The adult heart responds to increased workload with physiologic hypertrophy, cardiac stem cell activation, and new myocyte formation.

AIMS: It is a dogma of cardiovascular pathophysiology that the increased cardiac mass in response to increased workload is produced by the hypertrophy of the pre-existing myocytes. The role, if any, of adult-resident endogenous cardiac stem/progenitor cells (eCSCs) and new cardiomyocyte formation in physiological cardiac remodelling remains unexplored. METHODS AND RESULTS: In response to regular, intensity-controlled exercise training, adult rats respond with hypertrophy of the pre-existing myocytes. In addition, a significant number (∼7%) of smaller newly formed BrdU-positive cardiomyocytes are produced by the exercised animals. Capillary density significantly increased in exercised animals, balancing cardiomyogenesis with neo-angiogenesis. c-kit(pos) eCSCs increased their number and activated state in exercising vs. sedentary animals. c-kit(pos) eCSCs in exercised hearts showed an increased expression of transcription factors, indicative of their commitment to either the cardiomyocyte (Nkx2.5(pos)) or capillary (Ets-1(pos)) lineages. These adaptations were dependent on exercise duration and intensity. Insulin-like growth factor-1, transforming growth factor-ß1, neuregulin-1, bone morphogenetic protein-10, and periostin were significantly up-regulated in cardiomyocytes of exercised vs. sedentary animals. These factors differentially stimulated c-kit(pos) eCSC proliferation and commitment in vitro, pointing to a similar role in vivo. CONCLUSION: Intensity-controlled exercise training initiates myocardial remodelling through increased cardiomyocyte growth factor expression leading to cardiomyocyte hypertrophy and to activation and ensuing differentiation of c-kit(pos) eCSCs. This leads to the generation of new myocardial cells. These findings highlight the endogenous regenerative capacity of the adult heart, represented by the eCSCs, and the fact that the physiological cardiac adaptation to exercise stress is a combination of cardiomyocyte hypertrophy and hyperplasia (cardiomyocytes and capillaries).

Mechanical dyssynchrony precedes QRS widening in ATP-sensitive K⁺ channel-deficient dilated cardiomyopathy.

BACKGROUND: Contractile discordance exacerbates cardiac dysfunction, aggravating heart failure outcome. Dissecting the genesis of mechanical dyssynchrony would enable an early diagnosis before advanced disease. METHODS AND RESULTS: High-resolution speckle-tracking echocardiography was applied in a knockout murine surrogate of adult-onset human cardiomyopathy caused by mutations in cardioprotective ATP-sensitive K(+) (K(ATP)) channels. Preceding the established criteria of cardiac dyssynchrony, multiparametric speckle-based strain resolved nascent erosion of dysfunctional regions within cardiomyopathic ventricles of the K(ATP) channel-null mutant exposed to hemodynamic stress. Not observed in wild-type counterparts, intraventricular disparity in wall motion, validated by the degree, direction, and delay of myocardial speckle patterns, unmasked the disease substrate from asymptomatic to overt heart failure. Mechanical dyssynchrony preceded widening of the QRS complex and exercise intolerance and progressed into global myocardial discoordination and decompensated cardiac pump function, precipitating a low output syndrome. CONCLUSIONS: The present study, with the use of high-resolution imaging, prospectively resolved the origin and extent of intraventricular motion disparity in a K(ATP) channel-knockout model of dilated cardiomyopathy. Mechanical dyssynchrony established as an early marker of cardiomyopathic disease offers novel insight into the pathodynamics of dyssynchronous heart failure.

Endogenous cardiac stem cell activation by insulin-like growth factor-1/hepatocyte growth factor intracoronary injection fosters survival and regeneration of the infarcted pig heart.

OBJECTIVES: The purpose of this study was to test the ability of insulin-like growth factor (IGF)-1/hepatocyte growth factor (HGF) to activate resident endogenous porcine cardiac stem/progenitor cells (epCSCs) and to promote myocardial repair through a clinically applicable intracoronary injection protocol in a pig model of myocardial infarction (MI) relevant to human disease. BACKGROUND: In rodents, cardiac stem/progenitor cell (CSC) transplantation as well as in situ activation through intramyocardial injection of specific growth factors has been shown to result in myocardial regeneration after acute myocardial infarction (AMI). METHODS: Acute MI was induced in pigs by a 60-min percutaneous transluminal coronary angiography left anterior descending artery occlusion. The IGF-1 and HGF were co-administered through the infarct-related artery in a single dose (ranging from 0.5 to 2 µg HGF and 2 to 8 µg IGF-1) 30 min after coronary reperfusion. Pigs were sacrificed 21 days later for dose-response relationship evaluation by immunohistopathology or 2 months later for cardiac function evaluation by cardiac magnetic resonance imaging. RESULTS: The IGF-1/HGF activated c-kit positive-CD45 negative epCSCs and increased their myogenic differentiation in vitro. The IGF-1/HGF, in a dose-dependent manner, improved cardiomyocyte survival, and reduced fibrosis and cardiomyocyte reactive hypertrophy. It significantly increased c-kit positive-CD45 negative epCSC number and fostered the generation of new myocardium (myocytes and microvasculature) in infarcted and peri-infarct/border regions at 21 and 60 days after AMI. The IGF-1/HGF reduced infarct size and improved left ventricular function at 2 months after AMI. CONCLUSIONS: In an animal model of AMI relevant to the human disease, intracoronary administration of IGF-1/HGF is a practical and effective strategy to reduce pathological cardiac remodeling, induce myocardial regeneration, and improve ventricular function.

Acute beta-adrenergic overload produces myocyte damage through calcium leakage from the ryanodine receptor 2 but spares cardiac stem cells.

A hyperadrenergic state is a seminal aspect of chronic heart failure. Also, "Takotsubo stress cardiomyopathy," is associated with increased plasma catecholamine levels. The mechanisms of myocyte damage secondary to excess catecholamine exposure as well as the consequence of this neurohumoral burst on cardiac stem cells (CSCs) are unknown. Cardiomyocytes and CSCs were exposed to high doses of isoproterenol (ISO), in vivo and in vitro. Male Wistar rats received a single injection of ISO (5 mg kg-1) and were sacrificed 1, 3, and 6 days later. In comparison with controls, LV function was impaired in rats 1 day after ISO and started to improve at 3 days. The fraction of dead myocytes peaked 1 day after ISO and decreased thereafter. ISO administration resulted in significant ryanodine receptor 2 (RyR2) hyperphosphorylation and RyR2-calstabin dissociation. JTV519, a RyR2 stabilizer, prevented the ISO-induced death of adult myocytes in vitro. In contrast, CSCs were resistant to the acute neurohumoral overload. Indeed, CSCs expressed a decreased and inverted complement of beta1/beta2-adrenoreceptors and absence of RyR2, which may explain their survival to ISO insult. Thus, a single injection of ISO causes diffuse myocyte death through Ca2+ leakage secondary to the acutely dysfunctional RyR2. CSCs are resistant to the noxious effects of an acute hyperadrenergic state and through their activation participate in the response to the ISO-induced myocardial injury. The latter could contribute to the ability of the myocardium to rapidly recover from acute hyperadrenergic damage.