Essential role of diastolic oscillatory potentials in adrenergic control of guinea pig sino-atrial node discharge

Background: The diastolic oscillatory after-potential Vos and pre-potential ThVos play an essential role in the pacemaker mechanism of sino-atrial node (SAN). The aim of this study was to investigate whether these oscillatory potentials are also involved in adrenergic control of SAN discharge. Methods: Vos and ThVos were visualized by superfusing guinea pig SAN in high [K+]o. The actions of adrenergic agonists on oscillatory potentials were studied by means of a microelectrode technique. Statistical significance was determined by means of Student's paired t-test. Results: In non-spontaneous SAN, norepinephrine (NE) decreased the resting potential into a voltage range ("oscillatory zone") where increasingly larger ThVos appeared and initiated spontaneous discharge. In slowly discharging SAN, NE gradually increased the rate by increasing the amplitude and slope of earlier-occurring ThVos and of Vos until these oscillations fused with initial diastolic depolarization (DD1). In the presence of NE, sudden fast rhythms were initiated by large Vos that entered a more negative oscillatory zone and initiated a large ThVos. Recovery from NE exposure involved the converse changes. The β-adrenergic agonist isoproterenol had similar actions. Increasing calcium load by decreasing high [K+]o, by fast drive or by recovery in Tyrode solution led to growth of Vos and ThVos which abruptly fused when a fast sudden rhythm was induced. Low [Ca2+]o antagonized the adrenergic actions. Cesium (a blocker of If) induced spontaneous discharge in quiescent SAN through ThVos. In spontaneous SAN, Cs+increased Vos and ThVos, thereby increasing the rate. Cs+ did not hinder the positive chronotropic action of NE. Barium increased the rate, as Cs+ did. Conclusion: Adrenergic agonists: (i) initiate SAN discharge by decreasing the resting potential and inducing ThVos; (ii) gradually accelerate SAN rate by predominantly increasing size and slope of earlier and more negative ThVos; (iii) can induce sudden fast rhythms through the abrupt fusion of large Vos with large ThVos; (iv) increase Vos and ThVosby increasing cellular calcium; and (v) do not modify the oscillatory potentials by means of the hyperpolarization-activated current If. The results provide evidence for novel mechanisms by which the SAN dominant pacemaker activity is initiated and enhanced by adrenergic agonists

Phenotypically Heterogeneous Podoplanin-expressing Cell Populations Are Associated with the Lymphatic Vessel Growth and Fibrogenic Responses in the Acutely and Chronically Infarcted Myocardium

Cardiac lymphatic vasculature undergoes substantial expansion in response to myocardial infarction (MI). However, there is limited information on the cellular mechanisms mediating post-MI lymphangiogenesis and accompanying fibrosis in the infarcted adult heart. Using a mouse model of permanent coronary artery ligation, we examined spatiotemporal changes in the expression of lymphendothelial and mesenchymal markers in the acutely and chronically infarcted myocardium. We found that at the time of wound granulation, a three-fold increase in the frequency of podoplanin-labeled cells occurred in the infarcted hearts compared to non-operated and sham-operated counterparts. Podoplanin immunoreactivity detected LYVE-1-positive lymphatic vessels, as well as masses of LYVE-1-negative cells dispersed between myocytes, predominantly in the vicinity of the infarcted region. Podoplanin-carrying populations displayed a mesenchymal progenitor marker PDGFRalpha, and intermittently expressed Prox-1, a master regulator of the lymphatic endothelial fate. At the stages of scar formation and maturation, concomitantly with the enlargement of lymphatic network in the injured myocardium, the podoplanin-rich LYVE-1-negative multicellular assemblies were apparent in the fibrotic area, aligned with extracellular matrix deposits, or located in immediate proximity to activated blood vessels with high VEGFR-2 content. Of note, these podoplanin-containing cells acquired the expression of PDGFRbeta or a hematoendothelial epitope CD34. Although Prox-1 labeling was abundant in the area affected by MI, the podoplanin-presenting cells were not consistently Prox-1-positive. The concordance of podoplanin with VEGFR-3 similarly varied. Thus, our data reveal previously unknown phenotypic and structural heterogeneity within the podoplanin-positive cell compartment in the infarcted heart, and suggest an alternate ability of podoplanin-presenting cardiac cells to generate lymphatic endothelium and pro-fibrotic cells, contributing to scar development

Phenotypically Heterogeneous Podoplanin-expressing Cell Populations Are Associated with the Lymphatic Vessel Growth and Fibrogenic Responses in the Acutely and Chronically Infarcted Myocardium

Cardiac lymphatic vasculature undergoes substantial expansion in response to myocardial infarction (MI). However, there is limited information on the cellular mechanisms mediating post-MI lymphangiogenesis and accompanying fibrosis in the infarcted adult heart. Using a mouse model of permanent coronary artery ligation, we examined spatiotemporal changes in the expression of lymphendothelial and mesenchymal markers in the acutely and chronically infarcted myocardium. We found that at the time of wound granulation, a three-fold increase in the frequency of podoplanin-labeled cells occurred in the infarcted hearts compared to non-operated and sham-operated counterparts. Podoplanin immunoreactivity detected LYVE-1-positive lymphatic vessels, as well as masses of LYVE-1-negative cells dispersed between myocytes, predominantly in the vicinity of the infarcted region. Podoplanin-carrying populations displayed a mesenchymal progenitor marker PDGFRalpha, and intermittently expressed Prox-1, a master regulator of the lymphatic endothelial fate. At the stages of scar formation and maturation, concomitantly with the enlargement of lymphatic network in the injured myocardium, the podoplanin-rich LYVE-1-negative multicellular assemblies were apparent in the fibrotic area, aligned with extracellular matrix deposits, or located in immediate proximity to activated blood vessels with high VEGFR-2 content. Of note, these podoplanin-containing cells acquired the expression of PDGFRbeta or a hematoendothelial epitope CD34. Although Prox-1 labeling was abundant in the area affected by MI, the podoplanin-presenting cells were not consistently Prox-1-positive. The concordance of podoplanin with VEGFR-3 similarly varied. Thus, our data reveal previously unknown phenotypic and structural heterogeneity within the podoplanin-positive cell compartment in the infarcted heart, and suggest an alternate ability of podoplanin-presenting cardiac cells to generate lymphatic endothelium and pro-fibrotic cells, contributing to scar development

Reduction in Kv Current Enhances the Temporal Dispersion of the Action Potential in Diabetic Myocytes: Insights From a Novel Repolarization Algorithm

BACKGROUND: Diabetes is associated with prolongation of the QT interval of the electrocardiogram and enhanced dispersion of ventricular repolarization, factors that, together with atherosclerosis and myocardial ischemia, may promote the occurrence of electrical disorders. Thus, we tested the possibility that alterations in transmembrane ionic currents reduce the repolarization reserve of myocytes, leading to action potential (AP) prolongation and enhanced beat-to-beat variability of repolarization. METHODS AND RESULTS: Diabetes was induced in mice with streptozotocin (STZ), and effects of hyperglycemia on electrical properties of whole heart and myocytes were studied with respect to an untreated control group (Ctrl) using electrocardiographic recordings in vivo, ex vivo perfused hearts, and single-cell patch-clamp analysis. Additionally, a newly developed algorithm was introduced to obtain detailed information of the impact of high glucose on AP profile. Compared to Ctrl, hyperglycemia in STZ-treated animals was coupled with prolongation of the QT interval, enhanced temporal dispersion of electrical recovery, and susceptibility to ventricular arrhythmias, defects observed, in part, in the Akita mutant mouse model of type I diabetes. AP was prolonged and beat-to-beat variability of repolarization was enhanced in diabetic myocytes, with respect to Ctrl cells. Density of Kv K(+) and L-type Ca(2+) currents were decreased in STZ myocytes, in comparison to cells from normoglycemic mice. Pharmacological reduction of Kv currents in Ctrl cells lengthened AP duration and increased temporal dispersion of repolarization, reiterating features identified in diabetic myocytes. CONCLUSIONS: Reductions in the repolarizing K(+) currents may contribute to electrical disturbances of the diabetic heart

Dynamic Regulation of Vascular Myosin Light Chain (MYL9) with Injury and Aging

Aging-associated changes in the cardiovascular system increase the risk for disease development and lead to profound alterations in vascular reactivity and stiffness. Elucidating the molecular response of arteries to injury and age will help understand the exaggerated remodeling of aging vessels.We studied the gene expression profile in a model of mechanical vascular injury in the iliac artery of aging (22 months old) and young rats (4 months old). We investigated aging-related variations in gene expression at 30 min, 3 d and 7 d post injury. We found that the Myosin Light Chain gene (MYL9) was the only gene differentially expressed in the aged versus young injured arteries at all time points studied, peaking at day 3 after injury (4.6 fold upregulation (p<0.05) in the smooth muscle cell layers. We confirmed this finding on an aging aortic microarray experiment available through NCBI's GEO database. We found that Myl9 was consistently upregulated with age in healthy rat aortas. To determine the arterial localization of Myl9 with age and injury, we performed immunohistochemistry for Myl9 in rat iliac arteries and found that in healthy and injured (30 days post injury) arteries, Myl9 expression increased with age in the endothelial layers.The consistent upregulation of the myosin light chain protein (Myl9) with age and injury in arterial tissue draws attention to the increased vascular permeability and to the age-caused predisposition to arterial constriction after balloon angioplasty

S-diclofenac Protects against Doxorubicin-Induced Cardiomyopathy in Mice via Ameliorating Cardiac Gap Junction Remodeling

Hydrogen sulfide (H2S), as a novel gaseous mediator, plays important roles in mammalian cardiovascular tissues. In the present study, we investigated the cardioprotective effect of S-diclofenac (2-[(2,6-dichlorophenyl)amino] benzeneacetic acid 4-(3H-1,2,dithiol-3-thione-5-yl)phenyl ester), a novel H2S-releasing derivative of diclofenac, in a murine model of doxorubicin-induced cardiomyopathy. After a single dose injection of doxorubicin (15 mg/kg, i.p.), male C57BL/6J mice were given daily treatment of S-diclofenac (25 and 50 µmol/kg, i.p.), diclofenac (25 and 50 µmol/kg, i.p.), NaHS (50 µmol/kg, i.p.), or same volume of vehicle. The cardioprotective effect of S-diclofenac was observed after 14 days. It showed that S-diclofenac, but not diclofenac, dose-dependently inhibited the doxorubicin-induced downregulation of cardiac gap junction proteins (connexin 43 and connexin 45) and thus reversed the remodeling of gap junctions in hearts. It also dose-dependently suppressed doxorubicin-induced activation of JNK in hearts. Furthermore, S-diclofenac produced a dose-dependent anti-inflammatory and anti-oxidative effect in this model. As a result, S-diclofenac significantly attenuated doxorubicin-related cardiac injury and cardiac dysfunction, and improved the survival rate of mice with doxorubicin-induced cardiomyopathy. These effects of S-diclofenac were mimicked in large part by NaHS. Therefore, we propose that H2S released from S-diclofenac in vivo contributes to the protective effect in doxorubicin-induced cardiomyopathy. These data also provide evidence for a critical role of H2S in the pathogenesis of doxorubicin-induced cardiomyopathy

Adult cardiac stem cell aging: A reversible stochastic phenomenon?

Aging is by far the dominant risk factor for the development of cardiovascular diseases, whose prevalence dramatically increases with increasing age reaching epidemic proportions. In the elderly, pathologic cellular and molecular changes in cardiac tissue homeostasis and response to injury result in progressive deteriorations in the structure and function of the heart. Although the phenotypes of cardiac aging have been the subject of intense study, the recent discovery that cardiac homeostasis during mammalian lifespan is maintained and regulated by regenerative events associated with endogenous cardiac stem cell (CSC) activation has produced a crucial reconsideration of the biology of the adult and aged mammalian myocardium. The classical notion of the adult heart as a static organ, in terms of cell turnover and renewal, has now been replaced by a dynamic model in which cardiac cells continuously die and are then replaced by CSC progeny differentiation. However, CSCs are not immortal. They undergo cellular senescence characterized by increased ROS production and oxidative stress and loss of telomere/telomerase integrity in response to a variety of physiological and pathological demands with aging. Nevertheless, the old myocardium preserves an endogenous functionally competent CSC cohort which appears to be resistant to the senescent phenotype occurring with aging. The latter envisions the phenomenon of CSC ageing as a result of a stochastic and therefore reversible cell autonomous process. However, CSC aging could be a programmed cell cycle-dependent process, which affects all or most of the endogenous CSC population. The latter would infer that the loss of CSC regenerative capacity with aging is an inevitable phenomenon that cannot be rescued by stimulating their growth, which would only speed their progressive exhaustion. The resolution of these two biological views will be crucial to design and develop effective CSC-based interventions to counteract cardiac aging not only improving health span of the elderly but also extending lifespan by delaying cardiovascular disease-related deaths

Phenotypically heterogeneous podoplanin-expressing cell populations are associated with the lymphatic vessel growth and fibrogenic responses in the acutely and chronically infarcted myocardium

Cardiac lymphatic vasculature undergoes substantial expansion in response to myocardial infarction (MI). However, there is limited information on the cellular mechanisms mediating post-MI lymphangiogenesis and accompanying fibrosis in the infarcted adult heart. Using a mouse model of permanent coronary artery ligation, we examined spatiotemporal changes in the expression of lymphendothelial and mesenchymal markers in the acutely and chronically infarcted myocardium. We found that at the time of wound granulation, a three-fold increase in the frequency of podoplanin-labeled cells occurred in the infarcted hearts compared to non-operated and sham-operated counterparts. Podoplanin immunoreactivity detected LYVE-1-positive lymphatic vessels, as well as masses of LYVE-1-negative cells dispersed between myocytes, predominantly in the vicinity of the infarcted region. Podoplanin-carrying populations displayed a mesenchymal progenitor marker PDGFRα, and intermittently expressed Prox-1, a master regulator of the lymphatic endothelial fate. At the stages of scar formation and maturation, concomitantly with the enlargement of lymphatic network in the injured myocardium, the podoplanin-rich LYVE-1-negative multicellular assemblies were apparent in the fibrotic area, aligned with extracellular matrix deposits, or located in immediate proximity to activated blood vessels with high VEGFR-2 content. Of note, these podoplanin-containing cells acquired the expression of PDGFRβ or a hematoendothelial epitope CD34. Although Prox-1 labeling was abundant in the area affected by MI, the podoplanin-presenting cells were not consistently Prox-1-positive. The concordance of podoplanin with VEGFR-3 similarly varied. Thus, our data reveal previously unknown phenotypic and structural heterogeneity within the podoplanin-positive cell compartment in the infarcted heart, and suggest an alternate ability of podoplanin-presenting cardiac cells to generate lymphatic endothelium and pro-fibrotic cells, contributing to scar development