; the latter occurred concurrent with increased message abundance for the neuronal Scn3a mRNA, suggesting increased abundance of tetrodotoxin-sensitive NaV 1.3 protein and yet its exclusion from the region of the intercalated disc. Ventricular myocytes from cardiac-specific adult Scn1b null animals showed increased Scn3a message, prolonged action potential repolarization, presence of delayed after-depolarizations and triggered beats, delayed Ca(2+) transients and frequent spontaneous Ca(2+) release events and at the whole heart level, increased susceptibility to polymorphic ventricular arrhythmias. Most alterations in Ca(2+) homeostasis were prevented by 100 nM tetrodotoxin. Our results suggest that life-threatening arrhythmias in patients with mutations in Scn1b, a gene classically defined as ancillary to the Na(+) channel α subunit, can be partly consequent to disrupted intracellular Ca(2+) homeostasis in ventricular myocytes.

TRPV2 is critical for the maintenance of cardiac structure and function in mice

The heart has a dynamic compensatory mechanism for haemodynamic stress. However, the molecular details of how mechanical forces are transduced in the heart are unclear. Here we show that the transient receptor potential, vanilloid family type 2 (TRPV2) cation channel is critical for the maintenance of cardiac structure and function. Within 4 days of eliminating TRPV2 from hearts of the adult mice, cardiac function declines severely, with disorganization of the intercalated discs that support mechanical coupling with neighbouring myocytes and myocardial conduction defects. After 9 days, cell shortening and Ca²⁺ handling by single myocytes are impaired in TRPV2-deficient hearts. TRPV2-deficient neonatal cardiomyocytes form no intercalated discs and show no extracellular Ca²⁺-dependent intracellular Ca²⁺ increase and insulin-like growth factor (IGF-1) secretion in response to stretch stimulation. We further demonstrate that IGF-1 receptor/PI3K/Akt pathway signalling is significantly downregulated in TRPV2-deficient hearts, and that IGF-1 administration partially prevents chamber dilation and impairment in cardiac pump function in these hearts. Our results improve our understanding of the molecular processes underlying the maintenance of cardiac structure and function.

The TRPV2 calcium channel can be activated by mechanical stretch and may act as a mechanoreceptor in tissues. Here the authors deplete the TRPV2 calcium channel from the hearts of adult mice, showing that TRPV2 is important for the maintenance of cardiac structure and function.

Overexpression of the Na+/K+ ATPase α2 but not α1 isoform attenuates pathological cardiac hypertrophy and remodeling

RATIONALE

The Na+ / K+ ATPase (NKA) directly regulates intracellular Na+ levels, which in turn indirectly regulates Ca2+ levels by proximally controlling flux through the Na+ / Ca2+ exchanger (NCX1). Elevated Na+ levels have been reported during heart failure, which permits some degree of reverse-mode Ca2+ entry through NCX1, as well as less efficient Ca2+ clearance.

OBJECTIVE

To determine whether maintaining lower intracellular Na+ levels by NKA overexpression in the heart would enhance forward-mode Ca2+ clearance and prevent reverse-mode Ca2+ entry through NCX1 to protect the heart.

METHODS AND RESULTS

Cardiac-specific transgenic mice overexpressing either NKA-α1 or NKA-α2 were generated and subjected to pressure overload hypertrophic stimulation. We found that although increased expression of NKA-α1 had no protective effect, overexpression of NKA-α2 significantly decreased cardiac hypertrophy after pressure overload in mice at 2, 10, and 16 weeks of stimulation. Remarkably, total NKA protein expression and activity were not altered in either of these 2 transgenic models because increased expression of one isoform led to a concomitant decrease in the other endogenous isoform. NKA-α2 overexpression but not NKA-α1 led to significantly faster removal of bulk Ca2+ from the cytosol in a manner requiring NCX1 activity. Mechanistically, overexpressed NKA-α2 showed greater affinity for Na+ compared with NKA-α1, leading to more efficient clearance of this ion. Furthermore, overexpression of NKA-α2 but not NKA-α1 was coupled to a decrease in phospholemman expression and phosphorylation, which would favor greater NKA activity, NCX1 activity, and Ca2+ removal.

CONCLUSIONS

Our results suggest that the protective effect produced by increased expression of NKA-α2 on the heart after pressure overload is due to more efficient Ca2+ clearance because this isoform of NKA preferentially enhances NCX1 activity compared with NKA-α1.

Mechanisms of altered Ca²⁺ handling in heart failure

Ca²⁺ plays a crucial role in connecting membrane excitability with contraction in myocardium. The hallmark features of heart failure are mechanical dysfunction and arrhythmias; defective intracellular Ca²⁺ homeostasis is a central cause of contractile dysfunction and arrhythmias in failing myocardium. Defective Ca²⁺ homeostasis in heart failure can result from pathological alteration in the expression and activity of an increasingly understood collection of Ca²⁺ homeostatic and structural proteins, ion channels, and enzymes. This review focuses on the molecular mechanisms of defective Ca²⁺ cycling in heart failure and considers how fundamental understanding of these pathways may translate into novel and innovative therapies.

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."

Intracellular Na⁺ and cardiac metabolism

In heart failure, alterations of excitation-contraction underlie contractile dysfunction. One important defect is an elevation of the intracellular Na(+) concentration in cardiac myocytes ([Na(+)]i), which has an important impact on cytosolic and mitochondrial Ca(2+) homeostasis. While elevated [Na(+)]i is thought to compensate for decreased Ca(2+) load of the sarcoplasmic reticulum (SR), it yet negatively affects energy supply-and-demand matching and can even induce mitochondrial oxidative stress. Here, we review the mechanisms underlying these pathophysiological changes. The chain of events may constitute a vicious cycle of ion dysregulation, oxidative stress and energetic deficit, resembling characteristic cellular deficits that are considered key hallmarks of the failing heart. This article is part of a Special Issue entitled "Na(+) Regulation in Cardiac Myocytes".

The sodium-potassium pump controls the intrinsic firing of the cerebellar Purkinje neuron

In vitro, cerebellar Purkinje cells can intrinsically fire action potentials in a repeating trimodal or bimodal pattern. The trimodal pattern consists of tonic spiking, bursting, and quiescence. The bimodal pattern consists of tonic spiking and quiescence. It is unclear how these firing patterns are generated and what determines which firing pattern is selected. We have constructed a realistic biophysical Purkinje cell model that can replicate these patterns. In this model, Na(+)/K(+) pump activity sets the Purkinje cell's operating mode. From rat cerebellar slices we present Purkinje whole cell recordings in the presence of ouabain, which irreversibly blocks the Na(+)/K(+) pump. The model can replicate these recordings. We propose that Na(+)/K(+) pump activity controls the intrinsic firing mode of cerbellar Purkinje cells.

Can the sodium-calcium exchanger initiate or suppress calcium sparks in cardiac myocytes?

Positive feedback of Calcium (Ca)-induced Ca release is the mechanism of Ca spark formation in cardiac myocytes. To initiate this process, a certain amount of Ca in the cleft space is necessary. When the membrane potential becomes higher during excitation-contraction coupling, Ca can enter through both Ca current (I(CaL)) and sodium-calcium exchanger (NCX) and may activate ryanodine receptors to initiate a Ca spark. On the other hand, at the resting membrane potential (V(m) ~-80 mV), NCX removes Ca from the cell (forward mode). If Ca released from the sarcoplasmic reticulum is quickly removed via forward mode NCX before Ca-induced Ca release starts, the Ca release becomes nonspark Ca leak. This would also be influenced by the cleft/noncleft distribution of NCX, which is unknown. Using a physiologically detailed mathematical model of subcellular Ca cycling, we analyze how NCX strength and distribution alter Ca spark formation. During excitation-contraction coupling, most Ca sparks are induced by I(CaL) with very few due to NCX current. At the resting membrane potential if most NCX is localized to the cleft, spontaneous Ca sparks are significantly reduced.

Regulation of the cardiac sodium pump

In cardiac muscle, the sarcolemmal sodium/potassium ATPase is the principal quantitative means of active transport at the myocyte cell surface, and its activity is essential for maintaining the trans-sarcolemmal sodium gradient that drives ion exchange and transport processes that are critical for cardiac function. The 72-residue phosphoprotein phospholemman regulates the sodium pump in the heart: unphosphorylated phospholemman inhibits the pump, and phospholemman phosphorylation increases pump activity. Phospholemman is subject to a remarkable plethora of post-translational modifications for such a small protein: the combination of three phosphorylation sites, two palmitoylation sites, and one glutathionylation site means that phospholemman integrates multiple signaling events to control the cardiac sodium pump. Since misregulation of cytosolic sodium contributes to contractile and metabolic dysfunction during cardiac failure, a complete understanding of the mechanisms that control the cardiac sodium pump is vital. This review explores our current understanding of these mechanisms.

Coordinating electrical activity of the heart: ankyrin polypeptides in human cardiac disease

INTRODUCTION

Over the past ten years, ankyrin polypeptides have emerged as players in cardiac excitation-contraction coupling. Once thought to solely play a structural role, loss-of-function variants of genes encoding ankyrin polypeptides have highlighted how this protein mediates subcellular localization of various electrical components of the excitation-contraction coupling machinery. Evidence has revealed how disruption of this localization is the primary cause of various cardiomyopathies, ranging from long-QT syndrome 4, to sinus node disease, to more common forms of arrhythmias.

AREAS COVERED

The roles of ankyrin polypeptides in excitation-contraction coupling in the heart and the development of ankyrin-specific cardiomyopathies. How ankyrin polypeptides may be involved in structural and electrical remodeling of the heart, post-myocardial infarct. How ankyrin interactions with membrane-bound ion channels may regulate these channels' response to stimuli. New data, which offers the potential for unique therapies, for not only combating heart disease, but also for wider applications to various disease states.

EXPERT OPINION

The ankyrin family of adapter proteins is emerging as an intimate player in cardiac excitation-contraction coupling. Until recently, these proteins have gone largely unappreciated for their importance in proper cardiac function. New insights into how these proteins function within the heart are offering potentially new avenues for therapies against cardiomyopathy.

Impact of sarcoplasmic reticulum calcium release on calcium dynamics and action potential morphology in human atrial myocytes: a computational study

Electrophysiological studies of the human heart face the fundamental challenge that experimental data can be acquired only from patients with underlying heart disease. Regarding human atria, there exist sizable gaps in the understanding of the functional role of cellular Ca²+ dynamics, which differ crucially from that of ventricular cells, in the modulation of excitation-contraction coupling. Accordingly, the objective of this study was to develop a mathematical model of the human atrial myocyte that, in addition to the sarcolemmal (SL) ion currents, accounts for the heterogeneity of intracellular Ca²+ dynamics emerging from a structurally detailed sarcoplasmic reticulum (SR). Based on the simulation results, our model convincingly reproduces the principal characteristics of Ca²+ dynamics: 1) the biphasic increment during the upstroke of the Ca²+ transient resulting from the delay between the peripheral and central SR Ca²+ release, and 2) the relative contribution of SL Ca²+ current and SR Ca²+ release to the Ca²+ transient. In line with experimental findings, the model also replicates the strong impact of intracellular Ca²+ dynamics on the shape of the action potential. The simulation results suggest that the peripheral SR Ca²+ release sites define the interface between Ca²+ and AP, whereas the central release sites are important for the fire-diffuse-fire propagation of Ca²+ diffusion. Furthermore, our analysis predicts that the modulation of the action potential duration due to increasing heart rate is largely mediated by changes in the intracellular Na+ concentration. Finally, the results indicate that the SR Ca²+ release is a strong modulator of AP duration and, consequently, myocyte refractoriness/excitability. We conclude that the developed model is robust and reproduces many fundamental aspects of the tight coupling between SL ion currents and intracellular Ca²+ signaling. Thus, the model provides a useful framework for future studies of excitation-contraction coupling in human atrial myocytes.

Loss of the AE3 anion exchanger in a hypertrophic cardiomyopathy model causes rapid decompensation and heart failure

The AE3 Cl(-)/HCO(3)(-) exchanger is abundantly expressed in the sarcolemma of cardiomyocytes, where it mediates Cl(-)-uptake and HCO(3)(-)-extrusion. Inhibition of AE3-mediated Cl(-)/HCO(3)(-) exchange has been suggested to protect against cardiac hypertrophy; however, other studies indicate that AE3 might be necessary for optimal cardiac function. To test these hypotheses we crossed AE3-null mice, which appear phenotypically normal, with a hypertrophic cardiomyopathy mouse model carrying a Glu180Gly mutation in α-tropomyosin (TM180). Loss of AE3 had no effect on hypertrophy; however, survival of TM180/AE3 double mutants was sharply reduced compared with TM180 single mutants. Analysis of cardiac performance revealed impaired cardiac function in TM180 and TM180/AE3 mutants. TM180/AE3 double mutants were more severely affected and exhibited little response to β-adrenergic stimulation, a likely consequence of their more rapid progression to heart failure. Increased expression of calmodulin-dependent kinase II and protein phosphatase 1 and differences in methylation and localization of protein phosphatase 2A were observed, but were similar in single and double mutants. Phosphorylation of phospholamban on Ser16 was sharply increased in both single and double mutants relative to wild-type hearts under basal conditions, leading to reduced reserve capacity for β-adrenergic stimulation of phospholamban phosphorylation. Imaging analysis of isolated myocytes revealed reductions in amplitude and decay of Ca(2+) transients in both mutants, with greater reductions in TM180/AE3 mutants, consistent with the greater severity of their heart failure phenotype. Thus, in the TM180 cardiomyopathy model, loss of AE3 had no apparent anti-hypertrophic effect and led to more rapid decompensation and heart failure.

Transient receptor potential channel 6-mediated, localized cytosolic [Na+] transients drive Na+/Ca2+ exchanger-mediated Ca2+ entry in purinergically stimulated aorta smooth muscle cells

The Na+/Ca2+ exchanger (NCX) is increasingly recognized as a physiological mediator of Ca2+ influx and significantly contributes to salt-sensitive hypertension. We recently reported that Ca2+ influx by the NCX (1) is the primary mechanism of Ca2+ entry in purinergically stimulated rat aorta smooth muscle cells and (2) requires functional coupling with transient receptor potential channel 6 nonselective cation channels. Using the Na+ indicator CoroNa Green, we now directly observed and characterized the localized cytosolic [Na+] ([Na+]i) elevations that have long been hypothesized to underlie physiological NCX reversal but that have never been directly shown. Stimulation of rat aorta smooth muscle cells caused both global and monotonic [Na+]i elevations and localized [Na+]i transients (LNats) at the cell periphery. Inhibition of nonselective cation channels with SKF-96365 (50 micromol/L) and 2-amino-4-phosphonobutyrate (75 micromol/L) reduced both global and localized [Na+]i elevations in response to ATP (1 mmol/L). This effect was mimicked by expression of a dominant negative construct of transient receptor potential channel 6. Selective inhibition of NCX-mediated Ca2+ entry with KB-R7943 (10 micromol/L) enhanced the LNats, whereas the global cytosolic [Na+] signal was unaffected. Inhibition of mitochondrial Na+ uptake with CGP-37157 (10 micromol/L) increased both LNats and global cytosolic [Na+] elevations. These findings directly demonstrate NCX regulation by LNats, which are restricted to subsarcolemmal, cytoplasmic microdomains. Analysis of the LNats, which facilitate Ca2+ entry via NCX, suggests that mitochondria limit the cytosolic diffusion of LNats generated by agonist-mediated activation of transient receptor potential channel 6-containing channels.

Electrogenic Na+/Ca2+-exchange of nerve and muscle cells

The plasma membrane Na(+)/Ca(2+)-exchanger is a bi-directional electrogenic (3Na(+):1Ca(2+)) and voltage-sensitive ion transport mechanism, which is mainly responsible for Ca(2+)-extrusion. The Na(+)-gradient, required for normal mode operation, is created by the Na(+)-pump, which is also electrogenic (3Na(+):2K(+)) and voltage-sensitive. The Na(+)/Ca(2+)-exchanger operational modes are very similar to those of the Na(+)-pump, except that the uncoupled flux (Na(+)-influx or -efflux?) is missing. The reversal potential of the exchanger is around -40 mV; therefore, during the upstroke of the AP it is probably transiently activated, leading to Ca(2+)-influx. The Na(+)/Ca(2+)-exchange is regulated by transported and non-transported external and internal cations, and shows ATP(i)-, pH- and temperature-dependence. The main problem in determining the role of Na(+)/Ca(2+)-exchange in excitation-secretion/contraction coupling is the lack of specific (mode-selective) blockers. During recent years, evidence has been accumulated for co-localisation of the Na(+)-pump, and the Na(+)/Ca(2+)-exchanger and their possible functional interaction in the "restricted" or "fuzzy space." In cardiac failure, the Na(+)-pump is down-regulated, while the exchanger is up-regulated. If the exchanger is working in normal mode (Ca(2+)-extrusion) during most of the cardiac cycle, upregulation of the exchanger may result in SR Ca(2+)-store depletion and further impairment in contractility. If so, a normal mode selective Na(+)/Ca(2+)-exchange inhibitor would be useful therapy for decompensation, and unlike CGs would not increase internal Na(+). In peripheral sympathetic nerves, pre-synaptic alpha(2)-receptors may regulate not only the VSCCs but possibly the reverse Na(+)/Ca(2+)-exchange as well.

Interaction between Na+/K+-pump and Na+/Ca2+-exchanger modulates intercellular communication

Ouabain, a specific inhibitor of the Na(+)/K(+)-pump, has previously been shown to interfere with intercellular communication. Here we test the hypothesis that the communication between vascular smooth muscle cells is regulated through an interaction between the Na(+)/K(+)-pump and the Na(+)/Ca(2+)-exchanger leading to an increase in the intracellular calcium concentration ([Ca(2+)](i)) in discrete areas near the plasma membrane. [Ca(2+)](i) in smooth muscle cells was imaged in cultured rat aortic smooth muscle cell pairs (A7r5) and in rat mesenteric small artery segments simultaneously with force. In A7r5 coupling between cells was estimated by measuring membrane capacitance. Smooth muscle cells were uncoupled when the Na(+)/K(+)-pump was inhibited either by a low concentration of ouabain, which also caused a localized increase of [Ca(2+)](i) near the membrane, or by ATP depletion. Reduction of Na(+)/K(+)-pump activity by removal of extracellular potassium ([K(+)](o)) also uncoupled cells, but only after inhibition of K(ATP) channels. Inhibition of the Na(+)/Ca(2+)-exchange activity by SEA0400 or by a reduction of the equilibrium potential (making it more negative) also uncoupled the cells. Depletion of intracellular Na(+) and clamping of [Ca(2+)](i) at low concentrations prevented the uncoupling. The experiments suggest that the Na(+)/K(+)-pump may affect gap junction conductivity via localized changes in [Ca(2+)](i) through modulation of Na(+)/Ca(2+)-exchanger activity.

Contribution of the Na+/Ca2+ exchanger to rapid Ca2+ release in cardiomyocytes

Trigger Ca(2+) is considered to be the Ca(2+) current through the L-type Ca(2+) channel (LTCC) that causes release of Ca(2+) from the sarcoplasmic reticulum. However, cell contraction also occurs in the absence of the LTCC current (I(Ca)). In this article, we investigate the contribution of the Na(+)/Ca(2+) exchanger (NCX) to the trigger Ca(2+). Experimental data from rat cardiomyocytes using confocal microscopy indicating that inhibition of reverse mode Na(+)/Ca(2+) exchange delays the Ca(2+) transient by 3-4 ms served as a basis for the mathematical model. A detailed computational model of the dyadic cleft (fuzzy space) is presented where the diffusion of both Na(+) and Ca(2+) is taken into account. Ionic channels are included at discrete locations, making it possible to study the effect of channel position and colocalization. The simulations indicate that if a Na(+) channel is present in the fuzzy space, the NCX is able to bring enough Ca(2+) into the cell to affect the timing of release. However, this critically depends on channel placement and local diffusion properties. With fuzzy space diffusion in the order of four orders of magnitude lower than in water, triggering through LTCC alone was up to 5 ms slower than with the presence of a Na(+) channel and NCX.

Sodium calcium exchange as a target for antiarrhythmic therapy

In search of better antiarrhythmic therapy, targeting the Na/Ca exchanger is an option to be explored. The rationale is that increased activity of the Na/Ca exchanger has been implicated in arrhythmogenesis in a number of conditions. The evidence is strong for triggered arrhythmias related to Ca2+ overload, due to increased Na+ load or during adrenergic stimulation; the Na/Ca exchanger may be important in triggered arrhythmias in heart failure and in atrial fibrillation. There is also evidence for a less direct role of the Na/Ca exchanger in contributing to remodelling processes. In this chapter, we review this evidence and discuss the consequences of inhibition of Na/Ca exchange in the perspective of its physiological role in Ca2+ homeostasis. We summarize the current data on the use of available blockers of Na/Ca exchange and propose a framework for further study and development of such drugs. Very selective agents have great potential as tools for further study of the role the Na/Ca exchanger plays in arrhythmogenesis. For therapy, they may have their specific indications, but they carry the risk of increasing Ca2+ load of the cell. Agents with a broader action that includes Ca2+ channel block may have advantages in other conditions, e.g. with Ca2+ overload. Additional actions such as block of K+ channels, which may be unwanted in e.g. heart failure, may be used to advantage as well.