The physiological significance of a coronary stenosis differentially affects contractility and mitochondrial function in viable chronically dysfunctional myocardium

Myocardial Injury, Troponin Release and Cardiomyocyte Death in Brief Ischemia, Failure and Ventricular Remodeling

Troponin released from irreversibly injured myocytes is the gold standard biomarker for the rapid identification of an acute coronary syndrome. In acute myocardial infarction, necrotic cell death is characterized by sarcolemmal disruption in response to a critical level of energy depletion after more than 15-minutes of ischemia. While troponin I and T are highly specific for cardiomyocyte death, high-sensitivity assays have demonstrated that measurable circulating levels of troponin are present in the majority of normal subjects. In addition, transient as well as chronic elevations have been demonstrated in many disease states not clearly associated with myocardial ischemia. The latter observations have given rise to the clinical concept of myocardial injury. This review will summarize evidence supporting the notion that circulating troponin levels parallel the extent of myocyte apoptosis in normal ventricular remodeling and in pathophysiological conditions not associated with infarction or necrosis. It will review the evidence that myocyte apoptosis can be accelerated by both diastolic strain from elevated ventricular preload as well as systolic strain from dyskinesis after brief episodes of ischemia too short to cause a critical level of myocyte energy depletion. We then show how chronic, low rates of myocyte apoptosis from endogenous myocyte turnover, repetitive ischemia or repetitive elevations in LV diastolic pressure can lead to significant myocyte loss in the absence of neurohormonal stimulation. Finally, we posit that the differential response to strain-induced injury in heart failure may determine whether progressive myocyte loss and HFrEF or interstitial fibrosis and HFpEF become the heart failure phenotype.

Myocardial stunning and hibernation revisited

Unlike acute myocardial infarction with reperfusion, in which infarct size is the end point reflecting irreversible injury, myocardial stunning and hibernation result from reversible myocardial ischaemia-reperfusion injury, and contractile dysfunction is the obvious end point. Stunned myocardium is characterized by a disproportionately long-lasting, yet fully reversible, contractile dysfunction that follows brief bouts of myocardial ischaemia. Reperfusion precipitates a burst of reactive oxygen species formation and alterations in excitation-contraction coupling, which interact and cause the contractile dysfunction. Hibernating myocardium is characterized by reduced regional contractile function and blood flow, which both recover after reperfusion or revascularization. Short-term myocardial hibernation is an adaptation of contractile function to the reduced blood flow such that energy and substrate metabolism recover during the ongoing ischaemia. Chronic myocardial hibernation is characterized by severe morphological alterations and altered expression of metabolic and pro-survival proteins. Myocardial stunning is observed clinically and must be recognized but is rarely haemodynamically compromising and does not require treatment. Myocardial hibernation is clinically identified with the use of imaging techniques, and the myocardium recovers after revascularization. Several trials in the past two decades have challenged the superiority of revascularization over medical therapy for symptomatic relief and prognosis in patients with chronic coronary syndromes. A better understanding of the pathophysiology of myocardial stunning and hibernation is important for a more precise indication of revascularization and its consequences. Therefore, this Review summarizes the current knowledge of the pathophysiology of these characteristic reperfusion phenomena and highlights their clinical implications.

Quantitative proteomic and phosphoproteomic profiling of ischemic myocardial stunning in swine

Despite decades of research on the pathophysiology of myocardial stunning, protein changes and/or phosphorylation status underlying alterations in cardiac function/structure remain inadequately understood. Here, we utilized comprehensive and quantitative proteomic and phosphoproteomic approaches to explore molecular mechanisms of myocardial stunning in swine. The closed-chest swine (n = 5 pigs) were subjected to a 10-min left anterior descending coronary artery (LAD) occlusion producing regional myocardial stunning. Tissues from the ischemic LAD region and a remote nonischemic area of the left ventricle were collected 1 h after reperfusion. Ion current-based proteomics (IonStar) and quantitative phosphoproteomics were employed in parallel to identify alterations in protein level and site-specific phosphorylation changes. A novel swine heart protein database exhibiting high accuracy and low redundancy was developed here to facilitate comprehensive study. Further informatic investigations identified potential protein-protein interactions in stunned myocardium. In total, we quantified 2,099 protein groups and 4,699 phosphorylation sites with only 0.4% missing values. Proteomic analyses revealed downregulation of contractile function and extracellular matrix remodeling. Meanwhile, alterations in phosphorylation linked with contractile dysfunction and apoptotic cell death were uncovered. NetworKIN/STRING analysis predicted regulatory kinases responsible for altered phosphosites, such as protein kinase C-mediated phosphorylation of cardiac troponin I-S199 and CaMKII-mediated phosphorylation of phospholamban-T17. In summary, the ion current-based proteomics and phosphoproteomics reliably identified novel alterations in protein content and phosphorylation contributing to contractile dysfunction, extracellular matrix (ECM) damage, and programmed cell death in stunned myocardium, which corroborate well with our physiological observations. Moreover, this work developed a comprehensive database of the swine heart proteome, a highly valuable resource for future translational research in porcine models with cardiovascular diseases.NEW & NOTEWORTHY We first used ion current-based proteomics and phosphoproteomics to reliably identify novel alterations in protein expression and phosphorylation contributing to contractile dysfunction, extracellular matrix (ECM) damage, and programmed cell death in stunned myocardium and developed a comprehensive swine heart-specific proteome database, which provides a valuable resource for future research in porcine models of cardiovascular diseases.

Influence of increased heart rate and aortic pressure on resting indices of functional coronary stenosis severity

Baseline assessment of functional stenosis severity has been proposed as a practical alternative to hyperemic indices. However, intact autoregulation mechanisms may affect intracoronary hemodynamics. The aim of this study was to investigate the effect of changes in aortic pressure (Pa) and heart rate (HR) on baseline coronary hemodynamics and functional stenosis assessment. In 15 patients (55 ± 3% diameter stenosis) Pa, intracoronary pressure (Pd) and flow velocity were obtained at control, and during atrial pacing at 120 bpm, increased Pa (+30 mmHg) with intravenous phenylephrine (PE), and elevated Pa while pacing at sinus heart rate (PE + sHR). We derived rate pressure product (RPP = systolic Pa × HR), baseline microvascular resistance (BMR = Pd/velocity), and stenosis resistance [BSR = (Pa − Pd)/velocity] as well as whole-cycle Pd/Pa. Tachycardia (120 ± 1 bpm) raised RPP by 74% vs. control. Accordingly, BMR decreased by 27% (p < 0.01) and velocity increased by 36% (p < 0.05), while Pd/Pa decreased by 0.05 ± 0.02 (p < 0.05) and BSR remained similar to control. Raising Pa to 121 ± 3 mmHg (PE) with concomitant reflex bradycardia increased BMR by 26% (p < 0.001) at essentially unchanged RPP and velocity. Consequently, BSR and Pd/Pa were only marginally affected. During PE + sHR, velocity increased by 21% (p < 0.01) attributable to a 46% higher RPP (p < 0.001). However, BMR, BSR, and Pd/Pa remained statistically unaffected. Nonetheless, the interventions tended to increase functional stenosis severity, causing Pd/Pa and BSR of borderline lesions to cross the diagnostic threshold. In conclusion, coronary microvascular adaptation to physiological conditions affecting metabolic demand at rest influences intracoronary hemodynamics, which may lead to altered basal stenosis indices used for clinical decision-making.

Comprehensive mathematical model of oxidative phosphorylation valid for physiological and pathological conditions

We developed a mathematical model of oxidative phosphorylation (OXPHOS) that allows for a precise description of mitochondrial function with respect to the respiratory flux and the ATP production. The model reproduced flux-force relationships under various experimental conditions (state 3 and 4, uncoupling, and shortage of respiratory substrate) as well as time courses, exhibiting correct P/O ratios. The model was able to reproduce experimental threshold curves for perturbations of the respiratory chain complexes, the F₁ F₀ -ATP synthase, the ADP/ATP carrier, the phosphate/OH carrier, and the proton leak. Thus, the model is well suited to study complex interactions within the OXPHOS system, especially with respect to physiological adaptations or pathological modifications, influencing substrate and product affinities or maximal catalytic rates. Moreover, it could be a useful tool to study the role of OXPHOS and its capacity to compensate or enhance physiopathologies of the mitochondrial and cellular energy metabolism.

Reduced mitochondrial respiration in the ischemic as well as in the remote nonischemic region in postmyocardial infarction remodeling

Scarring and remodeling of the left ventricle (LV) after myocardial infarction (MI) results in ischemic cardiomyopathy with reduced contractile function. Regional differences related to persisting ischemia may exist. We investigated the hypothesis that mitochondrial function and structure is altered in the myocardium adjacent to MI with reduced perfusion (MI_adjacent) and less so in the remote, nonischemic myocardium (MI_remote). We used a pig model of chronic coronary stenosis and MI (n = 13). Functional and perfusion MR imaging 6 wk after intervention showed reduced ejection fraction and increased global wall stress compared with sham-operated animals (Sham; n = 14). Regional strain in MI_adjacent was reduced with reduced contractile reserve; in MI_remote strain was also reduced but responsive to dobutamine and perfusion was normal compared with Sham. Capillary density was unchanged. Cardiac myocytes isolated from both regions had reduced basal and maximal oxygen consumption rate, as well as through complex I and II, but complex IV activity was unchanged. Reduced respiration was not associated with detectable reduction of mitochondrial density. There was no significant change in AMPK or glucose transporter expression levels, but glycogen content was significantly increased in both MI_adjacent and MI_remote Glycogen accumulation was predominantly perinuclear; mitochondria in this area were smaller but only in MI_adjacent where also subsarcolemmal mitochondria were smaller. In conclusion, after MI reduction of mitochondrial respiration and glycogen accumulation occur in all LV regions suggesting that reduced perfusion does not lead to additional specific changes and that increased hemodynamic load is the major driver for changes in mitochondrial function.

Pioglitazone increases PGC1-α signaling within chronically ischemic myocardium

The peroxisome proliferator-activated receptor (PPAR)-γ drug pioglitazone (PIO) has been shown to protect tissue against oxidant stress. In a swine model of chronic myocardial ischemia, we tested whether PIO increases PGC1-α signaling and the expression of mitochondrial antioxidant peptides. Eighteen pigs underwent a thoracotomy with placement of a fixed constrictor around the LAD artery. At 8 weeks, diet was supplemented with either PIO (3 mg/kg) or placebo for 4 weeks. Regional myocardial function and blood flow were determined at the time of the terminal study. PGC1-α expression was quantified from nuclear membranes by gels and respiration, oxidant stress markers and proteomics by iTRAQ were determined from isolated mitochondria. In the chronically ischemic LAD region, wall thickening from the PIO and control groups was 42 ± 6 and 45 ± 5 %, respectively (NS) with no intergroup differences in basal blood flow (0.72 ± 0.04 versus 0.74 ± 0.04 ml/min g, respectively; NS). In the PIO group, the expression of nuclear bound PGC1-α was higher (11.3 ± 2.6 versus 4.4 ± 1.4 AU; P < 0.05) and the content of mitochondrial antioxidant peptides including superoxide dismutase 2, aldose reductase, glutathione S-transferase and thioredoxin reductase were greater than controls. Although isolated mitochondria from the PIO group showed lower state 3 respiration (102 ± 13 versus 161 ± 22 nmol/min mg; P < 0.05), no differences in oxidant stress were noted by protein carbonyl (1.7 ± 0.7 versus 1.1 ± 0.1 nmol/mg). Chronic pioglitazone does not reduce regional myocardial blood flow or function in a swine model of chronic myocardial ischemia, but may have an important role in increasing expression of antioxidant proteins through PGC1-α signaling.

Revascularization of chronic hibernating myocardium stimulates myocyte proliferation and partially reverses chronic adaptations to ischemia

BACKGROUND

The time course and extent of recovery after revascularization of viable dysfunctional myocardium are variable. Although fibrosis is a major determinant, myocyte structural and molecular remodeling may also play important roles.

OBJECTIVES

This study sought to determine whether persistent myocyte loss and/or irreversibility of protein changes that develop in hibernating myocardium have an impact on functional recovery in the absence of infarction.

METHODS

Swine implanted with a chronic left anterior descending artery (LAD) stenosis to produce hibernating myocardium underwent percutaneous revascularization, with serial functional recovery evaluated for 1 month (n = 12). Myocardial tissue was evaluated to assess myocyte size, nuclear density, and proliferation indexes in comparison with those of normal animals and nonrevascularized controls. Proteomic analysis by 2-dimensional differential in-gel electrophoresis was used to determine the reversibility of molecular adaptations of hibernating myocytes.

RESULTS

At 3 months, physiological features of hibernating myocardium were confirmed, with depressed LAD wall thickening and no significant infarction. Revascularization normalized LAD flow reserve, with no immediate change in LAD wall thickening. Regional LAD wall thickening slowly improved but remained depressed 1 month post-percutaneous coronary intervention. Surprisingly, revascularization was associated with histological evidence of myocytes re-entering the growth phase of the cell cycle and increases in the number of c-Kit(+) cells. Myocyte nuclear density returned to normal, whereas regional myocyte hypertrophy regressed. Proteomic analysis demonstrated heterogeneous effects of revascularization. Up-regulated stress and cytoskeletal proteins normalized, whereas reduced contractile and metabolic proteins persisted.

CONCLUSIONS

Delayed recovery of hibernating myocardium in the absence of scar may reflect persistent reductions in the amounts of contractile and metabolic proteins. Although revascularization appeared to stimulate myocyte proliferation, the persistence of small immature myocytes may have contributed to delayed functional recovery.

Reproducible ion-current-based approach for 24-plex comparison of the tissue proteomes of hibernating versus normal myocardium in swine models

Hibernating myocardium is an adaptive response to repetitive myocardial ischemia that is clinically common, but the mechanism of adaptation is poorly understood. Here we compared the proteomes of hibernating versus normal myocardium in a porcine model with 24 biological replicates. Using the ion-current-based proteomic strategy optimized in this study to expand upon previous proteomic work, we identified differentially expressed proteins in new molecular pathways of cardiovascular interest. The methodological strategy includes efficient extraction with detergent cocktail; precipitation/digestion procedure with high, quantitative peptide recovery; reproducible nano-LC/MS analysis on a long, heated column packed with small particles; and quantification based on ion-current peak areas. Under the optimized conditions, high efficiency and reproducibility were achieved for each step, which enabled a reliable comparison of 24 the myocardial samples. To achieve confident discovery of differentially regulated proteins in hibernating myocardium, we used highly stringent criteria to define “quantifiable proteins”. These included the filtering criteria of low peptide FDR and S/N > 10 for peptide ion currents, and each protein was quantified independently from ≥2 distinct peptides. For a broad methodological validation, the quantitative results were compared with a parallel, well-validated 2D-DIGE analysis of the same model. Excellent agreement between the two orthogonal methods was observed (R = 0.74), and the ion-current-based method quantified almost one order of magnitude more proteins. In hibernating myocardium, 225 significantly altered proteins were discovered with a low false-discovery rate (∼3%). These proteins are involved in biological processes including metabolism, apoptosis, stress response, contraction, cytoskeleton, transcription, and translation. This provides compelling evidence that hibernating myocardium adapts to chronic ischemia. The major metabolic mechanisms include a down-regulation of mitochondrial respiration and an increase in glycolysis. Meanwhile, cardioprotective and cytoskeletal proteins are increased, while cardiomyocyte contractile proteins are reduced. These intrinsic adaptations to regional ischemia maintain long-term cardiomyocyte viability at the expense of contractile function.