Effects of regular exercise on ventricular myocyte biomechanics and KATP channel function

Cardiomyocyte Adaptation to Exercise: K+ Channels, Contractility and Ischemic Injury

Cardiovascular disease is a leading cause of morbidity and mortality, and exercise-training (TRN) is known to reduce risk factors and protect the heart from ischemia and reperfusion injury. Though the cardioprotective effects of exercise are well-documented, underlying mechanisms are not well understood. This review highlights recent findings and focuses on cardiac factors with emphasis on K+ channel control of the action potential duration (APD), β-adrenergic and adenosine regulation of cardiomyocyte function, and mitochondrial Ca2+ regulation. TRN-induced prolongation and shortening of the APD at low and high activation rates, respectively, is discussed in the context of a reduced response of the sarcolemma delayed rectifier potassium channel (IK) and increased content and activation of the sarcolemma KATP channel. A proposed mechanism underlying the latter is presented, including the phosphatidylinositol-3kinase/protein kinase B pathway. TRN induced increases in cardiomyocyte contractility and the response to adrenergic agonists are discussed. The TRN-induced protection from reperfusion injury is highlighted by the increased content and activation of the sarcolemma KATP channel and the increased phosphorylated glycogen synthase kinase-3β, which aid in preventing mitochondrial Ca2+ overload and mitochondria-triggered apoptosis. Finally, a brief section is presented on the increased incidences of atrial fibrillation associated with age and in life-long exercisers.

Cardiac repolarisation indices are associated with oxygen consumption during maximal exercise test in highly-trained cross-country skiers

The objective of this study was to test the vectorelectrocardiographic T-wave characteristics for their associations with oxygen consumption (VO2) and physical performance during a maximal cardiopulmonary exercise test (CPET) in highly trained cross-country skiers. Male highly trained cross-country skiers (n = 30) performed the maximal CPET on the bicycle ergospirometric "Oxycon Pro" system with simultaneous oxygen consumption (VO2) and electrocardiogram recording. The measurements were done at rest; the stage preceding anaerobic threshold (preAnT); peak load; and recovery. The anaerobic threshold was estimated by respiratory exchange ratio. Physical performance was estimated by maximal oxygen consumption (VO2max/kg). VECG characteristics were calculated using Kors transformation procedure. During the test, the magnitudes of T-vector, Tx and Ty components decreased until preAnT, then stayed relatively stable until peak load, and reversed during recovery. In univariate linear regression analysis, T-vector amplitude and Tx, Ty and Tz magnitudes were associated with VO2/kg during the test (p < 0.010). The baseline T-vector characteristics were not associated with physical performance. At the preAnT stage, Tx and T-vector amplitude were associated with VO2max/kg (RC 12.70, 95% CI 0.68-24.73, p = 0.039 and RC 10.64, 95% CI 1.62-19.67, p = 0.023, respectively).

Altered contractility, Ca2+ transients, and cell morphology seen in a patient-specific iPSC-CM model of Ebstein's anomaly with left ventricular noncompaction

Patients with two congenital heart diseases (CHDs), Ebstein's anomaly (EA) and left ventricular noncompaction (LVNC), suffer higher morbidity than either CHD alone. The genetic etiology and pathogenesis of combined EA/LVNC remain largely unknown. We investigated a familial EA/LVNC case associated with a variant (p.R237C) in the gene encoding Kelch-like protein 26 (KLHL26) by differentiating induced pluripotent stem cells (iPSCs) generated from affected and unaffected family members into cardiomyocytes (iPSC-CMs) and assessing iPSC-CM morphology, function, gene expression, and protein abundance. Compared with unaffected iPSC-CMs, CMs containing the KLHL26 (p.R237C) variant exhibited aberrant morphology including distended endo(sarco)plasmic reticulum (ER/SR) and dysmorphic mitochondria and aberrant function that included decreased contractions per minute, altered calcium transients, and increased proliferation. Pathway enrichment analyses based on RNASeq data indicated that the "structural constituent of muscle" pathway was suppressed, whereas the "ER lumen" pathway was activated. Taken together, these findings suggest that iPSC-CMs containing this KLHL26 (p.R237C) variant develop dysregulated ER/SR, calcium signaling, contractility, and proliferation.NEW & NOTEWORTHY We demonstrate here that iPSCs derived from patients with Ebstein's anomaly and left ventricular noncompaction, when differentiated into cardiomyocytes, display significant structural and functional changes that offer insight into disease pathogenesis, including altered ER/SR and mitochondrial morphology, contractility, and calcium signaling.

State of Knowledge on Molecular Adaptations to Exercise in Humans: Historical Perspectives and Future Directions

For centuries, regular exercise has been acknowledged as a potent stimulus to promote, maintain, and restore healthy functioning of nearly every physiological system of the human body. With advancing understanding of the complexity of human physiology, continually evolving methodological possibilities, and an increasingly dire public health situation, the study of exercise as a preventative or therapeutic treatment has never been more interdisciplinary, or more impactful. During the early stages of the NIH Common Fund Molecular Transducers of Physical Activity Consortium (MoTrPAC) Initiative, the field is well-positioned to build substantially upon the existing understanding of the mechanisms underlying benefits associated with exercise. Thus, we present a comprehensive body of the knowledge detailing the current literature basis surrounding the molecular adaptations to exercise in humans to provide a view of the state of the field at this critical juncture, as well as a resource for scientists bringing external expertise to the field of exercise physiology. In reviewing current literature related to molecular and cellular processes underlying exercise-induced benefits and adaptations, we also draw attention to existing knowledge gaps warranting continued research effort. © 2021 American Physiological Society. Compr Physiol 12:3193-3279, 2022.

Does endurance training prior to ovariectomy protect against myocardial contractility dysfunction in rats?

BACKGROUND

Endurance training prevents cardiac dysfunction induced by menopause, but to date, no studies compared the effects of endurance training prior to menopause versus estrogen replacement therapy during menopause on heart function of rats.

METHODS

Female Wistar rats aged three months were randomly assigned into three groups: Untrained ovariectomized rats (UN-OVX), untrained ovariectomized rats treated with estradiol (UN-OVX-E2), and ovariectomized rats previously exercised (EX-OVX). The endurance training protocol consisted of running on a treadmill at 60-70% of maximal aerobic capacity, 60 min per day, five days per week, for eight weeks. Estradiol replacement therapy consisted of silastic capsules containing the hormone for twelve days. After euthanasia, hearts were harvested, weighed and cardiac function was evaluated by the Langendorff technique.

RESULTS

Both cardiac contractility and relaxation indexes improved similarly in the EX-OVX and UN-OVX-E2 rats compared to UN-OVX.

CONCLUSION

Our findings reveal similar beneficial effects between endurance training previously to menopause and estradiol replacement therapy during menopause on cardiac function of rats.

Nicorandil Affects Mitochondrial Respiratory Chain Function by Increasing Complex III Activity and ROS Production in Skeletal Muscle Mitochondria

Adenosine triphosphate (ATP)-dependent potassium channels openers (K_ATP) protect skeletal muscle against function impairment through the activation of the mitochondrial K_ATP channels (mitoK_ATP). Previous reports suggest that modulators of the mitochondrial K_ATP channels have additional effects on isolated mitochondria. To determine whether the K_ATP channel opener nicorandil has non-specific effects that explain its protective effect through the mitochondrial function, chicken muscle mitochondria were isolated, and respiration rate was determined pollarographically. The activity of the electron transport chain (ETC) complexes (I-IV) was measured using a spectrophotometric method. Reactive oxygen species (ROS) levels and lipid peroxidation were assessed using flow cytometry and thiobarbituric acid assay, respectively. Both K_ATP channel opener nicorandil and K_ATP channel blocker 5-hydroxydecanoate (5-HD) decreased mitochondrial respiration; nicorandil increased complex III activity and decreased complex IV activity. The effects of nicorandil on complex III were antagonized by 5-HD. Nicorandil increased ROS levels, effect reverted by either 5-HD or the antioxidant N-2-mercaptopropionyl glycine (MPG). None of these drugs affected lipid peroxidation levels. These findings suggest that K_ATP channel opener nicorandil increases mitochondrial ROS production from complex III. This results by partially blocking electron flow in the complex IV, setting electron carriers in a more reduced state, which is favored by the increase in complex III activity by nicorandil. Overall, our study showed that nicorandil like other mitochondrial K_ATP channel openers might not act through mitoK_ATP channel activation.

Contractility of Induced Pluripotent Stem Cell-Cardiomyocytes With an MYH6 Head Domain Variant Associated With Hypoplastic Left Heart Syndrome

Hypoplastic left heart syndrome (HLHS) is a clinically and anatomically severe form of congenital heart disease; however, its etiology remains largely unknown. We previously demonstrated that genetic variants in the MYH6 gene are significantly associated with HLHS. Additionally, induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) from an HLHS-affected family trio (affected parent, unaffected parent, affected proband) carrying an MYH6-R443P head domain variant demonstrated dysmorphic sarcomere structure and increased compensatory MYH7 expression. Analysis of iPSC-CMs derived from the HLHS trio revealed that only beta myosin heavy chain expression was observed in CMs carrying the MYH6-R443P variant after differentiation day 15 (D15). Functional assessments performed between D20-D23 revealed that MYH6-R443P variant CMs contracted more slowly (40 ± 2 vs. 47 ± 2 contractions/min, P < 0.05), shortened less (5.6 ± 0.5 vs. 8.1 ± 0.7% of cell length, P < 0.05), and exhibited slower shortening rates (19.9 ± 1.7 vs. 28.1 ± 2.5 μm/s, P < 0.05) and relaxation rates (11.0 ± 0.9 vs. 19.7 ± 2.0 μm/s, P < 0.05). Treatment with isoproterenol had no effect on iPSC-CM mechanics. Using CRISPR/Cas9 gene editing technology, introduction of the R443P variant into the unaffected parent's iPSCs recapitulated the phenotype of the proband's iPSC-CMs, and conversely, correction of the R443P variant in the proband's iPSCs rescued the cardiomyogenic differentiation, sarcomere organization, slower contraction (P < 0.05) and decreased velocity phenotypes (P < 0.0001). This is the first report to identify that cardiac tissues from HLHS patients with MYH6 variants can exhibit sarcomere disorganization in atrial but not ventricular tissues. This new discovery was not unexpected, since MYH6 is expressed predominantly in the postnatal atria in humans. These findings demonstrate the feasibility of employing patient-derived iPSC-CMs, in combination with patient cardiac tissues, to gain mechanistic insight into how genetic variants can lead to HLHS. Results from this study suggest that decreased contractility of CMs due to sarcomere disorganization in the atria may effect hemodynamic changes preventing development of a normal left ventricle.

Cardiomyocyte slowly activating delayed rectifier potassium channel: regulation by exercise and β-adrenergic signaling

Results demonstrate that exercise training (TRN) downregulates ventricular IKs channel current and the channel’s responsiveness to β-agonist factors mediated by TRN-induced decline in channel subunits KCNQ1 and KCNE1 and the A-kinase anchoring protein yotiao. The reduced IKs current helps explain the TRN-induced prolongation of the action potential in basal conditions and, coupled with previously reported upregulation of the KATP channel, results in a more efficient heart that is better able to respond to beat-by-beat changes in metabolism.

Ischemic Heart Disease and Heart Failure: Role of Coronary Ion Channels

Heart failure is a complex syndrome responsible for high rates of death and hospitalization. Ischemic heart disease is one of the most frequent causes of heart failure and it is normally attributed to coronary artery disease, defined by the presence of one or more obstructive plaques, which determine a reduced coronary blood flow, causing myocardial ischemia and consequent heart failure. However, coronary obstruction is only an element of a complex pathophysiological process that leads to myocardial ischemia. In the literature, attention paid to the role of microcirculation, in the pathophysiology of ischemic heart disease and heart failure, is growing. Coronary microvascular dysfunction determines an inability of coronary circulation to satisfy myocardial metabolic demands, due to the imbalance of coronary blood flow regulatory mechanisms, including ion channels, leading to the development of hypoxia, fibrosis and tissue death, which may determine a loss of myocardial function, even beyond the presence of atherosclerotic epicardial plaques. For this reason, ion channels may represent the link among coronary microvascular dysfunction, ischemic heart disease and consequent heart failure.

Differential Modulation of IK and ICa,L Channels in High-Fat Diet-Induced Obese Guinea Pig Atria

Obesity mechanisms that make atrial tissue vulnerable to arrhythmia are poorly understood. Voltage-dependent potassium (I_K, I_Kur, and I_K1) and L-type calcium currents (I_Ca,L) are electrically relevant and represent key substrates for modulation in obesity. We investigated whether electrical remodeling produced by high-fat diet (HFD) alone or in concert with acute atrial stimulation were different. Electrophysiology was used to assess atrial electrical function after short-term HFD-feeding in guinea pigs. HFD atria displayed spontaneous beats, increased I_K (I_Kr + I_Ks) and decreased I_Ca,L densities. Only with pacing did a reduction in I_Kur and increased I_K1 phenotype emerge, leading to a further shortening of action potential duration. Computer modeling studies further indicate that the measured changes in potassium and calcium current densities contribute prominently to shortened atrial action potential duration in human heart. Our data are the first to show that multiple mechanisms (shortened action potential duration, early afterdepolarizations and increased incidence of spontaneous beats) may underlie initiation of supraventricular arrhythmias in obese guinea pig hearts. These results offer different mechanistic insights with implications for obese patients harboring supraventricular arrhythmias.