The unique functions of cardiac troponin I in the control of cardiac muscle contraction and relaxation

We review development of evidence and current perceptions of the multiple and significant functions of cardiac troponin I in regulation and modulation of cardiac function. Our emphasis is on the unique structure function relations of the cardiac isoform of troponin I, especially regions containing sites of phosphorylation. The data indicate that modifications of specific regions cardiac troponin I by phosphorylations either promote or reduce cardiac contractility. Thus, a homeostatic balance in these phosphorylations is an important aspect of control of cardiac function. A new concept is the idea that the homeostatic mechanisms may involve modifications of intra-molecular interactions in cardiac troponin I.     

Cardiac ryanodine receptor is absent in type I slow skeletal muscle fibers: immunochemical and ryanodine binding studies

Cardiac ryanodine receptor was purified from canine ventricle as a single polypeptide of Mr 400,000 by a stepwise sucrose density gradient centrifugation and heparin-Sepharose CL-4B column chromatography. The [3H]ryanodine binding capacity (Bmax) was 60-fold enriched from cardiac microsomes without a change in affinity for [3H]ryanodine. The purity of the final preparation was determined to be greater than 95% by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Using this purified preparation as an antigen, we produced six monoclonal antibodies which immunoprecipitated the cardiac ryanodine receptor. Three of these antibodies recognized the cardiac receptor on immunoblot analysis. In contrast, no protein in the microsomes isolated from Type I (slow) or Type II (fast) skeletal muscles was recognized by these antibodies. The [3H]ryanodine binding to cardiac and skeletal muscle microsomes was dependent on free Ca2+ concentration. In skeletal muscle microsomes, the [3H]ryanodine binding was remarkably enhanced by the addition of ATP or KCl and inhibited by high free Ca2+, whereas it was less sensitive to these agents in cardiac microsomes. All of these results clearly demonstrate that the cardiac ryanodine receptor is different from the skeletal muscle receptors and is not present even in Type I (slow) skeletal muscle fibers, in which cardiac isoforms of some of the muscle proteins are constitutively expressed.     

Significance of nitric oxide function in development of cardiac hypertrophy under condition of experimental renal hypertension

The development of cardiac hypertrophy was studied under condition of experimental renal hypertension on the rat. The number of cardiac nitric oxide synthase (NOS)-positive neurones increased simultaneously with the increase in NOS-activity in these neurones. A connection was found between the development of cardiac hypertrophy and the activity of NOS in cardiomiocytes. The involvement of NO in the development cardiac hypertrophy as auto- and paracrine regulator is supposed.     

Growth factor for cardiac hypertrophy

Cardiac size can be regulated by the balance in activity between cardiac growth factors and inhibiting factors, chalones. This study was undertaken to verify the role of the cardiac growth factor and its purification from hypertrophied hearts. For this propose the hypertrophied hearts of renovascular hypertensive rats were used. The purification was made by using an isoelectric focusing chromatography and the HPLC method. We examined the cardiac growth effect of the isolated fractions with cultured chicken embryonic cardiac myocytes. Simultaneously, the influence of these fractions on the cardiac cell cycle was examined by DNA analysis with the flow cytometric method. If the hearts were overloaded due to hypertension, the growth of the cardiac size could be induced by increased the level of five proteins with different molecular weight and with an isoelectric point of 8.3. The significant growth activities were observed at these five proteins compared to the absence of the fractions. For the appearance of these growth effect, it is necessary that the structure of the protein is there fundamentally as a form with a molecular weight of 27 k dalton. After addition of these isolated fractions, BrdU content is S and G2 phases by flow cytometry was increased. This change indicates that the cardiac myocytes are stimulated in form DNA synthesis.     

Optocardiography: In vivo measurements of the insect cardiac activity with a new optical method

A new method allowing long-term in vivo evaluation of the insect cardiac activity is described. Comparison with other methods and experimental results indicate the advantages of the new method over classical techniques. The basic cardiac cycle and cardiac activity patterns as recorded by this method are shown. The independent cardiac activity of different chambers of the dorsal vessel of Schistocerca gregaria is demonstrated. A preliminary explanation of these events is given.     

SHP-2 is required for the maintenance of cardiac progenitors

The isolation and culturing of cardiac progenitor cells has demonstrated that growth factor signaling is required to maintain cardiac cell survival and proliferation. In this study, we demonstrate in Xenopus that SHP-2 activity is required for the maintenance of cardiac precursors in vivo. In the absence of SHP-2 signaling, cardiac progenitor cells downregulate genes associated with early heart development and fail to initiate cardiac differentiation. We further show that this requirement for SHP-2 is restricted to cardiac precursor cells undergoing active proliferation. By demonstrating that SHP-2 is phosphorylated on Y542/Y580 and that it binds to FRS-2, we place SHP-2 in the FGF pathway during early embryonic heart development. Furthermore, we demonstrate that inhibition of FGF signaling mimics the cellular and biochemical effects of SHP-2 inhibition and that these effects can be rescued by constitutively active/Noonan-syndrome-associated forms of SHP-2. Collectively, these results show that SHP-2 functions within the FGF/MAPK pathway to maintain survival of proliferating populations of cardiac progenitor cells.     

TBX5 is required for embryonic cardiac cell cycle progression

Despite the critical importance of TBX5 in normal development and disease, relatively little is known about the mechanisms by which TBX5 functions in the embryonic heart. Our present studies demonstrate that TBX5 is necessary to control the length of the embryonic cardiac cell cycle, with depletion of TBX5 leading to cardiac cell cycle arrest in late G(1)- or early S-phase. Blocking cell cycle progression by TBX5 depletion leads to a decrease in cardiac cell number, an alteration in the timing of the cardiac differentiation program, defects in cardiac sarcomere formation, and ultimately, to cardiac programmed cell death. In these studies we have also established that terminally differentiated cardiomyocytes retain the capacity to undergo cell division. We further show that TBX5 is sufficient to determine the length of the embryonic cardiac cell cycle and the timing of the cardiac differentiation program. Thus, these studies establish a role for TBX5 in regulating the progression of the cardiac cell cycle.     

Neuregulin-1alpha and beta isoform expression in cardiac microvascular endothelial cells and function in cardiac myocytes in vitro

Neuregulins (NRGs) are a family of alternatively spliced growth factors that act through receptor tyrosine kinases of the epidermal growth factor (EGF) receptor family in diverse tissues. The NRG-erbB signaling axis is a critical mediator of cardiac development, and growing evidence supports a role for this system in the intricate cross-talk between the microvascular endothelium and myocytes in the adult heart. The purpose of this study was first to examine the expression of splice variants of the NRG1 gene in adult rat cardiac microvascular endothelial cells and second to compare the function of these variants in cardiac myocytes. We demonstrate that cardiac microvascular endothelial cells in rat culture express multiple Type I NRG1 gene products, including both alpha and beta variants. Comparison of the activity of recombinant NRG1alpha and NRG1beta EGF-like domain proteins in cardiac myocytes shows that the beta ligand is a more potent activator of receptor phosphorylation and intracellular signaling than the alpha ligand, and only the beta ligand stimulated glucose uptake and protein synthesis in these culture conditions. Thus, cardiac microvascular endothelial cells express multiple NRG1 isotypes, but only beta-variants are biologically active on cardiac myocytes.     

A TRP to cardiac fibroblast differentiation

The differentiation of cardiac fibroblasts to myofibroblasts is one of the key events during cardiac remodeling, however, the molecular mechanism underlying this process is not well known. Calcium signaling plays an important role in the regulation of cardiac fibroblast function, but its role in the differentiation of fibroblasts is undefined. Recently four Transient Receptor Potential (TRP) channels TRPM7, TRPC3, TRPC6 and TRPV4 were shown to be crucial for the differentiation of cardiac fibroblasts to myofibroblasts. This addendum sums up the roles described for these four TRP channels in cardiac fibroblast differentiation, and discusses the possible molecular mechanisms underlying this process and its relevance for cardiac remodeling in disease.     

Interleukin-6 and cardiac operations

Interleukin (IL)-6 is a pleiotropic inflammatory cytokine with both pro- and anti-inflammatory capacities, produced by different cells and tissues, such as leukocytes, adipocytes, and endothelium. From the viewpoint of cardiologists, this cytokine is a reliable biomarker of cardiac dysfunction, occurrence of atrial fibrillation, cardiac myxoma with recurrence, remote metastasis or embolization, and atherosclerotic processes. Although IL-6 levels were detected in patients undergoing cardiac operations and reported sporadically, the perioperative kinetics of IL-6 in cardiac surgical patients was insufficiently elaborated. The influencing factors, clinical implications, and causative effects of IL-6 on clinical outcomes and potential treatment choices among cardiac surgical patients remained to be clarified as well. The purpose of this article is to discuss these aspects of IL-6 in patients undergoing a cardiac operation.     

Allicin protects against cardiac hypertrophy and fibrosis via attenuating reactive oxygen species-dependent signaling pathways

Increased oxidative stress has been associated with the pathogenesis of chronic cardiac hypertrophy and heart failure. Since allicin suppresses oxidative stress in vitro and in vivo, we hypothesized that allicin would inhibit cardiac hypertrophy through blocking oxidative stress-dependent signaling. We examined this hypothesis using primary cultured cardiac myocytes and fibroblasts and one well-established animal model of cardiac hypertrophy. Our results showed that allicin markedly inhibited hypertrophic responses induced by Ang II or pressure overload. The increased reactive oxygen species (ROS) generation and NADPH oxidase activity were significantly suppressed by allicin. Our further investigation revealed this inhibitory effect on cardiac hypertrophy was mediated by blocking the activation of ROS-dependent ERK1/2, JNK1/2 and AKT signaling pathways. Additional experiments demonstrated allicin abrogated inflammation and fibrosis by blocking the activation of nuclear factor-κB and Smad 2/3 signaling, respectively. The combination of these effects resulted in preserved cardiac function in response to cardiac stimuli. Consequently, these findings indicated that allicin protected cardiac function and prevented the development of cardiac hypertrophy through ROS-dependent mechanism involving multiple intracellular signaling.     

Neurohormonal regulation of myocardial cell apoptosis during the development of heart failure

Adult cardiac myocytes are terminally differentiated cells that are no longer able to divide. Accumulating data support the idea that apoptosis in these cells is involved in the transition from cardiac compensation to decompensated heart failure. Since a number of neurohormonal factors are activated in this state, these factors may be involved in the positive and negative regulation of apoptosis in cardiac myocytes. beta1-Adrenergic receptor and angiotensin type 1 receptor pathways, nitric oxide and natriuretic peptides are involved in the induction of apoptosis in these cells, while alpha1- and beta2-adrenergic receptor and endothelin-1 type A receptor pathways and gp130-related cytokines are antiapoptotic. The myocardial protection of the latter is mediated, at least in part, through mitogen-activated protein kinase-dependent pathways, compatible with the findings in other cell types. In contrast, signaling pathways leading to apoptosis in cardiac myocytes are distinct from those in other cell types. The cAMP/PKA pathway induces apoptosis in cardiac myocytes and blocks apoptosis in other cell types. The p300 protein, a coactivator of p53, mediates apoptosis in fibroblasts but appears to play a protective role in differentiated cardiac myocytes. The inhibition of myocardial cell apoptosis in heart failure may be achieved by directly blocking apoptosis signaling pathways or by modulating neurohormonal factors involved in their regulation. These may provide novel therapeutic strategies in some forms of heart failure.     

Cardiac GPCRs: GPCR signaling in healthy and failing hearts

G protein-coupled receptors (GPCRs) are widely implicated in human heart disease, making them an important target for cardiac drug therapy. The most commonly studied and clinically targeted cardiac GPCRs include the adrenergic, angiotensin, endothelin, and adenosine receptors. Treatment options focusing on the complex and integrated signaling pathways of these GPCRs are critical for the understanding and amelioration of heart disease. The focus of this review is to highlight the most commonly studied and clinically targeted cardiac GPCRs, placing emphasis on their common signaling components implicated in cardiac disease.     

Alpha-adrenergic receptor modulation of the intrathoracic efferent sympathetic nervous system regulating the canine heart

The augmentation of ventricular inotropism induced by electrical stimulation of acutely decentralized efferent sympathetic preganglionic axons was reduced, but still present, following administration of hexamethonium (10 mg/kg i.v.). While hexamethonium continued to be administered, the cardiac augmentations so induced were enhanced significantly following administration of the alpha-adrenergic receptor blocking agent, phentolamine myselate (1 mg/kg i.v.). Stimulation of the sympathetic efferent postganglionic axons in cardiopulmonary nerves induced cardiac augmentations that were unchanged following administration of these agents singly or together. The cardiac augmentations induced by stimulation of efferent preganglionic sympathetic axons were unchanged when phentolamine was administered alone. The augmentations of cardiac inotropism induced by efferent postganglionic sympathetic axonal stimulation were decreased following local administration of the beta-adrenergic antagonist timolol into the ipsilateral stellate and middle cervical ganglia. Thereafter, these augmentations were unchanged following the subsequent intravenous administration of phentolamine. It is concluded that the activation of cardiac neurons in the stellate and middle cervical ganglia by stimulation of efferent preganglionic sympathetic axons can be modified by alpha-adrenergic receptors and that these effects are dependent upon beta-adrenergic receptors, not nicotinic ones, in intrathoracic ganglia.