CaMKII inhibition targeted to the sarcoplasmic reticulum inhibits frequency-dependent acceleration of relaxation and Ca2+ current facilitation

Enhanced capacity for CaMKII signaling mitigates calcium release related contractile fatigue with high intensity exercise

BACKGROUND

We tested whether enhancing the capacity for calcium/calmodulin-dependent protein kinase type II (CaMKII) signaling would delay fatigue of excitation-induced calcium release and improve contractile characteristics of skeletal muscle during fatiguing exercise.

METHODS

Fast and slow type muscle, gastrocnemius medialis (GM) and soleus (SOL), of rats and mouse interosseus (IO) muscle fibers, were transfected with pcDNA3-based plasmids for rat α and β CaMKII or empty controls. Levels of CaMKII, its T287-phosphorylation (pT287-CaMKII), and phosphorylation of components of calcium release and re-uptake, ryanodine receptor 1 (pS2843-RyR1) and phospholamban (pT17-PLN), were quantified biochemically. Sarcoplasmic calcium in transfected muscle fibers was monitored microscopically during trains of electrical excitation based on Fluo-4 FF fluorescence (n = 5-7). Effects of low- (n = 6) and high- (n = 8) intensity exercise on pT287-CaMKII and contractile characteristics were studied in situ.

RESULTS

Co-transfection with αCaMKII-pcDNA3/βCaMKII-pcDNA3 increased α and βCaMKII levels in SOL (+45.8 %, +250.5 %) and GM (+40.4 %, +89.9 %) muscle fibers compared to control transfection. High-intensity exercise increased pT287-βCaMKII and pS2843-RyR1 levels in SOL (+269 %, +151 %) and GM (+354 %, +119 %), but decreased pT287-αCaMKII and p17-PLN levels in GM compared to SOL (-76 % vs. +166 %; 0 % vs. +128 %). α/β CaMKII overexpression attenuated the decline of calcium release in muscle fibers with repeated excitation, and mitigated exercise-induced deterioration of rates in force production, and passive force, in a muscle-dependent manner, in correlation with pS2843-RyR1 and pT17-PLN levels (|r| > 0.7).

CONCLUSION

Enhanced capacity for α/β CaMKII signaling improves fatigue-resistance of active and passive contractile muscle properties in association with RyR1- and PLN-related improvements in sarcoplasmic calcium release.

Reduced O-GlcNAcylation diminishes cardiomyocyte Ca2+ dependent facilitation and frequency dependent acceleration of relaxation

Ca2+ dependent facilitation (CDF) and frequency dependent acceleration of relaxation (FDAR) are regulatory mechanisms that potentiate cardiomyocyte Ca2+ channel function and increase the rate of Ca2+ sequestration following a Ca2+-release event, respectively, when depolarization frequency increases. CDF and FDAR likely evolved to maintain EC coupling at increased heart rates. Ca2+/calmodulin-dependent kinase II (CaMKII) was shown to be indispensable to both; however, the mechanisms remain to be completely elucidated. CaMKII activity can be modulated by post-translational modifications but if and how these modifications impact CDF and FDAR is unknown. Intracellular O-linked glycosylation (O-GlcNAcylation) is a post-translational modification that acts as a signaling molecule and metabolic sensor. In hyperglycemic conditions, CaMKII was shown to be O-GlcNAcylated resulting in pathologic activity. Here we sought to investigate whether O-GlcNAcylation impacts CDF and FDAR through modulation of CaMKII activity in a pseudo-physiologic setting. Using voltage-clamp and Ca2+ photometry we show that cardiomyocyte CDF and FDAR are significantly diminished in conditions of reduced O-GlcNAcylation. Immunoblot showed that CaMKIIδ and calmodulin expression are increased but the autophosphorylation of CaMKIIδ and the muscle cell-specific CaMKIIβ isoform are reduced by 75% or more when O-GlcNAcylation is inhibited. We also show that the enzyme responsible for O-GlcNAcylation (OGT) can likely be localized in the dyad space and/or at the cardiac sarcoplasmic reticulum and is precipitated by calmodulin in a Ca2+ dependent manner. These findings will have important implications for our understanding of how CaMKII and OGT interact to impact cardiomyocyte EC coupling in normal physiologic settings as well as in disease states where CaMKII and OGT may be aberrantly regulated.

Effects of Cobra Cardiotoxins on Intracellular Calcium and the Contracture of Rat Cardiomyocytes Depend on Their Structural Types

Cardiotoxins (CaTx) of the three-finger toxin family are one of the main components of cobra venoms. Depending on the structure of the N-terminal or the central polypeptide loop, they are classified into either group I and II or P- and S-types, respectively, and toxins of different groups or types interact with lipid membranes variably. While their main target in the organism is the cardiovascular system, there is no data on the effects of CaTxs from different groups or types on cardiomyocytes. To evaluate these effects, a fluorescence measurement of intracellular Ca²⁺ concentration and an assessment of the rat cardiomyocytes' shape were used. The obtained results showed that CaTxs of group I containing two adjacent proline residues in the N-terminal loop were less toxic to cardiomyocytes than group II toxins and that CaTxs of S-type were less active than P-type ones. The highest activity was observed for Naja oxiana cobra cardiotoxin 2, which is of P-type and belongs to group II. For the first time, the effects of CaTxs of different groups and types on the cardiomyocytes were studied, and the data obtained showed that the CaTx toxicity to cardiomyocytes depends on the structures both of the N-terminal and central polypeptide loops.

L-type Ca2+ channel recovery from inactivation in rabbit atrial myocytes

Adaptation of the myocardium to varying workloads critically depends on the recovery from inactivation (RFI) of L-type Ca²⁺ channels (LCCs) which provide the trigger for cardiac contraction. The goal of the present study was a comprehensive investigation of LCC RFI in atrial myocytes. The study was performed on voltage-clamped rabbit atrial myocytes using a double pulse protocol with variable diastolic intervals in cells held at physiological holding potentials, with intact intracellular Ca²⁺ release, and preserved Na⁺ current and Na⁺ /Ca²⁺ exchanger (NCX) activity. We demonstrate that the kinetics of RFI of LCCs are co-regulated by several factors including resting membrane potential, [Ca²⁺ ]_i , Na⁺ influx, and activity of CaMKII. In addition, activation of CaMKII resulted in increased I_Ca amplitude at higher pacing rates. Pharmacological inhibition of NCX failed to have any significant effect on RFI, indicating that impaired removal of Ca²⁺ by NCX has little effect on LCC recovery. Finally, RFI of intracellular Ca²⁺ release was substantially slower than LCC RFI, suggesting that inactivation kinetics of LCC do not significantly contribute to the beat-to-beat refractoriness of SR Ca²⁺ release. The study demonstrates that CaMKII and intracellular Ca²⁺ dynamics play a central role in modulation of LCC activity in atrial myocytes during increased workloads that could have important consequences under pathological conditions such as atrial fibrillations, where Ca²⁺ cycling and CaMKII activity are altered.

AKAP18δ Anchors and Regulates CaMKII Activity at Phospholamban-SERCA2 and RYR

BACKGROUND

The sarcoplasmic reticulum (SR) Ca²⁺-ATPase 2 (SERCA2) mediates Ca²⁺ reuptake into SR and thereby promotes cardiomyocyte relaxation, whereas the ryanodine receptor (RYR) mediates Ca²⁺ release from SR and triggers contraction. Ca²⁺/CaMKII (CaM [calmodulin]-dependent protein kinase II) regulates activities of SERCA2 through phosphorylation of PLN (phospholamban) and RYR through direct phosphorylation. However, the mechanisms for CaMKIIδ anchoring to SERCA2-PLN and RYR and its regulation by local Ca²⁺ signals remain elusive. The objective of this study was to investigate CaMKIIδ anchoring and regulation at SERCA2-PLN and RYR.

METHODS

A role for AKAP18δ (A-kinase anchoring protein 18δ) in CaMKIIδ anchoring and regulation was analyzed by bioinformatics, peptide arrays, cell-permeant peptide technology, immunoprecipitations, pull downs, transfections, immunoblotting, proximity ligation, FRET-based CaMKII activity and ELISA-based assays, whole cell and SR vesicle fluorescence imaging, high-resolution microscopy, adenovirus transduction, adenoassociated virus injection, structural modeling, surface plasmon resonance, and alpha screen technology.

RESULTS

Our results show that AKAP18δ anchors and directly regulates CaMKIIδ activity at SERCA2-PLN and RYR, via 2 distinct AKAP18δ regions. An N-terminal region (AKAP18δ-N) inhibited CaMKIIδ through binding of a region homologous to the natural CaMKII inhibitor peptide and the Thr17-PLN region. AKAP18δ-N also bound CaM, introducing a second level of control. Conversely, AKAP18δ-C, which shares homology to neuronal CaMKIIα activator peptide (N2B-s), activated CaMKIIδ by lowering the apparent Ca²⁺ threshold for kinase activation and inducing CaM trapping. While AKAP18δ-C facilitated faster Ca²⁺ reuptake by SERCA2 and Ca²⁺ release through RYR, AKAP18δ-N had opposite effects. We propose a model where the 2 unique AKAP18δ regions fine-tune Ca²⁺-frequency-dependent activation of CaMKIIδ at SERCA2-PLN and RYR.

CONCLUSIONS

AKAP18δ anchors and functionally regulates CaMKII activity at PLN-SERCA2 and RYR, indicating a crucial role of AKAP18δ in regulation of the heartbeat. To our knowledge, this is the first protein shown to enhance CaMKII activity in heart and also the first AKAP (A-kinase anchoring protein) reported to anchor a CaMKII isoform, defining AKAP18δ also as a CaM-KAP.

Implications of SGLT Inhibition on Redox Signalling in Atrial Fibrillation

Atrial fibrillation (AF) is the most common sustained (atrial) arrhythmia, a considerable global health burden and often associated with heart failure. Perturbations of redox signalling in cardiomyocytes provide a cellular substrate for the manifestation and maintenance of atrial arrhythmias. Several clinical trials have shown that treatment with sodium-glucose linked transporter inhibitors (SGLTi) improves mortality and hospitalisation in heart failure patients independent of the presence of diabetes. Post hoc analysis of the DECLARE-TIMI 58 trial showed a 19% reduction in AF in patients with diabetes mellitus (hazard ratio, 0.81 (95% confidence interval: 0.68-0.95), n = 17.160) upon treatment with SGLTi, regardless of pre-existing AF or heart failure and independent from blood pressure or renal function. Accordingly, ongoing experimental work suggests that SGLTi not only positively impact heart failure but also counteract cellular ROS production in cardiomyocytes, thereby potentially altering atrial remodelling and reducing AF burden. In this article, we review recent studies investigating the effect of SGLTi on cellular processes closely interlinked with redox balance and their potential effects on the onset and progression of AF. Despite promising insight into SGLTi effect on Ca²⁺ cycling, Na⁺ balance, inflammatory and fibrotic signalling, mitochondrial function and energy balance and their potential effect on AF, the data are not yet conclusive and the importance of individual pathways for human AF remains to be established. Lastly, an overview of clinical studies investigating SGLTi in the context of AF is provided.

Ryanodine receptor remodeling in cardiomyopathy and muscular dystrophy caused by lamin A/C gene mutation

Mutations in the lamin A/C gene (LMNA), which encodes A-type lamins, cause several diseases called laminopathies, the most common of which is dilated cardiomyopathy with muscular dystrophy. The role of Ca2+ regulation in these diseases remain poorly understood. We now show biochemical remodeling of the ryanodine receptor (RyR)/intracellular Ca2+ release channel in heart samples from human subjects with LMNA mutations, including protein kinase A-catalyzed phosphorylation, oxidation and depletion of the stabilizing subunit calstabin. In the LmnaH222P/H222P murine model of Emery-Dreifuss muscular dystrophy caused by LMNA mutation, we demonstrate an age-dependent biochemical remodeling of RyR2 in the heart and RyR1 in skeletal muscle. This RyR remodeling is associated with heart and skeletal muscle dysfunction. Defective heart and muscle function are ameliorated by treatment with a novel Rycal small molecule drug (S107) that fixes 'leaky' RyRs. SMAD3 phosphorylation is increased in hearts and diaphragms of LmnaH222P/H222P mice, which enhances NADPH oxidase binding to RyR channels, contributing to their oxidation. There is also increased generalized protein oxidation, increased calcium/calmodulin-dependent protein kinase II-catalyzed phosphorylation of RyRs and increased protein kinase A activity in these tissues. Our data show that RyR remodeling plays a role in cardiomyopathy and skeletal muscle dysfunction caused by LMNA mutation and identify these Ca2+ channels as a potential therapeutic target.

Long-Time Phase Correlations Reveal Regulation of Beating Cardiomyocytes

Spontaneous contractions of cardiomyocytes are driven by calcium oscillations due to the activity of ionic calcium channels and pumps. The beating phase is related to the time-dependent deviation of the oscillations from their average frequency, due to noise and the resulting cellular response. Here, we demonstrate experimentally that, in addition to the short-time (1-2 Hz), beat-to-beat variability, there are long-time correlations (tens of minutes) in the beating phase dynamics of isolated cardiomyocytes. Our theoretical model relates these long-time correlations to cellular regulation that restores the frequency to its average, homeostatic value in response to stochastic perturbations.

A Mathematical Model of the Mouse Atrial Myocyte With Inter-Atrial Electrophysiological Heterogeneity

Biophysically detailed mathematical models of cardiac electrophysiology provide an alternative to experimental approaches for investigating possible ionic mechanisms underlying the genesis of electrical action potentials and their propagation through the heart. The aim of this study was to develop a biophysically detailed mathematical model of the action potentials of mouse atrial myocytes, a popular experimental model for elucidating molecular and cellular mechanisms of arrhythmogenesis. Based on experimental data from isolated mouse atrial cardiomyocytes, a set of mathematical equations for describing the biophysical properties of membrane ion channel currents, intracellular Ca²⁺ handling, and Ca²⁺-calmodulin activated protein kinase II and β-adrenergic signaling pathways were developed. Wherever possible, membrane ion channel currents were modeled using Markov chain formalisms, allowing detailed representation of channel kinetics. The model also considered heterogeneous electrophysiological properties between the left and the right atrial cardiomyocytes. The developed model was validated by its ability to reproduce the characteristics of action potentials and Ca²⁺ transients, matching quantitatively to experimental data. Using the model, the functional roles of four K⁺ channel currents in atrial action potential were evaluated by channel block simulations, results of which were quantitatively in agreement with existent experimental data. To conclude, this newly developed model of mouse atrial cardiomyocytes provides a powerful tool for investigating possible ion channel mechanisms of atrial electrical activity at the cellular level and can be further used to investigate mechanisms underlying atrial arrhythmogenesis.

Ketamine‑induced bladder dysfunction is associated with extracellular matrix accumulation and impairment of calcium signaling in a mouse model

Due to the rising abuse of ketamine usage in recent years, ketamine-induced urinary tract syndrome has received increasing attention. The present study aimed to investigate the molecular mechanism underlying ketamine-associated cystitis in a mouse model. Female C57BL/6 mice were randomly divided into two groups: One group was treated with ketamine (100 mg/kg/day of ketamine for 20 weeks), whereas, the control group was treated with saline solution. In each group, micturition frequency and urine volume were examined to assess urinary voiding functions. Mouse bladders were extracted and samples were examined for pathological and morphological alterations using hematoxylin and eosin staining, Masson's trichrome staining and scanning electron microscopy. A cDNA microarray was conducted to investigate the differentially expressed genes following treatment with ketamine. The results suggested that bladder hyperactivity increased in the mice treated with ketamine. Furthermore, treatment with ketamine resulted in a smooth apical epithelial surface, subepithelial vascular congestion and lymphoplasmacytic aggregation. Microarray analysis identified a number of genes involved in extracellular matrix accumulation, which is associated with connective tissue fibrosis progression, and in calcium signaling regulation, that was associated with urinary bladder smooth muscle contraction. Collectively, the present results suggested that these differentially expressed genes may serve critical roles in ketamine-induced alterations of micturition patterns and urothelial pathogenesis. Furthermore, the present findings may provide a theoretical basis for the development of effective therapies to treat ketamine-induced urinary tract syndrome.

SERCA is critical to control the Bowditch effect in the heart

The Bowditch effect or staircase phenomenon is the increment or reduction of contractile force when heart rate increases, defined as either a positive or negative staircase. The healthy and failing human heart both show positive or negative staircase, respectively, but the causes of these distinct cardiac responses are unclear. Different experimental approaches indicate that while the level of Ca2+ in the sarcoplasmic reticulum is critical, the molecular mechanisms are unclear. Here, we demonstrate that Drosophila melanogaster shows a negative staircase which is associated to a slight but significant frequency-dependent acceleration of relaxation (FDAR) at the highest stimulation frequencies tested. We further showed that the type of staircase is oppositely modified by two distinct SERCA mutations. The dominant conditional mutation SERCAA617T induced positive staircase and arrhythmia, while SERCAE442K accentuated the negative staircase of wild type. At the stimulation frequencies tested, no significant FDAR could be appreciated in mutant flies. The present results provide evidence that two individual mutations directly modify the type of staircase occurring within the heart and suggest an important role of SERCA in regulating the Bowditch effect.

Research advances in kinase enzymes and inhibitors for cardiovascular disease treatment

The targeting of protein kinases has great future potential for the design of new drugs against cardiovascular diseases (CVDs). Enormous efforts have been made toward achieving this aim. Unfortunately, kinase inhibitors designed to treat CVDs have suffered from numerous limitations such as poor selectivity, bad permeability and toxicity. So, where are we now in terms of discovering effective kinase targeting drugs to treat CVDs? Various drug design techniques have been approached for this purpose since the discovery of the inhibitory activity of Staurosporine against protein kinase C in 1986. This review aims to provide context for the status of several emerging classes of direct kinase modulators to treat CVDs and discuss challenges that are preventing scientists from finding new kinase drugs to treat heart disease.

The calcium-frequency response in the rat ventricular myocyte: an experimental and modelling study

KEY POINTS

In the majority of species, including humans, increased heart rate increases cardiac contractility. This change is known as the force-frequency response (FFR). The majority of mammals have a positive force-frequency relationship (FFR). In rat the FFR is controversial. We derive a species- and temperature-specific data-driven model of the rat ventricular myocyte. As a measure of the FFR, we test the effects of changes in frequency and extracellular calcium on the calcium-frequency response (CFR) in our model and three altered models. The results show a biphasic peak calcium-frequency response, due to biphasic behaviour of the ryanodine receptor and the combined effect of the rapid calmodulin buffer and the frequency-dependent increase in diastolic calcium. Alterations to the model reveal that inclusion of Ca(2+) /calmodulin-dependent protein kinase II (CAMKII)-mediated L-type channel and transient outward K(+) current activity enhances the positive magnitude calcium-frequency response, and the absence of CAMKII-mediated increase in activity of the sarco/endoplasmic reticulum Ca(2+) -ATPase induces a negative magnitude calcium-frequency response.

ABSTRACT

An increase in heart rate affects the strength of cardiac contraction by altering the Ca(2+) transient as a response to physiological demands. This is described by the force-frequency response (FFR), a change in developed force with pacing frequency. The majority of mammals, including humans, have a positive FFR, and cardiac contraction strength increases with heart rate. However, the rat and mouse are exceptions, with the majority of studies reporting a negative FFR, while others report either a biphasic or a positive FFR. Understanding the differences in the FFR between humans and rats is fundamental to interpreting rat-based experimental findings in the context of human physiology. We have developed a novel model of rat ventricular electrophysiology and calcium dynamics, derived predominantly from experimental data recorded under physiological conditions. As a measure of FFR, we tested the effects of changes in stimulation frequency and extracellular calcium concentration on the simulated Ca(2+) transient characteristics and showed a biphasic peak calcium-frequency relationship, consistent with recent observations of a shift from negative to positive FFR when approaching the rat physiological frequency range. We tested the hypotheses that (1) inhibition of Ca(2+) /calmodulin-dependent protein kinase II (CAMKII)-mediated increase in sarco/endoplasmic reticulum Ca(2+) -ATPase (SERCA) activity, (2) CAMKII modulation of SERCA, L-type channel and transient outward K(+) current activity and (3) Na(+) /K(+) pump dynamics play a significant role in the rat FFR. The results reveal a major role for CAMKII modulation of SERCA in the peak Ca(2+) -frequency response, driven most significantly by the cytosolic calcium buffering system and changes in diastolic Ca(2+) .

Exercise training reverses myocardial dysfunction induced by CaMKIIδC overexpression by restoring Ca2+ homeostasis

Several conditions of heart disease, including heart failure and diabetic cardiomyopathy, are associated with upregulation of cytosolic Ca(2+)/calmodulin-dependent protein kinase II (CaMKIIδC) activity. In the heart, CaMKIIδC isoform targets several proteins involved in intracellular Ca(2+) homeostasis. We hypothesized that high-intensity endurance training activates mechanisms that enable a rescue of dysfunctional cardiomyocyte Ca(2+) handling and thereby ameliorate cardiac dysfunction despite continuous and chronic elevated levels of CaMKIIδC CaMKIIδC transgenic (TG) and wild-type (WT) mice performed aerobic interval exercise training over 6 wk. Cardiac function was measured by echocardiography in vivo, and cardiomyocyte shortening and intracellular Ca(2+) handling were measured in vitro. TG mice had reduced global cardiac function, cardiomyocyte shortening (47% reduced compared with WT, P < 0.01), and impaired Ca(2+) homeostasis. Despite no change in the chronic elevated levels of CaMKIIδC, exercise improved global cardiac function, restored cardiomyocyte shortening, and reestablished Ca(2+) homeostasis to values not different from WT. The key features to explain restored Ca(2+) homeostasis after exercise training were increased L-type Ca(2+) current density and flux by 79 and 85%, respectively (P < 0.01), increased sarcoplasmic reticulum (SR) Ca(2+)-ATPase (SERCA2a) function by 50% (P < 0.01), and reduced diastolic SR Ca(2+) leak by 73% (P < 0.01), compared with sedentary TG mice. In conclusion, exercise training improves global cardiac function as well as cardiomyocyte function in the presence of a maintained high CaMKII activity. The main mechanisms of exercise-induced improvements in TG CaMKIIδC mice are mediated via increased L-type Ca(2+) channel currents and improved SR Ca(2+) handling by restoration of SERCA2a function in addition to reduced diastolic SR Ca(2+) leak.

CaMKII-dependent myofilament Ca2+ desensitization contributes to the frequency-dependent acceleration of relaxation

BACKGROUND

Previous studies suggest that CaMKII activity is required for frequency-dependent acceleration of relaxation (FDAR) in ventricular myocytes. We propose that the underlying mechanism involves CaMKII-dependent regulation of myofilament Ca(2+) sensitivity.

METHODS AND RESULTS

Cardiac function was measured in mice using murine echo machine. [Ca(2+)]i and sarcomere length were measured by IonOptix Ca(2+) image system. Increasing pacing rate from 0.5 to 4 Hz in left ventricular myocytes induced frequency-dependent myofilament Ca(2+) desensitization (FDMCD) and FDAR. Acute inhibition of PKA or PKC had no effect, whereas CaMKII inhibition abolished both FDMCD and FDAR. Co-immunoprecipitation of CaMKII and troponin I (TnI) has been detected and CaMKII inhibition significantly reduced serine residue phosphorylation of TnI. Finally, chronic inhibition of CaMKII in vivo reduced TnI phosphorylation and abolished both FDAR and FDMCD, leading to impaired diastolic function.

CONCLUSIONS

Our results suggest that CaMKII-dependent TnI phosphorylation is involved in FDMCD and the consequent FDAR and that CaMKII inhibition removes this mechanism and thus induces diastolic dysfunction.

How cardiomyocyte excitation, calcium release and contraction become altered with age

Cardiovascular disease is the main cause of death globally, accounting for over 17 million deaths each year. As the incidence of cardiovascular disease rises markedly with age, the overall risk of cardiovascular disease is expected to increase dramatically with the aging of the population such that by 2030 it could account for over 23 million deaths per year. It is therefore vitally important to understand how the heart remodels in response to normal aging for at least two reasons: i) to understand why the aged heart is increasingly susceptible to disease; and ii) since it may be possible to modify treatment of disease in older adults if the underlying substrate upon which the disease first develops is fully understood. It is well known that age modulates cardiac function at the level of the individual cardiomyocyte. Generally, in males, aging reduces cell shortening, which is associated with a decrease in the amplitude of the systolic Ca(2+) transient. This may arise due to a decrease in peak L-type Ca(2+) current. Sarcoplasmic reticulum (SR) Ca(2+) load appears to be maintained during normal aging but evidence suggests that SR function is disrupted, such that the rate of sarco/endoplasmic reticulum Ca(2+)-ATPase (SERCA)-mediated Ca(2+) removal is reduced and the properties of SR Ca(2+) release in terms of Ca(2+) sparks are altered. Interestingly, Ca(2+) handling is modulated by age to a lesser degree in females. Here we review how cellular contraction is altered as a result of the aging process by considering expression levels and functional properties of key proteins involved in controlling intracellular Ca(2+). We consider how changes in both electrical properties and intracellular Ca(2+) handling may interact to modulate cardiomyocyte contraction. We also reflect on why cardiovascular risk may differ between the sexes by highlighting sex-specific variation in the age-associated remodeling process. This article is part of a Special Issue entitled CV Aging.

Chasing cardiac physiology and pathology down the CaMKII cascade

Calcium dynamics is central in cardiac physiology, as the key event leading to the excitation-contraction coupling (ECC) and relaxation processes. The primary function of Ca(2+) in the heart is the control of mechanical activity developed by the myofibril contractile apparatus. This key role of Ca(2+) signaling explains the subtle and critical control of important events of ECC and relaxation, such as Ca(2+) influx and SR Ca(2+) release and uptake. The multifunctional Ca(2+)-calmodulin-dependent protein kinase II (CaMKII) is a signaling molecule that regulates a diverse array of proteins involved not only in ECC and relaxation but also in cell death, transcriptional activation of hypertrophy, inflammation, and arrhythmias. CaMKII activity is triggered by an increase in intracellular Ca(2+) levels. This activity can be sustained, creating molecular memory after the decline in Ca(2+) concentration, by autophosphorylation of the enzyme, as well as by oxidation, glycosylation, and nitrosylation at different sites of the regulatory domain of the kinase. CaMKII activity is enhanced in several cardiac diseases, altering the signaling pathways by which CaMKII regulates the different fundamental proteins involved in functional and transcriptional cardiac processes. Dysregulation of these pathways constitutes a central mechanism of various cardiac disease phenomena, like apoptosis and necrosis during ischemia/reperfusion injury, digitalis exposure, post-acidosis and heart failure arrhythmias, or cardiac hypertrophy. Here we summarize significant aspects of the molecular physiology of CaMKII and provide a conceptual framework for understanding the role of the CaMKII cascade on Ca(2+) regulation and dysregulation in cardiac health and disease.

Suppression of Rad leads to arrhythmogenesis via PKA-mediated phosphorylation of ryanodine receptor activity in the heart

Ras-related small G-protein Rad plays a critical role in generating arrhythmias via regulation of the L-type Ca(2+) channel (LTCC). The aim was to demonstrate the role of Rad in intracellular calcium homeostasis by cardiac-Specific dominant-negative suppression of Rad. Transgenic (TG) mice overexpressing dominant-negative mutant Rad (S105N Rad TG) were generated. To measure intracellular Ca(2+) concentration ([Ca(2+)]i), we recorded [Ca(2+)]i transients and Ca(2+) sparks from isolated cardiomyocytes using confocal microscopy. The mean [Ca(2+)]i transient amplitude was significantly increased in S105N Rad TG cardiomyocytes, compared with control littermate mouse cells. The frequency of Ca(2+) sparks was also significantly higher in TG cells than in control cells, although there were no significant differences in amplitude. The sarcoplasmic reticulum Ca(2+) content was not altered in the S105N Rad TG cells, as assessed by measuring caffeine-induced [Ca(2+)]i transient. In contrast, phosphorylation of Ser(2809) on the cardiac ryanodine receptor (RyR2) was significantly enhanced in TG mouse hearts compared with controls. Additionally, the Rad-mediated RyR2 phosphorylation was regulated via a direct interaction of Rad with protein kinase A (PKA).

Ca(2+) current facilitation is CaMKII-dependent and has arrhythmogenic consequences

The cardiac voltage gated Ca²⁺ current (I_Ca) is critical to the electrophysiological properties, excitation-contraction coupling, mitochondrial energetics, and transcriptional regulation in heart. Thus, it is not surprising that cardiac I_Ca is regulated by numerous pathways. This review will focus on changes in I_Ca that occur during the cardiac action potential (AP), with particular attention to Ca²⁺-dependent inactivation (CDI), Ca²⁺-dependent facilitation (CDF) and how calmodulin (CaM) and Ca²⁺-CaM dependent protein kinase (CaMKII) participate in the regulation of Ca²⁺ current during the cardiac AP. CDI depends on CaM pre-bound to the C-terminal of the L-type Ca²⁺ channel, such that Ca²⁺ influx and Ca²⁺ released from the sarcoplasmic reticulum bind to that CaM and cause CDI. In cardiac myocytes CDI normally pre-dominates over voltage-dependent inactivation. The decrease in I_Ca via CDI provides direct negative feedback on the overall Ca²⁺ influx during a single beat, when myocyte Ca²⁺ loading is high. CDF builds up over several beats, depends on CaMKII-dependent Ca²⁺ channel phosphorylation, and results in a staircase of increasing I_Ca peak, with progressively slower inactivation. CDF and CDI co-exist and in combination may fine-tune the I_Ca waveform during the cardiac AP. CDF may partially compensate for the tendency for Ca²⁺ channel availability to decrease at higher heart rates because of accumulating inactivation. CDF may also allow some reactivation of I_Ca during long duration cardiac APs, and contribute to early afterdepolarizations, a form of triggered arrhythmias.

The role of CaMKII regulation of phospholamban activity in heart disease

Phospholamban (PLN) is a phosphoprotein in cardiac sarcoplasmic reticulum (SR) that is a reversible regulator of the Ca²⁺-ATPase (SERCA2a) activity and cardiac contractility. Dephosphorylated PLN inhibits SERCA2a and PLN phosphorylation, at either Ser¹⁶ by PKA or Thr¹⁷ by Ca²⁺-calmodulin-dependent protein kinase (CaMKII), reverses this inhibition. Through this mechanism, PLN is a key modulator of SR Ca²⁺ uptake, Ca²⁺ load, contractility, and relaxation. PLN phosphorylation is also the main determinant of β1-adrenergic responses in the heart. Although phosphorylation of Thr¹⁷ by CaMKII contributes to this effect, its role is subordinate to the PKA-dependent increase in cytosolic Ca²⁺, necessary to activate CaMKII. Furthermore, the effects of PLN and its phosphorylation on cardiac function are subject to additional regulation by its interacting partners, the anti-apoptotic HAX-1 protein and Gm or the anchoring unit of protein phosphatase 1. Regulation of PLN activity by this multimeric complex becomes even more important in pathological conditions, characterized by aberrant Ca²⁺-cycling. In this scenario, CaMKII-dependent PLN phosphorylation has been associated with protective effects in both acidosis and ischemia/reperfusion. However, the beneficial effects of increasing SR Ca²⁺ uptake through PLN phosphorylation may be lost or even become deleterious, when these occur in association with alterations in SR Ca²⁺ leak. Moreover, a major characteristic in human and experimental heart failure (HF) is depressed SR Ca²⁺ uptake, associated with decreased SERCA2a levels and dephosphorylation of PLN, leading to decreased SR Ca²⁺ load and impaired contractility. Thus, the strategy of altering SERCA2a and/or PLN levels or activity to restore perturbed SR Ca²⁺ uptake is a potential therapeutic tool for HF treatment. We will review here the role of CaMKII-dependent phosphorylation of PLN at Thr¹⁷ on cardiac function under physiological and pathological conditions.

Rad GTPase deletion increases L-type calcium channel current leading to increased cardiac contraction

BACKGROUND

The small GTPase Rad is a negative regulator of voltage-dependent L-type calcium channel current (ICaL); however, the effects of Rad ablation on cardiomyocyte function are unknown. The objective of this study is to test the hypothesis that Rad-depletion causes positive inotropic effects without inducing cardiac hypertrophy.

METHODS AND RESULTS

Ventricular myocytes from adult Rad(-/-) mice were isolated and evaluated by patch-clamp recordings for I(Ca,L) and action potentials, Ca(2+) transients, and sarcomere shortening. Maximum I(CaL) is elevated in Rad(-/-) (maximal conductance 0.35 ± 0.04 picoSiemens/picoFarad (pS/pF) wild-type; 0.61 ± 0.14 pS/pF Rad(-/-)), decay kinetics are faster, and I(Ca,L) activates at lower voltages (activation midpoint -7.2 ± 0.6 wild-type; -11.7 ± 0.9 Rad(-/-)) mimicking effects of β-adrenergic receptor stimulation. Diastolic and twitch calcium are elevated in Rad(-/-) (F340/380: 1.03 diastolic and 0.35 twitch for wild-type; 1.47 diastolic and 0.736 twitch for Rad(-/-)) and sarcomere shortening is enhanced (4.31% wild-type; 14.13% Rad(-/-)) at lower pacing frequencies. Consequentially, frequency-dependence of Ca(2+) transients is less in Rad(-/-), and the frequency dependence of relaxation is also blunted. In isolated working hearts, similar results were obtained; chiefly, +dP/dt was elevated at baseline and developed pressure was relatively nonresponsive to acute β-adrenergic receptor stimulation. In single cells, at subphysiological frequencies, nonstimulated calmodulin-dependent protein kinase-sensitive calcium release is observed. Remarkably, Rad(-/-) hearts did not show hypertrophic growth despite elevated levels of diastolic calcium.

CONCLUSIONS

This study demonstrates that the depletion of Rad GTPase is equivalent to sympathomimetic β-adrenergic receptor, without stimulating cardiac hypertrophy. Thus, targeting Rad GTPase is a novel potential therapeutic target for Ca(2+)-homeostasis-driven positive inotropic support of the heart.

Atrial myocyte function and Ca2+ handling is associated with inborn aerobic capacity

Background

Although high aerobic capacity is associated with effective cardiac function, the effect of aerobic capacity on atrial function, especially in terms of cellular mechanisms, is not known. We aimed to investigate whether rats with low inborn maximal oxygen uptake (VO_{2 max}) had impaired atrial myocyte contractile function when compared to rats with high inborn VO_{2 max}.

Methods and Results

Atrial myocyte function was depressed in Low Capacity Runners (LCR) relative to High Capacity Runners (HCR) which was associated with impaired Ca²⁺ handling. Fractional shortening was 52% lower at 2 Hz and 60% lower at 5 Hz stimulation while time to 50% relengthening was 43% prolonged and 55% prolonged, respectively. Differences in Ca²⁺ amplitude and diastolic Ca²⁺ level were observed at 5 Hz stimulation where Ca²⁺ amplitude was 70% lower and diastolic Ca²⁺ level was 11% higher in LCR rats. Prolonged time to 50% Ca²⁺ decay was associated with reduced sarcoplasmic reticulum (SR) Ca²⁺ ATPase function in LCR (39%). Na⁺/Ca²⁺ exchanger activity was comparable between the groups. Diastolic SR Ca²⁺ leak was increased by 109%. This could be partly explained by increased ryanodine receptors phosphorylation at the Ca²⁺-calmodulin-dependent protein kinase-II specific Ser-2814 site in LCR rats. T-tubules were present in 68% of HCR cells whereas only 33% LCR cells had these structures. In HCR, the significantly higher numbers of cells with T-tubules were combined with greater numbers of myocytes where Ca²⁺ release in the cell occurred simultaneously in central and peripheral regions, giving rise to faster and more spatial homogenous Ca²⁺-signal onset.

Conclusion

This data demonstrates that contrasting for low or high aerobic capacity leads to diverse functional and structural remodelling of atrial myocytes, with impaired contractile function in LCR compared to HCR rats.

Rate-dependent Ca2+ signalling underlying the force-frequency response in rat ventricular myocytes: a coupled electromechanical modeling study

BACKGROUND

Rate-dependent effects on the Ca2+ sub-system in a rat ventricular myocyte are investigated. Here, we employ a deterministic mathematical model describing various Ca2+ signalling pathways under voltage clamp (VC) conditions, to better understand the important role of calmodulin (CaM) in modulating the key control variables Ca2+/calmodulin-dependent protein kinase-II (CaMKII), calcineurin (CaN), and cyclic adenosine monophosphate (cAMP) as they affect various intracellular targets. In particular, we study the frequency dependence of the peak force generated by the myofilaments, the force-frequency response (FFR).

METHODS

Our cell model incorporates frequency-dependent CaM-mediated spatially heterogenous interaction of CaMKII and CaN with their principal targets (dihydropyridine (DHPR) and ryanodine (RyR) receptors and the SERCA pump). It also accounts for the rate-dependent effects of phospholamban (PLB) on the SERCA pump; the rate-dependent role of cAMP in up-regulation of the L-type Ca2+ channel (ICa,L); and the enhancement in SERCA pump activity via phosphorylation of PLB.

RESULTS

Our model reproduces positive peak FFR observed in rat ventricular myocytes during voltage-clamp studies both in the presence/absence of cAMP mediated β-adrenergic stimulation. This study provides quantitative insight into the rate-dependence of Ca2+-induced Ca2+-release (CICR) by investigating the frequency-dependence of the trigger current (ICa,L) and RyR-release. It also highlights the relative role of the sodium-calcium exchanger (NCX) and the SERCA pump at higher frequencies, as well as the rate-dependence of sarcoplasmic reticulum (SR) Ca2+ content. A rigorous Ca2+ balance imposed on our investigation of these Ca2+ signalling pathways clarifies their individual roles. Here, we present a coupled electromechanical study emphasizing the rate-dependence of isometric force developed and also investigate the temperature-dependence of FFR.

CONCLUSIONS

Our model provides mechanistic biophysically based explanations for the rate-dependence of CICR, generating useful and testable hypotheses. Although rat ventricular myocytes exhibit a positive peak FFR in the presence/absence of beta-adrenergic stimulation, they show a characteristic increase in the positive slope in FFR due to the presence of Norepinephrine or Isoproterenol. Our study identifies cAMP-mediated stimulation, and rate-dependent CaMKII-mediated up-regulation of ICa,L as the key mechanisms underlying the aforementioned positive FFR.

Trypanosoma brucei cathepsin-L increases arrhythmogenic sarcoplasmic reticulum-mediated calcium release in rat cardiomyocytes

Aims

African trypanosomiasis, caused by Trypanosoma brucei species, leads to both neurological and cardiac dysfunction and can be fatal if untreated. While the neurological-related pathogenesis is well studied, the cardiac pathogenesis remains unknown. The current study exposed isolated ventricular cardiomyocytes and adult rat hearts to T. brucei to test whether trypanosomes can alter cardiac function independent of a systemic inflammatory/immune response.

Methods and results

Using confocal imaging, T. brucei and T. brucei culture media (supernatant) caused an increased frequency of arrhythmogenic spontaneous diastolic sarcoplasmic reticulum (SR)-mediated Ca²⁺ release (Ca²⁺ waves) in isolated adult rat ventricular cardiomyocytes. Studies utilising inhibitors, recombinant protein and RNAi all demonstrated that this altered SR function was due to T. brucei cathepsin-L (TbCatL). Separate experiments revealed that TbCatL induced a 10–15% increase of SERCA activity but reduced SR Ca²⁺ content, suggesting a concomitant increased SR-mediated Ca²⁺ leak. This conclusion was supported by data demonstrating that TbCatL increased Ca²⁺ wave frequency. These effects were abolished by autocamtide-2-related inhibitory peptide, highlighting a role for CaMKII in the TbCatL action on SR function. Isolated Langendorff perfused whole heart experiments confirmed that supernatant caused an increased number of arrhythmic events.

Conclusion

These data demonstrate for the first time that African trypanosomes alter cardiac function independent of a systemic immune response, via a mechanism involving extracellular cathepsin-L-mediated changes in SR function.

A Memory Molecule, Ca(2+)/Calmodulin-Dependent Protein Kinase II and Redox Stress; Key Factors for Arrhythmias in a Diseased Heart

Arrhythmias can develop in various cardiac diseases, such as ischemic heart disease, cardiomyopathy and congenital heart disease. It can also contribute to the aggravation of heart failure and sudden cardiac death. Redox stress and Ca²⁺ overload are thought to be the important triggering factors in the generation of arrhythmias in failing myocardium. From recent studies, it appears evident that Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) plays a central role in the arrhythmogenic processes in heart failure by sensing intracellular Ca²⁺ and redox stress, affecting individual ion channels and thereby leading to electrical instability in the heart. CaMKII, a multifunctional serine/threonine kinase, is an abundant molecule in the neuron and the heart. It has a specific property as "a memory molecule" such that the binding of calcified calmodulin (Ca²⁺/CaM) to the regulatory domain on CaMKII initially activates this enzyme. Further, it allows autophosphorylation of T287 or oxidation of M281/282 in the regulatory domain, resulting in sustained activation of CaMKII even after the dissociation of Ca²⁺/CaM. This review provides the understanding of both the structural and functional properties of CaMKII, the experimental findings of the interactions between CaMKII, redox stress and individual ion channels, and the evidences proving the potential participation of CaMKII and oxidative stress in the diverse arrhythmogenic processes in a diseased heart.

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.

JTV519 (K201) reduces sarcoplasmic reticulum Ca²⁺ leak and improves diastolic function in vitro in murine and human non-failing myocardium

BACKGROUND AND PURPOSE

Ca²⁺ leak from the sarcoplasmic reticulum (SR) via ryanodine receptors (RyR2s) contributes to cardiomyocyte dysfunction. RyR2 Ca²⁺ leak has been related to RyR2 phosphorylation. In these conditions, JTV519 (K201), a 1,4-benzothiazepine derivative and multi-channel blocker, stabilizes RyR2s and decrease SR Ca²⁺ leak. We investigated whether JTV519 stabilizes RyR2s without increasing RyR2 phosphorylation in mice and in non-failing human myocardium and explored underlying mechanisms.

EXPERIMENTAL APPROACH

SR Ca²⁺ leak was induced by ouabain in murine cardiomyocytes. [Ca²⁺]-transients, SR Ca²⁺ load and RyR2-mediated Ca²⁺ leak (sparks/waves) were quantified, with or without JTV519 (1 µmol·L⁻¹). Contribution of Ca²⁺ -/calmodulin-dependent kinase II (CaMKII) was assessed by KN-93 and Western blot (RyR2-Ser(2814) phosphorylation). Effects of JTV519 on contractile force were investigated in non-failing human ventricular trabeculae.

KEY RESULTS

Ouabain increased systolic and diastolic cytosolic [Ca²⁺](i) , SR [Ca²⁺], and SR Ca²⁺ leak (Ca²⁺ spark (SparkF) and Ca²⁺ wave frequency), independently of CaMKII and RyR-Ser(2814) phosphorylation. JTV519 decreased SparkF but also SR Ca²⁺ load. At matched SR [Ca²⁺], Ca²⁺ leak was significantly reduced by JTV519, but it had no effect on fractional Ca²⁺ release or Ca²⁺ wave propagation velocity. In human muscle, JTV519 was negatively inotropic at baseline but significantly enhanced ouabain-induced force and reduced its deleterious effects on diastolic function.

CONCLUSIONS AND IMPLICATIONS

JTV519 was effective in reducing SR Ca²⁺ leak by specifically regulating RyR2 opening at diastolic [Ca²⁺](i) in the absence of increased RyR2 phosphorylation at Ser(2814) , extending the potential use of JTV519 to conditions of acute cellular Ca²⁺ overload.

Ginkgo biloba extract EGB761 protects against aging-associated diastolic dysfunction in cardiomyocytes of D-galactose-induced aging rat

The aim of the present study was to make use of the artificially induced aging model cardiomyocytes to further investigate potential anti-aging-associated cellular diastolic dysfunction effects of EGB761 and explore underlying molecular mechanisms. Cultured rat primary cardiomyocytes were treated with either D-galactose or D-galactose combined with EGB761 for 48 h. After treatment, the percentage of cells positive for SA-β-gal, AGEs production, cardiac sarcoplasmic reticulum calcium pump (SERCA) activity, the myocardial sarcoplasmic reticulum calcium uptake, and relative protein levels were measured. Our results demonstrated that in vitro stimulation with D-galactose induced AGEs production. The addition of EGB761 significantly decreased the number of cells positive for SA-β-gal. Furthermore, decreased diastolic [Ca²⁺]_i, curtailment of the time from the maximum concentration of Ca²⁺ to the baseline level and increased reuptake of Ca²⁺ stores in the SR were also observed. In addition, the level of p-Ser16-PLN protein as well as SERCA was markedly increased. The study indicated that EGb761 alleviates formation of AGEs products on SERCA2a in order to mitigate myocardial stiffness on one hand; on other hand, improve SERCA2a function through increase the amount of Ser16 sites PLN phosphorylation, which two hands finally led to ameliorate diastolic dysfunction of aging cardiomyocytes.

Staurosporine inhibits frequency-dependent myofilament desensitization in intact rabbit cardiac trabeculae

Myofilament calcium sensitivity decreases with frequency in intact healthy rabbit trabeculae and associates with Troponin I and Myosin light chain-2 phosphorylation. We here tested whether serine-threonine kinase activity is primarily responsible for this frequency-dependent modulations of myofilament calcium sensitivity. Right ventricular trabeculae were isolated from New Zealand White rabbit hearts and iontophoretically loaded with bis-fura-2. Twitch force-calcium relationships and steady state force-calcium relationships were measured at frequencies of 1 and 4 Hz at 37 °C. Staurosporine (100 nM), a nonspecific serine-threonine kinase inhibitor, or vehicle (DMSO) was included in the superfusion solution before and during the contractures. Staurosporine had no frequency-dependent effect on force development, kinetics, calcium transient amplitude, or rate of calcium transient decline. The shift in the pCa₅₀ of the force-calcium relationship was significant from 6.05 ± 0.04 at 1 Hz versus 5.88 ± 0.06 at 4 Hz under control conditions (vehicle, P < 0.001) but not in presence of staurosporine (5.89 ± 0.08 at 1 Hz versus 5.94 ± 0.07 at 4 Hz, P = NS). Phosphoprotein analysis (Pro-Q Diamond stain) confirmed that staurosporine significantly blunted the frequency-dependent phosphorylation at Troponin I and Myosin light chain-2. We conclude that frequency-dependent modulation of calcium sensitivity is mediated through a kinase-specific effect involving phosphorylation of myofilament proteins.

Excitation-contraction coupling in ventricular myocytes is enhanced by paracrine signaling from mesenchymal stem cells

In clinical trials mesenchymal stem cells (MSCs) are transplanted into cardiac ischemic regions to decrease infarct size and improve contractility. However, the mechanism and time course of MSC-mediated cardioprotection are incompletely understood. We tested the hypothesis that paracrine signaling by MSCs promotes changes in cardiac excitation-contraction (EC) coupling that protects myocytes from cell death and enhances contractility. Isolated mouse ventricular myocytes (VMs) were treated with control tyrode, MSC conditioned-tyrode (ConT) or co-cultured with MSCs. The Ca handling properties of VMs were monitored by laser scanning confocal microscopy and whole cell voltage clamp. ConT superfusion of VMs resulted in a time dependent increase of the Ca transient amplitude (ConT(15min): ΔF/F(0)=3.52±0.38, n=14; Ctrl(15min): ΔF/F(0)=2.41±0.35, n=14) and acceleration of the Ca transient decay (τ: ConT: 269±18ms n=14; vs. Ctrl: 315±57ms, n=14). Voltage clamp recordings confirmed a ConT induced increase in I(Ca,L) (ConT: -5.9±0.5 pA/pF n=11; vs. Ctrl: -4.04±0.3 pA/pF, n=12). The change of τ resulted from increased SERCA activity. Changes in the Ca transient amplitude and τ were prevented by the PI3K inhibitors Wortmannin (100nmol/L) and LY294002 (10μmol/L) and the Akt inhibitor V (20μmol/L) indicating regulation through PI3K signal transduction and Akt activation which was confirmed by western blotting. A change in τ was also prevented in eNOS(-/-) myocytes or by inhibition of eNOS suggesting an NO mediated regulation of SERCA activity. Since paracrine signaling further resulted in increased survival of VMs we propose that the Akt induced change in Ca signaling is also a mechanism by which MSCs mediate an anti-apoptotic effect.

Cardiac calmodulin kinase: a potential target for drug design

Therapeutic strategy for cardiac arrhythmias has undergone a remarkable change during the last decades. Currently implantable cardioverter defibrillator therapy is considered to be the most effective therapeutic method to treat malignant arrhythmias. Some even argue that there is no room for antiarrhythmic drug therapy in the age of implantable cardioverter defibrillators. However, in clinical practice, antiarrhythmic drug therapies are frequently needed, because implantable cardioverter defibrillators are not effective in certain types of arrhythmias (i.e. premature ventricular beats or atrial fibrillation). Furthermore, given the staggering cost of device therapy, it is economically imperative to develop alternative effective treatments. Cardiac ion channels are the target of a number of current treatment strategies, but therapies based on ion channel blockers only resulted in moderate success. Furthermore, these drugs are associated with an increased risk of proarrhythmia, systemic toxicity, and increased defibrillation threshold. In many cases, certain ion channel blockers were found to increase mortality. Other drug classes such as ßblockers, angiotensin-converting enzyme inhibitors, aldosterone antagonists, and statins appear to have proven efficacy for reducing cardiac mortality. These facts forced researchers to shift the focus of their research to molecular targets that act upstream of ion channels. One of these potential targets is calcium/calmodulin-dependent kinase II (CaMKII). Several lines of evidence converge to suggest that CaMKII inhibition may provide an effective treatment strategy for heart diseases. (1) Recent studies have elucidated that CaMKII plays a key role in modulating cardiac function and regulating hypertrophy development. (2) CaMKII activity has been found elevated in the failing hearts from human patients and animal models. (3) Inhibition of CaMKII activity has been shown to mitigate hypertrophy, prevent functional remodeling and reduce arrhythmogenic activity. In this review, we will discuss the structural and functional properties of CaMKII, the modes of its activation and the functional consequences of CaMKII activity on ion channels.

Sustained CaMKII activity mediates transient oxidative stress-induced long-term facilitation of L-type Ca(2+) current in cardiomyocytes

Oxidative stress remodels Ca(2+) signaling in cardiomyocytes, which promotes altered heart function in various heart diseases. Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) was shown to be activated by oxidation, but whether and how CaMKII links oxidative stress to pathophysiological long-term changes in Ca(2+) signaling remain unknown. Here, we present evidence demonstrating the role of CaMKII in transient oxidative stress-induced long-term facilitation (LTF) of L-type Ca(2+) current (I(Ca,L)) in rat cardiomyocytes. A 5-min exposure of 1mM H(2)O(2) induced an increase in I(Ca,L), and this increase was sustained for ~1h. The CaMKII inhibitor KN-93 fully reversed H(2)O(2)-induced LTF of I(Ca,L), indicating that sustained CaMKII activity underlies this oxidative stress-induced memory. Simultaneous inhibition of oxidation and autophosphorylation of CaMKII prevented the maintenance of LTF, suggesting that both mechanisms contribute to sustained CaMKII activity. We further found that sarcoplasmic reticulum Ca(2+) release and mitochondrial ROS generation have critical roles in sustaining CaMKII activity via autophosphorylation- and oxidation-dependent mechanisms. Finally, we show that long-term remodeling of the cardiac action potential is induced by H(2)O(2) via CaMKII. In conclusion, CaMKII and mitochondria confer oxidative stress-induced pathological cellular memory that leads to cardiac arrhythmia.

CaMKII in the cardiovascular system: sensing redox states

The multifunctional Ca(2+)- and calmodulin-dependent protein kinase II (CaMKII) is now recognized to play a central role in pathological events in the cardiovascular system. CaMKII has diverse downstream targets that promote vascular disease, heart failure, and arrhythmias, so improved understanding of CaMKII signaling has the potential to lead to new therapies for cardiovascular disease. CaMKII is a multimeric serine-threonine kinase that is initially activated by binding calcified calmodulin (Ca(2+)/CaM). Under conditions of sustained exposure to elevated Ca(2+)/CaM, CaMKII transitions into a Ca(2+)/CaM-autonomous enzyme by two distinct but parallel processes. Autophosphorylation of threonine-287 in the CaMKII regulatory domain "traps" CaMKII into an open configuration even after Ca(2+)/CaM unbinding. More recently, our group identified a pair of methionines (281/282) in the CaMKII regulatory domain that undergo a partially reversible oxidation which, like autophosphorylation, prevents CaMKII from inactivating after Ca(2+)/CaM unbinding. Here we review roles of CaMKII in cardiovascular disease with an eye to understanding how CaMKII may act as a transduction signal to connect pro-oxidant conditions into specific downstream pathological effects that are relevant to rare and common forms of cardiovascular disease.

Calcium-calmodulin dependent protein kinase II (CaMKII): a main signal responsible for early reperfusion arrhythmias

To explore whether CaMKII-dependent phosphorylation events mediate reperfusion arrhythmias, Langendorff perfused hearts were submitted to global ischemia/reperfusion. Epicardial monophasic or transmembrane action potentials and contractility were recorded. In rat hearts, reperfusion significantly increased the number of premature beats (PBs) relative to pre-ischemic values. This arrhythmic pattern was associated with a significant increase in CaMKII-dependent phosphorylation of Ser2814 on Ca(2+)-release channels (RyR2) and Thr17 on phospholamban (PLN) at the sarcoplasmic reticulum (SR). These phenomena could be prevented by the CaMKII-inhibitor KN-93. In transgenic mice with targeted inhibition of CaMKII at the SR membranes (SR-AIP), PBs were significantly decreased from 31±6 to 5±1 beats/3min with a virtually complete disappearance of early-afterdepolarizations (EADs). In mice with genetic mutation of the CaMKII phosphorylation site on RyR2 (RyR2-S2814A), PBs decreased by 51.0±14.7%. In contrast, the number of PBs upon reperfusion did not change in transgenic mice with ablation of both PLN phosphorylation sites (PLN-DM). The experiments in SR-AIP mice, in which the CaMKII inhibitor peptide is anchored in the SR membrane but also inhibits CaMKII regulation of L-type Ca(2+) channels, indicated a critical role of CaMKII-dependent phosphorylation of SR proteins and/or L-type Ca(2+) channels in reperfusion arrhythmias. The experiments in RyR2-S2814A further indicate that up to 60% of PBs related to CaMKII are dependent on the phosphorylation of RyR2-Ser2814 site and could be ascribed to delayed-afterdepolarizations (DADs). Moreover, phosphorylation of PLN-Thr17 and L-type Ca(2+) channels might contribute to reperfusion-induced PBs, by increasing SR Ca(2+) content and Ca(2+) influx.

In vitro studies of early cardiac remodeling: impact on contraction and calcium handling

Cardiac remodeling, hypertrophy, and alterations in calcium signaling are changes of the heart that often lead to failure. After a hypertrophic stimulus, the heart progresses through a state of compensated hypertrophy which over time leads to decompensated hypertrophy or failure. It is at this point that a cardiac transplant is required for survival making early detection imperative. Current experimental systems used to study the remodeling of the heart include in vivo systems (the whole body), isolated organ and sub-organ tissue, and the individual cardiac muscle cells and organelles.. During pathological remodeling there is a derangement in the intracellular calcium handling processes. These derangements are thought to lead to a dysregulation of contractile output. Hence, understanding the mechanism between remodeling and dysregulation is of great interest in the cardiac field and will ultimately help in the development of future treatment and early detection. This review will center on changes in contraction and calcium handling in early cardiac remodeling, with a specific focus on findings in two different in vitro model systems: multicellular and individual cell preparations.

Synergy between CaMKII substrates and β-adrenergic signaling in regulation of cardiac myocyte Ca(2+) handling

Cardiac excitation-contraction coupling is a highly coordinated process that is controlled by protein kinase signaling pathways, including Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) and protein kinase A (PKA). Increased CaMKII expression and activity (as occurs during heart failure) destabilizes EC coupling and may lead to sudden cardiac death. To better understand mechanisms of cardiac CaMKII function, we integrated dynamic CaMKII-dependent regulation of key Ca(2+) handling targets with previously validated models of cardiac EC coupling, Ca(2+)/calmodulin-dependent activation of CaMKII, and β-adrenergic activation of PKA. Model predictions are validated against CaMKII-overexpression data from rabbit ventricular myocytes. The model demonstrates how overall changes to Ca(2+) handling during CaMKII overexpression are explained by interactions between individual CaMKII substrates. CaMKII and PKA activities during β-adrenergic stimulation may synergistically facilitate inotropic responses and contribute to a CaMKII-Ca(2+)-CaMKII feedback loop. CaMKII regulated early frequency-dependent acceleration of relaxation and EC coupling gain (which was highly sarcoplasmic reticulum Ca(2+) load-dependent). Additionally, the model identifies CaMKII-dependent ryanodine receptor hyperphosphorylation as a proarrhythmogenic trigger. In summary, we developed a detailed computational model of CaMKII and PKA signaling in cardiac myocytes that provides unique insights into their regulation of normal and pathological Ca(2+) handling.

Impact of long-term caloric restriction on cardiac senescence: caloric restriction ameliorates cardiac diastolic dysfunction associated with aging

Approximately half of older patients with congestive heart failure have normal left ventricular (LV) systolic but abnormal LV diastolic function. In mammalian hearts, aging is associated with LV diastolic dysfunction. Caloric restriction (CR) is expected to retard cellular senescence and to attenuate the physiological decline in organ function. Therefore, the aim of the present study was to investigate the impact of long-term CR on cardiac senescence, in particular the effect of CR on LV diastolic dysfunction associated with aging. Male 8-month-old Fischer344 rats were divided into ad libitum fed and CR (40% energy reduction) groups. LV function was evaluated by echocardiography and cardiac senescence was compared between the two groups at the age of 30-month-old. (1) Echocardiography showed similar LV systolic function, but better LV diastolic function in the CR group. (2) Histological analysis revealed that CR attenuated the accumulation of senescence-associated β-galactosidase and lipofuscin and reduced myocyte apoptosis. (3) In measurements of [Ca(2+)](i) transients, the time to 50% relaxation was significantly smaller in the CR group, whereas F/F(0) was similar. (4) CR attenuated the decrease in sarcoplasmic reticulum calcium ATPase 2 protein with aging. (5) CR suppressed the mammalian target of rapamycin (mTOR) pathway and increased the ratio of conjugated to cytosolic light chain 3, suggesting that autophagy is enhanced in the CR hearts. In conclusion, CR improves diastolic function in the senescent myocardium by amelioration of the age-associated deterioration in intracellular Ca(2+) handling. Enhanced autophagy via the suppression of mTOR during CR may retard cardiac senescence.

SR-targeted CaMKII inhibition improves SR Ca²+ handling, but accelerates cardiac remodeling in mice overexpressing CaMKIIδC

Cardiac myocyte overexpression of CaMKIIδ(C) leads to cardiac hypertrophy and heart failure (HF) possibly caused by altered myocyte Ca(2+) handling. A central defect might be the marked CaMKII-induced increase in diastolic sarcoplasmic reticulum (SR) Ca(2+) leak which decreases SR Ca(2+) load and Ca(2+) transient amplitude. We hypothesized that inhibition of CaMKII near the SR membrane would decrease the leak, improve Ca(2+) handling and prevent the development of contractile dysfunction and HF. To test this hypothesis we crossbred CaMKIIδ(C) overexpressing mice (CaMK) with mice expressing the CaMKII-inhibitor AIP targeted to the SR via a modified phospholamban (PLB)-transmembrane-domain (SR-AIP). There was a selective decrease in the amount of activated CaMKII in the microsomal (SR/membrane) fraction prepared from these double-transgenic mice (CaMK/SR-AIP) mice. In ventricular cardiomyocytes from CaMK/SR-AIP mice, SR Ca(2+) leak, assessed both as diastolic Ca(2+) shift into SR upon tetracaine in intact myocytes or integrated Ca(2+) spark release in permeabilized myocytes, was significantly reduced. The reduced leak was accompanied by enhanced SR Ca(2+) load and twitch amplitude in double-transgenic mice (vs. CaMK), without changes in SERCA expression or NCX function. However, despite the improved myocyte Ca(2+) handling, cardiac hypertrophy and remodeling was accelerated in CaMK/SR-AIP and cardiac function worsened. We conclude that while inhibition of SR localized CaMKII in CaMK mice improves Ca(2+) handling, it does not necessarily rescue the HF phenotype. This implies that a non-SR CaMKIIδ(C) exerts SR-independent effects that contribute to hypertrophy and HF, and this CaMKII pathway may be exacerbated by the global enhancement of Ca transients.

Myocardial contraction-relaxation coupling

Since the pioneering work of Henry Pickering Bowditch in the late 1800s to early 1900s, cardiac muscle contraction has remained an intensely studied topic for several reasons. The heart is located centrally in our body, and its pumping motion demands the attention of the observer. The contraction of the heart encompasses a complex interplay of mechanical, chemical, and electrical properties, and its function can thus be studied from any of these viewpoints. In addition, diseases of the heart are currently killing more people in the Westernized world than any other disease. When combined with the increasing emphasis of research to be clinically relevant, this contributes to the heart remaining a topic of continued basic and clinical investigation. Yet, there are significant aspects of cardiac muscle contraction that are still not well understood. A big complication of the study of cardiac muscle contraction is that there exists no equilibrium among many of the important governing parameters, which include pre- and afterload, intracellular ion concentrations, membrane potential, and velocity and direction of movement. Thus the classic approach of perturbing an equilibrium or a steady state to learn about the role of the perturbing factor in the system cannot be unambiguously interpreted, since each of the parameters that govern contraction are constantly changing, as well as constantly changing their interaction with each other. In this review, presented as the 54th Bowditch Lecture at Experimental Biology meeting in Anaheim in April 2010, I will revisit several governing factors of cardiac muscle relaxation by applying newly developed tools and protocols to isolated cardiac muscle tissue in which the dynamic interactions between the governing factors of contraction and relaxation can be studied.

A mathematical model of the murine ventricular myocyte: a data-driven biophysically based approach applied to mice overexpressing the canine NCX isoform

Mathematical modeling of Ca(2+) dynamics in the heart has the potential to provide an integrated understanding of Ca(2+)-handling mechanisms. However, many previous published models used heterogeneous experimental data sources from a variety of animals and temperatures to characterize model parameters and motivate model equations. This methodology limits the direct comparison of these models with any particular experimental data set. To directly address this issue, in this study, we present a biophysically based model of Ca(2+) dynamics directly fitted to experimental data collected in left ventricular myocytes isolated from the C57BL/6 mouse, the most commonly used genetic background for genetically modified mice in studies of heart diseases. This Ca(2+) dynamics model was then integrated into an existing mouse cardiac electrophysiology model, which was reparameterized using experimental data recorded at consistent and physiological temperatures. The model was validated against the experimentally observed frequency response of Ca(2+) dynamics, action potential shape, dependence of action potential duration on cycle length, and electrical restitution. Using this framework, the implications of cardiac Na(+)/Ca(2+) exchanger (NCX) overexpression in transgenic mice were investigated. These simulations showed that heterozygous overexpression of the canine cardiac NCX increases intracellular Ca(2+) concentration transient magnitude and sarcoplasmic reticulum Ca(2+) loading, in agreement with experimental observations, whereas acute overexpression of the murine cardiac NCX results in a significant loss of Ca(2+) from the cell and, hence, depressed sarcoplasmic reticulum Ca(2+) load and intracellular Ca(2+) concentration transient magnitude. From this analysis, we conclude that these differences are primarily due to the presence of allosteric regulation in the canine cardiac NCX, which has not been observed experimentally in the wild-type mouse heart.

Alterations of L-type calcium current and cardiac function in CaMKII{delta} knockout mice

RATIONALE

Recent studies have highlighted important roles of CaMKII in regulating Ca(2+) handling and excitation-contraction coupling. However, the cardiac effect of chronic CaMKII inhibition has not been well understood.

OBJECTIVE

We have tested the alterations of L-type calcium current (I(Ca)) and cardiac function in CaMKIIdelta knockout (KO) mouse left ventricle (LV).

METHODS AND RESULTS

We used the patch-clamp method to record I(Ca) in ventricular myocytes and found that in KO LV, basal I(Ca) was significantly increased without changing the transmural gradient of I(Ca) distribution. Substitution of Ba(2+) for Ca(2+) showed similar increase in I(Ba). There was no change in the voltage dependence of I(Ca) activation and inactivation. I(Ca) recovery from inactivation, however, was significantly slowed. In KO LV, the Ca(2+)-dependent I(Ca) facilitation (CDF) and I(Ca) response to isoproterenol (ISO) were significantly reduced. However, ISO response was reversed by beta2-adrenergic receptor (AR) inhibition. Western blots showed a decrease in beta1-AR and an increase in Ca(v)1.2, beta2-AR, and Galphai3 protein levels. Ca(2+) transient and sarcomere shortening in KO myocytes were unchanged at 1-Hz but reduced at 3-Hz stimulation. Echocardiography in conscious mice revealed an increased basal contractility in KO mice. However, cardiac reserve to work load and beta-adrenergic stimulation was reduced. Surprisingly, KO mice showed a reduced heart rate in response to work load or beta-adrenergic stimulation.

CONCLUSIONS

Our results implicate physiological CaMKII activity in maintaining normal I(Ca), Ca(2+) handling, excitation-contraction coupling, and the in vivo heart function in response to cardiac stress.

Regulation of myocyte contraction via neuronal nitric oxide synthase: role of ryanodine receptor S-nitrosylation

The sarcoplasmic reticulum (SR) Ca(2+) release channel (ryanodine receptor, RyR2) has been proposed to be an end target of neuronal nitric oxide synthase (NOS1) signalling. The purpose of this study is to investigate the mechanism of NOS1 modulation of RyR2 activity and the corresponding effect on myocyte function. Myocytes were isolated from NOS1 knockout (NOS1(/)) and wild-type mice. NOS1(/) myocytes displayed a decreased fractional SR Ca(2+) release, NOS1 knockout also led to reduced RyR2 S-nitrosylation levels. RyR2 channels from NOS1(/) hearts had decreased RyR2 open probability. Additionally, knockout of NOS1 led to a decrease in [(3)H]ryanodine binding, Ca(2+) spark frequency (CaSpF) and a rightward shift in the SR Ca(2+) leak/load relationship. Similar effects were observed with acute inhibition of NOS1. These data are indicative of decreased RyR2 activity in myocytes with NOS1 knockout or acute inhibition. Interestingly, the NO donor and nitrosylating agent SNAP reversed the depressed RyR2 open probability, the reduced CaSpF, and caused a leftward shift in the leak/load relationship in NOS1(/) myocytes. SNAP also normalized Ca(2+) transient and cell shortening amplitudes and SR fractional release in myocytes with NOS1 knockout or acute inhibition. Furthermore, SNAP was able to normalize the RyR2 S-nitrosylation levels. These data suggest that NOS1 signalling increases RyR2 activity via S-nitrosylation, which contributes to the NOS1-induced positive inotropic effect. Thus, RyR2 is an important end target of NOS1.

Facilitation of murine cardiac L-type Ca(v)1.2 channel is modulated by calmodulin kinase II-dependent phosphorylation of S1512 and S1570

Activity-dependent means of altering calcium (Ca(2)(+)) influx are assumed to be of great physiological consequence, although definitive tests of this assumption have only begun to emerge. Facilitation and inactivation offer two opposing, activity-dependent means of altering Ca(2+) influx via cardiac Ca(v)1.2 calcium channels. Voltage- and frequency-dependent facilitation of Ca(v)1.2 has been reported to depend on Calmodulin (CaM) and/or the activity of Calmodulin kinase II (CaMKII). Several sites within the cardiac L-type calcium channel complex have been proposed as the targets of CaMKII. Here, we generated mice with knockin mutations of alpha(1)1.2 S1512 and S1570 phosphorylation sites [sine facilitation (SF) mice]. Homocygote SF mice were viable and reproduced in a Mendelian ratio. Voltage-dependent facilitation in ventricular cardiomyocytes carrying the SF mutation was decreased from 1.58- to 1.18-fold. The CaMKII inhibitor KN-93 reduced facilitation to 1.28 in control cardiomyocytes. SF mutation negatively shifted the voltage-dependent inactivation and slowed recovery from inactivation, thereby making fewer channels available for activation. Telemetric ECG recordings at different heart rates showed that QT time decreased significantly more in SF than in control mice at higher rates. Our results strongly support the notion that CaMKII-dependent phosphorylation of Cav1.2 at S1512 and S1570 mediates Ca(2+) current facilitation in the murine heart.

Role of CaMKIIdelta phosphorylation of the cardiac ryanodine receptor in the force frequency relationship and heart failure

The force frequency relationship (FFR), first described by Bowditch 139 years ago as the observation that myocardial contractility increases proportionally with increasing heart rate, is an important mediator of enhanced cardiac output during exercise. Individuals with heart failure have defective positive FFR that impairs their cardiac function in response to stress, and the degree of positive FFR deficiency correlates with heart failure progression. We have identified a mechanism for FFR involving heart rate dependent phosphorylation of the major cardiac sarcoplasmic reticulum calcium release channel/ryanodine receptor (RyR2), at Ser2814, by calcium/calmodulin-dependent serine/threonine kinase-delta (CaMKIIdelta). Mice engineered with an RyR2-S2814A mutation have RyR2 channels that cannot be phosphorylated by CaMKIIdelta, and exhibit a blunted positive FFR. Ex vivo hearts from RyR2-S2814A mice also have blunted positive FFR, and cardiomyocytes isolated from the RyR2-S2814A mice exhibit impaired rate-dependent enhancement of cytosolic calcium levels and fractional shortening. The cardiac RyR2 macromolecular complexes isolated from murine and human failing hearts have reduced CaMKIIdelta levels. These data indicate that CaMKIIdelta phosphorylation of RyR2 plays an important role in mediating positive FFR in the heart, and that defective regulation of RyR2 by CaMKIIdelta-mediated phosphorylation is associated with the loss of positive FFR in failing hearts.

The signalling pathway of CaMKII-mediated apoptosis and necrosis in the ischemia/reperfusion injury

Ca(2+)-calmodulin-dependent protein kinase II (CaMKII) plays an important role mediating apoptosis/necrosis during ischemia-reperfusion (IR). We explored the mechanisms of this deleterious effect. Langendorff perfused rat and transgenic mice hearts with CaMKII inhibition targeted to sarcoplasmic reticulum (SR-AIP) were subjected to global IR. The onset of reperfusion increased the phosphorylation of Thr(17) site of phospholamban, without changes in total protein, consistent with an increase in CaMKII activity. Instead, there was a proportional decrease in the phosphorylation of Ser2815 site of ryanodine receptors (RyR2) and the amount of RyR2 at the onset of reperfusion, i.e. the ratio Ser2815/RyR2 did not change. Inhibition of the reverse Na(+)/Ca(2+)exchanger (NCX) mode (KBR7943) diminished phospholamban phosphorylation, reduced apoptosis/necrosis and enhanced mechanical recovery. CaMKII-inhibition (KN-93), significantly decreased phospholamban phosphorylation, infarct area, lactate dehydrogenase release (LDH) (necrosis), TUNEL positive nuclei, caspase-3 activity, Bax/Bcl-2 ratio and Ca(2+)-induced mitochondrial swelling (apoptosis), and increased contractile recovery when compared with non-treated IR hearts or IR hearts pretreated with the inactive analog, KN-92. Blocking SR Ca(2+) loading and release (thapsigargin/dantrolene), mitochondrial Ca(2+) uniporter (ruthenium red/RU360), or mitochondrial permeability transition pore (cyclosporine A), significantly decreased infarct size, LDH release and apoptosis. SR-AIP hearts failed to show an increase in the phosphorylation of Thr(17) of phospholamban at the onset of reflow and exhibited a significant decrease in infarct size, apoptosis and necrosis respect to controls. The results reveal an apoptotic-necrotic pathway mediated by CaMKII-dependent phosphorylations at the SR, which involves the reverse NCX mode and the mitochondria as trigger and end effectors, respectively, of the cascade.

A random cycle length approach for assessment of myocardial contraction in isolated rabbit myocardium

It is well known that the strength of cardiac contraction is dependent on the cycle length, evidenced by the force-frequency relationship (FFR) and the existence of postrest potentiation (PRP). Because the contractile strength of the steady-state FFR and force-interval relationship involve instant intrinsic responses to cycle length as well as slower acting components such as posttranslational modification-based mechanisms, it remains unclear how cycle length intrinsically affects cardiac contraction and relaxation. To dissect the impact of cycle length changes from slower acting signaling components associated with persisting changes in cycle length, we developed a novel technique/protocol to study cycle length-dependent effects on cardiac function; twitch contractions of right ventricular rabbit trabeculae at different cycle lengths were randomized around a steady-state frequency. Patterns of cycle lengths that resulted in changes in force and/or relaxation times can now be identified and analyzed. Using this novel protocol, taking under 10 min to complete, we found that the duration of the cycle length before a twitch contraction ("primary" cycle length) positively correlated with force. In sharp contrast, the cycle length one ("secondary") or two ("tertiary") beats before the analyzed twitch correlated negatively with force. Using this protocol, we can quantify the intrinsic effect of cycle length on contractile strength while avoiding rundown and lengthiness that are often complications of FFR and PRP assessments. The data show that the history of up to three cycle lengths before a contraction influences myocardial contractility and that primary cycle length affects cardiac twitch dynamics in the opposite direction from secondary/tertiary cycle lengths.

Visualizing sodium dynamics in isolated cardiomyocytes using fluorescent nanosensors

Regulation of sodium flux across the cell membrane plays a vital role in the generation of action potentials and regulation of membrane excitability in cells such as cardiomyocytes and neurons. Alteration of sodium channel function has been implicated in diseases such as epilepsy, long QT syndrome, and heart failure. However, single cell imaging of sodium dynamics has been limited due to the narrow selection of fluorescent sodium indicators available to researchers. Here we report, the detection of spatially defined sodium activity during action potentials. Fluorescent nanosensors that measure sodium in real-time, are reversible and are completely selective over other cations such as potassium that were used to image sodium. The use of the nanosensors in vitro was validated by determining drug-induced activation in heterologous cells transfected with the voltage-gated sodium channel Na(V)1.7. Spatial information of sodium concentrations during action potentials will provide insight at the cellular level on the role of sodium and how slight changes in sodium channel function can affect the entirety of an action potential.