Left-Ventricular Energetics in Pulmonary Arterial Hypertension-Induced Right-Ventricular Hypertrophic Failure

The Monocrotaline Rat Model of Right Heart Disease Induced by Pulmonary Artery Hypertension

Pulmonary artery hypertension (PAH) is characterised by increased pulmonary vascular resistance (PVR) resulting in elevated pressure in the pulmonary artery supplying the pulmonary circulation. Disease of the right ventricle (RV) often manifests as a result of PAH placing excessive pressure on the right side of the heart. Although a relatively rare disease in humans, the impact of sustained PAH is severe, with poor outcomes even in treated individuals. As PAH develops, the blood flow is restricted through the pulmonary arteries and the right ventricle hypertrophies due to the increased strain of pumping blood through the pulmonary circulation. With time, RV hypertrophy progresses to right heart failure, impacting the supply of blood to the left ventricle and systemic circulation. Although right heart failure can currently be treated, it cannot be cured. There is therefore a need for more research into the physiological changes that cause the heart to fail under pressure overload. This review aims to evaluate the monocrotaline (MCT) rat model of PAH as a means of studying the cellular mechanisms associated with the development of RV hypertrophy and right heart failure.

Resolving an inconsistency in the estimation of the energy for excitation of cardiac muscle contraction

In the excitation of muscle contraction, calcium ions interact with transmembrane transporters. This process is accompanied by energy consumption and heat liberation. To quantify this activation energy or heat in the heart or cardiac muscle, two non-pharmacological approaches can be used. In one approach using the "pressure-volume area" concept, the same estimate of activation energy is obtained regardless of the mode of contraction (either isovolumic/isometric or ejecting/shortening). In the other approach, an accurate estimate of activation energy is obtained only when the muscle contracts isometrically. If the contraction involves muscle shortening, then an additional component of heat associated with shortening is liberated, over and above that of activation. The present study thus examines the reconcilability of the two approaches by performing experiments on isolated muscles measuring contractile force and heat output. A framework was devised from the experimental data to allow us to replicate several mechanoenergetics results gleaned from the literature. From these replications, we conclude that the choice of initial muscle length (or ventricular volume) underlies the divergence of the two approaches in the estimation of activation energy when the mode of contraction involves shortening (ejection). At low initial muscle lengths, the heat of shortening is relatively small, which can lead to the misconception that activation energy is contraction mode independent. In fact, because cardiac muscle liberates heat of shortening when allowed to shorten, estimation of activation heat must be performed only under isometric (isovolumic) contractions. We thus recommend caution when estimating activation energy using the "pressure-volume area" concept.

Microglia-derived TNF-α contributes to RVLM neuronal mitochondrial dysfunction via blocking the AMPK-Sirt3 pathway in stress-induced hypertension

BACKGROUND

Neuroinflammation in the rostral ventrolateral medulla (RVLM) has been associated with the pathogenesis of stress-induced hypertension (SIH). Neuronal mitochondrial dysfunction is involved in many pathological and physiological processes. However, the impact of neuroinflammation on neuronal mitochondrial homeostasis and the involved signaling pathway in the RVLM during SIH are largely unknown.

METHODS

The morphology and phenotype of microglia and the neuronal mitochondrial injury in vivo were analyzed by immunofluorescence, Western blot, RT-qPCR, transmission electron microscopy, and kit detection. The underlying mechanisms of microglia-derived tumor necrosis factor-α (TNF-α) on neuronal mitochondrial function were investigated through in vitro and in vivo experiments such as immunofluorescence and Western blot. The effect of TNF-α on blood pressure (BP) regulation was determined in vivo via intra-RVLM microinjection of TNF-α receptor antagonist R7050.

RESULTS

The results demonstrated that BP, heart rate (HR), renal sympathetic nerve activity (RSNA), plasma norepinephrine (NE), and electroencephalogram (EEG) power increased in SIH rats. Furthermore, the branching complexity of microglia in the RVLM of SIH rats decreased and polarized into M1 phenotype, accompanied by upregulation of TNF-α. Increased neuronal mitochondria injury was observed in the RVLM of SIH rats. Mechanistically, Sirtuin 3 (Sirt3) and p-AMPK expression were markedly downregulated in both SIH rats and TNF-α-treated N2a cells. AMPK activator A769662 upregulated AMPK-Sirt3 signaling pathway and consequently reversed TNF-α-induced mitochondrial dysfunction. Microinjection of TNF-α receptor antagonist R7050 into the RVLM of SIH rats significantly inhibited the biological activities of TNF-α, increased p-AMPK and Sirt3 levels, and alleviated neuronal mitochondrial injury, thereby reducing c-FOS expression, RSNA, plasma NE, and BP.

CONCLUSIONS

This study revealed that microglia-derived TNF-α in the RVLM impairs neuronal mitochondrial function in SIH possibly through inhibiting the AMPK-Sirt3 pathway. Therefore, microglia-derived TNF-α in the RVLM may be a possible therapeutic target for the intervention of SIH.

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.

Ventricular Function and Cardio-Ankle Vascular Index in Patients With Pulmonary Artery Hypertension

Aim

This study aims to evaluate the left ventricle (LV) systolic and diastolic function in patients with idiopathic pulmonary arterial hypertension (IPAH) and its correlation with systemic arterial stiffness assessed by cardio-ankle vascular index (CAVI).

Patient and methods

We included 37 patients with IPAH and 20 healthy people matched by age. All patients were assessed: vital signs, 6-minute walk test, NT-proBNP level, the CAVI, the right ventricular (RV) and LV function parameters, including ejection time (ET), tissue speckle-tracking values - global longitudinal strain (GLS) and strain rate (SR).

Results

The groups were matched by age, gender, BMI, office SBP and DBP. Patients with IPAH had higher heart rate, NT-proBNP level and lower ferritin level, GFR (CKD-EPI), SaO2 than healthy people. The mean CAVIleft was higher in IPAH patients than in the control group- 8.7±1.1 vs 7.5±0.9, P=0.007. Healthy people had significantly less E/e' and lower IVRT. LVET and RVET were shorter in IPAH patients. Patients with IPAH had mean LVGLS -(-17.6±4.8%) and 35.1% of them were with LVGLS ≤16% compared to healthy people -(-21.8±1.4%) and 0%, respectively. LVSR was significant less in IPAH patients, but in the normal range. We found significant correlations of CAVI with age, history of syncope, bilirubin, uric acid, total cholesterol, cardiac output, cardiac index, RVET, LVET and E/A. Multiple linear regression confirmed the independent significance for age (β=0.083±0.023, CI 0.033-0.133) and RVET (β=-0.018±0.005, CI -0.029 to -0.008) only. The risk to have CAVI ≥8 increased in 5.8 times in IPAH patients with RVET <248 ms (P=0.046). CAVI did not correlate with LVGLS and LVSR.

Conclusion

Significant worse systolic and diastolic LV functions were stated in pulmonary hypertensive patients compared to the control group. No LV GLS, no LV SR had significant associations with arterial stiffness evaluated by CAVI.

Increased SERCA2a sub-cellular heterogeneity in right-ventricular heart failure inhibits excitation-contraction coupling and modulates arrhythmogenic dynamics

The intracellular calcium handling system of cardiomyocytes is responsible for controlling excitation-contraction coupling (ECC) and has been linked to pro-arrhythmogenic cellular phenomena in conditions such as heart failure (HF). SERCA2a, responsible for intracellular uptake, is a primary regulator of calcium homeostasis, and remodelling of its function has been proposed as a causal factor underlying cellular and tissue dysfunction in disease. Whereas adaptations to the global (i.e. whole-cell) expression of SERCA2a have been previously investigated in the context of multiple diseases, the role of its spatial profile in the sub-cellular volume has yet to be elucidated. We present an approach to characterize the sub-cellular heterogeneity of SERCA2a and apply this approach to quantify adaptations to the length-scale of heterogeneity (the distance over which expression is correlated) associated with right-ventricular (RV)-HF. These characterizations informed simulations to predict the functional implications of this heterogeneity, and its remodelling in disease, on ECC, the dynamics of calcium-transient alternans and the emergence of spontaneous triggered activity. Image analysis reveals that RV-HF is associated with an increase in length-scale and its inter-cellular variability; simulations predict that this increase in length-scale can reduce ECC and critically modulate the vulnerability to both alternans and triggered activity. This article is part of the theme issue 'The cardiomyocyte: new revelations on the interplay between architecture and function in growth, health, and disease'.

Work-loop contractions reveal that the afterload-dependent time course of cardiac Ca2+ transients is modulated by preload

Preload and afterload dictate the dynamics of the cyclical work-loop contraction that the heart undergoes in vivo. Cellular Ca²⁺ dynamics drive contraction, but the effects of afterload alone on the Ca²⁺ transient are inconclusive. To our knowledge, no study has investigated whether the putative afterload dependence of the Ca²⁺ transient is preload dependent. This study is designed to provide the first insight into the Ca²⁺ handling of cardiac trabeculae undergoing work-loop contractions, with the aim to examine whether the conflicting afterload dependency of the Ca²⁺ transient can be accounted for by considering preload under isometric and physiological work-loop contractions. Thus, we subjected ex vivo rat right-ventricular trabeculae, loaded with the fluorescent dye Fura-2, to work-loop contractions over a wide range of afterloads at two preloads while measuring stress, length changes, and Ca²⁺ transients. Work-loop control was implemented with a real-time Windkessel model to mimic the contraction patterns of the heart in vivo. We extracted a range of metrics from the measured steady-state twitch stress and Ca²⁺ transients, including the amplitudes, time courses, rates of rise, and integrals. Results show that parameters of stress were afterload and preload dependent. In contrast, the parameters associated with Ca²⁺ transients displayed a mixed dependence on afterload and preload. Most notably, its time course was afterload dependent, an effect augmented at the greater preload. This study reveals that the afterload dependence of cardiac Ca²⁺ transients is modulated by preload, which brings the study of Ca²⁺ transients during isometric contractions into question when aiming to understand physiological Ca²⁺ handling.NEW & NOTEWORTHY This study is the first examination of Ca²⁺ handling in trabeculae undergoing work-loop contractions. These data reveal that reducing preload diminishes the influence of afterload on the decay phase of the cardiac Ca²⁺ transient. This is significant as it reconciles inconsistencies in the literature regarding the influence of external loads on cardiac Ca²⁺ handling. Furthermore, these findings highlight discrepancies between Ca²⁺ handling during isometric and work-loop contractions in cardiac trabeculae operating at their optimal length.

Crossbridge thermodynamics in pulmonary arterial hypertensive right-ventricular failure

Right-ventricular (RV) failure is an event consequent to pathological RV hypertrophy commonly resulting from pulmonary arterial hypertension. This pathology is well characterized by RV diastolic dysfunction, impaired ejection, and reduced mechanical efficiency. However, whether the dynamic stiffness and cross-bridge thermodynamics in the failing RV muscles are compromised remains uncertain. Pulmonary arterial hypertension was induced in the rat by injection of monocrotaline, and RV trabeculae were isolated from RV failing rats. Cross-bridge mechano-energetics were characterized by subjecting the trabeculae to two interventions: 1) force-length work-loop contractions over a range of afterloads while measuring heat output, followed by careful partitioning of heat components into activation heat and cross-bridge heat to separately assess mechanical efficiency and cross-bridge efficiency, and 2) sinusoidal-perturbation of muscle length while trabeculae were actively contracting to interrogate cross-bridge dynamic stiffness. We found that reduced mechanical efficiency is correlated with increased passive stress, reduced shortening, and elevated activation heat. In contrast, the thermodynamics, specifically the efficiency of, and the stiffness characteristics of, cross bridges did not differ between the control and failing trabeculae and were not correlated with elevated passive stress or reduced shortening. We thus conclude that, despite diastolic dysfunction and mechanical inefficiency, cross-bridge stiffness and thermodynamics are unaffected in RV failure following pulmonary arterial hypertension.

NEW & NOTEWORTHY This study characterizes cross-bridge mechano-energetics and dynamic stiffness of right-ventricular trabeculae isolated from a rat model of pulmonary hypertensive right-ventricular failure. Failing trabeculae showed increased passive force but normal active force. Their lower mechanical efficiency is found to be driven by an increase in the energy expenditure arising from contractile activation. This does not reflect a change in their cross-bridge stiffness and efficiency.

The synergistic effects of TGF-β1 and RUNX2 on enamel mineralization through regulating ODAPH expression during the maturation stage

Transforming growth factor β1 (TGF-β1) and Runt-related transcription factor 2 (RUNX2) are critical factors promoting enamel development and maturation. Our previous studies reported that absence of TGF-β1 or RUNX2 resulted in abnormal secretion and absorption of enamel matrix proteins. However, the mechanism remained enigmatic. In this study, TGF-β1^-/-Runx2^-/- and TGF-β1^+/-Runx2^+/- mice were successfully generated to clarify the relationship between TGF-β1 and RUNX2 during amelogenesis. Lower mineralization was observed in TGF-β1^-/-Runx2^-/- and TGF-β1^+/-Runx2^+/- mice than single gene deficient mice. Micro-computed tomography (μCT) revealed a lower ratio of enamel to dentin density in TGF-β1^-/-Runx2^-/- mice. Although μCT elucidated a relatively constant enamel thickness, variation was identified by scanning electron microscopy, which revealed that TGF-β1^-/-Runx2^-/- mice were more vulnerable to acid etching with lower degree of enamel mineralization. Furthermore, the double gene knock-out mice exhibited more serious enamel dysplasia than the single gene deficient mice. Hematoxylin-eosin staining revealed abnormalities in ameloblast morphology and arrangement in TGF-β1^-/-Runx2^-/- mice, which was accompanied by the absence of atypical basal lamina (BL) and the ectopic of enamel matrix. Odontogenesis-associated phosphoprotein (ODAPH) has been identified as a component of an atypical BL. The protein and mRNA expression of ODAPH were down-regulated. In summary, TGF-β1 and RUNX2 might synergistically regulate enamel mineralization through the downstream target gene Odaph. However, the specific mechanism by which TGF-β1 and RUNX2 promote mineralization remains to be further studied.

Pioglitazone restores phosphorylation of downregulated caveolin-1 in right ventricle of monocrotaline-induced pulmonary hypertension

BACKGROUND

Caveolin-1 (cav-1) plays a role in pulmonary arterial hypertension (PAH). Monocrotaline (MCT)-induced PAH is characterized by a loss of cav-1 in pulmonary arteries; however, less is known regarding its role in the hypertrophied right ventricle (RV). We aimed to characterize the role of cav-1 and Hsp90 in the RV of MCT-induced PAH and their impact on endothelial nitric oxide synthase (eNOS). Additionally, we focused on restoration of cav-1 expression with pioglitazone administration.

METHODS

Male 12-week-old Wistar rats were injected subcutaneously with monocrotaline (60 mg/kg). Selected proteins (cav-1, eNOS, pSer1177eNOS, Hsp90) and mRNAs (cav-1α, cav-1β, eNOS) were determined in the RV and left ventricle (LV) 4 weeks later. In a separate MCT-induced PAH study, pioglitazone (10 mg/kg/d, orally) administration started on day 14 after MCT.

RESULTS

MCT induced RV hypertrophy and lung enlargement. Cav-1 and pTyr14cav-1 were decreased in RV. Caveolin-1α (cav-1α) and caveolin-1β (cav-1β) mRNAs were decreased in both ventricles. Hsp90 protein was increased in RV. eNOS and pSer1177eNOS proteins were unchanged in the ventricles. eNOS mRNA was reduced in RV. Pioglitazone treatment increased oxygen saturation and pTyr14cav-1 vs. MCT group.

CONCLUSIONS

Restoration of pTyr14cav-1 did not lead to amelioration of the disease, nor did it prevent RV hypertrophy and fibrosis, which was indicated by an increase in Acta2, Nppb, Col3a1, and Tgfβ1 mRNA.

Right-sided heart failure is also associated with transverse tubule remodeling in the left ventricle

Right-sided heart failure is a common consequence of pulmonary arterial hypertension. Overloading the right ventricle results in right ventricular hypertrophy, which progresses to failure in a process characterized by impaired Ca2+ dynamics and force production that is linked with transverse (t)-tubule remodeling. This also unloads the left ventricle, which consequently atrophies. Experimental left-ventricular unloading can result in t-tubule remodeling, but it is currently unclear if this occurs in right-sided heart failure. In this work, we used a model of monocrotaline (MCT)-induced right heart failure in male rats, using confocal microscopy to investigate cellular remodeling of t-tubules, junctophilin-2 (JPH2), and ryanodine receptor-2 (RyR2). We examined remodeling across tissue anatomical regions of both ventricles: in trabeculae, papillary muscles, and free walls. Our analyses revealed that MCT hearts demonstrated a significant loss of t-tubule periodicity, disruption of the normal sarcomere striated pattern with JPH2 labeling, and also a disorganized striated pattern of RyR2, a feature not previously reported in right heart failure. Remodeling of JPH2 and RyR2 in the MCT heart was more pronounced in papillary muscles and trabeculae compared with free walls, particularly in the left ventricle. We find that these structures, commonly used as ex vivo muscle preparations, are more sensitive to the disease process.NEW & NOTEWORTHY In this work, we demonstrate that t-tubule remodeling occurs in the atrophied left ventricle as well as the overloaded right ventricle after right-side heart failure. Moreover, we identify that t-tubule remodeling in both ventricles is linked to sarcoplasmic reticulum remodeling as indicated by decreased labeling periodicity of both the Ca2+ release channel, RyR2, and the cardiac junction-forming protein, JPH2, that forms a link between the sarcoplasmic reticulum and sarcolemma. Studies developing treatments for right-sided heart failure should consider effects on both the right and left ventricle.

Echocardiography in Pulmonary Arterial Hypertension: Is It Time to Reconsider Its Prognostic Utility?

Pulmonary arterial hypertension (PAH) is characterized by an insult in the pulmonary vasculature, with subsequent right ventricular (RV) adaptation to the increased afterload that ultimately leads to RV failure. The awareness of the importance of RV function in PAH has increased considerably because right heart failure is the predominant cause of death in PAH patients. Given its wide availability and reduced cost, echocardiography is of paramount importance in the evaluation of the right heart in PAH. Several echocardiographic parameters have been shown to have prognostic implications in PAH; however, the role of echocardiography in the risk assessment of the PAH patient is limited under the current guidelines. This review discusses the echocardiographic evaluation of the RV in PAH and during therapy, and its prognostic implications, as well as the potential significant role of repeated echocardiographic assessment in the follow-up of patients with PAH.

Current Understanding of the Right Ventricle Structure and Function in Pulmonary Arterial Hypertension

Pulmonary arterial hypertension (PAH) is a disease resulting in increased right ventricular (RV) afterload and RV remodeling. PAH results in altered RV structure and function at different scales from organ-level hemodynamics to tissue-level biomechanical properties, fiber-level architecture, and cardiomyocyte-level contractility. Biomechanical analysis of RV pathophysiology has drawn significant attention over the past years and recent work has found a close link between RV biomechanics and physiological function. Building upon previously developed techniques, biomechanical studies have employed multi-scale analysis frameworks to investigate the underlying mechanisms of RV remodeling in PAH and effects of potential therapeutic interventions on these mechanisms. In this review, we discuss the current understanding of RV structure and function in PAH, highlighting the findings from recent studies on the biomechanics of RV remodeling at organ, tissue, fiber, and cellular levels. Recent progress in understanding the underlying mechanisms of RV remodeling in PAH, and effects of potential therapeutics, will be highlighted from a biomechanical perspective. The clinical relevance of RV biomechanics in PAH will be discussed, followed by addressing the current knowledge gaps and providing suggested directions for future research.

RhoA: a dubious molecule in cardiac pathophysiology

The Ras homolog gene family member A (RhoA) is the founding member of Rho GTPase superfamily originally studied in cancer cells where it was found to stimulate cell cycle progression and migration. RhoA acts as a master switch control of actin dynamics essential for maintaining cytoarchitecture of a cell. In the last two decades, however, RhoA has been coined and increasingly investigated as an essential molecule involved in signal transduction and regulation of gene transcription thereby affecting physiological functions such as cell division, survival, proliferation and migration. RhoA has been shown to play an important role in cardiac remodeling and cardiomyopathies; underlying mechanisms are however still poorly understood since the results derived from in vitro and in vivo experiments are still inconclusive. Interestingly its role in the development of cardiomyopathies or heart failure remains largely unclear due to anomalies in the current data available that indicate both cardioprotective and deleterious effects. In this review, we aimed to outline the molecular mechanisms of RhoA activation, to give an overview of its regulators, and the probable mechanisms of signal transduction leading to RhoA activation and induction of downstream effector pathways and corresponding cellular responses in cardiac (patho)physiology. Furthermore, we discuss the existing studies assessing the presented results and shedding light on the often-ambiguous data. Overall, we provide an update of the molecular, physiological and pathological functions of RhoA in the heart and its potential in cardiac therapeutics.

Protective effect of baicalin against pulmonary arterial hypertension vascular remodeling through regulation of TNF-α signaling pathway

Pulmonary arterial hypertension (PAH) is a progressive cardiovascular disease with high mortality. However, there were no efficient medical drugs for PAH to enormously improve the survival and quality of life measures. The present study aimed to explore the protective effect of baicalin against experimental PAH in vivo and vitro. All the experimental rats received intraperitoneal injection of monocrotaline (MCT) to induce PAH model. Baicalin was given by intragastric administration from 2 days after MCT injection. Forty animals were randomly divided into four groups: Control, MCT, saline-, and baicalin-treated groups (n = 10 in each). Post-operation, hemodynamic data, and index of right ventricular hypertrophy (RVHI) were recorded to evaluate the inhibition of baicalin on MCT-induced PAH. Furthermore, pulmonary artery smooth muscle cells (PASMCs) model induced by tumor necrosis factor-α (TNF-α) was used to observe the inhibition of vascular cells proliferation in vitro. The results demonstrated that baicalin significantly attenuated MCT-induced right ventricular systolic pressure (RVSP), the index of right ventricular hypertrophy, and vessel wall thickness; inhibit inflammatory and cell proliferation induced by MCT or TNF-α, respectively. In addition, we found that baicalin might protect against experimental PAH via regulating the TNF-α/BMPR2 signaling pathway.

Exercise intolerance establishment in pulmonary hypertension: Preventive effect of aerobic exercise training

AIMS

1) Characterize the progression of exercise intolerance in monocrotaline-induced pulmonary hypertension (PH) in mice and 2) evaluate the therapeutic effect of aerobic exercise training (AET) on counteracting skeletal and cardiac dysfunction in PH.

MAIN METHODS

Wild type C57BL6/J mice were studied in two different time points: 2 months and 4 months. Exercise tolerance was evaluated by graded treadmill exercise test. The AET was performed in the last month of treatment of 4 months' time point. Cardiac function was evaluated by echocardiography. Skeletal muscle cross-sectional area was assessed by immunofluorescence. The diameter of cardiomyocytes and pulmonary edema were quantified by staining with hematoxylin-eosin. The variables were compared among the groups by two-way ANOVA or non-paired Student's t-test. Significance level was set at p < 0.05.

KEY FINDINGS

After 2 months of MCT treatment, mice presented pulmonary edema, right cardiac dysfunction and left ventricle hypertrophy. After 4 months of MCT treatment, mice showed pulmonary edema, right and left cardiac dysfunction and remodeling associated with exercise intolerance and skeletal muscle atrophy. AET was able to reverse cardiac left ventricle dysfunction and remodeling, prevent exercise intolerance and skeletal muscle dysfunction. Thus, our data provide evidence of skeletal muscle abnormalities on advanced PH. AET was efficient in inducing an anti-cardiac remodeling effect besides preventing exercise intolerance.

SIGNIFICANCE

Our study provides a robust model of PH in mice, as well as highlights the importance of AET as a preventive strategy for exercise intolerance and, skeletal and cardiac muscle abnormalities in PH.

Compensating for changes in heart muscle resting heat production in a microcalorimeter

The heat production of cardiac muscle, determined by calorimetry, can be used as a measure of cardiac metabolism. However, heat produced while a muscle is actively-shortening, thereby performing force-length work, comprises both active and basal metabolic processes. In this paper, we present a method for post-experimental processing of calorimetric measurements of muscle heat production, that uncovers and compensates for the measured basal heat rate during work. In this method, the relationships between muscle length, velocity of length change and muscle heat output are coupled with a simulation of the measurement instrument, providing a model-based estimate of change of measured basal heat while the muscle is performing work. We demonstrate the use of this technique in an experiment conducted on a working cardiac muscle sample. The ability to identify the various components of heat release in these muscles provides useful insight into their mechanical and energetic capabilities.

Changes in heart morphometric parameters over the course of a monocrotaline-induced pulmonary arterial hypertension rat model

Background

Aim of this study was to assess changes in cardiac morphometric parameters at different stages of pulmonary arterial hypertension (PAH) using a monocrotaline-induced rat model.

Methods

Four groups were distinguished: I–control, non-PAH (n = 18); II–early PAH (n = 12); III–end-stage PAH (n = 23); and IV–end-stage PAH with myocarditis (n = 7).

Results

Performed over the course of PAH in vivo echocardiography showed significant thickening of the right ventricle free wall (end-diastolic dimension), tricuspid annular plane systolic excursion reduction and decrease in pulmonary artery acceleration time normalized to cycle length. No differences in end-diastolic left ventricle free wall thickness measured in echocardiography was observed between groups. Significant increase of right ventricle and decrease of left ventricle systolic pressure was observed over the development of PAH. Thickening and weight increase (241.2% increase) of the right ventricle free wall and significant dilatation of the right ventricle was observed over the course of PAH (p < 0.001). Reduction in the left ventricle free wall thickness was also observed in end-stage PAH (p < 0.001). Significant trend in the left ventricle free wall weight decrease was observed over the course of PAH (p < 0.001, 24.3% reduction). Calculated right/left ventricle free wall weight ratio gradually increased over PAH stages (p < 0.001). The reduction of left ventricle diameter was observed in rats with end-stage PAH both with and without myocarditis (p < 0.001).

Conclusions

PAH leads to multidimensional changes in morphometric cardiac parameters. Right ventricle morphological and functional failure develop gradually from early stage of PAH, while left ventricle changes develop at the end stages of PAH.

Response of non-failing hypertrophic rat hearts to prostaglandin F2α

Background

Prostaglandin F2α (PGF2α) has a positively inotropic effect on right ventricular (RV) trabeculae from healthy adult rat hearts, and may therefore be therapeutically useful as a non-catecholaminergic inotrope. These provide additional contractile support for the heart without the added energetic demand of increased heart rate, and are also suitable for patients with reduced β adrenergic receptor (β-AR) responsiveness, or impaired mitochondrial energy supply. However, the response of hypertrophied rat hearts to PGF2α has not previously been examined. Our aim was therefore to determine the effect of PGF2α on isolated perfused rat hearts with RV hypertrophy following induction of pulmonary artery hypertension.

Methods

Male Wistar rats (300 g) were injected with either 60 mg kg⁻¹ of monocrotaline (MCT, n = 10) or sterile saline as control (CON, n = 11). Four weeks post injection; hearts were isolated and Langendorff-perfused in sinus rhythm. Measurement of left ventricular (LV) pressure and the electrocardiogram were made and the response to 0.3 μM PGF2α was determined.

Results

PGF2α increased LV developed pressure in CON and in 60% MCT hearts, with no change in heart rate. However, 40% of MCT hearts developed arrhythmias during the peak inotropic response. For comparison, the response to 0.03 μM isoproterenol (ISO) was also investigated. Peak LV pressure developed sooner in response to ISO compared to PGF2α in both rat groups, although the inotropic response to ISO was reduced in MCT hearts. Analysis of fixed ventricular tissue confirmed that only RV myocytes were hypertrophied in MCT hearts. Our study showed that PGF2α was positively inotropic for healthy hearts, but found it generated arrhythmias in 40% of MCT hearts at the dose investigated. However, a more physiological dose of PGF2α may be a useful alternative without the added energetic cost of catecholaminergic inotropes.

•

PGF2α elicits a positive inotropic response in isolated, perfused healthy and hypertrophic rat hearts, with no chronotropic effects, unlike β-AR stimulation.

•

The dose of 0.3 μM PGF2α investigated also triggered sustained, slow onset, arrhythmic activity in 40% of hypertrophic MCT hearts.

•

The peak inotropic response to PGF2α is slower to establish in comparison to the characteristic response to β-AR stimulation, which suggests PGF2α acts via a separate signalling pathway within cardiomyocytes.

•

Hypertrophic MCT hearts had a reduced inotropic response to β-AR stimulation, which illustrates the importance of developing non-catecholaminergic inotropes which will eliminate the increased energetic cost and improve myocardial performance.

Mechanical loading of isolated cardiac muscle with a real-time computed Windkessel model of the vasculature impedance

To date, the mechanical loads imposed on isolated cardiac muscle tissue in vitro have been oversimplified. Researchers typically applied loads that are time-invariant, resulting in either isometric and auxotonic contractions, or flat-topped (isotonic shortening) work-loops. These contraction types do not fully capture the dynamic response of contracting tissues adapting to a variable load, such as is experienced by ventricular tissue in vivo. In this study, we have successfully developed a loading system that presents a model-based, time-varying, continuously updated, load to cardiac tissue preparations. We combined a Windkessel model of vascular fluid impedance together with Laplace's Law and encoded it in a real-time hardware-based force-length control system. Experiments were carried out on isolated rat left ventricular trabeculae; we directly compare the work-loops arising from this protocol with those of a typical simplified isotonic shortening work-loop system. We found that, under body conditions, cardiac trabeculae achieved greater mechanical work output against our new loading system, than with the simplified isotonic work-loop protocol. We further tested whether loading the tissue with a mechanical impedance defined by "diseased" Windkessel model parameters had an effect on the performance of healthy trabeculae. We found that trabecula shortening decreased when applying the set of Windkessel parameters describing the hypertensive condition, and increased in the hypotensive state. Our implementation of a real-time model of arterial characteristics provides an improved, physiologically derived, instantly calculated load for use in studying isolated cardiac muscle, and is readily applicable to study various disease conditions.

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.

Solving a century-old conundrum underlying cardiac force-length relations

In the late 19th century, Otto Frank presented a diagram (Frank O. Z Biol 37: 483-526, 1899) showing that cardiac end-systolic pressure-volume relations are dependent on the mode of contraction: one for isovolumic contractions that locate above that for afterloaded ejecting contractions. Conflicting results to Frank's have been subsequently demonstrated in various species, both within and among preparations, ranging from the whole hearts to single myocytes, showing a single pressure-volume or force-length relation that is independent of the mode of contraction. Numerous explanations for these conflicting results have been proposed but are mutually contradictory and hence unsatisfying. The present study aimed to explore how these conflicting findings can be reconciled. We thus explored the cardiac force-length relation across a wide spectrum of both preloads and afterloads, encompassing the physiological working range. Experiments were performed using isolated ventricular trabeculae at physiological temperature and stimulus frequency. The force-length relation obtained from isometric contractions was indeed located above a family of those obtained from shortening contractions. Low preload conditions rendered the relation contraction mode independent. High afterload conditions also showed a comparable effect. Our exploration allowed us to reveal the loading conditions that can explain the apparent single, contraction mode-independent, force-length relation that is in contrast with that presented by Frank. Resolving this century-old cardiac conundrum highlights the caution that must be taken when using the end-systolic force-length relation to illustrate as well as to understand the concepts of the Frank-Starling law of the heart, "potential energy," and cardiac contractility. NEW & NOTEWORTHY Our exploration of the cardiac force-length relation under wide ranges of preload and afterload has allowed us to reconcile conflicting results in the literature regarding its length dependency. We show that the relation is dependent on the mode of contraction but can appear to be otherwise under certain conditions. This finding highlights the need for caution when using the force-length relation to understand key concepts in cardiac physiology.

Pulmonary arterial hypertension reduces energy efficiency of right, but not left, rat ventricular trabeculae

KEY POINTS

Pulmonary arterial hypertension (PAH) triggers right ventricle (RV) hypertrophy and left ventricle (LV) atrophy, which progressively leads to heart failure. We designed experiments under conditions mimicking those encountered by the heart in vivo that allowed us to investigate whether consequent structural and functional remodelling of the ventricles affects their respective energy efficiencies. We found that peak work output was lower in RV trabeculae from PAH rats due to reduced extent and velocity of shortening. However, their suprabasal enthalpy was unaffected due to increased activation heat, resulting in reduced suprabasal efficiency. There was no effect of PAH on LV suprabasal efficiency. We conclude that the mechanism underlying the reduced energy efficiency of hypertrophied RV tissues is attributable to the increased energy cost of Ca²⁺ cycling, whereas atrophied LV tissues still maintain normal mechano-energetic performance.

ABSTRACT

Pulmonary arterial hypertension (PAH) greatly increases the afterload on the right ventricle (RV), triggering RV hypertrophy, which progressively leads to RV failure. In contrast, the disease reduces the passive filling pressure of the left ventricle (LV), resulting in LV atrophy. We investigated whether these distinct structural and functional consequences to the ventricles affect their respective energy efficiencies. We studied trabeculae isolated from both ventricles of Wistar rats with monocrotaline-induced PAH and their respective Control groups. Trabeculae were mounted in a calorimeter at 37°C. While contracting at 5 Hz, they were subjected to stress-length work-loops over a wide range of afterloads. They were subsequently required to undergo a series of isometric contractions at various muscle lengths. In both protocols, stress production, length change and suprabasal heat output were simultaneously measured. We found that RV trabeculae from PAH rats generated higher activation heat, but developed normal active stress. Their peak external work output was lower due to reduced extent and velocity of shortening. Despite lower peak work output, suprabasal enthalpy was unaffected, thereby rendering suprabasal efficiency lower. Crossbridge efficiency, however, was unaffected. In contrast, LV trabeculae from PAH rats maintained normal mechano-energetic performance. Pulmonary arterial hypertension reduces the suprabasal energy efficiency of hypertrophied right ventricular tissues as a consequence of the increased energy cost of Ca²⁺ cycling.