Maternal exercise preserves offspring cardiovascular health via oxidative regulation of the ryanodine receptor

OBJECTIVE

The intrauterine environment during pregnancy is a critical factor in the development of obesity, diabetes, and cardiovascular disease in offspring. Maternal exercise prevents the detrimental effects of a maternal high fat diet on the metabolic health in adult offspring, but the effects of maternal exercise on offspring cardiovascular health have not been thoroughly investigated.

METHODS

To determine the effects of maternal exercise on offspring cardiovascular health, female mice were fed a chow (C; 21% kcal from fat) or high-fat (H; 60% kcal from fat) diet and further subdivided into sedentary (CS, HS) or wheel exercised (CW, HW) prior to pregnancy and throughout gestation. Offspring were maintained in a sedentary state and chow-fed throughout 52 weeks of age and subjected to serial echocardiography and cardiomyocyte isolation for functional and mechanistic studies.

RESULTS

High-fat fed sedentary dams (HS) produced female offspring with reduced ejection fraction (EF) compared to offspring from chow-fed dams (CS), but EF was preserved in offspring from high-fat fed exercised dams (HW) throughout 52 weeks of age. Cardiomyocytes from HW female offspring had increased kinetics, calcium cycling, and respiration compared to CS and HS offspring. HS offspring had increased oxidation of the RyR2 in cardiomyocytes coupled with increased baseline sarcomere length, resulting in RyR2 overactivity, which was negated in female HW offspring.

CONCLUSIONS

These data suggest a role for maternal exercise to protect against the detrimental effects of a maternal high-fat diet on female offspring cardiac health. Maternal exercise improved female offspring cardiomyocyte contraction, calcium cycling, respiration, RyR2 oxidation, and RyR2 activity. These data present an important, translatable role for maternal exercise to preserve cardiac health of female offspring and provide insight on mechanisms to prevent the transmission of cardiovascular diseases to subsequent generations.

Sexual dimorphism in bidirectional SR-mitochondria crosstalk in ventricular cardiomyocytes

Calcium transfer into the mitochondrial matrix during sarcoplasmic reticulum (SR) Ca2+ release is essential to boost energy production in ventricular cardiomyocytes (VCMs) and match increased metabolic demand. Mitochondria from female hearts exhibit lower mito-[Ca2+] and produce less reactive oxygen species (ROS) compared to males, without change in respiration capacity. We hypothesized that in female VCMs, more efficient electron transport chain (ETC) organization into supercomplexes offsets the deficit in mito-Ca2+ accumulation, thereby reducing ROS production and stress-induced intracellular Ca2+ mishandling. Experiments using mitochondria-targeted biosensors confirmed lower mito-ROS and mito-[Ca2+] in female rat VCMs challenged with β-adrenergic agonist isoproterenol compared to males. Biochemical studies revealed decreased mitochondria Ca2+ uniporter expression and increased supercomplex assembly in rat and human female ventricular tissues vs male. Importantly, western blot analysis showed higher expression levels of COX7RP, an estrogen-dependent supercomplex assembly factor in female heart tissues vs males. Furthermore, COX7RP was decreased in hearts from aged and ovariectomized female rats. COX7RP overexpression in male VCMs increased mitochondrial supercomplexes, reduced mito-ROS and spontaneous SR Ca2+ release in response to ISO. Conversely, shRNA-mediated knockdown of COX7RP in female VCMs reduced supercomplexes and increased mito-ROS, promoting intracellular Ca2+ mishandling. Compared to males, mitochondria in female VCMs exhibit higher ETC subunit incorporation into supercomplexes, supporting more efficient electron transport. Such organization coupled to lower levels of mito-[Ca2+] limits mito-ROS under stress conditions and lowers propensity to pro-arrhythmic spontaneous SR Ca2+ release. We conclude that sexual dimorphism in mito-Ca2+ handling and ETC organization may contribute to cardioprotection in healthy premenopausal females.

STIM1 ablation impairs exercise-induced physiological cardiac hypertrophy and dysregulates autophagy in mouse hearts

Cardiac stromal interaction molecule 1 (STIM1), a key mediator of store-operated Ca2+ entry (SOCE), is a known determinant of cardiomyocyte pathological growth in hypertrophic cardiomyopathy. We examined the role of STIM1 and SOCE in response to exercise-dependent physiological hypertrophy. Wild-type (WT) mice subjected to exercise training (WT-Ex) showed a significant increase in exercise capacity and heart weight compared with sedentary (WT-Sed) mice. Moreover, myocytes from WT-Ex hearts displayed an increase in length, but not width, compared with WT-Sed myocytes. Conversely, exercised cardiac-specific STIM1 knock-out mice (cSTIM1KO-Ex), although displaying significant increase in heart weight and cardiac dilation, evidenced no changes in myocyte size and displayed a decreased exercise capacity, impaired cardiac function, and premature death compared with sedentary cardiac-specific STIM1 knock-out mice (cSTIM1KO-Sed). Confocal Ca2+ imaging demonstrated enhanced SOCE in WT-Ex myocytes compared with WT-Sed myocytes with no measurable SOCE detected in cSTIM1KO myocytes. Exercise training induced a significant increase in cardiac phospho-Akt Ser473 in WT mice but not in cSTIM1KO mice. No differences were observed in phosphorylation of mammalian target of rapamycin (mTOR) and glycogen synthase kinase (GSK) in exercised versus sedentary cSTIM1KO mice hearts. cSTIM1KO-Sed mice showed increased basal MAPK phosphorylation compared with WT-Sed that was not altered by exercise training. Finally, histological analysis revealed exercise resulted in increased autophagy in cSTIM1KO but not in WT myocytes. Taken together, our results suggest that adaptive cardiac hypertrophy in response to exercise training involves STIM1-mediated SOCE. Our results demonstrate that STIM1 is involved in and essential for the myocyte longitudinal growth and mTOR activation in response to endurance exercise training.NEW & NOTEWORTHY Store-operated Ca2+ entry (SOCE) has been implicated in pathological cardiac hypertrophy; however, its role in physiological hypertrophy is unknown. Here we report that SOCE is also essential for physiological cardiac hypertrophy and functional adaptations in response to endurance exercise. These adaptations were associated with activation of AKT/mTOR pathway and curtailed cardiac autophagy and degeneration. Thus, SOCE is a common mechanism and an important bifurcation point for signaling paths involved in physiological and pathological hypertrophy.

NaV1.6 dysregulation within myocardial T-tubules by D96V calmodulin enhances proarrhythmic sodium and calcium mishandling

Calmodulin (CaM) plays critical roles in cardiomyocytes, regulating Na+ (NaV) and L-type Ca2+ channels (LTCC). LTCC dysregulation by mutant CaMs has been implicated in action potential duration (APD) prolongation and arrhythmogenic long QT (LQT) syndrome. Intriguingly, D96V-CaM prolongs APD more than other LQT-associated CaMs despite inducing comparable levels of LTCC dysfunction, suggesting dysregulation of other depolarizing channels. Here, we provide evidence implicating NaV dysregulation within transverse (T)-tubules in D96V-CaM-associated arrhythmias. D96V-CaM induces pro-arrhythmic late Na+ current (INa) by impairing inactivation of NaV1.6, but not the predominant cardiac NaV isoform, NaV1.5. We investigated arrhythmia mechanisms using mice with cardiac-specific expression of D96V-CaM (cD96V). Super-resolution microscopy revealed close proximity of NaV1.6 and RyR2 within T-tubules. NaV1.6 density within these regions increased in cD96V relative to WT. Consistent with NaV1.6 dysregulation by D96V-CaM in these regions, we observed increased late NaV activity in T-tubules. The resulting late INa promoted aberrant Ca2+ release and prolonged APD in myocytes, leading to LQT and ventricular tachycardia (VT) in vivo. Cardiac-specific NaV1.6 knockout protected cD96V mice from increased T-tubular late NaV activity, and its arrhythmogenic consequences. In summary, we demonstrate that D96V-CaM promotes arrhythmias by dysregulating LTCC and NaV1.6 within T-tubules and thereby, facilitating aberrant Ca2+ release.

Mitochondrial calpain inhibition restores defective SR-mitochondrial crosstalk in CPVT rat myocytes

Cardiac RYR2-mediated sarcoplasmic Ca2+ (SR) release is essential for matching increased energy demand during fight-or-flight response with mitochondrial metabolic output by delivering Ca2+ into the mitochondrial matrix to activate Ca2+-dependent Krebs cycle dehydrogenases. RYR2 complex gain-of-function mutations associated with catecholaminergic polymorphic ventricular tachycardia (CPVT) have been linked to mitochondrial structural damage and enhanced production of reactive oxygen species (ROS). Despite being critical for arrhythmogenesis in CPVT, the exact causes of these phenomena remain undetermined. Taking advantage of a new rat model of CPVT induced by heterozygous RYR2 gain-of-function mutation S2222L, we tested how RYR2 overactivity alters mitochondrial Ca2+ and ROS handling, and how these changes cause mitochondrial structural defects. Injection of epinephrine (1 mg/kg) and caffeine (120 mg/kg) induced bigamy and bidirectional VT in vivo in 100% of CPVT rats. Simultaneous whole-cell patch clamp and confocal Ca2+-imaging demonstrated that under β-adrenergic stimulation with isoproterenol (50 nM), CPVT ventricular myocytes (VMs) exhibited severe Ca2+ mishandling and high propensity for generation of spontaneous Ca2+ waves (SCWs) that cause arrhythmogenic afterdepolarizations. Diminished Ca2+ transient amplitude in CPVT VMs resulted in a significant reduction in mitochondrial matrix-[Ca2+], and thereby a mito-ROS surge, visualized using matrix-targeted biosensors mtRCaMP1h and MLS-HyPer, respectively. Importantly, using novel Ca2+-biosensors targeted to intermembrane space (IMS-GECO), we uncovered that [Ca2+] in this compartment reaches 1 µM, sufficient for activation of Ca2+-dependent protease μ-calpain. Adenoviral overexpression of IMS-targeted calpastatin, an endogenous calpain inhibitor, reduced mito-ROS, restored cytosolic Ca2+ transient amplitude and SR Ca2+ content, and reduced RYR2-mediated SCWs in CPVT VMs. These changes were paralleled by restored expression levels of OPA1, a mitochondrial structural protein responsible for tight cristae organization. Our data suggest that enhanced mito-ROS due to matrix-[Ca2+] reduction in CPVT VMs and unexpectedly high IMS-[Ca2+] promotes IMS-calpain-mediated degradation of OPA1, resulting in mitochondrial structural damage that contributes to proarrhythmic remodeling.

Abstract P1083: Calcium-dependent Proteolysis Of Mitochondrial Opa1 By Calpain Contributes To Arrhythmogenesis In Catecholaminergic Polymorphic Ventricular Tachycardia

Intracellular Ca 2+ cycling in ventricular myocytes (VMs) is tightly linked to mitochondrial metabolic output, and disturbances in this coupling contribute to cardiac arrhythmias. We recently demonstrated in the heritable arrhythmia syndrome catecholaminergic polymorphic ventricular tachycardia (CPVT), the arrhythmogenic phenotype is revealed by oxidation of the cardiac ryanodine receptor (RyR2) calcium release channel by mitochondrial reactive oxygen species (mito-ROS), secondary to causative genetic mutation. However, exact mechanisms linking RyR2 dysfunction to mitochondrial injury remain unclear. We hypothesized that RyR2 hyperactivity activates the intermembrane space (IMS)-residing mitochondrial Ca 2+ -dependent protease , driving proteolysis of OPA1, a protein responsible for cristae flattening. This results in altered mitochondrial cristae diameter and mito-ROS production, exacerbating the arrhythmogenic CPVT phenotype. To test this, we created a unique CPVT RyR2-S2222L (+/-) rat model. Electron microscopy studies revealed a significant increase in cristae diameters in Langendorff-perfused CPVT rat hearts. Western blot analysis showed significantly reduced OPA1 expression in isolated CPVT VMs, suggesting that Ca 2+ -dependent proteolysis of OPA1 could underlie these ultrastructural changes. Pilot experiments suggest specific inhibition of via adeno-associated viral injection of IMS-targeted calpastatin (CAST, 10 12 VP, 4 weeks) can reduce ventricular tachycardia incidence of CPVT rats in vivo. Adenoviral expression of IMS-CAST in isolated VMs restored OPA1 expression levels and significantly improved intracellular Ca 2+ cycling in CPVT, suggesting a marked improvement in RyR2 function. Furthermore, genetic probes mtRCamp1h and HyPer-7 revealed that IMS- inhibition significantly improves mitochondrial matrix Ca 2+ homeostasis and mito-ROS in CPVT VMs, respectively. Importantly, these beneficial effects were lost in VMs with shRNA-mediated knockdown of OPA1. Our data suggest that enhanced activity of RyR2 promotes IMS- -mediated degradation of OPA1, resulting in mitochondrial structural damage that contributes to proarrhythmic remodeling in CPVT.

Ero1α-Dependent ERp44 Dissociation From RyR2 Contributes to Cardiac Arrhythmia

BACKGROUND

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.

METHODS

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).

RESULTS

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.

CONCLUSIONS

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.

MCU overexpression evokes disparate dose-dependent effects on mito-ROS and spontaneous Ca2+ release in hypertrophic rat cardiomyocytes

Cardiac dysfunction in heart failure (HF) and diabetic cardiomyopathy (DCM) is associated with aberrant intracellular Ca2+ handling and impaired mitochondrial function accompanied with reduced mitochondrial calcium concentration (mito-[Ca2+]). Pharmacological or genetic facilitation of mito-Ca2+ uptake was shown to restore Ca2+ transient amplitude in DCM and HF, improving contractility. However, recent reports suggest that pharmacological enhancement of mito-Ca2+ uptake can exacerbate ryanodine receptor-mediated spontaneous sarcoplasmic reticulum (SR) Ca2+ release in ventricular myocytes (VMs) from diseased animals, increasing propensity to stress-induced ventricular tachyarrhythmia. To test whether chronic recovery of mito-[Ca2+] restores systolic Ca2+ release without adverse effects in diastole, we overexpressed mitochondrial Ca2+ uniporter (MCU) in VMs from male rat hearts with hypertrophy induced by thoracic aortic banding (TAB). Measurement of mito-[Ca2+] using genetic probe mtRCamp1h revealed that mito-[Ca2+] in TAB VMs paced at 2 Hz under β-adrenergic stimulation is lower compared with shams. Adenoviral 2.5-fold MCU overexpression in TAB VMs fully restored mito-[Ca2+]. However, it failed to improve cytosolic Ca2+ handling and reduce proarrhythmic spontaneous Ca2+ waves. Furthermore, mitochondrial-targeted genetic probes MLS-HyPer7 and OMM-HyPer revealed a significant increase in emission of reactive oxygen species (ROS) in TAB VMs with 2.5-fold MCU overexpression. Conversely, 1.5-fold MCU overexpression in TABs, that led to partial restoration of mito-[Ca2+], reduced mitochondria-derived reactive oxygen species (mito-ROS) and spontaneous Ca2+ waves. Our findings emphasize the key role of elevated mito-ROS in disease-related proarrhythmic Ca2+ mishandling. These data establish nonlinear mito-[Ca2+]/mito-ROS relationship, whereby partial restoration of mito-[Ca2+] in diseased VMs is protective, whereas further enhancement of MCU-mediated Ca2+ uptake exacerbates damaging mito-ROS emission.NEW & NOTEWORTHY Defective intracellular Ca2+ homeostasis and aberrant mitochondrial function are common features in cardiac disease. Here, we directly compared potential benefits of mito-ROS scavenging and restoration of mito-Ca2+ uptake by overexpressing MCU in ventricular myocytes from hypertrophic rat hearts. Experiments using novel mito-ROS and Ca2+ biosensors demonstrated that mito-ROS scavenging rescued both cytosolic and mito-Ca2+ homeostasis, whereas moderate and high MCU overexpression demonstrated disparate effects on mito-ROS emission, with only a moderate increase in MCU being beneficial.

Sarcoplasmic reticulum-mitochondria communication; implications for cardiac arrhythmia

Sudden cardiac death due to ventricular tachyarrhythmias remains the major cause of mortality in the world. Heart failure, diabetic cardiomyopathy, old age-related cardiac dysfunction and inherited disorders are associated with enhanced propensity to malignant cardiac arrhythmias. Both defective mitochondrial function and abnormal intracellular Ca²⁺ homeostasis have been established as the key contributing factors in the pathophysiology and arrhythmogenesis in these conditions. This article reviews current advances in understanding of bidirectional control of ryanodine receptor-mediated sarcoplasmic reticulum Ca²⁺ release and mitochondrial function, and how defects in crosstalk between these two organelles increase arrhythmic risk in cardiac disease.

Pyridostigmine improves cardiac function and rhythmicity through RyR2 stabilization and inhibition of STIM1-mediated calcium entry in heart failure

Heart failure (HF) is characterized by asymmetrical autonomic balance. Treatments to restore parasympathetic activity in human heart failure trials have shown beneficial effects. However, mechanisms of parasympathetic-mediated improvement in cardiac function remain unclear. The present study examined the effects and underpinning mechanisms of chronic treatment with the cholinesterase inhibitor, pyridostigmine (PYR), in pressure overload HF induced by transverse aortic constriction (TAC) in mice. TAC mice exhibited characteristic adverse structural (left ventricular hypertrophy) and functional remodelling (reduced ejection fraction, altered myocyte calcium (Ca) handling, increased arrhythmogenesis) with enhanced predisposition to arrhythmogenic aberrant sarcoplasmic reticulum (SR) Ca release, cardiac ryanodine receptor (RyR2) hyper-phosphorylation and up-regulated store-operated Ca entry (SOCE). PYR treatment resulted in improved cardiac contractile performance and rhythmic activity relative to untreated TAC mice. Chronic PYR treatment inhibited altered intracellular Ca handling by alleviating aberrant Ca release and diminishing pathologically enhanced SOCE in TAC myocytes. At the molecular level, these PYR-induced changes in Ca handling were associated with reductions of pathologically enhanced phosphorylation of RyR2 serine-2814 and STIM1 expression in HF myocytes. These results suggest that chronic cholinergic augmentation alleviates HF via normalization of both canonical RyR2-mediated SR Ca release and non-canonical hypertrophic Ca signaling via STIM1-dependent SOCE.

Interleukin-1β, Oxidative Stress, and Abnormal Calcium Handling Mediate Diabetic Arrhythmic Risk

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Diabetes-induced arrhythmic risk involved activation of innate immunity, elevation of IL-1β, mitochondrial oxidative stress, SR calcium release channel oxidation, and QT prolongation.

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Diabetes-induced arrhythmic risk could be inhibited by IL-1β antagonism, mitoROS scavenging, and SR calcium release stabilization.

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The relationship of inflammation and arrhythmic risk may account for increased susceptibility of diabetic patients to the effects of COVID-19.

Diabetes mellitus (DM) is associated with increased arrhythmia. Type 2 DM (T2DM) mice showed prolonged QT interval and increased ventricular arrhythmic inducibility, accompanied by elevated cardiac interleukin (IL)-1β, increased mitochondrial reactive oxygen species (mitoROS), and oxidation of the sarcoplasmic reticulum (SR) Ca²⁺ release channel (ryanodine receptor 2 [RyR2]). Inhibiting IL-1β and mitoROS reduced RyR2 oxidation and the ventricular arrhythmia in DM. Inhibiting SR Ca2⁺ leak by stabilizing the oxidized RyR2 channel reversed the diabetic arrhythmic risk. In conclusion, cardiac IL-1β mediated the DM-associated arrhythmia through mitoROS generation that enhances SR Ca²⁺ leak. The mechanistic link between inflammation and arrhythmias provides new therapeutic options.

Increased RyR2 activity is exacerbated by calcium leak-induced mitochondrial ROS

Cardiac disease is associated with deleterious emission of mitochondrial reactive oxygen species (mito-ROS), as well as enhanced oxidation and activity of the sarcoplasmic reticulum (SR) Ca²⁺ release channel, the ryanodine receptor (RyR2). The transfer of Ca²⁺ from the SR via RyR2 to mitochondria is thought to play a key role in matching increased metabolic demand during stress. In this study, we investigated whether augmented RyR2 activity results in self-imposed exacerbation of SR Ca²⁺ leak, via altered SR-mitochondrial Ca²⁺ transfer and elevated mito-ROS emission. Fluorescent indicators and spatially restricted genetic ROS probes revealed that both pharmacologically and genetically enhanced RyR2 activity, in ventricular myocytes from rats and catecholaminergic polymorphic ventricular tachycardia (CPVT) mice, respectively, resulted in increased ROS emission under β-adrenergic stimulation. Expression of mitochondrial Ca²⁺ probe mtRCamp1h revealed diminished net mitochondrial [Ca²⁺] with enhanced SR Ca²⁺ leak, accompanied by depolarization of the mitochondrial matrix. While this may serve as a protective mechanism to prevent mitochondrial Ca²⁺ overload, protection is not complete and enhanced mito-ROS emission resulted in oxidation of RyR2, further amplifying proarrhythmic SR Ca²⁺ release. Importantly, the effects of augmented RyR2 activity could be attenuated by mitochondrial ROS scavenging, and experiments with dominant-negative paralogs of the mitochondrial Ca²⁺ uniporter (MCU) supported the hypothesis that SR-mitochondria Ca²⁺ transfer is essential for the increase in mito-ROS. We conclude that in a process whereby leak begets leak, augmented RyR2 activity modulates mitochondrial Ca²⁺ handling, promoting mito-ROS emission and driving further channel activity in a proarrhythmic feedback cycle in the diseased heart.

Transient Outward K+ Current (Ito) Underlies the Right Ventricular Initiation of Polymorphic Ventricular Tachycardia in a Transgenic Rabbit Model of Long-QT Syndrome Type 1

BACKGROUND

Sudden death in long-QT syndrome type 1 (LQT1), an inherited disease caused by loss-of-function mutations in KCNQ1, is triggered by early afterdepolarizations (EADs) that initiate polymorphic ventricular tachycardia (pVT). We investigated ionic mechanisms that underlie pVT in LQT1 using a transgenic rabbit model of LQT1.

METHODS

Optical mapping, cellular patch clamping, and computer modeling were used to elucidate the mechanisms of EADs in transgenic LQT1 rabbits.

RESULTS

The results showed that shorter action potential duration in the right ventricle (RV) was associated with focal activity during pVT initiation. RV cardiomyocytes demonstrated higher incidence of EADs under 50 nmol/L isoproterenol. Voltage-clamp studies revealed that the transient outward potassium current (I_to) magnitude was 28% greater in RV associated with KChiP2 but with no differences in terms of calcium-cycling kinetics and other sarcolemmal currents. Perfusing with the I_to blocker 4-aminopyridine changed the initial focal sites of pVT from the RV to the left ventricle, corroborating the role of I_to in pVT initiation. Computer modeling showed that EADs occur preferentially in the RV because of the larger conductance of the slow-inactivating component of I_to, which repolarizes the membrane potential sufficiently rapidly to allow reactivation of I_Ca,L before I_Kr has had sufficient time to activate.

CONCLUSIONS

I_to heterogeneity creates both triggers and an arrhythmogenic substrate in LQT1. In the absence of I_Ks, I_to interactions with I_Ca,L and I_Kr promote EADs in the RV while prolonging action potential duration in the left ventricle. This heterogeneity of action potential enhances dispersion of refractoriness and facilitates conduction blocks that initiate pVTs.

PKA phosphorylation underlies functional recruitment of sarcolemmal SK2 channels in ventricular myocytes from hypertrophic hearts

KEY POINTS

Small-conductance Ca²⁺ -activated K⁺ (SK) channels expressed in ventricular myocytes are dormant in health, yet become functional in cardiac disease. SK channels are voltage independent and their gating is controlled by intracellular [Ca²⁺ ] in a biphasic manner. Submicromolar [Ca²⁺ ] activates the channel via constitutively-bound calmodulin, whereas higher [Ca²⁺ ] exerts inhibitory effect during depolarization. Using a rat model of cardiac hypertrophy induced by thoracic aortic banding, we found that functional upregulation of SK2 channels in hypertrophic rat ventricular cardiomyocytes is driven by protein kinase A (PKA) phosphorylation. Using site-directed mutagenesis, we identified serine-465 as the site conferring PKA-dependent effects on SK2 channel function. PKA phosphorylation attenuates I_SK rectification by reducing the Ca²⁺ /voltage-dependent inhibition of SK channels without changing their sensitivity to activating submicromolar [Ca²⁺ ]_i . This mechanism underlies the functional recruitment of SK channels not only in cardiac disease, but also in normal physiology, contributing to repolarization under conditions of enhanced adrenergic drive.

ABSTRACT

Small-conductance Ca²⁺ -activated K⁺ (SK) channels expressed in ventricular myocytes (VMs) are dormant in health, yet become functional in cardiac disease. We aimed to test the hypothesis that post-translational modification of SK channels under conditions accompanied by enhanced adrenergic drive plays a central role in disease-related activation of the channels. We investigated this phenomenon using a rat model of hypertrophy induced by thoracic aortic banding (TAB). Western blot analysis using anti-pan-serine/threonine antibodies demonstrated enhanced phosphorylation of immunoprecipitated SK2 channels in VMs from TAB rats vs. Shams, which was reversible by incubation of the VMs with PKA inhibitor H89 (1 μmol L^-1 ). Patch clamped VMs under basal conditions from TABs but not Shams exhibited outward current sensitive to the specific SK inhibitor apamin (100 nmol L^-1 ), which was eliminated by inhibition of PKA (1 μmol L^-1 ). Beta-adrenergic stimulation (isoproterenol, 100 nmol L^-1 ) evoked I_SK in VMs from Shams, resulting in shortening of action potentials in VMs and ex vivo optically mapped Sham hearts. Using adenoviral gene transfer, wild-type and mutant SK2 channels were overexpressed in adult rat VMs, revealing serine-465 as the site that elicits PKA-dependent phosphorylation effects on SK2 channel function. Concurrent confocal Ca²⁺ imaging experiments established that PKA phosphorylation lessens rectification of I_SK via reduction Ca²⁺ /voltage-dependent inhibition of the channels at high [Ca²⁺ ] without affecting their sensitivity to activation by Ca²⁺ in the submicromolar range. In conclusion, upregulation of SK channels in diseased VMs is mediated by hyperadrenergic drive in cardiac hypertrophy, with functional effects on the channel conferred by PKA-dependent phosphorylation at serine-465.

Pharmacological Modulation of Mitochondrial Ca2+ Content Regulates Sarcoplasmic Reticulum Ca2+ Release via Oxidation of the Ryanodine Receptor by Mitochondria-Derived Reactive Oxygen Species

In a physiological setting, mitochondria increase oxidative phosphorylation during periods of stress to meet increased metabolic demand. This in part is mediated via enhanced mitochondrial Ca2+ uptake, an important regulator of cellular ATP homeostasis. In a pathophysiological setting pharmacological modulation of mitochondrial Ca2+ uptake or retention has been suggested as a therapeutic strategy to improve metabolic homeostasis or attenuate Ca2+-dependent arrhythmias in cardiac disease states. To explore the consequences of mitochondrial Ca2+ accumulation, we tested the effects of kaempferol, an activator of mitochondrial Ca2+ uniporter (MCU), CGP-37157, an inhibitor of mitochondrial Na+/Ca2+ exchanger, and MCU inhibitor Ru360 in rat ventricular myocytes (VMs) from control rats and rats with hypertrophy induced by thoracic aortic banding (TAB). In periodically paced VMs under β-adrenergic stimulation, treatment with kaempferol (10 μmol/L) or CGP-37157 (1 μmol/L) enhanced mitochondrial Ca2+ accumulation monitored by mitochondrial-targeted Ca2+ biosensor mtRCamp1h. Experiments with mitochondrial membrane potential-sensitive dye TMRM revealed this was accompanied by depolarization of the mitochondrial matrix. Using redox-sensitive OMM-HyPer and ERroGFP_iE biosensors, we found treatment with kaempferol or CGP-37157 increased the levels of reactive oxygen species (ROS) in mitochondria and the sarcoplasmic reticulum (SR), respectively. Confocal Ca2+ imaging showed that accelerated Ca2+ accumulation reduced Ca2+ transient amplitude and promoted generation of spontaneous Ca2+ waves in VMs paced under ISO, suggestive of abnormally high activity of the SR Ca2+ release channel ryanodine receptor (RyR). Western blot analyses showed increased RyR oxidation after treatment with kaempferol or CGP-37157 vs. controls. Furthermore, in freshly isolated TAB VMs, confocal Ca2+ imaging demonstrated that enhancement of mitochondrial Ca2+ accumulation further perturbed global Ca2+ handling, increasing the number of cells exhibiting spontaneous Ca2+ waves, shortening RyR refractoriness and decreasing SR Ca2+ content. In ex vivo optically mapped TAB hearts, kaempferol exacerbated proarrhythmic phenotype. On the contrary, incubation of cells with MCU inhibitor Ru360 (2 μmol/L, 30 min) normalized RyR oxidation state, improved intracellular Ca2+ homeostasis and reduced triggered activity in ex vivo TAB hearts. These findings suggest facilitation of mitochondrial Ca2+ uptake in cardiac disease can exacerbate proarrhythmic disturbances in Ca2+ homeostasis via ROS and enhanced activity of oxidized RyRs, while strategies to reduce mitochondrial Ca2+ accumulation can be protective.

SK channel enhancers attenuate Ca2+-dependent arrhythmia in hypertrophic hearts by regulating mito-ROS-dependent oxidation and activity of RyR

Aims

Plasmamembrane small conductance Ca2+-activated K+ (SK) channels were implicated in ventricular arrhythmias in infarcted and failing hearts. Recently, SK channels were detected in the inner mitochondria membrane (IMM) (mSK), and their activation protected from acute ischaemia-reperfusion injury by reducing intracellular levels of reactive oxygen species (ROS). We hypothesized that mSK play an important role in regulating mitochondrial function in chronic cardiac diseases. We investigated the role of mSK channels in Ca2+-dependent ventricular arrhythmia using rat model of cardiac hypertrophy induced by banding of the ascending aorta thoracic aortic banding (TAB).

Methods and results

Dual Ca2+ and membrane potential optical mapping of whole hearts derived from TAB rats revealed that membrane-permeable SK enhancer NS309 (2 μM) improved aberrant Ca2+ homeostasis and abolished VT/VF induced by β-adrenergic stimulation. Using whole cell patch-clamp and confocal Ca2+ imaging of cardiomyocytes derived from TAB hearts (TCMs) we found that membrane-permeable SK enhancers NS309 and CyPPA (10 μM) attenuated frequency of spontaneous Ca2+ waves and delayed afterdepolarizations. Furthermore, mSK inhibition enhanced (UCL-1684, 1 μM); while activation reduced mitochondrial ROS production in TCMs measured with MitoSOX. Protein oxidation assays demonstrated that increased oxidation of ryanodine receptors (RyRs) in TCMs was reversed by SK enhancers. Experiments in permeabilized TCMs showed that SK enhancers restored SR Ca2+ content, suggestive of substantial improvement in RyR function.

Conclusion

These data suggest that enhancement of mSK channels in hypertrophic rat hearts protects from Ca2+-dependent arrhythmia and suggest that the protection is mediated via decreased mitochondrial ROS and subsequent decreased oxidation of reactive cysteines in RyR, which ultimately leads to stabilization of RyR-mediated Ca2+ release.

Abstract 473: Differential Regulation of Sk Channels by CamKII and Pyk2 Under Adrenergic Stimulation

Introduction: Small conductance calcium (Ca)-activated K (SK) channels present in the plasmalemma of ventricular cardiomyocytes (VCMs) are dormant in health and become functional in cardiac disease. We recently showed that SK channels contribute to VCM repolarization in a rat model of hypertrophy induced by thoracic aortic banding (TAB), despite reduced expression levels, suggestive of increased activity of the channels. However, the mechanisms responsible for disease-associated functional upregulation of the channels are yet to be defined.

Hypothesis: Functional recruitment of SK channels in cardiac hypertrophy is caused by enhancement of adrenergic signaling cascades.

Methods: The effects of α1 adrenergic stimulation by phenylephrine (10 μM) + propranolol (100 μM) were studied in patch-clamped rat VCMs overexpressing rat SK2 after 48 hrs in culture. Calcium transients were recorded simultaneously using confocal microscopy and were used as a ramp [Ca] to assess possible changes in Ca sensitivity of the channel. Phosphorylation of SKs by Ser/Thr and tyrosine kinases, phosphorylation of Calmodulin at Thr-79, expression levels and phosphorylation of CaMKII and tyrosine kinase Pyk2 were assessed in cultured rat VCMs and freshly isolated VCMs from TABs and Shams using Western blot analysis.

Results: WB analysis showed increased Ser/Thr and Tyr phosphorylation of SK2 in TABs. Alpha1 stimulation of rat VCMs overexpressing rat SK2 mimicked this pattern, increasing I SK at high voltages in parallel with the increase in Ca transient amplitude and reduction in Ca sensitivity of the channels. Overexpression of Pyk2 inhibitor CRNK did not restore I SK despite reduction in Ca transient amplitude because of attenuating α1-agonist mediated decrease in Ca sensitivity of SKs. Application of CaMKII inhibitor KN93 fully reversed α1-mediated I SK increase at high voltages without affecting Ca sensitivity of the channel.

Conclusion: Upregulation of SKs in hypertrophic hearts is likely caused by CaMKII-dependent phosphorylation which reduces voltage-dependent block of the channels. Pyk2 inhibition may present therapeutic potential by attenuating negative effects on SK Ca sensitivity thereby increasing repolarization reserve diminished in cardiac disease.

Abstract 431: Upregulation of Small Conductance Ca 2+ Activated K + Channels Promotes Formation of Mitochondrial Supercomplexes and Increases Respiration Reserve

Introduction: Small conductance calcium (Ca 2+ )-activated K + (SK) channels are present in the inner mitochondria membrane of ventricular cardiomyocytes. We recently reported that pharmacological enhancement of mito-SKs protects from Ca 2+ dependent arrhythmia by reducing production of reactive oxygen species (ROS) by mitochondria thereby normalizing oxidation state and activity of Ca 2+ release channels ryanodine receptors in hypertrophic rat hearts. Recent advances demonstrate that respiratory components of the mitochondrial electron transport chain can organize into supercomplexes (SCs) resulting in more efficient electron transfer and reduced generation of ROS.

Hypothesis: Functional upregulation of mito-SK channels protects from Ca 2+ -dependent arrhythmia via promotion of mitochondrial SC formation and improved respiration.

Methods: H9C2 myoblasts were infected with adenoviral vectors to overexpress SK2 and Cox7rp. After 48 hours in culture, cell respiration was measured using Seahorse XFe Analyzer; ROS was measured using general ROS indicator CFDA; and mitochondria were isolated to assess SC formation using Blue-Native electrophoresis (BN-PAGE). The effects of SK enhancer NS309 (2 μM, 10 min.) on SC formation were studied in Langendorff perfused rat hearts with hypertrophy induced by thoracic aortic banding (TAB).

Results: rSK2 overexpression in H9C2s reduced intracellular ROS ~ 30%, enhanced maximum respiration rate vs. controls (0.74±0.04 and 0.58±0.03 pM/min/μg respectively, p<0.05); and caused a robust shift (>2 fold) of complex IV towards large mitochondrial SCs (~1.2 mDa). As a control, similar results were obtained with overexpression of Cox7rp protein that promotes SC formation. In addition, BN-PAGE experiments in isolated rat hearts demonstrated that robust decrease in high MW Complexes I and IV in hypertrophy was fully restored to control levels upon treatment with SK enhancer NS309.

Conclusion: Upregulation of SK channels reduces mito-ROS by an increase in respiration reserve which is linked to enhanced mitochondrial SC formation. Our data suggest that targeting mito-SK channels has therapeutic potential to treat cardiac arrhythmia in conditions accompanied by oxidative stress.

The role of spatial organization of Ca2+ release sites in the generation of arrhythmogenic diastolic Ca2+ release in myocytes from failing hearts

In heart failure (HF), dysregulated cardiac ryanodine receptors (RyR2) contribute to the generation of diastolic Ca²⁺ waves (DCWs), thereby predisposing adrenergically stressed failing hearts to life-threatening arrhythmias. However, the specific cellular, subcellular, and molecular defects that account for cardiac arrhythmia in HF remain to be elucidated. Patch-clamp techniques and confocal Ca²⁺ imaging were applied to study spatially defined Ca²⁺ handling in ventricular myocytes isolated from normal (control) and failing canine hearts. Based on their activation time upon electrical stimulation, Ca²⁺ release sites were categorized as coupled, located in close proximity to the sarcolemmal Ca²⁺ channels, and uncoupled, the Ca²⁺ channel-free non-junctional Ca²⁺ release units. In control myocytes, stimulation of β-adrenergic receptors with isoproterenol (Iso) resulted in a preferential increase in Ca²⁺ spark rate at uncoupled sites. This site-specific effect of Iso was eliminated by the phosphatase inhibitor okadaic acid, which caused similar facilitation of Ca²⁺ sparks at coupled and uncoupled sites. Iso-challenged HF myocytes exhibited increased predisposition to DCWs compared to control myocytes. In addition, the overall frequency of Ca²⁺ sparks was increased in HF cells due to preferential stimulation of coupled sites. Furthermore, coupled sites exhibited accelerated recovery from functional refractoriness in HF myocytes compared to control myocytes. Spatially resolved subcellular Ca²⁺ mapping revealed that DCWs predominantly originated from coupled sites. Inhibition of CaMKII suppressed DCWs and prevented preferential stimulation of coupled sites in Iso-challenged HF myocytes. These results suggest that CaMKII- (and phosphatase)-dependent dysregulation of junctional Ca²⁺ release sites contributes to Ca²⁺-dependent arrhythmogenesis in HF.

Hyperphosphorylation of RyRs underlies triggered activity in transgenic rabbit model of LQT2 syndrome

RATIONALE

Loss-of-function mutations in human ether go-go (HERG) potassium channels underlie long QT syndrome type 2 (LQT2) and are associated with fatal ventricular tachyarrhythmia. Previously, most studies focused on plasma membrane-related pathways involved in arrhythmogenesis in long QT syndrome, whereas proarrhythmic changes in intracellular Ca(2+) handling remained unexplored.

OBJECTIVE

We investigated the remodeling of Ca(2+) homeostasis in ventricular cardiomyocytes derived from transgenic rabbit model of LQT2 to determine whether these changes contribute to triggered activity in the form of early after depolarizations (EADs).

METHODS AND RESULTS

Confocal Ca(2+) imaging revealed decrease in amplitude of Ca(2+) transients and sarcoplasmic reticulum Ca(2+) content in LQT2 myocytes. Experiments using sarcoplasmic reticulum-entrapped Ca(2+) indicator demonstrated enhanced ryanodine receptor (RyR)-mediated sarcoplasmic reticulum Ca(2+) leak in LQT2 cells. Western blot analyses showed increased phosphorylation of RyR in LQT2 myocytes versus controls. Coimmunoprecipitation experiments demonstrated loss of protein phosphatases type 1 and type 2 from the RyR complex. Stimulation of LQT2 cells with β-adrenergic agonist isoproterenol resulted in prolongation of the plateau of action potentials accompanied by aberrant Ca(2+) releases and EADs, which were abolished by inhibition of Ca(2+)/calmodulin-dependent protein kinase type 2. Computer simulations showed that late aberrant Ca(2+) releases caused by RyR hyperactivity promote EADs and underlie the enhanced triggered activity through increased forward mode of Na(+)/Ca(2+) exchanger type 1.

CONCLUSIONS

Hyperactive, hyperphosphorylated RyRs because of reduced local phosphatase activity enhance triggered activity in LQT2 syndrome. EADs are promoted by aberrant RyR-mediated Ca(2+) releases that are present despite a reduction of sarcoplasmic reticulum content. Those releases increase forward mode Na(+)/Ca(2+) exchanger type 1, thereby slowing repolarization and enabling L-type Ca(2+) current reactivation.

Sarcoplasmic reticulum Ca²⁺ release is both necessary and sufficient for SK channel activation in ventricular myocytes

SK channels are upregulated in human patients and animal models of heart failure (HF). However, their activation mechanism and function in ventricular myocytes remain poorly understood. We aim to test the hypotheses that activation of SK channels in ventricular myocytes requires Ca(2+) release from sarcoplasmic reticulum (SR) and that SK currents contribute to reducing triggered activity. SK2 channels were overexpressed in adult rat ventricular myocytes using adenovirus gene transfer. Simultaneous patch clamp and confocal Ca(2+) imaging experiments in SK2-overexpressing cells demonstrated that depolarizations resulted in Ca(2+)-dependent outward currents sensitive to SK inhibitor apamin. SR Ca(2+) release induced by rapid application of 10 mM caffeine evoked repolarizing SK currents, whereas complete depletion of SR Ca(2+) content eliminated SK currents in response to depolarizations, despite intact Ca(2+) influx through L-type Ca(2+) channels. Furthermore, voltage-clamp experiments showed that SK channels can be activated by global spontaneous SR Ca(2+) release events Ca(2+) waves (SCWs). Current-clamp experiments revealed that SK overexpression reduces the amplitude of delayed afterdepolarizations (DADs) resulting from SCWs and shortens action potential duration. Immunolocalization studies showed that overexpressed SK channels are distributed both at external sarcolemmal membranes and along the Z-lines, resembling the distribution of endogenous SK channels. In summary, SR Ca(2+) release is both necessary and sufficient for the activation of SK channels in rat ventricular myocytes. SK currents contribute to repolarization during action potentials and attenuate DADs driven by SCWs. Thus SK upregulation in HF may have an anti-arrhythmic effect by reducing triggered activity.

Dietary omega-3 fatty acids promote arrhythmogenic remodeling of cellular Ca2+ handling in a postinfarction model of sudden cardiac death

It has been proposed that dietary omega-3 polyunsaturated fatty acids (n-3 PUFAs) can reduce the risk of ventricular arrhythmias in post-MI patients. Abnormal Ca(2+) handling has been implicated in the genesis of post-MI ventricular arrhythmias. Therefore, we tested the hypothesis that dietary n-3 PUFAs alter the vulnerability of ventricular myocytes to cellular arrhythmia by stabilizing intracellular Ca(2+) cycling. To test this hypothesis, we used a canine model of post-MI ventricular fibrillation (VF) and assigned the animals to either placebo (1 g/day corn oil) or n-3 PUFAs (1-4 g/day) groups. Using Ca(2+) imaging techniques, we examined the intracellular Ca(2+) handling in myocytes isolated from post-MI hearts resistant (VF-) and susceptible (VF+) to VF. Frequency of occurrence of diastolic Ca(2+) waves (DCWs) in VF+ myocytes from placebo group was significantly higher than in placebo-treated VF- myocytes. n-3 PUFA treatment did not decrease frequency of DCWs in VF+ myocytes. In contrast, VF- myocytes from the n-3 PUFA group had a significantly higher frequency of DCWs than myocytes from the placebo group. In addition, n-3 PUFA treatment increased beat-to-beat alterations in the amplitude of Ca(2+) transients (Ca(2+) alternans) in VF- myocytes. These n-3 PUFAs effects in VF- myocytes were associated with an increased Ca(2+) spark frequency and reduced sarcoplasmic reticulum Ca(2+) content, indicative of increased activity of ryanodine receptors. Thus, dietary n-3 PUFAs do not alleviate intracellular Ca(2+) cycling remodeling in myocytes isolated from post-MI VF+ hearts. Furthermore, dietary n-3 PUFAs increase vulnerability of ventricular myocytes to cellular arrhythmia in post-MI VF- hearts by destabilizing intracellular Ca(2+) handling.

Redox modification of ryanodine receptors by mitochondria-derived reactive oxygen species contributes to aberrant Ca2+ handling in ageing rabbit hearts

Ageing is associated with a blunted response to sympathetic stimulation and an increased risk of arrhythmia and sudden cardiac death. Aberrant calcium (Ca(2+)) handling is an important contributor to the electrical and contractile dysfunction associated with ageing. Yet, the specific molecular mechanisms underlying abnormal Ca(2+) handling in ageing heart remain poorly understood. In this study, we used ventricular myocytes isolated from young (5-9 months) and old (4-6 years) rabbit hearts to test the hypothesis that changes in Ca(2+) homeostasis are caused by post-translational modification of ryanodine receptors (RyRs) by mitochondria-derived reactive oxygen species (ROS) generated in the ageing heart. Changes in parameters of Ca(2+) handling were determined by measuring cytosolic and intra-sarcoplasmic reticulum (SR) Ca(2+) dynamics in intact and permeabilized ventricular myocytes using confocal microscopy. We also measured age-related changes in ROS production and mitochondria membrane potential using a ROS-sensitive dye and a mitochondrial voltage-sensitive fluorescent indicator, respectively. In permeablized myocytes, ageing did not change SERCA activity and spark frequency but decreased spark amplitude and SR Ca(2+) load suggesting increased RyR activity. Treatment with the antioxidant dithiothreitol reduced RyR-mediated SR Ca(2+) leak in permeabilized myocytes from old rabbit hearts to the level comparable to young. Moreover, myocytes from old rabbits had more depolarized mitochondria membrane potential and increased rate of ROS production. Under β-adrenergic stimulation, Ca(2+) transient amplitude, SR Ca(2+) load, and latency of pro-arrhythmic spontaneous Ca(2+) waves (SCWs) were decreased while RyR-mediated SR Ca(2+) leak was increased in cardiomyocytes from old rabbits. Additionally, with β-adrenergic stimulation, scavenging of mitochondrial ROS in myocytes from old rabbit hearts restored redox status of RyRs, which reduced SR Ca(2+) leak, ablated most SCWs, and increased latency to levels comparable to young. These data indicate that an age-associated increase of ROS production by mitochondria leads to the thiol-oxidation of RyRs, which underlies the hyperactivity of RyRs and thereby shortened refractoriness of Ca(2+) release in cardiomyocytes from the ageing heart. This mechanism probably plays an important role in the increased incidence of arrhythmia and sudden death in the ageing population.

Endurance exercise training normalizes repolarization and calcium-handling abnormalities, preventing ventricular fibrillation in a model of sudden cardiac death

The risk of sudden cardiac death is increased following myocardial infarction. Exercise training reduces arrhythmia susceptibility, but the mechanism is unknown. We used a canine model of sudden cardiac death (healed infarction, with ventricular tachyarrhythmias induced by an exercise plus ischemia test, VF+); we previously reported that endurance exercise training was antiarrhythmic in this model (Billman GE. Am J Physiol Heart Circ Physiol 297: H1171-H1193, 2009). A total of 41 VF+ animals were studied, after random assignment to 10 wk of endurance exercise training (EET; n = 21) or a matched sedentary period (n = 20). Following (>1 wk) the final attempted arrhythmia induction, isolated myocytes were used to test the hypotheses that the endurance exercise-induced antiarrhythmic effects resulted from normalization of cellular electrophysiology and/or normalization of calcium handling. EET prevented VF and shortened in vivo repolarization (P < 0.05). EET normalized action potential duration and variability compared with the sedentary group. EET resulted in a further decrement in transient outward current compared with the sedentary VF+ group (P < 0.05). Sedentary VF+ dogs had a significant reduction in repolarizing K(+) current, which was restored by exercise training (P < 0.05). Compared with controls, myocytes from the sedentary VF+ group displayed calcium alternans, increased calcium spark frequency, and increased phosphorylation of S2814 on ryanodine receptor 2. These abnormalities in intracellular calcium handling were attenuated by exercise training (P < 0.05). Exercise training prevented ischemically induced VF, in association with a combination of beneficial effects on cellular electrophysiology and calcium handling.