Beta1-adrenoceptor antagonist, metoprolol attenuates cardiac myocyte Ca2+ handling dysfunction in rats with pulmonary artery hypertension

Toxicity Risks Associated With the Beta-Blocker Metoprolol in Marine and Freshwater Organisms: A Review

The detection of pharmaceuticals in aquatic ecosystems has generated concern for wildlife and human health over the past several decades. β-adrenergic blocking agents are a class of drugs designed to treat cardiovascular diseases and high blood pressure. Metoprolol is a second-generation β1-adrenergic receptor inhibitor detected in effluent derived from sewage treatment plants. Our review presents an updated survey of the current state of knowledge regarding the sources, occurrence, and toxicity of metoprolol in aquatic ecosystems. We further aimed to summarize the current literature on the presence of metoprolol in various classes of aquatic species and to consider the trophic transfer of these contaminants in marine mammals. The biological impacts of metoprolol have been reported in 20 aquatic organisms, with a primary focus on cardiac function and oxidative stress. Our review reveals that concentrations of metoprolol that cause toxicity in aquatic species are above levels that are typical of marine and freshwater environments. Future studies should investigate the effects of metoprolol at lower concentrations in aquatic organisms. Other recommendations include (1) a further focus on noncardiac endpoints, because computational assessments of currently available molecular data identify gonadotropins, vitellogenin, collagen, and cytokines as potential targets of modulation, and (2) development of adverse outcome pathways for cardiac dysfunction in aquatic species to improve our understanding of molecular interactions and outcomes following exposure. As the next generation of β-blockers is developed, continued diligence is needed for assessing environmental impacts in aquatic ecosystems to determine their potential accumulation and long-term effects on wildlife and humans. Environ Toxicol Chem 2024;43:2530-2544. © 2024 SETAC.

Remodelling of T-Tubules and Associated Calcium Handling Dysfunction in Heart Failure: Mechanisms and Therapeutic Insights

In cardiomyocytes, transverse tubules (T-tubules) are sarcolemmal invaginations that facilitate excitation-contraction coupling and diastolic function. The clinical significance of T-tubules has become evident in that their remodelling is recognised as a hallmark feature of heart failure (HF) and a key contributor to disrupted Ca2+ homeostasis, compromised cardiac function, and arrhythmogenesis. Further investigations have revealed that T-tubule remodelling is particularly pronounced in HF with reduced ejection fraction (HFrEF), but not in HF with preserved ejection fraction, implying that T-tubule remodelling may play a crucial pathophysiologic role in HFrEF. While research on the functional importance of T-tubules is ongoing, T-tubule remodelling has been found to be reversible. That finding has triggered a surge in studies aimed at identifying specific therapeutic approaches for HFrEF. This review discusses the functional importance of T-tubules and their microdomains, the pathophysiology of T-tubule remodelling, and the potential mechanisms of current HFrEF therapeutic approaches in reversing T-tubule alterations. We also highlight discrepancies regarding the roles of T-tubule proteins in the recovery process across studies to offer valuable insights for future research.

Effects of dopamine β-hydroxylase inhibition in pressure overload-induced right ventricular failure

Activation of the sympathetic nervous system is observed in pulmonary arterial hypertension patients. This study investigates whether inhibiting the conversion of dopamine into noradrenaline by dopamine β-hydroxylase (DβH) inhibition with BIA 21-5337 improved right ventricular (RV) function or remodeling in pressure overload-induced RV failure. RV failure was induced in male Wistar rats by pulmonary trunk banding (PTB). Two weeks after the procedure, PTB rats were randomized to vehicle (n = 8) or BIA 21-5337 (n = 11) treatment. An additional PTB group treated with ivabradine (n = 11) was included to control for the potential heart rate-reducing effects of BIA 21-5337. A sham group (n = 6) received vehicle treatment. After 5 weeks of treatment, RV function was assessed by echocardiography, magnetic resonance imaging, and invasive pressure-volume measurements before rats were euthanized. RV myocardium was analyzed to evaluate RV remodeling. PTB caused a fourfold increase in RV afterload which led to RV dysfunction, remodeling, and failure. Treatment with BIA 21-5337 reduced adrenal gland DβH activity and 24-h urinary noradrenaline levels confirming relevant physiological response to the treatment. At end-of-study, there were no differences in RV function or RV remodeling between BIA 21-5337 and vehicle-treated rats. In conclusion, treatment with BIA 21-5337 did not have any beneficial-nor adverse-effects on the development of RV failure after PTB despite reduced adrenal gland DβH activity.

Sex-dependent remodeling of right ventricular function in a rat model of pulmonary arterial hypertension

Right ventricular (RV) function is an important prognostic indicator for pulmonary arterial hypertension (PAH), a vasculopathy that primarily and disproportionally affects women with distinct pre- and postmenopausal clinical outcomes. However, most animal studies have overlooked the impact of sex and ovarian hormones on RV remodeling in PAH. Here, we combined invasive measurements of RV hemodynamics and morphology with computational models of RV biomechanics in sugen-hypoxia (SuHx)-treated male, ovary-intact female, and ovariectomized female rats. Despite similar pressure overload levels, SuHx induced increases in end-diastolic elastance and passive myocardial stiffening, notably in male SuHx animals, corresponding to elevated diastolic intracellular calcium. Increases in end-systolic chamber elastance were largely explained by myocardial hypertrophy in male and ovary-intact female rats, whereas ovariectomized females exhibited contractility recruitment via calcium transient augmentation. Ovary-intact female rats primarily responded with hypertrophy, showing fewer myocardial mechanical alterations and less stiffening. These findings highlight sex-related RV remodeling differences in rats, affecting systolic and diastolic RV function in PAH.NEW & NOTEWORTHY Combining hemodynamic and morphological measurements from male, female, and ovariectomized female pulmonary arterial hypertension (PAH) rats revealed distinct adaptation mechanisms despite similar pressure overload. Males showed the most diastolic stiffening. Ovariectomized females had enhanced myocyte contractility and calcium transient upregulation. Ovary-intact females primarily responded with hypertrophy, experiencing milder passive myocardial stiffening and no changes in myocyte shortening. These findings suggest potential sex-specific pathways in right ventricular (RV) adaptation to PAH, with implications for targeted interventions.

Beta Blockade Prevents Cardiac Morphological and Molecular Remodelling in Experimental Uremia

Heart failure and chronic kidney disease (CKD) share several mediators of cardiac pathological remodelling. Akin to heart failure, this remodelling sets in motion a vicious cycle of progressive pathological hypertrophy and myocardial dysfunction in CKD. Several decades of heart failure research have shown that beta blockade is a powerful tool in preventing cardiac remodelling and breaking this vicious cycle. This phenomenon remains hitherto untested in CKD. Therefore, we set out to test the hypothesis that beta blockade prevents cardiac pathological remodelling in experimental uremia. Wistar rats had subtotal nephrectomy or sham surgery and were followed up for 10 weeks. The animals were randomly allocated to the beta blocker metoprolol (10 mg/kg/day) or vehicle. In vivo and in vitro cardiac assessments were performed. Cardiac tissue was extracted, and protein expression was quantified using immunoblotting. Histological analyses were performed to quantify myocardial fibrosis. Beta blockade attenuated cardiac pathological remodelling in nephrectomised animals. The echocardiographic left ventricular mass and the heart weight to tibial length ratio were significantly lower in nephrectomised animals treated with metoprolol. Furthermore, beta blockade attenuated myocardial fibrosis associated with subtotal nephrectomy. In addition, the Ca++- calmodulin-dependent kinase II (CAMKII) pathway was shown to be activated in uremia and attenuated by beta blockade, offering a potential mechanism of action. In conclusion, beta blockade attenuated hypertrophic signalling pathways and ameliorated cardiac pathological remodelling in experimental uremia. The study provides a strong scientific rationale for repurposing beta blockers, a tried and tested treatment in heart failure, for the benefit of patients with CKD.

OP2113, a new drug for chronic hypoxia-induced pulmonary hypertension treatment in rat

BACKGROUND AND PURPOSE

Pulmonary hypertension (PH) is a cardiovascular disease characterised by an increase in pulmonary arterial (PA) resistance leading to right ventricular (RV) failure. Reactive oxygen species (ROS) play a major role in PH. OP2113 is a drug with beneficial effects on cardiac injuries that targets mitochondrial ROS. The aim of the study was to address the in vivo therapeutic effect of OP2113 in PH.

EXPERIMENTAL APPROACH

PH was induced by 3 weeks of chronic hypoxia (CH-PH) in rats treated with OP2113 or its vehicle via subcutaneous osmotic mini-pumps. Haemodynamic parameters and both PA and heart remodelling were assessed. Reactivity was quantified in PA rings and in RV or left ventricular (LV) cardiomyocytes. Oxidative stress was detected by electron paramagnetic resonance and western blotting. Mitochondrial mass and respiration were measured by western blotting and oxygraphy, respectively.

KEY RESULTS

In CH-PH rats, OP2113 reduced the mean PA pressure, PA remodelling, PA hyperreactivity in response to 5-HT, the contraction slowdown in RV and LV and increased the mitochondrial mass in RV. Interestingly, OP2113 had no effect on haemodynamic parameters, both PA and RV wall thickness and PA reactivity, in control rats. Whereas oxidative stress was evidenced by an increase in protein carbonylation in CH-PH, this was not affected by OP2113.

CONCLUSION AND IMPLICATIONS

Our study provides evidence for a selective protective effect of OP2113 in vivo on alterations in both PA and RV from CH-PH rats without side effects in control rats.

Correlative super-resolution analysis of cardiac calcium sparks and their molecular origins in health and disease

Rapid release of calcium from internal stores via ryanodine receptors (RyRs) is one of the fastest types of cytoplasmic second messenger signalling in excitable cells. In the heart, rapid summation of the elementary events of calcium release, 'calcium sparks', determine the contraction of the myocardium. We adapted a correlative super-resolution microscopy protocol to correlate sub-plasmalemmal spontaneous calcium sparks in rat right ventricular myocytes with the local nanoscale RyR2 positions. This revealed a steep relationship between the integral of a calcium spark and the sum of the local RyR2s. Segmentation of recurring spark sites showed evidence of repeated and triggered saltatory activation of multiple local RyR2 clusters. In myocytes taken from failing right ventricles, RyR2 clusters themselves showed a dissipated morphology and fragmented (smaller) clusters. They also featured greater heterogeneity in both the spark properties and the relationship between the integral of the calcium spark and the local ensemble of RyR2s. While fragmented (smaller) RyR2 clusters were rarely observed directly underlying the larger sparks or the recurring spark sites, local interrogation of the channel-to-channel distances confirmed a clear link between the positions of each calcium spark and the tight, non-random clustering of the local RyR2 in both healthy and failing ventricles.

Computational modeling of electromechanical coupling in human cardiomyocyte applied to study hypertrophic cardiomyopathy and its drug response

BACKGROUND AND OBJECTIVE

Knowledge of electromechanical coupling in cardiomyocyte and how it is influenced by various pathophysiological factors is fundamental to understanding the pathogenesis of myocardial disease and its response to medication, which is however hard to be thoroughly addressed by clinical/experimental studies due to technical limitations. At this point, computational modeling offers an alternative approach. The main objective of the study was to develop a computational model capable of simulating the process of electromechanical coupling and quantifying the roles of various factors in play in the human left ventricular cardiomyocyte.

METHODS

A new electrophysiological model was firstly built by combining several existing electrophysiological models and incorporating the mechanism of electrophysiological homeostasis, which was subsequently coupled to models representing the cross-bridge dynamics and active force generation during excitation-contraction coupling and the passive mechanical properties of cardiomyocyte to yield an integrative electromechanical model. Model parameters were calibrated or optimized based on a large amount of experimental data. The resulting model was applied to delineate the characteristics of electromechanical coupling and explore underlying determinant factors in hypertrophic cardiomyopathy (HCM) cardiomyocyte, as well as quantify their changes in response to different medications.

RESULTS

Model predictions captured the major electromechanical characteristics of cardiomyocyte under both normal physiological and HCM conditions. In comparison with normal cardiomyocyte, HCM cardiomyocyte suffered from systemic changes in both electrophysiological and mechanical variables. Numerical simulations of drug response revealed that Mavacamten and Metoprolol could both reduce the active contractility and alleviate calcium overload but had marked differential influences on many other electromechanical variables, which theoretically explained why the two drugs have differential therapeutic effects. In addition, our numerical experiments demonstrated the important role of compensatory ion transport in maintaining electrophysiological homeostasis and regulating cytoplasmic volume.

CONCLUSIONS

A sophisticated computational model has the advantage of providing quantitative and integrative insights for understanding the pathogenesis and drug responses of HCM or other myocardial diseases at the level of cardiomyocyte, and hence may contribute as a useful complement to clinical/experimental studies. The model may also be coupled to tissue- or organ-level models to strengthen the physiological implications of macro-scale numerical simulations.

Increased Mitochondrial Calcium Fluxes in Hypertrophic Right Ventricular Cardiomyocytes from a Rat Model of Pulmonary Artery Hypertension

Pulmonary artery hypertension causes right ventricular hypertrophy which rapidly progresses to heart failure with underlying cardiac mitochondrial dysfunction. Prior to failure, there are alterations in cytosolic Ca²⁺ handling that might impact mitochondrial function in the compensatory phase of RV hypertrophy. Our aims, therefore, were (i) to measure beat-to-beat mitochondrial Ca²⁺ fluxes, and (ii) to determine mitochondrial abundance and function in non-failing, hypertrophic cardiomyocytes. Male Wistar rats were injected with either saline (CON) or monocrotaline (MCT) to induce pulmonary artery hypertension and RV hypertrophy after four weeks. Cytosolic Ca²⁺ ([Ca²⁺]_cyto) transients were obtained in isolated right ventricular (RV) cardiomyocytes, and mitochondrial Ca²⁺ ([Ca²⁺]_mito) was recorded in separate RV cardiomyocytes. The distribution and abundance of key proteins was determined using confocal and stimulated emission depletion (STED) microscopy. The RV mitochondrial function was also assessed in RV homogenates using oxygraphy. The MCT cardiomyocytes had increased area, larger [Ca²⁺]_cyto transients, increased Ca²⁺ store content, and faster trans-sarcolemmal Ca²⁺ extrusion relative to CON. The MCT cardiomyocytes also had larger [Ca²⁺]_mito transients. STED images detected increased mitochondrial protein abundance (TOM20 clusters per μm²) in MCT, yet no difference was found when comparing mitochondrial respiration and membrane potential between the groups. We suggest that the larger [Ca²⁺]_mito transients compensate to match ATP supply to the increased energy demands of hypertrophic cardiomyocytes.

Three-dimensional visualization of the cardiac ryanodine receptor clusters and the molecular-scale fraying of dyads

Clusters of ryanodine receptor calcium channels (RyRs) form the primary molecular machinery of intracellular calcium signalling in cardiomyocytes. While a range of optical super-resolution microscopy techniques have revealed the nanoscale structure of these clusters, the three-dimensional (3D) nanoscale topologies of the clusters have remained mostly unresolved. In this paper, we demonstrate the exploitation of molecular-scale resolution in enhanced expansion microscopy (EExM) along with various 2D and 3D visualization strategies to observe the topological complexities, geometries and molecular sub-domains within the RyR clusters. Notably, we observed sub-domains containing RyR-binding protein junctophilin-2 (JPH2) occupying the central regions of RyR clusters in the deeper interior of the myocytes (including dyads), while the poles were typically devoid of JPH2, lending to a looser RyR arrangement. By contrast, peripheral RyR clusters exhibited variable co-clustering patterns and ratios between RyR and JPH2. EExM images of dyadic RyR clusters in right ventricular (RV) myocytes isolated from rats with monocrotaline-induced RV failure revealed hallmarks of RyR cluster fragmentation accompanied by breaches in the JPH2 sub-domains. Frayed RyR patterns observed adjacent to these constitute new evidence that the destabilization of the RyR arrays inside the JPH2 sub-domains may seed the primordial foci of dyad remodelling observed in heart failure. This article is part of the theme issue 'The cardiomyocyte: new revelations on the interplay between architecture and function in growth, health, and disease'.

Dapagliflozin reduces the vulnerability of rats with pulmonary arterial hypertension-induced right heart failure to ventricular arrhythmia by restoring calcium handling

Background

Malignant ventricular arrhythmia (VA) is a major contributor to sudden cardiac death (SCD) in patients with pulmonary arterial hypertension (PAH)-induced right heart failure (RHF). Recently, dapagliflozin (DAPA), a sodium/glucose cotransporter-2 inhibitor (SGLT2i), has been found to exhibit cardioprotective effects in patients with left ventricular systolic dysfunction. In this study, we examined the effects of DAPA on VA vulnerability in a rat model of PAH-induced RHF.

Methods

Rats randomly received monocrotaline (MCT, 60 mg/kg) or vehicle via a single intraperitoneal injection. A day later, MCT-injected rats were randomly treated with placebo, low-dose DAPA (1 mg/kg/day), or high-dose (3 mg/kg/day) DAPA orally for 35 days. Echocardiographic analysis, haemodynamic experiments, and histological assessments were subsequently performed to confirm the presence of PAH-induced RHF. Right ventricle (RV) expression of calcium (Ca²⁺) handling proteins were detected via Western blotting. RV expression of connexin 43 (Cx43) was determined via immunohistochemical staining. An optical mapping study was performed to assess the electrophysiological characteristics in isolated hearts. Cellular Ca²⁺ imaging from RV cardiomyocytes (RVCMs) was recorded using Fura-2 AM or Fluo-4 AM.

Results

High-dose DAPA treatment attenuated RV structural remodelling, improved RV function, alleviated Cx43 remodelling, increased the conduction velocity, restored the expression of key Ca²⁺ handling proteins, increased the threshold for Ca²⁺ and action potential duration (APD) alternans, decreased susceptibility to spatially discordant APD alternans and spontaneous Ca²⁺ events, promoted cellular Ca²⁺ handling, and reduced VA vulnerability in PAH-induced RHF rats. Low-dose DAPA treatment also showed antiarrhythmic effects in hearts with PAH-induced RHF, although with a lower level of efficacy.

Conclusion

DAPA administration reduced VA vulnerability in rats with PAH-induced RHF by improving RVCM Ca²⁺ handling.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12933-022-01614-5.

Molecular, Subcellular, and Arrhythmogenic Mechanisms in Genetic RyR2 Disease

The ryanodine receptor (RyR2) has a critical role in controlling Ca²⁺ release from the sarcoplasmic reticulum (SR) throughout the cardiac cycle. RyR2 protein has multiple functional domains with specific roles, and four of these RyR2 protomers are required to form the quaternary structure that comprises the functional channel. Numerous mutations in the gene encoding RyR2 protein have been identified and many are linked to a wide spectrum of arrhythmic heart disease. Gain of function mutations (GoF) result in a hyperactive channel that causes excessive spontaneous SR Ca²⁺ release. This is the predominant cause of the inherited syndrome catecholaminergic polymorphic ventricular tachycardia (CPVT). Recently, rare hypoactive loss of function (LoF) mutations have been identified that produce atypical effects on cardiac Ca²⁺ handling that has been termed calcium release deficiency syndrome (CRDS). Aberrant Ca²⁺ release resulting from both GoF and LoF mutations can result in arrhythmias through the Na⁺/Ca²⁺ exchange mechanism. This mini-review discusses recent findings regarding the role of RyR2 domains and endogenous regulators that influence RyR2 gating normally and with GoF/LoF mutations. The arrhythmogenic consequences of GoF/LoF mutations will then be discussed at the macromolecular and cellular level.

miR-1226-3p Promotes eNOS Expression of Pulmonary Arterial Endothelial Cells to Mitigate Hypertension in Rats via Targeting Profilin-1

In pulmonary arterial hypertension (PAH), microRNAs (miRNAs) are related with dysfunction of pulmonary arterial endothelial cells. miR-1226-3p was found to be downregulated in the serum of PAH patients, while few studies have illustrated the regulation mechanism of miR-1226-3p on PAH. In this study, we aimed to systematically investigate the role of miR-1226-3p in PAH. Sprague-Dawley (SD) rats were treated with monocrotaline (MCT) to establish the PAH models. The right ventricular systolic pressure (RVSP), ratio of the right ventricle to the left ventricle with septum (RV/(LV+S) ratio), and nitric oxide (NO) content were used to reflect the symptom of the rats. The rat models were used to observe the regulation mechanism of miR-1226-3p on PAH, and dual-luciferase reporter assay was used to verify the binding effect of miR-1226-3p to Pfn1. Besides, the qRT-PCR and western blot were used to measure the expression levels of miR-1226-3p and some keys proteins such as eNOS and Pfn1, respectively. The results showed that the PAH models were established successfully. The RVSP levels and the RV/(LV+S) ratio of the PAH rats were higher than those indexes in normal rats, while the NO content showed the opposite trends. Besides, the decreased miR-1226-3p and eNOS were, respectively, found in the PAH rats and rPAECs, and overexpressed miR-1226-3p could reverse the disadvantages of the PAH rats including increased RVSP, high RV/(LV+S) ratio, and decreased NO content. Furthermore, miR-1226-3p could directly target the 3'-UTR of Profilin-1 (Pfn1). Overexpressed Pfn1 led to decreased eNOS, while miR-1226-3p could partly inhibit the expression of Pfn1 and increase the expression level of eNOS in rPAECs. In summary, this study suggests miR-1226-3p as a protector to increase eNOS, improve NO content in rPAECs of the PAH rats via targeting Pfn, and finally protect the rats from the injury induced by PAH.

Neurohormonal modulation in pulmonary arterial hypertension

Pulmonary hypertension is a fatal condition of elevated pulmonary pressures, complicated by right heart failure. Pulmonary hypertension appears in various forms; one of those is pulmonary arterial hypertension (PAH) and is particularly characterised by progressive remodelling and obstruction of the smaller pulmonary vessels. Neurohormonal imbalance in PAH patients is associated with worse prognosis and survival. In this back-to-basics article on neurohormonal modulation in PAH, we provide an overview of the pharmacological and nonpharmacological strategies that have been tested pre-clinically and clinically. The benefit of neurohormonal modulation strategies in PAH patients has been limited by lack of insight into how the neurohormonal system is changed throughout the disease and difficulties in translation from animal models to human trials. We propose that longitudinal and individual assessments of neurohormonal status are required to improve the timing and specificity of neurohormonal modulation strategies. Ongoing developments in imaging techniques such as positron emission tomography may become helpful to determine neurohormonal status in PAH patients in different disease stages and optimise individual treatment responses.

Continuous Aerobic Exercise Prevents Detrimental Remodeling and Right Heart Myocyte Contraction and Calcium Cycling Dysfunction in Pulmonary Artery Hypertension

ABSTRACT

Pulmonary artery hypertension (PAH) imposes right heart and lung detrimental remodeling which impairs cardiac contractility, physical effort tolerance, and survival. The effects of an early moderate-intensity continuous aerobic exercise training on the right ventricle and lung structure, and on contractility and the calcium (Ca2+) transient in isolated myocytes from rats with severe PAH induced by monocrotaline were analyzed. Rats were divided into control sedentary (CS), control exercise (CE), monocrotaline sedentary (MS), and monocrotaline exercise (ME) groups. Animals from control exercise and ME groups underwent a moderate-intensity aerobic exercise on a treadmill (60 min/d; 60% intensity) for 32 days, after a monocrotaline (60 mg/kg body weight i.p.) or saline injection. The pulmonary artery resistance was higher in MS than in control sedentary (1.36-fold) and was reduced by 39.39% in ME compared with MS. Compared with MS, the ME group presented reduced alveolus (17%) and blood vessel (46%) wall, fibrosis (25.37%) and type I collagen content (55.78%), and increased alveolus (52.96%) and blood vessel (146.97%) lumen. In the right ventricle, the ME group exhibited diminished hypertrophy index (25.53%) and type I collagen content (40.42%) and improved myocyte contraction [ie, reduced times to peak (29.27%) and to 50% relax (13.79%)] and intracellular Ca2+ transient [ie, decreased times to peak (16.06%) and to 50% decay (7.41%)] compared with MS. Thus, early moderate-intensity continuous aerobic exercise prevents detrimental remodeling in the right heart and lung increases in the pulmonary artery resistance and dysfunction in single myocyte contraction and Ca2+ cycling in this model.

Nanoscale Study of Calcium Handling Remodeling in Right Ventricular Cardiomyocytes Following Pulmonary Hypertension

Pulmonary hypertension is a complex disorder characterized by pulmonary vascular remodeling and right ventricular hypertrophy, leading to right heart failure. The mechanisms underlying this process are not well understood. We hypothesize that the structural remodeling occurring in the cardiomyocytes of the right ventricle affects the cytosolic Ca²⁺ handling leading to arrhythmias. After 12 days of monocrotaline-induced pulmonary hypertension in rats, epicardial mapping showed electrical remodeling in both ventricles. In myocytes isolated from the hypertensive rats, a combination of high-speed camera and confocal line-scan documented a prolongation of Ca²⁺ transients along with a higher local Ca²⁺-release activity. These Ca²⁺ transients were less synchronous than in controls, likely due to disorganized transverse-axial tubular system. In fact, following pulmonary hypertension, hypertrophied right ventricular myocytes showed significantly reduced number of transverse tubules and increased number of axial tubules; however, Stimulation Emission Depletion microscopy demonstrated that the colocalization of L-type Ca²⁺ channels and RyR2 (ryanodine receptor 2) remained unchanged. Finally, Stimulation Emission Depletion microscopy and super-resolution scanning patch-clamp analysis uncovered a decrease in the density of active L-type Ca²⁺ channels in right ventricular myocytes with an elevated open probability of the T-tubule anchored channels. This may represent a general mechanism of how nanoscale structural changes at the early stage of pulmonary hypertension impact on the development of the end stage failing phenotype in the right ventricle.

The Protective Effect of Qishen Granule on Heart Failure after Myocardial Infarction through Regulation of Calcium Homeostasis

Qishen granule (QSG) is a frequently prescribed traditional Chinese medicine formula, which improves heart function in patients with heart failure (HF). However, the cardioprotective mechanisms of QSG have not been fully understood. The current study aimed to elucidate whether the effect of QSG is mediated by ameliorating cytoplasmic calcium (Ca²⁺) overload in cardiomyocytes. The HF rat model was induced by left anterior descending (LAD) artery ligation surgery. Rats were randomly divided into sham, model, QSG-low dosage (QSG-L) treatment, QSG-high dosage (QSG-H) treatment, and positive drug (diltiazem) treatment groups. 28 days after surgery, cardiac functions were assessed by echocardiography. Levels of norepinephrine (NE) and angiotensin II (AngII) in the plasma were evaluated. Expressions of critical proteins in the calcium signaling pathway, including cell membrane calcium channel CaV1.2, sarcoendoplasmic reticulum ATPase 2a (SERCA2a), calcium/calmodulin-dependent protein kinase type II (CaMKII), and protein phosphatase calcineurin (CaN), were measured by Western blotting (WB) and immunohistochemistry (IHC). Echocardiography showed that left ventricular ejection fraction (EF) and fractional shortening (FS) value significantly decreased in the model group compared to the sham group, and illustrating heart function was severely impaired. Furthermore, levels of NE and AngII in the plasma were dramatically increased. Expressions of CaV1.2, CaMKII, and CaN in the cardiomyocytes were upregulated, and expressions of SERCA2a were downregulated in the model group. After treatment with QSG, both EF and FS values were increased. QSG significantly reduced levels of NE and AngII in the plasma. In particular, QSG prevented cytoplasmic Ca²⁺ overload by downregulating expression of CaV1.2 and upregulating expression of SERCA2a. Meanwhile, expressions of CaMKII and CaN were inhibited by QSG treatment. In conclusion, QSG could effectively promote heart function in HF rats by restoring cardiac Ca²⁺ homeostasis. These findings revealed novel therapeutic mechanisms of QSG and provided potential targets in the treatment of HF.

Metoprolol alleviates arginine vasopressin-induced cardiomyocyte hypertrophy by upregulating the AKT1-SERCA2 cascade in H9C2 cells

BACKGROUND

Arginine vasopressin (AVP) is elevated in patients with heart failure, and the increase in the AVP concentration in plasma is positively correlated with disease severity and mortality. Metoprolol (Met) is a beta blocker that is widely used in the clinic to treat pathological cardiac hypertrophy and to improve heart function. However, the specific mechanism by which Met alleviates AVP-induced pathological cardiac hypertrophy is still unknown. Our current study aimed to evaluate the inhibitory effects of Met on AVP-induced cardiomyocyte hypertrophy and the underlying mechanisms.

METHODS

AVP alone or AVP plus Met was added to the wild type or AKT1-overexpressing rat cardiac H9C2 cell line. The cell surface areas and ANP/BNP/β-MHC expressions were used to evaluate the levels of hypertrophy. Western bolting was used to analyze AKT1/P-AKT1, AKT2/P-AKT2, total AKT, SERCA2, and Phospholamban (PLN) expression. Fluo3-AM was used to measure the intracellular Ca2+ stores.

RESULTS

In the current study, we found that AKT1 but not AKT2 mediated the pathogenesis of AVP-induced cardiomyocyte hypertrophy. Sustained stimulation (48 h) with AVP led to hypertrophy in the H9C2 rat cardiomyocytes, resulting in the downregulation of AKT1 (0.48 fold compared to control) and SERCA2 (0.62 fold), the upregulation of PLN (1.32 fold), and the increase in the cytoplasmic calcium concentration (1.52 fold). In addition, AKT1 overexpression increased the expression of SERCA2 (1.34 fold) and decreased the expression of PLN (0.48 fold) in the H9C2 cells. Moreover, we found that Met could attenuate the AVP-induced changes in AKT1, SERCA2 and PLN expression and decreased the cytoplasmic calcium concentration in the H9C2 cells.

CONCLUSIONS

Our results demonstrated that the AKT1-SERCA2 cascade served as an important regulatory pathway in AVP-induced pathological cardiac hypertrophy.

Energy Metabolism in the Failing Right Ventricle: Limitations of Oxygen Delivery and the Creatine Kinase System

Pulmonary arterial hypertension (PAH) results in hypertrophic remodeling of the right ventricle (RV) to overcome increased pulmonary pressure. This increases the O2 consumption of the myocardium, and without a concomitant increase in energy generation, a mismatch with demand may occur. Eventually, RV function can no longer be sustained, and RV failure occurs. Beta-adrenergic blockers (BB) are thought to improve survival in left heart failure, in part by reducing energy expenditure and hypertrophy, however they are not currently a therapy for PAH. The monocrotaline (MCT) rat model of PAH was used to investigate the consequence of RV failure on myocardial oxygenation and mitochondrial function. A second group of MCT rats was treated daily with the beta-1 blocker metoprolol (MCT + BB). Histology confirmed reduced capillary density and increased capillary supply area without indications of capillary rarefaction in MCT rats. A computer model of O2 flux was applied to the experimentally recorded capillary locations and predicted a reduction in mean tissue PO2 in MCT rats. The fraction of hypoxic tissue (defined as PO2 < 0.5 mmHg) was reduced following beta-1 blocker (BB) treatment. The functionality of the creatine kinase (CK) energy shuttle was measured in permeabilized RV myocytes by sequential ADP titrations in the presence and absence of creatine. Creatine significantly decreased the KmADP in cells from saline-injected control (CON) rats, but not MCT rats. The difference in KmADP with or without creatine was not different in MCT + BB cells compared to CON or MCT cells. Improved myocardial energetics could contribute to improved survival of PAH with chronic BB treatment.

Mitochondrial function remains impaired in the hypertrophied right ventricle of pulmonary hypertensive rats following short duration metoprolol treatment

Pulmonary hypertension (PH) increases the work of the right ventricle (RV) and causes right-sided heart failure. This study examined RV mitochondrial function and ADP transfer in PH animals advancing to right heart failure, and investigated a potential therapy with the specific β1-adrenergic-blocker metoprolol. Adult Wistar rats (317 ± 4 g) were injected either with monocrotaline (MCT, 60 mg kg-1) to induce PH, or with an equivalent volume of saline for controls (CON). At three weeks post-injection the MCT rats began oral metoprolol (10 mg kg-1 day-1-) or placebo treatment until heart failure was observed in the MCT group. Mitochondrial function was then measured using high-resolution respirometry from permeabilised RV fibres. Relative to controls, MCT animals had impaired mitochondrial function but maintained coupling between myofibrillar ATPases and mitochondria, despite an increase in ADP diffusion distances. Cardiomyocytes from the RV of MCT rats were enlarged, primarily due to an increase in myofibrillar protein. The ratio of mitochondria per myofilament area was decreased in both MCT groups (p ≤ 0.05) in comparison to control (CON: 1.03 ± 0.04; MCT: 0.74 ± 0.04; MCT + BB: 0.74 ± 0.03). This not only implicates impaired energy production in PH, but also increases the diffusion distance for metabolites within the MCT cardiomyocytes, adding an additional hindrance to energy supply. Together, these changes may limit energy supply in MCT rat hearts, particularly at high cardiac workloads. Metoprolol treatment did not delay the onset of heart failure symptoms, improve mitochondrial function, or regress RV hypertrophy.

Three-Dimensional and Chemical Mapping of Intracellular Signaling Nanodomains in Health and Disease with Enhanced Expansion Microscopy

Nanodomains are intracellular foci which transduce signals between major cellular compartments. One of the most ubiquitous signal transducers, the ryanodine receptor (RyR) calcium channel, is tightly clustered within these nanodomains. Super-resolution microscopy has previously been used to visualize RyR clusters near the cell surface. A majority of nanodomains located deeper within cells have remained unresolved due to limited imaging depths and axial resolution of these modalities. A series of enhancements made to expansion microscopy allowed individual RyRs to be resolved within planar nanodomains at the cell periphery and the curved nanodomains located deeper within the interiors of cardiomyocytes. With a resolution of ∼ 15 nm, we localized both the position of RyRs and their individual phosphorylation for the residue Ser2808. With a three-dimensional imaging protocol, we observed disturbances to the RyR arrays in the nanometer scale which accompanied right-heart failure caused by pulmonary hypertension. The disease coincided with a distinct gradient of RyR hyperphosphorylation from the edge of the nanodomain toward the center, not seen in healthy cells. This spatial profile appeared to contrast distinctly from that sustained by the cells during acute, physiological hyperphosphorylation when they were stimulated with a β-adrenergic agonist. Simulations of RyR arrays based on the experimentally determined channel positions and phosphorylation signatures showed how the nanoscale dispersal of the RyRs during pathology diminishes its intrinsic likelihood to ignite a calcium signal. It also revealed that the natural topography of RyR phosphorylation could offset potential heterogeneity in nanodomain excitability which may arise from such RyR reorganization.

The Role of G Protein-Coupled Receptors in the Right Ventricle in Pulmonary Hypertension

Pressure overload of the right ventricle (RV) in pulmonary arterial hypertension (PAH) leads to RV remodeling and failure, an important determinant of outcome in patients with PAH. Several G protein-coupled receptors (GPCRs) are differentially regulated in the RV myocardium, contributing to the pathogenesis of RV adverse remodeling and dysfunction. Many pharmacological agents that target GPCRs have been demonstrated to result in beneficial effects on left ventricular (LV) failure, such as beta-adrenergic receptor and angiotensin receptor antagonists. However, the role of such drugs on RV remodeling and performance is not known at this time. Moreover, many of these same receptors are also expressed in the pulmonary vasculature, which could result in complex effects in PAH. This manuscript reviews the role of GPCRs in the RV remodeling and dysfunction and discusses activating and blocking GPCR signaling to potentially attenuate remodeling while promoting improvements of RV function in PAH.