The effects of daily dose of intense exercise on cardiac responses and atrial fibrillation.

Atrial fibrillation (AF) is a supraventricular tachyarrhythmia that is strongly associated with cardiovascular (CV) disease and sedentary lifestyles. Despite the benefits of exercise on overall health, AF incidence in high-level endurance athletes rivals that of CV disease patients, suggesting a J-shaped relationship with AF. To investigate the dependence of AF vulnerability on exercise, we varied daily swim durations (120, 180 or 240 min day-1 ) in 7-week-old male CD1 mice. We assessed mice after performing equivalent amounts of cumulative work during swimming (i.e. ∼700 L O2  kg-1 ), as determined from O2 consumption rates ( V ̇ O 2 ${\dot V_{{{\mathrm{O}}_2}}}$ ). The mean V ̇ O 2 ${\dot V_{{{\mathrm{O}}_2}}}$ during exercise increased progressively throughout the training period and was indistinguishable between the swim groups. Consistent with similar improvements in aerobic conditioning induced by swimming, skeletal muscle mitochondria content increased (P = 0.027) indistinguishably between exercise groups. Physiological ventricular remodelling, characterized by mild hypertrophy and left ventricular dilatation, was also similar between exercised mice without evidence of ventricular arrhythmia inducibility. By contrast, prolongation of daily swim durations caused progressive and vagal-dependent heart rate reductions (P = 0.008), as well as increased (P = 0.005) AF vulnerability. As expected, vagal inhibition prolonged (P = 0.013) atrial refractoriness, leading to reduced AF vulnerability, although still inducible in the 180 and 240 min swim groups. Accordingly, daily swim dose progressively increased atrial hypertrophy (P = 0.003), fibrosis (P < 0.001) and macrophage accumulation (P = 0.006) without differentially affecting the ventricular tissue properties. Thus, increasing daily exercise duration drives progressively adverse atrial-specific remodelling and vagal-dependent AF vulnerability despite robust and beneficial aerobic conditioning and physiological remodelling of ventricles and skeletal muscle. KEY POINTS: Previous studies have suggested that a J-shaped dose-response relationship exists between physical activity and cardiovascular health outcomes, with moderate exercise providing protection against many cardiovascular disease conditions, whereas chronic endurance exercise can promote atrial fibrillation (AF). We found that AF vulnerability increased alongside elevated atrial hypertrophy, fibrosis and inflammation as daily swim exercise durations in mice were prolonged (i.e. ≥180 min day-1 for 6 weeks). The MET-h week-1 (based on O2  measurements during swimming) needed to induce increased AF vulnerability mirrored the levels linked to AF in athletes. These adverse atria effects associated with excessive daily exercise occurred despite improved aerobic conditioning, skeletal muscle adaptation and physiological ventricular remodelling. We suggest that atrial-specific changes observed with exercise arise from excessive elevations in venous filling pressures during prolonged exercise bouts, which we argue has implications for all AF patients because elevated atrial pressures occur in most cardiovascular disease conditions as well as ageing which are linked to AF.

Cardiomyocyte and endothelial cells play distinct roles in the tumour necrosis factor (TNF)-dependent atrial responses and increased atrial fibrillation vulnerability induced by endurance exercise training in mice.

AIMS: Endurance exercise is associated with an increased risk of atrial fibrillation (AF). We previously established that adverse atrial remodelling and AF susceptibility induced by intense exercise in mice require the mechanosensitive and pro-inflammatory cytokine tumour necrosis factor (TNF). The cellular and mechanistic basis for these TNF-mediated effects is unknown. METHODS AND RESULTS: We studied the impact of Tnf excision, in either atrial cardiomyocytes or endothelial cells (using Cre-recombinase expression controlled by Nppa or Tie2 promoters, respectively), on the cardiac responses to six weeks of intense swim exercise training. TNF ablation, in either cell type, had no impact on the changes in heart rate, autonomic tone, or left ventricular structure and function induced by exercise training. Tnf excision in atrial cardiomyocytes did, however, prevent atrial hypertrophy, fibrosis, and macrophage infiltration as well as conduction slowing and increased AF susceptibility arising from exercise training. In contrast, endothelial-specific excision only reduced the training-induced atrial hypertrophy. Consistent with these cell-specific effects of Tnf excision, inducing TNF loss from atrial cardiomyocytes prevented activation of p38MAPKinase, a strain-dependent downstream mediator of TNF signalling, without affecting the atrial stretch as assessed by atrial pressures induced by exercise. Despite TNF's established role in innate immune responses and inflammation, neither acute nor chronic exercise training caused measurable NLRP3 inflammasome activation. CONCLUSIONS: Our findings demonstrate that adverse atrial remodelling and AF vulnerability induced by intense exercise require TNF in atrial cardiomyocytes whereas the impact of endothelial-derived TNF is limited to hypertrophy modulation. The implications of the cell autonomous effects of TNF and crosstalk between cells in the atria are discussed.

Dissecting the Roles of the Autonomic Nervous System and Physical Activity on Circadian Heart Rate Fluctuations in Mice.

Heart rate (HR) and blood pressure as well as adverse cardiovascular events show clear circadian patterns, which are linked to interdependent daily variations in physical activity and cardiac autonomic nerve system (ANS) activity. We set out to assess the relative contributions of the ANS (alone) and physical activity to circadian HR fluctuations. To do so, we measured HR (beats per minute, bpm) in mice that were either immobilized using isoflurane anesthesia or free-moving. Nonlinear fits of HR data to sine functions revealed that anesthetized mice display brisk circadian HR fluctuations with amplitudes of 47.1±7.4bpm with the highest HRs in middle of the dark (active) period (ZT 18: 589±46bpm) and lowest HRs in the middle of the light (rest) period (ZT 6: 497±54bpm). The circadian HR fluctuations were reduced by ~70% following blockade of cardiac parasympathetic nervous activity (PNA) with atropine while declining by <15% following cardiac sympathetic nerve activity (SNA) blockade with propranolol. Small HR fluctuation amplitudes (11.6±5.9bpm) remained after complete cardiac ANS blockade. Remarkably, circadian HR fluctuation amplitudes in freely moving, telemetrized mice were only ~32% larger than in anesthetized mice. However, after gaining access to running wheels for 1week, circadian HR fluctuations increase to 102.9±12.1bpm and this is linked directly to increased O2 consumption during running. We conclude that, independent of physical activity, the ANS is a major determinant of circadian HR variations with PNA playing a dominant role compared to SNA. The effects of physical activity to the daily HR variations are remarkably small unless mice get access to running wheels.

Differential negative effects of acute exhaustive swim exercise on the right ventricle are associated with disproportionate hemodynamic loading.

Acute exhaustive endurance exercise can differentially impact the right ventricle (RV) versus the left ventricle (LV). However, the hemodynamic basis for these differences and its impact on postexercise recovery remain unclear. Therefore, we assessed cardiac structure and function along with hemodynamic properties of mice subjected to single bouts (216 ± 8 min) of exhaustive swimming (ES). One-hour after ES, LVs displayed mild diastolic impairment compared with that in sedentary (SED) mice. Following dobutamine administration to assess functional reserve, diastolic and systolic function were slightly impaired. Twenty-four hours after ES, LV function was largely indistinguishable from that in SED. By contrast, 1-h post swim, RVs showed pronounced impairment of diastolic and systolic function with and without dobutamine, which persisted 24 h later. The degree of RV impairment correlated with the time-to-exhaustion. To identify hemodynamic factors mediating chamber-specific responses to ES, LV pressure was recorded during swimming. Swimming initiated immediate increases in heart rates (HRs), systolic pressure, dP/dtmax and -dP/dtmin, which remained stable for ∼45 min. LV end-diastolic pressures (LVEDP) increased to ≥45 mmHg during the first 10 min and subsequently declined. After 45 min, HR and -dP/dtmin declined, which correlated with gradual elevations in LVEDP (to ∼45 mmHg) as mice approached exhaustion. All parameters rapidly normalized postexercise. Consistent with human studies, our findings demonstrate a disproportionate negative impact of acute exhaustive exercise on RVs that persisted for at least 24 h. We speculate that the differential effects of exhaustive exercise on the ventricles arise from a ∼2-fold greater hemodynamic load in the RV than in LV originating from profound elevations in LVEDPs as mice approach exhaustion.NEW & NOTEWORTHY Acute exhaustive exercise differentially impacts the right ventricle (RV) versus left ventricle (LV), yet the underlying hemodynamic basis remains unclear. Using pressure-volume analyses and pressure-telemetry implantation in mice, we confirmed a marked disproportionate and persistent negative impact of exhaustive exercise on the RV. These differences in responses of the ventricles to exhaustive exercise are of clinical relevance, reflecting ∼2-fold greater hemodynamic RV loads versus LVs arising from massive (∼45 mmHg) increases in LV end-diastolic pressures at exhaustion.

Revisiting right anterior oblique projections for the triangle of Koch: implications from computed tomography.

BACKGROUND: Variability in the anatomy and orientation of the triangle of Koch (TK) complicates ablation procedures involving the atrioventricular (AV) node. We used CT angiography (CTA) to assess the anatomical TK orientation, the CS ostium direction, and the relationship between the two, and we validated an individualized CS-guided projection during ablation procedures. METHODS: In 104 patients without structural heart disease undergoing computed tomography (CT) angiography, TK orientations were determined in relation to the coronary sinus ostium (CSo) as well as two standard right anterior oblique (RAO) projection angles (30o and 45o) commonly used in ablation procedures. RESULTS: A CS-guided RAO projection (RAOCS) was shown to best track the orientation of the TK compared to RAO30° and 45°, with TK orientation strongly correlating with the CSo direction (r = 0.86, P < 0.001). In addition, the mean relative difference between the angle of the CSo and TK orientation was 5.54 ± 0.48°, consistent with a reduction in the degree of image shortening compared to traditional RAOs. Moreover, in vivo validation following ablation revealed that using a CS-guided projection limited the degree of on-screen image shortening compared to both the RAO30° and 45° in 25 patients with catheter ablation procedures. CONCLUSION: In hearts with a normal structure, the CSo direction offers a reliable predictor of the TK orientation which can be used to guide the projection of the TK during ablation procedures.

Impairments in left ventricular mitochondrial bioenergetics precede overt cardiac dysfunction and remodelling in Duchenne muscular dystrophy.

KEY POINTS: Ninety-eight per cent of patients with Duchenne muscular dystrophy (DMD) develop cardiomyopathy, with 40% developing heart failure. While increased propensity for mitochondrial induction of cell death has been observed in left ventricle, it remains unknown whether this is linked to impaired mitochondrial respiratory control and elevated H2 O2 emission prior to the onset of cardiomyopathy. Classic mouse models of DMD demonstrate hyper-regeneration in skeletal muscle which may mask mitochondrial abnormalities. Using a model with less regenerative capacity that is more akin to DMD patients, we observed elevated left ventricular mitochondrial H2 O2 and impaired oxidative phosphorylation in the absence of cardiac remodelling or overt cardiac dysfunction at 4 weeks. These impairments were associated with dysfunctions at complex I, governance by ADP and creatine-dependent phosphate shuttling, which results in a less efficient response to energy demands. Mitochondria may be a therapeutic target for the treatment of cardiomyopathy in DMD. ABSTRACT: In Duchenne muscular dystrophy (DMD), mitochondrial dysfunction is predicted as a response to numerous cellular stressors, yet the contribution of mitochondria to the onset of cardiomyopathy remains unknown. To resolve this uncertainty, we designed in vitro assessments of mitochondrial bioenergetics to model mitochondrial control parameters that influence cardiac function. Both left ventricular mitochondrial responsiveness to the central bioenergetic controller ADP and the ability of creatine to facilitate mitochondrial-cytoplasmic phosphate shuttling were assessed. These measurements were performed in D2.B10-DMDmdx /2J mice - a model that demonstrates skeletal muscle atrophy and weakness due to limited regenerative capacities and cardiomyopathy more akin to people with DMD than classic models. At 4 weeks of age, there was no evidence of cardiac remodelling or cardiac dysfunction despite impairments in ADP-stimulated respiration and ADP attenuation of H2 O2 emission. These impairments were seen at both submaximal and maximal ADP concentrations despite no reductions in mitochondrial content markers. The ability of creatine to enhance ADP's control of mitochondrial bioenergetics was also impaired, suggesting an impairment in mitochondrial creatine kinase-dependent phosphate shuttling. Susceptibly to permeability transition pore opening and the subsequent activation of cell death pathways remained unchanged. Mitochondrial H2 O2 emission was elevated despite no change in markers of irreversible oxidative damage, suggesting alternative redox signalling mechanisms should be explored. These findings demonstrate that selective mitochondrial dysfunction precedes the onset of overt cardiomyopathy in D2.mdx mice, suggesting that improving mitochondrial bioenergetics by restoring ADP, creatine-dependent phosphate shuttling and complex I should be considered for treating DMD patients.

Phosphodiesterase type 3A (PDE3A), but not type 3B (PDE3B), contributes to the adverse cardiac remodeling induced by pressure overload.

Phosphodiesterase type 3 (PDE3) inhibitors block the cAMP hydrolyzing activity of both PDE3 isoforms, PDE3A and PDE3B, which have distinct roles in the heart. Although PDE3 inhibitors improve cardiac function in heart disease patients, they also increase mortality. Nevertheless, PDE3 inhibitors can provide benefit to non-ischemic heart disease patients and are used extensively to treat heart failure in dogs. Since the isoform-dependence of the complex cardiac actions of PDE3 inhibition in diseased hearts remains unknown, we assessed the effects of PDE3 inhibitors as well as gene ablation of PDE3A or PDEB in mice following the induction of non-ischemic heart disease by pressure-overload with transverse-aortic constriction (TAC). As expected, after 6 weeks of TAC, mice exhibited left ventricular contractile dysfunction, dilation, hypertrophy and interstitial fibrosis, in association with increased macrophage numbers, activation of p38 MAPK and elevated PDE3 activity. Chronic PDE3 inhibition with milrinone (MIL), at doses that did not affect either cardiac contractility or arterial blood pressure, profoundly attenuated the adverse ventricular remodeling, reduced macrophage number and diminished p38-MAPK activation induced by TAC. Surprisingly, whole-body ablation of PDE3A, but not PDE3B, provided similar protection against TAC-induced adverse ventricular remodeling, and the addition of MIL to mice lacking PDE3A provided no further protection. Our results support the conclusion that PDE3A plays an important role in adverse cardiac remodeling induced by chronic pressure overload in mice, although the underlying biochemical mechanisms remain to be fully elucidated. The implications of this conclusion on the clinical use of PDE3 inhibitors are discussed.

Inhibition of soluble TNFα prevents adverse atrial remodeling and atrial arrhythmia susceptibility induced in mice by endurance exercise.

Intense endurance exercise is linked to atrial fibrillation (AF). We established previously that interventions that simultaneously interfere with TNFα signaling, mediated via both the enzymatically liberated soluble and membrane-bound forms of TNFα, prevent atrial remodeling and AF vulnerability in exercised mice. To investigate which signaling modality underlies this protection, we treated exercised mice with XPRO®1595, a selective dominant-negative inhibitor of solTNFα. In male CD1 mice, 6 weeks of intense swim exercise induced reductions in heart rate, increased cardiac vagal tone, left ventricular (LV) dilation and enhanced LV function. By contrast, exercise induced hypertrophy, fibrosis, and increased inflammatory cell infiltrates in atria, and these changes were associated with increased AF susceptibility in isolated atria as well as mice, with and without parasympathetic nerve blockade. Although XPRO treatment had no effect on the beneficial physiological changes induced by exercise, it protected against adverse atrial changes as well as AF susceptibility. Our results establish that soluble TNFα is required for exercise-induced increases in AF vulnerability, which is linked to fibrosis, inflammation, and enlargement of the atria, but largely independent of changes in vagal tone.

Changes in Heart Rate and Its Regulation by the Autonomic Nervous System Do Not Differ Between Forced and Voluntary Exercise in Mice.

Most exercise studies in mice have relied on forced training which can introduce psychological stress. Consequently, the utility of mouse models for understanding exercise-mediated effects in humans, particularly autonomic nervous system (ANS) remodeling, have been challenged. We compared the effects of voluntary free-wheel running vs. non-voluntary swimming on heart function in mice with a focus on the regulation of heart rate (HR) by the ANS. Under conditions where the total excess O2 consumption associated with exercise was comparable, the two exercise models led to similar improvements in ventricular function as well as comparable reductions in HR and its control by parasympathetic nervous activity (PNA) and sympathetic nervous activity (SNA), compared to sedentary mice. Both exercise models also increased HR variability (HRV) by similar amounts, independent of HR reductions. In all mice, HRV depended primarily on PNA, with SNA weakly affecting HRV at low frequencies. The differences in both HR and HRV between exercised vs. sedentary mice were eliminated by autonomic blockade, consistent with the similar intrinsic beating rates observed in atria isolated from exercised vs. sedentary mice. In conclusion, both forced and voluntary exercise induce comparable ventricular physiological remodeling as well as HR reductions and HR-independent enhancements of HRV which were both primarily dependent on increased PNA. NEW AND NOTEWORTHY: -No previous mouse studies have compared the effects of forced and voluntary exercise on the heart function and its modulation by the autonomic nervous system (ANS).-Both voluntary free-wheel running and forced swimming induced similar improvements in ventricular contractile function, reductions in heart rate (HR) and enhancements of HR variability (HRV).-HR regulation in exercised mice was linked to increased parasympathetic nerve activity and reduced sympathetic nerve activity.- HRV was independent of HR and depended primarily on PNA in both exercised and sedentary mice.- Complete cardiac autonomic blockade eliminated differences in both HR and HRV between exercised and sedentary mice.

Targeted disruption of PDE3B, but not PDE3A, protects murine heart from ischemia/reperfusion injury.

Although inhibition of cyclic nucleotide phosphodiesterase type 3 (PDE3) has been reported to protect rodent heart against ischemia/reperfusion (I/R) injury, neither the specific PDE3 isoform involved nor the underlying mechanisms have been identified. Targeted disruption of PDE3 subfamily B (PDE3B), but not of PDE3 subfamily A (PDE3A), protected mouse heart from I/R injury in vivo and in vitro, with reduced infarct size and improved cardiac function. The cardioprotective effect in PDE3B(-/-) heart was reversed by blocking cAMP-dependent PKA and by paxilline, an inhibitor of mitochondrial calcium-activated K channels, the opening of which is potentiated by cAMP/PKA signaling. Compared with WT mitochondria, PDE3B(-/-) mitochondria were enriched in antiapoptotic Bcl-2, produced less reactive oxygen species, and more frequently contacted transverse tubules where PDE3B was localized with caveolin-3. Moreover, a PDE3B(-/-) mitochondrial fraction containing connexin-43 and caveolin-3 was more resistant to Ca(2+)-induced opening of the mitochondrial permeability transition pore. Proteomics analyses indicated that PDE3B(-/-) heart mitochondria fractions were enriched in buoyant ischemia-induced caveolin-3-enriched fractions (ICEFs) containing cardioprotective proteins. Accumulation of proteins into ICEFs was PKA dependent and was achieved by ischemic preconditioning or treatment of WT heart with the PDE3 inhibitor cilostamide. Taken together, these findings indicate that PDE3B deletion confers cardioprotective effects because of cAMP/PKA-induced preconditioning, which is associated with the accumulation of proteins with cardioprotective function in ICEFs. To our knowledge, our study is the first to define a role for PDE3B in cardioprotection against I/R injury and suggests PDE3B as a target for cardiovascular therapies.

Phosphodiesterase type 3A regulates basal myocardial contractility through interacting with sarcoplasmic reticulum calcium ATPase type 2a signaling complexes in mouse heart.

RATIONALE: cAMP is an important regulator of myocardial function, and regulation of cAMP hydrolysis by cyclic nucleotide phosphodiesterases (PDEs) is a critical determinant of the amplitude, duration, and compartmentation of cAMP-mediated signaling. The role of different PDE isozymes, particularly PDE3A vs PDE3B, in the regulation of heart function remains unclear. OBJECTIVE: To determine the relative contribution of PDE3A vs PDE3B isozymes in the regulation of heart function and to dissect the molecular basis for this regulation. METHODS AND RESULTS: Compared with wild-type littermates, cardiac contractility and relaxation were enhanced in isolated hearts from PDE3A(-/-), but not PDE3B(-/-), mice. Furthermore, PDE3 inhibition had no effect on PDE3A(-/-) hearts but increased contractility in wild-type (as expected) and PDE3B(-/-) hearts to levels indistinguishable from PDE3A(-/-). The enhanced contractility in PDE3A(-/-) hearts was associated with cAMP-dependent elevations in Ca(2+) transient amplitudes and increased sarcoplasmic reticulum (SR) Ca(2+) content, without changes in L-type Ca(2+) currents of cardiomyocytes, as well as with increased SR Ca(2+)-ATPase type 2a activity, SR Ca(2+) uptake rates, and phospholamban phosphorylation in SR fractions. Consistent with these observations, PDE3 activity was reduced ≈8-fold in SR fractions from PDE3A(-/-) hearts. Coimmunoprecipitation experiments further revealed that PDE3A associates with both SR calcium ATPase type 2a and phospholamban in a complex that also contains A-kinase anchoring protein-18, protein kinase type A-RII, and protein phosphatase type 2A. CONCLUSIONS: Our data support the conclusion that PDE3A is the primary PDE3 isozyme modulating basal contractility and SR Ca(2+) content by regulating cAMP in microdomains containing macromolecular complexes of SR calcium ATPase type 2a-phospholamban-PDE3A.