A context-specific cardiac ß-catenin and GATA4 interaction influences TCF7L2 occupancy and remodels chromatin driving disease progression in the adult heart.

Chromatin remodelling precedes transcriptional and structural changes in heart failure. A body of work suggests roles for the developmental Wnt signalling pathway in cardiac remodelling. Hitherto, there is no evidence supporting a direct role of Wnt nuclear components in regulating chromatin landscapes in this process. We show that transcriptionally active, nuclear, phosphorylated(p)Ser675-ß-catenin and TCF7L2 are upregulated in diseased murine and human cardiac ventricles. We report that inducible cardiomyocytes (CM)-specific pSer675-ß-catenin accumulation mimics the disease situation by triggering TCF7L2 expression. This enhances active chromatin, characterized by increased H3K27ac and TCF7L2 occupancies to cardiac developmental and remodelling genes in vivo. Accordingly, transcriptomic analysis of ß-catenin stabilized hearts shows a strong recapitulation of cardiac developmental processes like cell cycling and cytoskeletal remodelling. Mechanistically, TCF7L2 co-occupies distal genomic regions with cardiac transcription factors NKX2-5 and GATA4 in stabilized-ß-catenin hearts. Validation assays revealed a previously unrecognized function of GATA4 as a cardiac repressor of the TCF7L2/ß-catenin complex in vivo, thereby defining a transcriptional switch controlling disease progression. Conversely, preventing ß-catenin activation post-pressure-overload results in a downregulation of these novel TCF7L2-targets and rescues cardiac function. Thus, we present a novel role for TCF7L2/ß-catenin in CMs-specific chromatin modulation, which could be exploited for manipulating the ubiquitous Wnt pathway.

Protein kinase/phosphatase balance mediates the effects of increased late sodium current on ventricular calcium cycling.

Increased late sodium current (late INa) is an important arrhythmogenic trigger in cardiac disease. It prolongs cardiac action potential and leads to an increased SR Ca2+ leak. This study investigates the contribution of Ca2+/Calmodulin-dependent kinase II (CaMKII), protein kinase A (PKA) and conversely acting protein phosphatases 1 and 2A (PP1, PP2A) to this subcellular crosstalk. Augmentation of late INa (ATX-II) in murine cardiomyocytes led to an increase of diastolic Ca2+ spark frequency and amplitudes of Ca2+ transients but did not affect SR Ca2+ load. Interestingly, inhibition of both, CaMKII and PKA, attenuated the late INa-dependent induction of the SR Ca2+ leak. PKA inhibition additionally reduced the amplitudes of systolic Ca2+ transients. FRET-measurements revealed increased levels of cAMP upon late INa augmentation, which could be prevented by simultaneous inhibition of Na+/Ca2+-exchanger (NCX) suggesting that PKA is activated by Ca2+-dependent cAMP-production. Whereas inhibition of PP2A showed no effect on late INa-dependent alterations of Ca2+ cycling, additional inhibition of PP1 further increased the SR Ca2+ leak. In line with this, selective activation of PP1 yielded a strong reduction of the late INa-induced SR Ca2+ leak and did not affect systolic Ca2+ release. This study indicates that phosphatase/kinase-balance is perturbed upon increased Na+ influx leading to disruption of ventricular Ca2+ cycling via CaMKII- and PKA-dependent pathways. Importantly, an activation of PP1 at RyR2 may represent a promising new toehold to counteract pathologically increased kinase activity.

Sex-dependent alterations of Ca2+ cycling in human cardiac hypertrophy and heart failure.

AIMS: Clinical studies have shown differences in the propensity for malignant ventricular arrhythmias between women and men suffering from cardiomyopathies and heart failure (HF). This is clinically relevant as it impacts therapies like prophylactic implantable cardioverter-defibrillator implantation but the pathomechanisms are unknown. As an increased sarcoplasmic reticulum (SR) Ca(2+) leak is arrhythmogenic, it could represent a cellular basis for this paradox. METHODS/RESULTS: We evaluated the SR Ca(2+) leak with respect to sex differences in (i) afterload-induced cardiac hypertrophy (Hy) with preserved left ventricular (LV) function and (ii) end-stage HF. Cardiac function did not differ between sexes in both cardiac pathologies. Human cardiomyocytes isolated from female patients with Hy showed a significantly lower Ca(2+) spark frequency (CaSpF, confocal microscopy, Fluo3-AM) compared with men (P < 0.05). As Ca(2+) spark width and duration were similar in women and men, this difference in CaSpF did not yet translate into a significant difference of the calculated SR Ca(2+) leak between both sexes at this stage of disease (P = 0.14). Epifluorescence measurements (Fura2-AM) revealed comparable Ca(2+) cycling properties (diastolic Ca(2+) levels, amplitude of systolic Ca(2+) transients, SR Ca(2+) load) in patients of both sexes suffering from Hy. Additionally, the increased diastolic CaSpF in male patients with Hy did not yet translate into an elevated ratio of cells showing arrhythmic events (Ca(2+) waves, spontaneous Ca(2+) transients) (P = 0.77). In the transition to HF, both sexes showed an increase of the CaSpF (P < 0.05) and the sex dependence was even more pronounced. Female patients had a 69 ± 10% lower SR Ca(2+) leak (P < 0.05), which now even translated into a lower ratio of arrhythmic cells in female HF patients compared with men (P < 0.001). CONCLUSION: These data show that the SR Ca(2+) leak is lower in women than in men with comparable cardiac impairment. Since the SR Ca(2+) leak triggers delayed afterdepolarizations, our findings may explain why women are less prone to ventricular arrhythmias and confirm the rationale of therapeutic measures reducing the SR Ca(2+) leak.

Inhibition of CaMKII Attenuates Progressing Disruption of Ca(2+) Homeostasis Upon Left Ventricular Assist Device Implantation in Human Heart Failure.

In heart failure, left ventricular assist device (LVAD) implantation is performed to ensure sufficient cardiac output. Whereas some patients are subsequently weaned from LVAD support, other patients still need heart transplantation. To elucidate underlying mechanisms, we assessed the arrhythmogenic SR-Ca(2+) leak at the time of LVAD implantation (HF-Im) and heart transplantation (HF-Tx) and evaluated the effects of CaMKII-inhibition. Human left-ventricular cardiomyocytes were isolated, paced at 1 Hz for 10 beats to ensure SR-Ca(2+) loading and scanned for diastolic Ca(2+) sparks (confocal microscopy). In HF-Im, the high diastolic spark frequency (CaSpF) of 0.76 ± 0.12 × 100 µm(-1) × s(-1) could be reduced to 0.48 ± 0.10 × 100 µm(-1) × s(-1) by CaMKII inhibition (AIP, 1 µM). The amplitude of Ca(2+) sparks, width, and length was not significantly altered. In sum, CaMKII inhibition yielded a clear tendency toward a reduction of the SR-Ca(2+) leak (n cells/patients = 76/6 vs. 108/6, P = 0.08). In HF-Tx, we detected an even higher CaSpF of 1.00 ± 0.10 100 µm(-1) × s(-1) and a higher SR-Ca(2+) leak compared with HF-Im (increase by 81 ± 33%, n cells/patients = 156/7 vs. 130/7, P < 0.05), which fits to the further decreased LV function. Here, CaMKII inhibition likewise reduced CaSpF (0.35 ± 0.09 100 µm(-1) × s(-1,) P = 0.06) and significantly reduced spark duration (n sparks/patients = 58/3 vs. 159/3, P < 0.05). Conclusively, the SR-Ca(2+) leak was reduced by 69 ± 12% in HF-Tx upon CaMKII inhibition (n cells/patients = 53/3 vs. 91/3, P < 0.05). These data show that the SR-Ca(2+) leak correlates with the development of LV function after LVAD implantation and may represent an important pathomechanism. The fact that CaMKII inhibition reduces the SR-Ca(2+) leak in HF-Tx suggests that CaMKII inhibition may be a promising option to beneficially influence clinical course after LVAD implantation.

Enhanced late INa induces proarrhythmogenic SR Ca leak in a CaMKII-dependent manner.

OBJECTIVE: Enhanced late Na current (late INa) induces Na-dependent Ca overload as well as proarrhythmogenic events on the cellular level that include spatio-temporally uncoordinated diastolic Ca release from the sarcoplasmic reticulum (SR) and delayed afterdepolarizations (DADs). The Ca/calmodulin-dependent protein kinase II (CaMKII) gets activated upon increases in [Ca]i and mediates diastolic SR Ca leak as well as DADs. RATIONALE: We hypothesized that increased late INa (in disease-comparable ranges) exerts proarrhythmogenic events in isolated ventricular mouse myocytes in a manner depending on CaMKII-dependent SR Ca leak. We further tested whether inhibition of disease-related late INa may reduce proarrhythmogenic SR Ca leak in myocytes from failing human hearts. METHODS: Ventricular myocytes were isolated from healthy wildtype (WT), failing CaMKIIδC transgenic (TG) mouse, and failing human hearts. ATX-II (0.25-10 nmol/L) was used to enhance late INa. Spontaneous Ca loss from the SR during diastole (Ca sparks), DADs, non-triggered diastolic Ca transients in myocytes and premature beats of isometrically twitching papillary muscles were used as readouts for proarrhythmogenic events. CaMKII autophosphorylation was assessed by immunoblots. Late INa was inhibited using ranolazine (Ran, 10 µmol/L) or TTX (2 µmol/L), and CaMKII by KN-93 (1 µmol/L) or AIP (1 µmol/L). RESULTS: In WT myocytes, sub-nanomolar ATX-II exposure (0.5 nmol/L) enhanced late INa by ~60%, which resulted in increased diastolic SR Ca loss despite unaltered SR Ca content. In parallel, DADs and non-triggered diastolic Ca transients arose. Inhibition of enhanced late INa by RAN or TTX significantly attenuated diastolic SR Ca loss and suppressed DADs as well as mechanical alternans in mouse and diastolic SR Ca loss in failing human myocytes. ATX-II caused Ca-dependent CaMKII-activation without changes in protein expression, which was reversible by Ran or AIP. Conversely, CaMKII-inhibition decreased diastolic SR Ca loss, DADs and non-triggered diastolic Ca transients despite ATX-II-exposure. Finally, failing mouse myocytes with increased CaMKII activity (TG CaMKIIδC) showed an even aggravated diastolic SR Ca loss that was associated with an increased frequency of non-triggered diastolic Ca transients upon enhanced late INa. CONCLUSIONS: Increased late INa (in disease-comparable ranges) induces proarrhythmogenic events during diastole in healthy and failing mouse myocytes, which are mediated via CaMKII-dependent SR Ca loss. Inhibition of late INa not only attenuated these cellular arrhythmias in mouse myocytes but also in failing human myocytes indicating some antiarrhythmic potential for an inhibition of the elevated late INa/CaMKII signaling pathway in this setting.

Ca2+/calmodulin-dependent protein kinase II and protein kinase A differentially regulate sarcoplasmic reticulum Ca2+ leak in human cardiac pathology.

BACKGROUND: Sarcoplasmic reticulum (SR) Ca(2+) leak through ryanodine receptor type 2 (RyR2) dysfunction is of major pathophysiological relevance in human heart failure (HF); however, mechanisms underlying progressive RyR2 dysregulation from cardiac hypertrophy to HF are still controversial. METHODS AND RESULTS: We investigated healthy control myocardium (n=5) and myocardium from patients with compensated hypertrophy (n=25) and HF (n=32). In hypertrophy, Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) and protein kinase A (PKA) both phosphorylated RyR2 at levels that were not different from healthy myocardium. Accordingly, inhibitors of these kinases reduced the SR Ca(2+) leak. In HF, however, the SR Ca(2+) leak was nearly doubled compared with hypertrophy, which led to reduced systolic Ca(2+) transients, a depletion of SR Ca(2+) storage and elevated diastolic Ca(2+) levels. This was accompanied by a significantly increased CaMKII-dependent phosphorylation of RyR2. In contrast, PKA-dependent RyR2 phosphorylation was not increased in HF and was independent of previous ß-blocker treatment. In HF, CaMKII inhibition but not inhibition of PKA yielded a reduction of the SR Ca(2+) leak. Moreover, PKA inhibition further reduced SR Ca(2+) load and systolic Ca(2+) transients. CONCLUSIONS: In human hypertrophy, both CaMKII and PKA functionally regulate RyR2 and may induce SR Ca(2+) leak. In the transition from hypertrophy to HF, the diastolic Ca(2+) leak increases and disturbed Ca(2+) cycling occurs. This is associated with an increase in CaMKII- but not PKA-dependent RyR2 phosphorylation. CaMKII inhibition may thus reflect a promising therapeutic target for the treatment of arrhythmias and contractile dysfunction.

Role of late sodium current as a potential arrhythmogenic mechanism in the progression of pressure-induced heart disease.

The aim of the study was to determine the characteristics of the late Na current (INaL) and its arrhythmogenic potential in the progression of pressure-induced heart disease. Transverse aortic constriction (TAC) was used to induce pressure overload in mice. After one week the hearts developed isolated hypertrophy with preserved systolic contractility. In patch-clamp experiments both, INaL and the action potential duration (APD90) were unchanged. In contrast, after five weeks animals developed heart failure with prolonged APDs and slowed INaL decay time which could be normalized by addition of the INaL inhibitor ranolazine (Ran) or by the Ca/calmodulin-dependent protein kinase II (CaMKII) inhibitor AIP. Accordingly the APD90 could be significantly abbreviated by Ran, tetrodotoxin and the CaMKII inhibitor AIP. Isoproterenol increased the number of delayed afterdepolarizations (DAD) in myocytes from failing but not sham hearts. Application of either Ran or AIP prevented the occurrence of DADs. Moreover, the incidence of triggered activity was significantly increased in TAC myocytes and was largely prevented by Ran and AIP. Western blot analyses indicate that increased CaMKII activity and a hyperphosphorylation of the Nav1.5 at the CaMKII phosphorylation site (Ser571) paralleled our functional observations five weeks after TAC surgery. In pressure overload-induced heart failure a CaMKII-dependent augmentation of INaL plays a crucial role in the AP prolongation and generation of cellular arrhythmogenic triggers, which cannot be found in early and still compensated hypertrophy. Inhibition of INaL and CaMKII exerts potent antiarrhythmic effects and might therefore be of potential therapeutic interest. This article is part of a Special Issue entitled "Na(+) Regulation in Cardiac Myocytes".

Atrioventricular 2:1-conduction via an accessory pathway during left atrial flutter unmasking WPW syndrome: a case report.

Background: Implantable cardioverter defibrillators (ICDs) are most effective in treating sudden cardiac death. However, accurate diagnostic workup of broad complex tachycardia is crucial to ensure correct indication for ICD treatment and to avoid unnecessary invasive treatment and device-associated morbidity. Case summary: We present a case of atypical atrial flutter with 2:1 atrioventricular (AV) conduction via a left-posterior accessory pathway (AP), leading to the diagnosis of Wolff-Parkinson-White (WPW) syndrome. Upon admission, the 72-year-old patient showed a regular broad complex tachycardia with superior axis and positive concordance in precordial leads, suggestive of either ventricular tachycardia (VT), antidromic AV re-entrant tachycardia (AVRT), or supraventricular tachycardia with antegrade conduction via a left-posterior AP. Interrogation of the two-chamber ICD, which was very likely implanted unjustified in a peripheral clinic before, revealed atrial flutter with 2:1 AV conduction. Remarkably, after the restoration of sinus rhythm, no classic echocardiogram (ECG) criteria for preexcitation syndrome were detected. An invasive electrophysiological study proved the diagnosis of a bidirectionally conducting, left-posterior AP, which was successfully ablated. Discussion: Differential diagnosis of broad complex tachycardia with superior axis and positive concordance of chest leads consists of i) VT with a left ventricular exit at the posterior mitral annulus, ii) antidromic AVRT involving a left-posterior AP, and iii) supraventricular tachycardia predominantly conducted via a left-posterior AP. The absence of classic ECG criteria for preexcitation syndrome does not rule out AP sufficiently, highlighting the importance of minimal surface-ECG preexcitation criteria. In the case of detection of minimal surface-ECG preexcitation criteria, administration of adenosine rules out or proves the existence of an AP noninvasively and cost-effectively.

Sacubitrilat reduces pro-arrhythmogenic sarcoplasmic reticulum Ca2+ leak in human ventricular cardiomyocytes of patients with end-stage heart failure.

AIMS: Inhibition of neprilysin and angiotensin II receptor by sacubitril/valsartan (Val) (LCZ696) reduces mortality in heart failure (HF) patients compared with sole inhibition of renin-angiotensin system. Beneficial effects of increased natriuretic peptide levels upon neprilysin inhibition have been proposed, whereas direct effects of sacubitrilat (Sac) (LBQ657) on myocardial Ca2+ cycling remain elusive. METHODS AND RESULTS: Confocal microscopy (Fluo-4 AM) was used to investigate pro-arrhythmogenic sarcoplasmic reticulum (SR) Ca2+ leak in freshly isolated murine and human ventricular cardiomyocytes (CMs) upon Sac (40 µmol/L)/Val (13 µmol/L) treatment. The concentrations of Sac and Val equalled plasma concentrations of LCZ696 treatment used in PARADIGM-HF trial. Epifluorescence microscopy measurements (Fura-2 AM) were performed to investigate effects on systolic Ca2+ release, SR Ca2+ load, and Ca2+ -transient kinetics in freshly isolated murine ventricular CMs. The impact of Sac on myocardial contractility was evaluated using in toto-isolated, isometrically twitching ventricular trabeculae from human hearts with end-stage HF. Under basal conditions, the combination of Sac/Val did not influence diastolic Ca2+ -spark frequency (CaSpF) nor pro-arrhythmogenic SR Ca2 leak in isolated murine ventricular CMs (n CMs/hearts = 80/7 vs. 100/7, P = 0.91/0.99). In contrast, Sac/Val treatment reduced CaSpF by 35 ± 9% and SR Ca2+ leak by 45 ± 9% in CMs put under catecholaminergic stress (isoproterenol 30 nmol/L, n = 81/7 vs. 62/7, P < 0.001 each). This could be attributed to Sac, as sole Sac treatment also reduced both parameters by similar degrees (reduction of CaSpF by 57 ± 7% and SR Ca2+ leak by 76 ± 5%; n = 101/4 vs. 108/4, P < 0.01 each), whereas sole Val treatment did not. Systolic Ca2+ release, SR Ca2+ load, and Ca2+ -transient kinetics including SERCA activity (kSERCA ) were not compromised by Sac in isolated murine CMs (n = 41/6 vs. 39/6). Importantly, the combination of Sac/Val and Sac alone also reduced diastolic CaSpF and SR Ca2+ leak (reduction by 74 ± 7%) in human left ventricular CMs from patients with end-stage HF (n = 71/8 vs. 78/8, P < 0.05 each). Myocardial contractility of human ventricular trabeculae was not acutely affected by Sac treatment as the developed force remained unchanged over a time course of 30 min (n trabeculae/hearts = 3/3 vs. 4/3). CONCLUSION: This study demonstrates that neprilysin inhibitor Sac directly improves Ca2+ homeostasis in human end-stage HF by reducing pro-arrhythmogenic SR Ca2+ leak without acutely affecting systolic Ca2+ release and inotropy. These effects might contribute to the mortality benefits observed in the PARADIGM-HF trial.

Antiarrhythmic effects of dantrolene in human diseased cardiomyocytes.

BACKGROUND: Cardiac type 2 ryanodine receptors (RyR2s) play a pivotal role in cellular electrophysiology and contractility. Increased RyR2-mediated diastolic sarcoplasmic reticulum (SR) Ca2+ release is linked to heart failure (HF) and arrhythmias. Dantrolene, a drug used for the treatment of malignant hyperthermia, is known to stabilize RyRs in skeletal muscle. OBJECTIVE: The purpose of this study was to investigate the effects of dantrolene on arrhythmogenic triggers and contractile function in human atrial fibrillation (AF) and HF cardiomyocytes (CM). METHODS: Human CM were isolated from either patients with HF (ventricular) or patients with AF (atrial), and Ca2+ imaging, patch-clamp, or muscle strip experiments were performed. RESULTS: After exposure to dantrolene, human atrial AF and left ventricular HF CM showed significant reductions in proarrhythmic SR Ca2+ spark frequency and diastolic SR Ca2+ leak. Moreover, dantrolene decreased the frequency of Ca2+ waves and spontaneous Ca2+ transients in HF CM. Patch-clamp experiments revealed that dantrolene significantly suppressed delayed afterdepolarizations in HF and AF CM. Importantly, dantrolene had no effect on action potential duration in AF or in HF CM. In addition, dantrolene had neutral effects on contractile force of human isometrically twitching ventricular HF trabeculae. CONCLUSION: Our study showed that dantrolene beneficially influenced disrupted SR Ca2+ homeostasis in human HF and AF CM. Cellular arrhythmogenic triggers were potently suppressed by dantrolene, whereas action potential duration and contractility were not affected. As a clinically approved drug for the treatment of malignant hyperthermia, dantrolene may be a potential antiarrhythmic drug for patients with rhythm disorders and merits further clinical investigation.

Interactions of Calcium Fluctuations during Cardiomyocyte Contraction with Real-Time cAMP Dynamics Detected by FRET.

Calcium (Ca2+) and 3',5'-cyclic adenosine monophosphate (cAMP) play a critical role for cardiac excitation-contraction-coupling. Both second messengers are known to interact with each other, for example via Ca2+-dependent modulation of phosphodiesterase 1 (PDE1) and adenylyl cyclase 5/6 (AC 5/6) activities, which is supposed to occur especially at the local level in distinct subcellular microdomains. Currently, many studies analyze global and local cAMP signaling and its regulation in resting cardiomyocytes devoid of electrical stimulation. For example, Förster resonance energy transfer (FRET) microscopy is a popular approach for visualization of real time cAMP dynamics performed in resting cardiomyocytes to avoid potential contractility-related movement artifacts. However, it is unknown whether such data are comparable with the cell behavior under more physiologically relevant conditions during contraction. Here, we directly compare the cAMP-FRET responses to AC stimulation and PDE inhibition in resting vs. paced adult mouse ventricular cardiomyocytes for both cytosolic and subsarcolemmal microdomains. Interestingly, no significant differences in cAMP dynamics could be detected after ß-adrenergic (isoproterenol) stimulation, suggesting low impact of rapidly changing contractile Ca2+ concentrations on cytosolic cAMP levels associated with AC activation. However, the contribution of the calcium-dependent PDE1, but not of the Ca2+-insensitive PDE4, to the regulation of cAMP levels after forskolin stimulation was significantly increased. This increase could be mimicked by pretreatment of resting cells with Ca2+ elevating agents. Ca2+ imaging demonstrated significantly higher amplitudes of Ca2+ transients in forskolin than in isoproterenol stimulated cells, suggesting that forskolin stimulation might lead to stronger activation of PDE1. In conclusion, changes in intracellular Ca2+ during cardiomyocyte contraction dynamically interact with cAMP levels, especially after strong AC stimulation. The use of resting cells for FRET-based measurements of cAMP can be justified under ß-adrenergic stimulation, while the reliable analysis of PDE1 effects may require electric field stimulation.

Late INa increases diastolic SR-Ca2+-leak in atrial myocardium by activating PKA and CaMKII.

AIMS: Enhanced cardiac late Na current (late INa) and increased sarcoplasmic reticulum (SR)-Ca(2+)-leak are both highly arrhythmogenic. This study seeks to identify signalling pathways interconnecting late INa and SR-Ca(2+)-leak in atrial cardiomyocytes (CMs). METHODS AND RESULTS: In murine atrial CMs, SR-Ca(2+)-leak was increased by the late INa enhancer Anemonia sulcata toxin II (ATX-II). An inhibition of Ca(2+)/calmodulin-dependent protein kinase II (Autocamide-2-related inhibitory peptide), protein kinase A (H89), or late INa (Ranolazine or Tetrodotoxin) all prevented ATX-II-dependent SR-Ca(2+)-leak. The SR-Ca(2+)-leak induction by ATX-II was not detected when either the Na(+)/Ca(2+) exchanger was inhibited (KBR) or in CaMKIIδc-knockout mice. FRET measurements revealed increased cAMP levels upon ATX-II stimulation, which could be prevented by inhibition of adenylyl cyclases (ACs) 5 and 6 (NKY 80) but not by inhibition of phosphodiesterases (IBMX), suggesting PKA activation via an AC-dependent increase of cAMP levels. Western blots showed late INa-dependent hyperphosphorylation of CaMKII as well as PKA target sites at ryanodine receptor type-2 (-S2814 and -S2808) and phospholamban (-Thr17, -S16). Enhancement of late INa did not alter Ca(2+)-transient amplitude or SR-Ca(2+)-load. However, upon late INa activation and simultaneous CaMKII inhibition, Ca(2+)-transient amplitude and SR-Ca(2+)-load were increased, whereas PKA inhibition reduced Ca(2+)-transient amplitude and load and additionally slowed Ca(2+) elimination. In atrial CMs from patients with atrial fibrillation, inhibition of late INa, CaMKII, or PKA reduced the SR-Ca(2+)-leak. CONCLUSION: Late INa exerts distinct effects on Ca(2+) homeostasis in atrial myocardium through activation of CaMKII and PKA. Inhibition of late INa represents a potential approach to attenuate CaMKII activation and decreases SR-Ca(2+)-leak in atrial rhythm disorders. The interconnection with the cAMP/PKA system further increases the antiarrhythmic potential of late INa inhibition.

Ca(2+) /calmodulin-dependent protein kinase II equally induces sarcoplasmic reticulum Ca(2+) leak in human ischaemic and dilated cardiomyopathy.

AIMS: The sarcoplasmic reticulum (SR) Ca(2+) leak is an important pathomechanism in heart failure (HF). It has been suggested that Ca(2+) /calmodulin-dependent protein kinase II (CaMKII) is only relevant for the induction of the SR Ca(2+) leak in non-ischaemic but not in ischaemic HF. Therefore, we investigated CaMKII and its targets as well as the functional effects of CaMKII inhibition in human ischaemic cardiomyopathy (ICM, n = 37) and dilated cardiomyopathy (DCM, n = 40). METHODS AND RESULTS: Western blots showed a significantly increased expression (by 54 ± 9%) and autophosphorylation at Thr286 (by 129 ± 29%, P < 0.05 each) of CaMKII in HF compared with healthy myocardium. However, no significant difference could be detected in ICM compared with DCM as to the expression and autophosphorylation of CaMKII nor the phosphorylation of the target sites ryanodine receptor 2 (RyR2)-S2809, RyR2-S2815, and phospholamban-Thr17. Isolated human cardiomyocytes (CMs) of patients with DCM and ICM showed a similar frequency of diastolic Ca(2+) sparks (confocal microscopy) as well as of major arrhythmic events (Ca(2+) waves, spontaneous Ca(2+) transients). Despite a slightly smaller size of Ca(2+) sparks in DCM (P < 0.01), the calculated SR Ca(2+) leak [Ca(2+) spark frequecy (CaSpF) × amplitude × width × duration] did not differ between CMs of ICM vs. DCM. Importantly, CaMKII inhibition by autocamide-2-related inhibitory peptide (AIP, 1 µmol/L) reduced the SR Ca(2+) leak by ∼80% in both aetiologies (P < 0.05 each) and effectively decreased the ratio of arrhythmic cells (P < 0.05). CONCLUSION: Functional and molecular measures of the SR Ca(2+) leak are comparable in human ICM and DCM. CaMKII is equally responsible for the induction of the 'RyR2 leakiness' in both pathologies. Thus, CaMKII inhibition as a therapeutic measure may not be restricted to patients suffering from DCM but rather may be beneficial for the majority of HF patients.