Cardiac unloading alters contractility and calcium homeostasis in ventricular myocytes

Calsyntenin-1 Promotes Doxorubicin-induced Dilated Cardiomyopathy in Rats

PURPOSE

Doxorubicin is an important cancer chemotherapeutic agent with severe cardiotoxic effects that eventually lead to dilated cardiomyopathy (DCM). Calsyntenin-1(CLSTN1) plays a critical role in the nervous system, but its relevance in cardiovascular diseases is unknown. We investigated the significance of CLSTN1 in doxorubicin-induced DCM.

METHODS

CLSTN1 expression in doxorubicin-induced DCM rats and H9c2 cells was determined using western blotting. To further explore the functions of CLSTN1, a cardiac-specific CLSTN1 overexpression rat model was constructed. The rats were subjected to analysis using echocardiographic, hemodynamic, and electrocardiographic parameters. Potential downstream molecules in CLSTN1 overexpression heart tissue were investigated using proteomics and western blotting. Finally, a knockdown of CLSTN1 was constructed to investigate the rescue function on doxorubicin-induced cell toxicity.

RESULTS

CLSTN1 protein expression increased drastically in doxorubicin-induced DCM rats and H9c2 cells. Under doxorubicin treatment, CLSTN1 protein-specific overexpression in the heart muscle promoted cardiac chamber enlargement and heart failure, while the knockdown of CLSTN1 reduced doxorubicin-induced cardiomyocyte toxicity in vitro. At the mechanistic level, overexpression of CLSTN1 downregulated SERCA2 expression and increased the phosphorylation levels of PI3K-Akt and CaMK2.

CONCLUSION

Our findings demonstrated that CLSTN1 promotes the pathogenesis of doxorubicin-induced DCM. CLSTN1 could be a therapeutic target to prevent the development of doxorubicin-induced DCM.

Myocardial Biomechanics and the Consequent Differentially Expressed Genes of the Left Atrial Ligation Chick Embryonic Model of Hypoplastic Left Heart Syndrome

Left atrial ligation (LAL) of the chick embryonic heart is a model of the hypoplastic left heart syndrome (HLHS) where a purely mechanical intervention without genetic or pharmacological manipulation is employed to initiate cardiac malformation. It is thus a key model for understanding the biomechanical origins of HLHS. However, its myocardial mechanics and subsequent gene expressions are not well-understood. We performed finite element (FE) modeling and single-cell RNA sequencing to address this. 4D high-frequency ultrasound imaging of chick embryonic hearts at HH25 (ED 4.5) were obtained for both LAL and control. Motion tracking was performed to quantify strains. Image-based FE modeling was conducted, using the direction of the smallest strain eigenvector as the orientations of contractions, the Guccione active tension model and a Fung-type transversely isotropic passive stiffness model that was determined via micro-pipette aspiration. Single-cell RNA sequencing of left ventricle (LV) heart tissues was performed for normal and LAL embryos at HH30 (ED 6.5) and differentially expressed genes (DEG) were identified.After LAL, LV thickness increased by 33%, strains in the myofiber direction increased by 42%, while stresses in the myofiber direction decreased by 50%. These were likely related to the reduction in ventricular preload and underloading of the LV due to LAL. RNA-seq data revealed potentially related DEG in myocytes, including mechano-sensing genes (Cadherins, NOTCH1, etc.), myosin contractility genes (MLCK, MLCP, etc.), calcium signaling genes (PI3K, PMCA, etc.), and genes related to fibrosis and fibroelastosis (TGF-β, BMP, etc.). We elucidated the changes to the myocardial biomechanics brought by LAL and the corresponding changes to myocyte gene expressions. These data may be useful in identifying the mechanobiological pathways of HLHS.

TRPC Channels in Cardiac Plasticity

The heart flexibly changes its structure in response to changing environments and oxygen/nutrition demands of the body. Increased and decreased mechanical loading induces hypertrophy and atrophy of cardiomyocytes, respectively. In physiological conditions, these structural changes of the heart are reversible. However, chronic stresses such as hypertension or cancer cachexia cause irreversible remodeling of the heart, leading to heart failure. Accumulating evidence indicates that calcium dyshomeostasis and aberrant reactive oxygen species production cause pathological heart remodeling. Canonical transient receptor potential (TRPC) is a nonselective cation channel subfamily whose multimodal activation or modulation of channel activity play important roles in a plethora of cellular physiology. Roles of TRPC channels in cardiac physiology have been reported in pathological cardiac remodeling. In this review, we summarize recent findings regarding the importance of TRPC channels in flexible cardiac remodeling (i.e., cardiac plasticity) in response to environmental stresses and discuss questions that should be addressed in the near future.

Intraventricular placement of a spring expander does not attenuate cardiac atrophy of the healthy heart induced by unloading via heterotopic heart transplantation

An important complication of the prolonged left ventricle assist device support in patients with heart failure is unloading-induced cardiac atrophy which proved resistant to various treatments. Heterotopic heart transplantation (HTx) is the usual experimental model to study this process. We showed previously that implantation of the newly designed intraventricular spring expander can attenuate the atrophy when examined after HTx in the failing heart (derived from animals with established heart failure). The present study aimed to examine if enhanced isovolumic loading achieved by implantation of the expander would attenuate cardiac post-HTx atrophy also in the healthy heart. Cardiac atrophy was assessed as the ratio of the transplanted-to-native heart weight (HW) and its degree was determined on days 7, 14, 21 and 28 after HTx. The transplantation resulted in 32±3, 46±2, 48±3 and 46±3 % HW loss when measured at the four time points; implantation of the expander had no significant effect on these decreases. We conclude that enhanced isovolumic loading achieved by intraventricular implantation of the expander does not attenuate the development of cardiac atrophy after HTx in the healthy heart. This indicates that such an approach does not represent a useful therapeutic measure to attenuate the development of unloading-induced cardiac atrophy.

Review of a causal role of fructose-containing sugars in myocardial susceptibility to ischemia/reperfusion injury

In 2012, the World Health Organization Global Status Report on noncommunicable diseases showed that 7.4 million deaths were due to ischemic heart disease. Consequently, cardiovascular disease is a significant health burden, especially when partnered with comorbidities such as obesity, metabolic syndrome, and type 2 diabetes mellitus. Of note, these diseases can all be induced or exacerbated by diet. Carbohydrates, in particular, fructose and glucose, generally form the largest part of the human diet. Accumulating evidence from animal studies suggests that if large amounts of fructose are consumed either in isolation or in combination with glucose (fructose-containing sugars), myocardial susceptibility to ischemia/reperfusion (I/R) injury increases. However, the underlying mechanisms that predisposes the myocardium to I/R injury in the fructose model are not elucidated, and no single mechanistic pathway has been described. Based on all available data on this topic, this review describes previously investigated mechanisms and highlights 3 main mechanistic pathways whereby fructose has shown to increase myocardial susceptibility to I/R injury. These pathways include (1) increased reactive oxygen species, resulting in reduced nitric oxide synthase and coronary flow; (2) elevated plasma fatty acids and insulin, leading to increased cardiac triglyceride content and lipotoxicity; and (3) disrupted myocardial calcium handling/homeostasis. Moreover, we highlight various factors that should be taken into account when the fructose animal model is used, such as rat strain, treatment periods, and doses. We argue that failure to do so would result in erratic inferences drawn from the existing body of evidence on fructose animal models.

Development of a Vascularized Heterotopic Neonatal Rat Heart Transplantation Model

BACKGROUND/PURPOSE

Rodent adult-to-adult heterotopic heart transplantation is a well-established animal model, and the detailed surgical technique with several modifications has been previously described. In immature donor organ transplantation, however, the surgical technique needs to be revised given the smaller size and fragility of the donor graft. Here, we report our surgical technique for heterotopic abdominal (AHTx) and femoral (FHTx) neonatal rat heart transplantation based on an experience of over 300 cases.

METHODS

Heterotopic heart transplantation was conducted in syngeneic Lewis rats. Neonatal rats (postnatal day 2-4) served as donors. AHTx was performed by utilizing the conventional adult-to-adult transplant method with specific modifications for optimal aortotomy and venous anastomosis. In the FHTx, the donor heart was vascularized by connecting the donor's aorta and pulmonary artery to the recipient's right femoral artery and vein, respectively, in an end-to-end manner. A specifically fashioned butterfly-shaped rubber sheet was used to align the target vessels properly. The transplanted graft was visually assessed for its viability and was accepted as a technical success when the viability met specific criteria. Successfully transplanted grafts were subject to further postoperative evaluation. Forty cases (AHTx and FHTx; n = 20 each) were compared regarding perioperative parameters and outcomes.

RESULTS

Both models were technically feasible (success rate: AHTx 75% vs. FHTx 70%) by refining the conventional heterotopic transplant technique. Injury to the fragile donor aorta and congestion of the graft due to suboptimal venous connection were predominant causes of failure, leading to refractory bleeding and poor graft viability. Although the FHTx required significantly longer operation time and graft ischemic time, the in situ graft viabilities were comparable. The FHTx provided better postoperative monitoring as it enabled daily graft palpation and better echocardiographic visualization.

CONCLUSIONS

We describe detailed surgical techniques for AHTx and FHTx while addressing neonatal donor-specific issues. Following our recommendations potentially reduces the learning curve to achieve reliable and reproducible results with these challenging animal models.

MAFbx/Atrogin-1 is required for atrophic remodeling of the unloaded heart

BACKGROUND

Mechanical unloading of the failing human heart induces profound cardiac changes resulting in the reversal of a distorted structure and function. In this process, cardiomyocytes break down unneeded proteins and replace those with new ones. The specificity of protein degradation via the ubiquitin proteasome system is regulated by ubiquitin ligases. Over-expressing the ubiquitin ligase MAFbx/Atrogin-1 in the heart inhibits the development of cardiac hypertrophy, but the role of MAFbx/Atrogin-1 in the unloaded heart is not known.

METHODS AND RESULTS

Mechanical unloading, by heterotopic transplantation, decreased heart weight and cardiomyocyte cross-sectional area in wild type mouse hearts. Unexpectedly, MAFbx/Atrogin-1(-/-) hearts hypertrophied after transplantation (n=8-10). Proteasome activity and markers of autophagy were increased to the same extent in WT and MAFbx/Atrogin-1(-/-) hearts after transplantation (unloading). Calcineurin, a regulator of cardiac hypertrophy, was only upregulated in MAFbx/Atrogin-1(-/-) transplanted hearts, while the mTOR pathway was similarly activated in unloaded WT and MAFbx/Atrogin-1(-/-) hearts. MAFbx/Atrogin-1(-/-) cardiomyocytes exhibited increased calcineurin protein expression, NFAT transcriptional activity, and protein synthesis rates, while inhibition of calcineurin normalized NFAT activity and protein synthesis. Lastly, mechanical unloading of failing human hearts with a left ventricular assist device (n=18) also increased MAFbx/Atrogin-1 protein levels and expression of NFAT regulated genes.

CONCLUSIONS

MAFbx/Atrogin-1 is required for atrophic remodeling of the heart. During unloading, MAFbx/Atrogin-1 represses calcineurin-induced cardiac hypertrophy. Therefore, MAFbx/Atrogin-1 not only regulates protein degradation, but also reduces protein synthesis, exerting a dual role in regulating cardiac mass.

Taking pressure off the heart: the ins and outs of atrophic remodelling

Our work on atrophic remodelling of the heart has led us to appreciate the simple principles in biology: (i) the dynamic nature of intracellular protein turnover, (ii) the return to the foetal gene programme when the heart remodels, and (iii) the adaptive changes of cardiac metabolism. Although the molecular mechanisms of cardiac hypertrophy are many, much less is known regarding the molecular mechanisms of cardiac atrophy. We state the case that knowing more about mechanisms of atrophic remodelling may provide insights into cellular consequences of metabolic and haemodynamic unloading of the stressed heart. Overall we strive to find an answer to the question: 'What makes the failing heart shrink and become stronger?' We speculate that signals arising from intermediary metabolism of energy-providing substrates are likely candidates.

Transmural heterogeneity of repolarization and Ca2+ handling in a model of mouse ventricular tissue

Mouse hearts have a diversity of action potentials (APs) generated by the cardiac myocytes from different regions. Recent evidence shows that cells from the epicardial and endocardial regions of the mouse ventricle have a diversity in Ca(2+) handling properties as well as K(+) current expression. To examine the mechanisms of AP generation, propagation, and stability in transmurally heterogeneous tissue, we developed a comprehensive model of the mouse cardiac cells from the epicardial and endocardial regions of the heart. Our computer model simulates the following differences between epicardial and endocardial myocytes: 1) AP duration is longer in endocardial and shorter in epicardial myocytes, 2) diastolic and systolic intracellular Ca(2+) concentration and intracellular Ca(2+) concentration transients are higher in paced endocardial and lower in epicardial myocytes, 3) Ca(2+) release rate is about two times larger in endocardial than in epicardial myocytes, and 4) Na(+)/Ca(2+) exchanger rate is greater in epicardial than in endocardial myocytes. Isolated epicardial cells showed a higher threshold for stability of AP generation but more complex patterns of AP duration at fast pacing rates. AP propagation velocities in the model of two-dimensional tissue are close to those measured experimentally. Simulations show that heterogeneity of repolarization and Ca(2+) handling are sustained across the mouse ventricular wall. Stability analysis of AP propagation in the two-dimensional model showed the generation of Ca(2+) alternans and more complex transmurally heterogeneous irregular structures of repolarization and intracellular Ca(2+) transients at fast pacing rates.

Contemporary use of ventricular assist devices

The introduction of the heart lung machine more than 50 years ago proved in principle that heart function can be replaced, albeit for short periods. This was followed by attempts to produce total or partial artificial hearts that could function for prolonged periods of time. Progress in this field has been intermittent but has accelerated considerably in the past 10 years, with ventricular assist devices (VADs) reaching an impressive degree of sophistication and complexity owing to the contributions from clinicians, engineers, scientists, industrialists, and others. This review describes the currently available types of VADs, their current clinical use, the patient selection process, the trend toward use of VADs in patients with less severe heart failure, and the use of VADs for myocardial recovery in combination with novel pharmacological strategies, gene therapy, and cell therapy.

Prolonged mechanical unloading affects cardiomyocyte excitation-contraction coupling, transverse-tubule structure, and the cell surface

Prolonged mechanical unloading (UN) of the heart is associated with detrimental changes to the structure and function of cardiomyocytes. The mechanisms underlying these changes are unknown. In this study, we report the influence of UN on excitation-contraction coupling, Ca(2+)-induced Ca(2+) release (CICR) in particular, and transverse (t)-tubule structure. UN was induced in male Lewis rat hearts by heterotopic abdominal heart transplantation. Left ventricular cardiomyocytes were isolated from the transplanted hearts after 4 wk and studied using whole-cell patch clamping, confocal microscopy, and scanning ion conductance microscopy (SICM). Recipient hearts were used as control (C). UN reduced the volume of cardiomyocytes by 56.5% compared with C (UN, n=90; C, n=59; P<0.001). The variance of time-to-peak of the Ca(2+) transients was significantly increased in unloaded cardiomyocytes (UN 227.4+/-24.9 ms(2), n=42 vs. C 157.8+/-18.0 ms(2), n=40; P<0.05). UN did not alter the action potential morphology or whole-cell L-type Ca(2+) current compared with C, but caused a significantly higher Ca(2+) spark frequency (UN 3.718+/-0.85 events/100 mum/s, n=47 vs. C 0.908+/-0.186 events/100 microm/s, n=45; P<0.05). Confocal studies showed irregular distribution of the t tubules (power of the normal t-tubule frequency: UN 8.13+/-1.12x10(5), n=57 vs. C 20.60+/- 3.174x10(5), n=56; P<0.001) and SICM studies revealed a profound disruption to the openings of the t tubules and the cell surface in unloaded cardiomyocytes. We show that UN leads to a functional uncoupling of the CICR process and identify disruption of the t-tubule-sarcoplasmic reticulum interaction as a possible mechanism.

Transfer of bone marrow progenitors prevents coronary insufficiency and systolic dysfunction in the mechanical unloaded heart in mice

BACKGROUND

Left ventricular-assist device (LVAD) can lead to improvement of cardiac performance in a subset of patients, but chronic mechanical unloading in this fashion may result in left ventricular (LV)-atrophy and impaired functional recovery. Here, we evaluate the efficacy of transferring bone-marrow KSL cells (Lin-/c-kit+/Sca1+), a fraction containing endothelial progenitor cells, for preventing LV-atrophy and malfunction in a mouse model of mechanical unloading of the heart.

MATERIALS AND METHODS

Recipients of an isogenic heart transplant received intramyocardial isogenic KSL cells or PBS in three different locations of the left ventricle (LV). Coronary blood flow and LV systolic function were evaluated by echocardiography, and morphologic changes were analyzed on d 7 and 56.

RESULTS

PBS-treated mice showed severe systolic dysfunction and large thrombi in LV at both time points. In contrast, KSL cell transfer markedly reduced systolic dysfunction and thrombus size. Furthermore, in comparison with PBS control, KSL recipients had increased coronary blood flow (3-fold, P < 0.01) accompanied by increased LV capillary density and muscle mass.

CONCLUSIONS

These results indicate that intramyocardial transfer of bone marrow KSL cells significantly protects against coronary insufficiency and systolic dysfunction in the chronic LV-unloading heart, suggesting that this approach may have clinical potential as a combination therapy with LVAD.

The role of the cardiac Na+/Ca2+ exchanger in reverse remodeling: relevance for LVAD-recovery

Different strategies can, at least in certain conditions, prevent or reverse myocardial remodeling due to heart failure and induce myocardial functional improvement. Na+/Ca2+ exchanger (NCX) is considered a major player in the pathophysiology of heart failure but its role in reverse remodeling is unknown. A combination of mechanical unloading by left ventricular assist devices (LVADs) and pharmacological therapy has been shown to induce clinical recovery in a limited number of patients with end-stage heart failure. In myocytes isolated from these patients we found that, after LVAD treatment, NCX1/SERCA2a mRNA was 38% higher than at device implant. We studied the ability of NCX to extrude Ca2+ during caffeine-induced SR Ca2+ release in isolated ventricular myocytes from these patients. The time constant of decline was slower in heart failure. In myocytes from patients with clinical recovery following mechanical and pharmacological treatment, NCX1-mediated Ca2+ extrusion was faster compared with myocytes from patient who, despite identical treatment, did not recover. We propose that increased NCX function may be associated with reverse remodeling in patients and that factors that regulate NCX function (i.e., phosphorylation or intracellular [Na+]) other than NCX expression levels alone, may have detrimental consequences on cardiac function.

Determination of optimal duration of mechanical unloading for failing hearts to achieve bridge to recovery in a rat heterotopic heart transplantation model

BACKGROUND

Mechanical unloading (MU) of a failing heart using a left ventricular assist device (LVAD) can lead to "bridge to recovery" in some patients. However, it is still unknown how to determine when to withdraw assistance. We sought to determine the optimal duration of MU by investigating its short- and long-term effects using a rat model of heterotopic heart transplantation.

METHODS

Heart failure (HF) was induced in Lewis rats by ligating the left anterior descending artery. In the MU-HF groups, failing hearts were harvested and heterotopically transplanted. In the non-unloaded HF groups and the control group, hearts were not transplanted. After 2, 4 and 8 weeks, we evaluated papillary muscle function, histologic change and cardiac gene expression. Normal hearts served as the control group.

RESULTS

In the MU-HF groups, papillary muscle function improved significantly in the early period of unloading. It peaked and normalized at 4 weeks of unloading, but decreased to 50% the level of a normal heart at 8 weeks. In parallel with papillary muscle function, expression of brain natriuretic peptide (BNP) mRNA and SERCA2a mRNA normalized at 2 and 4 weeks of unloading, respectively, but deteriorated after 4 weeks. Cardiomyocyte hypertrophy was normalized at 2 weeks of unloading, but extended unloading induced cardiac atrophy. Myocardial fibrosis increased after unloading.

CONCLUSIONS

Mechanical unloading of the failing heart can help normalize cardiac function, cardiomyocyte hypertrophy and cardiac gene expression for an optimal duration (<4 weeks), but this normalization deteriorates with prolonged support.

Atrial BNP endocrine function during chronic unloading of the normal canine heart

The goal of the study was to define the effect of chronic unloading of the normal heart on atrial endocrine function with a focus on brain natriuretic peptide (BNP), specifically addressing the role of load and neurohumoral stimulation. Although produced primarily by atrial myocardium in the normal heart, controversy persists with regard to load-dependent vs. neurohumoral mechanisms controlling atrial BNP synthesis and storage. We used a unique canine model of chronic unloading of the heart produced by thoracic inferior vena caval constriction (TIVCC), which also resulted in activation of plasma endothelin (ET-1), ANG II, and norepinephrine (NE), known activators of BNP synthesis, compared with sham. TIVCC was produced by banding of the inferior vena cava for 10 days (n = 6), whereas in control (n = 5) the band was not constricted (sham). In a third group (n = 7), the band was released on day 11, thus acutely reloading the heart. Chronic TIVCC decreased cardiac output and right atrial pressure with a decrease in atrial mass index consistent with atrial atrophy. Atrial BNP mRNA decreased compared with sham. Immunoelectron microscopy revealed an increase in BNP in atrial granules consistent with increased storage. Acute reloading increased cardiac filling pressures and resulted in an increase in plasma BNP. We conclude that chronic unloading of the normal heart results in atrial atrophic remodeling and in suppression of atrial BNP mRNA despite intense stimulation by ET, ANG II, and NE, underscoring the primacy of load in the control of atrial endocrine function and structure.

Mechanical unloading improves intracellular Ca2+ regulation in rats with doxorubicin-induced cardiomyopathy

OBJECTIVES

We sought to assess whether mechanical unloading has beneficial effects on cardiomyocytes from doxorubicin-induced cardiomyopathy in rats.

BACKGROUND

Mechanical unloading by a left ventricular assist device (LVAD) improves the cardiac function of terminal heart failure in humans. However, previous animal studies have failed to demonstrate beneficial effects of mechanical unloading in the myocardium.

METHODS

The effects of mechanical unloading by heterotopic abdominal heart transplantation were evaluated in the myocardium from doxorubicin-treated rats by analyzing the intracellular free calcium level ([Ca(2+)](i)) and the levels of intracellular Ca(2+)-regulatory proteins.

RESULTS

In doxorubicin-treated rats, the duration of cell shortening and [Ca(2+)](i) transients in cardiomyocytes was prolonged (432 +/- 28.2% of control in 50% relaxation time; 184 +/- 10.5% of control in [Ca(2+)](i) 50% decay time). Such prolonged time courses significantly recovered after mechanical unloading (114 +/- 10.4% of control in 50% relaxation time; 114 +/- 5.8% of control in 50% decay time). These effects were accompanied by an increase in sarcoplasmic reticulum Ca(2+) ATPase (SERCA2a) protein levels (0.97 +/- 0.05 in unloaded hearts vs. 0.41+/- 0.09 in non-unloaded hearts). The levels of other intracellular Ca(2+)-regulatory proteins (phospholamban and ryanodine receptor) were not altered after mechanical unloading in doxorubicin-treated hearts. These parameters in unloaded hearts without doxorubicin treatment were similar to normal hearts.

CONCLUSIONS

Mechanical unloading increases functional sarcoplasmic reticulum Ca(2+) ATPase and improves [Ca(2+)](i) handling and contractility in rats with doxorubicin-induced cardiomyopathy. These beneficial effects of mechanical unloading were not observed in normal hearts.

Mechanotransduction in cardiac myocytes

Cardiac myocytes react to diverse mechanical demands with a multitude of transient and long-term responses to normalize the cellular mechanical environment. Several stretch-activated signaling pathways have been identified, most prominently guanine nucleotide binding proteins (G-proteins), mitogen-activated protein kinases (MAPK), Janus-associated kinase/signal transducers and activators of transcription (JAK/STAT), protein kinase C (PKC), calcineurin, intracellular calcium regulation, and several autocrine and paracrine factors. Multiple levels of crosstalk exist between pathways. The cellular response to changes in the mechanical environment can lead to cardiac myocyte hypertrophy, cellular growth that can be accompanied by pathological myocyte dysfunction, and tissue fibrosis. Several candidates for the primary mechanosensor in cardiac myocytes have been identified, ranging from stretch-activated ion channels in the membrane to yet-unknown mechanosensitive mechanisms in the nucleus. New and refined experimental techniques will exploit advances in molecular biology and biological imaging to study mechanotransduction in isolated cells and genetically engineered mice to explore the function of individual proteins.

Computer model of action potential of mouse ventricular myocytes

We have developed a mathematical model of the mouse ventricular myocyte action potential (AP) from voltage-clamp data of the underlying currents and Ca2+ transients. Wherever possible, we used Markov models to represent the molecular structure and function of ion channels. The model includes detailed intracellular Ca2+ dynamics, with simulations of localized events such as sarcoplasmic Ca2+ release into a small intracellular volume bounded by the sarcolemma and sarcoplasmic reticulum. Transporter-mediated Ca2+ fluxes from the bulk cytosol are closely matched to the experimentally reported values and predict stimulation rate-dependent changes in Ca2+ transients. Our model reproduces the properties of cardiac myocytes from two different regions of the heart: the apex and the septum. The septum has a relatively prolonged AP, which reflects a relatively small contribution from the rapid transient outward K+ current in the septum. The attribution of putative molecular bases for several of the component currents enables our mouse model to be used to simulate the behavior of genetically modified transgenic mice.

The future of mechanical circulatory support

Heart failure is one of the most important causes of morbidity and mortality in adults and the elderly. In the United States, an estimated 5 million persons already have heart failure, and more than 500,000 new cases are being diagnosed each year [ 1]. Today, cardiovascular physicians can choose from a wide range of mechanical circulatory systems, depending on the desired degree of support, length of support, extent of postoperative mobility and other factors. This article describes the growing problem of heart failure and the future prospects for patients with heart disease. It discusses current mechanical circulatory support devices and their changing applications, newer devices still in the experimental stages, and some hurdles to the use of mechanical circulatory support.

Regional passive ventricular stress-strain relations during development of altered loads in chick embryo

Mechanical load influences embryonic ventricular growth, morphogenesis, and function. However, little is known about changes in regional passive ventricular properties during the development of altered mechanical loading conditions in the embryo. We tested the hypothesis that regional mechanical loads are a critical determinant of embryonic ventricular passive properties. We measured biaxial passive right and left ventricular (RV and LV, respectively) stress-strain relations in chick embryos at Hamburger-Hamilton stages 21 and 27 after conotruncal banding (CTB) to increase biventricular pressure load or left atrial ligation (LAL) to reduce LV volume load and increase RV volume load. In the RV, wall strains at end-diastolic (ED) pressure normalized whereas ED stresses increased after either CTB or LAL during development. In the left ventricle, both ED strain and stress normalized after CTB, whereas both remained reduced with significantly increased myocardial stiffness after LAL. These results suggest that the embryonic ventricle adapts to chronically altered mechanical loading conditions by changing specific RV and LV passive properties. Thus regional mechanical load has a critical role during cardiogenesis.

Preserved contractile function despite atrophic remodeling in unloaded rat hearts

The present study was designed to determine whether myocardial atrophy is necessarily associated with changes in cardiac contractility. Myocardial unloading of normal hearts was produced via heterotopic transplantation in rats. Contractions of isolated myocytes (1.2 mM Ca2+; 37 degrees C) were assessed during field stimulation (0.5, 1.0, and 2.0 Hz), and papillary muscle contractions were assessed during direct stimulation (2.0 mM Ca2+; 37 degrees C; 0.5 Hz). Hemodynamic unloading was associated with a 41% decrease in median myocyte volume and proportional decreases in myocyte length and width. Nevertheless, atrophic myocytes had normal fractional shortening, time to peak contraction, and relaxation times. Despite decreases in absolute maximal force generation (F(max)), there were no differences in F(max)/ area in papillary muscles isolated from unloaded transplanted hearts. Therefore, atrophic remodeling after unloading is associated with intact contractile function in isolated myocytes and papillary muscles when contractile indexes are normalized to account for reductions in cell length and cross-sectional area, respectively. Nevertheless, in the absence of compensatory increases in contractile function, reductions in myocardial mass will lead to impaired overall work capacity.