Increased Sarcoplasmic Reticulum Ca Research Articles

Ca2+ storage function is altered in the sarcoplasmic reticulum of skeletal muscle lacking mitsugumin 23.

Mitsugumin 23 (MG23) has been identified as a ball-shaped cation channel in the sarcoplasmic reticulum (SR) but its physiological role remains unclear. This study aimed to examine the contribution of MG23 to Ca2+ storage function in skeletal muscle by using Mg23-knockout (Mg23-/-) mice. There was no difference in the isometric specific force of the extensor digitorum longus (EDL) and soleus (SOL) muscles between Mg23-/- and wild-type (Wt) mice. In Mg23-/- mice, the calsequestrin 2 content in the EDL muscle and SR Ca2+-ATPase 2 content in the SOL were increased. We have examined SR and myofibril functions using mechanically skinned fibers and determined their fiber types based on the response to Sr2+, which showed that Mg23-/- mice, compared with Wt, had: 1) elevated total Ca2+ content in the membranous components including SR, mitochondria, and transverse tubular system referred to as endogenous Ca2+ content, in both type I and II fibers of the EDL and SOL; 2) increased maximal Ca2+ content in both type I and II fibers of the EDL and SOL; 3) decreased SR Ca2+ leakage in type I fibers of the SOL; and 4) enhanced SR Ca2+ uptake in type I fibers of the SOL, although myofibril function was not different in both type I and II fibers of the SOL and EDL muscles. These results suggest that MG23 decreases SR Ca2+ storage in both type I and type II fibers, likely due to increased SR Ca2+ leakage.NEW & NOTEWORTHY The function of calcium storage within sarcoplasmic reticulum (SR) plays a pivotal role in influencing the health and disease states of skeletal muscle. In the present study, we demonstrated that mitsgumin 23, a novel non-selective cation channel, modifies SR Ca2+ storage in skeletal muscle fibers. These findings provide valuable insights into the physiological regulation of Ca2+ in skeletal muscle, offering significant potential for uncovering the mechanisms underlying muscle fatigue, muscle adaptation, and muscle diseases.

Lysosomal Ca2+-flux contributes to abnormal automaticity-related arrhythmia in ventricular cardiomyopathy.

Abnormal automaticity contributes to ventricular arrhythmias during cardiac ischemia. This mechanism involves Ca2+ handling at the membrane and the sarcoplasmic reticulum (SR). We have described a contribution of mitochondrial Ca2+ flux to automaticity. Recent publications showed that lysosomal Ca2+ release influences SR Ca2+ release. Therefore, we determined if lysosomal Ca2+ flux could modify abnormal automaticity in ventricular cardiomyocytes. Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) and cardiomyocytes (CMs) derived from infarcted mice were studied. The spontaneous beating rate of CMs was determined from action potentials or cytoplasmic Ca2+ transients. The hiPSC-CMs transfected with lysosomal transient receptor potential mucolipin Ca2+ release channel (TRPML1) siRNA had significantly reduced automaticity and a significantly increased SR Ca2+ release time constant. Moreover, the TRPML1 specific agonist, ML-SA1, accelerated the spontaneous beating while a TRPML1 specific antagonist, ML-SI1 substantially decreased this frequency. Proteasome inhibition by MG132 activated lysosomal transcription factor EB (TFEB). TFEB activation increased lysosomes and resulted in increased automaticity. Proteasome inhibition had the opposite effect. Blocking the lysosomal two-pore Ca2+ channel (TPC2) or inhibiting the V-type ATPase responsible for lysosomal Ca2+ loading had similar effects on automaticity. Furthermore, inhibition lysosomal Ca2+ release reduced the abnormal automaticity in adult cardiomyocytes isolated after myocardial infarction. As expected based upon hiPSC-CMs and mouse cardiomyocytes data, the protein level of TRPML1 in heart failure (HF) patients with tachycardia (VT) was significantly enhanced when compared with that in HF patients without VT. Lysosomes Ca2+-cycling modulated automaticity in ventricular hiPSC-CMs or cardiomyocytes from infarcted mouse hearts and contributed to HF patient VT events, implying that changes in lysosomal Ca2+ handling may play an important role in abnormal automaticity related arrhythmia.

Evolutionary isolation of ryanodine receptor isoform 1 for muscle-based thermogenesis in mammals

Resting skeletal muscle generates heat for endothermy in mammals but not amphibians, while both use the same Ca2+-handling proteins and membrane structures to conduct excitation-contraction coupling apart from having different ryanodine receptor (RyR) isoforms for Ca2+ release. The sarcoplasmic reticulum (SR) generates heat following Adenosine triphosphate (ATP) hydrolysis at the Ca2+ pump, which is amplified by increasing RyR1 Ca2+ leak in mammals, subsequently increasing cytoplasmic [Ca2+] ([Ca2+]cyto). For thermogenesis to be functional, rising [Ca2+]cyto must not interfere with cytoplasmic effectors of the sympathetic nervous system (SNS) that likely increase RyR1 Ca2+ leak; nor should it compromise the muscle remaining relaxed. To achieve this, Ca2+ activated, regenerative Ca2+ release that is robust in lower vertebrates needs to be suppressed in mammals. However, it has not been clear whether: i) the RyR1 can be opened by local increases in [Ca2+]cyto; and ii) downstream effectors of the SNS increase RyR Ca2+ leak and subsequently, heat generation. By positioning amphibian and malignant hyperthermia-susceptible human-skinned muscle fibers perpendicularly, we induced abrupt rises in [Ca2+]cyto under identical conditions optimized for activating regenerative Ca2+ release as Ca2+ waves passed through the junction of fibers. Only mammalian fibers showed resistance to rising [Ca2+]cyto, resulting in increased SR Ca2+ load and leak. Fiber heat output was increased by cyclic adenosine monophosphate (cAMP)-induced RyR1 phosphorylation at Ser2844 and Ca2+ leak, indicating likely SNS regulation of thermogenesis. Thermogenesis occurred despite the absence of SR Ca2+ pump regulator sarcolipin. Thus, evolutionary isolation of RyR1 provided increased dynamic range for thermogenesis with sensitivity to cAMP, supporting endothermy.

Ziprasidone Induces Rabbit Atrium Arrhythmogenesis via Modification of Oxidative Stress and Sodium/Calcium Homeostasis.

Background: Atypical antipsychotics increase the risk of atrial arrhythmias and sudden cardiac death. This study investigated whether ziprasidone, a second-generation antipsychotic, affected intracellular Ca2+ and Na+ regulation and oxidative stress, providing proarrhythmogenic substrates in atriums. Methods: Electromechanical analyses of rabbit atrial tissues were conducted. Intracellular Ca2+ monitoring using Fluo-3, the patch-clamp method for ionic current recordings, and a fluorescence study for the detection of reactive oxygen species and intracellular Na+ levels were conducted in enzymatically dissociated atrial myocytes. Results: Ziprasidone-treated atriums showed sustained triggered activities after rapid pacing, which were inhibited by KN-93 and ranolazine. A reduced peak L-type Ca2+ channel current and enhanced late Na+ current were observed in ziprasidone-treated atrial myocytes, together with an increased cytosolic Na+ level. KN-93 suppressed the enhanced late Na+ current in ziprasidone-treated atrial myocytes. Atrial myocytes treated with ziprasidone showed reduced Ca2+ transient amplitudes and sarcoplasmic reticulum (SR) Ca2+ stores, and increased SR Ca2+ leakage. Cytosolic and mitochondrial reactive oxygen species production was increased in atrial myocytes treated with ziprasidone. TNF-α and NLRP3 were upregulated in ziprasidone-treated myocytes, and the level of phosphorylated calcium/calmodulin-dependent protein kinase II protein was increased. Conclusions: Our results suggest that ziprasidone increases the occurrence of atrial triggered activity and causes intracellular Ca2+ and Na+ dysregulation, which may result from enhanced oxidative stress and activation of the TNF-α/NLRP3 inflammasome pathway in ziprasidone-treated myocytes.

Aldosterone-Induced Sarco/Endoplasmic Reticulum Ca2+ Pump Upregulation Counterbalances Cav1.2-Mediated Ca2+ Influx in Mesenteric Arteries.

In mesenteric arteries (MAs), aldosterone (ALDO) binds to the endogenous mineralocorticoid receptor (MR) and increases the expression of the voltage-gated L-type Cav1.2 channel, an essential ion channel for vascular contraction, sarcoplasmic reticulum (SR) Ca2+ store refilling, and Ca2+ spark generation. In mesenteric artery smooth muscle cells (MASMCs), Ca2+ influx through Cav1.2 is the indirect mechanism for triggering Ca2+ sparks. This process is facilitated by plasma membrane-sarcoplasmic reticulum (PM-SR) nanojunctions that drive Ca2+ from the extracellular space into the SR via Sarco/Endoplasmic Reticulum Ca2+ (SERCA) pump. Ca2+ sparks produced by clusters of Ryanodine receptors (RyRs) at PM-SR nanodomains, decrease contractility by activating large-conductance Ca2+-activated K+ channels (BKCa channels), which generate spontaneous transient outward currents (STOCs). Altogether, Cav1.2, SERCA pump, RyRs, and BKCa channels work as a functional unit at the PM-SR nanodomain, regulating intracellular Ca2+ and vascular function. However, the effect of the ALDO/MR signaling pathway on this functional unit has not been completely explored. Our results show that short-term exposure to ALDO (10 nM, 24 h) increased the expression of Cav1.2 in rat MAs. The depolarization-induced Ca2+ entry increased SR Ca2+ load, and the frequencies of both Ca2+ sparks and STOCs, while [Ca2+]cyt and vasoconstriction remained unaltered in Aldo-treated MAs. ALDO treatment significantly increased the mRNA and protein expression levels of the SERCA pump, which counterbalanced the augmented Cav1.2-mediated Ca2+ influx at the PM-SR nanodomain, increasing SR Ca2+ content, Ca2+ spark and STOC frequencies, and opposing to hyperpolarization-induced vasoconstriction while enhancing Acetylcholine-mediated vasorelaxation. This work provides novel evidence for short-term ALDO-induced upregulation of the functional unit comprising Cav1.2, SERCA2 pump, RyRs, and BKCa channels; in which the SERCA pump buffers ALDO-induced upregulation of Ca2+ entry at the superficial SR-PM nanodomain of MASMCs, preventing ALDO-triggered depolarization-induced vasoconstriction and enhancing vasodilation. Pathological conditions that lead to SERCA pump downregulation, for instance, chronic exposure to ALDO, might favor the development of ALDO/MR-mediated augmented vasoconstriction of mesenteric arteries.

Ero1α-Dependent ERp44 Dissociation From RyR2 Contributes to Cardiac Arrhythmia.

Oxidative stress in cardiac disease promotes proarrhythmic disturbances in Ca2+ homeostasis, impairing luminal Ca2+ regulation of the sarcoplasmic reticulum (SR) Ca2+ release channel, the RyR2 (ryanodine receptor), and increasing channel activity. However, exact mechanisms underlying redox-mediated increase of RyR2 function in cardiac disease remain elusive. We tested whether the oxidoreductase family of proteins that dynamically regulate the oxidative environment within the SR are involved in this process. A rat model of hypertrophy induced by thoracic aortic banding (TAB) was used for ex vivo whole heart optical mapping and for Ca2+ and reactive oxygen species imaging in isolated ventricular myocytes (VMs). The SR-targeted reactive oxygen species biosensor ERroGFP showed increased intra-SR oxidation in TAB VMs that was associated with increased expression of Ero1α (endoplasmic reticulum oxidoreductase 1 alpha). Pharmacological (EN460) or genetic Ero1α inhibition normalized SR redox state, increased Ca2+ transient amplitude and SR Ca2+ content, and reduced proarrhythmic spontaneous Ca2+ waves in TAB VMs under β-adrenergic stimulation (isoproterenol). Ero1α overexpression in Sham VMs had opposite effects. Ero1α inhibition attenuated Ca2+-dependent ventricular tachyarrhythmias in TAB hearts challenged with isoproterenol. Experiments in TAB VMs and human embryonic kidney 293 cells expressing human RyR2 revealed that an Ero1α-mediated increase in SR Ca2+-channel activity involves dissociation of intraluminal protein ERp44 (endoplasmic reticulum protein 44) from the RyR2 complex. Site-directed mutagenesis and molecular dynamics simulations demonstrated a novel redox-sensitive association of ERp44 with RyR2 mediated by intraluminal cysteine 4806. ERp44-RyR2 association in TAB VMs was restored by Ero1α inhibition, but not by reducing agent dithiothreitol, as hypo-oxidation precludes formation of covalent bond between RyR2 and ERp44. A novel axis of intraluminal interaction between RyR2, ERp44, and Ero1α has been identified. Ero1α inhibition exhibits promising therapeutic potential by stabilizing RyR2-ERp44 complex, thereby reducing spontaneous Ca2+ release and Ca2+-dependent tachyarrhythmias in hypertrophic hearts, without causing hypo-oxidative stress in the SR.

Chrysosplenol-C Increases Contraction by Augmentation of Sarcoplasmic Reticulum Ca2+ Loading and Release via Protein Kinase C in Rat Ventricular Myocytes.

Naturally found chrysosplenol-C (4',5,6-trihydroxy-3,3',7-trimethoxyflavone) increases the contractility of cardiac myocytes independent of β-adrenergic signaling. We investigated the cellular mechanism for chrysosplenol-C-induced positive inotropy. Global and local Ca2+ signals, L-type Ca2+ current (ICa), and contraction were measured from adult rat ventricular myocytes using two-dimensional confocal Ca2+ imaging, the whole-cell patch-clamp technique, and video-edge detection, respectively. Application of chrysosplenol-C reversibly increased Ca2+ transient magnitude with a maximal increase of ∼55% within 2- to 3-minute exposures (EC50 ≅ 21 μM). This chemical did not alter ICa and slightly increased diastolic Ca2+ level. The frequency and size of resting Ca2+ sparks were increased by chrysosplenol-C. Chrysosplenol-C significantly increased sarcoplasmic reticulum (SR) Ca2+ content but not fractional release. Pretreatment of protein kinase C (PKC) inhibitor but not Ca2+/calmodulin-dependent protein kinase II (CaMKII) inhibitor abolished the stimulatory effects of chrysosplenol-C on Ca2+ transients and Ca2+ sparks. Chrysosplenol-C-induced positive inotropy was removed by the inhibition of PKC but not CaMKII or phospholipase C. Western blotting assessment revealed that PKC-δ protein level in the membrane fractions significantly increase within 2 minutes after chrysosplenol-C exposure with a delayed (5-minute) increase in PKC-α levels in insoluble membrane. These results suggest that chrysosplenol-C enhances contractility via PKC (most likely PKC-δ)-dependent enhancement of SR Ca2+ releases in ventricular myocytes. SIGNIFICANCE STATEMENT: Study shows that chrysosplenol-C, a natural flavone showing a positive inotropic effect, increases SR Ca2+ releases on depolarizations and Ca2+ sparks with an increase of SR Ca2+ loading but not L-type Ca2+ current in ventricular myocytes. Chrysosplenol-C-induced enhancement in contraction is eliminated by PKC inhibition, and it is associated with redistributions of PKC to the membrane. These indicate that chrysosplenol-C enhances contraction via PKC-dependent augmentations of SR Ca2+ release and Ca2+ loading during action potentials.

Imperacalcin‐Induced Reduction of Sarcoplasmic Reticulum Ca 2+ in Intact Mouse Heart

Introduction Catecholaminergic polymorphic ventricular tachycardia (CPVT) is an inherited cardiac channelopathy, that often leads to sudden death due to arrhythmogenic episodes triggered by adrenergic stimulation. Previous studies have shown that CPVT is associated with mutations in the Ca2+ release channel of the sarcoplasmic reticulum (SR), the ryanodine receptor type-2 (RyR2). Loss of function mutations in the RyR2 result in abnormally low Ca2+ release which leads to an increase in SR Ca2+ content. It has been proposed that SR Ca2+ overload precipitates CPVT events by over activating the Na-Ca exchanger (NCX) which favors the development of arrhythmogenic delayed afterdepolarizations (DADs). Previous studies from our group and others have shown that the scorpion-derived peptide, imperacalcin (IpCa) is an effective agent to prevent/revert DADs in CPVT mouse models. The overall goal of this study was to test the hypothesis that the IpCa protects against CPVT by partially reducing SR Ca2+ content, therefore preventing the development of Ca2+ overload. To test this hypothesis, IpCa effect on SR Ca2+ content was evaluated by measuring SR Ca2+ restitution, which is the time-dependent recovery of SR Ca2+ release as a function of the stimulation frequency. If indeed, IpCa reduces SR Ca2+ content due to RyR2-mediated Ca2+ leak, SR Ca2+ restitution would be significantly shorter than in its absence. As a positive control, a known RyR2 agonist like caffeine, was used to define the effects of reducing SR Ca2+ content on SR Ca2+ restitution. Methods To test IpCa effects on SR Ca2+ restitution the pulsed local field fluorescence microscopy was used. Rhod-2AM-loaded hearts were perfused with either Tyrode, caffeine (1 mM), or IpCa (1 µM) and were stimulated at 2, 3, 4, 5, 6, 7, and 8 Hz while action potentials and intracellular Ca2+ transients were simultaneously recorded. SR Ca2+restitution was calculated from the ratio of the Ca2+ transient amplitude before and after the premature stimulation at the tested frequencies. Results At a constant stimulation frequency of 2 Hz, IpCa (1 µM; n=8) induced a 36% reduction of the Ca2+ transients amplitude, without affecting their kinetics or basal Ca2+. In contrast, caffeine (1 mM; n=10) affected basal Ca2+ (~3-fold increase), Ca2+ transients amplitude (~50% reduction), and their kinetics (~65% longer rise time & ~70% shorter decay & half-duration). As a function of the stimulation frequency, the SR Ca2+ exponential recovery (control t = 206±5 ms; n=6) was slightly accelerated by IpCa (t = 198±3 ms; n=4), & by caffeine (t = 183±4 ms; n=6). Conclusion Our results indicate that in intact mouse heart, the effects of IpCa on the intracellular Ca2+ dynamics are similar to those induced by caffeine in terms of reduction of SR Ca2+ content, without affecting the Ca2+ transient kinetics. Consistent with previous observations, our SR Ca2+ restitution results support the notion that IpCa partially depletes SR Ca2+ content, providing a plausible mechanism for its antiarrhythmic effects.

Orthograde signal of dihydropyridine receptor increases Ca2+ leakage after repeated contractions in rat fast-twitch muscles in vivo.

The purpose of this study was to investigate the mechanism underlying sarcoplasmic reticulum (SR) Ca2+ leakage after in vivo contractions. Rat gastrocnemius muscles were electrically stimulated in vivo, and then mechanically skinned fibers and SR microsomes were prepared from the muscles excised 30 min after repeated high-intensity contractions. The mechanically skinned fibers maintained the interaction between dihydropyridine receptors (DHPRs) and ryanodine receptors (RyRs), whereas the SR microsomes did not. Interestingly, skinned fibers from the stimulated muscles showed increased SR Ca2+ leakage, whereas Ca2+ leakage decreased in SR microsomes from the stimulated muscles. To enhance the orthograde signal of DHPRs, SR Ca2+ leakage in the skinned fiber was measured 1) under a continuously depolarized condition and 2) in the presence of nifedipine. As a result, in either of the two conditions, SR Ca2+ leakage in the rested fibers reached a level similar to that in the stimulated fibers. Furthermore, the increased SR Ca2+ leakage from the stimulated fibers was alleviated by treatment with 1 mM tetracaine (Tet) but not by treatment with 3 mM free Mg2+ (3 Mg). Tet exerted a greater inhibitory effect on the DHPR signal to RyR than 3 Mg, although their inhibitory effects on RyR were almost similar. These results suggest that the increased Ca2+ leakage after muscle contractions is mainly caused by the orthograde signal of DHPRs to RyRs.

Clusterin alleviates Cr(VI)-induced mitochondrial apoptosis in L02 hepatocytes via inhibition of Ca2+-ROS-Drp1-mitochondrial fission axis

Hexavalent chromium [Cr(VI)] is ubiquitous in the environment and is commonly used in various industrial processes. Clusterin (CLU) is an extracellular chaperone protein which exerts the anti-apoptotic function. In this study, we aimed to explore the effect of CLU on Cr(VI)-induced mitochondrial fission and apoptosis. We revealed that the apoptosis rate of L02 hepatocytes treated with Cr (VI) was increased. CLU over-expression could protect the hepatocytes from Cr(VI)-induced mitochondrial apoptosis. Furthermore, Cr(VI) triggered the intracellular calcium overload, resulting in the activation of xanthine oxidase (XO). Cr(VI) induced reactive oxygen species (ROS) overproduction, led to dynamin-related protein 1 (Drp1) translocation to mitochondria and the subsequent mitochondrial fission, contributing to the caspase-3-dependent mitochondrial apoptosis as evidenced by higher mitochondrial permeability transition pore (mPTP) opening rate, lower mitochondrial membrane potential (MMP), and more alanine transaminase (ALT)/aspartate transaminase (AST) leakage into the culture medium. However, CLU over-expression could trigger the AMP-activated protein kinase (AMPK) pathway, which was followed by the increase of sarcoplasmic reticulum Ca2+-ATPase (SERCA2a) expression. CLU-induced AMPK/SERCA2a activation attenuated calcium overload, caspase-3 activation, and ultimate mitochondrial apoptosis. All in all, the present study demonstrated that Cr(VI) induced hepatocytes apoptosis via Ca2+-ROS-Drp1-mitochondrial fission axis and CLU alleviated the mitochondrial apoptosis through activation of the AMPK/SERCA2a pathway.

CaMKIIδC Drives Early Adaptive Ca2+ Change and Late Eccentric Cardiac Hypertrophy.

CaMKII (Ca2+-Calmodulin dependent protein kinase) δC activation is implicated in pathological progression of heart failure (HF) and CaMKIIδC transgenic mice rapidly develop HF and arrhythmias. However, little is known about early spatio-temporal Ca2+ handling and CaMKII activation in hypertrophy and HF. To measure time- and location-dependent activation of CaMKIIδC signaling in adult ventricular cardiomyocytes, during transaortic constriction (TAC) and in CaMKIIδC transgenic mice. We used human tissue from nonfailing and HF hearts, 4 mouse lines: wild-type, KO (CaMKIIδ-knockout), CaMKIIδC transgenic in wild-type (TG), or KO background, and wild-type mice exposed to TAC. Confocal imaging and biochemistry revealed disproportional CaMKIIδC activation and accumulation in nuclear and perinuclear versus cytosolic regions at 5 days post-TAC. This CaMKIIδ activation caused a compensatory increase in sarcoplasmic reticulum Ca2+ content, Ca2+ transient amplitude, and [Ca2+] decline rates, with reduced phospholamban expression, all of which were most prominent near and in the nucleus. These early adaptive effects in TAC were entirely mimicked in young CaMKIIδ TG mice (6-8 weeks) where no overt cardiac dysfunction was present. The (peri)nuclear CaMKII accumulation also correlated with enhanced HDAC4 (histone deacetylase) nuclear export, creating a microdomain for transcriptional regulation. At longer times both TAC and TG mice progressed to overt HF (at 45 days and 11-13 weeks, respectively), during which time the compensatory Ca2+ transient effects reversed, but further increases in nuclear and time-averaged [Ca2+] and CaMKII activation occurred. CaMKIIδ TG mice lacking δB exhibited more severe HF, eccentric myocyte growth, and nuclear changes. Patient HF samples also showed greatly increased CaMKIIδ expression, especially for CaMKIIδC in nuclear fractions. We conclude that in early TAC perinuclear CaMKIIδC activation promotes adaptive increases in myocyte Ca2+ transients and nuclear transcriptional responses but that chronic progression of this nuclear Ca2+-CaMKIIδC axis contributes to eccentric hypertrophy and HF.

Heart failure-induced atrial remodelling promotes electrical and conduction alternans.

Heart failure (HF) is associated with an increased propensity for atrial fibrillation (AF), causing higher mortality than AF or HF alone. It is hypothesized that HF-induced remodelling of atrial cellular and tissue properties promotes the genesis of atrial action potential (AP) alternans and conduction alternans that perpetuate AF. However, the mechanism underlying the increased susceptibility to atrial alternans in HF remains incompletely elucidated. In this study, we investigated the effects of how HF-induced atrial cellular electrophysiological (with prolonged AP duration) and tissue structural (reduced cell-to-cell coupling caused by atrial fibrosis) remodelling can have an effect on the generation of atrial AP alternans and their conduction at the cellular and one-dimensional (1D) tissue levels. Simulation results showed that HF-induced atrial electrical remodelling prolonged AP duration, which was accompanied by an increased sarcoplasmic reticulum (SR) Ca2+ content and Ca2+ transient amplitude. Further analysis demonstrated that HF-induced atrial electrical remodelling increased susceptibility to atrial alternans mainly due to the increased sarcoplasmic reticulum Ca2+-ATPase (SERCA) Ca2+ reuptake, modulated by increased phospholamban (PLB) phosphorylation, and the decreased transient outward K+ current (Ito). The underlying mechanism has been suggested that the increased SR Ca2+ content and prolonged AP did not fully recover to their previous levels at the end of diastole, resulting in a smaller SR Ca2+ release and AP in the next beat. These produced Ca2+ transient alternans and AP alternans, and further caused AP alternans and Ca2+ transient alternans through Ca2+→AP coupling and AP→Ca2+ coupling, respectively. Simulation of a 1D tissue model showed that the combined action of HF-induced ion channel remodelling and a decrease in cell-to-cell coupling due to fibrosis increased the heart tissue’s susceptibility to the formation of spatially discordant alternans, resulting in an increased functional AP propagation dispersion, which is pro-arrhythmic. These findings provide insights into how HF promotes atrial arrhythmia in association with atrial alternans.

Stretch-induced sarcoplasmic reticulum calcium leak is causatively associated with atrial fibrillation in pressure-overloaded hearts.

Despite numerous reports documenting an important role of hypertension in the development of atrial fibrillation (AF), the detailed mechanism underlying the pathological process remains incompletely understood. Here, we aim to test the hypothesis that diastolic sarcoplasmic reticulum (SR) Ca2+ leak in atrial myocytes, induced by mechanical stretch due to elevated pressure in the left atrium (LA), plays an essential role in the AF development in pressure-overloaded hearts. Isolated mouse atrial myocytes subjected to acute axial stretch displayed an immediate elevation of SR Ca2+ leak. Using a mouse model of transverse aortic constriction (TAC), the relation between stretch, SR Ca2+ leak, and AF susceptibility was further tested. At 36 h post-TAC, SR Ca2+ leak in cardiomyocytes from the LA (with haemodynamic stress), but not right atrium (without haemodynamic stress), significantly increased, which was further elevated at 4 weeks post-TAC. Accordingly, AF susceptibility to atrial burst pacing in the 4-week TAC mice were also significantly increased, which was unaffected by inhibition of atrial fibrosis or inflammation via deletion of galectin-3. Western blotting revealed that type 2 ryanodine receptor (RyR2) in left atrial myocytes of TAC mice was oxidized due to activation and up-regulation of Nox2 and Nox4. Direct rescue of dysfunctional RyR2 with dantrolene or rycal S107 reduced diastolic SR Ca2+ leak in left atrial myocytes and prevented atrial burst pacing stimulated AF. Our study demonstrated for the first time the increased SR Ca2+ leak mediated by enhanced oxidative stress in left atrial myocytes that is causatively associated with higher AF susceptibility in pressure-overloaded hearts.

The effect of Sirt1 deficiency on Ca2+ and Na+ regulation in mouse ventricular myocytes.

This study addressed the hypothesis that cardiac Sirtuin 1 (Sirt1) deficiency alters cardiomyocyte Ca2+ and Na+ regulation, leading to cardiac dysfunction and arrhythmogenesis. We used mice with cardiac‐specific Sirt1 knockout (Sirt1−/−). Sirt1flox/flox mice were served as control. Sirt1−/− mice showed impaired cardiac ejection fraction with increased ventricular spontaneous activity and burst firing compared with those in control mice. The arrhythmic events were suppressed by KN93 and ranolazine. Reduction in Ca2+ transient amplitudes and sarcoplasmic reticulum (SR) Ca2+ stores, and increased SR Ca2+ leak were shown in the Sirt1−/− mice. Electrophysiological measurements were performed using patch‐clamp method. While L‐type Ca2+ current (I Ca, L) was smaller in Sirt1−/− myocytes, reverse‐mode Na+/Ca2+ exchanger (NCX) current was larger compared with those in control myocytes. Late Na+ current (I Na, L) was enhanced in the Sirt1−/− mice, alongside with elevated cytosolic Na+ level. Increased cytosolic and mitochondrial reactive oxygen species (ROS) were shown in Sirt1−/− mice. Sirt1−/− cardiomyocytes showed down‐regulation of L‐type Ca2+ channel α1c subunit (Cav1.2) and sarcoplasmic/endoplasmic reticulum Ca2+ ATPase 2a (SERCA2a), but up‐regulation of Ca2+/calmodulin‐dependent protein kinase II and NCX. In conclusions, these findings suggest that deficiency of Sirt1 impairs the regulation of intracellular Ca2+ and Na+ in cardiomyocytes, thereby provoking cardiac dysfunction and arrhythmogenesis.

Redox-Active Drug, MnTE-2-PyP5+, Prevents and Treats Cardiac Arrhythmias Preserving Heart Contractile Function.

Background Cardiomyopathies remain among the leading causes of death worldwide, despite all efforts and important advances in the development of cardiovascular therapeutics, demonstrating the need for new solutions. Herein, we describe the effects of the redox-active therapeutic Mn(III) meso-tetrakis(N-ethylpyridinium-2-yl)porphyrin, AEOL10113, BMX-010 (MnTE-2-PyP5+), on rat heart as an entry to new strategies to circumvent cardiomyopathies. Methods Wistar rats weighing 250-300 g were used in both in vitro and in vivo experiments, to analyze intracellular Ca2+ dynamics, L-type Ca2+ currents, Ca2+ spark frequency, intracellular reactive oxygen species (ROS) levels, and cardiomyocyte and cardiac contractility, in control and MnTE-2-PyP5+-treated cells, hearts, or animals. Cells and hearts were treated with 20 μM MnTE-2-PyP5+ and animals with 1 mg/kg, i.p. daily. Additionally, we performed electrocardiographic and echocardiographic analysis. Results Using isolated rat cardiomyocytes, we observed that MnTE-2-PyP5+ reduced intracellular Ca2+ transient amplitude, without altering cell contractility. Whereas MnTE-2-PyP5+ did not alter basal ROS levels, it was efficient in modulating cardiomyocyte redox state under stress conditions; MnTE-2-PyP5+ reduced Ca2+ spark frequency and increased sarcoplasmic reticulum (SR) Ca2+ load. Accordingly, analysis of isolated perfused rat hearts showed that MnTE-2-PyP5+ preserves cardiac function, increases SR Ca2+ load, and reduces arrhythmia index, indicating an antiarrhythmic effect. In vivo experiments showed that MnTE-2-PyP5+ treatment increased Ca2+ transient, preserved cardiac ejection fraction, and reduced arrhythmia index and duration. MnTE-2-PyP5+ was effective both to prevent and to treat cardiac arrhythmias. Conclusion MnTE-2-PyP5+ prevents and treats cardiac arrhythmias in rats. In contrast to most antiarrhythmic drugs, MnTE-2-PyP5+ preserves cardiac contractile function, arising, thus, as a prospective therapeutic for improvement of cardiac arrhythmia treatment.

Location and function of transient receptor potential canonical channel 1 in ventricular myocytes

Transient receptor potential canonical 1 (TRPC1) protein is abundantly expressed in cardiomyocytes. While TRPC1 is supposed to be critically involved in cardiac hypertrophy, its physiological role in cardiomyocytes is poorly understood. We investigated the subcellular location of TRPC1 and its contribution to Ca2+ signaling in mammalian ventricular myocytes. Immunolabeling, three-dimensional scanning confocal microscopy and quantitative colocalization analysis revealed an abundant intracellular location of TRPC1 in neonatal rat ventricular myocytes (NRVMs) and adult rabbit ventricular myocytes. TRPC1 was colocalized with intracellular proteins including sarco/endoplasmic reticulum Ca2+ ATPase 2 in the sarcoplasmic reticulum (SR). Colocalization with wheat germ agglutinin, which labels the glycocalyx and thus marks the sarcolemma including the transverse tubular system, was low. Super-resolution and immunoelectron microscopy supported the intracellular location of TRPC1. We investigated Ca2+ signaling in NRVMs after adenoviral TRPC1 overexpression or silencing. In NRVMs bathed in Na+ and Ca2+ free solution, TRPC1 overexpression and silencing was associated with a decreased and increased SR Ca2+ content, respectively. In isolated rabbit cardiomyocytes bathed in Na+ and Ca2+ free solution, we found an increased decay of the cytosolic Ca2+ concentration [Ca2+]i and increased SR Ca2+ content in the presence of the TRPC channel blocker SKF-96365. In a computational model of rabbit ventricular myocytes at physiological pacing rates, Ca2+ leak through SR TRPC channels increased the systolic and diastolic [Ca2+]i with only minor effects on the action potential and SR Ca2+ content. Our studies suggest that TRPC1 channels are localized in the SR, and not present in the sarcolemma of ventricular myocytes. The studies provide evidence for a role of TRPC1 as a contributor to SR Ca2+ leak in cardiomyocytes, which was previously explained by ryanodine receptors only. We propose that the findings will guide us to an understanding of TRPC1 channels as modulators of [Ca2+]i and contractility in cardiomyocytes.

Priority Strategy of Intracellular Ca2+ Homeostasis in Skeletal Muscle Fibers During the Multiple Stresses of Hibernation.

Intracellular calcium (Ca2+) homeostasis plays a vital role in the preservation of skeletal muscle. In view of the well-maintained skeletal muscle found in Daurian ground squirrels (Spermophilus dauricus) during hibernation, we hypothesized that hibernators possess unique strategies of intracellular Ca2+ homeostasis. Here, cytoplasmic, sarcoplasmic reticulum (SR), and mitochondrial Ca2+ levels, as well as the potential Ca2+ regulatory mechanisms, were investigated in skeletal muscle fibers of Daurian ground squirrels at different stages of hibernation. The results showed that cytoplasmic Ca2+ levels increased in the skeletal muscle fibers during late torpor (LT) and inter-bout arousal (IBA), and partially recovered when the animals re-entered torpor (early torpor, ET). Furthermore, compared with levels in the summer active or pre-hibernation state, the activity and protein expression levels of six major Ca2+ channels/proteins were up-regulated during hibernation, including the store-operated Ca2+ entry (SOCE), ryanodine receptor 1 (RyR1), leucine zipper-EF-hand containing transmembrane protein 1 (LETM1), SR Ca2+ ATPase 1 (SERCA1), mitochondrial calcium uniporter complex (MCU complex), and calmodulin (CALM). Among these, the increased extracellular Ca2+ influx mediated by SOCE, SR Ca2+ release mediated by RyR1, and mitochondrial Ca2+ extrusion mediated by LETM1 may be triggers for the periodic elevation in cytoplasmic Ca2+ levels observed during hibernation. Furthermore, the increased SR Ca2+ uptake through SERCA1, mitochondrial Ca2+ uptake induced by MCU, and elevated free Ca2+ binding capacity mediated by CALM may be vital strategies in hibernating ground squirrels to attenuate cytoplasmic Ca2+ levels and restore Ca2+ homeostasis during hibernation. Compared with that in LT or IBA, the decreased extracellular Ca2+ influx mediated by SOCE and elevated mitochondrial Ca2+ uptake induced by MCU may be important mechanisms for the partial cytoplasmic Ca2+ recovery in ET. Overall, under extreme conditions, hibernating ground squirrels still possess the ability to maintain intracellular Ca2+ homeostasis.

P3508A novel mouse model of sleep-disordered breathing is associated with contractile dysfunction and CaMKII overexpression

Abstract Background Patients with sleep-disordered breathing (SDB) develop arrhythmias and contractile dysfunction, but the mechanisms are poorly understood, possibly due to the lack of mouse models that mimic airway obstruction in spontaneously sleeping mice. Interestingly, New Zealand obese mice have been shown to develop airway obstruction with inspiratory flow limitation resulting in apneas, but these mice also develop diabetes. Purpose We developed a novel mouse model of increased airway obstruction in spontaneously sleeping lean mice and investigated the impact on sleep-related apneas and contractile function. Methods and results Male C57BL6 mice at 8–12 weeks of age were subjected to polytetrafluoroethylene (PTFE) injection (100 μl) into the tongue. This resulted in an increased tongue volume and reduced pharyngeal luminal diameter. Conscious mice behave normal and there was no difference in body weight between PTFE injected mice and untreated littermates (control). Whole body plethysmography was used to monitor spontaneous breathing for 8h in a quiet environment. Interestingly, compared to control, mice with PTFE injection showed a significantly increased frequency of apneas (lasting >1s, fig. A, * indicated P<0.05, t-test). Echocardiographic analysis revealed that ejection fraction was significantly reduced in PTFE-treated mice 8 weeks after surgery (vs. control, fig. B). In accordance, the developed force of isolated papillary muscles from hearts of PTFE mice was significantly reduced compared to control (fig. C). Ca/calmodulin-dependent protein kinase II (CaMKII) has been implicated in the development of heart failure. Intriguingly, compared to control, CaMKII expression was significantly increased in ventricular heart homogenates of PTFE-treated mice (fig. D). Moreover, the magnitude of CaMKII overexpression correlated significantly with the frequency of apneas (fig. E). Papillary muscle post-pause contractions can be used as measure of diastolic sarcoplasmic reticulum (SR) Ca leak, which is known to be enhanced by CaMKII. Compared to control, post-pause contraction amplitude was significantly smaller in PTFE-treated mice, indicating an increased SR Ca leak (fig. F). Figure 1 Conclusion PTFE injection in mice results in an increased frequency of spontaneous apneas. PTFE-treated mice develop mild contractile dysfunction, which is accompanied by CaMKII overexpression. This novel mouse model offers great opportunities for investigation of sleep-related breathing disorders. Acknowledgement/Funding This study was supported by the Medical Faculty of the University of Regensburg (ReForM programme).

Calcium Fluxes in Work-Related Muscle Disorder: Implications from a Rat Model.

Introduction Ca2+ regulatory excitation-contraction coupling properties are key topics of interest in the development of work-related muscle myalgia and may constitute an underlying cause of muscle pain and loss of force generating capacity. Method A well-established rat model of high repetition high force (HRHF) work was used to investigate if such exposure leads to an increase in cytosolic Ca2+ concentration ([Ca2+]i) and changes in sarcoplasmic reticulum (SR) vesicle Ca2+ uptake and release rates. Result Six weeks exposure of rats to HRHF increased indicators of fatigue, pain behaviors, and [Ca2+]i, the latter implied by around 50–100% increases in pCam, as well as in the Ca2+ handling proteins RyR1 and Casq1 accompanied by an ∼10% increased SR Ca2+ uptake rate in extensor and flexor muscles compared to those of control rats. This demonstrated a work-related altered myocellular Ca2+ regulation, SR Ca2+ handling, and SR protein expression. Discussion These disturbances may mirror intracellular changes in early stages of human work-related myalgic muscle. Increased uptake of Ca2+ into the SR may reflect an early adaptation to avoid a sustained detrimental increase in [Ca2+]i similar to the previous findings of deteriorated Ca2+ regulation and impaired function in fatigued human muscle.

Similar dysfunctional Ca2+ regulation in coronary smooth muscle from explanted human hearts and Ossabaw miniature swine strongly supports the translational relevance of this large animal model

IntroductionEstablishing a suitable animal model for metabolic syndrome (MetS) and coronary artery disease (CAD) is a challenge, as there are numerous risk factors and uncontrollable variables in the human population. However, Ossabaw miniature swine have been used with great success to recapitulate MetS and CAD in. The goal of this study was to determine whether the Ossabaw miniature swine is a clinically relevant large animal model on the cellular level, specifically with regards to intracellular Ca2+ ([Ca2+]i) handling, which is crucial for coronary smooth muscle (CSM) function.MethodsExplanted human hearts from 32 heart failure patients (22 male, 10 female; aged 53.4 ± 2.1 years) undergoing transplantation surgery at the Methodist Hospital in Indianapolis, IN were collected. Coronary arteries were excised and CSM cells were enzymatically dispersed and loaded with the fluorescent [Ca2+]i indicator fura‐2. [Ca2+]i levels were measured during depolarization‐induced Ca2+ influx and caffeine‐induced sarcoplasmic reticulum (SR) Ca2+ store release.ResultsIn explanted human hearts exhibiting either ischemic or dilated non‐ischemic cardiomyopathy, a greater coronary artery neointima formation was inversely correlated to depolarization‐induced Ca2+ influx and SR Ca2+ release in CSM cells. Furthermore, a greater number of MetS risk factors, including some factors individually (body mass index, LDL/HDL cholesterol ratio) was also correlated to decreased Ca2+ entry and, in patients with ischemic cardiomyopathy, decreased SR Ca2+ store release. Increased age, however, was correlated to an increased SR Ca2+ store release. These data are all consistent with previous studies of Ossabaw swine CSM.ConclusionThe similar CSM Ca2+ regulation in coronary arteries of humans and Ossabaw swine provides strong evidence for the phenomenal translational relevance of this large animal model, both on the whole‐animal level and the CSM [Ca2+]i signaling level.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.