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

Augmenting workload drives T-tubule assembly in developing cardiomyocytes

Contraction of cardiomyocytes is initiated at subcellular elements called dyads, where L-type Ca2+ channels in t-tubules are located within close proximity to ryanodine receptors in the sarcoplasmic reticulum. While evidence from small rodents indicates that dyads are assembled gradually in the developing heart, it is unclear how this process occurs in large mammals. We presently examined dyadic formation in fetal and newborn sheep (Ovis aries), and the regulation of this process by fetal cardiac workload. By employing advanced imaging methods, we demonstrated that t-tubule growth and dyadic assembly proceed gradually during fetal sheep development, from 93 days of gestational age until birth (147 days). This process parallels progressive increases in fetal systolic blood pressure, and includes step-wise colocalization of L-type Ca2+ channels and the Na+/Ca2+ exchanger with ryanodine receptors. These proteins are upregulated together with the dyadic anchor junctophilin-2 during development, alongside changes in the expression of amphiphysin-2 (BIN1) and its partner proteins myotubularin and dynamin-2. Increasing fetal systolic load by infusing plasma or occluding the post-ductal aorta accelerated t-tubule growth. Conversely, reducing fetal systolic load with infusion of enalaprilat, an angiotensin converting enzyme inhibitor, blunted t-tubule formation. Interestingly, altered t-tubule densities did not relate to changes in dyadic junctions, or marked changes in the expression of dyadic regulatory proteins, indicating that distinct signals are responsible for maturation of the sarcoplasmic reticulum. In conclusion, augmenting blood pressure and workload during normal fetal development critically promotes t-tubule growth, while additional signals contribute to dyadic assembly. KEY POINTS: T-tubule growth and dyadic assembly proceed gradually in cardiomyocytes during fetal sheep development, from 93 days of gestational age until the post-natal stage. Increasing fetal systolic load by infusing plasma or occluding the post-ductal aorta accelerated t-tubule growth and hypertrophy. In contrast, reducing fetal systolic load by enalaprilat infusion slowed t-tubule development and decreased cardiomyocyte size. Load-dependent modulation of t-tubule maturation was linked to altered expression patterns of the t-tubule regulatory proteins junctophilin-2 and amphiphysin-2 (BIN1) and its protein partners. Altered t-tubule densities did not influence dyadic formation, indicating that distinct signals are responsible for maturation of the sarcoplasmic reticulum.

Image-Driven Modeling of Nanoscopic Cardiac Function: Where Have We Come From, and Where Are We Going?

Complementary developments in microscopy and mathematical modeling have been critical to our understanding of cardiac excitation-contraction coupling. Historically, limitations imposed by the spatial or temporal resolution of imaging methods have been addressed through careful mathematical interrogation. Similarly, limitations imposed by computational power have been addressed by imaging macroscopic function in large subcellular domains or in whole myocytes. As both imaging resolution and computational tractability have improved, the two approaches have nearly merged in terms of the scales that they can each be used to interrogate. With this review we will provide an overview of these advances and their contribution to understanding ventricular myocyte function, including exciting developments over the last decade. We specifically focus on experimental methods that have pushed back limits of either spatial or temporal resolution of nanoscale imaging (e.g., DNA-PAINT), or have permitted high resolution imaging on large cellular volumes (e.g., serial scanning electron microscopy). We also review the progression of computational approaches used to integrate and interrogate these new experimental data sources, and comment on near-term advances that may unify understanding of the underlying biology. Finally, we comment on several outstanding questions in cardiac physiology that stand to benefit from a concerted and complementary application of these new experimental and computational methods.

The Physiology and Pathophysiology of T-Tubules in the Heart

In cardiomyocytes, invaginations of the sarcolemmal membrane called t-tubules are critically important for triggering contraction by excitation-contraction (EC) coupling. These structures form functional junctions with the sarcoplasmic reticulum (SR), and thereby enable close contact between L-type Ca²⁺ channels (LTCCs) and Ryanodine Receptors (RyRs). This arrangement in turn ensures efficient triggering of Ca²⁺ release, and contraction. While new data indicate that t-tubules are capable of exhibiting compensatory remodeling, they are also widely reported to be structurally and functionally compromised during disease, resulting in disrupted Ca²⁺ homeostasis, impaired systolic and/or diastolic function, and arrhythmogenesis. This review summarizes these findings, while highlighting an emerging appreciation of the distinct roles of t-tubules in the pathophysiology of heart failure with reduced and preserved ejection fraction (HFrEF and HFpEF). In this context, we review current understanding of the processes underlying t-tubule growth, maintenance, and degradation, underscoring the involvement of a variety of regulatory proteins, including junctophilin-2 (JPH2), amphiphysin-2 (BIN1), caveolin-3 (Cav3), and newer candidate proteins. Upstream regulation of t-tubule structure/function by cardiac workload and specifically ventricular wall stress is also discussed, alongside perspectives for novel strategies which may therapeutically target these mechanisms.

Design of a Proteolytically Stable Sodium-Calcium Exchanger 1 Activator Peptide for In Vivo Studies

The cardiac sodium–calcium exchanger (NCX1) is important for normal Na⁺- and Ca²⁺-homeostasis and cardiomyocyte relaxation and contraction. It has been suggested that NCX1 activity is reduced by phosphorylated phospholemman (pSer68-PLM); however its direct interaction with PLM is debated. Disruption of the potentially inhibitory pSer68-PLM-NCX1 interaction might be a therapeutic strategy to increase NCX1 activity in cardiac disease. In the present study, we aimed to analyze the binding affinities and kinetics of the PLM-NCX1 and pSer68-PLM-NCX1 interactions by surface plasmon resonance (SPR) and to develop a proteolytically stable NCX1 activator peptide for future in vivo studies. The cytoplasmic parts of PLM (PLM_cyt) and pSer68-PLM (pSer68-PLM_cyt) were found to bind strongly to the intracellular loop of NCX1 (NCX1_cyt) with similar K_D values of 4.1 ± 1.0 nM and 4.3 ± 1.9 nM, but the PLM_cyt-NCX1_cyt interaction showed higher on/off rates. To develop a proteolytically stable NCX1 activator, we took advantage of a previously designed, high-affinity PLM binding peptide (OPT) that was derived from the PLM binding region in NCX1 and that reverses the inhibitory PLM (S68D)-NCX1 interaction in HEK293. We performed N- and C-terminal truncations of OPT and identified PYKEIEQLIELANYQV as the minimum sequence required for pSer68-PLM binding. To increase peptide stability in human serum, we replaced the proline with an N-methyl-proline (NOPT) after identification of N-terminus as substitution tolerant by two-dimensional peptide array analysis. Mass spectrometry analysis revealed that the half-life of NOPT was increased 17-fold from that of OPT. NOPT pulled down endogenous PLM from rat left ventricle lysate and exhibited direct pSer68-PLM binding in an ELISA-based assay and bound to pSer68-PLM_cyt with a K_D of 129 nM. Excess NOPT also reduced the PLM_cyt-NCX1_cyt interaction in an ELISA-based competition assay, but in line with that NCX1 and PLM form oligomers, NOPT was not able to outcompete the physical interaction between endogenous full length proteins. Importantly, cell-permeable NOPT-TAT increased NCX1 activity in cardiomyocytes isolated from both SHAM-operated and aorta banded heart failure (HF) mice, indicating that NOPT disrupted the inhibitory pSer68-PLM-NCX1 interaction. In conclusion, we have developed a proteolytically stable NCX1-derived PLM binding peptide that upregulates NCX1 activity in SHAM and HF cardiomyocytes.

Hydrochlorothiazide therapy: impact on early recurrence of atrial fibrillation after catheter ablation?

BACKGROUND

Hypokalemia has been linked to electrocardiogram changes and afterdepolarization-mediated arrhythmias. However, the association between hypokalemia and atrial fibrillation (AF) has not been well studied. Hydrochlorothiazide (HCT) diuretic therapy was shown to be associated with hypokalemia in multiple studies. We aimed to determine whether HCT therapy is associated with early recurrence of AF after radiofrequency (RF) catheter ablation during a 3-month follow-up.

METHODS

We performed a retrospective registry analysis of our internal AF ablation registry, containing 807 consecutive patients that underwent RF ablation for symptomatic AF. Propensity score matching was used to match 156 patients on HCT therapy with 156 controls. Furthermore, we performed propensity score matching between the first and the fourth quartile of baseline serum potassium (K) concentrations in the initial population (N.=807).

RESULTS

We observed a small but statistically significant difference in baseline mean potassium levels between the HCT group and the control group (4.03 mmol/L vs. 4.19 mmol/L respectively, P=0.001). There was no difference in short term recurrence of atrial fibrillation in the HCT group compared to the propensity score matched control group (41.0% [N.=64] vs. 45.5% [N.=71], P=0.424). In the comparison between the first and the fourth quartile of baseline serum potassium values, no difference in AF recurrence (38.2% [N.=63] vs. 37.0% [N.=61], P=0.820) during a 3-month follow-up after ablation was observed between both groups.

CONCLUSIONS

Patients on HCT therapy showed no difference in short term recurrence of AF after ablation compared to propensity matched controls.

Calcium Channels and Calcium-Regulated Channels in Human Red Blood Cells

Free Calcium (Ca²⁺) is an important and universal signalling entity in all cells, red blood cells included. Although mature mammalian red blood cells are believed to not contain organelles as Ca²⁺ stores such as the endoplasmic reticulum or mitochondria, a 20,000-fold gradient based on a intracellular Ca²⁺ concentration of approximately 60 nM vs. an extracellular concentration of 1.2 mM makes Ca²⁺-permeable channels a major signalling tool of red blood cells. However, the internal Ca²⁺ concentration is tightly controlled, regulated and maintained primarily by the Ca²⁺ pumps PMCA1 and PMCA4. Within the last two decades it became evident that an increased intracellular Ca²⁺ is associated with red blood cell clearance in the spleen and promotes red blood cell aggregability and clot formation. In contrast to this rather uncontrolled deadly Ca²⁺ signals only recently it became evident, that a temporal increase in intracellular Ca²⁺ can also have positive effects such as the modulation of the red blood cells O₂ binding properties or even be vital for brief transient cellular volume adaptation when passing constrictions like small capillaries or slits in the spleen. Here we give an overview of Ca²⁺ channels and Ca²⁺-regulated channels in red blood cells, namely the Gárdos channel, the non-selective voltage dependent cation channel, Piezo1, the NMDA receptor, VDAC, TRPC channels, Ca_V2.1, a Ca²⁺-inhibited channel novel to red blood cells and i.a. relate these channels to the molecular unknown sickle cell disease conductance P_sickle. Particular attention is given to correlation of functional measurements with molecular entities as well as the physiological and pathophysiological function of these channels. This view is in constant progress and in particular the understanding of the interaction of several ion channels in a physiological context just started. This includes on the one hand channelopathies, where a mutation of the ion channel is the direct cause of the disease, like Hereditary Xerocytosis and the Gárdos Channelopathy. On the other hand it applies to red blood cell related diseases where an altered channel activity is a secondary effect like in sickle cell disease or thalassemia. Also these secondary effects should receive medical and pharmacologic attention because they can be crucial when it comes to the life-threatening symptoms of the disease.

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.

Hypokalemia-Induced Arrhythmias and Heart Failure: New Insights and Implications for Therapy

Routine use of diuretics and neurohumoral activation make hypokalemia (serum K⁺ < 3. 5 mM) a prevalent electrolyte disorder among heart failure patients, contributing to the increased risk of ventricular arrhythmias and sudden cardiac death in heart failure. Recent experimental studies have suggested that hypokalemia-induced arrhythmias are initiated by the reduced activity of the Na⁺/K⁺-ATPase (NKA), subsequently leading to Ca²⁺ overload, Ca²⁺/Calmodulin-dependent kinase II (CaMKII) activation, and development of afterdepolarizations. In this article, we review the current mechanistic evidence of hypokalemia-induced triggered arrhythmias and discuss how molecular changes in heart failure might lower the threshold for these arrhythmias. Finally, we discuss how recent insights into hypokalemia-induced arrhythmias could have potential implications for future antiarrhythmic treatment strategies.

Shining New Light on the Structural Determinants of Cardiac Couplon Function: Insights From Ten Years of Nanoscale Microscopy

Remodelling of the membranes and protein clustering patterns during the pathogenesis of cardiomyopathies has renewed the interest in spatial visualisation of these structures in cardiomyocytes. Coincidental emergence of single molecule (super-resolution) imaging and tomographic electron microscopy tools in the last decade have led to a number of new observations on the structural features of the couplons, the primary sites of excitation-contraction coupling in the heart. In particular, super-resolution and tomographic electron micrographs have revised and refined the classical views of the nanoscale geometries of couplons, t-tubules and the organisation of the principal calcium handling proteins in both healthy and failing hearts. These methods have also allowed the visualisation of some features which were too small to be detected with conventional microscopy tools. With new analytical capabilities such as single-protein mapping, in situ protein quantification, correlative and live cell imaging we are now observing an unprecedented interest in adapting these research tools across the cardiac biophysical research discipline. In this article, we review the depth of the new insights that have been enabled by these tools toward understanding the structure and function of the cardiac couplon. We outline the major challenges that remain in these experiments and emerging avenues of research which will be enabled by these technologies.

Regulation of Cardiomyocyte T-Tubular Structure: Opportunities for Therapy

PURPOSE OF REVIEW

Membrane invaginations called t-tubules play an integral role in triggering cardiomyocyte contraction, and their disruption during diseases such as heart failure critically impairs cardiac performance. In this review, we outline the growing understanding of the malleability of t-tubule structure and function, and highlight emerging t-tubule regulators which may be exploited for novel therapies.

RECENT FINDINGS

New technologies are revealing the nanometer scale organization of t-tubules, and their functional junctions with the sarcoplasmic reticulum called dyads, which generate Ca²⁺ sparks. Recent data have indicated that the dyadic anchoring protein junctophilin-2, and the membrane-bending protein BIN1 are key regulators of dyadic formation and maintenance. While the underlying signals which control expression and localization of these proteins remain unclear, accumulating data support an important role of myocardial workload. Although t-tubule alterations are believed to be a key cause of heart failure, the plasticity of these structures also creates an opportunity for therapy. Promising recent data suggest that such therapies may specifically target junctophilin-2, BIN1, and/or mechanotransduction.

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.

Murine Electrophysiological Models of Cardiac Arrhythmogenesis

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

Cardiomyocyte Ca2+ dynamics: clinical perspectives

In the heart, Ca(2+) signals regulate a variety of biological functions ranging from contractility to gene expression, cellular hypertrophy and death. In this review, we summarize the role of local Ca(2+) homeostasis in these processes in healthy cardiac muscle cells, and highlight how mismanaged Ca(2+) handling contributes to the pathophysiology of conditions such as cardiac arrhythmia, ischemic heart disease, cardiac hypertrophy and heart failure. Aiming to provide an introduction to the field with a clinical perspective, we also indicate how current and future therapies may modulate cardiomyocytes Ca(2+) handling for the treatment of patients.

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.

Calcium signalling in developing cardiomyocytes: implications for model systems and disease

Adult cardiomyocytes exhibit complex Ca(2+) homeostasis, enabling tight control of contraction and relaxation. This intricate regulatory system develops gradually, with progressive maturation of specialized structures and increasing capacity of Ca(2+) sources and sinks. In this review, we outline current understanding of these developmental processes, and draw parallels to pathophysiological conditions where cardiomyocytes exhibit a striking regression to an immature state of Ca(2+) homeostasis. We further highlight the importance of understanding developmental physiology when employing immature cardiomyocyte models such as cultured neonatal cells and stem cells.

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

Aim

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

Methods and results

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

Conclusion

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

The role of dyadic organization in regulation of sarcoplasmic reticulum Ca(2+) handling during rest in rabbit ventricular myocytes

The dyadic organization of ventricular myocytes ensures synchronized activation of sarcoplasmic reticulum (SR) Ca(2+) release during systole. However, it remains obscure how the dyadic organization affects SR Ca(2+) handling during diastole. By measuring intraluminal SR Ca(2+) ([Ca(2+)]SR) decline during rest in rabbit ventricular myocytes, we found that ∼76% of leaked SR Ca(2+) is extruded from the cytosol and only ∼24% is pumped back into the SR. Thus, the majority of Ca(2+) that leaks from the SR is removed from the cytosol before it can be sequestered back into the SR by the SR Ca(2+)-ATPase (SERCA). Detubulation decreased [Ca(2+)]SR decline during rest, thus making the leaked SR Ca(2+) more accessible for SERCA. These results suggest that Ca(2+) extrusion systems are localized in T-tubules. Inhibition of Na(+)-Ca(2+) exchanger (NCX) slowed [Ca(2+)]SR decline during rest by threefold, however did not prevent it. Depolarization of mitochondrial membrane potential during NCX inhibition completely prevented the rest-dependent [Ca(2+)]SR decline. Despite a significant SR Ca(2+) leak, Ca(2+) sparks were very rare events in control conditions. NCX inhibition or detubulation increased Ca(2+) spark activity independent of SR Ca(2+) load. Overall, these results indicate that during rest NCX effectively competes with SERCA for cytosolic Ca(2+) that leaks from the SR. This can be explained if the majority of SR Ca(2+) leak occurs through ryanodine receptors in the junctional SR that are located closely to NCX in the dyadic cleft. Such control of the dyadic [Ca(2+)] by NCX play a critical role in suppressing Ca(2+) sparks during rest.

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".

No rest for the weary: diastolic calcium homeostasis in the normal and failing myocardium

Following contraction of the heart, efficient relaxation (diastole) is essential for refilling the ventricles with blood. This review describes how ventricular relaxation is controlled by Ca(2+) homeostasis in cardiac muscle cells and how alterations in Ca(2+) cycling affect diastolic function in the normal and failing heart. These discussions illustrate that the diastolic phase is not simply a period of rest but rather involves highly regulated and dynamic Ca(2+) fluxes.

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.

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.

Modelling cardiac calcium sparks in a three-dimensional reconstruction of a calcium release unit

Triggered release of Ca2+ from an individual sarcoplasmic reticulum (SR) Ca(2+) release unit (CRU) is the fundamental event of cardiac excitation–contraction coupling, and spontaneous release events (sparks) are the major contributor to diastolic Ca(2+) leak in cardiomyocytes. Previous model studies have predicted that the duration and magnitude of the spark is determined by the local CRU geometry, as well as the localization and density of Ca(2+) handling proteins. We have created a detailed computational model of a CRU, and developed novel tools to generate the computational geometry from electron tomographic images. Ca(2+) diffusion was modelled within the SR and the cytosol to examine the effects of localization and density of the Na(+)/Ca(2+) exchanger, sarco/endoplasmic reticulum Ca(2+)-ATPase 2 (SERCA), and calsequestrin on spark dynamics. We reconcile previous model predictions of approximately 90% local Ca(2+) depletion in junctional SR, with experimental reports of about 40%. This analysis supports the hypothesis that dye kinetics and optical averaging effects can have a significant impact on measures of spark dynamics. Our model also predicts that distributing calsequestrin within non-junctional Z-disc SR compartments, in addition to the junctional compartment, prolongs spark release time as reported by Fluo5. By pumping Ca(2+) back into the SR during a release, SERCA is able to prolong a Ca(2+) spark, and this may contribute to SERCA-dependent changes in Ca(2+) wave speed. Finally, we show that including the Na(+)/Ca(2+) exchanger inside the dyadic cleft does not alter local [Ca(2+)] during a spark.

Methods in cardiomyocyte isolation, culture, and gene transfer

Since techniques for cardiomyocyte isolation were first developed 35 years ago, experiments on single myocytes have yielded great insight into their cellular and sub-cellular physiology. These studies have employed a broad range of techniques including electrophysiology, calcium imaging, cell mechanics, immunohistochemistry and protein biochemistry. More recently, techniques for cardiomyocyte culture have gained additional importance with the advent of gene transfer technology. While such studies require a high quality cardiomyocyte population, successful cell isolation and maintenance during culture remain challenging. In this review, we describe methods for the isolation of adult and neonatal ventricular myocytes from rat and mouse heart. This discussion outlines general principles for the beginner, but also provides detailed specific protocols and advice for common caveats. We additionally review methods for short-term myocyte culture, with particular attention given to the importance of substrate and media selection, and describe time-dependent alterations in myocyte physiology that should be anticipated. Gene transfer techniques for neonatal and adult cardiomyocytes are also reviewed, including methods for transfection (liposome, electroporation) and viral-based gene delivery.

β-Adrenoceptor/PKA-stimulation, Na(+)-Ca(2+) exchange and PKA-activated Cl(-) currents in rabbit cardiomyocytes: a conundrum

Investigations into the functional modulation of the cardiac Na(+)-Ca(2+) exchanger (NCX) by acute β-adrenoceptor/PKA stimulation have produced conflicting results. Here, we investigated (i) whether or not β-adrenoceptor activation/PKA stimulation activates current in rabbit cardiac myocytes under NCX-'selective' conditions and (ii) if so, whether a PKA-activated Cl(-)-current may contribute to the apparent modulation of NCX current (I(NCX)). Whole-cell voltage-clamp experiments were conducted at 37°C on rabbit ventricular and atrial myocytes. The β-adrenoceptor-activated currents both in NCX-'selective' and Cl(-)-selective recording conditions were found to be sensitive to 10mM Ni(2+). In contrast, the PKA-activated Cl(-) current was not sensitive to Ni(2+), when it was activated downstream to the β-adrenoceptors using 10μM forskolin (an adenylyl cyclase activator). When 10μM forskolin was applied under NCX-selective recording conditions, the Ni(2+)-sensitive current did not differ between control and forskolin. These findings suggest that in rabbit myocytes: (a) a PKA-activated Cl(-) current contributes to the Ni(2+)-sensitive current activated via β-adrenoceptor stimulation under recording conditions previously considered selective for I(NCX); (b) downstream activation of PKA does not augment Ni(2+)-sensitive I(NCX), when this is measured under conditions where the Ni(2+)-sensitive PKA-activated Cl(-) current is not present.

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

Ca2+ transients were activated in rabbit ventricular cells by a sequence of action potential shaped voltage clamps. After activating a series of control transients, Na+ currents (INa) were inactivated with a ramp from -80 to -40 mV (1.5 s) prior to the action potential clamp. The transients were detected with the calcium indicator Fluo-4 and an epifluorescence system. With zero Na+ in the pipette INa inactivation produced a decline in the SR Ca2+ release flux (measured as the maximum rate of rise of the transient) of 27 ± 4% (n = 9, P < 0.001) and a peak amplitude reduction of 10 ± 3% (n = 9, P < 0.05). With 5 mm Na+ in the pipette the reduction in release flux was greater (34 ± 4%, n = 4, P < 0.05). The ramp effectively inactivates INa without changing ICa, and there was no significant change in the transmembrane Ca2+ flux after the inactivation of INa. We next evoked action potentials under current clamp. TTX at 100 nm, which selectively blocks neuronal isoforms of Na+ channels, produced a decline in SR Ca2+ release flux of 35 ± 3% (n = 6, P < 0.001) and transient amplitude of 12 ± 2% (n = 6, P < 0.05). This effect was similar to the effect of INa inactivation on release flux. We conclude that a TTX-sensitive INa is essential for efficient triggering of SR Ca2+ release. We propose that neuronal Na+ channels residing within couplons activate sufficient reverse Na+-Ca2+ exchanger (NCX) to prime the junctional cleft with Ca2+. The results can be explained if non-linearities in excitation-contraction coupling mechanisms modify the coupling fidelity of ICa, which is known to be low at positive potentials.

Integrative systems models of cardiac excitation-contraction coupling

Excitation-contraction coupling in the cardiac myocyte is mediated by a number of highly integrated mechanisms of intracellular Ca²(+) transport. The complexity and integrative nature of heart cell electrophysiology and Ca²(+) cycling has led to an evolution of computational models that have played a crucial role in shaping our understanding of heart function. An important emerging theme in systems biology is that the detailed nature of local signaling events, such as those that occur in the cardiac dyad, have important consequences at higher biological scales. Multiscale modeling techniques have revealed many mechanistic links between microscale events, such as Ca²(+) binding to a channel protein, and macroscale phenomena, such as excitation-contraction coupling gain. Here, we review experimentally based multiscale computational models of excitation-contraction coupling and the insights that have been gained through their application.

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.

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.

Sodium-calcium exchange is essential for effective triggering of calcium release in mouse heart

In cardiac myocytes, excitation-contraction coupling depends upon sarcoplasmic reticular Ca2+ release triggered by Ca2+ influx through L-type Ca2+ channels. Although Na+-Ca2+ exchange (NCX) is essential for Ca2+ extrusion, its participation in the trigger process of excitation-contraction coupling is controversial. To investigate the role of NCX in triggering, we examined Ca2+ sparks in ventricular cardiomyocytes isolated from wild-type (WT) and cardiac-specific NCX knockout (KO) mice. Myocytes from young NCX KO mice are known to exhibit normal resting cytosolic Ca2+ and normal Ca2+ transients despite reduced L-type Ca2+ current. We loaded myocytes with fluo-3 to image Ca2+ sparks using confocal microscopy in line-scan mode. The frequency of spontaneous Ca2+ sparks was reduced in KO myocytes compared with WT. However, spark amplitude and width were increased in KO mice. Permeabilizing the myocytes with saponin eliminated differences between spontaneous sparks in WT and KO mice. These results suggest that sarcolemmal processes are responsible for the reduced spark frequency and increased spark width and amplitude in KO mice. When myocytes were loaded with 1 mM fluo-3 and 3 mM EGTA via the patch pipette to buffer diadic cleft Ca2+, the number of sparks triggered by action potentials was reduced by 60% in KO cells compared to WT cells, despite similar SR Ca2+ content in both cell types. When EGTA was omitted from the pipette solution, the number of sparks triggered in KO and WT myocytes was similar. Although the number of sparks was restored in KO cells, Ca2+ release was asynchronous. These results suggest that high subsarcolemmal Ca2+ is required to ensure synchronous triggering with short spark latency in the absence of NCX. In WT mice, high subsarcolemmal Ca2+ is not required for synchronous triggering, because NCX is capable of priming the diadic cleft with sufficient Ca2+ for normal triggering, even when subsarcolemmal Ca(2+) is lowered by EGTA. Thus, reducing subsarcolemmal Ca2+ with EGTA in NCX KO mice reveals the dependence of Ca2+ release on NCX.

Activation of reverse Na+-Ca2+ exchange by the Na+ current augments the cardiac Ca2+ transient: evidence from NCX knockout mice

The hypothesis that Na(+) influx during the action potential (AP) activates reverse Na(+)-Ca(2+) exchange (NCX) and subsequent entry of trigger Ca(2+) is controversial. We tested this hypothesis by monitoring intracellular Ca(2+) before and after selective inactivation of I(Na) prior to a simulated action potential in patch-clamped ventricular myocytes isolated from adult wild-type (WT) and NCX knockout (KO) mice. First, we inactivated I(Na) using a ramp prepulse to 45 mV. In WT cells, inactivation of I(Na) decreased the Ca(2+) transient amplitude by 51.1 +/- 4.6% (P < 0.001, n = 14) and reduced its maximum release flux by 53.0 +/- 4.6% (P < 0.001, n = 14). There was no effect on diastolic Ca(2+). In striking contrast, Ca(2+) transients in NCX KO cardiomyocytes were unaffected by the presence or absence of I(Na) (n = 8). We obtained similar results when measuring trigger Ca(2+) influx in myocytes with depleted sarcoplasmic reticulum. In WT cells, inactivation of I(Na) decreased trigger Ca(2+) influx by 37.8 +/- 6% and maximum rate of flux by 30.6 +/- 7.7% at 2.5 mm external Ca(2+) (P < 0.001 and P < 0.05, n = 9). This effect was again absent in the KO cells (n = 8). Second, exposure to 10 mum tetrodotoxin to block I(Na) also reduced the Ca(2+) transients in WT myocytes but not in NCX KO myocytes. We conclude that I(Na) and reverse NCX modulate Ca(2+) release in murine WT cardiomyocytes by augmenting the pool of Ca(2+) that triggers ryanodine receptors. This is an important mechanism for regulation of Ca(2+) release and contractility in murine heart.

There goes the neighborhood: pathological alterations in T-tubule morphology and consequences for cardiomyocyte Ca2+ handling

T-tubules are invaginations of the cardiomyocyte membrane into the cell interior which form a tortuous network. T-tubules provide proximity between the electrically excitable cell membrane and the sarcoplasmic reticulum, the main intracellular Ca²⁺ store. Tight coupling between the rapidly spreading action potential and Ca²⁺ release units in the SR membrane ensures synchronous Ca²⁺ release throughout the cardiomyocyte. This is a requirement for rapid and powerful contraction. In recent years, it has become clear that T-tubule structure and composition are altered in several pathological states which may importantly contribute to contractile defects in these conditions. In this review, we describe the “neighborhood” of proteins in the dyadic cleft which locally controls cardiomyocyte Ca²⁺ homeostasis and how alterations in T-tubule structure and composition may alter this neighborhood during heart failure, atrial fibrillation, and diabetic cardiomyopathy. Based on this evidence, we propose that T-tubules have the potential to serve as novel therapeutic targets.

Organization of ryanodine receptors, transverse tubules, and sodium-calcium exchanger in rat myocytes

Confocal and total internal reflection fluorescence imaging was used to examine the distribution of caveolin-3, sodium-calcium exchange (NCX) and ryanodine receptors (RyRs) in rat ventricular myocytes. Transverse and longitudinal optical sectioning shows that NCX is distributed widely along the transverse and longitudinal tubular system (t-system). The NCX labeling consisted of both punctate and distributed components, which partially colocalize with RyRs (27%). Surface membrane labeling showed a similar pattern but the fraction of RyR clusters containing NCX label was decreased and no nonpunctate labeling was observed. Sixteen percent of RyRs were not colocalized with the t-system and 1.6% of RyRs were found on longitudinal elements of the t-system. The surface distribution of RyR labeling was not generally consistent with circular patches of RyRs. This suggests that previous estimates for the number of RyRs in a junction (based on circular close-packed arrays) need to be revised. The observed distribution of caveolin-3 labeling was consistent with its exclusion from RyR clusters. Distance maps for all colocalization pairs were calculated to give the distance between centroids of punctate labeling and edges for distributed components. The possible roles for punctate NCX labeling are discussed.

Moderate heart dysfunction in mice with inducible cardiomyocyte-specific excision of the Serca2 gene

The sarco(endo)plasmic reticulum calcium ATPase 2 (SERCA2) transports Ca(2+) from cytosol into the sarcoplasmic reticulum (SR) of cardiomyocytes, thereby maintaining the store of releasable Ca(2+) necessary for contraction. Reduced SERCA function has been linked to heart failure, and loss of SERCA2 in the adult mammalian heart would be expected to cause immediate severe myocardial contractile dysfunction and death. We investigated heart function in adult mice with an inducible cardiomyocyte-specific excision of the Atp2a2 (Serca2) gene (SERCA2 KO). Seven weeks after induction of Serca2 gene excision, the mice displayed a substantial reduction in diastolic function with a 5-fold increase in the time constant of isovolumetric pressure decay (tau). However, already at 4 weeks following gene excision less than 5% SERCA2 protein was found in myocardial tissue. Surprisingly, heart function was only moderately impaired at this time point. Tissue Doppler imaging showed slightly reduced peak systolic tissue velocity and a less than 2-fold increase in tau was observed. The SR Ca(2+) content was dramatically reduced in cardiomyocytes from 4-week SERCA2 KO mice, and Ca(2+) transients were predominantly generated by enhanced Ca(2+) flux through L-type Ca(2+) channels and the Na(+)-Ca(2+) exchanger. Moreover, equivalent increases in cytosolic [Ca(2+)] in control and SERCA2 KO myocytes induced greater cell shortening in SERCA2 KO, suggesting enhanced myofilament responsiveness. Our data demonstrate that SR-independent Ca(2+) transport mechanisms temporarily can prevent major cardiac dysfunction despite a major reduction of SERCA2 in cardiomyocytes.

Distribution patterns of the Na+-Ca2+ exchanger and caveolin-3 in developing rabbit cardiomyocytes

In adult cardiac cells the established mechanism of excitation-contraction coupling is by calcium-induced calcium release (CICR) mediated by L-type Ca(2+) channels. However, in neonate cardiomyocytes, a CICR modality involving reverse mode Na(+)-Ca(2+) exchanger (NCX) activity predominates. This has been hypothesized to be due, in part, to the high expression levels of NCX in the neonate heart which drop several fold during ontogeny. Very little is known about the nature of NCX distribution within the cardiomyocyte and how this might change with development given the significant differences in gene expression. We investigated the spatial arrangements of NCX in developing rabbit ventricular myocytes with traditional as well as novel image processing and analysis techniques. Using image segmentation, colocalization analysis was conducted at the whole cell, compartmental (cell periphery and cell interior) and object levels. Because NCX has been suggested to colocalize with caveolin-3 (cav-3) and perhaps form a signaling unit within caveolae, the spatial relationship of NCX relative to cav-3 was also examined in detail. NCX and cav-3 objects were found to be isolated islands of lit voxels that are present after thresholding. These objects were categorized into non-colocalized (0%), lowly colocalized (<50%) and highly colocalized (>50%) subpopulations in both the interior and peripheral compartments. Our results show that NCX and cav-3 are distributed on the peripheral membrane as discrete objects and are not highly colocalized throughout development. 3D distance analysis revealed that NCX and cav-3 objects are organized with a longitudinal and lateral periodicity of about 1mum and that NCX and cav-3 cluster appear to be mutually exclusive on the cell periphery. We conclude that despite the very significant decrease in NCX expression with maturation, qualitatively there were no differences in NCX surface distribution or in the spatial relationship to caveolin 3.

Hypothermia increases the gain of excitation-contraction coupling in guinea pig ventricular myocytes

Components of excitation-contraction (EC)-coupling were compared at 37°C and 22°C to determine whether hypothermia altered the gain of EC coupling in guinea pig ventricular myocytes. Ca2+concentration (fura-2) and cell shortening (edge detector) were measured simultaneously. Hypothermia increased fractional shortening (8.3 ± 1.7 vs. 2.6 ± 0.3% at 37°C), Ca2+transients (157 ± 33 vs. 35 ± 5 nM at 37°C), and diastolic Ca2+(100 ± 9 vs. 60 ± 6 nM at 37°C) in field-stimulated myocytes (2 Hz). In experiments with high-resistance microelectrodes, the increase in contractions and Ca2+transients was accompanied by a twofold increase in action potential duration (APD). When voltage-clamp steps eliminated changes in APD, cooling still increased contractions and Ca2+transients. Hypothermia increased sarcoplasmic reticulum (SR) Ca2+stores (83 ± 17 at 37°C to 212 ± 50 nM, assessed with caffeine) and increased fractional SR Ca2+release twofold. In contrast, peak Ca2+current was much smaller at 22°C than at 37°C (1.3 ± 0.4 and 3.5 ± 0.7 pA/pF, respectively). In cells dialyzed with sodium-free pipette solutions to inhibit Ca2+influx via reverse-mode Na+/Ca2+exchange, hypothermia still increased contractions, Ca2+transients, SR stores, and fractional release but decreased the amplitude of Ca2+current. The rate of SR Ca2+release per unit Ca2+current, a measure of EC-coupling gain, was increased sixfold by hypothermia. This increase in gain occurred regardless of whether cells were dialyzed with sodium-free solutions. Thus an increase in EC-coupling gain contributes importantly to positive inotropic effects of hypothermia in the heart.

Calcium cycling and signaling in cardiac myocytes

Calcium (Ca) is a universal intracellular second messenger. In muscle, Ca is best known for its role in contractile activation. However, in recent years the critical role of Ca in other myocyte processes has become increasingly clear. This review focuses on Ca signaling in cardiac myocytes as pertaining to electrophysiology (including action potentials and arrhythmias), excitation-contraction coupling, modulation of contractile function, energy supply-demand balance (including mitochondrial function), cell death, and transcription regulation. Importantly, although such diverse Ca-dependent regulations occur simultaneously in a cell, the cell can distinguish distinct signals by local Ca or protein complexes and differential Ca signal integration.

Stochastic binding of Ca2+ ions in the dyadic cleft; continuous versus random walk description of diffusion

Ca(2+) signaling in the dyadic cleft in ventricular myocytes is fundamentally discrete and stochastic. We study the stochastic binding of single Ca(2+) ions to receptors in the cleft using two different models of diffusion: a stochastic and discrete Random Walk (RW) model, and a deterministic continuous model. We investigate whether the latter model, together with a stochastic receptor model, can reproduce binding events registered in fully stochastic RW simulations. By evaluating the continuous model goodness-of-fit for a large range of parameters, we present evidence that it can. Further, we show that the large fluctuations in binding rate observed at the level of single time-steps are integrated and smoothed at the larger timescale of binding events, which explains the continuous model goodness-of-fit. With these results we demonstrate that the stochasticity and discreteness of the Ca(2+) signaling in the dyadic cleft, determined by single binding events, can be described using a deterministic model of Ca(2+) diffusion together with a stochastic model of the binding events, for a specific range of physiological relevant parameters. Time-consuming RW simulations can thus be avoided. We also present a new analytical model of bimolecular binding probabilities, which we use in the RW simulations and the statistical analysis.

Three-dimensional distribution of cardiac Na+-Ca2+ exchanger and ryanodine receptor during development

Mechanisms of cardiac excitation-contraction coupling in neonates are still not clearly defined. Previous work in neonates shows reverse-mode Na(+)-Ca(2+) exchange to be the primary route of Ca(2+) entry during systole and the neonatal sarcoplasmic reticulum to have similar capability as that of adult in storing and releasing Ca(2+). We investigated Na(+)-Ca(2+) exchanger (NCX) and ryanodine receptor (RyR) distribution in developing ventricular myocytes using immunofluorescence, confocal microscopy, and digital image analysis. In neonates, both NCX and RyR clusters on the surface of the cell displayed a short longitudinal periodicity of approximately 0.7 microm. However, by adulthood, both proteins were also found in the interior. In the adult, clusters of NCX on the surface of the cell retained the approximately 0.7-microm periodicity whereas clusters of RyR adopted a longer longitudinal periodicity of approximately 2.0 microm. This suggests that neonatal myocytes also have a peri-M-line RyR distribution that is absent in adult myocytes. NCX and RyR colocalized voxel density was maximal in neonates and declined significantly with ontogeny. We conclude in newborns, Ca(2+) influx via NCX could potentially activate the dense network of peripheral Ca(2+) stores via peripheral couplings, evoking Ca(2+)-induced Ca(2+) release.

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.

Voltage dependence of cardiac excitation-contraction coupling: unitary Ca2+ current amplitude and open channel probability

Excitation-contraction coupling in cardiac myocytes occurs by Ca2+-induced Ca2+ release, where L-type Ca2+ current evokes a larger sarcoplasmic reticulum (SR) Ca2+ release. The Ca2+-induced Ca2+ release amplification factor or gain (SR Ca2+ release/I(Ca)) is usually assessed by the V(m) dependence of current and Ca2+ transients. Gain rises at negative V(m), as does single channel I(Ca) (i(Ca)), which has led to the suggestion that the increases of i(Ca) amplitude enhances gain at more negative V(m). However, I(Ca) = NP(o) x i(Ca) (where NP(o) is the number of open channels), and NP(o) and i(Ca) both depend on V(m). To assess how i(Ca) and NP(o) separately influence Ca2+-induced Ca2+ release, we measured I(Ca) and junctional SR Ca2+ release in voltage-clamped rat ventricular myocytes using "Ca2+ spikes" (confocal microscopy). To vary i(Ca) alone, we changed [Ca2+](o) rapidly at constant test V(m) (0 mV) or abruptly repolarized from +120 mV to different V(m) (at constant [Ca2+](o)). To vary NP(o) alone, we altered Ca2+ channel availability by varying holding V(m) (at constant test V(m)). Reducing either i(Ca) or NP(o) alone increased excitation-contraction coupling gain. Thus, increasing i(Ca) does not increase gain at progressively negative test V(m). Such enhanced gain depends on lower NP(o) and reduced redundant Ca2+ channel openings (per junction) and a consequently smaller denominator in the gain equation. Furthermore, modest i(Ca) (at V(m) = 0 mV) may still effectively trigger SR Ca2+ release, whereas at positive V(m) (and smaller i(Ca)), high and well-synchronized channel openings are required for efficient excitation-contraction coupling. At very positive V(m), reduced i(Ca) must explain reduced SR Ca2+ release.