Expressional analysis of the cardiac Na-Ca exchanger in rat development and senescence

In vitro characterization of Trypanosoma cruzi infection dynamics in skeletal and cardiac myotubes models suggests a potential cell-to-cell transmission in mediating cardiac pathology

Chagas disease predominantly affects the heart, esophagus, and colon in its chronic phase. However, the precise infection mechanisms of the causal agent Trypanosoma cruzi in these tissue types remain incompletely understood. This study investigated T. cruzi infection dynamics in skeletal (SM) and cardiac myotubes (CM) differentiated from H9c2(2-1) myoblasts (control). SM and CM were generated using 1% fetal bovine serum (FBS) without or with retinoic acid, respectively. Initial invasion efficiencies and numbers of released parasites were equivalent between undifferentiated and differentiated cells (~0.3-0.6%). Concomitantly, parasite motility patterns were similar across cell lines. However, CM demonstrated significantly higher infection kinetics over time, reaching 13.26% infected cells versus 3.12% for SM and 3.70% for myoblasts at later stages. Cellular automata modeling suggested an enhanced role for cell-to-cell transmission in driving the heightened parasitism observed in CM. The increased late-stage susceptibility of CM, potentially mediated by cell-to-cell transfer mechanisms of the parasite, aligns with reported clinical tropism patterns. The myotube infection models provide novel insights into Chagas disease pathogenesis that are not fully attainable through in vivo examination alone. Expanding knowledge in this area could aid therapeutic development for this neglected illness.

The Aging Heart: A Molecular and Clinical Challenge

Aging is associated with an increasing burden of morbidity, especially for cardiovascular diseases (CVDs). General cardiovascular risk factors, ischemic heart diseases, heart failure, arrhythmias, and cardiomyopathies present a significant prevalence in older people, and are characterized by peculiar clinical manifestations that have distinct features compared with the same conditions in a younger population. Remarkably, the aging heart phenotype in both healthy individuals and patients with CVD reflects modifications at the cellular level. An improvement in the knowledge of the physiological and pathological molecular mechanisms underlying cardiac aging could improve clinical management of older patients and offer new therapeutic targets.

Maturing heart muscle cells: Mechanisms and transcriptomic insights

Cardiomyocyte (CM) maturation is the transformation of differentiated fetal CMs into adult CMs that involves changes in morphology, cell function and metabolism, and the transcriptome. This process is, however, incomplete and ultimately arrested in pluripotent stem cell-derived CMs (PSC-CMs) in culture, which hinders their broad biomedical application. For this reason, enormous efforts are currently being made with the goal of generating mature PSC-CMs. In this review, we summarize key aspects of maturation observed in native CMs and discuss recent findings on the factors and mechanisms that regulate the process. Particular emphasis is put on transcriptional regulation and single-cell RNA-sequencing analysis that has emerged as a key tool to study time-series gene regulation and to determine the maturation state. We then discuss different biomimetic strategies to enhance PSC-CM maturation and discuss their effects at the single cell transcriptomic and functional levels.

Melatonin to Rescue the Aged Heart: Antiarrhythmic and Antioxidant Benefits

Aging comes with gradual loss of functions that increase the vulnerability to disease, senescence, and death. The mechanisms underlying these processes are linked to a prolonged imbalance between damage and repair. Damaging mechanisms include oxidative stress, mitochondrial dysfunction, chronodisruption, inflammation, and telomere attrition, as well as genetic and epigenetic alterations. Several endogenous tissue repairing mechanisms also decrease. These alterations associated with aging affect the entire organism. The most devastating manifestations involve the cardiovascular system and may lead to lethal cardiac arrhythmias. Together with structural remodeling, electrophysiological and intercellular communication alterations during aging predispose to arrhythmic events. Despite the knowledge on repairing mechanisms in the cardiovascular system, effective antiaging strategies able to reduce the risk of arrhythmias are still missing. Melatonin is a promising therapeutic candidate due to its pleiotropic actions. This indoleamine regulates chronobiology and endocrine physiology. Of relevance, melatonin is an antiaging, antioxidant, antiapoptotic, antiarrhythmic, immunomodulatory, and antiproliferative molecule. This review focuses on the protective effects of melatonin on age-induced cardiac functional and structural alterations, potentially becoming a new fountain of youth for the heart.

Cardiac transmembrane ion channels and action potentials: cellular physiology and arrhythmogenic behavior

Cardiac arrhythmias are among the leading causes of mortality. They often arise from alterations in the electrophysiological properties of cardiac cells and their underlying ionic mechanisms. It is therefore critical to further unravel the pathophysiology of the ionic basis of human cardiac electrophysiology in health and disease. In the first part of this review, current knowledge on the differences in ion channel expression and properties of the ionic processes that determine the morphology and properties of cardiac action potentials and calcium dynamics from cardiomyocytes in different regions of the heart are described. Then the cellular mechanisms promoting arrhythmias in congenital or acquired conditions of ion channel function (electrical remodeling) are discussed. The focus is on human-relevant findings obtained with clinical, experimental, and computational studies, given that interspecies differences make the extrapolation from animal experiments to human clinical settings difficult. Deepening the understanding of the diverse pathophysiology of human cellular electrophysiology will help in developing novel and effective antiarrhythmic strategies for specific subpopulations and disease conditions.

Sex-dependent effect of perinatal hypoxia on cardiac tolerance to oxygen deprivation in adults

Epidemiological studies have demonstrated a relationship between the adverse influence of perinatal development and increased risk of ischemic heart disease in adults. From negative factors to which the fetus is subjected, the most important is hypoxia. The fetus may experience hypoxic stress under different conditions, including pregnancy at high altitude, pregnancy with anemia, placental insufficiency, and heart, lung, and kidney disease. One of the most common insults during the early stages of postnatal development is hypoxemia due to congenital cyanotic heart defects. Experimental studies have demonstrated a link between early hypoxia and increased risk of ischemia/reperfusion injury (I/R) in adults. Furthermore, it has been observed that late myocardial effects of chronic hypoxia, experienced in early life, may be sex-dependent. Unlike in males, perinatal hypoxia significantly increased cardiac tolerance to acute I/R injury in adult females, expressed as decreased infarct size and lower incidence of ischemic arrhythmias. It was suggested that early hypoxia may result in sex-dependent programming of specific genes in the offspring with the consequence of increased cardiac susceptibility to I/R injury in adult males. These results would have important clinical implications, since cardiac sensitivity to oxygen deprivation in adult patients may be significantly influenced by perinatal hypoxia in a sex-dependent manner.

Long-term contractile activity and thyroid hormone supplementation produce engineered rat myocardium with adult-like structure and function

The field of cardiac tissue engineering has developed rapidly, but structural and functional immaturity of engineered heart tissues hinder their widespread use. Here, we show that a combination of low-rate (0.2 Hz) contractile activity and thyroid hormone (T3) supplementation significantly promote structural and functional maturation of engineered rat cardiac tissues ("cardiobundles"). The progressive maturation of cardiobundles during first 2 weeks of culture resulted in cell cycle exit and loss of spontaneous activity, which in longer culture yielded decreased contractile function. Maintaining a low level of contractile activity by 0.2 Hz pacing between culture weeks 3 and 5, combined with T3 treatment, yielded significant growth of cardiobundle and myocyte cross-sectional areas (by 68% and 32%, respectively), increased nuclei numbers (by 22%), improved twitch force (by 39%), shortened action potential duration (by 32%), polarized N-cadherin distribution, and switch from immature (slow skeletal) to mature (fast) cardiac troponin I isoform expression. Along with advanced functional output (conduction velocity 53.7 ± 0.8 cm/s, specific force 70.1 ± 5.8 mN/mm²), quantitative ultrastructural analyses revealed similar metrics and abundance of sarcomeres, T-tubules, M-bands, and intercalated disks compared to native age-matched (5-week) and adult (3-month) ventricular myocytes. Unlike 0.2 Hz regime, chronic 1 Hz pacing resulted in significant cardiomyocyte loss and formation of necrotic core despite the use of dynamic culture. Overall, our results demonstrate remarkable ultrastructural and functional maturation of neonatal rat cardiomyocytes in 3D culture and reveal importance of combined biophysical and hormonal inputs for in vitro engineering of adult-like myocardium.

STATEMENT OF SIGNIFICANCE

Compared to human stem cell-derived cardiomyocytes, neonatal rat ventricular myocytes show advanced maturation state which makes them suitable for in vitro studies of postnatal cardiac development. Still, maturation process from a neonatal to an adult cardiomyocyte has not been recapitulated in rodent cell cultures. Here, we show that low-frequency pacing and thyroid hormone supplementation of 3D engineered neonatal rat cardiac tissues synergistically yield significant increase in cell and tissue volume, robust formation of T-tubules and M-lines, improved sarcomere organization, and faster and more forceful contractions. To the best of our knowledge, 5-week old engineered cardiac tissues described in this study are the first that exhibit both ultrastructural and functional characteristics approaching or matching those of adult ventricular myocardium.

Postnatal developmental changes in the sensitivity of L-type Ca2+ channel to inhibition by verapamil in a mouse heart model

BackgroundIn the clinical setting, verapamil is contraindicated in neonates and infants, because of the perceived risk of hypotension or bradyarrhythmia. However, it remains unclear whether there is an age-dependent difference in the sensitivity of cardiac L-type Ca²⁺ channel current (I_Ca,L) to inhibition by verapamil.MethodsVentricular myocytes were enzymatically dissociated from the hearts of six different age groups (0, 7, 14, 21, 28 days, and 10-15 weeks) of mice, using a similar Langendorff-perfusion method. Whole-cell patch-clamp technique was applied to examine the sensitivity of I_Ca,L to inhibition, by three classes of structurally different L-type Ca²⁺ channel antagonists.ResultsVerapamil, nifedipine, and diltiazem concentration-dependently blocked the ventricular I_Ca,L in all six age groups. However, although nifedipine and diltiazem blocked ventricular I_Ca,L with a similar potency in all age groups, verapamil more potently blocked ventricular I_Ca,L in day 0, day 7, day 14, and day 21 mice, than in day 28, and 10-15-week mice.ConclusionIn a mouse heart model, ventricular I_Ca,L before the weaning age (~21 days of age) exhibited a higher sensitivity to inhibition by verapamil than that after the weaning age, which may explain one possible mechanism associated with the development of verapamil-induced hypotension in human neonates and infants.

Moderate Modulation of Cardiac PGC-1α Expression Partially Affects Age-Associated Transcriptional Remodeling of the Heart

Aging is associated with a decline in cardiac function due to a decreased myocardial reserve. This adverse cardiac remodeling comprises of a variety of changes, including a reduction in mitochondrial function and a decline in the expression of the peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α), a central regulator of mitochondrial biogenesis and metabolic adaptation in the myocardium. To study the etiological involvement of PGC-1α in cardiac aging, we used mouse models mimicking the modest down- and upregulation of this coactivator in the old and the exercised heart, respectively. Young mice with reduced cardiac expression of PGC-1α recapitulated part of the age-related impairment in mitochondrial gene expression, but otherwise did not aggravate the aging process. Inversely however, moderate overexpression of PGC-1α counteracts numerous key age-related remodeling changes, e.g., by improving blood pressure, age-associated apoptosis, and collagen accumulation, as well as in the expression of many, but not all cardiac genes involved in mitochondrial biogenesis, dynamics, metabolism, calcium handling and contractility. Thus, while the reduction of PGC-1α in the heart is insufficient to cause an aging phenotype, moderate overexpression reduces pathological remodeling of older hearts and could thereby contribute to the beneficial effects of exercise on cardiac function in aging.

Cardiomyocyte oxidants production may signal to T. cruzi intracellular development

Chagas disease, caused by the protozoan parasite Trypanosoma cruzi, presents a variable clinical course, varying from asymptomatic to serious debilitating pathologies with cardiac, digestive or cardio-digestive impairment. Previous studies using two clonal T. cruzi populations, Col1.7G2 (T. cruzi I) and JG (T. cruzi II) demonstrated that there was a differential tissue distribution of these parasites during infection in BALB/c mice, with predominance of JG in the heart. To date little is known about the mechanisms that determine this tissue selection. Upon infection, host cells respond producing several factors, such as reactive oxygen species (ROS), cytokines, among others. Herein and in agreement with previous data from the literature we show that JG presents a higher intracellular multiplication rate when compared to Col1.7G2. We also showed that upon infection cardiomyocytes in culture may increase the production of oxidative species and its levels are higher in cultures infected with JG, which expresses lower levels of antioxidant enzymes. Interestingly, inhibition of oxidative stress severely interferes with the intracellular multiplication rate of JG. Additionally, upon H2O2-treatment increase in intracellular Ca2+ and oxidants were observed only in JG epimastigotes. Data presented herein suggests that JG and Col1.7G2 may sense extracellular oxidants in a distinct manner, which would then interfere differently with their intracellular development in cardiomyocytes.

The Aging Heart

Aging results in progressive deteriorations in the structure and function of the heart and is a dominant risk factor for cardiovascular diseases, the leading cause of death in Western populations. Although the phenotypes of cardiac aging have been well characterized, the molecular mechanisms of cardiac aging are just beginning to be revealed. With the continuously growing elderly population, there is a great need for interventions in cardiac aging. This article will provide an overview of the phenotypic changes of cardiac aging, the molecular mechanisms underlying these changes, and will present some of the recent advances in the development of interventions to delay or reverse cardiac aging.

Neonatal hyper- and hypothyroidism alter the myoglobin gene expression program in adulthood

Myoglobin acts as an oxygen store and a reactive oxygen species acceptor in muscles. We examined myoglobin mRNA in rat cardiac ventricle and skeletal muscles during the first 42 days of life and the impact of transient neonatal hypo- and hyperthyroidism on the myoglobin gene expression pattern. Cardiac ventricle and skeletal muscles of Wistar rats at 7-42 days of life were quickly removed, and myoglobin mRNA was determined by Northern blot analysis. Rats were treated with propylthiouracil (5-10 mg/100 g) and triiodothyronine (0.5-50 µg/100 g) for 5, 15, or 30 days after birth to induce hypo- and hyperthyroidism and euthanized either just after treatment or at 90 days. During postnatal (P) days 7-28, the ventricle myoglobin mRNA remained unchanged, but it gradually increased in skeletal muscle (12-fold). Triiodothyronine treatment, from days P0-P5, increased the skeletal muscle myoglobin mRNA 1.5- to 4.5-fold; a 2.5-fold increase was observed in ventricle muscle, but only when triiodothyronine treatment was extended to day P15. Conversely, hypothyroidism at P5 markedly decreased (60%) ventricular myoglobin mRNA. Moreover, transient hyperthyroidism in the neonatal period increased ventricle myoglobin mRNA (2-fold), and decreased heart rate (5%), fast muscle myoglobin mRNA (30%) and body weight (20%) in adulthood. Transient hypothyroidism in the neonatal period also permanently decreased fast muscle myoglobin mRNA (30%) and body weight (14%). These results indicated that changes in triiodothyronine supply in the neonatal period alter the myoglobin expression program in ventricle and skeletal muscle, leading to specific physiological repercussions and alterations in other parameters in adulthood.

Calcium signalling in developing cardiomyocytes: implications for model systems and disease

Adult cardiomyocytes exhibit complex Ca(2+) homeostasis, enabling tight control of contraction and relaxation. This intricate regulatory system develops gradually, with progressive maturation of specialized structures and increasing capacity of Ca(2+) sources and sinks. In this review, we outline current understanding of these developmental processes, and draw parallels to pathophysiological conditions where cardiomyocytes exhibit a striking regression to an immature state of Ca(2+) homeostasis. We further highlight the importance of understanding developmental physiology when employing immature cardiomyocyte models such as cultured neonatal cells and stem cells.

Corticosteroids increase intracellular free sodium ion concentration via glucocorticoid receptor pathway in cultured neonatal rat cardiomyocytes

Background

Glucocorticoids as well as mineralocorticoid have been shown to play essential roles in the regulation of electrical and mechanical activities in cardiomyocytes. Excess of these hormones is an independent risk factor for cardiovascular disease. Intracellular sodium ([Na⁺]_i) kinetics are involved in cardiac diseases, including ischemia, heart failure and hypertrophy. However, intrinsic mediators that regulate [Na⁺]_i in cardiomyocytes have not been widely discussed. Moreover, the quantitative estimation of altered [Na⁺]_i in cultured cardiomyocytes and the association between the level of [Na⁺]_i and the severity of pathological conditions, such as hypertrophy, have not been precisely reported.

Methods and results

We herein demonstrate the quantitative estimation of [Na⁺]_i in cultured neonatal rat cardiomyocytes following 24 h of treatment with corticosterone, aldosterone and dexamethasone. The physiological concentration of glucocorticoids increased [Na⁺]_i up to approximately 2.5 mM (an almost 1.5-fold increase compared to the control) in a dose-dependent manner; this effect was blocked by a glucocorticoid receptor (GR) antagonist but not a mineralocorticoid receptor antagonist. Furthermore, glucocorticoids induced cardiac hypertrophy, and the hypertrophic gene expression was positively and significantly correlated with the level of [Na⁺]_i. Dexamethasone induced the upregulation of Na⁺/Ca^{2 +} exchanger 1 at the mRNA and protein levels.

Conclusions

The physiological concentration of glucocorticoids increases [Na⁺]_i via GR. The dexamethasone-induced upregulation of NCX1 is partly involved in the glucocorticoid-induced alteration of [Na⁺]_i in cardiomyocytes. These results provide new insight into the mechanisms by which glucocorticoid excess within a physiological concentration contributes to the development of cardiac pathology.

Cardiac remodeling and subcellular defects in heart failure due to myocardial infarction and aging

Although several risk factors including hypertension, cardiac hypertrophy, coronary artery disease, and diabetes are known to result in heart failure, elderly subjects are more susceptible to myocardial infarction and more likely to develop heart failure. This article is intended to discuss that cardiac dysfunction in hearts failing due to myocardial infarction and aging is associated with cardiac remodeling and defects in the subcellular organelles such as sarcolemma (SL), sarcoplasmic reticulum (SR), and myofibrils. Despite some differences in the pattern of heart failure due to myocardial infarction and aging with respect to their etiology and sequence of events, evidence has been presented to show that subcellular remodeling plays a critical role in the occurrence of intracellular Ca(2+)-overload and development of cardiac dysfunction in both types of failing heart. In particular, alterations in gene expression for SL and SR proteins induce Ca(2+)-handling abnormalities in cardiomyocytes, whereas those for myofibrillar proteins impair the interaction of Ca(2+) with myofibrils in hearts failing due to myocardial infarction and aging. In addition, different phosphorylation mechanisms, which regulate the activities of Ca(2+)-cycling proteins in SL and SR membranes as well as Ca(2+)-binding proteins in myofibrils, become defective in the failing heart. Accordingly, it is suggested that subcellular remodeling involving defects in Ca(2+)-handling and Ca(2+)-binding proteins as well as their regulatory mechanisms is intimately associated with cardiac remodeling and heart failure due to myocardial infarction and aging.

Mechanisms of arrhythmias and conduction disorders in older adults

Aging is associated with an increased prevalence of cardiac arrhythmias, which contribute to higher morbidity and mortality in the elderly. The frequency of cardiac arrhythmias, particularly atrial fibrillation and ventricular tachyarrhythmia, is projected to increase as the population ages, greatly impacting health care resource utilization. Several clinical factors associated with the risk of arrhythmias have been identified in the population, yet the molecular bases for the increased predisposition to arrhythmogenesis in the elderly are not fully understood. This review highlights the epidemiology of cardiac dysrhythmias, changes in cardiac structure and function associated with aging, and the basis for arrhythmogenesis in the elderly.

Use of antiarrhythmic drugs in elderly patients

Human aging is a global issue with important implications for current and future incidence and prevalence of health conditions and disability. Cardiac arrhythmias, including atrial fibrillation, sudden cardiac death, and bradycardia requiring pacemaker placement, all increase exponentially after the age of 60. It is important to distinguish between the normal, physiological consequences of aging on cardiac electrophysiology and the abnormal, pathological alterations. The age-related cardiac changes include ventricular hypertrophy, senile amyloidosis, cardiac valvular degenerative changes and annular calcification, fibrous infiltration of the conduction system, and loss of natural pacemaker cells and these changes could have a profound effect on the development of arrhythmias. The age-related cardiac electrophysiological changes include up- and down-regulation of specific ion channel expression and intracellular Ca(2+) overload which promote the development of cardiac arrhythmias. As ion channels are the substrates of antiarrhythmic drugs, it follows that the pharmacokinetics and pharmacodynamics of these drugs will also change with age. Aging alters the absorption, distribution, metabolism, and elimination of antiarrhythmic drugs, so liver and kidney function must be monitored to avoid potential adverse drug effects, and antiarrhythmic dosing may need to be adjusted for age. Elderly patients are also more susceptible to the side effects of many antiarrhythmics, including bradycardia, orthostatic hypotension, urinary retention, and falls. Moreover, the choice of antiarrhythmic drugs in the elderly patient is frequently complicated by the presence of co-morbid conditions and by polypharmacy, and the astute physician must pay careful attention to potential drug-drug interactions. Finally, it is important to remember that the use of antiarrhythmic drugs in elderly patients must be individualized and tailored to each patient's physiology, disease processes, and medication regimen.

Colocalization of voltage-gated Na+ channels with the Na+/Ca2+ exchanger in rabbit cardiomyocytes during development

Reverse-mode activity of the Na+/Ca2+ exchanger (NCX) has been previously shown to play a prominent role in excitation-contraction coupling in the neonatal rabbit heart, where we have proposed that a restricted subsarcolemmal domain allows a Na+ current to cause an elevation in the Na+ concentration sufficiently large to bring Ca2+ into the myocyte through reverse-mode NCX. In the present study, we tested the hypothesis that there is an overlapping expression and distribution of voltage-gated Na+ (Nav) channel isoforms and the NCX in the neonatal heart. For this purpose, Western blot analysis, immunocytochemistry, confocal microscopy, and image analyses were used. Here, we report the robust expression of skeletal Nav1.4 and cardiac Nav1.5 in neonatal myocytes. Both isoforms colocalized with the NCX, and Nav1.5-NCX colocalization was not statistically different from Nav1.4-NCX colocalization in the neonatal group. Western blot analysis also showed that Nav1.4 expression decreased by sixfold in the adult ( P < 0.01) and Nav1.1 expression decreased by ninefold ( P < 0.01), whereas Nav1.5 expression did not change. Although Nav1.4 underwent large changes in expression levels, the Nav1.4-NCX colocalization relationship did not change with age. In contrast, Nav1.5-NCX colocalization decreased ∼50% with development. Distance analysis indicated that the decrease in Nav1.5-NCX colocalization occurs due to a statistically significant increase in separation distances between Nav1.5 and NCX objects. Taken together, the robust expression of both Nav1.4 and Nav1.5 isoforms and their colocalization with the NCX in the neonatal heart provides structural support for Na+ current-induced Ca2+ entry through reverse-mode NCX. In contrast, this mechanism is likely less efficient in the adult heart because the expression of Nav1.4 and NCX is lower and the separation distance between Nav1.5 and NCX is larger.

Na+/Ca2+ exchanger is a determinant of excitation-contraction coupling in human embryonic stem cell-derived ventricular cardiomyocytes

In adult cardiomyocytes (CMs), the Na(+)/Ca(2+) exchanger (NCX) is a well-defined determinant of Ca(2+) homeostasis. Developmentally, global NCX knockout in mice leads to abnormal myofibrillar organization, electrical defects, and early embryonic death. Little is known about the expression and function of NCX in human heart development. Self-renewable, pluripotent human embryonic stem cells (hESCs) can serve as an excellent experimental model. However, hESC-derived CMs are highly heterogeneous. A stably lentivirus-transduced hESC line (MLC2v-dsRed) was generated to express dsRed under the transcriptional control of the ventricular-restricted myosin light chain-2v (MLC2v) promoter. Electrophysiologically, dsRed+ cells differentiated from MLC2vdsRed hESCs displayed ventricular action potentials (AP), exclusively. Neither atrial nor pacemaker APs were observed. While I(Ca-L), I(f), and I(Kr) were robustly expressed, I(Ks) and I(K1) were absent in dsRed+ ventricular hESCCMs. Upon differentiation (7+40 to +90 days), the basal [Ca(2+)](i), Ca(2+) transient amplitude, maximum upstroke, and decay velocities significantly increased (P < 0.05). The I(Ca-L) antagonizer nifedipine (1 microM) decreased the Ca(2+) transient amplitude (to approximately 30%) and slowed the kinetics (by approximately 2-fold), but Ca(2+) transients could still be elicited even after complete ICa-L blockade, suggesting the presence of additional Ca(2+) influx(es). Indeed, Ni(2+)-sensitive INCX could be recorded in 7+40- and +90-day dsRed+ hESC-CMs, and its densities increased from -1.2 +/- 0.6 pA/pF at -120 mV and 3.6 +/- 1.0 pA/pF at 60 mV by 6- and 2-folds, respectively. With higher [Ca(2+)](i), 7+90-day ventricular hESC-CMs spontaneously but irregularly fired transients upon a single stimulus under an external Na(+)-free condition; however, without extracellular Na(+), nifedipine could completely inhibit Ca(2+) transients. We conclude that I(NCX) is functionally expressed in developing ventricular hESC-CMs and contributes to their excitation-contraction coupling.

Cardiac aging in mice and humans: the role of mitochondrial oxidative stress

Age is a major risk factor for cardiovascular diseases, not only because it prolongs exposure to several other cardiovascular risks, but also owing to intrinsic cardiac aging, which reduces cardiac functional reserve, predisposes the heart to stress, and contributes to increased cardiovascular mortality in the elderly. Intrinsic cardiac aging in the murine model closely recapitulates age-related cardiac changes in humans, including left ventricular hypertrophy, fibrosis, and diastolic dysfunction. Cardiac aging in mice is accompanied by accumulation of mitochondrial protein oxidation, increased mitochondrial DNA mutations, increased mitochondrial biogenesis, as well as decreased cardiac SERCA2 protein. All of these age-related changes are significantly attenuated in mice overexpressing catalase targeted to mitochondria. These findings demonstrate the critical role of mitochondrial reactive oxygen species in cardiac aging and support the potential application of mitochondrial antioxidants to cardiac aging and age-related cardiovascular diseases.

Inhibition of L-type calcium currents by salusin-beta in rat cardiac ventricular myocytes

Salusin-beta is a new regulatory peptide relevant to the cardiovascular system and exerts negative inotropic effect on ventricular muscle. The purpose of the present study was to determine whether salusin-beta can inhibit cardiac L-type calcium channel current (I(Ca,L)). Using whole-cell voltage-clamp techniques, I(Ca,L) was measured in ventricular myocytes isolated from 12 to 16 weeks rats. Salusin-beta dose-dependently and reversibly reduced the magnitude of I(Ca,L) in rat ventricular myocytes. Neither threshold potential nor the peak potential of current-voltage relationship was affected. Salusin-beta increased the rate of I(Ca,L) inactivation without altering its gating properties. These results suggest salusin-beta inhibited I(Ca,L) by increasing the rate of I(Ca,L) inactivation and the inhibition of L-type Ca(2+) channels induced by salusin-beta may contribute to its negative inotropic effect on ventricular muscle.

Comparison of contractile behavior of native murine ventricular tissue and cardiomyocytes derived from embryonic or induced pluripotent stem cells

Cardiomyocytes generated from embryonic stem cells (ESCs) and induced pluripotent stem (iPS) cells are suggested for repopulation of destroyed myocardium. Because contractile properties are crucial for functional regeneration, we compared cardiomyocytes differentiated from ES cells (ESC-CMs) and iPS cells (iPS-CMs). Native myocardium served as control. Murine ESCs or iPS cells were differentiated 11 d in vitro and cocultured 5-7 d with irreversibly injured myocardial tissue slices. Vital embryonic ventricular tissue slices of similar age served for comparison. Force-frequency relationship (FFR), effects of Ca(2+), Ni(2+), nifedipine, ryanodine, beta-adrenergic, and muscarinic modulation were studied during loaded contractions. FFR was negative for ESC-CMs and iPS-CMs. FFR was positive for embryonic tissue and turned negative after treatment with ryanodine. In all groups, force of contraction and relaxation time increased with the concentration of Ca(2+) and decreased with nifedipine. Force was reduced by Ni(2+). Isoproterenol (1 microM) increased the force most pronounced in embryonic tissue (207+/-31%, n=7; ESC-CMs: 123+/-5%, n=4; iPS-CMs: 120+/-4%, n=8). EC(50) values were similar. Contractile properties of iPS-CMs and ESC-CMs were similar, but they were significantly different from ventricular tissue of comparable age. The results indicate immaturity of the sarcoplasmic reticulum and the beta-adrenergic response of iPS-CMs and ESC-CMs.

Ca2+removal mechanisms in mouse embryonic stem cell-derived cardiomyocytes

In mammalian adult cardiomyocytes, sarcoplasmic reticulum (SR) Ca2+-ATPase (SERCA) plays a major role in controlling the decline of cytosolic free Ca2+concentration ([Ca2+]i) in comparison with sarcolemmal Na+/Ca2+exchanger (NCX). However, the functional importance of SERCA and NCX in cytosolic Ca2+removal during early cardiomyogenesis is still debated. In this study, the functional contributions of Ca2+transporters to [Ca2+]idecline in mouse embryonic stem cell-derived cardiomyocytes (mESCMs), a suitable model for investigation of early cardiogenesis, at various differentiation stages were investigated. We estimated that even at early differentiation stages of mESCMs, SERCA was responsible for ∼76% of total Ca2+removal, while NCX was responsible for ∼21%. The contributions of SERCA and NCX to cytosolic Ca2+clearance were increased to ∼88% and decreased to ∼10%, respectively, at the late differentiation stage. Dynamical analysis of the transient decay phases in normal and Na+-free solutions suggests that the contribution of NCX to [Ca2+]idecline is more apparent in the terminal slow decay phase than that in the initial fast phase. When SR function was suppressed in type 2 ryanodine receptor-null mESCMs or with ryanodine receptor and SERCA inhibitors (ryanodine and thapsigargin), NCX acted as the main pathway for [Ca2+]idecline. We conclude that the rapid [Ca2+]idecline is mainly achieved by the SR uptake even at the early differentiation stage of mESCMs, while NCX acts as the main Ca2+remover when SR function is suppressed. These findings suggest a critical role of SR in the regulation of [Ca2+]ihomeostasis even in differentiating cardiomyocytes.

Enhanced proliferation of monolayer cultures of embryonic stem (ES) cell-derived cardiomyocytes following acute loss of retinoblastoma

Background

Cardiomyocyte (CM) cell cycle analysis has been impeded because of a reliance on primary neonatal cultures of poorly proliferating cells or chronic transgenic animal models with innate compensatory mechanisms.

Methodology/Principal Findings

We describe an in vitro model consisting of monolayer cultures of highly proliferative embryonic stem (ES) cell-derived CM. Following induction with ascorbate and selection with puromycin, early CM cultures are >98% pure, and at least 85% of the cells actively proliferate. During the proliferative stage, cells express high levels of E2F3a, B-Myb and phosphorylated forms of retinoblastoma (Rb), but with continued cultivation, cells stop dividing and mature functionally. This developmental transition is characterized by a switch from slow skeletal to cardiac TnI, an increase in binucleation, cardiac calsequestrin and hypophosphorylated Rb, a decrease in E2F3, B-Myb and atrial natriuretic factor, and the establishment of a more negative resting membrane potential. Although previous publications suggested that Rb was not necessary for cell cycle control in heart, we find following acute knockdown of Rb that this factor actively regulates progression through the G1 checkpoint and that its loss promotes proliferation at the expense of CM maturation.

Conclusions/Significance

We have established a unique model system for studying cardiac cell cycle progression, and show in contrast to previous reports that Rb actively regulates both cell cycle progression through the G1 checkpoint and maturation of heart cells. We conclude that this in vitro model will facilitate the analysis of cell cycle control mechanisms of CMs.

Excitation-contraction coupling of the mouse embryonic cardiomyocyte

In the mammalian embryo, the primitive tubular heart starts beating during the first trimester of gestation. These early heartbeats originate from calcium-induced contractions of the developing heart muscle cells. To explain the initiation of this activity, two ideas have been presented. One hypothesis supports the role of spontaneously activated voltage-gated calcium channels, whereas the other emphasizes the role of Ca(2+) release from intracellular stores initiating spontaneous intracellular calcium oscillations. We show with experiments that both of these mechanisms coexist and operate in mouse cardiomyocytes during embryonic days 9-11. Further, we characterize how inositol-3-phosphate receptors regulate the frequency of the sarcoplasmic reticulum calcium oscillations and thus the heartbeats. This study provides a novel view of the regulation of embryonic cardiomyocyte activity, explaining the functional versatility of developing cardiomyocytes and the origin and regulation of the embryonic heartbeat.

Mitochondrial channelopathies in aging

Defects in ion channels (channelopathies) are increasingly found in a large spectrum of human pathologies including aging. Mutations in genes encoding ion channel proteins, which disrupt channel function, are the most commonly identified cause of channelopathies. Mutations in associated proteins, alterations in the expression of ion channels, or changes in the activity of non-mutated channel genes or associated proteins can also produce acquired channelopathies. Mitochondria, the powerhouse of the cells, are considered to be the most important cellular organelles to contribute to aging mainly because of their role in the production of reactive oxygen species in the initiation of apoptotic cell remodeling and in efficient ATP synthesis. During the past 50 years, multiple ion channels or transporters have been found in mitochondria, and the relationship between the activity of these channels and cellular aging, as well as the overall cellular biological function, has been intensively studied in a number of cell types and animal models. In this review, we discuss the better characterized mitochondrial ion channels whose dysfunction (mitochondrial channelopathies) may affect or accelerate the aging processes. These channels include the mitochondrial ATP-sensitive potassium channel (mitoK(ATP)), Ca(2+) transporters, voltage-dependent anion channel, and the mitochondrial permeability transition pore (mitoPTP).

Phospholemman expression is high in the newborn rabbit heart and declines with postnatal maturation

Phospholemman (PLM) is a small sarcolemmal protein that modulates the activities of Na(+)/K(+)-ATPase and the Na(+)/Ca(2+) exchanger (NCX), thus contributing to the maintenance of intracellular Na(+) and Ca(2+) homeostasis. We characterized the expression and subcellular localization of PLM, NCX, and the Na(+)/K(+)-ATPase alpha1-subunit during perinatal development. Western blotting demonstrates that PLM (15kDa), NCX (120kDa), and Na(+)/K(+)-ATPase alpha-1 (approximately 100kDa) proteins are all more than 2-fold higher in ventricular membrane fractions from newborn rabbit hearts (1-4-day old) compared to adult hearts. Our immunocytochemistry data demonstrate that PLM, NCX, and Na(+)/K(+)-ATPase are all expressed at the sarcolemma of newborn ventricular myocytes. Taken together, our data indicate that PLM, NCX, and Na(+)/K(+)-ATPase alpha-1 proteins have similar developmental expression patterns in rabbit ventricular myocardium. Thus, PLM may have an important regulatory role in maintaining cardiac Na(+) and Ca(2+) homeostasis during perinatal maturation.

Cardiac overexpression of antioxidant catalase attenuates aging-induced cardiomyocyte relaxation dysfunction

Catalase, an enzyme which detoxifies H2O2, may interfere with cardiac aging. To test this hypothesis, contractile and intracellular Ca2+ properties were evaluated in cardiomyocytes from young (3-4 months) and old (26-28 months) FVB and transgenic mice with cardiac overexpression of catalase. Contractile indices analyzed included peak shortening (PS), time-to-90% PS (TPS90), time-to-90% relengthening (TR90), half-width duration (HWD), maximal velocity of shortening/relengthening (+/-dL/dt) and intracellular Ca2+ levels or decay rate. Levels of advanced glycation endproduct (AGE), Na+/Ca2+ exchanger (NCX), sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA2a), phospholamban (PLB), myosin heavy chain (MHC), membrane Ca2+ and K+ channels were measured by western blot. Catalase transgene prolonged survival while did not alter myocyte function by itself. Aging depressed+/-dL/dt, prolonged HWD, TR90 and intracellular Ca2+ decay without affecting other indices in FVB myocytes. Aged FVB myocytes exhibited a stepper decline in PS in response to elevated stimulus or a dampened rise in PS in response to elevated extracellular Ca2+ levels. Interestingly, aging-induced defects were nullified or significantly attenuated by catalase. AGE level was elevated by 5-fold in aged FVB compared with young FVB mice, which was reduced by catalase. Expression of SERCA2a, NCX and Kv1.2 K+ channel was significantly reduced although levels of PLB, L-type Ca2+ channel dihydropyridine receptor and beta-MHC isozyme remained unchanged in aged FVB hearts. Catalase restored NCX and Kv1.2 K+ channel but not SERCA2a level in aged mice. In summary, our data suggested that catalase protects cardiomyocytes from aging-induced contractile defect possibly via improved intracellular Ca2+ handling.

Functional expression of the Na+/Ca2+ exchanger in the embryonic mouse heart

The Na(+)/Ca(2+) exchanger (NCX) is one of the earliest functional genes and is currently assumed to compensate at least in part for the rudimentary sarcoplasmic reticulum in the developing mouse heart. However, to date little is known about the functional expression of NCX during development. This prompted us to investigate the NCX current (I(NCX)) in very early (embryonic day E8.5-E9.5 post coitum), early (E10.5-E11.5), middle (E13.5) and late (E16.5) stage mouse embryonic cardiomyocytes. For standard I(NCX) measurements, [Ca(2+)](i) was buffered to 150 nmol/l and voltage ramps were applied from +60 mV to -120 mV. At very early stages of development, we observed a prominent role of the I(NCX) Ca(2+) inward mode in elevating the cytosolic Ca(2+) concentration ([Ca(2+)](i)). Accordingly, a high I(NCX) density was observed (+60 mV: 4.6+/-0.7 pA/pF, n=14). Likewise, we found a strong Ca(2+) outward mode of I(NCX) (-120 mV: -3.9+/-0.7 pA/pF, n=14). At later stages, however, I(NCX) Ca(2+) inward mode was reduced by 54+/-6% (n=15, p<0.0001) in ventricular and 68+/-10% (n=9, p<0.0006) in atrial cells. For the outward mode, a reduction by 43+/-10% (n=15, p<0.01) in ventricular and 62+/-11% (n=9, p<0.004) in atrial cardiomyocytes was observed. By contrast, NCX isoform expression and the reversal potential did not significantly change during development. Thus, NCX displays a prominent Ca(2+) inward and outward mode during early embryonic heart development pointing to its important contribution to maintain [Ca(2+)](i) homeostasis. The functional and protein expression of NCX declines during further development.

Young MLP deficient mice show diastolic dysfunction before the onset of dilated cardiomyopathy

Targeted deletion of cytoskeletal muscle LIM protein (MLP) in mice consistently leads to dilated cardiomyopathy (DCM) after one or more months. However, next to nothing is known at present about the mechanisms of this process. We investigated whether diastolic performance including passive mechanics and systolic behavior are altered in 2-week-old MLP knockout (MLPKO) mice, in which heart size, fractional shortening and ejection fraction are still normal. Right ventricular trabeculae were isolated from 2-week-old MLPKO and wildtype mice and placed in an apparatus that allowed force measurements and sarcomere length measurements using laser diffraction. During a twitch from the unloaded state at 1 Hz, MLPKO muscles relengthened to slack length more slowly than controls, although the corresponding force relaxation time was unchanged. Active developed stress at a diastolic sarcomere length of 2.00 microm was preserved in MLPKO trabeculae over a wide range of pacing frequencies. Force relaxation under the same conditions was consistently prolonged compared with wildtype controls, whereas time to peak and maximum rate of force generation were not significantly altered. Ca2+ content of the sarcoplasmic reticulum (SR) and the quantities of Ca2+ handling proteins were similar in both genotypes. In summary, young MLPKO mice revealed substantial alterations in passive myocardial properties and relaxation time, but not in most systolic characteristics. These results indicate that the progression to heart failure in the MLPKO model may be driven by diastolic myocardial dysfunction and abnormal passive properties rather than systolic dysfunction.

Developmental regulation of intracellular calcium transients during cardiomyocyte differentiation of mouse embryonic stem cells

AIM

To investigate the developmental regulation of intracellular Ca2+ transients, an essential event in excitation-contraction coupling, during cardiomyocyte differentiation.

METHODS

Using the embryonic stem (ES) cell in vitro differentiation system and pharmacological intervention, we investigated the molecular and functional regulation of Ca2+ handling proteins on the Ca2+ transients at early, intermediate and later differentiation stages of ES cell-derived cardiomyocytes (ESCM).

RESULTS

Nifedipine, a selective antagonist of L-type Ca2+ channels, totally blocked Ca2+ transients even in the condition of field-electric stimulation in ESCM at three differentiation stages. The Ca2+ transients of ESCM were also inhibited by both ryanodine [an inhibitor of ryanodine receptors (RyRs)] and 2-aminoethoxydipheylborate [2-APB, an inhibitor of inositol-1,4,5-trisphosphate receptors (IP3Rs)]. The inhibitory effect of ryanodine increased with the time of differentiation, while the effect of 2-APB decreased with the differentiation. Thapsigargin, an inhibitor of SR Ca2+-pump ATPase, inhibited Ca2+ transients equally at three differentiation stages that matched the expression profile. Na+ free solution, which inhibits Na+-Ca2+ exchanger (NCX) to extrude Ca2+ from cytosol, did not affect the amplitude of Ca2+ transients of ESCM until the latter differentiation stage, but it significantly enhanced the basal Ca2+ concentration.

CONCLUSION

The Ca2+ transients in ESCM depend on both the sarcolemmal Ca2+ entry via L-type Ca2+ channels and the SR Ca2+ release from RyRs and IP3Rs even at the early differentiation stage; but NCX seems not to regulate the peak of Ca2+ transients until the latter differentiation stage.

Cardiac overexpression of alcohol dehydrogenase (ADH) alleviates aging-associated cardiomyocyte contractile dysfunction: role of intracellular Ca2+ cycling proteins

Aging is a complex biological process with contributions from a wide variety of genes including insulin-like growth factor I and alcohol dehydrogenase (ADH), which decline with advanced age. The goal of this study was to examine if ADH enzyme plays any role in cardiac aging. Ventricular myocytes were isolated from young (2-3 months old) or aged (26-28 months old) male FVB wild-type and cardiac-specific ADH (class I, isozyme type 1) transgenic mice. Mechanical properties were measured using an IonOptix system. Aged FVB myocytes displayed significantly reduced ADH activity compared with young ones, which was restored by the ADH transgene. Compared with young cardiomyocytes, aged FVB myocytes exhibited prolonged relengthening duration and a steaper decline in peak shortening amplitude in response to elevated electrical stimuli. Although ADH transgene itself did not alter mechanical properties in young mice, it rescued aging-associated diastolic dysfunction without affecting dampened contractile response to high stimulus frequency. Immunoblot analysis revealed reduced sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA2a) and Na(+)-Ca(2+) exchanger (NCX) levels in conjunction with enhanced phospholamban expression in aged FVB hearts. ADH transgene prevented aging-induced reduction in SERCA2a and NCX without affecting up-regulated phospholamban. Our data suggest that aging is associated with a reduced ADH enzymatic activity and diastolic dysfunction, which may be corrected with cardiac overexpression of the ADH enzyme. Alteration in cardiac Ca(2+) cycling proteins including SERCA2a and NCX may play a role in both pathogenesis of cardiac aging and the beneficial effect of ADH enzyme.

Na+/K+ ATPase and its functional coupling with Na+/Ca2+ exchanger in mouse embryonic stem cells during differentiation into cardiomyocytes

Cardiomyocytes derived from mouse embryonic stem (mES) cells have been demonstrated to exhibit a time-dependent expression of ion channels and signal transduction pathways in electrophysiological studies. However, ion transporters, such as Na+/K+ ATPase (Na+ pump) or Na+/Ca2+ exchanger, which play crucial roles for cardiac function, have not been well studied in this system. In this study, we investigated the functional expression of Na+/K+ ATPase and Na+/Ca2+ exchanger in mES cells during in vitro differentiation into cardiomyocytes, as well as the functional coupling between the two transporters. By measuring [Na+]i and Na+ pump current (Ip), it was shown that an ouabain-high sensitive Na+/K+ ATPase was expressed functionally in undifferentiated mES cells and these activities increased during a time course of differentiation. Using RT-PCR, the expression of mRNA for alpha1-subunit and alpha3-subunit of the Na+/K+ ATPase could be detected in both undifferentiated mES cells and derived cardiomyocytes. In contrast alpha2-subunit mRNA could be detected only in derived cardiomyocytes but not in undifferentiated mES cells. mRNA for the Na+/Ca2+ exchanger 1 isoform (NCX1) could be detected in undifferentiated mES cells and its expression levels seemed to gradually increase throughout the differentiation accompanied by increasing its Ca2+ extrusion function. At the middle stages of differentiation (after 10-day induction), more than 75% derived cardiomyocytes exhibited [Ca2+]i oscillations by blocking of Na+/K+ ATPase, suggesting the functional coupling with Na+/Ca2+ exchanger. From these results and RT-PCR analysis, we conclude that alpha2-subunit Na+/K+ ATPase mainly contributes to establish the functional coupling with NCX1 at the middle stages of differentiation of cardiomyocytes.

Regional expression of L-type calcium channel subunits during cardiac development

The contraction of cardiomyocytes is initiated by the entrance of extracellular calcium through specific calcium channels. Within the myocardium, L-type calcium channels are most abundant. In the heart, the main pore-forming subunit is the alpha1C, although there is a larger heterogeneity on auxiliary beta subunits. We have analyzed the distribution pattern of different alpha1C and beta subunits during cardiac development by immunohistochemistry. We observed homogeneous expression of alpha1C and beta subunits within the early tubular heart, whereas regional differences are observed during the late embryogenesis. beta2 and beta4 show differential expression within the embryonic myocardium. alpha1CD1 displays only a transient enhanced expression in the ventricular conduction system. In adult heart, the expression of the different calcium channel subunits analyzed is homogeneous along the entire myocardium except for alpha1CD1 that is practically undetectable. These findings suggest that beta subunits might play a major role in conferring calcium handling heterogeneity within the developing embryonic myocardium, while alpha1C subunits might contribute just transiently.

Effects of IgM from rheumatic fever patients on intracellular calcium levels of neonatal rat cardiac myocytes

Rheumatic fever (RF), a potential sequela of Streptococcus pyogenes pharyngitis, sometimes results in myocarditis and heart failure. Antibodies have been implicated in the pathogenesis of RF and anti-cardiac myosin antibody levels are elevated in RF patients. Since myocarditis is associated with altered cardiomyocyte calcium transients it was of interest to determine the direct effects of RF patient antibodies on calcium transients in cultured myocytes. RF patient polyclonal IgM treatment caused increased calcium retention by neonatal rat heart cells in vitro as determined with isotopically labeled calcium. Therefore, to further characterize this finding, calcium transients were evaluated by real time fluorescence spectroscopy and deconvolution imaging. RF patient polyclonal IgM produced increased calcium retention during the relaxation stage of the contraction cycle leading to a slowing of contraction rate, disorganized calcium transients, and eventual tetany. In contrast, calcium transient studies of cardiomyocytes following treatment with monoclonal anti-myosin antibodies revealed declining intracellular calcium levels, accompanied by disorganized transients and tetany. Treatment with both antibodies led to myocyte dysfunction and these novel findings suggest a role for antibodies in the pathogenesis of the myocarditis associated with rheumatic carditis.

Role of sodium-calcium exchanger (Ncx1) in embryonic heart development: a transgenic rescue?

Na(+)/Ca(2+) exchanger (Ncx-1) is highly expressed in cardiomyocytes, is thought to be required to maintain a low intracellular Ca(2+) concentration, and may play a role in excitation-contraction coupling. Significantly, targeted deletion of Ncx-1 results in Ncx1-null embryos that do not have a spontaneously beating heart and die in utero. Ultrastructural analysis revealed gross anomalies in the Ncx1-null contractile apparatus, but physiologic analysis showed normal field-stimulated Ca(2+) transients, suggesting that Ncx-1 function may not be critical for Ca(2+) extrusion from the cytosol as previously thought. Using caffeine to empty the intracellular Ca(2+) stores, we show that the sarcoplasmic reticulum is not fully functional within the 9.5-dpc mouse heart, indicating that the sarcoplasmic reticulum is unlikely to account for the unexpected maintenance of intracellular Ca(2+) homeostasis. Using the Ncx1-lacZ reporter, our data indicate restricted expression patterns of Ncx1 and that Ncx1 is highly expressed within the conduction system, suggesting Ncx1 may be required for spontaneous pacemaking activity. To test this hypothesis, we used transgenic mice overexpressing one of the two known adult Ncx1 isoforms under the control of the cardiac-specific a-myosin heavy chain promoter to restore Ncx1 expression within the Ncx1-null hearts. Results indicate that the transgenic re-expression of one Ncx1 isoform was unable to rescue the lethal null mutant phenotype. Furthermore, our in situ results indicate that both known adult Ncx1 isoforms are coexpressed within the embryonic heart, suggesting that effective transgenic rescue may require the presence of both isoforms within the developing heart.

Pathways regulating Na+/Ca2+ exchanger expression in the heart

The Na(+)/Ca(2+) exchanger (NCX1) is regulated at the transcriptional level in cardiac hypertrophy, ischemia, and failure. Following pressure overload, activation of MAPKs coincides with the kinetics of NCX1 gene upregulation in adult cardiocytes. Using adenoviral gene delivery, we begin to identify the molecular pathways responsible for upregulation of the exchanger gene. Inhibition of ERK with the MEK inhibitor UO126, the ERK protein phosphatase MKP-3, inhibited ERK activation, but only inhibited alpha-adrenergic-induced NCX1 upregulation by 30%. Overexpression of DN-JNK lowered basal NCX1 expression. Overexpression of activated MKK-3 was sufficient for alpha-adrenergic-stimulated upregulation of the reporter gene. Together, this data indicates that (1) JNK mediates basal cardiac expression of the NCX1 gene, (2) ERK and p38 play a role in alpha-adrenergic-stimulated NCX1 upregulation, and (3) p38 activation alone is sufficient for NCX1 upregulation.

Developmental changes of Ca(2+) handling in mouse ventricular cells from early embryo to adulthood

Transplant of immature cardiomyocytes is recently attracting a great deal of interest as a new experimental strategy for the treatment of failing hearts. Full understanding of normal cardiomyogenesis is essential to make this regenerative therapy feasible. We analyzed the molecular and functional changes of Ca(2+) handling proteins during development of the mouse heart from early embryo at 9.5 days postcoitum (dpc) through adulthood. From the early to the late (18 dpc) embryonic stage, mRNAs estimated by the real time PCR for ryanodine receptor (type 2, RyR2), sarcoplasmic reticulum (SR) Ca(2+) pump (type 2, SERCA2) and phospholamban (PLB) increased by 3-15 fold in the values normalized to GAPDH mRNA, although Na(+)/Ca(2+) exchanger (type 1, NCX1) mRNA was unchanged. After birth, there was a further increase in the mRNAs for RyR2, SERCA2 and PLB by 18-33 fold, but a 50% decrease in NCX1 mRNA. The protein levels of RyR2, SERCA2, PLB and NCX1, which were normalized to total protein, showed qualitatively parallel developmental changes. L-type Ca(2+) channel currents (I(Ca-L)) were increased during the development (1.3-fold at 18 dpc, 2.2-fold at adult stage, vs. 9.5 dpc). At 9.5 dpc, the Ca(2+) transient was, unlike adulthood, unaffected by the SR blockers, ryanodine (5 microM) and thapsigargin (2 microM), and also by a blocker of the Ca(2+) entry via Na(+)/Ca(2+) exchanger, KB-R 7943 (1 microM). The Ca(2+) transient was abolished after application of nisoldipine (5 microM). These results indicate that activator Ca(2+) for contraction in the early embryonic stage depends almost entirely on I(Ca-L).

Reduced calcium tolerance in rat cardiomyocytes after myocardial infarction

During ischaemia and reperfusion the intracellular Na+ concentration is elevated in the cardiomyocytes and the cells are depolarized, both favouring reverse mode Na,Ca-exchange loading of the cell with Ca2+. We examined whether cardiomyocytes from rats with congestive heart failure (CHF) and younger rats (HINCX) which both have a high expression of the Na,Ca-exchanger protein (NCX) showed reduced tolerance to extracellular Ca2+. The CHF was induced in Isofluran anaesthetized rats by left coronary artery ligation. Isolated cardiomyocytes were loaded with Fura-2AM and 140 mm Na+ and exposed to 0.05 mm Ca2+. Expression of the Na,Ca-exchanger protein was analysed. Fura-2 340/380 ratio rose more rapidly in HINCX and CHF than in SHAM, and the rise was abolished by Ni2+. Hypercontracture developed more frequently in HINCX and CHF than in SHAM cells. The amount of NCX was 54% higher in HINCX and 76% higher in CHF compared with SHAM. Na+-loaded cardiomyocytes from CHF and HINCX rats are more susceptible to Ca2+ overload than SHAM cells because of the increased capacity for Na,Ca-exchange.

The ryanodine receptor modulates the spontaneous beating rate of cardiomyocytes during development

In adult myocardium, the heartbeat originates from the sequential activation of ionic currents in pacemaker cells of the sinoatrial node. Ca(2+) release via the ryanodine receptor (RyR) modulates the rate at which these cells beat. In contrast, the mechanisms that regulate heart rate during early cardiac development are poorly understood. Embryonic stem (ES) cells can differentiate into spontaneously contracting myocytes whose beating rate increases with differentiation time. These cells thus offer an opportunity to determine the mechanisms that regulate heart rate during development. Here we show that the increase in heart rate with differentiation is markedly depressed in ES cell-derived cardiomyocytes with a functional knockout (KO) of the cardiac ryanodine receptor (RyR2). KO myocytes show a slowing of the rate of spontaneous diastolic depolarization and an absence of calcium sparks. The depressed rate of pacemaker potential can be mimicked in wild-type myocytes by ryanodine, and rescued in KO myocytes with herpes simplex virus (HSV)-1 amplicons containing full-length RyR2. We conclude that a functional RyR2 is crucial to the progressive increase in heart rate during differentiation of ES cell-derived cardiomyocytes, consistent with a mechanism that couples Ca(2+) release via RyR before an action potential with activation of an inward current that accelerates membrane depolarization.

Sodium calcium exchanger plays a key role in alteration of cardiac function in response to pressure overload

The Na+-Ca2+ exchanger (NCX) on the plasma membrane is thought to be the main calcium extrusion system from the cytosol to the extracellular space in many mammalian excitable cells, including cardiac myocytes. However, the pathophysiological role of NCX in the heart is still unclear because of the lack of known specific inhibitors of NCX. To determine the role of NCX in cardiac contraction and the development of cardiac hypertrophy, we imposed pressure overload on the heart of heterozygous NCX knockout (KO) mice by constricting transverse aorta, and examined cardiac function and morphology 3 wk after operation. Although there was no difference in cardiac function between sham-operated KO mice and sham-operated wild-type (WT) mice, KO mice showed higher left ventricular pressure and better systolic function than WT mice in response to pressure overload. Northern blot analysis revealed that mRNA levels of sarcoplasmic reticulum Ca2+-ATPase were reduced by pressure overload in left ventricles of WT but not of KO mice. However, hypertrophic changes with interstitial fibrosis were more prominent in KO mice than WT mice. These results suggest that reduction of NCX results in supernormalized cardiac function and causes marked cardiac hypertrophy in response to pressure overload.

Cardiovascular ageing in health sets the stage for cardiovascular disease

The incidence and prevalence of coronary disease, hypertension, heart failure and stroke increase exponentially with advancing age. While epidemiologic studies have discovered that aspects of lifestyle and genetics are risk factors for these diseases, age, per se, confers the major risk. Thus, it is reasonable to hypothesise that specific pathophysiological mechanisms that underlie these diseases become superimposed on cardiac and vascular substrates that have been modified by an 'ageing process', and that the latter modulates disease occurrence and severity. In order to unravel this age-disease interaction, the nature of the ageing process in the heart and vasculature requires elucidation. Some aspects of the current understanding of ageing of the heart and blood vessels in the absence of apparent disease are the focus of this review.

Age-associated cardiovascular changes in health: impact on cardiovascular disease in older persons

In the United States, cardiovascular disease, e.g., atherosclerosis and hypertension, that lead to heart failure and stroke, is the leading cause of mortality, accounting for over 40 percent of deaths in those aged 65 years and above. Over 80 percent of all cardio-vascular deaths occur in the same age group. Thus, age, per se, is the major risk factor for cardiovascular disease. Clinical manifestations and prognosis of these cardiovascular diseases likely become altered in older persons with advanced age because interactions occur between age-associated cardiovascular changes in health and specific pathophysiologic mechanisms that underlie a disease. A fundamental understanding of age-associated changes in cardiovascular structure and function ranging in scope from humans to molecules is required for effective and efficient prevention and treatment of cardiovascular disease in older persons. A sustained effort over the past two decades has been applied to characterize the multiple effects of aging in health on cardiovascular structure and function in a single study population, the Baltimore Longitudinal Study on Aging. In these studies, community dwelling, volunteer participants are rigorously screened to detect both clinical and occult cardiovascular disease and characterized with respect to lifestyle, e.g. exercise habits, in an attempt to deconvolute interactions among lifestyle, cardiovascular disease and the aging process in health. This review highlights some specific changes in resting cardiovascular structure and function and cardiovascular reserve capacity that occur with advancing age in healthy humans. Observations from relevant experiments in animal models have been integrated with those in humans to provide possible mechanistic insight.

A distant upstream region of the rat multipartite Na(+)-Ca(2+) exchanger NCX1 gene promoter is sufficient to confer cardiac-specific expression

The Na(+)-Ca(2+) exchanger (NCX) regulates intracellular calcium homeostasis. We report on an upstream region of the rat NCX1 multipartite promoter that is active in cardiac myocytes. Although inactive in most non-cardiac cell lines, its activity can be rescued by cotransfection with GATA-4 and -6, but not GATA-5 transcription factors. In transgenic mice and similar to endogenous NCX1 mRNA expression, the upstream promoter region directs uniform beta-galactosidase expression in cardiac myocytes from approximately 7.75dpc. In adult mouse hearts, promoter activity is, however, significantly reduced and heterogeneous, except in the conduction system (sinoatrial and atrioventricular node, atrioventricular bundles). The upstream NCX1 promoter region thus directs appropriate spatial and temporal control of cardiac expression throughout development.

Expression of Na+/Ca(2+) exchanger (NCX1) gene in the developmental mouse embryo and adult mouse brain

The Na(+)/Ca(2+) exchanger gene, NCX1, is widely expressed in many tissues, encoding several isoforms through alternative RNA splicing. NCX1 deficient mice are known to be lethal at embryonic day 9-10 (E9-10). However, its expression pattern during embryogenesis is largely unknown. Therefore, to identify and compare the localization and alternatively spliced isoforms of NCX1 mRNA expressed in the developmental stages, we analyzed the mouse embryo. Northern blot analysis demonstrated that NCX1 mRNA was expressed from the earliest stage examined, E7. In situ hybridization analysis revealed that NCX1 mRNA was expressed in the heart alone until E10.5. However, at E14.5 and 16.5, NCX1 mRNA was expressed not only in the heart, but also in neuronal cells. In addition, the expression of NCX1 mRNA in the adult brain was most abundant in the hippocampus. Using reverse transcription-polymerase chain reaction (RT-PCR), we also identified the alternatively spliced isoforms expressed during each developmental stage. The restricted expression of the NCX1 gene suggested that NCX1 may play an important role in the developing mouse embryo.