The Anrep effect: 100 years later

Substrate mechanics unveil early structural and functional pathology in iPSC micro-tissue models of hypertrophic cardiomyopathy

Hypertension is a major cause of morbidity and mortality in patients with hypertrophic cardiomyopathy (HCM), suggesting a potential role for mechanics in HCM pathogenesis. Here, we developed an in vitro physiological model to investigate how mechanics acts together with HCM-linked myosin binding protein C (MYBPC3) mutations to trigger disease. Micro-heart muscles (μHM) were engineered from induced pluripotent stem cell (iPSC)-derived cardiomyocytes bearing MYBPC3+/- mutations and challenged to contract against substrates of different elasticity. μHMs that worked against substrates with stiffness at or exceeding the stiffness of healthy adult heart muscle exhibited several hallmarks of HCM, including cellular hypertrophy, impaired contractile energetics, and maladaptive calcium handling. Remarkably, we discovered changes in troponin C and T localization in MYBPC3+/- μHM that were entirely absent in 2D culture. Pharmacologic studies suggested that excessive Ca2+ intake through membrane-embedded channels underlie the observed electrophysiological abnormalities. These results illustrate the power of physiologically relevant engineered tissue models to study inherited disease with iPSC technology.

Mechanisms maintaining right ventricular contractility-to-pulmonary arterial elastance ratio in VA ECMO: a retrospective animal data analysis of RV-PA coupling

BACKGROUND

To optimize right ventricular-pulmonary coupling during veno-arterial (VA) ECMO weaning, inotropes, vasopressors and/or vasodilators are used to change right ventricular (RV) function (contractility) and pulmonary artery (PA) elastance (afterload). RV-PA coupling is the ratio between right ventricular contractility and pulmonary vascular elastance and as such, is a measure of optimized crosstalk between ventricle and vasculature. Little is known about the physiology of RV-PA coupling during VA ECMO. This study describes adaptive mechanisms for maintaining RV-PA coupling resulting from changing pre- and afterload conditions in VA ECMO.

METHODS

In 13 pigs, extracorporeal flow was reduced from 4 to 1 L/min at baseline and increased afterload (pulmonary embolism and hypoxic vasoconstriction). Pressure and flow signals estimated right ventricular end-systolic elastance and pulmonary arterial elastance. Linear mixed-effect models estimated the association between conditions and elastance.

RESULTS

At no extracorporeal flow, end-systolic elastance increased from 0.83 [0.66 to 1.00] mmHg/mL at baseline by 0.44 [0.29 to 0.59] mmHg/mL with pulmonary embolism and by 1.36 [1.21 to 1.51] mmHg/mL with hypoxic pulmonary vasoconstriction (p < 0.001). Pulmonary arterial elastance increased from 0.39 [0.30 to 0.49] mmHg/mL at baseline by 0.36 [0.27 to 0.44] mmHg/mL with pulmonary embolism and by 0.75 [0.67 to 0.84] mmHg/mL with hypoxic pulmonary vasoconstriction (p < 0.001). Coupling remained unchanged (2.1 [1.8 to 2.3] mmHg/mL at baseline; - 0.1 [- 0.3 to 0.1] mmHg/mL increase with pulmonary embolism; - 0.2 [- 0.4 to 0.0] mmHg/mL with hypoxic pulmonary vasoconstriction, p > 0.05). Extracorporeal flow did not change coupling (0.0 [- 0.0 to 0.1] per change of 1 L/min, p > 0.05). End-diastolic volume increased with decreasing extracorporeal flow (7.2 [6.6 to 7.8] ml change per 1 L/min, p < 0.001).

CONCLUSIONS

The right ventricle dilates with increased preload and increases its contractility in response to afterload changes to maintain ventricular-arterial coupling during VA extracorporeal membrane oxygenation.

Regulation of cardiomyocyte t-tubule structure by preload and afterload: Roles in cardiac compensation and decompensation

Mechanical load is a potent regulator of cardiac structure and function. Although high workload during heart failure is associated with disruption of cardiomyocyte t-tubules and Ca2+ homeostasis, it remains unclear whether changes in preload and afterload may promote adaptive t-tubule remodelling. We examined this issue by first investigating isolated effects of stepwise increases in load in cultured rat papillary muscles. Both preload and afterload increases produced a biphasic response, with the highest t-tubule densities observed at moderate loads, whereas excessively low and high loads resulted in low t-tubule levels. To determine the baseline position of the heart on this bell-shaped curve, mice were subjected to mildly elevated preload or afterload (1 week of aortic shunt or banding). Both interventions resulted in compensated cardiac function linked to increased t-tubule density, consistent with ascension up the rising limb of the curve. Similar t-tubule proliferation was observed in human patients with moderately increased preload or afterload (mitral valve regurgitation, aortic stenosis). T-tubule growth was associated with larger Ca2+ transients, linked to upregulation of L-type Ca2+ channels, Na+-Ca2+ exchanger, mechanosensors and regulators of t-tubule structure. By contrast, marked elevation of cardiac load in rodents and patients advanced the heart down the declining limb of the t-tubule-load relationship. This bell-shaped relationship was lost in the absence of electrical stimulation, indicating a key role of systolic stress in controlling t-tubule plasticity. In conclusion, modest augmentation of workload promotes compensatory increases in t-tubule density and Ca2+ cycling, whereas this adaptation is reversed in overloaded hearts during heart failure progression. KEY POINTS: Excised papillary muscle experiments demonstrated a bell-shaped relationship between cardiomyocyte t-tubule density and workload (preload or afterload), which was only present when muscles were electrically stimulated. The in vivo heart at baseline is positioned on the rising phase of this curve because moderate increases in preload (mice with brief aortic shunt surgery, patients with mitral valve regurgitation) resulted in t-tubule growth. Moderate increases in afterload (mice and patients with mild aortic banding/stenosis) similarly increased t-tubule density. T-tubule proliferation was associated with larger Ca2+ transients, with upregulation of the L-type Ca2+ channel, Na+-Ca2+ exchanger, mechanosensors and regulators of t-tubule structure. By contrast, marked elevation of cardiac load in rodents and patients placed the heart on the declining phase of the t-tubule-load relationship, promoting heart failure progression. The dependence of t-tubule structure on preload and afterload thus enables both compensatory and maladaptive remodelling, in rodents and humans.

Intrinsic mechanisms of right ventricular autoregulation

To elucidate the adaptation of the right ventricle to acute and intermittently sustained afterload elevation, targeted preload reductions and afterload increases were implemented in a porcine model involving 12 pigs. Preload reduction was achieved via balloon occlusion of the inferior vena cava before, immediately and 5 min after acute afterload elevation induced by pulmonary artery occlusion or thromboxane A2 analog (U46619) infusion. Ventricular response was monitored by registration of pressure-volume (PV) loops using a conductance catheter. The end-systolic pressure-volume relationship (ESPVR) during pure preload reduction was adequately described by linear regression (mean and SEM slope of ESPVR (Ees) 0.414 ± 0.064 mmHg/ml), reflecting the classical Frank-Starling mechanism (FSM). The ESPVR during acute afterload elevation exhibited a biphasic trajectory with significantly distinct slopes (mean and SEM Ees bilin1: 1.256 ± 0.066 mmHg ml; Ees bilin2: 0.733 ± 0.063 mmHg ml, p < 0.001). The higher slope during the first phase in the absence of ventricular dilation could be explained by a reduced amount of shortening deactivation (SDA). The changes in PV-loops during the second phase were similar to those observed with a preload intervention. The persistent increase in afterload resulted in an increase in the slopes of ESPVR and preload recruitable stroke work (PRSW) with a slight decrease in filling state, indicating a relevant Anrep effect. This effect became more pronounced after 5 min or TXA infusion. This study demonstrates, for the first time, the relevance of intrinsic mechanisms of cardiac autoregulation in the right ventricle during the adaptation to load. The SDA, FSM, and Anrep effect could be differentiated and occurred successively, potentially with some overlap. Notably, the Anrep effect serves to prevent ventricular dilation.

Regulation of myocardial contraction as revealed by intracellular Ca2+ measurements using aequorin

Of the ions involved in myocardial function, Ca2+ is the most important. Ca2+ is crucial to the process that allows myocardium to repeatedly contract and relax in a well-organized fashion; it is the process called excitation-contraction coupling. In order, therefore, for accurate comprehension of the physiology of the heart, it is fundamentally important to understand the detailed mechanism by which the intracellular Ca2+ concentration is regulated to elicit excitation-contraction coupling. Aequorin was discovered by Shimomura, Johnson and Saiga in 1962. By taking advantage of the fact that aequorin emits blue light when it binds to Ca2+ within the physiologically relevant concentration range, in the 1970s and 1980s, physiologists microinjected it into myocardial preparations. By doing so, they proved that Ca2+ transients occur upon membrane depolarization, and tension development (i.e., actomyosin interaction) subsequently follows, dramatically advancing the research on cardiac excitation-contraction coupling.

Mechanical Resistance to Micro-Heart Tissue Contractility unveils early Structural and Functional Pathology in iPSC Models of Hypertrophic Cardiomyopathy

Hypertrophic cardiomyopathy is the most common cause of sudden death in the young. Because the disease exhibits variable penetrance, there are likely nongenetic factors that contribute to the manifestation of the disease phenotype. Clinically, hypertension is a major cause of morbidity and mortality in patients with HCM, suggesting a potential synergistic role for the sarcomeric mutations associated with HCM and mechanical stress on the heart. We developed an in vitro physiological model to investigate how the afterload that the heart muscle works against during contraction acts together with HCM-linked MYBPC3 mutations to trigger a disease phenotype. Micro-heart muscle arrays (μHM) were engineered from iPSC-derived cardiomyocytes bearing MYBPC3 loss-of-function mutations and challenged to contract against mechanical resistance with substrates stiffnesses ranging from the of embryonic hearts (0.4 kPa) up to the stiffness of fibrotic adult hearts (114 kPa). Whereas MYBPC3 +/- iPSC-cardiomyocytes showed little signs of disease pathology in standard 2D culture, μHMs that included components of afterload revealed several hallmarks of HCM, including cellular hypertrophy, impaired contractile energetics, and maladaptive calcium handling. Remarkably, we discovered changes in troponin C and T localization in the MYBPC3 +/- μHM that were entirely absent in 2D culture. Pharmacologic studies suggested that excessive Ca 2+ intake through membrane-embedded channels, rather than sarcoplasmic reticulum Ca 2+ ATPase (SERCA) dysfunction or Ca 2+ buffering at myofilaments underlie the observed electrophysiological abnormalities. These results illustrate the power of physiologically relevant engineered tissue models to study inherited disease mechanisms with iPSC technology.

Management of Acute Right Ventricular Failure

PURPOSE OF REVIEW

Acute right ventricular failure (RVF) is a frequent condition associated with high morbidity and mortality. This review aims to provide a current overview of the pathophysiology, presentation, and comprehensive management of acute RVF.

RECENT FINDINGS

Acute RVF is a common disease with a pathophysiology that is not completely understood. There is renewed interest in the right ventricle (RV). Some advances have been principally made in chronic right ventricular failure (e.g., pulmonary hypertension). Due to a lack of precise definition and diagnostic tools, acute RVF is poorly studied. Few advances have been made in this field. Acute RVF is a complex, frequent, and life-threatening condition with several etiologies. Transthoracic echocardiography (TTE) is the key diagnostic tool in search of the etiology. Management includes transfer to an expert center and admission to the intensive care unit (ICU) in most severe cases, etiological treatment, and general measures for RVF.

The effect of urgent blood pressure reduction on left atrial strain in patients with hypertensive attack : Blood pressure lowering affects LA strain

BACKGROUND

Left atrial (LA) strain is a robust measure of LA function and is a useful parameter to assess left ventricular filling pressure. While initially considered as a "load-independent" parameter of LA function, later studies have found that acute changes in LA preload may affect LA reservoir and contractile strains. Acute alterations in blood pressure (BP) induces a change in left ventricular (LV) filling pressure without imposing a volume load, thus providing an opportunity to assess the effects of the change in LA afterload on LA mechanics. This study aims to understand the effect of acute BP changes on LA strain.

METHODS

A total of 40 patients admitted to the emergency department with hypertensive urgency were included. All patients underwent a comprehensive echocardiographic examination including measurement of LA reservoir, conduit and contractile strains. A repeat set of measurements were obtained after BP lowering.

RESULTS

Average drop in mean BP following intervention was 18.1 ± 5.4%. LV end-systolic and end-diastolic volumes, as well as maximum and minimum LA volumes were decreased significantly after BP reduction. The absolute increases in reservoir and contractile strains were 2.3 ± 4.7% (7.9% ± 13.8% relative to baseline) and 2.5 ± 3.3% (13.5 ± 19.0% relative to baseline), respectively, with both changes being statistically significant (p = 0.003 for reservoir and p < 0.001 for contractile strains). There were no significant changes in conduit strain after BP intervention (p = 0.79). The change in both LA reservoir and contractile strains were more evident in those with a previous diagnosis of hypertension and those with a smaller degree of change in mean BP after intervention.

CONCLUSION

In patients with an acute hypertension, lowering BP leads to an acute improvement in LA reservoir and contractile strains. Thus, acute changes in systemic BP should be considered when LA mechanics are evaluated.

Nitrosylation of cardiac CaMKII at Cys290 mediates mechanical afterload-induced increases in Ca2+ transient and Ca2+ sparks

Cardiac mechanical afterload induces an intrinsic autoregulatory increase in myocyte Ca²⁺ dynamics and contractility to enhance contraction (known as the Anrep effect or slow force response). Our prior work has implicated both nitric oxide (NO) produced by NO synthase 1 (NOS1) and calcium/calmodulin-dependent protein kinase II (CaMKII) activity as required mediators of this form of mechano-chemo-transduction. To test whether a single S-nitrosylation site on CaMKIIδ (Cys290) mediates enhanced sarcoplasmic reticulum Ca²⁺ leak and afterload-induced increases in sarcoplasmic reticulum (SR) Ca²⁺ uptake and release, we created a novel CRISPR-based CaMKIIδ knock-in (KI) mouse with a Cys to Ala mutation at C290. These CaMKIIδ-C290A-KI mice exhibited normal cardiac morphometry and function, as well as basal myocyte Ca²⁺ transients (CaTs) and β-adrenergic responses. However, the NO donor S-nitrosoglutathione caused an acute increased Ca²⁺ spark frequency in wild-type (WT) myocytes that was absent in the CaMKIIδ-C290A-KI myocytes. Using our cell-in-gel system to exert multiaxial three-dimensional mechanical afterload on myocytes during contraction, we found that WT myocytes exhibited an afterload-induced increase in Ca²⁺ sparks and Ca²⁺ transient amplitude and rate of decline. These afterload-induced effects were prevented in both cardiac-specific CaMKIIδ knockout and point mutant CaMKIIδ-C290A-KI myocytes. We conclude that CaMKIIδ activation by S-nitrosylation at the C290 site is essential in mediating the intrinsic afterload-induced enhancement of myocyte SR Ca²⁺ uptake, release and Ca²⁺ transient amplitude (the Anrep effect). The data also indicate that NOS1 activation is upstream of S-nitrosylation at C290 of CaMKII, and that this molecular mechano-chemo-transduction pathway is beneficial in allowing the heart to increase contractility to limit the reduction in stroke volume when aortic pressure (afterload) is elevated. KEY POINTS: A novel CRISPR-based CaMKIIδ knock-in mouse was created in which kinase activation by S-nitrosylation at Cys290 (C290A) is prevented. How afterload affects Ca²⁺ signalling was measured in cardiac myocytes that were embedded in a hydrogel that imposes a three-dimensional afterload. This mechanical afterload induced an increase in Ca²⁺ transient amplitude and decay in wild-type myocytes, but not in cardiac-specific CaMKIIδ knockout or C290A knock-in myocytes. The CaMKIIδ-C290 S-nitrosylation site is essential for the afterload-induced enhancement of Ca²⁺ transient amplitude and Ca²⁺ sparks.

Pathophysiology and Management of Pulmonary Embolism

Pulmonary embolism (PE) is one of the most common etiologies of cardiovascular mortality. It could be linked to several risk factors including advanced age. The pathogenesis of PE is dictated by the Virchow's triad that includes venous stasis, endothelial injury, and a hypercoagulable state. The diagnosis of PE is difficult and is often missed due to the nonspecific symptomatology. Hypoxia is common in the setting of PE, and the degree of respiratory compromise is multifactorial and influenced by underlying cardiac function, clot location, and ability to compensate with respiratory mechanics. Right ventricular dysfunction/failure is the more profound cardiovascular impact of acute PE and occurs due to sudden increase in afterload. This is also the primary cause of death in PE. High clinical suspicion is required in those with risk factors and presenting signs or symptoms of venous thromboembolic disease, with validated clinical risk scores such as the Wells, Geneva, and pulmonary embolism rule out criteria in estimating the likelihood for PE. Advancement in capture time and wider availability of computed tomographic pulmonary angiography and D-dimer testing have further facilitated the rapid evaluation and diagnosis of suspected PE. Treatment is dependent on clinical presentation and initially involves providing adequate oxygenation and stabilizing hemodynamics. Anticoagulant therapy is indicated for the treatment of PE. Treatment is guided by presence or absence of shock and ranges from therapeutic anticoagulation to pharmacologic versus mechanical thrombectomy. The prognosis of patients can vary considerably depending on the cardiac and pulmonary status of patient and the size of the embolus.

Modeling Cardiomyocyte Mechanics and Autoregulation of Contractility by Mechano-Chemo-Transduction Feedback

The heart pumps blood into circulation against vascular resistance and actively regulates the contractile force to compensate for mechanical load changes. Our experimental data show that cardiomyocytes have a mechano-chemo-transduction (MCT) mechanism that increases intracellular Ca2+ transient to enhance contractility in response to increased mechanical load. This study advances the cardiac excitation- Ca2+ signaling-contraction (E-C) coupling model on conceptual and technical fronts. First, we developed analytical and computational models to perform 3-dimensional mechanical analysis of cardiomyocytes contracting in a viscoelastic medium under mechanical load. Next, we proposed an MCT feedback loop in the E-C coupling dynamic system to shift the feedforward paradigm of cardiac E-C coupling to an autoregulation model. Our combined modeling and experimental studies reveal that MCT enables autoregulation of E-C coupling and contractility in single cardiomyocytes, which underlies the heart’s intrinsic autoregulation in compensatory response to load changes in order to maintain the stroke volume and cardiac output.

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Excitation-contraction (E-C) coupling has mechano-chemo-transduction (MCT) feedback

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MCT feedback enables autoregulation of E-C coupling when contracting under load

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Models for 3D mechanical analyses of cardiomyocytes contraction

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Shifts the paradigm of cardiac E-C coupling from feedforward to autoregulation model

Piezo2 is not an indispensable mechanosensor in murine cardiomyocytes

A short-term increase in ventricular filling leads to an immediate (Frank-Starling mechanism) and a slower (Anrep effect) rise in cardiac contractility, while long-term increased cardiac load (e.g., in arterial hypertension) decreases contractility. Whether these answers to mechanical tension are mediated by specific sensors in cardiomyocytes remains elusive. In this study, the piezo2 protein was evaluated as a potential mechanosensor. Piezo2 was found to be upregulated in various rat and mouse cardiac tissues upon mechanical or pharmacological stress. To investigate its function, C57BL/6J mice with homozygous cardiomyocyte-specific piezo2 knockout [Piezo2-KO] were created. To this end, α-MHC-Cre mice were crossed with homozygous "floxed" piezo2 mice. α-MHC-Cre mice crossed with wildtype mice served as controls [WT-Cre⁺]. In cardiomyocytes of Piezo2-KO mice, piezo2 mRNA was reduced by > 90% and piezo2 protein was not detectable. Piezo2-KO mice displayed no morphological abnormalities or altered cardiac function under nonstressed conditions. In a subsequent step, hearts of Piezo2-KO or WT-Cre⁺-mice were stressed by either three weeks of increased afterload (angiotensin II, 2.5 mg/kg/day) or one week of hypercontractility (isoprenaline, 30 mg/kg/day). As expected, angiotensin II treatment in WT-Cre⁺-mice resulted in higher heart and lung weight (per body weight, + 38%, + 42%), lower ejection fraction and cardiac output (- 30%, - 39%) and higher left ventricular anterior and posterior wall thickness (+ 34%, + 37%), while isoprenaline led to higher heart weight (per body weight, + 25%) and higher heart rate and cardiac output (+ 24%, + 54%). The Piezo2-KO mice reacted similarly with the exception that the angiotensin II-induced increases in wall thickness were blunted and the isoprenaline-induced increase in cardiac output was slightly less pronounced. As cardiac function was neither severely affected under basal nor under stressed conditions in Piezo2-KO mice, we conclude that piezo2 is not an indispensable mechanosensor in cardiomyocytes.

Contractile Behavior of Right Atrial Myocardium of Healthy Rats and Rats with the Experimental Model of Pulmonary Hypertension

There is a lack of data about the contractile behavior of the right atrial myocardium in chronic pulmonary heart disease. We thoroughly characterized the contractility and Ca transient of isolated right atrial strips of healthy rats (CONT) and rats with the experimental model of monocrotaline-induced pulmonary hypertension (MCT) in steady state at different preloads (isometric force-length), during slow force response to stretch (SFR), and during post-rest potentiation after a period of absence of electrical stimulation (PRP). The preload-dependent changes in the isometric twitch and Ca transient did not differ between CONT and MCT rats while the kinetics of the twitch and Ca transient were noticeably slowed down in the MCT rats. The magnitude of SFR was significantly elevated in the MCT right atrial strips and this was accompanied by the significantly higher elevation of the Ca transient relative amplitude at the end of SFR. The slow changes in the contractility and Ca transient in the PRP protocol did not differ between CONT and MCT. In conclusion, the alterations in the contractility and Ca transient of the right atrial myocardium of monocrotaline-treated rats with pulmonary hypertension mostly concern the elevation in SFR. We hypothesize that this positive inotropic effect in the atrial myocardium may (partly) compensate the systolic deficiency of the right ventricular failing myocardium.

The effects of positive end-expiratory pressure on cardiac function: a comparative echocardiography-conductance catheter study

Background

Echocardiographic parameters of diastolic function depend on cardiac loading conditions, which are altered by positive pressure ventilation. The direct effects of positive end-expiratory pressure (PEEP) on cardiac diastolic function are unknown.

Methods

Twenty-five patients without apparent diastolic dysfunction undergoing coronary angiography were ventilated noninvasively at PEEPs of 0, 5, and 10 cmH₂O (in randomized order). Echocardiographic diastolic assessment and pressure–volume-loop analysis from conductance catheters were compared. The time constant for pressure decay (τ) was modeled with exponential decay. End-diastolic and end-systolic pressure volume relationships (EDPVRs and ESPVRs, respectively) from temporary caval occlusion were analyzed with generalized linear mixed-effects and linear mixed models. Transmural pressures were calculated using esophageal balloons.

Results

τ values for intracavitary cardiac pressure increased with the PEEP (n = 25; no PEEP, 44 ± 5 ms; 5 cmH₂O PEEP, 46 ± 6 ms; 10 cmH₂O PEEP, 45 ± 6 ms; p < 0.001). This increase disappeared when corrected for transmural pressure and diastole length. The transmural EDPVR was unaffected by PEEP. The ESPVR increased slightly with PEEP. Echocardiographic mitral inflow parameters and tissue Doppler values decreased with PEEP [peak E wave (n = 25): no PEEP, 0.76 ± 0.13 m/s; 5 cmH₂O PEEP, 0.74 ± 0.14 m/s; 10 cmH₂O PEEP, 0.68 ± 0.13 m/s; p = 0.016; peak A wave (n = 24): no PEEP, 0.74 ± 0.12 m/s; 5 cmH₂O PEEP, 0.7 ± 0.11 m/s; 10 cmH₂O PEEP, 0.67 ± 0.15 m/s; p = 0.014; E’ septal (n = 24): no PEEP, 0.085 ± 0.016 m/s; 5 cmH₂O PEEP, 0.08 ± 0.013 m/s; 10 cmH₂O PEEP, 0.075 ± 0.012 m/s; p = 0.002].

Conclusions

PEEP does not affect active diastolic relaxation or passive ventricular filling properties. Dynamic echocardiographic filling parameters may reflect changing loading conditions rather than intrinsic diastolic function. PEEP may have slight positive inotropic effects.

Clinical trial registration

https://clinicaltrials.gov/ct2/show/NCT02267291, registered 17. October 2014.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s00392-022-02014-1.

In Vivo Validation of a Cardiovascular Simulation Model in Pigs

Many computer simulation models of the cardiovascular system, of varying complexity and objectives, have been proposed in physiological science. Every model needs to be parameterized and evaluated individually. We conducted a porcine animal model to parameterize and evaluate a computer simulation model, recently proposed by our group. The results of an animal model, on thirteen healthy pigs, were used to generate consistent parameterization data for the full heart computer simulation model. To evaluate the simulation model, differences between the resulting simulation output and original animal data were analysed. The input parameters of the animal model, used to individualize the computer simulation, showed high interindividual variability (range of coefficient of variation: 10.1–84.5%), which was well-reflected by the resulting haemodynamic output parameters of the simulation (range of coefficient of variation: 12.6–45.7%). The overall bias between the animal and simulation model was low (mean: −3.24%, range: from −26.5 to 20.1%). The simulation model used in this study was able to adapt to the high physiological variability in the animal model. Possible reasons for the remaining differences between the animal and simulation model might be a static measurement error, unconsidered inaccuracies within the model, or unconsidered physiological interactions.

Emerging therapeutic targets for cardiac hypertrophy

INTRODUCTION

Cardiac hypertrophy is associated with adverse outcomes across cardiovascular disease states. Despite strides over the last three decades in identifying molecular and cellular mechanisms driving hypertrophy, the link between pathophysiological stress stimuli and specific myocyte/heart growth profiles remains unclear. Moreover, the optimal strategy for preventing pathology in the setting of hypertrophy remains controversial.

AREAS COVERED

This review discusses molecular mechanisms underlying cardiac hypertrophy with a focus on factors driving the orientation of myocyte growth and the impact on heart function. We highlight recent work showing a novel role for the spectrin-based cytoskeleton, emphasizing regulation of myocyte dimensions but not hypertrophy per se. Finally, we consider opportunities for directing the orientation of myocyte growth in response to hypertrophic stimuli as an alternative therapeutic approach. Relevant publications on the topic were identified through Pubmed with open-ended search dates.

EXPERT OPINION

To define new therapeutic avenues, more precision is required when describing changes in myocyte and heart structure/function in response to hypertrophic stimuli. Recent developments in computational modeling of hypertrophic networks, in concert with more refined experimental approaches will catalyze translational discovery to advance the field and further our understanding of cardiac hypertrophy and its relationship with heart disease.

Acute right ventricular failure associated with pulmonary hypertension in pediatrics: understanding the hemodynamic profiles

Pulmonary hypertension (PHTN) is a common pathology in pediatrics, arising from a diverse array of etiologies and manifesting in equally diverse patient populations. The inpatient management of these infants and children may be complicated by dynamic and at times severe increases in pulmonary vascular resistance (PVR) and right ventricular (RV) afterload. Yet absent are cognitively accessible heuristics in the field whereby providers can reconcile the various clinical manifestations they observe with an understanding of the cardiac physiology at play, and therefore, appropriate physiology-driven interventions. Described herein is a framework for understanding the pathophysiology of four clinical phenotypes which are driven by two echocardiographic patient characteristics: the presence or absence of an atrial communication and the capacity of the right ventricle to maintain ventricular-vascular coupling. Application of this paradigm may facilitate accurate interpretation of observed clinical data, and alignment of treatment strategies with the underlying pathophysiology.

Mechanoelectric coupling and arrhythmogenesis in cardiomyocytes contracting under mechanical afterload in a 3D viscoelastic hydrogel

The heart pumps blood against the mechanical afterload from arterial resistance, and increased afterload may alter cardiac electrophysiology and contribute to life-threatening arrhythmias. However, the cellular and molecular mechanisms underlying mechanoelectric coupling in cardiomyocytes remain unclear. We developed an innovative patch-clamp-in-gel technology to embed cardiomyocytes in a three-dimensional (3D) viscoelastic hydrogel that imposes an afterload during regular myocyte contraction. Here, we investigated how afterload affects action potentials, ionic currents, intracellular Ca2+ transients, and cell contraction of adult rabbit ventricular cardiomyocytes. We found that afterload prolonged action potential duration (APD), increased transient outward K+ current, decreased inward rectifier K+ current, and increased L-type Ca2+ current. Increased Ca2+ entry caused enhanced Ca2+ transients and contractility. Moreover, elevated afterload led to discordant alternans in APD and Ca2+ transient. Ca2+ alternans persisted under action potential clamp, indicating that the alternans was Ca2+ dependent. Furthermore, all these afterload effects were significantly attenuated by inhibiting nitric oxide synthase 1 (NOS1). Taken together, our data reveal a mechano-chemo-electrotransduction (MCET) mechanism that acutely transduces afterload through NOS1-nitric oxide signaling to modulate the action potential, Ca2+ transient, and contractility. The MCET pathway provides a feedback loop in excitation-Ca2+ signaling-contraction coupling, enabling autoregulation of contractility in cardiomyocytes in response to afterload. This MCET mechanism is integral to the individual cardiomyocyte (and thus the heart) to intrinsically enhance its contractility in response to the load against which it has to do work. While this MCET is largely compensatory for physiological load changes, it may also increase susceptibility to arrhythmias under excessive pathological loading.

Emergence of Mechano-Sensitive Contraction Autoregulation in Cardiomyocytes

The heart has two intrinsic mechanisms to enhance contractile strength that compensate for increased mechanical load to help maintain cardiac output. When vascular resistance increases the ventricular chamber initially expands causing an immediate length-dependent increase of contraction force via the Frank-Starling mechanism. Additionally, the stress-dependent Anrep effect slowly increases contraction force that results in the recovery of the chamber volume towards its initial state. The Anrep effect poses a paradox: how can the cardiomyocyte maintain higher contractility even after the cell length has recovered its initial length? Here we propose a surface mechanosensor model that enables the cardiomyocyte to sense different mechanical stresses at the same mechanical strain. The cell-surface mechanosensor is coupled to a mechano-chemo-transduction feedback mechanism involving three elements: surface mechanosensor strain, intracellular Ca2+ transient, and cell strain. We show that in this simple yet general system, contractility autoregulation naturally emerges, enabling the cardiomyocyte to maintain contraction amplitude despite changes in a range of afterloads. These nontrivial model predictions have been experimentally confirmed. Hence, this model provides a new conceptual framework for understanding the contractility autoregulation in cardiomyocytes, which contributes to the heart's intrinsic adaptivity to mechanical load changes in health and diseases.

Thioredoxin 1 (TRX1) Overexpression Cancels the Slow Force Response (SFR) Development

The stretch of cardiac muscle increases developed force in two phases. The first phase occurs immediately after stretch and is the expression of the Frank–Starling mechanism, while the second one or slow force response (SFR) occurs gradually and is due to an increase in the calcium transient amplitude. An important step in the chain of events leading to the SFR generation is the increased production of reactive oxygen species (ROS) leading to redox sensitive ERK1/2, p90RSK, and NHE1 phosphorylation/activation. Conversely, suppression of ROS production blunts the SFR. The purpose of this study was to explore whether overexpression of the ubiquitously expressed antioxidant molecule thioredoxin-1 (TRX1) affects the SFR development and NHE1 phosphorylation. We did not detect any change in basal phopho-ERK1/2, phopho-p90RSK, and NHE1 expression in mice with TRX1 overexpression compared to wild type (WT). Isolated papillary muscles from WT or TRX1-overexpressing mice were stretched from 92 to 98% of its maximal length. A prominent SFR was observed in WT mice that was completely canceled in TRX1 animals. Interestingly, myocardial stretch induced a significant increase in NHE1 phosphorylation in WT mice that was not detected in TRX1-overexpressing mice. These novel results suggest that magnification of cardiac antioxidant defense power by overexpression of TRX1 precludes NHE1 phosphorylation/activation after stretch, consequently blunting the SFR development.

The mechanosensitive Piezo1 channel mediates heart mechano-chemo transduction

The beating heart possesses the intrinsic ability to adapt cardiac output to changes in mechanical load. The century-old Frank-Starling law and Anrep effect have documented that stretching the heart during diastolic filling increases its contractile force. However, the molecular mechanotransduction mechanism and its impact on cardiac health and disease remain elusive. Here we show that the mechanically activated Piezo1 channel converts mechanical stretch of cardiomyocytes into Ca2+ and reactive oxygen species (ROS) signaling, which critically determines the mechanical activity of the heart. Either cardiac-specific knockout or overexpression of Piezo1 in mice results in defective Ca2+ and ROS signaling and the development of cardiomyopathy, demonstrating a homeostatic role of Piezo1. Piezo1 is pathologically upregulated in both mouse and human diseased hearts via an autonomic response of cardiomyocytes. Thus, Piezo1 serves as a key cardiac mechanotransducer for initiating mechano-chemo transduction and consequently maintaining normal heart function, and might represent a novel therapeutic target for treating human heart diseases.

Non-linearity of end-systolic pressure-volume relation in afterload increases is caused by an overlay of shortening deactivation and the Frank-Starling mechanism

The linearity and load insensitivity of the end-systolic pressure-volume-relationship (ESPVR), a parameter that describes the ventricular contractile state, are controversial. We hypothesize that linearity is influenced by a variable overlay of the intrinsic mechanism of autoregulation to afterload (shortening deactivation) and preload (Frank-Starling mechanism). To study the effect of different short-term loading alterations on the shape of the ESPVR, experiments on twenty-four healthy pigs were executed. Preload reductions, afterload increases and preload reductions while the afterload level was increased were performed. The ESPVR was described either by a linear or a bilinear regression through the end-systolic pressure volume (ES-PV) points. Increases in afterload caused a biphasic course of the ES-PV points, which led to a better fit of the bilinear ESPVRs (r2 0.929 linear ESPVR vs. r2 0.96 and 0.943 bilinear ESPVR). ES-PV points of a preload reduction on a normal and augmented afterload level could be well described by a linear regression (r2 0.974 linear ESPVR vs. r2 0.976 and 0.975 bilinear ESPVR). The intercept of the second ESPVR (V0) but not the slope demonstrated a significant linear correlation with the reached afterload level (effective arterial elastance Ea). Thus, the early response to load could be described by the fixed slope of the ESPVR and variable V0, which was determined by the actual afterload. The ESPVR is only apparently nonlinear, as its course over several heartbeats was affected by an overlay of SDA and FSM. These findings could be easily transferred to cardiovascular simulation models to improve their accuracy.

Mechanotranduction Pathways in the Regulation of Mitochondrial Homeostasis in Cardiomyocytes

Mitochondria are one of the most important organelles in cardiomyocytes. Mitochondrial homeostasis is necessary for the maintenance of normal heart function. Mitochondria perform four major biological processes in cardiomyocytes: mitochondrial dynamics, metabolic regulation, Ca2+ handling, and redox generation. Additionally, the cardiovascular system is quite sensitive in responding to changes in mechanical stress from internal and external environments. Several mechanotransduction pathways are involved in regulating the physiological and pathophysiological status of cardiomyocytes. Typically, the extracellular matrix generates a stress-loading gradient, which can be sensed by sensors located in cellular membranes, including biophysical and biochemical sensors. In subsequent stages, stress stimulation would regulate the transcription of mitochondrial related genes through intracellular transduction pathways. Emerging evidence reveals that mechanotransduction pathways have greatly impacted the regulation of mitochondrial homeostasis. Excessive mechanical stress loading contributes to impairing mitochondrial function, leading to cardiac disorder. Therefore, the concept of restoring mitochondrial function by shutting down the excessive mechanotransduction pathways is a promising therapeutic strategy for cardiovascular diseases. Recently, viral and non-viral protocols have shown potentials in application of gene therapy. This review examines the biological process of mechanotransduction pathways in regulating mitochondrial function in response to mechanical stress during the development of cardiomyopathy and heart failure. We also summarize gene therapy delivery protocols to explore treatments based on mechanical stress-induced mitochondrial dysfunction, to provide new integrative insights into cardiovascular diseases.

Effect of endothelin on sex-dependent regulation of tone in coronary resistance vessels

BACKGROUND/AIMS

Sex dependent differences in coronary artery vasoregulation may be due to variations in responses to endogenous vasoactive compounds including endothelin (ET-1) and nitric oxide (NO).

METHODS

Septal coronary arteries (<200 μm) from healthy, sexually mature male, female and ovariectomized (i.e. surgical menopause) Sprague-Dawley rats were used. Myogenic tone, measured by pressure myography, was initially determined for all vessel segments studied before and after exposure to the nonselective ET_A/ET_B receptor blocker, bosentan (1 μM). Vasoconstrictor responses (vascular endothelium intact) to cumulative ET-1 (10^-12 - 10^-9 M) were assessed in a separate set of septal coronary vessels. Additional studies, examined the vasoconstrictor effects of ET-1 after NO blockade with L-NAME (200 μM).

RESULTS

Myogenic tone was 26 ± 7% in male, 20 ± 7% in female (p = 0.04 versus male) and 24 ± 3% in ovariectomized (p = NS versus male/female) vessels. Antagonism of ET-1 receptors produced a greater reduction in myogenic tone in male, compared to female rats over a similar range of intraluminal pressure (20-80 mmHg). Robust constrictor responses to cumulative concentrations of ET-1 were observed in all vessels; however, male rats exhibited greater sensitivity to vasoconstrictor effects of ET-1. After exposure to L-NAME vessel responses to ET-1 were normalized in male and female (not studied in ovariectomized) groups.

CONCLUSIONS

These findings confirm marked sex differences for myogenic tone and vessel constrictor responses to ET-1 in coronary resistance vessels. Results also suggest greater sensitivity to vasoconstrictor effects of ET-1 in male coronary resistance vessels.

Dynamic right ventricular function response to incremental exercise in pulmonary hypertension

Pulmonary hypertension is a progressive disease whose survival is linked to adequate right ventricle adaptation to its afterload. In the current study, we performed an in-depth characterization of right ventricle function during maximum incremental exercise in patients with pulmonary hypertension and how it relates to exercise capacity. A total of 377 pulmonary hypertension patients who completed a maximum symptom-limited invasive cardiopulmonary exercise testing were evaluated to identify 45 patients with heart failure with preserved ejection fraction, 48 with exercise pulmonary hypertension, and 47 with established pulmonary arterial hypertension. These patients were compared to 17 age- and gender-matched normal controls. Load-adjusted right ventricle function was quantified as the ratio of right ventricle stroke work index to pulmonary arterial elastance. All patients with pulmonary hypertension had reduced peak VO₂ %predicted compared to controls. Right ventricle function deteriorated for all pulmonary hypertension groups by 50% of peak VO₂. Worsening of right ventricle function during freewheeling exercise was associated with greater reduction in peak VO₂ compared to those whose right ventricle function deteriorated at later exercise stages (i.e. min 1, 2, and 3). On multivariate analysis, reduced ratio of right ventricle stroke work index to arterial elastance was an independent predictor of peak VO₂ %predicted (β-Coefficient –5.46, 95% CI: –9.47 to –1.47, p = 0.01). Right ventricle function deteriorates early during incremental exercise in pulmonary hypertension, occurring by 50% of peak oxygen uptake. The current study demonstrates that right ventricle dysfunction is an early phenomenon during incremental exercise in pulmonary hypertension, occurring by 50% of peak oxygen uptake. The threshold at which right ventricle function is compromised during incremental exercise in pulmonary hypertension influences aerobic capacity and may help guide exercise strategies to mitigate dynamic worsening of right ventricle function during exercise training.

Mechanics of right ventricular dysfunction in pulmonary arterial hypertension and heart failure with preserved ejection fraction

Right ventricular (RV) dysfunction is the most important determinant of survival in patients with pulmonary hypertension (PH). The manifestations of RV dysfunction not only include changes in global RV systolic function but also abnormalities in the pattern of contraction and synchrony. The effects of PH on the right ventricle have been mainly studied in patients with pulmonary arterial hypertension (PAH). However, with the demographic shift towards an aging population, heart failure with preserved ejection fraction (HFpEF) has become an important etiology of PH in recent years. There are significant differences in RV mechanics, function and adaptation between patients with PAH and HFpEF (with or without PH), which are related to different patterns of remodeling and dysfunction. Due to the unique features of the RV chamber, its connection with the main pulmonary artery and the pulmonary circulation, an understanding of the mechanics of RV function and its clinical significance is mandatory for both entities. In this review, we describe the mechanics of the pressure overloaded right ventricle. We review the different mechanical components of RV dysfunction and ventricular dyssynchrony, followed by insights via analysis of pressure-volume loop, energetics and novel blood flow patterns, such as vortex imaging. We conduct an in-depth comparison of prevalence and characteristics of RV dysfunction in HFpEF and PAH, and summarize key outcome studies. Finally, we provide a perspective on needed and expected future work in the field of RV mechanics.

Differential Expression of the Angiotensin-(1-12)/Chymase Axis in Human Atrial Tissue

BACKGROUND

Heart chymase rather than angiotensin (Ang)-converting enzyme has higher specificity for Ang I conversion into Ang II in humans. A new pathway for direct cardiac Ang II generation has been revealed through the demonstration that Ang-(1-12) is cleaved by chymase to generate Ang II directly. Herein, we address whether Ang-(1-12), chymase messenger RNA (mRNA), and activity levels can be differentiated in human atrial tissue from normal and diseased hearts and if these measures associate with various pathologic heart conditions.

MATERIALS AND METHODS

Atrial appendages were collected from 11 nonfailing donor hearts and 111 patients undergoing heart surgery for the correction of valvular heart disease, resistant atrial fibrillation, or ischemic heart disease. Chymase mRNA was analyzed by real-time polymerase chain reaction and enzymatic activity by high-performance liquid chromatography using Ang-(1-12) as the substrate. Ang-(1-12) levels were determined by immunohistochemical staining.

RESULTS

Chymase gene transcripts, chymase activity, and immunoreactive Ang-(1-12) expression levels were higher in left atrial tissue compared with right atrial tissue, irrespective of cardiac disease. In addition, left atrial chymase mRNA expression was significantly higher in stroke versus nonstroke patients and in cardiac surgery patients who had a history of postoperative atrial fibrillation versus nonatrial fibrillation. Correlation analysis showed that left atrial chymase mRNA was positively related to left atrial enlargement, as determined by echocardiography.

CONCLUSIONS

As Ang-(1-12) expression and chymase gene transcripts and enzymatic activity levels were positively linked to left atrial size in patients with left ventricular heart disease, an important alternate Ang II forming pathway, via Ang-(1-12) and chymase, in maladaptive atrial and ventricular remodeling in humans is uncovered.

Mechano-electric and mechano-chemo-transduction in cardiomyocytes

Cardiac excitation-contraction (E-C) coupling is influenced by (at least) three dynamic systems that couple and feedback to one another (see Abstract Figure). Here we review the mechanical effects on cardiomyocytes that include mechano-electro-transduction (commonly referred to as mechano-electric coupling, MEC) and mechano-chemo-transduction (MCT) mechanisms at cell and molecular levels which couple to Ca2+ -electro and E-C coupling reviewed elsewhere. These feedback loops from muscle contraction and mechano-transduction to the Ca2+ homeodynamics and to the electrical excitation are essential for understanding the E-C coupling dynamic system and arrhythmogenesis in mechanically loaded hearts. This white paper comprises two parts, each reflecting key aspects from the 2018 UC Davis symposium: MEC (how mechanical load influences electrical dynamics) and MCT (how mechanical load alters cell signalling and Ca2+ dynamics). Of course, such separation is artificial since Ca2+ dynamics profoundly affect ion channels and electrogenic transporters and vice versa. In time, these dynamic systems and their interactions must become fully integrated, and that should be a goal for a comprehensive understanding of how mechanical load influences cell signalling, Ca2+ homeodynamics and electrical dynamics. In this white paper we emphasize current understanding, consensus, controversies and the pressing issues for future investigations. Space constraints make it impossible to cover all relevant articles in the field, so we will focus on the topics discussed at the symposium.

Myocardial Contractility: Historical and Contemporary Considerations

The term myocardial contractility is thought to have originated more than 125 years ago and has remained and enigma ever since. Although the term is frequently used in textbooks, editorials and contemporary manuscripts its definition remains illusive often being conflated with cardiac performance or inotropy. The absence of a universally accepted definition has led to confusion, disagreement and misconceptions among physiologists, cardiologists and safety pharmacologists regarding its definition particularly in light of new discoveries regarding the load dependent kinetics of cardiac contraction and their translation to cardiac force-velocity and ventricular pressure-volume measurements. Importantly, the Starling interpretation of force development is length-dependent while contractility is length independent. Most historical definitions employ an operational approach and define cardiac contractility in terms of the hearts mechanical properties independent of loading conditions. Literally defined the term contract infers that something has become smaller, shrunk or shortened. The addition of the suffix “ility” implies the quality of this process. The discovery and clinical investigation of small molecules that bind to sarcomeric proteins independently altering force or velocity requires that a modern definition of the term myocardial contractility be developed if the term is to persist. This review reconsiders the historical and contemporary interpretations of the terms cardiac performance and inotropy and recommends a modern definition of myocardial contractility as the preload, afterload and length-independent intrinsic kinetically controlled, chemo-mechanical processes responsible for the development of force and velocity.

Stretch-Induced Biased Signaling in Angiotensin II Type 1 and Apelin Receptors for the Mediation of Cardiac Contractility and Hypertrophy

The myocardium has an intrinsic ability to sense and respond to mechanical load in order to adapt to physiological demands. Primary examples are the augmentation of myocardial contractility in response to increased ventricular filling caused by either increased venous return (Frank-Starling law) or aortic resistance to ejection (the Anrep effect). Sustained mechanical overload, however, can induce pathological hypertrophy and dysfunction, resulting in heart failure and arrhythmias. It has been proposed that angiotensin II type 1 receptor (AT1R) and apelin receptor (APJ) are primary upstream actors in this acute myocardial autoregulation as well as the chronic maladaptive signaling program. These receptors are thought to have mechanosensing capacity through activation of intracellular signaling via G proteins and/or the multifunctional transducer protein, β-arrestin. Importantly, ligand and mechanical stimuli can selectively activate different downstream signaling pathways to promote inotropic, cardioprotective or cardiotoxic signaling. Studies to understand how AT1R and APJ integrate ligand and mechanical stimuli to bias downstream signaling are an important and novel area for the discovery of new therapeutics for heart failure. In this review, we provide an up-to-date understanding of AT1R and APJ signaling pathways activated by ligand versus mechanical stimuli, and their effects on inotropy and adaptive/maladaptive hypertrophy. We also discuss the possibility of targeting these signaling pathways for the development of novel heart failure therapeutics.

The role of mechanotransduction in heart failure pathobiology-a concise review

This review evaluates the role of mechanotransduction (MT) in heart failure (HF) pathobiology. Cardiac functional and structural modifications are regulated by biomechanical forces. Exposing cardiomyocytes and the myocardial tissue to altered biomechanical stress precipitates changes in the end-diastolic wall stress (EDWS). Thereby various interconnected biomolecular pathways, essentially mediated and orchestrated by MT, are launched and jointly contribute to adapt and remodel the myocardium. This cardiac MT-mediated feedback decisively determines the primary cardiac cellular and tissue response, the sort (concentric or eccentric) of hypertrophy/remodeling, to mechanical and/or hemodynamic alterations. Moreover, the altered EDWS affects the diastolic myocardial properties independent of the systolic function, and elevated EDWS causes diastolic dysfunction. The close interconnection between MT pathways and the cell nucleus, the genetic endowment, principally allows for the wide variety of phenotypic appearances. However, demographic, environmental features, comorbidities, and also the genetic make-up may modulate the phenotypic result. Cardiac MT takes a fundamental and superordinate position in the myocardial adaptation and remodeling processes in all HF categories and phenotypes. Therefore, the effects of MT should be integrated in all our scientific, clinical, and therapeutic considerations.

Stretch modulation of cardiac contractility: importance of myocyte calcium during the slow force response

The mechanical response of the heart to myocardial stretch has been understood since the work of muscle physiologists more than 100 years ago, whereby an increase in ventricular chamber filling during diastole increases the subsequent force of contraction. The stretch-induced increase in contraction is biphasic. There is an abrupt increase in the force that coincides with the stretch (the rapid response), which is then followed by a slower response that develops over several minutes (the slow force response, or SFR). The SFR is associated with a progressive increase in the magnitude of the Ca²⁺ transient, the event that initiates myocyte cross-bridge cycling and force development. However, the mechanisms underlying the stretch-dependent increase in the Ca²⁺ transient are still debated. This review outlines recent literature on the SFR and summarizes the different stretch-activated Ca²⁺ entry pathways. The SFR might result from a combination of several different cellular mechanisms initiated in response to activation of different cellular stretch sensors.

Control of cardiac contraction by sodium: Promises, reckonings, and new beginnings

Several generations of cardiac physiologists have verified that basal cardiac contractility depends strongly on the transsarcolemmal Na gradient, and the underlying molecular mechanisms that link cardiac excitation-contraction coupling (ECC) to the Na gradient have been elucidated in good detail for more than 30 years. In brief, small increases of cytoplasmic Na push cardiac (NCX1) Na/Ca exchangers to increase contractility by increasing the myocyte Ca load. Accordingly, basal cardiac contractility is expected to be physiologically regulated by pathways that modify the cardiac Na gradient and the function of Na transporters. Assuming that this expectation is correct, it remains to be elucidated how in detail signaling pathways affecting the cardiac Na gradient are controlled in response to changing cardiac output requirements. Some puzzle pieces that may facilitate progress are outlined in this short review. Key open issues include (1) whether the concept of local Na gradients is viable, (2) how in detail Na channels, Na transporters and Na/K pumps are regulated by lipids and metabolic processes, (3) the physiological roles of Na/K pump inactivation, and (4) the possibility that key diffusible signaling molecules remain to be discovered.

Impact of norepinephrine on right ventricular afterload and function in septic shock-a strain echocardiography study

Background

In this observational study, the effects of norepinephrine‐induced changes in mean arterial pressure (MAP) on right ventricular (RV) systolic function, afterload and pulmonary haemodynamics were studied in septic shock patients. We hypothesised that RV systolic function improves at higher doses of norepinephrine/MAP levels.

Methods

Eleven patients with septic shock requiring norepinephrine after fluid resuscitation were included <24 hours after ICU arrival. Study enrolment and insertion of a pulmonary artery catheter was performed after written informed consent from the next of kin. Norepinephrine infusion was titrated to target mean arterial pressures (MAP) of 60, 75 and 90 mmHg in a random sequential order. At each target MAP, strain—and conventional echocardiographic—and pulmonary haemodynamic variables were measured. RV afterload was assessed as effective pulmonary arterial elastance, (E_pa) and pulmonary vascular resistance index, (PVRI). RV free wall peak strain was the primary end‐point.

Results

At highest compared to lowest norepinephrine dose/MAP level, RV free wall peak strain increased from −19% to −25% (32%, P = .003), accompanied by increased tricuspid annular plane systolic excursion (22%, P = .01). At the highest norepinephrine dose/MAP, RV end‐diastolic area index (16%, P < .001), central venous pressure (38%, P < .001), stroke volume index (7%, P = .001), mean pulmonary artery pressure (19%, P < .001) and RV stroke work index (15%, P = .045) increased, with no effects on PVRI or E_pa. Cardiac index did not change, assessed by thermodilution (P = .079) and echocardiography (P = .054).

Conclusions

Higher doses of norepinephrine to a target MAP of 90 mm Hg improved RV systolic function while RV afterload was not affected.

A magnetics-based approach for fine-tuning afterload in engineered heart tissues

Afterload plays important roles during heart development and disease progression, however, studying these effects in a laboratory setting is challenging. Current techniques lack the ability to precisely and reversibly alter afterload over time. Here, we describe a magnetics-based approach for achieving this control and present results from experiments in which this device was employed to sequentially increase afterload applied to rat engineered heart tissues (rEHTs) over a 7-day period. The contractile properties of rEHTs grown on control posts marginally increased over the observation period. The average post deflection, fractional shortening, and twitch velocities measured for afterload-affected tissues initially followed this same trend, but fell below control tissue values at high magnitudes of afterload. However, the average force, force production rate, and force relaxation rate for these rEHTs were consistently up to 3-fold higher than in control tissues. Transcript levels of hypertrophic or fibrotic markers and cell size remained unaffected by afterload, suggesting that the increased force output was not accompanied by pathological remodeling. Accordingly, the increased force output was fully reversed to control levels during a stepwise decrease in afterload over 4 hours. Afterload application did not affect systolic or diastolic tissue lengths, indicating that the afterload system was likely not a source of changes in preload strain. In summary, the afterload system developed herein is capable of fine-tuning EHT afterload while simultaneously allowing optical force measurements. Using this system, we found that small daily alterations in afterload can enhance the contractile properties of rEHTs, while larger increases can have temporary undesirable effects. Overall, these findings demonstrate the significant role that afterload plays in cardiac force regulation. Future studies with this system may allow for novel insights into the mechanisms that underlie afterload-induced adaptations in cardiac force development.

Efficacy and safety of 20% albumin fluid loading in healthy subjects: a comparison of four resuscitation fluids

Recently, buffered salt solutions and 20% albumin (small volume resuscitation) have been advocated as an alternative fluid for intravenous resuscitation. The relative comparative efficacy and potential adverse effects of these solutions have not been evaluated. In a randomized, double blind, cross-over study of six healthy male subjects we compared the pulmonary and hemodynamic effects of intravenous administration of 30 ml/kg of 0.9% saline, Hartmann's solution and 4% albumin, and 6 ml/kg of 20% albumin (albumin dose equivalent). Lung tests (spirometry, ultrasound, impulse oscillometry, diffusion capacity, and plethysmography), two- to three-dimensional Doppler echocardiography, carotid applanation tonometry, blood gases, serum/urine markers of endothelial, and kidney injury were measured before and after each fluid bolus. Data were analyzed with repeated measures ANOVA with effect of fluid type examined as an interaction. Crystalloids caused lung edema [increase in ultrasound B line ( P = 0.006) and airway resistance ( P = 0.009)], but evidence of lung injury [increased angiopoietin-2 ( P = 0.019)] and glycocalyx injury [increased syndecan ( P = 0.026)] was only observed with 0.9% saline. The colloids caused greater left atrial stretch, decrease in lung volumes, and increase in diffusion capacity than the crystalloids, but without pulmonary edema. Stroke work increased proportionally to increase in preload with all four fluids ( R2 = 0.71). There was a greater increase in cardiac output and stroke volume after colloid administration, associated with a reduction in afterload. Hartmann’s solution did not significantly alter ventricular performance. Markers of kidney injury were not affected by any of the fluids administrated. Bolus administration of 20% albumin is both effective and safe in healthy subjects.

NEW & NOTEWORTHY Bolus administration of 20% albumin is both effective and safe in healthy subjects when compared with other commonly available crystalloids and colloidal solution.

Effects of the Inhibition of Late Sodium Current by GS967 on Stretch-Induced Changes in Cardiac Electrophysiology

PURPOSE

Mechanical stretch increases sodium and calcium entry into myocytes and activates the late sodium current. GS967, a triazolopyridine derivative, is a sodium channel blocker with preferential effects on the late sodium current. The present study evaluates whether GS967 inhibits or modulates the arrhythmogenic electrophysiological effects of myocardial stretch.

METHODS

Atrial and ventricular refractoriness and ventricular fibrillation modifications induced by acute stretch were studied in Langendorff-perfused rabbit hearts (n = 28) using epicardial multiple electrodes and high-resolution mapping techniques under control conditions and during the perfusion of GS967 at different concentrations (0.03, 0.1, and 0.3 μM).

RESULTS

On comparing ventricular refractoriness, conduction velocity and wavelength obtained before stretch had no significant changes under each GS967 concentration while atrial refractoriness increased under GS967 0.3 μM. Under GS967, the stretch-induced changes were attenuated, and no significant differences were observed between before and during stretch. GS967 0.3 μM diminished the normal stretch-induced changes resulting in longer (less shortened) atrial refractoriness (138 ± 26 ms vs 95 ± 9 ms; p < 0.01), ventricular refractoriness (155 ± 18 ms vs 124 ± 16 ms; p < 0.01) and increments in spectral concentration (23 ± 5% vs 17 ± 2%; p < 0.01), the fifth percentile of ventricular activation intervals (46 ± 8 ms vs 31 ± 3 ms; p < 0.05), and wavelength of ventricular fibrillation (2.5 ±0.5 cm vs 1.7 ± 0.3 cm; p < 0.05) during stretch. The stretch-induced increments in dominant frequency during ventricular fibrillation (control = 38%, 0.03 μM = 33%, 0.1 μM = 33%, 0.3 μM = 14%; p < 0.01) and the stretch-induced increments in arrhythmia complexity index (control = 62%, 0.03μM = 41%, 0.1 μM = 32%, 0.3 μM = 16%; p < 0.05) progressively decreased on increasing the GS967 concentration.

CONCLUSIONS

GS967 attenuates stretch-induced changes in cardiac electrophysiology.

The lack of slow force response in failing rat myocardium: role of stretch-induced modulation of Ca-TnC kinetics

The slow force response (SFR) to stretch is an important adaptive mechanism of the heart. The SFR may result in ~ 20-30% extra force but it is substantially attenuated in heart failure. We investigated the relation of SFR magnitude with Ca2+ transient decay in healthy (CONT) and monocrotaline-treated rats with heart failure (MCT). Right ventricular trabeculae were stretched from 85 to 95% of optimal length and held stretched for 10 min at 30 °C and 1 Hz. Isometric twitches and Ca2+ transients were collected on 2, 4, 6, 8, 10 min after stretch. The changes in peak tension and Ca2+ transient decay characteristics during SFR were evaluated as a percentage of the value measured immediately after stretch. The amount of Ca2+ utilized by TnC was indirectly evaluated using the methods of Ca2+ transient "bump" and "difference curve." The muscles of CONT rats produced positive SFR and they showed prominent functional relation between SFR magnitude and the magnitude (amplitude, integral intensity) of Ca2+ transient "bump" and "difference curve." The myocardium of MCT rats showed negative SFR to stretch (force decreased in time) which was not correlated well with the characteristics of Ca2+ transient decay, evaluated by the methods of "bump" and "difference curve." We conclude that the intracellular mechanisms of Ca2+ balancing during stretch-induced slow adaptation of myocardial contractility are disrupted in failing rat myocardium. The potential significance of our findings is that the deficiency of slow force response in diseased myocardium may be diminished under augmented kinetics of Ca-TnC interaction.

Arrhythmogenic Mechanisms in Heart Failure: Linking β-Adrenergic Stimulation, Stretch, and Calcium

Heart failure (HF) is associated with elevated sympathetic tone and mechanical load. Both systems activate signaling transduction pathways that increase cardiac output, but eventually become part of the disease process itself leading to further worsening of cardiac function. These alterations can adversely contribute to electrical instability, at least in part due to the modulation of Ca²⁺ handling at the level of the single cardiac myocyte. The major aim of this review is to provide a definitive overview of the links and cross talk between β-adrenergic stimulation, mechanical load, and arrhythmogenesis in the setting of HF. We will initially review the role of Ca²⁺ in the induction of both early and delayed afterdepolarizations, the role that β-adrenergic stimulation plays in the initiation of these and how the propensity for these may be altered in HF. We will then go onto reviewing the current data with regards to the link between mechanical load and afterdepolarizations, the associated mechano-sensitivity of the ryanodine receptor and other stretch activated channels that may be associated with HF-associated arrhythmias. Furthermore, we will discuss how alterations in local Ca²⁺ microdomains during the remodeling process associated the HF may contribute to the increased disposition for β-adrenergic or stretch induced arrhythmogenic triggers. Finally, the potential mechanisms linking β-adrenergic stimulation and mechanical stretch will be clarified, with the aim of finding common modalities of arrhythmogenesis that could be targeted by novel therapeutic agents in the setting of HF.

Noradrenaline modifies arterial reflection phenomena and left ventricular efficiency in septic shock patients: A prospective observational study

PURPOSE

To determine whether noradrenaline alters the arterial pressure reflection phenomena in septic shock patients and the effects on left ventricular (LV) efficiency.

MATERIAL AND METHODS

Thirty-seven septic shock patients with a planned change in noradrenaline dose. Timing and magnitude (Reflection Magnitude and Augmentation Index) of arterial reflections were evaluated. Total, steady, and oscillatory LV power (also expressed as fraction of the total power), subendocardial viability ratio (SEVR), energy efficiency and transmission ratios were used as a marker of LV efficiency.

RESULTS

An incremental change in noradrenaline increased Reflection Magnitude [0.28(0.09) to 0.31(0.1], Augmentation Index [-6.4(23.6) to 4.8(20.7)%], and LV total power [0.79(IQR:0.47-1) to 0.98(IQR:0.57-1.27)W], all p < 0.001; whereas decreased arrival time of reflected waves [from 95(87 to 121) to 83(79 to 101)ms; p < 0.001]. Variables of LV performance showed a decreased efficiency: oscillatory fraction and energy efficiency ratio increased [20.9(5.7) to 22.8(4.9)%, and 8.2(1.7) to 10.1(2) mW.min.litre^-1; p < 0.001, respectively]; and energy transmission ratio and SEVR decreased [73.8(9.9) to 72(9.8)% and 146(IQR:113-188) to 143(IQR:109-172)%, p = 0.003 and p = 0.041, respectively].

CONCLUSIONS

Noradrenaline increased reflection phenomena, increasing LV workload and worsening LV performance in septic shock patients. These conditions could explain the detrimental effects during long-term use of noradrenaline.

Effect of Calcium on Slow Force Responses in Isolated Right Ventricle Preparations of Healthy and Hypertrophied Myocardium in Male and Female Rats

Effect of different Ca²⁺ concentrations in the bathing solution [Ca²⁺]_o on the parameters of single isometric contraction and slow force response to stretching was studied in isolated preparations of healthy and hypertrophied myocardium of male and female Wistar rats. In all groups of experimental animals, the increase in calcium concentration was followed by a decrease in the myocardium slow response intensity. We revealed a complementary relationship between the current and medium-term systems of myocardial contractility regulation by the length of the myocardium aimed at the maintenance of the constant level during adaptation to the load. Slow responses of the hypertrophied rat heart myocardium were suppressed in comparison with those in the healthy myocardium and their intensity did not depend on animal sex.

Cardiac Ventricular Contractile Responses to Chronically Increased Afterload Secondary to Right Ventricular Outflow Obstruction in Patients With Tetralogy of Fallot

We examined the adaptive mechanism of the pulmonary ventricle (PV) in response to increased afterload secondary to pulmonary stenosis in tetralogy of Fallot (TOF, n = 47) and congenitally corrected transposition of the great arteries (cCTGA, n = 18), where the PV is morphologically different. We also elucidated the effects of such adaptation on systemic ventricular (SV) function. PV contractility, assessed by dp/dt_max, showed significant positive correlations with PV pressure (r = 0.82, p <0.01 for TOF and r = 0.78, p <0.01 for cCTGA) and pulmonary-to-systemic ventricular pressure ratio (r = 0.70, p <0.01 for TOF and r = 0.76, p <0.01 for cCTGA) in patients with both TOF and cCTGA. Notably, the slopes of these correlations were significantly higher in cCTGA than in TOF (p <0.01), suggesting enhanced contractile responses in cCTGA. Moreover, SV dp/dt_max showed significant positive correlations with PV dp/dt_max in patients with both TOF and cCTGA (r = 0.67, p <0.01 and r = 0.61, p <0.01, respectively), indicating positive ventricular-ventricular interaction. In this relationship, the slopes of correlations were significantly higher in TOF than in cCTGA (p = 0.024). These results, indicating different behaviors of PV contractile physiology and its interaction with the SV, may have important therapeutic implications when considering medical, catheter, and surgical interventions for pulmonary stenosis in these diseases. The results may also offer the potential for a new approach for improvement of prognosis, especially in cCTGA.