A comprehensive protocol combining in vivo and ex vivo electrophysiological experiments in an arrhythmogenic animal model.

Ventricular arrhythmias contribute significantly to cardiovascular mortality, with coronary artery disease as the predominant underlying cause. Understanding the mechanisms of arrhythmogenesis is essential to identify proarrhythmic factors and develop novel approaches for antiarrhythmic prophylaxis and treatment. Animal models are vital in basic research on cardiac arrhythmias, encompassing molecular, cellular, ex vivo whole heart, and in vivo models. Most studies use either in vivo protocols lacking important information on clinical relevance or exclusively ex vivo protocols, thereby missing the opportunity to explore underlying mechanisms. Consequently, interpretation may be difficult due to dissimilarities in animal models, interventions, and individual properties across animals. Moreover, proarrhythmic effects observed in vivo are often not replicated in corresponding ex vivo preparations during mechanistic studies. We have established a protocol to perform both an in vivo and ex vivo electrophysiological characterization in an arrhythmogenic rat model with heart failure following myocardial infarction. The same animal is followed throughout the experiment. In vivo methods involve intracardiac programmed electrical stimulation and external defibrillation to terminate sustained ventricular arrhythmia. Ex vivo methods conducted on the Langendorff-perfused heart include an electrophysiological study with optical mapping of regional action potentials, conduction velocities, and dispersion of electrophysiological properties. By exploring the retention of the in vivo proarrhythmic phenotype ex vivo, we aim to examine whether the subsequent ex vivo detailed measurements are relevant to in vivo pathological behavior. This protocol can enhance greater understanding of cardiac arrhythmias by providing a standardized, yet adaptable model for evaluating arrhythmogenicity or antiarrhythmic interventions in cardiac diseases.NEW & NOTEWORTHY Rodent models are widely used in arrhythmia research. However, most studies do not standardize clinically relevant in vivo and ex vivo techniques to support their conclusions. Here, we present a comprehensive electrophysiological protocol in an arrhythmogenic rat model, connecting in vivo and ex vivo programmed electrical stimulation with optical mapping. By establishing this protocol, we aim to facilitate the adoption of a standardized model for investigating arrhythmias, enhancing research rigor and comparability in this field.

Exercise training reveals micro-RNAs associated with improved cardiac function and electrophysiology in rats with heart failure after myocardial infarction.

AIMS: Endurance training improves aerobic fitness and cardiac function in individuals with heart failure. However, the underlying mechanisms are not well characterized. Exercise training could therefore act as a tool to discover novel targets for heart failure treatment. We aimed to associate changes in Ca2+ handling and electrophysiology with micro-RNA (miRNA) profile in exercise trained heart failure rats to establish which miRNAs induce heart failure-like effects in Ca2+ handling and electrophysiology. METHODS AND RESULTS: Post-myocardial infarction (MI) heart failure was induced in Sprague Dawley rats. Rats with MI were randomized to sedentary control (sed), moderate (mod)- or high-intensity (high) endurance training for 8 weeks. Exercise training improved cardiac function, Ca2+ handling and electrophysiology including reduced susceptibility to arrhythmia in an exercise intensity-dependent manner where high intensity gave a larger effect. Fifty-five miRNAs were significantly regulated (up or down) in MI-sed, of which 18 and 3 were changed towards Sham-sed in MI-high and MI-mod, respectively. Thereafter we experimentally altered expression of these "exercise-miRNAs" individually in human induced pluripotent stem cell-derived cardiomyocytes (hIPSC-CM) in the same direction as they were changed in MI. Of the "exercise-miRNAs", miR-214-3p prolonged AP duration, whereas miR-140 and miR-208a shortened AP duration. miR-497-5p prolonged Ca2+ release whereas miR-214-3p and miR-31a-5p prolonged Ca2+ decay. CONCLUSION: Using exercise training as a tool, we discovered that miR-214-3p, miR-497-5p, miR-31a-5p contribute to heart-failure like behaviour in Ca2+ handling and electrophysiology and could be potential treatment targets.

Two-photon excitation of FluoVolt allows improved interrogation of transmural electrophysiological function in the intact mouse heart.

BACKGROUND & AIMS: Two-photon excitation of voltage sensitive dyes (VSDs) can measure rapidly changing electrophysiological signals deep within intact cardiac tissue with improved three-dimensional resolution along with reduced photobleaching and photo-toxicity compared to conventional confocal microscopy. Recently, a category of VSDs has emerged which records membrane potentials by photo-induced electron transfer. FluoVolt is a novel VSD in this category which promises fast response and a 25% fractional change in fluorescence per 100 mV, making it an attractive optical probe for action potential (AP) recordings within intact cardiac tissue. The purpose of this study was to characterize the fluorescent properties of FluoVolt as well as its utility for deep tissue imaging. METHODS: Discrete tissue layers throughout the left ventricular wall of isolated perfused murine hearts loaded with FluoVolt or di-4-ANEPPS were sequentially excited with two-photon microscopy. RESULTS: FluoVolt loaded hearts suffered significantly fewer episodes of atrio-ventricular block compared to di-4-ANEPPS loaded hearts, indicating comparatively low toxicity of FluoVolt in the intact heart. APs recorded with FluoVolt were characterized by a lower signal-to-noise ratio and a higher dynamic range compared to APs recorded with di-4-ANEPPS. Although both depolarization and repolarization parameters were similar in APs recorded with either dye, FluoVolt allowed deeper tissue excitation with improved three-dimensional resolution due to reduced out-of-focus fluorescence generation under two-photon excitation. CONCLUSION: Our results demonstrate several advantages of two-photon excitation of FluoVolt in functional studies in intact heart preparations, including reduced toxicity and improved fluorescent properties.

Effect of exercise training on cardiac metabolism in rats with heart failure.

Objectives. Heart failure (HF) impairs resting myocardial energetics, myocardial mitochondrial performance, and maximal oxygen uptake (VO2max). Exercise training is included in most rehabilitation programs and benefits HF patients. However, the effect of exercise intensity on cardiac mitochondrial respiration and concentrations of the key bioenergetic metabolites phosphocreatine (PCr), adenosine triphosphate (ATP), and inorganic phosphate (Pi) is unclear. This study aimed to investigate the effects of exercise training at different intensities in rats with HF. Methods. Rats underwent myocardial infarction or sham operations and were divided into three subgroups: sedentary, moderate intensity, or high intensity. The impact of HF and 6 weeks of exercise training on energy metabolism was evaluated by 31P magnetic resonance spectroscopy and mitochondrial respirometry. The concentrations of PCr, ATP, and Pi were quantified by magnetic resonance spectroscopy. VO2max was measured by treadmill respirometry. Results. Exercise training increased VO2max in sham and HF. PCr/ATP ratio was reduced in HF (p < .01) and remained unchanged by exercise training. PCr concentration was significantly lower in HF compared to sham (p < .01). Moderate and high-intensity exercise training increased ATP in HF and sham. HF impaired complex I (CI) and complex II (p = .034) respiration. High-intensity exercise training recovered CI respiration in HF rats compared to HF sedentary (p = .014). Conclusions. Exercise training improved cardiac performance, as indicated by increased VO2max and higher exercise capacity, without changing the myocardial PCr/ATP ratio. These observations suggest that the PCr/ATP biomarker is not suited to evaluate the beneficial effects of exercise training in the heart. The exact mechanisms require further investigations, as exercise training did increase ATP levels and CI respiration.

Corrigendum: Characterization of Electrical Activity in Post-myocardial Infarction Scar Tissue in Rat Hearts Using Multiphoton Microscopy.

[This corrects the article DOI: 10.3389/fphys.2018.01454.].

The Effect of Exercise Training on Myocardial and Skeletal Muscle Metabolism by MR Spectroscopy in Rats with Heart Failure.

The metabolism and performance of myocardial and skeletal muscle are impaired in heart failure (HF) patients. Exercise training improves the performance and benefits the quality of life in HF patients. The purpose of the present study was to determine the metabolic profiles in myocardial and skeletal muscle in HF and exercise training using MRS, and thus to identify targets for clinical MRS in vivo. After surgically establishing HF in rats, we randomized the rats to exercise training programs of different intensities. After the final training session, rats were sacrificed and tissues from the myocardial and skeletal muscle were extracted. Magnetic resonance spectra were acquired from these extracts, and principal component and metabolic enrichment analysis were used to assess the differences in metabolic profiles. The results indicated that HF affected myocardial metabolism by changing multiple metabolites, whereas it had a limited effect on skeletal muscle metabolism. Moreover, exercise training mainly altered the metabolite distribution in skeletal muscle, indicating regulation of metabolic pathways of taurine and hypotaurine metabolism and carnitine synthesis.

High Intensity Interval Training Ameliorates Mitochondrial Dysfunction in the Left Ventricle of Mice with Type 2 Diabetes.

Both human and animal studies have shown mitochondrial and contractile dysfunction in hearts of type 2 diabetes mellitus (T2DM). Exercise training has shown positive effects on cardiac function, but its effect on the mitochondria have been insufficiently explored. The aim of this study was to assess the effect of exercise training on mitochondrial function in T2DM hearts. We divided T2DM mice (db/db) into a sedentary and an interval training group at 8 weeks of age and used heterozygote db/+ as controls. After 8 weeks of training, we evaluated mitochondrial structure and function, as well as the levels of mRNA and proteins involved in key metabolic processes from the left ventricle. db/db animals showed decreased oxidative phosphorylation capacity and fragmented mitochondria. Mitochondrial respiration showed a blunted response to Ca2+ along with reduced protein levels of the mitochondrial calcium uniporter. Exercise training ameliorated the reduced oxidative phosphorylation in complex (C) I + II, CII and CIV, but not CI or Ca2+ response. Mitochondrial fragmentation was partially restored. mRNA levels of isocitrate, succinate and oxoglutarate dehydrogenase were increased in db/db mice and normalized by exercise training. Exercise training induced an upregulation of two transcripts of peroxisome proliferator activated receptor gamma coactivator 1 alpha (PGC1α1 and PGC1α4) previously linked to endurance training adaptations and strength training adaptations, respectively. The T2DM heart showed mitochondrial dysfunction at multiple levels and exercise training ameliorated some, but not all mitochondrial dysfunctions.

Skeletal muscle metabolism in rats with low and high intrinsic aerobic capacity: Effect of aging and exercise training.

PURPOSE: Exercise training increases aerobic capacity and is beneficial for health, whereas low aerobic exercise capacity is a strong independent predictor of cardiovascular disease and premature death. The purpose of the present study was to determine the metabolic profiles in a rat model of inborn low versus high capacity runners (LCR/HCR) and to determine the effect of inborn aerobic capacity, aging, and exercise training on skeletal muscle metabolic profile. METHODS: LCR/HCR rats were randomized to high intensity low volume interval treadmill training twice a week or sedentary control for 3 or 11 months before they were sacrificed, at 9 and 18 months of age, respectively. Magnetic resonance spectra were acquired from soleus muscle extracts, and partial least square discriminative analysis was used to determine the differences in metabolic profile. RESULTS: Sedentary HCR rats had 54% and 30% higher VO2max compared to sedentary LCR rats at 9 months and 18 months, respectively. In HCR, exercise increased running speed significantly, and VO2max was higher at age of 9 months, compared to sedentary counterparts. In LCR, changes were small and did not reach the level of significance. The metabolic profile was significantly different in the LCR sedentary group compared to the HCR sedentary group at the age of 9 and 18 months, with higher glutamine and glutamate levels (9 months) and lower lactate level (18 months) in HCR. Irrespective of fitness level, aging was associated with increased soleus muscle concentrations of glycerophosphocholine and glucose. Interval training did not influence metabolic profiles in LCR or HCR rats at any age. CONCLUSION: Differences in inborn aerobic capacity gave the most marked contrasts in metabolic profile, there were also some changes with ageing. Low volume high intensity interval training twice a week had no detectable effect on metabolic profile.

Characterization of Electrical Activity in Post-myocardial Infarction Scar Tissue in Rat Hearts Using Multiphoton Microscopy.

Background: The origin of electrical behavior in post-myocardial infarction scar tissue is still under debate. This study aims to examine the extent and nature of the residual electrical activity within a stabilized ventricular infarct scar. Methods and Results: An apical infarct was induced in the left ventricle of Wistar rats by coronary artery occlusion. Five weeks post-procedure, hearts were Langendorff-perfused, and optically mapped using di-4-ANEPPS. Widefield imaging of optical action potentials (APs) on the left ventricular epicardial surface revealed uniform areas of electrical activity in both normal zone (NZ) and infarct border zone (BZ), but only limited areas of low-amplitude signals in the infarct zone (IZ). 2-photon (2P) excitation of di-4-ANEPPS and Fura-2/AM at discrete layers in the NZ revealed APs and Ca2+ transients (CaTs) to 500-600 µm below the epicardial surface. 2P imaging in the BZ revealed superficial connective tissue structures lacking APs or CaTs. At depths greater than approximately 300 µm, myocardial structures were evident that supported normal APs and CaTs. In the IZ, although 2P imaging did not reveal clear myocardial structures, low-amplitude AP signals were recorded at discrete layers. No discernible Ca2+ signals could be detected in the IZ. AP rise times in BZ were slower than NZ (3.50 ± 0.50 ms vs. 2.23 ± 0.28 ms) and further slowed in IZ (9.13 ± 0.56 ms). Widefield measurements of activation delay between NZ and BZ showed negligible difference (3.37 ± 1.55 ms), while delay values in IZ showed large variation (11.88 ± 9.43 ms). Conclusion: These AP measurements indicate that BZ consists of an electrically inert scar above relatively normal myocardium. Discrete areas/layers of IZ displayed entrained APs with altered electrophysiology, but the structure of this tissue remains to be elucidated.

Aerobic Interval Training Prevents Age-Dependent Vulnerability to Atrial Fibrillation in Rodents.

Aims: Increasing age is the most important risk factor for atrial fibrillation (AF). Very high doses of exercise training might increase AF risk, while moderate levels seem to be protective. This study aimed to examine the effects of age on vulnerability to AF and whether long-term aerobic interval training (AIT) could modify these effects. Methods: Nine months old, male Sprague-Dawley rats were randomized to AIT for 16 weeks (old-ex) or to a sedentary control group (old-sed), and compared to young sedentary males (young-sed). After the intervention, animals underwent echocardiography, testing of exercise capacity (VO2max), and electrophysiology with AF induction before ex vivo electrophysiology. Fibrosis quantification, immunohistochemistry and western blotting of atrial tissue were performed. Results: Sustained AF was induced in vivo in 4 of 11 old-sed animals, but none of the old-ex or young-sed rats (p = 0.006). VO2max was lower in old-sed, while old-ex had comparable results to young-sed. Fibrosis was increased in old-sed (p = 0.006), with similar results in old-ex. There was a significantly slower atrial conduction in old-sed (p = 0.038), with an increase in old-ex (p = 0.027). Action potential duration was unaltered in old-sed, but prolonged in old-ex (p = 0.036). There were no differences in amount of atrial connexin 43 between groups, but a lateralization in atrial cardiomyocytes of old-sed, with similar findings in old-ex. Conclusion: AF vulnerability was higher in old-sed animals, associated with increased atrial fibrosis, lateralization of connexin-43, and reduced atrial conduction velocity. AIT reduced the age-associated susceptibility to AF, possibly through increased conduction velocity and prolongation of action potentials.

Human cardiomyocyte calcium handling and transverse tubules in mid-stage of post-myocardial-infarction heart failure.

AIMS: Cellular processes in the heart rely mainly on studies from experimental animal models or explanted hearts from patients with terminal end-stage heart failure (HF). To address this limitation, we provide data on excitation contraction coupling, cardiomyocyte contraction and relaxation, and Ca2+ handling in post-myocardial-infarction (MI) patients at mid-stage of HF. METHODS AND RESULTS: Nine MI patients and eight control patients without MI (non-MI) were included. Biopsies were taken from the left ventricular myocardium and processed for further measurements with epifluorescence and confocal microscopy. Cardiomyocyte function was progressively impaired in MI cardiomyocytes compared with non-MI cardiomyocytes when increasing electrical stimulation towards frequencies that simulate heart rates during physical activity (2 Hz); at 3 Hz, we observed almost total breakdown of function in MI. Concurrently, we observed impaired Ca2+ handling with more spontaneous Ca2+ release events, increased diastolic Ca2+ , lower Ca2+ amplitude, and prolonged time to diastolic Ca2+ removal in MI (P < 0.01). Significantly reduced transverse-tubule density (-35%, P < 0.01) and sarcoplasmic reticulum Ca2+ adenosine triphosphatase 2a (SERCA2a) function (-26%, P < 0.01) in MI cardiomyocytes may explain the findings. Reduced protein phosphorylation of phospholamban (PLB) serine-16 and threonine-17 in MI provides further mechanisms to the reduced function. CONCLUSIONS: Depressed cardiomyocyte contraction and relaxation were associated with impaired intracellular Ca2+ handling due to impaired SERCA2a activity caused by a combination of alteration in the PLB/SERCA2a ratio and chronic dephosphorylation of PLB as well as loss of transverse tubules, which disrupts normal intracellular Ca2+ homeostasis and handling. This is the first study that presents these mechanisms from viable and intact cardiomyocytes isolated from the left ventricle of human hearts at mid-stage of post-MI HF.

Normal interventricular differences in tissue architecture underlie right ventricular susceptibility to conduction abnormalities in a mouse model of Brugada syndrome.

Aims: Loss-of-function of the cardiac sodium channel NaV1.5 is a common feature of Brugada syndrome. Arrhythmias arise preferentially from the right ventricle (RV) despite equivalent NaV1.5 downregulation in the left ventricle (LV). The reasons for increased RV sensitivity to NaV1.5 loss-of-function mutations remain unclear. Because ventricular electrical activation occurs predominantly in the transmural axis, we compare RV and LV transmural electrophysiology to determine the underlying cause of the asymmetrical conduction abnormalities in Scn5a haploinsufficient mice (Scn5a+/-). Methods and results: Optical mapping and two-photon microscopy in isolated-perfused mouse hearts demonstrated equivalent depression of transmural conduction velocity (CV) in the LV and RV of Scn5a+/- vs. wild-type littermates. Only RV transmural conduction was further impaired when challenged with increased pacing frequencies. Epicardial dispersion of activation and beat-to-beat variation in activation time were increased only in the RV of Scn5a+/- hearts. Analysis of confocal and histological images revealed larger intramural clefts between cardiomyocyte layers in the RV vs. LV, independent of genotype. Acute sodium current inhibition in wild type hearts using tetrodotoxin reproduced beat-to-beat activation variability and frequency-dependent CV slowing in the RV only, with the LV unaffected. The influence of clefts on conduction was examined using a two-dimensional monodomain computational model. When peak sodium channel conductance was reduced to 50% of normal the presence of clefts between cardiomyocyte layers reproduced the activation variability and conduction phenotype observed experimentally. Conclusions: Normal structural heterogeneities present in the RV are responsible for increased vulnerability to conduction slowing in the presence of reduced sodium channel function. Heterogeneous conduction slowing seen in the RV will predispose to functional block and the initiation of re-entrant ventricular arrhythmias.

Content of mitochondrial calcium uniporter (MCU) in cardiomyocytes is regulated by microRNA-1 in physiologic and pathologic hypertrophy.

The mitochondrial Ca2+ uniporter complex (MCUC) is a multimeric ion channel which, by tuning Ca2+ influx into the mitochondrial matrix, finely regulates metabolic energy production. In the heart, this dynamic control of mitochondrial Ca2+ uptake is fundamental for cardiomyocytes to adapt to either physiologic or pathologic stresses. Mitochondrial calcium uniporter (MCU), which is the core channel subunit of MCUC, has been shown to play a critical role in the response to ß-adrenoreceptor stimulation occurring during acute exercise. The molecular mechanisms underlying the regulation of MCU, in conditions requiring chronic increase in energy production, such as physiologic or pathologic cardiac growth, remain elusive. Here, we show that microRNA-1 (miR-1), a member of the muscle-specific microRNA (myomiR) family, is responsible for direct and selective targeting of MCU and inhibition of its translation, thereby affecting the capacity of the mitochondrial Ca2+ uptake machinery. Consistent with the role of miR-1 in heart development and cardiomyocyte hypertrophic remodeling, we additionally found that MCU levels are inversely related with the myomiR content, in murine and, remarkably, human hearts from both physiologic (i.e., postnatal development and exercise) and pathologic (i.e., pressure overload) myocardial hypertrophy. Interestingly, the persistent activation of ß-adrenoreceptors is likely one of the upstream repressors of miR-1 as treatment with ß-blockers in pressure-overloaded mouse hearts prevented its down-regulation and the consequent increase in MCU content. Altogether, these findings identify the miR-1/MCU axis as a factor in the dynamic adaptation of cardiac cells to hypertrophy.

Exercise induces cerebral VEGF and angiogenesis via the lactate receptor HCAR1.

Physical exercise can improve brain function and delay neurodegeneration; however, the initial signal from muscle to brain is unknown. Here we show that the lactate receptor (HCAR1) is highly enriched in pial fibroblast-like cells that line the vessels supplying blood to the brain, and in pericyte-like cells along intracerebral microvessels. Activation of HCAR1 enhances cerebral vascular endothelial growth factor A (VEGFA) and cerebral angiogenesis. High-intensity interval exercise (5 days weekly for 7 weeks), as well as L-lactate subcutaneous injection that leads to an increase in blood lactate levels similar to exercise, increases brain VEGFA protein and capillary density in wild-type mice, but not in knockout mice lacking HCAR1. In contrast, skeletal muscle shows no vascular HCAR1 expression and no HCAR1-dependent change in vascularization induced by exercise or lactate. Thus, we demonstrate that a substance released by exercising skeletal muscle induces supportive effects in brain through an identified receptor.

T cell costimulation blockade blunts pressure overload-induced heart failure.

Heart failure (HF) is a leading cause of mortality. Inflammation is implicated in HF, yet clinical trials targeting pro-inflammatory cytokines in HF were unsuccessful, possibly due to redundant functions of individual cytokines. Searching for better cardiac inflammation targets, here we link T cells with HF development in a mouse model of pathological cardiac hypertrophy and in human HF patients. T cell costimulation blockade, through FDA-approved rheumatoid arthritis drug abatacept, leads to highly significant delay in progression and decreased severity of cardiac dysfunction in the mouse HF model. The therapeutic effect occurs via inhibition of activation and cardiac infiltration of T cells and macrophages, leading to reduced cardiomyocyte death. Abatacept treatment also induces production of anti-inflammatory cytokine interleukin-10 (IL-10). IL-10-deficient mice are refractive to treatment, while protection could be rescued by transfer of IL-10-sufficient B cells. These results suggest that T cell costimulation blockade might be therapeutically exploited to treat HF.

Cardiac repair in guinea pigs with human engineered heart tissue from induced pluripotent stem cells.

Myocardial injury results in a loss of contractile tissue mass that, in the absence of efficient regeneration, is essentially irreversible. Transplantation of human pluripotent stem cell-derived cardiomyocytes has beneficial but variable effects. We created human engineered heart tissue (hEHT) strips from human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes and hiPSC-derived endothelial cells. The hEHTs were transplanted onto large defects (22% of the left ventricular wall, 35% decline in left ventricular function) of guinea pig hearts 7 days after cryoinjury, and the results were compared with those obtained with human endothelial cell patches (hEETs) or cell-free patches. Twenty-eight days after transplantation, the hearts repaired with hEHT strips exhibited, within the scar, human heart muscle grafts, which had remuscularized 12% of the infarct area. These grafts showed cardiomyocyte proliferation, vascularization, and evidence for electrical coupling to the intact heart tissue in a subset of engrafted hearts. hEHT strips improved left ventricular function by 31% compared to that before implantation, whereas the hEET or cell-free patches had no effect. Together, our study demonstrates that three-dimensional human heart muscle constructs can repair the injured heart.

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

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

Aerobic interval training reduces inducible ventricular arrhythmias in diabetic mice after myocardial infarction.

Diabetes mellitus (DM) increases the risk of heart failure after myocardial infarction (MI), and aggravates ventricular arrhythmias in heart failure patients. Although exercise training improves cardiac function in heart failure, it is still unclear how it benefits the diabetic heart after MI. To study the effects of aerobic interval training on cardiac function, susceptibility to inducible ventricular arrhythmias and cardiomyocyte calcium handling in DM mice after MI (DM-MI). Male type 2 DM mice (C57BLKS/J Lepr (db) /Lepr (db) ) underwent MI or sham surgery. One group of DM-MI mice was submitted to aerobic interval training running sessions during 6 weeks. Cardiac function and structure were assessed by echocardiography and magnetic resonance imaging, respectively. Ventricular arrhythmias were induced by high-frequency cardiac pacing in vivo. Protein expression was measured by Western blot. DM-MI mice displayed increased susceptibility for inducible ventricular arrhythmias and impaired diastolic function when compared to wild type-MI, which was associated with disruption of cardiomyocyte calcium handling and increased calcium leak from the sarcoplasmic reticulum. High-intensity exercise recovered cardiomyocyte function in vitro, reduced sarcoplasmic reticulum diastolic calcium leak and significantly reduced the incidence of inducible ventricular arrhythmias in vivo in DM-MI mice. Exercise training also normalized the expression profile of key proteins involved in cardiomyocyte calcium handling, suggesting a potential molecular mechanism for the benefits of exercise in DM-MI mice. High-intensity aerobic exercise training recovers cardiomyocyte function and reduces inducible ventricular arrhythmias in infarcted diabetic mice.

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

BACKGROUND: Although high aerobic capacity is associated with effective cardiac function, the effect of aerobic capacity on atrial function, especially in terms of cellular mechanisms, is not known. We aimed to investigate whether rats with low inborn maximal oxygen uptake (VO2 max) had impaired atrial myocyte contractile function when compared to rats with high inborn VO2 max. METHODS AND RESULTS: Atrial myocyte function was depressed in Low Capacity Runners (LCR) relative to High Capacity Runners (HCR) which was associated with impaired Ca(2+) handling. Fractional shortening was 52% lower at 2 Hz and 60% lower at 5 Hz stimulation while time to 50% relengthening was 43% prolonged and 55% prolonged, respectively. Differences in Ca(2+) amplitude and diastolic Ca(2+) level were observed at 5 Hz stimulation where Ca(2+) amplitude was 70% lower and diastolic Ca(2+) level was 11% higher in LCR rats. Prolonged time to 50% Ca(2+) decay was associated with reduced sarcoplasmic reticulum (SR) Ca(2+) ATPase function in LCR (39%). Na(+)/Ca(2+) exchanger activity was comparable between the groups. Diastolic SR Ca(2+) leak was increased by 109%. This could be partly explained by increased ryanodine receptors phosphorylation at the Ca(2+)-calmodulin-dependent protein kinase-II specific Ser-2814 site in LCR rats. T-tubules were present in 68% of HCR cells whereas only 33% LCR cells had these structures. In HCR, the significantly higher numbers of cells with T-tubules were combined with greater numbers of myocytes where Ca(2+) release in the cell occurred simultaneously in central and peripheral regions, giving rise to faster and more spatial homogenous Ca(2+)-signal onset. CONCLUSION: This data demonstrates that contrasting for low or high aerobic capacity leads to diverse functional and structural remodelling of atrial myocytes, with impaired contractile function in LCR compared to HCR rats.