The relative phospholamban and SERCA2 ratio: a critical determinant of myocardial contractility

Activation of the cannabinoid type 2 (CB2) receptor improves cardiac contractile performance in fish, Brycon amazonicus

In addition to their well-known classical effects, cannabinoid CB1 and CB2 receptors have also been involvement in both deleterious and protective actions on the heart under various pathological conditions. While the potential therapeutic applications of the endocannabinoid system in the context of cardiovascular function are indeed a viable prospect, significant debate exists within the literature regarding whether CB1, CB2, or a combination of both receptors exert a favorable influence on cardiac function. Hence, the aim of this study was to investigate the effects of CB1 + CB2 or CB2 agonists on cardiac excitation-contraction (E-C) coupling, utilizing fish (Brycon amazonicus) as an experimental model. The CB2 agonist elicited marked positive inotropic and lusitropic responses in isolated ventricular myocardium, induced cyclic adenosine 3',5'-monophosphate (cAMP) production, and upregulated critical Ca2+ handling proteins, such as sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) and Na+/Ca2+ exchanger (NCX). Our current study demonstrated, for the first time, that CB2 receptor activation-induced effects improved the efficiency of Ca2+ cycling, excitation-contraction coupling (E-C coupling), and cardiac performance in under physiological conditions. Hence, CB2 receptors could be considered a potential therapeutic target for modulating cardiac contractile dysfunctions.

Gene therapy for heart failure: A novel treatment for the age old disease

Across the globe, cardiovascular disease (CVD) is the leading cause of mortality. According to reports, around 6.2 million people in the United states have heart failure. Current standards of care for heart failure can delay but not prevent progression of disease. Gene therapy is one of the novel treatment modalities that promises to fill this limitation in the current standard of care for Heart Failure. In this paper we performed an extensive search of the literature on various advances made in gene therapy for heart failure till date. We review the delivery methods, targets, current applications, trials, limitations and feasibility of gene therapy for heart failure. Various methods have been employed till date for administering gene therapies including but not limited to arterial and venous infusion, direct myocardial injection and pericardial injection. Various strategies such as AC6 expression, S100A1 protein upregulation, VEGF-B and SDF-1 gene therapy have shown promise in recent preclinical trials. Furthermore, few studies even show that stimulation of cardiomyocyte proliferation such as through cyclin A2 overexpression is a realistic avenue. However, a considerable number of obstacles need to be overcome for gene therapy to be part of standard treatment of care such as definitive choice of gene, gene delivery systems and a suitable method for preclinical trials and clinical trials on patients. Considering the challenges and taking into account the recent advances in gene therapy research, there are encouraging signs to indicate gene therapy for heart failure to be a promising treatment modality for the future. However, the time and feasibility of this option remains in a situation of balance.

In Vivo Treatment with a Standardized Green Tea Extract Restores Cardiomyocyte Contractility in Diabetic Rats by Improving Mitochondrial Function through SIRT1 Activation

Background. Green tea catechins are known to promote mitochondrial function, and to modulate gene expression and signalling pathways that are altered in the diabetic heart. We thus evaluated the effectiveness of the in vivo administration of a standardized green tea extract (GTE) in restoring cardiac performance, in a rat model of early streptozotocin-induced diabetes, with a focus on the underlying mechanisms. Methods. Twenty-five male adult Wistar rats were studied: the control (n = 9), untreated diabetic animals (n = 7) and diabetic rats subjected to daily GTE administration for 28 days (n = 9). Isolated ventricular cardiomyocytes were used for ex vivo measurements of cell mechanics and calcium transients, and molecular assays, including the analysis of functional protein and specific miRNA expression. Results. GTE treatment induced an almost complete recovery of cardiomyocyte contractility that was markedly impaired in the diabetic cells, by preserving mitochondrial function and energy availability, and modulating the expression of the sarcoplasmic reticulum calcium ATPase and phospholamban. Increased Sirtuin 1 (SIRT1) expression and activity substantially contributed to the observed cardioprotective effects. Conclusions. The data supported the hypothesis that green tea dietary polyphenols, by targeting SIRT1, can constitute an adjuvant strategy for counteracting the initial damage of the diabetic heart, before the occurrence of diabetic cardiomyopathy.

Structure-Function Relationships and Modifications of Cardiac Sarcoplasmic Reticulum Ca2+-Transport

Sarcoplasmic reticulum (SR) is a specialized tubular network, which not only maintains the intracellular concentration of Ca2+ at a low level but is also known to release and accumulate Ca2+ for the occurrence of cardiac contraction and relaxation, respectively. This subcellular organelle is composed of several phospholipids and different Ca2+-cycling, Ca2+-binding and regulatory proteins, which work in a coordinated manner to determine its function in cardiomyocytes. Some of the major proteins in the cardiac SR membrane include Ca2+-pump ATPase (SERCA2), Ca2+-release protein (ryanodine receptor), calsequestrin (Ca2+-binding protein) and phospholamban (regulatory protein). The phosphorylation of SR Ca2+-cycling proteins by protein kinase A or Ca2+-calmodulin kinase (directly or indirectly) has been demonstrated to augment SR Ca2+-release and Ca2+-uptake activities and promote cardiac contraction and relaxation functions. The activation of phospholipases and proteases as well as changes in different gene expressions under different pathological conditions have been shown to alter the SR composition and produce Ca2+-handling abnormalities in cardiomyocytes for the development of cardiac dysfunction. The post-translational modifications of SR Ca2+ cycling proteins by processes such as oxidation, nitrosylation, glycosylation, lipidation, acetylation, sumoylation, and O GlcNacylation have also been reported to affect the SR Ca2+ release and uptake activities as well as cardiac contractile activity. The SR function in the heart is also influenced in association with changes in cardiac performance by several hormones including thyroid hormones and adiponectin as well as by exercise-training. On the basis of such observations, it is suggested that both Ca2+-cycling and regulatory proteins in the SR membranes are intimately involved in determining the status of cardiac function and are thus excellent targets for drug development for the treatment of heart disease.

Titin M-line insertion sequence 7 is required for proper cardiac function in mice

Titin is a giant sarcomeric protein that is involved in a large number of functions, with a primary role in skeletal and cardiac sarcomere organization and stiffness. The titin gene (TTN) is subject to various alternative splicing events, but in the region that is present at the M-line, the only exon that can be spliced out is Mex5, which encodes for the insertion sequence 7 (is7). Interestingly, in the heart, the majority of titin isoforms are Mex5+, suggesting a cardiac role for is7. Here, we performed comprehensive functional, histological, transcriptomic, microscopic and molecular analyses of a mouse model lacking the Ttn Mex5 exon (ΔMex5), and revealed that the absence of the is7 is causative for dilated cardiomyopathy. ΔMex5 mice showed altered cardiac function accompanied by increased fibrosis and ultrastructural alterations. Abnormal expression of excitation-contraction coupling proteins was also observed. The results reported here confirm the importance of the C-terminal region of titin in cardiac function and are the first to suggest a possible relationship between the is7 and excitation-contraction coupling. Finally, these findings give important insights for the identification of new targets in the treatment of titinopathies.

Alpha and beta myosin isoforms and human atrial and ventricular contraction

Human atrial and ventricular contractions have distinct mechanical characteristics including speed of contraction, volume of blood delivered and the range of pressure generated. Notably, the ventricle expresses predominantly β-cardiac myosin while the atrium expresses mostly the α-isoform. In recent years exploration of the properties of pure α- & β-myosin isoforms have been possible in solution, in isolated myocytes and myofibrils. This allows us to consider the extent to which the atrial vs ventricular mechanical characteristics are defined by the myosin isoform expressed, and how the isoform properties are matched to their physiological roles. To do this we Outline the essential feature of atrial and ventricular contraction; Explore the molecular structural and functional characteristics of the two myosin isoforms; Describe the contractile behaviour of myocytes and myofibrils expressing a single myosin isoform; Finally we outline the outstanding problems in defining the differences between the atria and ventricles. This allowed us consider what features of contraction can and cannot be ascribed to the myosin isoforms present in the atria and ventricles.

Effects of Standardized Green Tea Extract and Its Main Component, EGCG, on Mitochondrial Function and Contractile Performance of Healthy Rat Cardiomyocytes

We recently showed that the long-term in vivo administration of green tea catechin extract (GTE) resulted in hyperdynamic cardiomyocyte contractility. The present study investigates the mechanisms underlying GTE action in comparison to its major component, epigallocatechin-3-gallate (EGCG), given at the equivalent amount that would be in the entirety of GTE. Twenty-six male Wistar rats were given 40 mL/day of a tap water solution with either standardized GTE or pure EGCG for 4 weeks. Cardiomyocytes were then isolated for the study. Cellular bioenergetics was found to be significantly improved in both GTE- and EGCG-fed rats compared to that in controls as shown by measuring the maximal mitochondrial respiration rate and the cellular ATP level. Notably, the improvement of mitochondrial function was associated with increased levels of oxidative phosphorylation complexes, whereas the cellular mitochondrial mass was unchanged. However, only the GTE supplement improved cardiomyocyte mechanics and intracellular calcium dynamics, by lowering the expression of total phospholamban (PLB), which led to an increase of both the phosphorylated-PLB/PLB and the sarco-endoplasmic reticulum calcium ATPase/PLB ratios. Our findings suggest that GTE might be a valuable adjuvant tool for counteracting the occurrence and/or the progression of cardiomyopathies in which mitochondrial dysfunction and alteration of intracellular calcium dynamics constitute early pathogenic factors.

Novel Therapies for Prevention and Early Treatment of Cardiomyopathies

Heritable cardiomyopathies are a class of heart diseases caused by variations in a number of genetic loci. Genetic variants on one allele lead to either a degraded protein, which causes a haploinsufficiency of that protein, or a nonfunctioning protein that subverts the molecular system within which the protein works. Over years, both of these mechanisms eventually lead to diseased heart tissue and symptoms of a failing heart. Most cardiomyopathy treatments repurpose heart failure drugs to manage these symptoms and avoid adverse outcomes. There are few therapies that correct the underlying pathogenic genetic or molecular mechanism. This review will reflect on this unmet clinical need in genetic cardiomyopathies and consider a variety of therapies that address the mechanism of disease rather than patient symptoms. These therapies are genetic, targeting a defective gene or transcript, or ameliorating a genetic insufficiency. However, there are also a number of small molecules under exploration that modulate downstream faulty protein products affected in cardiomyopathies.

Unique Ca2+-Cycling Protein Abundance and Regulation Sustains Local Ca2+ Releases and Spontaneous Firing of Rabbit Sinoatrial Node Cells

Spontaneous beating of the heart pacemaker, the sinoatrial node, is generated by sinoatrial node cells (SANC) and caused by gradual change of the membrane potential called diastolic depolarization (DD). Submembrane local Ca²⁺ releases (LCR) from sarcoplasmic reticulum (SR) occur during late DD and activate an inward Na⁺/Ca²⁺ exchange current, which accelerates the DD rate leading to earlier occurrence of an action potential. A comparison of intrinsic SR Ca²⁺ cycling revealed that, at similar physiological Ca²⁺ concentrations, LCRs are large and rhythmic in permeabilized SANC, but small and random in permeabilized ventricular myocytes (VM). Permeabilized SANC spontaneously released more Ca²⁺ from SR than VM, despite comparable SR Ca²⁺ content in both cell types. In this review we discuss specific patterns of expression and distribution of SR Ca²⁺ cycling proteins (SR Ca²⁺ ATPase (SERCA2), phospholamban (PLB) and ryanodine receptors (RyR)) in SANC and ventricular myocytes. We link ability of SANC to generate larger and rhythmic LCRs with increased abundance of SERCA2, reduced abundance of the SERCA inhibitor PLB. In addition, an increase in intracellular [Ca²⁺] increases phosphorylation of both PLB and RyR exclusively in SANC. The differences in SR Ca²⁺ cycling protein expression between SANC and VM provide insights into diverse regulation of intrinsic SR Ca²⁺ cycling that drives automaticity of SANC.

Expression of cardiac insulin signalling genes and proteins in rats fed a high-sucrose diet: effect of bilberry anthocyanin extract

Scope

Insulin resistance is associated with impaired cardiac function, but the underlying molecular abnormalities are largely unexplained. Bilberry anthocyanin (BAcn) may be protective, as it appears to potentiate insulin action.

Methods

Rats were randomly allocated to control, sucrose-fed (SF) or sucrose-fed + BAcn diets (SF-A) for 15 weeks. Cardiac insulin signalling genes and proteins were quantified using reverse transcription quantitative real-time polymerase chain reaction and western blots.

Results

Glucose tolerance was not different with treatment. SF showed lower (p < 0.05) ferric reducing antioxidant power, which increased with BAcn. SF resulted in significantly decreased (p < 0.05) expression of 10 genes: acetyl-coenzyme A carboxylase alpha; V-Akt murine thymoma viral oncogene homolog 1; Bcl2-like 1; cytosine-cytosine-adenosine-adenosine-thymidine/enhancer binding protein; FK506 binding protein 12-rapamycin associated; glycerol-3-phosphate dehydrogenase 1 (soluble); solute carrier family 2 (facilitated glucose transporter), member 1, 4; hexokinase 2; and thyroglobulin. SF-A prevented these changes. Compared to SF-A, SF up-regulated (p < 0.05) complement factor D and phosphoinositide-3-kinase, regulatory subunit1 (α); sterol regulatory element binding transcription factor 1 was down-regulated (p < 0.05). SF increased (p < 0.05) cardiac phospholamban and decreased phosphorylated troponin I, which were not attenuated by BAcn. Compared to control or SF, SF-A resulted in significantly lower (p < 0.05) 5′-AMP-activated protein kinase.

Conclusions

SF lowered antioxidant capacity and changed the expression of insulin signalling genes, which were modulated by BAcn.

Molecular genetics and pathogenesis of cardiomyopathy

Cardiomyopathy is defined as a disease of functional impairment in the cardiac muscle and its etiology includes both extrinsic and intrinsic factors. Cardiomyopathy caused by the intrinsic factors is called as primary cardiomyopathy of which two major clinical phenotypes are hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM). Genetic approaches have revealed the disease genes for hereditary primary cardiomyopathy and functional studies have demonstrated that characteristic functional alterations induced by the disease-associated mutations are closely related to the clinical types, such that increased and decreased Ca(2+) sensitivities of muscle contraction are associated with HCM and DCM, respectively. In addition, recent studies have suggested that mutations in the Z-disc components found in HCM and DCM may result in increased and decreased stiffness of sarcomere, respectively. Moreover, functional analysis of mutations in the other components of cardiac muscle have suggested that the altered response to metabolic stresses is associated with cardiomyopathy, further indicating the heterogeneity in the etiology and pathogenesis of cardiomyopathy.

Sex Hormones Promote Opposite Effects on ACE and ACE2 Activity, Hypertrophy and Cardiac Contractility in Spontaneously Hypertensive Rats

Background

There is growing interest in sex differences and RAS components. However, whether gender influences cardiac angiotensin I-converting enzyme (ACE) and angiotensin-converting enzyme 2 (ACE2) activity is still unknown. In the present work, we determined the relationship between ACE and ACE2 activity, left ventricular function and gender in spontaneously hypertensive rats (SHRs).

Methodology / Principal Findings

Twelve-week-old female (F) and male (M) SHRs were divided into 2 experimental groups (n = 7 in each group): sham (S) and gonadectomized (G). Fifty days after gonadectomy, we measured positive and negative first derivatives (dP/dt maximum left ventricle (LV) and dP/dt minimum LV, respectively), hypertrophy (morphometric analysis) and ACE and ACE2 catalytic activity (fluorimetrically). Expression of calcium handling proteins was measured by western blot. Male rats exhibited higher cardiac ACE and ACE2 activity as well as hypertrophy compared to female rats. Orchiectomy decreased the activity of these enzymes and hypertrophy, while ovariectomy increased hypertrophy and ACE2, but did not change ACE activity. For cardiac function, the male sham group had a lower +dP/dt than the female sham group. After gonadectomy, the +dP/dt increased in males and reduced in females. The male sham group had a lower -dP/dt than the female group. After gonadectomy, the -dP/dt increased in the male and decreased in the female groups when compared to the sham group. No difference was observed among the groups in SERCA2a protein expression. Gonadectomy increased protein expression of PLB (phospholamban) and the PLB to SERCA2a ratio in female rats, but did not change in male rats.

Conclusion

Ovariectomy leads to increased cardiac hypertrophy, ACE2 activity, PLB expression and PLB to SERCA2a ratio, and worsening of hemodynamic variables, whereas in males the removal of testosterone has the opposite effects on RAS components.

Calcium signalling microdomains and the t-tubular system in atrial mycoytes: potential roles in cardiac disease and arrhythmias

The atria contribute 25% to ventricular stroke volume and are the site of the commonest cardiac arrhythmia, atrial fibrillation (AF). The initiation of contraction in the atria is similar to that in the ventricle involving a systolic rise of intracellular Ca(2+) concentration ([Ca(2+)](i)). There are, however, substantial inter-species differences in the way systolic Ca(2+) is regulated in atrial cells. These differences are a consequence of a well-developed and functionally relevant transverse (t)-tubule network in the atria of large mammals, including humans, and its virtual absence in smaller laboratory species such as the rat. Where T-tubules are absent, the systolic Ca(2+) transient results from a 'fire-diffuse-fire' sequential recruitment of Ca(2+) release sites from the cell edge to the centre and hence marked spatiotemporal heterogeneity of systolic Ca(2+). Conversely, the well-developed T-tubule network in large mammals ensures a near synchronous rise of [Ca(2+)](i). In addition to synchronizing the systolic rise of [Ca(2+)](i), the presence of T-tubules in the atria of large mammals, by virtue of localization of the L-type Ca(2+) channels and Na(+)-Ca(2+) exchanger antiporters on the T-tubules, may serve to, respectively, accelerate changes in the amplitude of the systolic Ca(2+) transient during inotropic manoeuvres and lower diastolic [Ca(2+)](i). On the other hand, the presence of T-tubules and hence wider cellular distribution of the Na(+)-Ca(2+) exchanger may predispose the atria of large mammals to Ca(2+)-dependent delayed afterdepolarizations (DADs); this may be a determining factor in why the atria of large mammals spontaneously develop and maintain AF.

A functional role for transverse (t-) tubules in the atria

Mammalian ventricular myocytes are characterised by the presence of an extensive transverse (t-) tubule network which is responsible for the synchronous rise of intracellular Ca(2+) concentration ([Ca(2+)]i) during systole. Disruption to the ventricular t-tubule network occurs in various cardiac pathologies and leads to heterogeneous changes of [Ca(2+)]i which are thought to contribute to the reduced contractility and increased susceptibility to arrhythmias of the diseased ventricle. Here we review evidence that, despite the long-held dogma of atrial cells having no or very few t-tubules, there is indeed an extensive and functionally significant t-tubule network present in atrial myocytes of large mammals including human. Moreover, the atrial t-tubule network is highly plastic in nature and undergoes far more extensive remodelling in heart disease than is the case in the ventricle with profound consequences for the resulting systolic Ca(2+) transient. In addition to considering the functional role of the t-tubule network in the healthy and diseased atria we also provide an overview of recent data concerning the putative factors controlling the formation of t-tubules and conclude by posing some important questions that currently remain to be addressed and whether or not targeting t-tubules offers potential novel therapeutic possibilities for heart disease.

Carvedilol prevents ovariectomy-induced myocardial contractile dysfunction in female rat

Carvedilol has beneficial effects on cardiac function in patients with heart failure but its effect on ovariectomy-induced myocardial contractile dysfunction remains unclear. Estrogen deficiency induces myocardial contractile dysfunction and increases cardiovascular disease risk in postmenopausal women. Our aim was to investigate whether carvedilol, a beta receptor blocker, would prevent ovariectomy-induced myocardial contractile dysfunction. Female rats (8 weeks old) that underwent bilateral ovariectomy were randomly assigned to receive daily treatment with carvedilol (OVX+CAR, 20 mg/kg), placebo (OVX) and SHAM for 58 days. Left ventricle papillary muscle was mounted for isometric tension recordings. The inotropic response to Ca²⁺ (0.62 to 3.75 mM) and isoproterenol (Iso 10⁻⁸ to 10⁻² M) were assessed. Expression of calcium handling proteins was measured by western blot analysis. Carvedilol treatment in the OVX animals: prevented weight gain and slight hypertrophy, restored the reduced positive inotropic responses to Ca²⁺ and isoproterenol, prevented the reduction in SERCA2a expression, abolished the increase in superoxide anion production, normalized the increase in p22^phox expression, and decreased serum angiotensin converting enzyme (ACE) activity. This study demonstrated that myocardial contractile dysfunction and SERCA2a down regulation were prevented by carvedilol treatment. Superoxide anion production and NADPH oxidase seem to be involved in this response.

Low glial angiotensinogen improves body habitus, diastolic function, and exercise tolerance in aging male rats

OBJECTIVES

Long-term systemic blockade of the renin-angiotensin system (RAS) with either an angiotensin (Ang) II type 1 receptor antagonist or an angiotensin-converting enzyme inhibitor attenuates age-related cardiac remodeling and oxidative damage, and improves myocardial relaxation. However, the role of the brain RAS in mediating the development of diastolic dysfunction during aging is not known. We hypothesized that low brain RAS protects against the development of age-related diastolic dysfunction and left ventricular remodeling.

METHODS

Sixty-week-old transgenic male ASrAOGEN rats (n =9), with normal circulating Ang II and functionally low brain Ang II, because of a GFAP promoter-linked angiotensinogen antisense targeted to glia, and age-matched and sex-matched Hannover Sprague-Dawley (SD; n= 9) rats, with normal levels of both circulating and brain Ang II, underwent echocardiograms to evaluate cardiac structure and function. Postmortem hearts were further compared for histological, molecular, and biochemical changes consistent with cardiac aging.

RESULTS

ASrAOGEN rats showed preserved systolic and diastolic function at mid-life and this was associated with a lower, more favorable ratio of the phospholamban-SERCA2 ratio, reduced incidence of histological changes in the left ventricle, and increased cardiac Ang-(1-7) when compared with the in-vivo functional, and ex-vivo structural and biochemical indices from age-matched SD rats. Moreover, ASrAOGEN rats had lower percent body fat and a superior exercise tolerance when compared with SD rats of the same age.

CONCLUSION

Our data indicate that the central RAS plays a role in the maintenance of diastolic function and exercise tolerance in mid-life and this may be related to effects on body habitus.

The ryanodine receptor channel as a molecular motif in atrial fibrillation: pathophysiological and therapeutic implications

Atrial fibrillation (AF) is the most common cardiac arrhythmia and is associated with substantial morbidity and mortality. It causes profound changes in sarcoplasmic reticulum (SR) Ca(2+) homeostasis, including ryanodine receptor channel dysfunction and diastolic SR Ca(2+) leak, which might contribute to both decreased contractile function and increased propensity to atrial arrhythmias. In this review, we will focus on the molecular basis of ryanodine receptor channel dysfunction and enhanced diastolic SR Ca(2+) leak in AF. The potential relevance of increased incidence of spontaneous SR Ca(2+) release for both AF induction and/or maintenance and the development of novel mechanism-based therapeutic approaches will be discussed.

Ion channels, transporters, and pumps as targets for heart failure therapy

Congestive heart failure is a leading cause of morbidity and mortality. Congestive heart failure is marked by atrial and ventricular enlargements and reduced cardiac contractility and an association with an increased incidence of atrial and ventricular arrhythmias and sudden cardiac death. Dysfunctional ion channel function is one of the major underlying mechanisms of the reduced contractility and arrhythmias. In this review, we explore the utility of ion channels, transporters, and pumps as targets for the treatment of heart failure, focusing predominantly on the treatment for reduced contractility and arrhythmias.

Molecular basis of hereditary cardiomyopathy: abnormalities in calcium sensitivity, stretch response, stress response and beyond

Cardiomyopathy is caused by functional abnormality of cardiac muscle. The functional abnormality involved in its etiology includes both extrinsic and intrinsic factors, and cardiomyopathy caused by the intrinsic factors is called as idiopathic or primary cardiomyopathy. There are several clinical types of primary cardiomyopathy including hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM). Linkage studies and candidate gene approaches have explored the disease genes for hereditary primary cardiomyopathy. The most notable finding was that mutations in the same disease gene can be found in different clinical types of cardiomyopathy. Functional analyses of disease-related mutations have revealed that characteristic functional alterations are associated with the clinical types, such that increased and decreased Ca(2+) sensitivity due to sarcomere mutations are associated with HCM and DCM, respectively. In addition, our recent studies have suggested that mutations in the Z-disc components found in HCM and DCM may result in increased and decreased stiffness of sarcomere; that is, stiff sarcomere and loose sarcomere, respectively, and hence altered stretch response. More recently, mutations in the components of I region were found in hereditary cardiomyopathy and the functional analyses of the mutations suggested that the altered stress response was associated with cardiomyopathy, further complicating the etiology and pathogenesis. However, elucidation of genetic etiology and functional alterations caused by the mutations shed lights on the new therapeutic approaches to hereditary cardiomyopathy, such that treatment of DCM with a Ca(2+) sensitizer prevented the disease in a mouse model.

Rescue of familial cardiomyopathies by modifications at the level of sarcomere and Ca2+ fluxes

Cardiomyopathies are a heterogeneous group of diseases of the myocardium associated with mechanical and/or electrical dysfunction that frequently show inappropriate ventricular hypertrophy or dilation. Current data suggest that numerous mutations in several genes can cause cardiomyopathies, and the severity of their phenotypes is also influenced by modifier genes. Two major types of inherited cardiomyopathies include familial hypertrophic cardiomyopathy (FHC) and dilated cardiomyopathy (DCM). FHC typically involves increased myofilament Ca(2+) sensitivity associated with diastolic dysfunction, whereas DCM often results in decreased myofilament Ca(2+) sensitivity and systolic dysfunction. Besides alterations in myofilament Ca(2+) sensitivity, alterations in the levels of Ca(2+)-handling proteins have also been described in both diseases. Recent work in animal models has attempted to rescue FHC and DCM via modifications at the myofilament level, altering Ca(2+) homeostasis by targeting Ca(2+)-handling proteins, such as the sarcoplasmic reticulum ATPase and phospholamban, or by interfering with the products of different modifiers genes. Although attempts to rescue cardiomyopathies in animal models have shown great promise, further studies are needed to validate these strategies in order to provide more effective and specific treatments.

Modelling sarcoplasmic reticulum calcium ATPase and its regulation in cardiac myocytes

When developing large-scale mathematical models of physiology, some reduction in complexity is necessarily required to maintain computational efficiency. A prime example of such an intricate cell is the cardiac myocyte. For the predictive power of the cardiomyocyte models, it is vital to accurately describe the calcium transport mechanisms, since they essentially link the electrical activation to contractility. The removal of calcium from the cytoplasm takes place mainly by the Na(+)/Ca(2+) exchanger, and the sarcoplasmic reticulum Ca(2+) ATPase (SERCA). In the present study, we review the properties of SERCA, its frequency-dependent and beta-adrenergic regulation, and the approaches of mathematical modelling that have been used to investigate its function. Furthermore, we present novel theoretical considerations that might prove useful for the elucidation of the role of SERCA in cardiac function, achieving a reduction in model complexity, but at the same time retaining the central aspects of its function. Our results indicate that to faithfully predict the physiological properties of SERCA, we should take into account the calcium-buffering effect and reversible function of the pump. This 'uncomplicated' modelling approach could be useful to other similar transport mechanisms as well.

Changes in gene expression during the formation of bioengineered heart muscle

A three-dimensional bioengineered heart muscle (BEHM) construct model had been previously developed, exhibiting contractile forces up to 800 microN. The interest of this study was to determine gene expression levels of biologic markers involved in calcium-handling between BEHM, cell monolayer, and neonatal heart. Cardiac cells were isolated from one litter of F344 rats and organized into groups (n = 5): 4-, 7-, 10-day BEHM and cell monolayer; BEHM was evaluated for cell viability and contractility. Groups were then analyzed for mRNA expression of calcium-handling proteins: myosin heavy chain (MHC) alpha and beta, Sarcoplasmic reticulum Ca++ ATPase (SERCA) 2, phospholamban (PBL), and ryanodine receptor. BEHM exhibited electrically stimulated active force (208 +/- 12 microN day 4, 361 +/- 22 microN day 7, and 344 +/- 29 microN day 10) and no decrease in cell number. Real-time polymerase chain reaction (PCR) showed an increase in gene expression of all calcium-handling proteins in BEHM at 7 and 10 days compared with monolayers, for example, comparing BEHM to monolayer (7 and 10 days, respectively), MHC-alpha: 2600-fold increase and a 100-fold increase; MHC-beta: 70-fold increase at 10 days; ryanodine receptor: 74-fold increase at 10 days; SERCA: 19-fold increase and sixfold increase; PBL: 158-fold increase and 24-fold increase. It was concluded that a three-dimensional environment is a better culturing condition of cardiac cells than a monolayer. Also, BEHM constructs demonstrated a high similarity to a native myocardium, and is, thus, a good starting foundation for engineered heart muscle.

Differences in intracellular calcium homeostasis between atrial and ventricular myocytes

The role that Ca(2+) plays in ventricular excitation contraction coupling is well defined and much is known about the marked differences in the spatiotemporal properties of the systolic Ca(2+) transient between atrial and ventricular myocytes. However, to date there has been no systematic appraisal of the Ca(2+) homeostatic mechanisms employed by atrial cells and how these compare to the ventricle. In the present study we sought to determine the fractional contributions made to the systolic Ca(2+) transient and the decay of [Ca(2+)](i) by the sarcoplasmic reticulum and sarcolemmal mechanisms. Experiments were performed on single myocytes isolated from the atria and ventricles of the rat. Intracellular Ca(2+) concentration, membrane currents, SR Ca(2+) content and cellular Ca(2+) buffering capacity were measured at 23 degrees C. Atrial cells had smaller systolic Ca(2+) transients (251+/-39 vs. 376+/-41 nmol x L(-1)) that decayed more rapidly (7.4+/-0.6 vs. 5.45+/-0.3 s(-1)). This was due primarily to an increased rate of SR mediated Ca(2+) uptake (k(SR), 6.88+/-0.6 vs. 4.57+/-0.3 s(-1)). SR Ca(2+) content was 289% greater and Ca(2+) buffering capacity was increased approximately 3-fold in atrial cells (B(max) 371.9+/-32.4 vs. 121.8+/-8 micromol x L(-1), all differences P<0.05). The fractional release of Ca(2+) from the SR was greater in atrial cells, although the gain of excitation contraction coupling was the same in both cell types. In summary our data demonstrate fundamental differences in Ca(2+) homeostasis between atrial and ventricular cells and we speculate that the increased SR Ca(2+) content may be significant in determining the increased prevalence of arrhythmias in the atria.

Designing heart performance by gene transfer

The birth of molecular cardiology can be traced to the development and implementation of high-fidelity genetic approaches for manipulating the heart. Recombinant viral vector-based technology offers a highly effective approach to genetically engineer cardiac muscle in vitro and in vivo. This review highlights discoveries made in cardiac muscle physiology through the use of targeted viral-mediated genetic modification. Here the history of cardiac gene transfer technology and the strengths and limitations of viral and nonviral vectors for gene delivery are reviewed. A comprehensive account is given of the application of gene transfer technology for studying key cardiac muscle targets including Ca(2+) handling, the sarcomere, the cytoskeleton, and signaling molecules and their posttranslational modifications. The primary objective of this review is to provide a thorough analysis of gene transfer studies for understanding cardiac physiology in health and disease. By comparing results obtained from gene transfer with those obtained from transgenesis and biophysical and biochemical methodologies, this review provides a global view of cardiac structure-function with an eye towards future areas of research. The data presented here serve as a basis for discovery of new therapeutic targets for remediation of acquired and inherited cardiac diseases.

Molecular etiology and pathogenesis of hereditary cardiomyopathy

Cardiomyopathy is defined as a cardiac disease caused by functional abnormality of cardiac muscle, and the etiology of the functional abnormality includes both extrinsic and intrinsic factors. Cardiomyopathy caused by the intrinsic factors is defined as idiopathic or primary cardiomyopathy, and there are several clinical phenotypes, including hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM). The major intrinsic factor is gene mutations, and linkage studies, as well as candidate gene approaches, have deciphered multiple disease genes for hereditary primary cardiomyopathy. Of note is that mutations in the same disease gene can be found in different clinical phenotypes of cardiomyopathy. Functional analyses of disease-related mutations have revealed that characteristic functional alterations are associated with the clinical phenotypes, such that increased and decreased Ca(2+) sensitivity because of sarcomere mutations are associated with HCM and DCM, respectively. In addition, recent data have suggested that mutations in the Z-disc components found in HCM and DCM may result in increased and decreased stiffness of the sarcomere (ie, stiff sarcomere and loose sarcomere, respectively). More recently, mutations in the components of the I region can be found in hereditary cardiomyopathy, further complicating the etiology of primary cardiomyopathy.

Gene therapy in heart failure

With increasing knowledge of basic molecular mechanisms governing the development of heart failure (HF), the possibility of specifically targeting key pathological players is evolving. Technology allowing for efficient in vivo transduction of myocardial tissue with long-term expression of a transgene enables translation of basic mechanistic knowledge into potential gene therapy approaches. Gene therapy in HF is in its infancy clinically with the predominant amount of experience being from animal models. Nevertheless, this challenging and promising field is gaining momentum as recent preclinical studies in larger animals have been carried out and, importantly, there are 2 newly initiated phase I clinical trials for HF gene therapy. To put it simply, 2 parameters are needed for achieving success with HF gene therapy: (1) clearly identified detrimental/beneficial molecular targets; and (2) the means to manipulate these targets at a molecular level in a sufficient number of cardiac cells. However, several obstacles do exist on our way to efficient and safe gene transfer to human myocardium. Some of these obstacles are discussed in this review; however, it primarily focuses on the molecular target systems that have been subjected to intense investigation over the last decade in an attempt to make gene therapy for human HF a reality.

Neuronal nitric oxide synthase signaling within cardiac myocytes targets phospholamban

Studies have shown that neuronal nitric oxide synthase (nNOS, NOS1) knockout mice (NOS1-/-) have increased or decreased contractility, but consistently have found a slowed rate of intracellular Ca2+ ([Ca2+]i) decline and relengthening. Contraction and [Ca2+]i decline are determined by many factors, one of which is phospholamban (PLB). The purpose of this study is to determine the involvement of PLB in the NOS1-mediated effects. Force-frequency experiments were performed in trabeculae isolated from NOS1-/- and wild-type (WT) mice. We also simultaneously measured Ca2+ transients (Fluo-4) and cell shortening (edge detection) in myocytes isolated from WT, NOS1-/-, and PLB-/- mice. NOS1-/- trabeculae had a blunted force-frequency response and prolonged relaxation. We observed similar effects in myocytes with NOS1 knockout or specific NOS1 inhibition with S-methyl-l-thiocitrulline (SMLT) in WT myocytes (i.e., decreased Ca2+ transient and cell shortening amplitudes and prolonged decline of [Ca2+]i). Alternatively, NOS1 inhibition with SMLT in PLB-/- myocytes had no effect. Acute inhibition of NOS1 with SMLT in WT myocytes also decreased basal PLB serine16 phosphorylation. Furthermore, there was a decreased SR Ca2+ load with NOS1 knockout or inhibition, which is consistent with the negative contractile effects. Perfusion with FeTPPS (peroxynitrite decomposition catalyst) mimicked the effects of NOS1 knockout or inhibition. beta-Adrenergic stimulation restored the slowed [Ca2+]i decline in NOS1-/- myocytes, but a blunted contraction remained, suggesting additional protein target(s). In summary, NOS1 inhibition or knockout leads to decreased contraction and slowed [Ca2+]i decline, and this effect is absent in PLB-/- myocytes. Thus NOS1 signaling modulates PLB serine16 phosphorylation, in part, via peroxynitrite.

eNOS deficient mice develop progressive cardiac hypertrophy with altered cytokine and calcium handling protein expression

Although studies have shown that endothelial nitric oxide synthase (eNOS) homozygous knockout mice (eNOS-/-) develop left ventricular (LV) hypertrophy, well compensated at least to 24 wks, uncertainty still exists as to the cardiac functional and molecular mechanistic consequences of eNOS deficiency at later time-points. To bridge the gap in existent data, we examined whole hearts from eNOS-/- and age-matched wild-type (WT) control mice ranging in age from 18 to 52 wks for macroscopic and microscopic histopathology, LV mRNA and protein expression using RNA Dot blots and Western blots, respectively, and LV function using isolated perfused work-performing heart preparations. Heart weight to body weight (HW/BW in mg/g) ratio increased significantly as eNOS-/- mice aged (82.2%, P < 0.001). Multi-focal replacement fibrosis and myocyte degeneration/death were first apparent in eNOS-/- mouse hearts at 40 wks. Progressive increases in LV atrial natriuretic factor (ANF) and alpha-skeletal actin mRNA levels both correlated significantly with increasing HW/BW ratio in aged eNOS-/- mice (r = 0.722 and r = 0.648, respectively; P < 0.001). At 52 wks eNOS-/- mouse hearts exhibited basal LV hypercontractility yet blunted beta adrenergic receptor (betaAR) responsiveness that coincided with a significant reduction in the LV ratio of phospholamban to sarcoplasmic reticulum Ca2+-ATPase-2a protein levels and was preceded by a significant upregulation in LV steady-state mRNA and protein levels of the 28 kDa membrane-bound form of tumor necrosis factor-alpha. We conclude that absence of eNOS in eNOS-/- mice results in a progressive concentric hypertrophic cardiac phenotype that is functionally compensated with decreased betaAR responsiveness, and is associated with a potential cytokine-mediated alteration of calcium handling protein expression.

Tight interplay between the Ca2+ affinity of the cardiac SERCA2 Ca2+ pump and the SERCA2 expression level

A reduced activity of the sarcoplasmic reticulum Ca2+ pump SERCA2a is a hallmark of cardiac dysfunction in heart failure. In SERCA2b/b mice, the normal SERCA2a isoform is replaced by SERCA2b, displaying a higher Ca2+ affinity. This elicited decreased cardiac SERCA2 expression and cardiac hypertrophy. Here, the interplay was studied between the increased Ca2+ affinity and a reduced expression of the pump and its role in the cardiac remodeling was investigated. First, SERCA2b/b mice were crossed with SERCA2b transgenes to boost cardiac SERCA2b expression. However, the enforced expression of SERCA2b was spontaneously countered by an increased inhibition by phospholamban (PLB), reducing the pump's Ca2+ affinity. Moreover, the higher SERCA2 content did not prevent hypertrophy. Second, we studied heterozygous SERCA2b/WT mice, which also express lower SERCA2 levels compared to wild-type. Hypertrophy was not observed. In heterozygotes, SERCA2b expression was specifically suppressed, explaining the reduced SERCA2 content. The SERCA2b/WT model strikingly differs from the homozygote models because SERCA2a (not SERCA2b) is the major isoform and because the inhibition of the pump by PLB is decreased instead of being increased. Thus, a tight correlation exists between the SERCA2 levels and Ca2+ affinity (controlled by PLB). This compensatory response may be important to prevent cardiac remodeling.

Sarcolipin and phospholamban as regulators of cardiac sarcoplasmic reticulum Ca2+ ATPase

The cardiac sarcoplasmic reticulum calcium ATPase (SERCA2a) plays a critical role in maintaining the intracellular calcium homeostasis during cardiac contraction and relaxation. It has been well documented over the years that altered expression and activity of SERCA2a can lead to systolic and diastolic dysfunction. The activity of SERCA2a is regulated by two structurally similar proteins, phospholamban (PLB) and sarcolipin (SLN). Although, the relevance of PLB has been extensively studied over the years, the role SLN in cardiac physiology is an emerging field of study. This review focuses on the advances in the understanding of the regulation of SERCA2a by SLN and PLB. In particular, it highlights the similarities and differences between the two proteins and their roles in cardiac patho-physiology.

Diseases associated with altered ryanodine receptor activity

Mutations in two intracellular Ca2+ release channels or ryanodine receptors (RyR1 and RyR2) are associated with a number of human skeletal and cardiac diseases. This chapter discusses these diseases in terms of known mechanisms, controversies, and unanswered questions. We also compare the cardiac and skeletal muscle diseases to explore common mechanisms.

Calcium signaling phenomena in heart diseases: a perspective

Ca(2+) is a major intracellular messenger and nature has evolved multiple mechanisms to regulate free intracellular (Ca(2+))(i) level in situ. The Ca(2+) signal inducing contraction in cardiac muscle originates from two sources. Ca(2+) enters the cell through voltage dependent Ca(2+) channels. This Ca(2+) binds to and activates Ca(2+) release channels (ryanodine receptors) of the sarcoplasmic reticulum (SR) through a Ca(2+) induced Ca(2+) release (CICR) process. Entry of Ca(2+) with each contraction requires an equal amount of Ca(2+) extrusion within a single heartbeat to maintain Ca(2+) homeostasis and to ensure relaxation. Cardiac Ca(2+) extrusion mechanisms are mainly contributed by Na(+)/Ca(2+) exchanger and ATP dependent Ca(2+) pump (Ca(2+)-ATPase). These transport systems are important determinants of (Ca(2+))(i) level and cardiac contractility. Altered intracellular Ca(2+) handling importantly contributes to impaired contractility in heart failure. Chronic hyperactivity of the beta-adrenergic signaling pathway results in PKA-hyperphosphorylation of the cardiac RyR/intracellular Ca(2+) release channels. Numerous signaling molecules have been implicated in the development of hypertrophy and failure, including the beta-adrenergic receptor, protein kinase C, Gq, and the down stream effectors such as mitogen activated protein kinases pathways, and the Ca(2+) regulated phosphatase calcineurin. A number of signaling pathways have now been identified that may be key regulators of changes in myocardial structure and function in response to mutations in structural components of the cardiomyocytes. Myocardial structure and signal transduction are now merging into a common field of research that will lead to a more complete understanding of the molecular mechanisms that underlie heart diseases. Recent progress in molecular cardiology makes it possible to envision a new therapeutic approach to heart failure (HF), targeting key molecules involved in intracellular Ca(2+) handling such as RyR, SERCA2a, and PLN. Controlling these molecular functions by different agents have been found to be beneficial in some experimental conditions.

Enhancement of adenoviral gene transfer to adult rat cardiomyocytes in vivo by immobilization and ultrasound treatment of the heart

Direct injection of adenoviral vectors into ventricular myocardium in vivo produces local transfection of cells including cardiomyocytes. The use of vectors coexpressing GFP with the gene of interest allows subsequent identification of transfected myocytes isolated from the heart some days later, and examination of their function in cell bath experiments. We have injected vectors for antisense to phospholamban, or a control virus for expression of GFP only, into adult rat heart in vivo and then removed the heart and isolated ventricular myocytes 7 days later. Brief immobilization of the ventricle during and after injection using a haemoclip increased the number of transfected rod-shaped, viable myocytes from 1.7 +/- 0.8% (n = 8) to 5.6 +/- 0.8% (n = 9). This was further increased to 13.2 +/- 1.1% (n = 8) by the application of ultrasound pulses to the site before and after injection. Phospholamban antisense increased contraction amplitude and accelerated myocyte relengthening or decline of the Ca(2+) transient in transfected myocytes, while GFP control did not. Qualitative and quantitative effects of phospholamban downregulation were comparable between in vivo and in vitro transfections. This technique will have a number of uses, including production of transfected myocytes without the problem of culture-induced changes in contractility.

Intracellular calcium release and cardiac disease

Intracellular calcium release channels are present on sarcoplasmic and endoplasmic reticuli (SR, ER) of all cell types. There are two classes of these channels: ryanodine receptors (RyR) and inositol 1,4,5-trisphosphate receptors (IP3R). RyRs are required for excitation-contraction (EC) coupling in striated (cardiac and skeletal) muscles. RyRs are made up of macromolecular signaling complexes that contain large cytoplasmic domains, which serve as scaffolds for proteins that regulate the function of the channel. These regulatory proteins include calstabin1/calstabin2 (FKBP12/FKBP12.6), a 12/12.6 kDa subunit that stabilizes the closed state of the channel and prevents aberrant calcium leak from the SR. Kinases and phosphatases are targeted to RyR2 channels and modulate RyR2 function in response to extracellular signals. In the classic fight or flight stress response, phosphorylation of RyR channels by protein kinase A reduces the affinity for calstabin and activates the channels leading to increased SR calcium release. In heart failure, a cardiac insult causes a mismatch between blood supply and metabolic demands of organs. The chronically activated fight or flight response leads to leaky channels, altered calcium signaling, and contractile dysfunction and cardiac arrhythmias.

Construction of phospholamban antisense RNA recombinant adeno-associated virus vector and its effects in rat cardiomyocytes

AIM

To construct a recombinant adeno-associated virus (rAAV) vector containing gene encoding phospholamban antisense RNA (asPLB), and analyse its effect on expression of PLB, expression and activity of sarco-endoplasmic reticulum Ca2+-ATPase (SERCA), and the change of intracellular free Ca2+ concentration ([Ca2+]i) in rat cardiomyocytes.

METHODS

The target gene encoding PLB antisense RNA was inserted inversely into the adeno-associated virus plasmid pAAV-MCS digested by corresponding restricted endonuclease enzyme. The recombinant plasmid and pAAV-RC and pHelper were co-transfected into 293 cell. At the same time, a viral production positive control (rAAV-LacZ) and negative control were performed. The recombinant viruses were used to transfect the cultured rat cardiomyocytes. Site beta-Galactosidase staining were performed to observe the transfer efficiency. Reverse transcription-PCR and Western blot were used to determine the mRNA and protein expression of PLB and SERCA. The activity of SERCA and the [Ca2+]i were measured.

RESULTS

The rAAV vectors were constructed successfully and were transfected into rat cardiomyocytes effectively. The PLB mRNA and protein expression were reduced in rat cardiomyocytes transfected by rAAV-asPLB compared with controls. The activity of SERCA was increased. In rest state, the level of [Ca2+]i in the rAAV-asPLB transfected group decreased. The level of [Ca2+]i increased when induced by isoproterenol.

CONCLUSION

AAV-asPLB vector was constructed successfully, which disrupted the expression of PLB, enhanced the activity of SERCA, reduced the resting [Ca2+]i, and improved the cardiac function.

Role of Diastole in Left Ventricular Function, I: Biochemical and Biomechanical Events

Left ventricular diastolic function plays an important role in cardiac physiology. Lusitropy, the ability of the cardiac myocytes to relax, is affected by both biochemical events within the myocyte and biomechanical events in the left ventricle. β-Adrenergic stimulation alters diastole by enhancing the phosphorylation of phospholamban, a substrate within the myocyte that increases the uptake of calcium ions into the sarcoplasmic reticulum, increasing the rate of relaxation. Troponin I, a regulatory protein involved in the coupling of excitation to contraction, is vital to maintaining the diastolic state; depletion of troponin I can produce diastolic dysfunction. Other biochemical events, such as defects in the voltage-sensitive release mechanism or in inositol triphosphate calcium release channels, have also been implicated in altering diastolic tone. Extracellular collagen determines myocardial stiffness; impaired glucose tolerance can induce an increase in collagen cross-linking and lead to higher end-diastolic pressures. The passive properties of the left ventricle are most accurately measured during the diastasis and atrial contraction phases of diastole. These phases of the cardiac cycle are the least affected by volume status, afterload, inherent viscoelasticity, and the inotropic state of the myocardium. Diastolic abnormalities can be conceptualized by using pressure-volume loops that illustrate myocardial work and both diastolic and systolic pressure-volume relationships. The pressure-volume model is an educational tool that can be used to demonstrate isolated changes in preload, afterload, inotropy, and lusitropy and their interaction.

The metabolic and cardiovascular effects of hyperthyroidism are largely independent of beta-adrenergic stimulation

Hyperthyroidism and states of adrenergic hyperactivity have many common clinical features, suggesting similar pathogenic mechanisms of action. The widespread use of beta-adrenergic receptor (betaAR) antagonists (beta-blockers) to treat hyperthyroidism has led to the belief that the physiological consequences of thyroid hormone (TH) excess are mediated in part via catecholamine signaling through betaARs. To test this hypothesis, we compared the response to TH excess in mice lacking the three known betaARs (beta-less) vs. wild-type (WT) mice. Although beta-less mice had a lower heart rate at baseline in comparison to WT mice, the metabolic and cardiovascular responses to hyperthyroidism were equivalent in both WT and beta-less mice. These data indicate that the metabolic and cardiovascular effects of TH excess are largely independent of betaARs. These findings suggest that the efficacy of clinical treatment of hyperthyroidism with beta-blockers is due to antagonism of sympathetic signaling, and that this process functions independently of TH action.

Gene transfer in cardiac myocytes

Congestive heart failure (CHF) represents an enormous clinical problem and remains a leading cause of death despite advances in treatment. New treatments significantly impact mortality and disease course; they do not cure the underlying pathology. Gene transfer, the ability to genetically reprogram the heart in relevant cardiovascular disease models, allows testing the role of specific molecular pathways in disease pathogenesis. Potential therapeutic intervention targets can be then identified and approached with the full spectrum of therapeutic options, including traditional pharmacology, targeted synthesis of small molecule agonists or antagonists, biological agents (cells, antibodies, genetic material), or gene-based therapy. Lessons gleaned from gene transfer experiments on local modulation of cardiac genetic programs will guide attempts to transform early investigations into established therapy.

Impact of estrogen replacement on ventricular myocyte contractile function and protein kinase B/Akt activation

Women with functional ovaries have a lower cardiovascular risk than men and postmenopausal women. However, estrogen replacement therapy remains controversial. This study examined the effect of ovarian hormone deficiency and estrogen replacement on ventricular myocyte contractile function and PKB/Akt activation. Nulliparous female rats were subjected to bilateral ovariectomy (Ovx) or sham operation (sham). A subgroup of Ovx rats received estrogen (E(2)) replacement (40 microg. kg(-1). day(-1)) for 8 weeks. Mechanical and intracellular Ca(2+) properties were evaluated including peak shortening (PS), time to PS (TPS), time to 90% relengthening (TR(90)), maximal velocity of shortening/relengthening (+/-dL/dt), fura 2 fluorescence intensity (FFI), and decay rate. Levels of sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA2a), phospholamban (PLB), and Akt were assessed by Western blot. Ovx promoted body weight gain associated with reduced serum E(2) and uterine weight, all of which were abolished by E(2). Ovx depressed PS and +/-dL/dt, prolonged TPS, TR(90), and decay rate, and enhanced resting FFI, all of which, with the exception of TPS, were restored by E(2). Ovx did not alter the levels of SERCA2a, PLB, and total Akt, but significantly reduced Akt activation [phosphorylated Akt (pAkt)], pAkt/Akt, and the SERCA2a-to-PLB ratio. These alterations in protein expression were restored by E(2). E(2) enhanced PS and +dL/dt in vitro, which was abolished by the E(2) receptor antagonist ICI-182780. Ovx reduced myocyte Ca(2+) responsiveness and lessened stimulating frequency-induced decline in PS, both ablated by E(2). These data suggest that mechanical and protein functions of ventricular myocytes are directly regulated by E(2).

Impaired cardiac function and IGF-I response in myocytes from calmodulin-diabetic mice: role of Akt and RhoA

This study characterized the cardiac contractile function and IGF-I response in a transgenic diabetic mouse model. Mechanical properties were evaluated in cardiac myocytes from OVE26 diabetic and FVB wild-type mice, including peak shortening (PS), time to PS (TPS), time to 90% relengthening (TR(90)) and maximal velocity of shortening/relengthening (+/-dL/dt). Intracellular Ca(2+) was evaluated as Ca(2+)-induced Ca(2+) release [difference in fura 2 fluorescent intensity (Delta FFI)] and fluorescence decay rate (tau). Sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA)2a, phospholamban (PLB), Na(+)-Ca(2+) exchanger (NCX), GLUT4, and the serine-threonine kinase Akt were assessed by Western blot. RhoA and IGF-I/IGF-I receptor mRNA levels were determined by RT-PCR and Northern blot. OVE26 myocytes displayed decreased PS, +/-dL/dt, and Delta FFI associated with prolonged TPS, TR(90), and tau. SERCA2a, NCX, and Akt activation were reduced, whereas PLB and RhoA were enhanced in OVE26 hearts. GLUT4 was unchanged. IGF-I enhanced PS and Delta FFI in FVB but not OVE26 myocytes. IGF-I mRNA was increased, but IGF-I receptor mRNA was reduced in OVE26 hearts and livers. These results validate diabetic cardiomyopathy in OVE26 mice due to reduced SERCA2, NCX, IGF-I response, and Akt activation associated with enhanced RhoA level, suggesting a therapeutic potential for Akt and RhoA.

Evaluation of left ventricular diastolic function from spectral and color M-mode Doppler in genetically altered mice

Doppler indices of transmitral flow are commonly used to assess noninvasively left ventricular (LV) diastolic function in species larger than mice. The objective of our study was to characterize patterns of LV diastolic function in 2 genetically altered mouse models using Doppler- and color M-mode echocardiography. Phospholamban (PLB) reversibly inhibits the sarcoplasmic reticulum Ca(2+) ATPase (SERCA) and is a key regulator of myocardial relaxation. Twelve-week-old PLB knockout mice (PLB/KO) were examined in parallel with age-matched transgenic mice expressing a mutant form of PLB (PLB/N27A) that exhibited superinhibition of SERCA. Transmitral Doppler flow indexes, including isovolumic relaxation time, the ratio of peak early-to-late filling velocities, and deceleration time of peak early transmitral velocity indicate impaired diastolic filling in the PLB/N27A mutants, but improved LV diastolic function in the PLB/KO mice. In addition, a relatively load-independent parameter of LV relaxation measured by color M-mode Doppler, the propagation velocity of early flow into the LV cavity confirmed the observed differences. We conclude that transmitral filling patterns and color M-mode flow propagation velocity reflect changes in myocardial relaxation in mice with genetically altered levels of PLB and may be useful tools to characterize LV diastolic function in other mouse models of disease.

IGF-I attenuates diabetes-induced cardiac contractile dysfunction in ventricular myocytes

Diabetic cardiomyopathy is characterized by impaired ventricular contraction and altered function of insulin-like growth factor I (IGF-I), a key factor for cardiac growth and function. Endogenous IGF-I has been shown to alleviate diabetic cardiomyopathy. This study was designed to evaluate exogenous IGF-I treatment on the development of diabetic cardiomyopathy. Adult rats were divided into four groups: control, control + IGF-I, diabetic, and diabetic + IGF-I. Streptozotocin (STZ; 55 mg/kg) was used to induce experimental diabetes immediately followed by a 7-wk IGF-I (3 mg. kg(-1). day(-1) ip) treatment. Mechanical properties were assessed in ventricular myocytes including peak shortening (PS), time-to-PS (TPS), time-to-90% relengthening (TR(90)) and maximal velocities of shortening/relengthening (+/-dL/dt). Intracellular Ca(2+) transients were evaluated as Ca(2+)-induced Ca(2+) release and Ca(2+) clearing constant. Levels of sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA), phospholamban (PLB), and glucose transporter (GLUT4) were assessed by Western blot. STZ caused significant weight loss and elevated blood glucose, demonstrating the diabetic status. The diabetic state is associated with reduced serum IGF-I levels, which were restored by IGF-I treatment. Diabetic myocytes showed reduced PS and +/-dL/dt as well as prolonged TPS, TR(90), and intracellular Ca(2+) clearing compared with control. IGF-I treatment prevented the diabetes-induced abnormalities in PS, +/-dL/dt, TR(90), and Ca(2+) clearing but not TPS. The levels of SERCA and GLUT4, but not PLB, were significantly reduced in diabetic hearts compared with controls. IGF-I treatment restored the diabetes-induced decline in SERCA, whereas it had no effect on GLUT4 and PLB levels. These results suggest that exogenous IGF-I treatment may ameliorate contractile disturbances in cardiomyocytes from diabetic animals and could provide therapeutic potential in the treatment of diabetic cardiomyopathy.