Orai1–STIM1 Regulates Increased Ca2+ Mobilization, Leading to Contractile Duchenne Muscular Dystrophy Phenotypes in Patient-Derived Induced Pluripotent Stem Cells

Ca2+ overload is one of the factors leading to Duchenne muscular dystrophy (DMD) pathogenesis. However, the molecular targets of dystrophin deficiency-dependent Ca2+ overload and the correlation between Ca2+ overload and contractile DMD phenotypes in in vitro human models remain largely elusive. In this study, we utilized DMD patient-derived induced pluripotent stem cells (iPSCs) to differentiate myotubes using doxycycline-inducible MyoD overexpression, and searched for a target molecule that mediates dystrophin deficiency-dependent Ca2+ overload using commercially available chemicals and siRNAs. We found that several store-operated Ca2+ channel (SOC) inhibitors effectively prevented Ca2+ overload and identified that STIM1–Orai1 is a molecular target of SOCs. These findings were further confirmed by demonstrating that STIM1–Orai1 inhibitors, CM4620, AnCoA4, and GSK797A, prevented Ca2+ overload in dystrophic myotubes. Finally, we evaluated CM4620, AnCoA4, and GSK7975A activities using a previously reported model recapitulating a muscle fatigue-like decline in contractile performance in DMD. All three chemicals ameliorated the decline in contractile performance, indicating that modulating STIM1–Orai1-mediated Ca2+ overload is effective in rescuing contractile phenotypes. In conclusion, SOCs are major contributors to dystrophin deficiency-dependent Ca2+ overload through STIM1–Orai1 as molecular mediators. Modulating STIM1–Orai1 activity was effective in ameliorating the decline in contractile performance in DMD.

Potassium-induced potentiation of subtetanic force in rat skeletal muscles: influences of β2-activation, lactic acid, and temperature

Purpose: Moderate elevations of [K+]o occur during exercise and have been shown to potentiate force during contractions elicited with subtetanic frequencies. Here, we investigated whether lactic acid (reduced chloride conductance), β2-adrenoceptor activation, and increased temperature would influence the potentiating effect of potassium in slow- and fast-twitch muscle. Methods: Isometric contractions were elicited by electrical stimulation at various frequencies in isolated rat soleus and extensor digitorum longus (EDL) muscles incubated at normal (4 mM) or elevated K+, in combination with either salbutamol (5 μM), lactic acid (18.1 mM), 9-AC (25 μM) or increased temperature (30 to 35°C). Results: Elevating [K+] from 4 mM to 7 mM (soleus) and 10 mM (EDL) potentiated isometric twitch and subtetanic force while slightly reducing tetanic. In EDL, salbutamol further augmented twitch force (+27±3 %, P<0.001) and subtetanic force (+22±4 %, P<0.001). In contrast, salbutamol reduced subtetanic force (-28±6 %, P<0.001) in soleus muscles. Lactic acid and 9-AC had no significant effects on isometric force of muscles already exposed to moderate elevations of [K+]o. The potentiating effect of elevated [K+]o was still well maintained at 35°C. Conclusion: Addition of salbutamol exerts a further force-potentiating effect in fast-twitch but not in slow-twitch muscles already potentiated by moderately elevated [K+]o, whilst neither lactic acid, 9-AC nor increased temperature exerts any further augmentation. However, the potentiating effect of elevated [K+]o was still maintained in the presence of these, thus emphasizing the positive influence of moderately elevated [K+]o for contractile performance during exercise.

Different Impact of Heart Rate Variability in the Deep Cerebral and Central Hemodynamics at Rest: An in silico Investigation

Background: Heart rate variability (HRV), defined as the variability between consecutive heartbeats, is a surrogate measure of cardiac vagal tone. It is widely accepted that a decreased HRV is associated to several risk factors and cardiovascular diseases. However, a possible association between HRV and altered cerebral hemodynamics is still debated, suffering from HRV short-term measures and the paucity of high-resolution deep cerebral data. We propose a computational approach to evaluate the deep cerebral and central hemodynamics subject to physiological alterations of HRV in an ideal young healthy patient at rest.Methods: The cardiovascular-cerebral model is composed by electrical components able to reproduce the response of the different cardiovascular regions and their features. The model was validated over more than thirty studies and recently exploited to understand the hemodynamic mechanisms between cardiac arrythmia and cognitive deficit. Three configurations (baseline, increased HRV, and decreased HRV) are built based on the standard deviation (SDNN) of RR beats. For each configuration, 5,000 RR beats are simulated to investigate the occurrence of extreme values, alteration of the regular hemodynamics pattern, and variation of mean perfusion/pressure levels.Results: In the cerebral circulation, our results show that HRV has overall a stronger impact on pressure than flow rate mean values but similarly alters pressure and flow rate in terms of extreme events. By comparing reduced and increased HRV, this latter induces a higher probability of altered mean and extreme values, and is therefore more detrimental at distal cerebral level. On the contrary, at central level a decreased HRV induces a higher cardiac effort without improving the mechano-contractile performance, thus overall reducing the heart efficiency.Conclusions: Present results suggest that: (i) the increase of HRV per se does not seem to be sufficient to trigger a better cerebral hemodynamic response; (ii) by accounting for both central and cerebral circulations, the optimal HRV configuration is found at baseline. Given the relation inversely linking HRV and HR, the presence of this optimal condition can contribute to explain why the mean HR of the general population settles around the baseline value (70 bpm).

Lower-limb muscle function is influenced by changing mechanical demands in cycling

Although cycling is often considered a seemingly simple, reciprocal task, muscles must adapt their function to satisfy changes in mechanical demands induced by higher crank torques and faster pedalling cadences. We examined if muscle function was sensitive to these changes in mechanical demands across a wide range of pedalling conditions. We collected experimental data of cycling where crank torque and pedalling cadence were independently varied from 13-44 Nm and 60-140 RPM. These data were used in conjunction with musculoskeletal simulations and a recently developed functional index-based approach to characterise the role of the human lower-limb muscles. We found that in muscles that generate most of the mechanical power and work during cycling, greater crank torque induced shifts towards greater muscle activation, greater positive muscle-tendon unit (MTU) work and a more motor-like function, particularly in the limb extensors. Conversely, with faster pedalling cadence, the same muscles exhibited a phase advance in muscle activity prior to crank top dead centre, which led to greater negative MTU power and work and shifted the muscles to contract with more spring-like behaviour. Our results illustrate the capacity for muscles to adapt their function to satisfy the mechanical demands of the task, even during highly constrained reciprocal tasks such as cycling. Understanding how muscles shift their contractile performance under varied mechanical and environmental demands may inform decisions on how to optimise pedalling performance and to design targeted cycling rehabilitation therapies for muscle-specific injuries or deficits.

Abstract 15877: Chamber Volumes and Deformation Measures are Abnormal in Chemotherapy-Naïve Cancer Patients: Potential Implications for Surveillance and Definitions of Cardiotoxicity

Background: Cancer Therapeutics-related Cardiac Dysfunction (CTRCD) is defined by an interval drop in contractile performance to below reference lower limits of normal. This definition assumes healthy reference data appropriately represent referral populations with active cancer. However, the influence of active cancer on cardiac chamber volumes and contractile performance has not been established. Using cardiac magnetic resonance (CMR), we studied chamber volume- and deformation-based markers in ~400 cancer patients with comparison to ~100 healthy controls. Methods: 394 active cancer patients referred for first-time anthracycline-based chemotherapy and 102 healthy volunteers (HV) were recruited. Both underwent identical CMR protocols with quantification of chamber volumes and ejection fraction (EF). Left ventricular (LV) mechanics were also assessed by 3D myocardial deformation analysis (3D-MDA), providing global longitudinal, circumferential, radial and principal peak-systolic strain amplitude and rate. Results: The mean age was 53.8±13 years (78% female), with 64% having breast cancer and 36% lymphoma. Table 1 summarizes CMR findings of cancer patients versus HV, stratified by sex. Chamber volumes were significantly smaller while LV mass was significantly greater versus HV. LV EF and global longitudinal (GLS) were similar. However, cancer patients demonstrated significantly higher radial, circumferential and maximal principal strain amplitude. Peak-systolic strain rates were also consistently elevated. Conclusion: Chemotherapy-naïve cancer patients have smaller chamber volumes, greater LV mass, and higher radial, circumferential, and maximal principal strain versus healthy subjects. LV EF and peak GLS remain similar, and therefore are most appropriate to define CTRCD. However, an altered state of cardiac health is apparent by all other CMR-based markers, reflecting a unique cardiac phenotype of patients with active cancer.

Assessment of left ventricular systolic function by non-invasive pressure-strain loop area in young male strength athletes

Background The health of athletes has been recognized as a worldwide public concern with more reported sudden cardiac deaths (SCD). Therefore, early detection of abnormal heart function in athletes can help reduce the risk of exercise. A novel valid non-invasive method to evaluate left ventricular (LV) myocardial work (MW) using LV pressure-strain loop (PSL), was used in this paper to explore LV systolic function in young male strength athletes. Methods Thirty-six professional young male strength athletes (the athlete group) and 32 healthy, age-matched young men (the control group) were involved in the study. The LVMW parameters were calculated as the area of PSL by two-dimensional speckle tracking echocardiography (2D-STE) and peak systolic LV pressure. The differences between two groups of data and the predictive efficacy of MW parameters for LV systolic function were analyzed. Results The athlete group had significantly higher values of global wasted myocardial work (GWW) and peak strain dispersion (PSD) than did the control group (P<0.05). Global myocardial work index (GWI), global constructive myocardial work (GCW) and global longitudinal strain (GLS) were lower in the athlete group than that in the control group, although statistical significance was not reached (P>0.05). Due to the proportion of GWW and GCW, statistically significant reduction was found in global myocardial work efficiency (GWE) in the athlete group. Conventional echocardiography parameters were well correlated with GWW and GWE (P<0.05). The best predictor of LV myocardial contractile performance in the athletes using receiver operating characteristic curve (ROC) was GWE, with the area under ROC (AUC) of 0.733, sensitivity of 83.3% and specificity of 59.4%. Conclusions Subclinical changes have appeared in the hearts of young male strength athletes after long-term intensive exercise and LVMW parameters by PSL play an important role in the evaluation of athlete’s LV contractile performance.

Assessment of left ventricular systolic function by non-invasive pressure-strain loop area in young male strength athletes

BackgroundThe health of athletes has been recognized as a worldwide public concern with more reported sudden cardiac deaths (SCD). Therefore, early detection of abnormal heart function in athletes can help reduce the risk of exercise. A novel valid non-invasive method to evaluate left ventricular (LV) myocardial work (MW) using LV pressure-strain loop (PSL), was used in this paper to explore LV systolic function in young male strength athletes.Methods 36 professional young male strength athletes (the athlete group) and 32 healthy, age-matched young men (the control group) were involved in the study. The LVMW parameters were calculated as the area of PSL by two-dimensional speckle tracking echocardiography (2D-STE) and peak systolic LV pressure. The differences between two groups of data and the predictive efficacy of MW parameters for LV systolic function were analyzed.Results The athlete group had significantly higher values of global wasted myocardial work (GWW) and peak strain dispersion (PSD) than did the control group (P＜0.05).Global myocardial work index (GWI), global constructive myocardial work (GCW) and global longitudinal strain (GLS) were lower in the athlete group than that in the control group, although statistical significance was not reached (P＞0.05). Due to the proportion of GWW and GCW, statistically significant reduction was found in global myocardial work efficiency (GWE) in the athlete group. Conventional echocardiography parameters were well correlated with GWW and GWE (P＜0.05). The best predictor of LV myocardial contractile performance in the athletes using receiver operating characteristic curve (ROC) was GWE, with the area under ROC (AUC) of 0.733, sensitivity of 83.3% and specificity of 59.4%. ConclusionsSubclinical changes have appeared in the hearts of young male strength athletes after long-term intensive exercise and LVMW parameters by PSL play an important role in the evaluation of athlete’s LV contractile performance.