Phospholamban: a major determinant of the cardiac force-frequency relationship

Anti-bradycardia pacing-impact on patients with HFpEF: a systematic review

Heart failure with preserved ejection fraction (HFpEF) has become an emerging concern. The protective effect of bradycardia in patients with reduced ejection fraction using beta-blockers or ivabradine does not improve symptoms in HFpEF. This review aims to assess current data regarding the impact of anti-bradycardia pacing in patients with HFpEF. A search was conducted on PubMed, ScienceDirect, Springer, and Wiley Online Library, selecting studies from 2013 to 2023. Relevant and eligible prospective studies and randomized controlled trials were included. Functional status, quality of life, and echocardiographic parameters were assessed. Six studies conformed to the selection criteria. Four were prospective studies with a total of 90 patients analyzed. Two were randomized controlled trials with a total of 129 patients assessed. The 6-min walk test (6MWT) and the Minnesota Living with Heart Failure Questionnaire (MLHFQ) score improved in all prospective studies. My-PACE trial showed improvements in MLHFQ score (p < 0.001), significant relative lowering in NT-proBNP levels (p = 0.02), and an increased mean daily activity in the personalized accelerated pacing group compared to usual care. RAPID-HF trial proved that pacemaker implantation to enhance exercise heart rate (HR) did not improve exercise capacity and was associated with increased adverse events. HFpEF requires a more individualized approach and quality of life management. This review demonstrates that higher resting HR by atrial pacing may improve symptoms and even outcomes in HFpEF, while a higher adaptive rate during exertion has not been proven beneficial.

Effects of the Heterodimeric Neurotoxic Phospholipase A2 from the Venom of Vipera nikolskii on the Contractility of Rat Papillary Muscles and Thoracic Aortas

Phospholipases A2 (PLA2s) are a large family of snake toxins manifesting diverse biological effects, which are not always related to phospholipolytic activity. Snake venom PLA2s (svPLA2s) are extracellular proteins with a molecular mass of 13-14 kDa. They are present in venoms in the form of monomers, dimers, and larger oligomers. The cardiovascular system is one of the multiple svPLA2 targets in prey organisms. The results obtained previously on the cardiovascular effects of monomeric svPLA2s were inconsistent, while the data on the dimeric svPLA2 crotoxin from the rattlesnake Crotalus durissus terrificus showed that it significantly reduced the contractile force of guinea pig hearts. Here, we studied the effects of the heterodimeric svPLA2 HDP-1 from the viper Vipera nikolskii on papillary muscle (PM) contractility and the tension of the aortic rings (ARs). HDP-1 is structurally different from crotoxin, and over a wide range of concentrations, it produced a long-term, stable, positive inotropic effect in PMs, which did not turn into contractures at the concentrations studied. This also distinguishes HDP-1 from the monomeric svPLA2s, which at high concentrations inhibited cardiac function. HDP-1, when acting on ARs preconstricted with 10 μM phenylephrine, induced a vasorelaxant effect, similar to some other svPLA2s. These are the first indications of the cardiac and vascular effects of true vipers' heterodimeric svPLA2s.

Personalized accelerated physiologic pacing

Heart failure with preserved ejection fraction (HFpEF) is increasingly prevalent with a high socioeconomic burden. Pharmacological heart rate lowering was recommended to improve ventricular filling in HFpEF. This article discusses the misperceptions that have resulted in an overprescription of beta-blockers, which in all likelihood have untoward effects on patients with HFpEF, even if they have atrial fibrillation or coronary artery disease as a comorbidity. Directly contradicting the lower heart rate paradigm, faster heart rates provide haemodynamic and structural benefits, amongst which lower cardiac filling pressures and improved ventricular capacitance may be most important. Safe delivery of this therapeutic approach is feasible with atrial and ventricular conduction system pacing that aims to emulate or enhance cardiac excitation to maximize the haemodynamic benefits of accelerated pacing. This conceptual framework was first tested in the myPACE randomized controlled trial of patients with pre-existing pacemakers and preclinical or overt HFpEF. This article provides the background and path towards this treatment approach.

Dilated cardiomyopathy variant R14del increases phospholamban pentamer stability, blunting dynamic regulation of cardiac calcium handling

The sarco(endo)plasmic reticulum Ca ²⁺ ATPase (SERCA) is a membrane transporter that creates and maintains intracellular Ca ²⁺ stores. In the heart, SERCA is regulated by an inhibitory interaction with the monomeric form of the transmembrane micropeptide phospholamban (PLB). PLB also forms avid homo-pentamers, and dynamic exchange of PLB between pentamers and the regulatory complex with SERCA is an important determinant of cardiac responsiveness to exercise. Here, we investigated two naturally occurring pathogenic mutations of PLB, a cysteine substitution of arginine 9 (R9C) and an in-frame deletion of arginine 14 (R14del). Both mutations are associated with dilated cardiomyopathy. We previously showed that the R9C mutation causes disulfide crosslinking and hyperstabilization of pentamers. While the pathogenic mechanism of R14del is unclear, we hypothesized that this mutation may also alter PLB homo-oligomerization and disrupt the PLB-SERCA regulatory interaction. SDS-PAGE revealed a significantly increased pentamer:monomer ratio for R14del-PLB when compared to WT-PLB. In addition, we quantified homo-oligomerization and SERCA-binding in live cells using fluorescence resonance energy transfer (FRET) microscopy. R14del-PLB showed an increased affinity for homo-oligomerization and decreased binding affinity for SERCA compared to WT, suggesting that, like R9C, the R14del mutation stabilizes PLB in its pentameric form, decreasing its ability to regulate SERCA. Moreover, the R14del mutation reduces the rate of PLB unbinding from the pentamer after a transient Ca ²⁺ elevation, limiting the rate of re-binding to SERCA. A computational model predicted that hyperstabilization of PLB pentamers by R14del impairs the ability of cardiac Ca ²⁺ handling to respond to changing heart rates between rest and exercise. We postulate that impaired responsiveness to physiological stress contributes to arrhythmogenesis in human carriers of the R14del mutation.

Physiological calcium combined with electrical pacing accelerates maturation of human engineered heart tissue

Human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) have wide potential application in basic research, drug discovery, and regenerative medicine, but functional maturation remains challenging. Here, we present a method whereby maturation of hiPSC-CMs can be accelerated by simultaneous application of physiological Ca2+ and frequency-ramped electrical pacing in culture. This combination produces positive force-frequency behavior, physiological twitch kinetics, robust β-adrenergic response, improved Ca2+ handling, and cardiac troponin I expression within 25 days. This study provides insights into the role of Ca2+ in hiPSC-CM maturation and offers a scalable platform for translational and clinical research.

Inhibitory and stimulatory micropeptides preferentially bind to different conformations of the cardiac calcium pump

The ATP-dependent ion pump sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA) sequesters Ca²⁺ in the endoplasmic reticulum to establish a reservoir for cell signaling. Because of its central importance in physiology, the activity of this transporter is tightly controlled via direct interactions with tissue-specific regulatory micropeptides that tune SERCA function to match changing physiological conditions. In the heart, the micropeptide phospholamban (PLB) inhibits SERCA, while dwarf open reading frame (DWORF) stimulates SERCA. These competing interactions determine cardiac performance by modulating the amplitude of Ca²⁺ signals that drive the contraction/relaxation cycle. We hypothesized that the functions of these peptides may relate to their reciprocal preferences for SERCA binding; SERCA binds PLB more avidly at low cytoplasmic [Ca²⁺] but binds DWORF better when [Ca²⁺] is high. In the present study, we demonstrated this opposing Ca²⁺ sensitivity is due to preferential binding of DWORF and PLB to different intermediate states that SERCA samples during the Ca²⁺ transport cycle. We show PLB binds best to the SERCA E1-ATP state, which prevails at low [Ca²⁺]. In contrast, DWORF binds most avidly to E1P and E2P states that are more populated when Ca²⁺ is elevated. Moreover, FRET microscopy revealed dynamic shifts in SERCA–micropeptide binding equilibria during cellular Ca²⁺ elevations. A computational model showed that DWORF exaggerates changes in PLB–SERCA binding during the cardiac cycle. These results suggest a mechanistic basis for inhibitory versus stimulatory micropeptide function, as well as a new role for DWORF as a modulator of dynamic oscillations of PLB–SERCA regulatory interactions.

Long-Term Cultivation of Human Atrial Myocardium

Organotypic culture of human ventricular myocardium is emerging in basic and translational cardiac research. However, few institutions have access to human ventricular tissue, whereas atrial tissue is more commonly available and important for studying atrial physiology. This study presents a method for long-term cultivation of beating human atrial myocardium. After written informed consent, tissues from the right-atrial appendage were obtained from patients with sinus rhythm undergoing open heart surgery with cardiopulmonary bypass. Trabeculae (pectinate muscles) prepared from the samples were installed into cultivation chambers at 37°C with a diastolic preload of 500 μN. After 2 days with 0.5 Hz pacing, stimulation frequency was set to 1 Hz. Contractile force was monitored continuously. Beta-adrenergic response, refractory period (RP) and maximum captured frequency (f_max) were assessed periodically. After cultivation, viability and electromechanical function were investigated, as well as the expression of several genes important for intracellular Ca²⁺ cycling and electrophysiology. Tissue microstructure was analyzed by confocal microscopy. We cultivated 19 constantly beating trabeculae from 8 patient samples for 12 days and 4 trabeculae from 3 specimen for 21 days. Functional parameters were compared directly after installation (0 d) with those after 12 d in culture. Contraction force was 384 ± 69 μN at 0 d and 255 ± 90 μN at 12 d (p = 0.8, n = 22), RP 480 ± 97 ms and 408 ± 78 ms (p = 0.3, n = 9), f_max 3.0 ± 0.5 Hz and 3.8 ± 0.5 Hz (p = 0.18, n = 9), respectively. Application of 100 nM isoprenaline to 11 trabeculae at 7 d increased contraction force from 168 ± 35 μN to 361 ± 60 μN (p < 0.01), f_max from 6.4 ± 0.6 Hz to 8.5 ± 0.4 Hz (p < 0.01) and lowered RP from 319 ± 22 ms to 223 ± 15 ms. CACNA1c (L-type Ca²⁺ channel subunit) and GJA1 (connexin-43) mRNA expressions were not significantly altered at 12 d vs 0 d, while ATP2A (SERCA) and KCNJ4 (Kir2.3) were downregulated, and KCNJ2 (Kir2.1) was upregulated. Simultaneous Ca²⁺ imaging and force recording showed preserved excitation-contraction coupling in cultivated trabeculae. Confocal microscopy indicated preserved cardiomyocyte structure, unaltered amounts of extracellular matrix and gap junctions. MTT assays confirmed viability at 12 d. We established a workflow that allows for stable cultivation and functional analysis of beating human atrial myocardium for up to 3 weeks. This method may lead to novel insights into the physiology and pathophysiology of human atrial myocardium.

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.

Personalized pacing for diastolic dysfunction and heart failure with preserved ejection fraction: Design and rationale for the myPACE randomized controlled trial

Background

Patients with pacemakers and heart failure with preserved ejection fraction (HFpEF) or isolated diastolic dysfunction (DD) may benefit from a higher backup heart rate (HR) setting compared with the standard setting of 60 bpm.

Objective

The purpose of this study was to assess the effects of a personalized backup HR setting (myPACE group) compared with 60 bpm (control group).

Methods

In this prospective, blinded, randomized controlled study, pacemaker patients with DD or HFpEF and atrial pacing with intrinsic ventricular conduction or conduction system or biventricular pacing are randomized to the myPACE group or control group for 1 year. The primary outcome is the change in Minnesota Living with Heart Failure Questionnaire (MLHFQ) scores. Secondary endpoints include changes in N-terminal pro–brain natriuretic peptide levels, physical and emotional MLHFQ subscores, and pacemaker-detected atrial arrhythmia burden, patient activity levels, and thoracic impedance; hospitalization for heart failure, atrial fibrillation, cerebrovascular accident, or myocardial infarction; and loop diuretic or antiarrhythmic medication initiation or up-titration. A sample size of 118 subjects is expected to allow detection of a 5-point change in MLHFQ score in an intention-to-treat analysis and allow initial assessment of clinical outcomes and subgroup analyses.

Results

Enrollment began in July 2019. As of November 2020, 107 subjects have been enrolled. It is projected that the 1-year follow-up will be completed by December 2021.

Conclusion

Atrial pacing with intrinsic ventricular conduction or advanced ventricular pacing at a higher, personalized backup HR may be a therapeutic target for patients with isolated DD or HFpEF. The myPACE trial is designed to test this hypothesis.

SERCA2a-phospholamban interaction monitored by an interposed circularly permutated green fluorescent protein

The interaction of phospholamban (PLB) and the sarcoplasmic reticulum Ca²⁺-ATPase (SERCA2a) is a key regulator of cardiac contractility and a therapeutic target in heart failure (HF). PLB-mediated increases in SERCA2a activity improve cardiac function and HF. Clinically, this mechanism can only be exploited by a general activation of the proteinkinase A (PKA), which is associated with side effects and adverse clinical outcomes. A selective interference of the PLB-SERCA2a interaction is desirable but will require novel tools that allow for an integrated assessment of this interaction under both physiological and pathophysiological conditions. A circularly permutated green fluorescent protein (cpGFP) was interposed between SERCA2a and PLB to result into a single SERCA2a-cpGFP-PLB recombinant protein (SGP). Expression, phosphorylation, fluorescence, and function of SGP were evaluated. Expression of SGP-cDNA results in a functional recombinant protein at the predicted molecular weight. The PLB domain of SGP retains its ability to polymerize and can be phosphorylated by PKA activation. This increases the fluorescent yield of SGP by between 10% and 165% depending on cell line and conditions. In conclusion, a single recombinant fusion protein that combines SERCA2a, a circularly permutated green fluorescent protein, and PLB can be expressed in cells and can be phosphorylated at the PLB domain that markedly increases the fluorescence yield. SGP is a novel cellular SERCA2a-PLB interaction monitor.NEW & NOTEWORTHY This study describes the design and characterization of a novel biosensor that can visualize the interaction of SERCA2a and phospholamban (PLB). The biosensor combines SERCA2a, a circularly permutated green fluorescent protein, and PLB into one recombinant protein (SGP). Proteinkinase A activation results in phosphorylation of the PLB domain and is associated with a marked increase in the fluorescence yield to allow for real-time monitoring of the SERCA2a and PLB interaction in cells.

JNK2, a Newly-Identified SERCA2 Enhancer, Augments an Arrhythmic [Ca2+]SR Leak-Load Relationship

RATIONALE

We recently discovered pivotal contributions of stress kinase JNK2 (c-Jun N-terminal kinase isoform 2) in increased risk of atrial fibrillation through enhanced diastolic sarcoplasmic reticulum (SR) calcium (Ca2+) leak via RyR2 (ryanodine receptor isoform 2). However, the role of JNK2 in the function of the SERCA2 (SR Ca2+-ATPase), essential in maintaining SR Ca2+ content cycling during each heartbeat, is completely unknown.

OBJECTIVE

To test the hypothesis that JNK2 increases SERCA2 activity SR Ca2+ content and exacerbates an arrhythmic SR Ca2+ content leak-load relationship.

METHODS AND RESULTS

We used confocal Ca2+ imaging in myocytes and HEK-RyR2 (ryanodine receptor isoform 2-expressing human embryonic kidney 293 cells) cells, biochemistry, dual Ca2+/voltage optical mapping in intact hearts from alcohol-exposed or aged mice (where JNK2 is activated). We found that JNK2, but not JNK1 (c-Jun N-terminal kinase isoform 1), increased SERCA2 uptake and consequently elevated SR Ca2+ content load. JNK2 also associates with and phosphorylates SERCA2 proteins. JNK2 causally enhances SERCA2-ATPase activity via increased maximal rate, without altering Ca2+ affinity. Unlike the CaMKII (Ca2+/calmodulin-dependent kinase II)-dependent JNK2 action in SR Ca2+ leak, JNK2-driven SERCA2 function was CaMKII independent (not prevented by CaMKII inhibition). With CaMKII blocked, the JNK2-driven SR Ca2+ loading alone did not significantly raise leak. However, with JNK2-CaMKII-driven SR Ca2+ leak present, the JNK2-enhanced SR Ca2+ uptake limited leak-induced reduction in SR Ca2+, normalizing Ca2+ transient amplitude, but at a higher arrhythmogenic SR Ca2+ leak. JNK2-specific inhibition completely normalized SR Ca2+ handling, attenuated arrhythmic Ca2+ activities, and alleviated atrial fibrillation susceptibility in aged and alcohol-exposed myocytes and intact hearts.

CONCLUSIONS

We have identified a novel JNK2-induced activation of SERCA2. The dual action of JNK2 in CaMKII-dependent arrhythmic SR Ca2+ leak and a CaMKII-independent uptake exacerbates atrial arrhythmogenicity, while helping to maintain normal levels of Ca2+ transients and heart function. JNK2 modulation may be a novel therapeutic target for atrial fibrillation prevention and treatment.

Heart Rate-Induced Myocardial Ca2+ Retention and Left Ventricular Volume Loss in Patients With Heart Failure With Preserved Ejection Fraction

Background

Increases in heart rate are thought to result in incomplete left ventricular (LV) relaxation and elevated filling pressures in patients with heart failure with preserved ejection fraction (HFpEF). Experimental studies in isolated human myocardium have suggested that incomplete relaxation is a result of cellular Ca²⁺ overload caused by increased myocardial Na⁺ levels. We tested these heart rate paradigms in patients with HFpEF and referent controls without hypertension.

Methods and Results

In 22 fully sedated and instrumented patients (12 controls and 10 patients with HFpEF) in sinus rhythm with a preserved ejection fraction (≥50%) we assessed left‐sided filling pressures and volumes in sinus rhythm and with atrial pacing (95 beats per minute and 125 beats per minute) before atrial fibrillation ablation. Coronary sinus blood samples and flow measurements were also obtained. Seven women and 15 men were studied (aged 59±10 years, ejection fraction 61%±4%). Patients with HFpEF had a history of hypertension, dyspnea on exertion, concentric LV remodeling and a dilated left atrium, whereas controls did not. Pacing at 125 beats per minute lowered the mean LV end‐diastolic pressure in both groups (controls −4.3±4.1 mm Hg versus patients with HFpEF −8.5±6.0 mm Hg, P=0.08). Pacing also reduced LV end‐diastolic volumes. The volume loss was about twice as much in the HFpEF group (controls −15%±14% versus patients with HFpEF −32%±11%, P=0.009). Coronary venous [Ca²⁺] increased after pacing at 125 beats per minute in patients with HFpEF but not in controls. [Na⁺] did not change.

Conclusions

Higher resting heart rates are associated with lower filling pressures in patients with and without HFpEF. Incomplete relaxation and LV filling at high heart rates lead to a reduction in LV volumes that is more pronounced in patients with HFpEF and may be associated with myocardial Ca²⁺ retention.

Effects of a Higher Heart Rate on Quality of Life and Functional Capacity in Patients With Left Ventricular Diastolic Dysfunction

There is no evidence-based treatment for heart failure with preserved ejection fraction. Although lower heart rates (HRs) provide an unequivocal benefit for patients with HF with reduced ejection fraction, higher HR might convey important hemodynamic and substrate-modifying benefits in patients with diastolic dysfunction. In a prospective study of 20 stable outpatients with diastolic dysfunction and pacemakers, we evaluated the effects of a 4-week increase in the lower pacemaker rate to 80 beats/min followed by reversal to the previous lower HR setting from weeks 4 to 6. We assessed quality of life (Minnesota Living with Heart Failure Questionnaire), 6-minute walk test and N-terminal pro-brain natriuretic peptide (NT-proBNP) levels. Pacing at 80 beats/min significantly improved quality of life and the 6-minute walk test (p ≤0.05). There was a strong positive correlation between the pacing-induced changes in NT-proBNP and baseline QRS intervals (r² = 0.31, p <0.01). Stratification by QRS duration revealed that pacing at 80 beats/min led to -21 ± 26% reduction in NT-proBNP in patients with QRS ≤150 ms, whereas QRS >150 ms was associated with a 26 ± 35% increase in NT-proBNP (p <0.01). Patients physiologically paced from the conduction system had a -46 ± 26% reduction in NT-proBNP at 80 beats/min as compared with 4 ± 26% and 13 ± 26% change with pacing from the right atrial appendage and right ventricular apical septum (p_interaction = 0.04). In conclusion, increasing the lower rate setting of pacemakers to 80 beats/min in patients with diastolic dysfunction improves quality of life, functional capacity, and NT-proBNP for those patients with a baseline QRS ≤150 ms. These findings suggest that higher HRs may provide meaningful benefits to patients with left ventricular diastolic dysfunction and heart failure with preserved ejection fraction.

Force-frequency relationship and early relaxation kinetics are preserved upon sarcoplasmic blockade in human myocardium

In this study, we investigated the quantitative and qualitative role of the sarcoplasmic reticulum (SR) in the regulation of the force-frequency relationship (FFR). We blocked the function of SR with cyclopiazonic acid (CPA) and ryanodine and measured twitch kinetics and developed force at various stimulation frequencies in nonfailing and failing intact human right ventricular trabeculae. We found that developed forces are only slightly reduced upon SR blockade, while the positive FFR in nonfailing trabeculae and negative FFR in failing trabeculae were both preserved. The contraction kinetics (dF/dt, dF/dt/F, and time to peak), however, were significantly slower at all frequencies tested. Kinetics of first 50% of relaxation (RT50) was not affected by SR blockade. Kinetics of entire relaxation process (RT90) was overall slower at low frequencies, but not at high frequencies. From our findings, we conclude that the SR is not essential for FFR, and its role in regulation of FFR lies mostly in contraction kinetics. Unlike small rodents, human myocardium contractile function is near-normal in absence of a functional SR with little changes in contractile force, and with preservation with the main regulation of FFR.

Effect of Valsartan on Sarcoplasmic Reticulum Ca2+-ATPase Pump of the Left Ventricular Myocardium in Rats with Heart Failure with Preserved Ejection Fraction

Objectives

The aim was to investigate the effects of valsartan on the sarcoplasmic reticulum Ca²⁺-ATPase pump (SERCA) and L-type Ca²⁺ channel current (I_CaL) of the left ventricular myocardium in rats with heart failure with preserved ejection fraction.

Methods

The 30-week-old male spontaneously hypertensive rats (SHRs) are randomly divided into the non-Valsartan and Valsartan groups, and the 30-week-old male Wistar-Kyoto rats served as control rats. The expression of SERCA is measured by Western blot. The I_CaL is measured by whole-cell patch clamp. The left ventricular end-diastolic pressure and left ventricular relaxation time constant quantity are measured at the same time.

Results

The left ventricular end-diastolic pressure is much higher in SHRs compared with that in control rats (p < 0.01). The left ventricular relaxation time constant quantity is markedly extended in SHRs compared with control rats (p < 0.01). Valsartan cannot increase the expression of SERCA nor decrease the density of I_CaL compared with the non-Valsartan group (p > 0.05).

Conclusions

Valsartan has no effect on SERCA and I_CaL of the left ventricular myocardium in rats with heart failure with preserved ejection fraction.

Contractility Measurements on Isolated Papillary Muscles for the Investigation of Cardiac Inotropy in Mice

Papillary muscle isolated from adult mouse hearts can be used to study cardiac contractility during different physiological/pathological conditions. The contractile characteristics can be evaluated independently of external influences such as vascular tonus or neurohumoral status. It depicts a scientific approach between single cell measurements with isolated cardiac myocytes and in vivo studies like echocardiography. Thus, papillary muscle preparations serve as an excellent model to study cardiac physiology/pathophysiology and can be used for investigations like the modulation by pharmacological agents or the exploration of transgenic animal models. Here, we describe a method of isolating the murine left anterior papillary muscle to investigate cardiac contractility in an organ bath setup. In contrast to a muscle strip preparation isolated from the ventricular wall, the papillary muscle can be prepared in toto without damaging the muscle tissue severely. The organ bath setup consists of several temperature-controlled, gassed and electrode-equipped organ bath chambers. The isolated papillary muscle is fixed in the organ bath chamber and electrically stimulated. The evoked twitch force is recorded using a pressure transducer and parameters such as twitch force amplitude and twitch kinetics are analyzed. Different experimental protocols can be performed to investigate the calcium- and frequency-dependent contractility as well as dose-response curves of contractile agents such as catecholamines or other pharmaceuticals. Additionally, pathologic conditions like acute ischemia can be simulated.

Computational analysis of the regulation of Ca(2+) dynamics in rat ventricular myocytes

Force-frequency relationships of isolated cardiac myocytes show complex behaviors that are thought to be specific to both the species and the conditions associated with the experimental preparation. Ca(2+) signaling plays an important role in shaping the force-frequency relationship, and understanding the properties of the force-frequency relationship in vivo requires an understanding of Ca(2+) dynamics under physiologically relevant conditions. Ca(2+) signaling is itself a complicated process that is best understood on a quantitative level via biophysically based computational simulation. Although a large number of models are available in the literature, the models are often a conglomeration of components parameterized to data of incompatible species and/or experimental conditions. In addition, few models account for modulation of Ca(2+) dynamics via β-adrenergic and calmodulin-dependent protein kinase II (CaMKII) signaling pathways even though they are hypothesized to play an important regulatory role in vivo. Both protein-kinase-A and CaMKII are known to phosphorylate a variety of targets known to be involved in Ca(2+) signaling, but the effects of these pathways on the frequency- and inotrope-dependence of Ca(2+) dynamics are not currently well understood. In order to better understand Ca(2+) dynamics under physiological conditions relevant to rat, a previous computational model is adapted and re-parameterized to a self-consistent dataset obtained under physiological temperature and pacing frequency and updated to include β-adrenergic and CaMKII regulatory pathways. The necessity of specific effector mechanisms of these pathways in capturing inotrope- and frequency-dependence of the data is tested by attempting to fit the data while including and/or excluding those effector components. We find that: (1) β-adrenergic-mediated phosphorylation of the L-type calcium channel (LCC) (and not of phospholamban (PLB)) is sufficient to explain the inotrope-dependence; and (2) that CaMKII-mediated regulation of neither the LCC nor of PLB is required to explain the frequency-dependence of the data.

A Spatially Detailed Model of Isometric Contraction Based on Competitive Binding of Troponin I Explains Cooperative Interactions between Tropomyosin and Crossbridges

Biophysical models of cardiac tension development provide a succinct representation of our understanding of force generation in the heart. The link between protein kinetics and interactions that gives rise to high cooperativity is not yet fully explained from experiments or previous biophysical models. We propose a biophysical ODE-based representation of cross-bridge (XB), tropomyosin and troponin within a contractile regulatory unit (RU) to investigate the mechanisms behind cooperative activation, as well as the role of cooperativity in dynamic tension generation across different species. The model includes cooperative interactions between regulatory units (RU-RU), between crossbridges (XB-XB), as well more complex interactions between crossbridges and regulatory units (XB-RU interactions). For the steady-state force-calcium relationship, our framework predicts that: (1) XB-RU effects are key in shifting the half-activation value of the force-calcium relationship towards lower [Ca²⁺], but have only small effects on cooperativity. (2) XB-XB effects approximately double the duty ratio of myosin, but do not significantly affect cooperativity. (3) RU-RU effects derived from the long-range action of tropomyosin are a major factor in cooperative activation, with each additional unblocked RU increasing the rate of additional RU’s unblocking. (4) Myosin affinity for short (1–4 RU) unblocked stretches of actin of is very low, and the resulting suppression of force at low [Ca²⁺] is a major contributor in the biphasic force-calcium relationship. We also reproduce isometric tension development across mouse, rat and human at physiological temperature and pacing rate, and conclude that species differences require only changes in myosin affinity and troponin I/troponin C affinity. Furthermore, we show that the calcium dependence of the rate of tension redevelopment k_tr is explained by transient blocking of RU’s by a temporary decrease in XB-RU effects.

Force generation in cardiac muscle cells is driven by changes in calcium concentration. Relatively small changes in the calcium concentration over the course of a heart beat lead to the large changes in force required to fully contract and relax the heart. This is known as ‘cooperative activation’, and involves a complex interaction of several proteins involved in contraction. Current computer models which reproduce force generation often do not represent these processes explicitly, and stochastic approaches that do tend to require large amounts of computational power to solve, which limit the range of investigations in which they can be used. We have created an new computational model that captures the underlying physiological processes in more detail, and is more efficient than stochastic approaches, while still being able to run a large range of simulations. The model is able to explain the biological processes leading to the cooperative activation of muscle. In addition, the model reproduces how this cooperative activation translates to normal muscle function to generate force from changes in calcium across three different species.

Phospholamban C-terminal residues are critical determinants of the structure and function of the calcium ATPase regulatory complex

To determine the structural and regulatory role of the C-terminal residues of phospholamban (PLB) in the membranes of living cells, we fused fluorescent protein tags to PLB and sarco/endoplasmic reticulum calcium ATPase (SERCA). Alanine substitution of PLB C-terminal residues significantly altered fluorescence resonance energy transfer (FRET) from PLB to PLB and SERCA to PLB, suggesting a change in quaternary conformation of PLB pentamer and SERCA-PLB regulatory complex. Val to Ala substitution at position 49 (V49A) had particularly large effects on PLB pentamer structure and PLB-SERCA regulatory complex conformation, increasing and decreasing probe separation distance, respectively. We also quantified a decrease in oligomerization affinity, an increase in binding affinity of V49A-PLB for SERCA, and a gain of inhibitory function as quantified by calcium-dependent ATPase activity. Notably, deletion of only a few C-terminal residues resulted in significant loss of PLB membrane anchoring and mislocalization to the cytoplasm and nucleus. C-terminal truncations also resulted in progressive loss of PLB-PLB FRET due to a decrease in the apparent affinity of PLB oligomerization. We quantified a similar decrease in the binding affinity of truncated PLB for SERCA and loss of inhibitory potency. However, despite decreased SERCA-PLB binding, intermolecular FRET for Val(49)-stop (V49X) truncation mutant was paradoxically increased as a result of an 11.3-Å decrease in the distance between donor and acceptor fluorophores. We conclude that PLB C-terminal residues are critical for localization, oligomerization, and regulatory function. In particular, the PLB C terminus is an important determinant of the quaternary structure of the SERCA regulatory complex.

The heart in Duchenne muscular dystrophy: early detection of contractile performance alteration

Progressive cardiomyopathy is a major cause of death in Duchenne muscular dystrophy (DMD) patients. Coupling between Ca(2+) handling and contractile properties in dystrophic hearts is poorly understood. It is also not clear whether developing cardiac failure is dominated by alterations in Ca(2+) pathways or more related to the contractile apparatus. We simultaneously recorded force and Ca(2+) transients in field-stimulated papillary muscles from young (10-14 weeks) wild-type (wt) and dystrophic mdx mice. Force amplitudes were fivefold reduced in mdx muscles despite only 30% reduction in fura-2 ratio amplitudes. This indicated mechanisms other than systolic Ca(2+) to additionally account for force decrements in mdx muscles. pCa-force relations revealed decreased mdx myofibrillar Ca(2+) sensitivity. 'In vitro' motility assays, studied in mdx hearts here for the first time, showed significantly slower sliding velocities. mdx MLC/MHC isoforms were not grossly altered. Dystrophic hearts showed echocardiography signs of early ventricular wall hypertrophy with a significantly enlarged end-diastolic diameter 'in vivo'. However, fractional shortening was still comparable to wt mice. Changes in the contractile apparatus satisfactorily explained force drop in mdx hearts. We give first evidence of early hypertrophy in mdx mice and possible mechanisms for already functional impairment of cardiac muscle in DMD.

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.

Intracellular dyssynchrony of diastolic cytosolic [Ca²⁺] decay in ventricular cardiomyocytes in cardiac remodeling and human heart failure

RATIONALE

Synchronized release of Ca²⁺ into the cytosol during each cardiac cycle determines cardiomyocyte contraction.

OBJECTIVE

We investigated synchrony of cytosolic [Ca²⁺] decay during diastole and the impact of cardiac remodeling.

METHODS AND RESULTS

Local cytosolic [Ca²⁺] transients (1-µm intervals) were recorded in murine, porcine, and human ventricular single cardiomyocytes. We identified intracellular regions of slow (slowCaR) and fast (fastCaR) [Ca²⁺] decay based on the local time constants of decay (TAUlocal). The SD of TAUlocal as a measure of dyssynchrony was not related to the amplitude or the timing of local Ca²⁺ release. Stimulation of sarcoplasmic reticulum Ca²⁺ ATPase with forskolin or istaroxime accelerated and its inhibition with cyclopiazonic acid slowed TAUlocal significantly more in slowCaR, thus altering the relationship between SD of TAUlocal and global [Ca²⁺] decay (TAUglobal). Na⁺/Ca²⁺ exchanger inhibitor SEA0400 prolonged TAUlocal similarly in slowCaR and fastCaR. FastCaR were associated with increased mitochondrial density and were more sensitive to the mitochondrial Ca²⁺ uniporter blocker Ru360. Variation in TAUlocal was higher in pig and human cardiomyocytes and higher with increased stimulation frequency (2 Hz). TAUlocal correlated with local sarcomere relengthening. In mice with myocardial hypertrophy after transverse aortic constriction, in pigs with chronic myocardial ischemia, and in end-stage human heart failure, variation in TAUlocal was increased and related to cardiomyocyte hypertrophy and increased mitochondrial density.

CONCLUSIONS

In cardiomyocytes, cytosolic [Ca²⁺] decay is regulated locally and related to local sarcomere relengthening. Dyssynchronous intracellular [Ca²⁺] decay in cardiac remodeling and end-stage heart failure suggests a novel mechanism of cellular contractile dysfunction.

Endocardial endothelium is a key determinant of force-frequency relationship in rat ventricular myocardium

We tested the hypothesis that removing endocardial endothelium (EE) negatively impacts the force-frequency relationship (FFR) of ventricular myocardium and dissected the signaling that underlies this phenomenon. EE of rat trabeculae was selectively damaged by brief (<1 s) exposure to 0.1% Triton X-100. Force, intracellular Ca(2+) transient (iCa(2+)), and activity of protein kinase A (PKA) and protein kinase C (PKC) were determined. In control muscles, force and iCa(2+) increased as the stimulation frequency increased in steps of 0.5 Hz up to 3.0 Hz. However, EE-denuded (EED) muscles exhibited a markedly blunted FFR. Neither isoproterenol (ISO; 0.1-5 nmol/l) nor endothelin-1 (ET-1; 10-100 nmol/l) alone restored the slope of FFR in EED muscles. Intriguingly, however, a positive FFR was restored in EED preparations by combining low concentrations of ISO (0.1 nmol/l) and ET-1 (20 nmol/l). In intact muscles, PKA and PKC activity increased proportionally with the increase in frequency. This effect was completely lost in EED muscles. Again, combining ISO and ET-1 fully restored the frequency-dependent rise in PKA and PKC activity in EED muscles. In conclusion, selective damage of EE leads to significantly blunted FFR. A combination of low concentrations of ISO and ET-1 successfully restores FFR in EED muscles. The interdependence of ISO and ET-1 in this process indicates cross-talk between the β1-PKA and ET-1-PKC pathways for a normal (positive) FFR. The results also imply that dysfunction of EE and/or EE-myocyte coupling may contribute to flat (or even negative) FFR in heart failure.

An analysis of deformation-dependent electromechanical coupling in the mouse heart

To investigate the effects of the coupling between excitation and contraction on whole-organ function, we have developed a novel biophysically based multiscale electromechanical model of the murine heart. Through comparison with a comprehensive in vivo experimental data set, we show good agreement with pressure and volume measurements at both physiological temperatures and physiological pacing frequencies. This whole-organ model was used to investigate the effects of material and haemodynamic properties introduced at the tissue level, as well as emergent function of our novel cell contraction model. Through a comprehensive sensitivity analysis at both the cellular and whole organ level, we demonstrate the sensitivity of the model's results to its parameters and the constraining effect of experimental data. These results demonstrate the fundamental importance of length- and velocity-dependent feedback to the cellular scale for whole-organ function, and we show that a strong velocity dependence of tension is essential for explaining the differences between measured single cell tension and whole-organ pressure transients.

CaMKII effects on inotropic but not lusitropic force frequency responses require phospholamban

Increasing heart rate enhances cardiac contractility (force frequency relationship, FFR) and accelerates cardiac relaxation (frequency-dependent acceleration of relaxation, FDAR). The positive FFR together with FDAR promotes rapid filling and ejection of blood from the left ventricle (LV) at higher heart rates. Recent studies indicate that the multifunctional Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is involved in regulating FFR and FDAR. We used isolated perfused mouse hearts to study the mechanisms of FFR and FDAR in different genetic models, including transgenic myocardial CaMKII inhibition (AC3-I) and phospholmban knockout (PLN(-/-)). When the rate was increased from 360 beats/min to 630 beats/min in wild type mouse hearts, the LV developed pressure (LVDP) and the maximum rate of increase in pressure (dP/dt max) increased by 37.6 ± 4.7% and 77.0 ± 8.1%, respectively. However, hearts from AC3-I littermates showed no increase of LVDP and a relatively modest (20.4 ± 3.9%) increase in dP/dt max. PLN(-/-) hearts had a negative FFR, and myocardial AC3-I expression did not change the FFR in PLN(-/-) mice. PLN(-/-) mouse hearts did not exhibit FDAR, while PLN(-/-) mice with myocardial AC3-I expression showed further frequency dependent reductions in cardiac relaxation, suggesting that CaMKII targets in addition to PLN were critical to myocardial relaxation. We incubated a constitutively active form of CaMKII with chemically-skinned myocardium and found that several myofilament proteins were phosphorylated by CaMKII. However, CaMKII did not affect myofilament calcium sensitivity. Our study shows that CaMKII plays an important role in modulating FFR and FDAR in murine hearts and suggest that PLN is a critical target for CaMKII effects on FFR, while CaMKII effects on FDAR partially require PLN-alternative targets.

Toward an integrative computational model of the Guinea pig cardiac myocyte

The local control theory of excitation-contraction (EC) coupling asserts that regulation of calcium (Ca²⁺) release occurs at the nanodomain level, where openings of single L-type Ca²⁺ channels (LCCs) trigger openings of small clusters of ryanodine receptors (RyRs) co-localized within the dyad. A consequence of local control is that the whole-cell Ca²⁺ transient is a smooth continuous function of influx of Ca²⁺ through LCCs. While this so-called graded release property has been known for some time, its functional importance to the integrated behavior of the cardiac ventricular myocyte has not been fully appreciated. We previously formulated a biophysically based model, in which LCCs and RyRs interact via a coarse-grained representation of the dyadic space. The model captures key features of local control using a low-dimensional system of ordinary differential equations. Voltage-dependent gain and graded Ca²⁺ release are emergent properties of this model by virtue of the fact that model formulation is closely based on the sub-cellular basis of local control. In this current work, we have incorporated this graded release model into a prior model of guinea pig ventricular myocyte electrophysiology, metabolism, and isometric force production. The resulting integrative model predicts the experimentally observed causal relationship between action potential (AP) shape and timing of Ca²⁺ and force transients, a relationship that is not explained by models lacking the graded release property. Model results suggest that even relatively subtle changes in AP morphology that may result, for example, from remodeling of membrane transporter expression in disease or spatial variation in cell properties, may have major impact on the temporal waveform of Ca²⁺ transients, thus influencing tissue level electromechanical function.

Phospholamban and cardiac function: a comparative perspective in vertebrates

Phospholamban (PLN) is a small phosphoprotein closely associated with the cardiac sarcoplasmic reticulum (SR). Dephosphorylated PLN tonically inhibits the SR Ca-ATPase (SERCA2a), while phosphorylation at Ser16 by PKA and Thr17 by Ca(2+) /calmodulin-dependent protein kinase (CaMKII) relieves the inhibition, and this increases SR Ca(2+) uptake. For this reason, PLN is one of the major determinants of cardiac contractility and relaxation. In this review, we attempted to highlight the functional significance of PLN in vertebrate cardiac physiology. We will refer to the huge literature on mammals in order to describe the molecular characteristics of this protein, its interaction with SERCA2a and its role in the regulation of the mechanic and the electric performance of the heart under basal conditions, in the presence of chemical and physical stresses, such as β-adrenergic stimulation, response to stretch, force-frequency relationship and intracellular acidosis. Our aim is to provide the basis to discuss the role of PLN also on the cardiac function of nonmammalian vertebrates, because so far this aspect has been almost neglected. Accordingly, when possible, the literature on PLN will be analysed taking into account the nonuniform cardiac structural and functional characteristics encountered in ectothermic vertebrates, such as the peculiar and variable organization of the SR, the large spectrum of response to stresses and the disaptive absence of crucial proteins (i.e. haemoglobinless and myoglobinless species).

Calmodulin kinase II, sarcoplasmic reticulum Ca2+ leak, and atrial fibrillation

Although it is generally accepted that excitation-contraction coupling is defective in patients with atrial fibrillation, the underlying cellular mechanisms remain incompletely understood. Recent studies suggest that abnormal sarcoplasmic reticulum calcium "leak" via ryanodine receptors contributes to atrial arrhythmogenesis. Increased activity of the enzyme calmodulin kinase II (CaMKII) and, specifically, enhanced CaMKII phosphorylation of ryanodine receptors appear to play a critical role in the induction and perhaps maintenance of atrial fibrillation. In this review, we will summarize new insights into the role of enhanced CaMKII in sarcoplasmic reticulum calcium leak and atrial arrhythmogenesis during atrial fibrillation.

Calcium sensitivity, force frequency relationship and cardiac troponin I: critical role of PKA and PKC phosphorylation sites

Transgenic models with pseudo phosphorylation mutants of troponin I, PKA sites at Ser 22 and 23 (cTnIDD(22,23) mice) or PKC sites at Ser 42 and 44 (cTnIAD(22,23)DD(42,44)) displayed differential force-frequency relationships and afterload relaxation delay in vivo. We hypothesized that cTnI PKA and PKC phosphomimics impact cardiac muscle rate-related developed twitch force and relaxation kinetics in opposite directions. cTnIDD(22,23) transgenic mice produce a force frequency relationship (FFR) equivalent to control NTG albeit at lower peak [Ca(2+)](i), while cTnIAD(22,23)DD(42,44) TG mice had a flat FFR with normal peak systolic [Ca(2+)](i), thus suggestive of diminished responsiveness to [Ca(2+)](i) at higher frequencies. Force-[Ca(2+)](i) hysteresis analysis revealed that cTnIDD(22,23) mice have a combined enhanced myofilament calcium peak response with an enhanced slope of force development and decline per unit of [Ca(2+)](i), whereas cTnIAD(22,23)DD(42,44) transgenic mice showed the opposite. The computational ECME model predicts that the TG lines may be distinct from each other due to different rate constants for association/dissociation of Ca(2+) at the regulatory site of cTnC. Our data indicate that cTnI phosphorylation at PKA sites plays a critical role in the FFR by increasing relative myofilament responsiveness, and results in a distinctive transition between activation and relaxation, as displayed by force-[Ca(2+)](i) hysteresis loops. These findings may have important implications for understanding the specific contribution of cTnI to beta-adrenergic inotropy and lusitropy and to adverse contractile effects of PKC activation, which is relevant during heart failure development.

Characterization of calumenin-SERCA2 interaction in mouse cardiac sarcoplasmic reticulum

Calumenin is a multiple EF-hand Ca(2+)-binding protein localized in the sarcoplasmic reticulum (SR) with C-terminal SR retention signal HDEF. Recently, we showed evidence that calumenin interacts with SERCA2 in rat cardiac SR (Sahoo, S. K., and Kim, D. H. (2008) Mol. Cells 26, 265-269). The present study was undertaken to further characterize the association of calumenin with SERCA2 in mouse heart by various gene manipulation approaches. Immunocytochemical analysis showed that calumenin and SERCA2 were partially co-localized in HL-1 cells. Knockdown (KD) of calumenin was conducted in HL-1 cells and 80% reduction of calumenin did not induce any expressional changes of other Ca(2+)-cycling proteins. But it enhanced Ca(2+) transient amplitude and showed shortened time to reach peak and decreased time to reach 50% of baseline. Oxalate-supported Ca(2+) uptake showed increased Ca(2+) sensitivity of SERCA2 in calumenin KD HL-1 cells. Calumenin and SERCA2 interaction was significantly lower in the presence of thapsigargin, vanadate, or ATP, as compared with 1.3 mum Ca(2+), suggesting that the interaction is favored in the E1 state of SERCA2. A glutathione S-transferase-pulldown assay of calumenin deletion fragments and SERCA2 luminal domains suggested that regions of 132-222 amino acids of calumenin and 853-892 amino acids of SERCA2-L4 are the major binding partners. On the basis of our in vitro binding data and available information on three-dimensional structure of Ca(2+)-ATPases, a molecular model was proposed for the interaction between calumenin and SERCA2. Taken together, the present results suggest that calumenin is a novel regulator of SERCA2, and its expressional changes are tightly coupled with Ca(2+)-cycling of cardiomyocytes.

Differential effects of phospholamban and Ca2+/calmodulin-dependent kinase II on [Ca2+]i transients in cardiac myocytes at physiological stimulation frequencies

In cardiac myocytes, the activity of the Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is hypothesized to regulate Ca(2+) release from and Ca(2+) uptake into the sarcoplasmic reticulum via the phosphorylation of the ryanodine receptor 2 and phospholamban (PLN), respectively. We tested the role of CaMKII and PLN on the frequency adaptation of cytosolic Ca(2+) concentration ([Ca(2+)](i)) transients in nearly 500 isolated cardiac myocytes from transgenic mice chronically expressing a specific CaMKII inhibitor, interbred into wild-type or PLN null backgrounds under physiologically relevant pacing conditions (frequencies from 0.2 to 10 Hz and at 37 degrees C). When compared with that of mice lacking PLN only, the combined chronic CaMKII inhibition and PLN ablation decreased the maximum Ca(2+) release rate by more than 50% at 10 Hz. Although PLN ablation increased the rate of Ca(2+) uptake at all frequencies, its combination with CaMKII inhibition did not prevent a frequency-dependent reduction of the amplitude and the duration of the [Ca(2+)](i) transient. High stimulation frequencies in the physiological range diminished the effects of PLN ablation on the decay time constant and on the maximum decay rate of the [Ca(2+)](i) transient, indicating that the PLN-mediated feedback on [Ca(2+)](i) removal is limited by high stimulation frequencies. Taken together, our results suggest that in isolated mouse ventricular cardiac myocytes, the combined chronic CaMKII inhibition and PLN ablation slowed Ca(2+) release at physiological frequencies: the frequency-dependent decay of the amplitude and shortening of the [Ca(2+)](i) transient occurs independent of chronic CaMKII inhibition and PLN ablation, and the PLN-mediated regulation of Ca(2+) uptake is diminished at higher stimulation frequencies within the physiological range.

Time-course of changes in cardiac function during recovery after acute exercise

Exercise-induced cardiac dysfunction (EICD) has been observed immediately following exhaustive exercise in trained individuals, but limited and conflicting data are available regarding EICD in a previously untrained population days after an exhaustive exercise bout. The purpose of this study was to examine the effects of a single bout of acute exercise on cardiac function during the 72 h after exercise and identify potential contributing mechanisms. After completing an acute exercise bout on a motorized treadmill (25 m/min, 5% grade, 60 min), rats were sacrificed immediately, 24 h, 48 h, or 72 h after the exercise bout. At the scheduled time of sacrifice, hearts were isolated and perfused for determination of ex vivo cardiac function, and examined for malondialdehyde (MDA), a lipid peroxidation index, and antioxidant potential (AOP). During the 48 h post exercise, left ventricular developed pressure decreased by 30%, dP/dtmax declined by 37%, and dP/dtmin showed a 34% decrease (p < 0.05). By 72 h, cardiac function had returned to control levels. MDA was increased immediately after the exercise bout and at the 24 and 48 h intervals (p < 0.05). Conversely, AOP progressively decreased at the 24 and 48 h intervals. As with cardiac function, MDA and AOP had returned to control levels by 72 h post-exercise. These data indicate that a single bout of prolonged, moderately intense exercise performed by previously sedentary rats impaired cardiac function for up to 48 h. This decrement in cardiac function was associated with increased lipid peroxidation and decreased antioxidant potential.

Determinants of frequency-dependent contraction and relaxation of mammalian myocardium

An increase in heart rate is the primary mechanism that up-regulates cardiac output during conditions such as exercise and stress. When the heart rate increases, cardiac output increases due to (1) an increased number of beats per time period, and (2) the fact that myocardium generates a higher level of force. In this review, we focus on the underlying mechanisms that are at the basis of frequency-dependent activation of the heart. In addition to increased force development, the kinetics of both cardiac activation and relaxation are faster. This is crucial, as in between successive beats there is less time, and cardiac output can only be maintained if the ventricle can fill adequately. We will discuss the cellular mechanisms that are involved in the regulation of rate-dependent changes in kinetics, with a focus on changes that occur in regulation of the intracellular calcium transient, and the changes in the myofilament responsiveness that occur when the heart rate changes.

Frequency-dependent acceleration of relaxation involves decreased myofilament calcium sensitivity

The force-frequency relationship is an intrinsic modulator of cardiac contractility and relaxation. Force of contraction increases with frequency, while simultaneously a frequency-dependent acceleration of relaxation occurs. While frequency dependency of calcium handling and sarcoplasmic reticulum calcium load have been well described, it remains unknown whether frequency-dependent changes in myofilament calcium sensitivity occur. We hypothesized that an increase in heart rate that results in acceleration of relaxation is accompanied by a proportional decrease in myofilament calcium sensitivity. To test our hypothesis, ultrathin right ventricular trabeculae were isolated from New Zealand White rabbit hearts and iontophorically loaded with the calcium indicator bis-fura 2. Twitch and intracellular calcium handling parameters were measured and showed a robust increase in twitch force, acceleration of relaxation, and rise in both diastolic and systolic intracellular calcium concentration with increased frequency. Steady-state force-intracellular calcium concentration relationships were measured at frequencies 1, 2, 3, and 4 Hz at 37 degrees C using potassium-induced contractures. EC(50) significantly and gradually increased with frequency, from 475 +/- 64 nM at 1 Hz to 1,004 +/- 142 nM at 4 Hz (P < 0.05) and correlated with the corresponding changes in half relaxation time. No significant changes in maximal active force development or in the myofilament cooperativity coefficient were found. Myofilament protein phosphorylation was assessed using Pro-Q Diamond staining on protein gels of trabeculae frozen at either 1 or 4 Hz, revealing troponin I and myosin light chain-2 phosphorylation associated with the myofilament desensitization. We conclude that myofilament calcium sensitivity is substantially and significantly decreased at higher frequencies, playing a prominent role in frequency-dependent acceleration of relaxation.

Heart failure-associated alterations in troponin I phosphorylation impair ventricular relaxation-afterload and force-frequency responses and systolic function

Recent studies have found that selective stimulation of troponin (Tn)I protein kinase A (PKA) phosphorylation enhances heart rate-dependent inotropy and blunts relaxation delay coupled to increased afterload. However, in failing hearts, TnI phosphorylation by PKA declines while protein kinase C (PKC) activity is enhanced, potentially augmenting TnI PKC phosphorylation. Accordingly, we hypothesized that these site-specific changes deleteriously affect both rate-responsive cardiac function and afterload dependence of relaxation, both prominent phenotypic features of the failing heart. A transgenic (TG) mouse model was generated in which PKA-TnI sites were mutated to mimic partial dephosphorylation (Ser22 to Ala; Ser23 to Asp) and dominant PKC sites were mutated to mimic constitutive phosphorylation (Ser42 and Ser44 to Asp). The two highest-expressing lines were further characterized. TG mice had reduced fractional shortening of 34.7 ± 1.4% vs. 41.3 ± 2.0% ( P = 0.018) and slight chamber dilation on echocardiography. In vivo cardiac pressure-volume studies revealed near doubling of isovolumic relaxation prolongation with increasing afterload in TG animals ( P < 0.001), and this remained elevated despite isoproterenol infusion (PKA stimulation). Increasing heart rate from 400 to 700 beats/min elevated contractility 13% in TG hearts, nearly half the response observed in nontransgenic animals ( P = 0.005). This blunted frequency response was normalized by isoproterenol infusion. Abnormal TnI phosphorylation observed in cardiac failure may explain exacerbated relaxation delay in response to increased afterload and contribute to blunted chronotropic reserve.

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

Cardiac Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) in heart has been implicated in Ca(2+) current (I(Ca)) facilitation, enhanced sarcoplasmic reticulum (SR) Ca(2+) release and frequency-dependent acceleration of relaxation (FDAR) via enhanced SR Ca(2+) uptake. However, questions remain about how CaMKII may work in these three processes. Here we tested the role of CaMKII in these processes using transgenic mice (SR-AIP) that express four concatenated repeats of the CaMKII inhibitory peptide AIP selectively in the SR membrane. Wild type mice (WT) and mice expressing AIP exclusively in the nucleus (NLS-AIP) served as controls. Increasing stimulation frequency produced typical FDAR in WT and NLS-AIP, but FDAR was markedly inhibited in SR-AIP. Quantitative analysis of cytosolic Ca(2+) removal during [Ca(2+)](i) decline revealed that FDAR is due to an increased apparent V(max) of SERCA. CaMKII-dependent RyR phosphorylation at Ser2815 and SR Ca(2+) leak was both decreased in SR-AIP vs. WT. This decrease in SR Ca(2+) leak may partly balance the reduced SERCA activity leading to relatively unaltered SR-Ca(2+) load in SR-AIP vs. WT myocytes. Surprisingly, CaMKII regulation of the L-type Ca(2+) channel (I(Ca) facilitation and recovery from inactivation) was abolished by the SR-targeted CaMKII inhibition in SR-AIP mice. Inhibition of CaMKII effects on I(Ca) and RyR function by the SR-localized AIP places physical constraints on the localization of these proteins at the junctional microdomain. Thus SR-targeted CaMKII inhibition can directly inhibit the activation of SR Ca(2+) uptake, SR Ca(2+) release and I(Ca) by CaMKII, effects which have all been implicated in triggered arrhythmias.

Effects of halothane, sevoflurane and desflurane on the force-frequency relation in the dog heart in vivo

PURPOSE

Frequency potentiation is the increase in force of contraction induced by an increased heart rate (HR). This positive staircase phenomenon has been attributed to changes in Ca2+ entry and loading of intracellular Ca2+ stores. Volatile anesthetics interfere with Ca2+ homeostasis of cardiomyocytes. We hypothesized that frequency potentiation is altered by volatile anesthetics and investigated the influence of halothane (H), sevoflurane (S) and desflurane (D) on the positive staircase phenomenon in dogsin vivo.

METHODS

Dogs were chronically instrumented for measurement of left ventricular (LV) pressure and cardiac output. Heart rate was increased by atrial pacing from 120 to 220 beats·min^-1 and the LV maximal rate of pressure increase (dP/ dt_max) was determined as an index of myocardial performance. Measurements were performed in conscious dogs and during anesthesia with 1.0 minimal alveolar concentrations of each of the three inhaled anesthetics.

RESULTS

Increasing HR from 120 to 220 beats·min^-1 increased dP/dt_max from 3394 ± 786 (mean ± SD) to 3798 ± 810 mmHg sec-1 in conscious dogs. All anesthetics reduced dP/dt_max during baseline (at 120 beatss·min^-1: H, 1745 ± 340 mmHgs·sec^-1; S, 1882 ± 418; D, 1928 ± 454, allP < 0.05vs awake) but did not influence the frequency potentiation of dP/dtmax (at 220 beatss·min^-1: H, 1981 ± 587 mmHgs·sec^-1; S, 2187 ± 787; D, 2307 ± 691). The slope of the regression line correlating dP/dt_max and HR was not different between awake and anesetized dogs. Increasing HR did not influence cardiac output in awake or anesthetized dogs.

CONCLUSION

These results indicate that volatile anesthetics do not alter the force-frequency relation in dogs in vivo.

Deletion of the inducible 70-kDa heat shock protein genes in mice impairs cardiac contractile function and calcium handling associated with hypertrophy

BACKGROUND

Hspa1a and Hspa1b genes encode stress-inducible 70-kDa heat shock proteins (Hsp70) that protect cells from insults such as ischemia. Mice with null mutations of both genes (KO) were generated, and their cardiac phenotype was explored.

METHODS AND RESULTS

Heart rate and blood pressures were normal in the KO mice. Hearts from KO mice were more susceptible to both functional and cellular damage by ischemia/reperfusion. Cardiac hypertrophy developed in Hsp70-KO mice. Ca2+ transients in cardiomyocytes of KO mice showed a delayed (120%) calcium decline and decreased sarcoplasmic reticulum calcium content. Cell shortening was decreased by 35%, and rates of contraction and relaxation were slower by 40%. These alterations can be attributed to the absence of Hsp70 because viral expression of Hsp70 in KO cultured cardiomyocytes restored these parameters. One mechanism underlying myocyte dysfunction could be decreased SERCA2a expression. This hypothesis was supported by a prolonged calcium decline and decreased SERCA2a protein. Viral SERCA2a expression restored contractility and Ca2+ transients. We examined the involvement of Jun N-terminal kinase (JNK), p38-mitogen-activated protein kinase (p38-MAPK), Raf-1, and extracellular signal-regulated kinase (ERK) in SERCA2a downregulation and the cardiac phenotype of KO mice. Levels of phosphorylated JNK, p38-MAPK, Raf-1, and ERK were elevated in KO hearts. Activation of the Raf-1-ERK pathway in normal cardiomyocytes resulted in decreased SERCA2a.

CONCLUSIONS

Absence of Hsp70 leads to dysfunctional cardiomyocytes and impaired stress response of Hsp70-KO hearts against ischemia/reperfusion. In addition, deletion of Hsp70 genes might induce cardiac dysfunction and development of cardiac hypertrophy through the activation of JNK, p38-MAPK, Raf-1, and ERK.

Improvement of cardiac sarcoplasmic reticulum calcium cycling in dogs with heart failure following long-term therapy with the Acorn Cardiac Support Device

Abnormal Ca(2+)-homeostasis is a hall-marked characteristic of the failing heart. In the normal myocardium, the sarcoplasmic reticulum (SR) is a principal organelle that controls intracellular Ca(2+) concentration during the cardiac cycle. The SR consists of longitudinal and terminal cisternea regions. The former contains the Ca(2+)-ATPase pump or SERCA-2a whose function is to transport cytosolic Ca(2+) into the lumen of the SR during diastole and whose activity is regulated by reversible phosphorylation of the endogenously SR-bound phospholamban (PLB). The SR's terminal cisternea region contains ryanodine-sensitive Ca(2+)-release channels (RR), the activity of which is regulated by direct and indirect reversible phosphorylation. These channels release the SR-stored Ca(2+) during contraction. We have shown that in left ventricular (LV) myocardium from dogs with coronary microembolization-induced heart failure, ability of the SR to sequester and release Ca(2+) during the cardiac cycles is impaired. This abnormality was associated with reduced expression (protein and mRNA) levels of Ca(2+)-ATPase, PLB, and reduced PLB phosphorylation. Long-term therapy with the Acorn Cardiac Support Device (CSD) is associated with restoration of the ability of the SR to sequester Ca(2+). This improvement in SR function following chronic CSD therapy was due primarily to increased affinity of the SERCA-2a for calcium. The later was associated with (1) increased phosphorylation of PLB at serine 16 and threonine 17, (2) unchanged protein expression of PLB and (3) unchanged protein expression of SERCA-2a in LV myocardium of CSD-treated dogs compared to controls. This review summarizes our current understanding of the role of the CSD in modulating SR calcium cycling in an experimental canine model of chronic heart failure produced by multiple sequential intracoronary microembolizations.

ATP splitting by half the cross-bridges can explain the twitch energetics of mouse papillary muscle

The aim of this study was to quantify the fraction of cross-bridges that cycle during a cardiac twitch. Measurements of the energetics of contracting left ventricular mouse papillary muscle were made in vitro (27 degrees C) using the myothermic technique. Enthalpy output was partitioned into force-dependent and force-independent components using 2,3-butanedione monoxime (BDM) to selectively inhibit cross-bridge cycling. For isometric contractions and a contraction frequency of 2 Hz the net enthalpy output was 5.7 +/- 0.8 mJ g(-1) twitch(-1) and initial enthalpy output was 2.3 +/- 0.3 mJ g(-1) twitch(-1) (n = 11). Assuming that low concentrations of BDM did not affect Ca2+ cycling, force-independent enthalpy output was 18.6 +/- 1.9% (n = 7) of the initial enthalpy output. Enthalpy output decreased with increased contraction frequency but was independent of shortening velocity. On the basis of these values, it was calculated that the twitch energetics were consistent with ATP splitting by half the cross-bridges and the pumping of one Ca2+ into the sarcoplasmic reticulum for every three cross-bridge cycles. The simplest interpretation is that half the cross-bridges completed one ATP-splitting cycle in each twitch. The lack of influence of shortening velocity on energy cost supports the idea that the amount of energy to be used is determined early in a twitch and is not greatly influenced by events that occur during the contraction.

The presence of Lys27 instead of Asn27 in human phospholamban promotes sarcoplasmic reticulum Ca2+-ATPase superinhibition and cardiac remodeling

BACKGROUND

Phospholamban (PLN) is an inhibitor of the Ca2+ affinity of sarcoplasmic reticulum (SR) Ca2+-ATPase (SERCA2). The amino acid sequence of PLN is highly conserved, and although all species contain asparagine (Asn), human PLN is unique in containing lysine (Lys) at amino acid 27.

METHODS AND RESULTS

Human PLN was introduced in the null background. Expression of human PLN, at similar levels to mouse wild-type PLN, resulted in significant decreases in the affinity of SERCA2 for Ca2+, attributed to unique spatial conformation of this PLN form and increases in its monomeric active unit compared with mouse PLN. The increased inhibition by human PLN was associated with attenuated cardiac contractility in the intact-animal, organ, and cardiomyocyte levels and with depressed calcium kinetics. These inhibitory effects could not be fully reversed even on maximal isoproterenol stimulation. There were no alterations in the expression levels of SERCA2, calsequestrin, ryanodine receptor, and FKBP12, although the sodium/calcium exchanger and the L-type Ca2+ channel expression levels were upregulated. The depressed function resulted in increased heart/body weight ratios and phosphorylation levels of Akt, p38, and Erk1/2.

CONCLUSIONS

Human PLN may play a more inhibitory role than that of other species in Ca2+ cycling. Expression of human PLN in the mouse is compensated by alterations in Ca2+-handling proteins and cardiac remodeling in an effort to normalize cardiac contractility. Thus, the unique amino acid sequence of human PLN may be critical in maintaining a high cardiac reserve, which is of paramount importance in the regulation of human cardiac function.

Modelling diffusive O(2) supply to isolated preparations of mammalian skeletal and cardiac muscle

The purpose of this study was to use A. V. Hill's equation describing diffusion of O(2) into cylindrical muscles to assess the adequacy of O(2) supply for commonly used isolated preparations of mammalian cardiac and skeletal muscles. The diffusion equation was solved numerically to give the maximum, steady state O(2) diffusion distances (i.e. the distance from the surface of the muscle to the radial location where P(O(2)) is 0) for both resting and contracting muscles and for a range of temperatures. Non-steady state solutions for the rest-to-work transition were also determined to estimate how long contractile activity could be continued before anoxia develops at the muscle centre. The influence on muscle oxygenation of myoglobin-facilitated O(2) diffusion was also assessed. The analysis was performed for typical sized, whole muscles from adult rats and mice, for frog sartorius muscle and for a range of temperatures. Muscle O(2) consumption rates were taken from the literature. The results indicated that (1) diffusive O(2) supply would be adequate to support resting metabolism of soleus and EDL muscles of rat and mouse but may not be adequate to support the transient high resting metabolic rate of papillary muscles shortly after dissection, (2) during steady contractile activity of soleus and EDL muscles, particularly those from the rat, over a reasonable range of duty cycles, adequate O(2) supply could only be ensured if the radii of preparations was substantially smaller than those of whole muscles and (3) for cardiac muscles, diffusive O(2) supply could only support steady-state metabolism at twitch frequencies <1 Hz for whole papillary muscles from rat and <3 Hz for those from mouse. Reducing experimental temperature markedly enhances O(2) supply to skeletal, but not cardiac, muscle. O(2) supply from myoglobin had only minimal effects on oxygenation under typical isolated muscle conditions.

Reduced oxygen supply explains the negative force-frequency relation and the positive inotropic effect of adenosine in buffer-perfused hearts

In isolated rat hearts perfused with HEPES and red blood cell-enriched buffers, we examined changes in left ventricular pressure induced by increases in heart rate or infusion of adenosine to investigate whether the negative force-frequency relation and the positive inotropic effect of adenosine are related to an inadequate oxygen supply provided by crystalloid perfusates. Hearts perfused with HEPES buffer at a constant flow demonstrated a negative force-frequency relation, whereas hearts perfused with red blood cell-enriched buffer exhibited a positive force-frequency relation. In contrast, HEPES buffer-perfused hearts showed a concentration-dependent increase in left ventricular systolic pressure [EC50 = 7.0 ± 1.2 nM, maximal effect (Emax) = 104 ± 2 and 84 ± 2 mmHg at 0.1 μM and baseline, respectively] in response to adenosine, whereas hearts perfused with red blood cell-enriched buffer showed no change in left ventricular pressure. The positive inotropic effect of adenosine correlated with the simultaneous reduction in heart rate ( r = 0.67, P < 0.01; EC50 = 3.8 ± 1.4 nM, baseline 228 ± 21 beats/min to a minimum of 183 ± 22 beats/min at 0.1 μM) and was abolished in isolated hearts paced to suppress the adenosine-induced bradycardia. In conclusion, these results indicate that the negative force-frequency relation and the positive inotropic effect of adenosine in the isolated rat heart are related to myocardial hypoxia, rather than functional peculiarities of the rat heart.

Resting metabolism of mouse papillary muscle

The aims of this study were to measure the resting metabolic rate of isolated mouse papillary muscles and to determine whether diffusive O2 supply is adequate to support the resting metabolism. Resting metabolism of left ventricular papillary muscles was measured in vitro (27 degrees C) using the myothermic technique. The rate of resting metabolism declined exponentially with time towards a steady value, with a time constant of 18+/-2 min (n=13). There was no alteration in isometric force output during this time. The magnitude of the resting metabolism, which depended inversely on muscle mass, more than doubled following a change in substrate from glucose to pyruvate and was increased 2.5-fold when the osmolarity of the bathing solution was increased by addition of 300 mM sucrose. Addition of 30 mM 2, 3-butanedione monoxime affected neither the time course of the decline in metabolic rate nor the eventual steady value. Analysis of the diffusive oxygen supply to the isolated preparation indicated that small papillary muscles (mass <1 mg), which have a very high resting metabolic rate early in an experiment, are unlikely to be adequately oxygenated.

Frequency-dependent acceleration of relaxation in mammalian heart: a property not relying on phospholamban and SERCA2a phosphorylation

An increase in stimulation frequency causes an acceleration of myocardial relaxation (FDAR). Several mechanisms have been postulated to explain this effect, among which is the Ca(2+)-calmodulin-dependent protein kinase (CaMKII)-dependent phosphorylation of the Thr(17) site of phospholamban (PLN). To gain further insights into the mechanisms of FDAR, we studied the FDAR and the phosphorylation of PLN residues in perfused rat hearts, cat papillary muscles and isolated cat myocytes. This allowed us to sweep over a wide range of frequencies, in species with either positive or negative force-frequency relationships, as well as to explore the FDAR under isometric (or isovolumic) and isotonic conditions. Results were compared with those produced by isoprenaline, an intervention known to accelerate relaxation (IDAR) via PLN phosphorylation. While IDAR occurs tightly associated with a significant increase in the phosphorylation of Ser(16) and Thr(17) of PLN, FDAR occurs without significant changes in the phosphorylation of PLN residues in the intact heart and cat papillary muscles. Moreover, in intact hearts, FDAR was not associated with any significant change in the CaMKII-dependent phosphorylation of sarcoplasmic/endoplasmic Ca(2+) ATPase (SERCA2a), and was not affected by the presence of the CaMKII inhibitor, KN-93. In isolated myocytes, FDAR occurred associated with an increase in Thr(17) phosphorylation. However, for a similar relaxant effect produced by isoprenaline, the phosphorylation of PLN (Ser(16) and Thr(17)) was significantly higher in the presence of the beta-agonist. Moreover, the time course of Thr(17) phosphorylation was significantly delayed with respect to the onset of FDAR. In contrast, the time course of Ser(16) phosphorylation, the first residue that becomes phosphorylated with isoprenaline, was temporally associated with IDAR. Furthermore, KN-93 significantly decreased the phosphorylation of Thr(17) that was evoked by increasing the stimulation frequency, but failed to affect FDAR. Taken together, the results provide direct evidence indicating that CaMKII phosphorylation pathways are not involved in FDAR and that FDAR and IDAR do not share a common underlying mechanism. More likely, a CaMKII-independent mechanism could be involved, whereby increasing stimulation frequency would disrupt the SERCA2a-PLN interaction, leading to an increase in SR Ca(2+) uptake and myocardial relaxation.