Human pediatric and adult ventricular cardiomyocytes in culture: assessment of phenotypic changes with passaging

From Bench to Bedside: Translational Approaches to Cardiotoxicity in Breast Cancer, Lung Cancer, and Lymphoma Therapies

Cardiotoxicity represents a critical challenge in cancer therapy, particularly in the treatment of thoracic tumors, such as lung cancer and lymphomas, as well as breast cancer. These malignancies stand out for their high prevalence and the widespread use of cardiotoxic treatments, such as chemotherapy, radiotherapy, and immunotherapy. This work underscores the importance of preclinical models in uncovering the mechanisms of cardiotoxicity and developing targeted prevention and mitigation strategies. In vitro models provide valuable insights into cellular processes, enabling the observation of changes in cell viability and function following exposure to various drugs or ionizing radiation. Complementarily, in vivo animal models offer a broader perspective, allowing for evaluating of both short- and long-term effects and a better understanding of chronic toxicity and cardiac diseases. By integrating these approaches, researchers can identify potential mechanisms of cardiotoxicity and devise effective prevention strategies. This analysis highlights the central role of preclinical models in advancing knowledge of cardiotoxic effects associated with common therapeutic regimens for thoracic and breast cancers.

Compatibility and function of human induced pluripotent stem cell derived cardiomyocytes on an electrospun nanofibrous scaffold, generated from an ionomeric polyurethane composite

Synthetic scaffolds are needed for generating organized neo-myocardium constructs to promote functional tissue repair. This study investigated the biocompatibility of an elastomeric electrospun degradable polar/hydrophobic/ionic polyurethane (D-PHI) composite scaffold with human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs). The composite material was electrospun to generate scaffolds, with nanofibres oriented in aligned or random directions. These features enabled the authors to evaluate the effect of characteristic elements which mimic that of the native extracellular matrix (alignment, chemical heterogeneity, and fiber topography) on hiPSC-CMs activity. The functional nature of the hiPSC-CM cultured on gelatin and Matrigel-coated scaffolds were assessed, investigating the influence of protein interactions with the synthetic substrate on subsequent cell phenotype. After 7 days of culture, high hiPSC-CM viability was observed on the scaffolds. The cells on the aligned scaffold were elongated and demonstrated aligned sarcomeres that oriented parallel to the direction of the fibers, while the cells on random scaffolds and a tissue culture polystyrene (TCPS) control did not exhibit such an organized morphology. The hiPSC-CMs cultured on the scaffolds and TCPS expressed similar levels of cardiac troponin-T, but there was a higher expression of ventricular myosin light chain-2 on the D-PHI composite scaffolds versus TCPS, indicating a higher proportion of hiPSC-CM exhibiting a ventricular cardiomyocyte like phenotype. Within 7 days, the hiPSC-CMs on aligned scaffolds and TCPS beat synchronously and had similar conductive velocities. These preliminary results show that aligned D-PHI elastomeric scaffolds allow hiPSC-CMs to demonstrate important cardiomyocytes characteristics, critical to enabling their future potential use for cardiac tissue regeneration.

Mass-Spectrometry-Based Functional Proteomic and Phosphoproteomic Technologies and Their Application for Analyzing Ex Vivo and In Vitro Models of Hypertrophic Cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is an autosomal dominant disease thought to be principally caused by mutations in sarcomeric proteins. Despite extensive genetic analysis, there are no comprehensive molecular frameworks for how single mutations in contractile proteins result in the diverse assortment of cellular, phenotypic, and pathobiological cascades seen in HCM. Molecular profiling and system biology approaches are powerful tools for elucidating, quantifying, and interpreting dynamic signaling pathways and differential macromolecule expression profiles for a wide range of sample types, including cardiomyopathy. Cutting-edge approaches combine high-performance analytical instrumentation (e.g., mass spectrometry) with computational methods (e.g., bioinformatics) to study the comparative activity of biochemical pathways based on relative abundances of functionally linked proteins of interest. Cardiac research is poised to benefit enormously from the application of this toolkit to cardiac tissue models, which recapitulate key aspects of pathogenesis. In this review, we evaluate state-of-the-art mass-spectrometry-based proteomic and phosphoproteomic technologies and their application to in vitro and ex vivo models of HCM for global mapping of macromolecular alterations driving disease progression, emphasizing their potential for defining the components of basic biological systems, the fundamental mechanistic basis of HCM pathogenesis, and treating the ensuing varied clinical outcomes seen among affected patient cohorts.

Increased Retention of Cardiac Cells to a Glass Substrate through Streptavidin-Biotin Affinity

In vitro analysis of primary isolated adult cardiomyocyte physiological processes often involves optical imaging of dye-loaded cells on a glass substrate. However, when exposed to rapid solution changes, primary cardiomyocytes often move to compromise quantitative measures. Improved immobilization of cells to glass would permit higher throughput assays. Here, we engineer the peripheral membrane of cardiomyocytes with biotin to anchor cardiomyocytes to borosilicate glass coverslips functionalized with streptavidin. We use a rat cardiac myoblast cell line to determine general relationships between processing conditions, ligand density on the cell and the glass substrate, cellular function, and cell retention under shear flow. Use of the streptavidin-biotin system allows for more than 80% retention of cardiac myoblasts under conventional rinsing procedures, while unmodified cells are largely rinsed away. The adhesion system enables the in-field retention of cardiac cells during rapid fluid changes using traditional pipetting or a modern microfluidic system at a flow rate of 160 mL/min. Under fluid flow, the surface-engineered primary adult cardiomyocytes are retained in the field of view of the microscope, while unmodified cells are rinsed away. Importantly, the engineered cardiomyocytes are functional following adhesion to the glass substrate, where contractions are readily observed. When applying this adhesion system to cardiomyocyte functional analysis, we measure calcium release transients by caffeine induction at an 80% success rate compared to 20% without surface engineering.

Design of synthetic microenvironments to promote the maturation of human pluripotent stem cell derived cardiomyocytes

Cardiomyocyte like cells derived from human pluripotent stem cells (hPSC-CMs) have a good application perspective in many fields such as disease modeling, drug screening and clinical treatment. However, these are severely hampered by the fact that hPSC-CMs are immature compared to adult human cardiomyocytes. Therefore, many approaches such as genetic manipulation, biochemical factors supplement, mechanical stress, electrical stimulation and three-dimensional culture have been developed to promote the maturation of hPSC-CMs. Recently, establishing in vitro synthetic artificial microenvironments based on the in vivo development program of cardiomyocytes has achieved much attention due to their inherent properties such as stiffness, plasticity, nanotopography and chemical functionality. In this review, the achievements and deficiency of reported synthetic microenvironments that mainly discussed comprehensive biological, chemical, and physical factors, as well as three-dimensional culture were mainly discussed, which have significance to improve the microenvironment design and accelerate the maturation of hPSC-CMs.

Towards chamber specific heart-on-a-chip for drug testing applications

Modeling of human organs has long been a task for scientists in order to lower the costs of therapeutic development and understand the pathological onset of human disease. For decades, despite marked differences in genetics and etiology, animal models remained the norm for drug discovery and disease modeling. Innovative biofabrication techniques have facilitated the development of organ-on-a-chip technology that has great potential to complement conventional animal models. However, human organ as a whole, more specifically the human heart, is difficult to regenerate in vitro, in terms of its chamber specific orientation and its electrical functional complexity. Recent progress with the development of induced pluripotent stem cell differentiation protocols, made recapitulating the complexity of the human heart possible through the generation of cells representative of atrial & ventricular tissue, the sinoatrial node, atrioventricular node and Purkinje fibers. Current heart-on-a-chip approaches incorporate biological, electrical, mechanical, and topographical cues to facilitate tissue maturation, therefore improving the predictive power for the chamber-specific therapeutic effects targeting adult human. In this review, we will give a summary of current advances in heart-on-a-chip technology and provide a comprehensive outlook on the challenges involved in the development of human physiologically relevant heart-on-a-chip.

Learn from Your Elders: Developmental Biology Lessons to Guide Maturation of Stem Cell-Derived Cardiomyocytes

Human pluripotent stem cells (hPSCs) offer a multifaceted platform to study cardiac developmental biology, understand disease mechanisms, and develop novel therapies. Remarkable progress over the last two decades has led to methods to obtain highly pure hPSC-derived cardiomyocytes (hPSC-CMs) with reasonable ease and scalability. Nevertheless, a major bottleneck for the translational application of hPSC-CMs is their immature phenotype, resembling that of early fetal cardiomyocytes. Overall, bona fide maturation of hPSC-CMs represents one of the most significant goals facing the field today. Developmental biology studies have been pivotal in understanding the mechanisms to differentiate hPSC-CMs. Similarly, evaluation of developmental cues such as electrical and mechanical activities or neurohormonal and metabolic stimulations revealed the importance of these pathways in cardiomyocyte physiological maturation. Those signals cooperate and dictate the size and the performance of the developing heart. Likewise, this orchestra of stimuli is important in promoting hPSC-CM maturation, as demonstrated by current in vitro maturation approaches. Different shades of adult-like phenotype are achieved by prolonging the time in culture, electromechanical stimulation, patterned substrates, microRNA manipulation, neurohormonal or metabolic stimulation, and generation of human-engineered heart tissue (hEHT). However, mirroring this extremely dynamic environment is challenging, and reproducibility and scalability of these approaches represent the major obstacles for an efficient production of mature hPSC-CMs. For this reason, understanding the pattern behind the mechanisms elicited during the late gestational and early postnatal stages not only will provide new insights into postnatal development but also potentially offer new scalable and efficient approaches to mature hPSC-CMs.

Maturation of Pluripotent Stem Cell-Derived Cardiomyocytes: a Critical Step for Drug Development and Cell Therapy

Cardiomyocytes derived from human pluripotent stem cells (hPSCs) are emerging as an invaluable alternative to primarily sourced cardiomyocytes. The potentially unlimited number of hPSC-derived cardiomyocytes (hPSC-CMs) that may be obtained in vitro facilitates high-throughput applications like cell transplantation for myocardial repair, cardiotoxicity testing during drug development, and patient-specific disease modeling. Despite promising progress in these areas, a major disadvantage that limits the use of hPSC-CMs is their immaturity. Improvements to the maturity of hPSC-CMs are necessary to capture physiologically relevant responses. Herein, we review and discuss the different maturation strategies undertaken by others to improve the morphology, contractility, electrophysiology, and metabolism of these derived cardiomyocytes.

Engineered Microenvironments for Maturation of Stem Cell Derived Cardiac Myocytes

Through the use of stem cell-derived cardiac myocytes, tissue-engineered human myocardial constructs are poised for modeling normal and diseased physiology of the heart, as well as discovery of novel drugs and therapeutic targets in a human relevant manner. This review highlights the recent bioengineering efforts to recapitulate microenvironmental cues to further the maturation state of newly differentiated cardiac myocytes. These techniques include long-term culture, co-culture, exposure to mechanical stimuli, 3D culture, cell-matrix interactions, and electrical stimulation. Each of these methods has produced various degrees of maturation; however, a standardized measure for cardiomyocyte maturation is not yet widely accepted by the scientific community.

Naturally Engineered Maturation of Cardiomyocytes

Ischemic heart disease remains one of the most prominent causes of mortalities worldwide with heart transplantation being the gold-standard treatment option. However, due to the major limitations associated with heart transplants, such as an inadequate supply and heart rejection, there remains a significant clinical need for a viable cardiac regenerative therapy to restore native myocardial function. Over the course of the previous several decades, researchers have made prominent advances in the field of cardiac regeneration with the creation of in vitro human pluripotent stem cell-derived cardiomyocyte tissue engineered constructs. However, these engineered constructs exhibit a functionally immature, disorganized, fetal-like phenotype that is not equivalent physiologically to native adult cardiac tissue. Due to this major limitation, many recent studies have investigated approaches to improve pluripotent stem cell-derived cardiomyocyte maturation to close this large functionality gap between engineered and native cardiac tissue. This review integrates the natural developmental mechanisms of cardiomyocyte structural and functional maturation. The variety of ways researchers have attempted to improve cardiomyocyte maturation in vitro by mimicking natural development, known as natural engineering, is readily discussed. The main focus of this review involves the synergistic role of electrical and mechanical stimulation, extracellular matrix interactions, and non-cardiomyocyte interactions in facilitating cardiomyocyte maturation. Overall, even with these current natural engineering approaches, pluripotent stem cell-derived cardiomyocytes within three-dimensional engineered heart tissue still remain mostly within the early to late fetal stages of cardiomyocyte maturity. Therefore, although the end goal is to achieve adult phenotypic maturity, more emphasis must be placed on elucidating how the in vivo fetal microenvironment drives cardiomyocyte maturation. This information can then be utilized to develop natural engineering approaches that can emulate this fetal microenvironment and thus make prominent progress in pluripotent stem cell-derived maturity toward a more clinically relevant model for cardiac regeneration.

Geometry-dependent functional changes in iPSC-derived cardiomyocytes probed by functional imaging and RNA sequencing

Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CM) are a promising platform for cardiac studies in vitro, and possibly for tissue repair in humans. However, hiPSC-CM cells tend to retain morphology, metabolism, patterns of gene expression, and electrophysiology similar to that of embryonic cardiomyocytes. We grew hiPSC-CM in patterned islands of different sizes and shapes, and measured the effect of island geometry on action potential waveform and calcium dynamics using optical recordings of voltage and calcium from 970 islands of different sizes. hiPSC-CM in larger islands showed electrical and calcium dynamics indicative of greater functional maturity. We then compared transcriptional signatures of the small and large islands against a developmental time course of cardiac differentiation. Although island size had little effect on expression of most genes whose levels differed between hiPSC-CM and adult primary CM, we identified a subset of genes for which island size drove the majority (58%) of the changes associated with functional maturation. Finally, we patterned hiPSC-CM on islands with a variety of shapes to probe the relative contributions of soluble factors, electrical coupling, and direct cell-cell contacts to the functional maturation. Collectively, our data show that optical electrophysiology is a powerful tool for assaying hiPSC-CM maturation, and that island size powerfully drives activation of a subset of genes involved in cardiac maturation.

Bioengineering Approaches to Mature Human Pluripotent Stem Cell-Derived Cardiomyocytes

Human pluripotent stem cell-derived cardiomyocytes (hPSC-CM) represent a potential unlimited cell supply for cardiac tissue engineering and possibly regenerative medicine applications. However, hPSC-CMs produced by current protocols are not representative of native adult human cardiomyocytes as they display immature gene expression profile, structure and function. In order to improve hPSC-CM maturity and function, various approaches have been developed, including genetic manipulations to induce gene expression, delivery of biochemical factors, such as triiodothyronine and alpha-adrenergic agonist phenylephrine, induction of cell alignment in 3D tissues, mechanical stress as a mimic of cardiac load and electrical stimulation/pacing or a combination of these. In this mini review, we discuss biomimetic strategies for the maturation for hPSC-CMs with a particular focus on electromechanical conditioning methods.

Cell alignment induced by anisotropic electrospun fibrous scaffolds alone has limited effect on cardiomyocyte maturation

Enhancing the maturation of human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) will facilitate their applications in disease modeling and drug discovery. Previous studies suggest that cell alignment could enhance hPSC-CM maturation; however, the robustness of this approach has not been well investigated. To this end, we examined if the anisotropic orientation of hPSC-CMs imposed by the underlying aligned fibers within a 3D microenvironment could improve the maturation of hPSC-CMs. Enriched hPSC-CMs were cultured for two weeks on Matrigel-coated anisotropic (aligned) and isotropic (random) polycaprolactone (PCL) fibrous scaffolds, as well as tissue culture polystyrenes (TCPs) as a control. As expected, hPSC-CMs grown on the two types of fibrous scaffolds exhibited anisotropic and isotropic orientations, respectively. Similar to cells on TCPs, hPSC-CMs cultured on these scaffolds expressed CM-associated proteins and were pharmacologically responsive to adrenergic receptor agonists, a muscarinic agonist, and a gap junction uncoupler in a dose-dependent manner. Although hPSC-CMs grown on anisotropic fibrous scaffolds displayed the highest expression of genes encoding a number of sarcomere proteins, calcium handling proteins and ion channels, their calcium transient kinetics were slower than cells grown on TCPs. These results suggest that electrospun anisotropic fibrous scaffolds, as a single method, have limited effect on improving the maturation of hPSC-CMs.

Maturing human pluripotent stem cell-derived cardiomyocytes in human engineered cardiac tissues

Engineering functional human cardiac tissue that mimics the native adult morphological and functional phenotype has been a long held objective. In the last 5 years, the field of cardiac tissue engineering has transitioned from cardiac tissues derived from various animal species to the production of the first generation of human engineered cardiac tissues (hECTs), due to recent advances in human stem cell biology. Despite this progress, the hECTs generated to date remain immature relative to the native adult myocardium. In this review, we focus on the maturation challenge in the context of hECTs, the present state of the art, and future perspectives in terms of regenerative medicine, drug discovery, preclinical safety testing and pathophysiological studies.

Cryopreservation of Human Pluripotent Stem Cell-Derived Cardiomyocytes: Strategies, Challenges, and Future Directions

In recent years, human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) have emerged as a vital cell source for in vitro modeling of genetic cardiovascular disorders, drug screening, and in vivo cardiac regeneration research. Looking forward, the ability to efficiently cryopreserve hPSC-CMs without compromising their normal biochemical and physiologic functions will dramatically facilitate their various biomedical applications. Although working protocols for freezing, storing, and thawing hPSC-CMs have been established, the question remains as to whether they are optimal. In this chapter, we discuss our current understanding of cryopreservation appertaining to hPSC-CMs, and proffer key questions regarding the mechanical, contractile, and regenerative properties of cryopreserved hPSC-CMs.

Functional maturation of human pluripotent stem cell derived cardiomyocytes in vitro--correlation between contraction force and electrophysiology

Cardiomyocytes from human pluripotent stem cells (hPSC-CM) have many potential applications in disease modelling and drug target discovery but their phenotypic similarity to early fetal stages of cardiac development limits their applicability. In this study we compared contraction stresses of hPSC-CM to 2nd trimester human fetal derived cardiomyocytes (hFetal-CM) by imaging displacement of fluorescent beads by single contracting hPSC-CM, aligned by microcontact-printing on polyacrylamide gels. hPSC-CM showed distinctly lower contraction stress than cardiomyocytes isolated from hFetal-CM. To improve maturation of hPSC-CM in vitro we made use of commercial media optimized for cardiomyocyte maturation, which promoted significantly higher contraction stress in hPSC-compared with hFetal-CM. Accordingly, other features of cardiomyocyte maturation were observed, most strikingly increased upstroke velocities and action potential amplitudes, lower resting membrane potentials, improved sarcomeric organization and alterations in cardiac-specific gene expression. Performing contraction force and electrophysiology measurements on individual cardiomyocytes revealed strong correlations between an increase in contraction force and a rise of the upstroke velocity and action potential amplitude and with a decrease in the resting membrane potential. We showed that under standard differentiation conditions hPSC-CM display lower contractile force than primary hFetal-CM and identified conditions under which a commercially available culture medium could induce molecular, morphological and functional maturation of hPSC-CM in vitro. These results are an important contribution for full implementation of hPSC-CM in cardiac disease modelling and drug discovery.

Long-term primary cultures of human adult atrial cardiac myocytes: cell viability, structural properties and BNP secretion in vitro

BACKGROUND

Human adult cardiomyocytes (CM) have been used in short-term cultures for in vitro studies of the adult myocardium. However, little information is available regarding human adult CMs cultured for long term (>2 weeks).

METHODS

Human adult CMs were isolated from atrial specimens of 43 patients undergoing cardiopulmonary bypass surgery. Cell viability, cytoskeletal properties, intercellular junctional mediators and responsiveness to extracellular stimuli were monitored in CM cultures for 8 weeks.

RESULTS

Absolute numbers of CMs decreased through the first 2 weeks, with substantially lower rates of cell loss thereafter. Apoptosis predominated over necrosis as the principal mode of cell death, affecting 4.1+/-1.6% of freshly dissociated cells, that declined in culture (3.6+/-1.0% week 1, 1.3+/-0.5% week 2). CMs maintained rod-shaped morphology and cross-striated expression pattern of sarcomeric proteins desmin and beta-myosin heavy chain for the first 4 weeks. Levels of desmin remained stable on first 3 weeks, but declined thereafter. CMs expressed cardiac-specific adherence molecule N-cadherin throughout the culture duration, indicating conserved contractile potential. CMs remained functional early in culture, as indicated by BNP secretion, with maximal levels on 1st week that declined gradually by week 4. Cell responsiveness to metabolic stresses (serum deprivation) was detected, inducing an early (6 h) 1.8-fold increase in levels of BNP.

CONCLUSION

Long-term cultured human adult CMs maintain morphological integrity, adult-type cytoskeletal protein expression, cell-cell communication potential and functionality for 3-4 weeks in vitro.

Immunosuppression in cardiac graft rejection: a human in vitro model to study the potential use of new immunomodulatory drugs

CXCL10-CXCR3 axis plays a pivotal role in cardiac allograft rejection, so that targeting CXCL10 without inducing generalized immunosuppression may be of therapeutic significance in allotransplantation. Since the role of resident cells in cardiac rejection is still unclear, we aimed to establish reliable human cardiomyocyte cultures to investigate Th1 cytokine-mediated response in allograft rejection. We used human fetal cardiomyocytes (Hfcm) isolated from fetal hearts, obtained after legal abortions. Hfcm expressed specific cardiac lineage markers, specific cardiac structural proteins, typical cardiac currents and generated ventricular action potentials. Thus, Hfcm represent a reliable in vitro tool for allograft rejection research, since they resemble the features of mature cells. Hfcm secreted CXCL10 in response to IFNgamma and TNFalphaalpha; this effect was magnified by cytokine combination. Cytokine synergy was associated to a significant TNFalpha-induced up-regulation of IFNgammaR. The response of Hfcm to some currently used immunosuppressive drugs compared to rosiglitazone, a peroxisome proliferator-activated receptor gamma agonist and Th1-mediated response inhibitor, was also evaluated. Only micophenolic acid and rosiglitazone halved CXCL10 secretion by Hfcm. Given the pivotal role of IFNgamma-induced chemokines in Th1-mediated allograft rejection, these preliminary results suggest that the combined effects of immunosuppressive agents and rosiglitazone could be potentially beneficial to patients receiving heart transplants.

Formation of three-dimensional fetal myocardial tissue cultures from rat for long-term cultivation

Three-dimensional cardiomyocyte cultures offer new possibilities for the analysis of cardiac cell differentiation, spatial cellular arrangement, and time-specific gene expression in a tissue-like environment. We present a new method for generating homogenous and robust cardiomyocyte tissue cultures with good long-term viability. Ventricular heart cells prepared from fetal rats at embryonic day 13 were cultured in a scaffold-free two-step process. To optimize the cell culture model, several digestion protocols and culture conditions were tested. After digestion of fetal cardiac ventricles, the resultant cell suspension of isolated cardiocytes was shaken to initialize cell aggregate formation. In the second step, these three-dimensional cell aggregates were transferred onto a microporous membrane to allow further microstructure formation. Autonomously beating cultures possessed more than 25 cell layers and a homogenous distribution of cardiomyocytes without central necrosis after 8 weeks in vitro. The cardiomyocytes showed contractile elements, desmosomes, and gap junctions analyzed by immunohistochemistry and electron microscopy. The beat frequency could be modulated by adrenergic agonist and antagonist. Adenoviral green fluorescent protein transfer into cardiomyocytes was possible and highly effective. This three-dimensional tissue model proved to be useful for studying cell-cell interactions and cell differentiation processes in a three-dimensional cell arrangement.

Stem-like cells traffic from heart ex vivo, expand in vitro, and can be transplanted in vivo

BACKGROUND

Cells with stem cell surface markers have been identified in heart tissue. Early indications suggest that these are cardiac progenitor cells that could contribute to cardiac repair/regeneration. Clinically relevant therapeutic strategies based on these cells will require improved methods for their isolation and characterization of determinants of their mobilization, proliferation and differentiation.

METHODS

An ex vivo culture system was developed that promotes trafficking of progenitor-like cells from mouse ventricles to a culture surface. Cells that "trafficked" from cardiac tissue were phenotyped by flow cytometry and immunohistochemistry.

RESULTS

Morphologically distinct cells spontaneously trafficked from mouse ventricular tissue, adhered in culture, and proliferated for up to 4 weeks in Dulbecco's minimal essential media supplemented with fetal calf serum. After 4 weeks in culture, cell number declined. Co-culture with unfractionated bone marrow restored the proliferation of these trafficked cells. A significant population of the trafficked cells expressed a phenotype consistent with that of a myogenic progenitor such as: c-kit+, Sca-1+, CD45-, CD34-, CD90.2-, MyoD1-, desmin-, muscle-specific actin-, and, infrequently, myogenin+. An expanded population of trafficked cells from ventricles of mice expressing green fluorescent protein (GFP+) and containing cardiac-derived progenitor cells were injected into the pericardial space of GFP- mice. GFP+ cells trafficked throughout the heart but retained a primitive undifferentiated morphology. However, when injected into the pericardial space of Apo-E-deficient mice with coronary vasculopathy, progenitor-like cells trafficked into myocardium, and GFP+ cells differentiated into vessel-lining endothelial cells and, rarely, smooth muscle and cardiomyocytes.

CONCLUSIONS

Progenitor-like cells in the heart can be mobilized by tissue injury to spontaneously traffic from cardiac tissue and can expand in culture by co-culture with bone marrow. When re-infused by pericardiocentesis, these primitive cells traffic into heart, retain immature morphology, but are capable of undergoing injury-induced differentiation. The novel method described herein permits further characterization of cardiac-derived progenitor cells, which are a candidate for cardiac regeneration strategies.

Cardiac remodeling and failure: from molecules to man (Part III)

Given the lack of a unified theory of heart failure, future research efforts will be required to unify and synthesize our current understanding of the multiple mechanisms that control remodeling in the failing heart. Matrix remodeling and the associated activation of inflammatory cytokines and MMPs have emerged as key pathways in the development of heart failure. As such, attempts to understand the integrated control of ECM homeostasis with the bioactivation of inflammatory cytokines may be of particular relevance to the development of effective anti-remodeling approaches. Notably, the implantation of isolated populations of cells in failing myocardium has a profound and consistent anti-remodeling effect that limits the progression to CHF. These observations were consistently identified in numerous studies using diverse experimental animal models and varied cell types. Accordingly, multicenter clinical trials are underway, and the preliminary data in patients with CHF are encouraging. Despite the enormous promise of cell transplantation to restore and regenerate failing myocardium, the mechanisms underlying these profound biological effects are not understood. An improved understanding of the myocardial response to cell implantation, particularly on parameters of matrix remodeling, may help unify our current understanding of the progression of heart failure and optimize the development of this technique for its evolving therapeutic use. The following review outlines recent advances in medical and surgical approaches to control the remodeling process that underlies the progression of heart failure.

Establishment of Vessel-Like Structures in Long-Term Three-Dimensional Tissue Culture of Myocardium: An Electron Microscopy Study

To assess whether long-term three-dimensional (3D) tissue culture of myocardium enables the in vitro establishment of vessel-like structures, myocardial tissue from newborn mice was incubated under conditions of 3D culture for at least 3 weeks, and studied by phase-contrast microscopy, conventional histology, immunohistochemistry, and electron microscopy. During 3 weeks of culture, a mean 24.35 +/- 3.74% of all aggregates contracted spontaneously. The contracting aggregates displayed a tissue-like architecture with small basal and apical zones, and a large central zone. The basal and apical zone consisted of immature mesenchymal cells. The underlying shell of the aggregate contained many cardiomyocytes. Vessel-like structures were found concentrated within the aggregates. Immunohistochemistry showed that up to 15% of the cells in the central zone of the aggregate were positive for the endothelial-specific BS-I lectin. Vessel-like structures were formed by cells, which often showed intracytoplasmatic lumena. Surrounding the neocapillaries, structures of a rudimentary basal membrane could be detected. A 3D culture of myocardial tissue permits the establishment of a rudimentary capillary network within the tissue aggregates, which presumably guarantees a sufficient tissue perfusion up to a maximum aggregate diameter of approximately 900 microm.

Leptin increases cardiomyocyte hyperplasia via extracellular signal-regulated kinase- and phosphatidylinositol 3-kinase-dependent signaling pathways

Obesity is a major risk factor for the development of heart failure. Importantly, it is now appreciated that a change in the number of myocytes is one of multiple structural and functional alterations (remodeling) leading to heart failure. Here we investigate the effect of leptin, the product of the obese (ob) gene, on proliferation of human and murine cardiomyocytes. Leptin caused a time- and dose-dependent significant increase in proliferation of HL-1 cells that was inhibited by preincubation with PD98059 and LY294002, suggesting that leptin mediated proliferation via extracellular signal-regulated kinase-1/2- and phosphatidylinositol-3-kinase-dependent signaling pathways. We confirmed that leptin activates both extracellular signal-regulated kinase-1/2 phosphorylation and association of phosphatidylinositol-3-kinase (regulatory p85 subunit) with phosphotyrosine immunoprecipitates. We also examined bromodeoxyuridine incorporation as a measure of new DNA synthesis and demonstrated a stimulatory effect of leptin in both HL-1 cells and human cardiomyocytes. Bromodeoxyuridine incorporation in HL-1 cells was inhibited by PD98059 and LY294002. Our results establish a mitogenic effect of leptin in cardiomyocytes and provide additional evidence for a potential direct link between leptin and cardiac remodeling in obesity.

Restoration and regeneration of failing myocardium with cell transplantation and tissue engineering

Cell transplantation and the creation of bioengineered cardiovascular tissues are novel biologic approaches to restore and regenerate failing myocardium. These rapidly evolving therapies may complement and enhance other mechanical and surgical interventions for patients with congestive heart failure, providing cardiac surgeons with a wider range of treatments for patients at risk of congestive heart failure. Proof-of-concept studies have been performed in several experimental animal models of human cardiovascular disease, such as myocardial infarction and dilated cardiomyopathy. Although the exact mechanisms are unclear, cell transplantation restores cardiac function and limits ventricular dilatation. Clinical cell transplantation has been performed in a limited number of patients with encouraging preliminary results. In contrast, bioengineered muscle grafting is largely experimental but offers the promise of myocardial regeneration by replacing irreversibly damaged myocardium with healthy autologous tissue to facilitate more extensive ventricular remodeling surgery.

Significant improvement of heart function by cotransplantation of human mesenchymal stem cells and fetal cardiomyocytes in postinfarcted pigs

BACKGROUND

Viable cardiomyocytes after myocardial infarction (MI) are unable to repair the necrotic myocardium due to their limited capability of regeneration. The present study investigated whether intramyocardial transplantation of human mesenchymal stem cells (hMSCs) or cotransplantation of hMSCs plus human fetal cardiomyocytes (hFCs; 1:1) reconstituted impaired myocardium and improved cardiac function in MI pigs.

METHODS AND RESULTS

Cultured hMSCs were transfected with green fluorescent protein (GFP). Six weeks after MI induction and cell transplantation, cardiac function was significantly improved in MI pigs transplanted with hMSCs alone. However, the improvement was even markedly greater in MI pigs cotransplanted with hMSCs plus hFCs. Histological examination demonstrated that transplantation of hMSCs alone or hMSCs plus hFCs formed GFP-positive engrafts in infarcted myocardium. In addition, immunostaining for cardiac alpha-myosin heavy chain and troponin I showed positive stains in infarcted regions transplanted with hMSCs alone or hMSCs plus hFCs.

CONCLUSIONS

Our data demonstrate that transplantation of hMSCs alone improved cardiac function in MI pigs with a markedly greater improvement from cotransplantation of hMSCs plus hFCs. This improvement might result from myocardial regeneration and angiogenesis in injured hearts by engrafted cells.

The scarless heart

Over the past several years many mechanisms by which myocardial replacement could be achieved have been described. These include resident cardiac stem cells or circulating stem cells that can either differentiate into, or fuse to cardiomyocytes, or mature cells that can transdifferentiate into cardiomyocytes. However, the fact remains that after injury to the heart, the overriding response is scar formation with little myocardial replacement. One exception to this response is the MRL mouse, which heals with little scarring and shows nearly full myocardial replacement after injury. Results obtained with this model will be discussed.

Mechanical Stretch Regimen Enhances the Formation of Bioengineered Autologous Cardiac Muscle Grafts

Background Surgical repair of congenital and acquired cardiac defects may be enhanced by the use of autologous bioengineered muscle grafts. These tissue-engineered constructs are not optimal in their formation and function. We hypothesized that a mechanical stretch regimen applied to human heart cells that were seeded on a three-dimensional gelatin scaffold (Gelfoam) would improve tissue formation and enhance graft strength.

Methods and Results Heart cells from children undergoing repair of Tetralogy of Fallot were isolated and cultured. Heart cells were seeded on gelatin-matrix scaffolds (Gelfoam) and subjected to cyclical mechanical stress (n=7) using the Bio-Stretch Apparatus (80 cycles/minute for 14 days). Control scaffolds (n=7) were maintained under identical conditions but without cyclical stretch. Cell counting, histology, and computerized image analysis determined cell proliferation and their spatial distribution within the tissue-engineered grafts. Collagen matrix formation and organization was determined with polarized light and laser confocal microscopy. Uniaxial tensile testing assessed tissue-engineered graft function. Human heart cells proliferated within the gelatin scaffold. Remarkably, grafts that were subjected to cyclical stretch demonstrated increased cell proliferation and a marked improvement of cell distribution. Collagen matrix formation and organization was enhanced by mechanical stretch. Both maximal tensile strength and resistance to stretch were improved by cyclical mechanical stretch.

Conclusion The cyclical mechanical stretch regimen enhanced the formation of a three-dimensional tissue-engineered cardiac graft by improving the proliferation and distribution of seeded human heart cells and by stimulating organized matrix formation resulting in an order of magnitude increase in the mechanical strength of the graft.

The use of ex vivo gene transfer based on muscle-derived stem cells for cardiovascular medicine

Cell transplantation is a potential therapy for patients suffering from congestive heart failure. Many cell types have been experimentally tested for their ability to improve cardiac function. In this review, we discuss the potential of cell transplantation into the heart using various cell sources and introduce an attractive new cell source: Muscle-derived stem cells (MDSCs) are capable of delivering therapeutic genes and potentially differentiating toward a cardiomyocyte lineage within an injected heart. MDSCs are an attractive, alternate cell source because in addition to being multipotent (i.e., capable of differentiating into various lineages), they are easily accessible via simple biopsy of the patient's own muscle. This review will describe the isolation and unique characteristics of MDSCs and outline their potential use in regenerative medicine.

The doxorubicin cardioprotective agent dexrazoxane (ICRF-187) induces endopolyploidy in rat neonatal myocytes through inhibition of DNA topoisomerase II

Dexrazoxane (ICRF-187), which is clinically used to reduce doxorubicin-induced cardiotoxicity, is also a potent catalytic inhibitor of DNA topoisomerase II. In this study we showed that dexrazoxane inhibited the division of neonatal rat ventricular myocytes in culture, and resulted in nuclear multilobulation (demonstrated by three-dimensional reconstruction of confocal images) and marked increases in nuclear size and DNA ploidy levels (as shown by flow cytometry). It was concluded that dexrazoxane interfered with cell division in cardiac myocytes by virtue of its ability to inhibit topoisomerase II.

L-arginine protects human heart cells from low-volume anoxia and reoxygenation

Protective effects of L-arginine were evaluated in a human ventricular heart cell model of low-volume anoxia and reoxygenation independent of alternate cell types. Cell cultures were subjected to 90 min of low-volume anoxia and 30 min of reoxygenation. L-Arginine (0-5.0 mM) was administered during the preanoxic period or the reoxygenation phase. Nitric oxide (NO) production, NO synthase (NOS) activity, cGMP levels, and cellular injury were assessed. To evaluate the effects of the L-arginine on cell signaling, the effects of the NOS antagonist N(G)-nitro-L-arginine methyl ester, NO donor S-nitroso-N-acetyl-penicillamine, guanylate cyclase inhibitor methylene blue, cGMP analog 8-bromo-cGMP, and ATP-sensitive K+ channel antagonist glibenclamide were examined. Our data indicate that low-volume anoxia and reoxygenation increased NOS activity and facilitated the conversion of L-arginine to NO, which provided protection against cellular injury in a dose-dependent fashion. In addition, L-arginine cardioprotection was achieved by the activation of guanylate cyclase, leading to increased cGMP levels in human heart cells. This action involves a glibenclamide-sensitive, NO-cGMP-dependent pathway.

Selection of ventricular-like cardiomyocytes from ES cells in vitro

Ischemic disorders of the heart can cause an irreversible loss of cardiomyocytes resulting in a substantial decrease of cardiac output. The therapy of choice is heart transplantation, a technique that is hampered by the low number of donor organs. In the present study, we describe the specific labeling, rapid but gentle purification and characterization of cardiomyocytes derived from mouse pluripotent embryonic stem (ES) cells. To isolate the subpopulation of ventricular-like cardiomyocytes, ES cells were stable transfected with the enhanced green fluorescent protein (EGFP) under transcriptional control of the ventricular-specific 2.1 kb myosin light chain-2v (MLC-2v) promoter and the 0.5 kb enhancer element of the cytomegalovirus (CMV(enh).). First fluorescent cells were detected at day 6 + 8 of differentiation within EBs. Four weeks after initiation of differentiation 25% of the cardiomyocyte population displayed fluorescence. Immunohistochemistry revealed the exclusive cardiomyogenic nature of EGFP-positive cells. This was further corroborated by electrophysiological studies where preferentially ventricular phenotypes, but no pacemaker-like cardiomyocytes, were detected among the EGFP-positive population. The enzymatic digestion of EBs, followed by Percoll gradient centrifugation and fluorescence-activated cell sorting, resulted in a 97% pure population of cardiomyocytes. Based on this study, ventricular-like cardiomyocytes can be generated in vitro from EBs and labeled using CMV(enh)./MLC-2v-driven marker genes facilitating an efficient purification. This method may become an important tool for future cell replacement therapy of ischemic cardiomyopathy especially after the proof of somatic differentiation of human ES cells in vitro.

Construction of a bioengineered cardiac graft

OBJECTIVES

Currently available graft materials for repair of congenital heart defects cause significant morbidity and mortality because of their lack of growth potential. An autologous cell-seeded graft may improve patient outcomes. We report our initial experience with the construction of a biodegradable graft seeded with cultured rat or human cells and identify their 3-dimensional growth characteristics.

METHODS

Fetal rat ventricular cardiomyocytes, stomach smooth muscle cells, skin fibroblasts, and adult human atrial and ventricular cardiomyocytes were isolated and cultured in vitro. These cells were injected into or laid onto biodegradable gelatin meshes, and their rate of proliferation and spatial location within the mesh was evaluated by using a cell counter and histologic analysis.

RESULTS

Rat cardiomyocytes, smooth muscle cells, and fibroblasts demonstrated steady proliferation over 3 to 4 weeks. The gelatin mesh was slowly degraded, but this process was most rapid after seeding with fibroblasts. Human atrial cardiomyocytes proliferated within the gelatin meshes but at a slower rate than that of fetal rat cardiomyocytes. Human ventricular cardiomyocytes survived within the gelatin mesh matrix but did not increase in number during the 2-week duration of evaluation. Grafts seeded with rat ventricular cells exhibited spontaneous rhythmic contractility. All cell types preferentially migrated to the uppermost surface of each graft and formed a 300- to 500-microm thick layer.

CONCLUSIONS

Fetal rat ventricular cardiomyocytes, gastric smooth muscle cells, skin fibroblasts, and adult human atrial cardiomyocytes can grow in a 3-dimensional pattern within a biodegradable gelatin mesh. Similar autologous cell-seeded constructs may eventually be applied to repair congenital heart defects.

Autologous porcine heart cell transplantation improved heart function after a myocardial infarction

OBJECTIVE

Fetal cardiomyocyte transplantation improved heart function after cardiac injury. However, cellular allografts were rejected despite cyclosporine (INN: ciclosporin) therapy. We therefore evaluated autologous heart cell transplantation in an adult swine model of a myocardial infarction.

METHODS

In 16 adult swine a myocardial infarction was created by occlusion of the distal left anterior descending coronary artery by an intraluminal coil. Four weeks after infarction, technetium 99m-sestamibi single photon emission tomography showed minimal perfusion and viability in the infarcted region. Porcine heart cells were isolated and cultured from the interventricular septum at the time of infarction and grown in vitro for 4 weeks. Through a left thoracotomy, either cells (N = 8) or culture medium (N = 8) was injected into the infarct zone.

RESULTS

Four weeks after cell transplantation, technetium 99m-sestamibi single photon emission tomography demonstrated greater wall motion scores in the pigs receiving transplantation than in control animals (P =.01). Pigs receiving transplantation were more likely to have an improvement in perfusion scores (P =.03). Preload recruitable stroke work (P =.009) and end-systolic elastance (P =. 02) were greater in the pigs receiving transplantation than in control animals. Scar areas were not different, but scar thickness was greater (P =.02) in pigs receiving transplantation. Cells labeled with bromodeoxyuridine in vitro could be identified in the infarct zone 4 weeks after transplantation. Swine receiving transplantation gained more weight than control animals (P =.02).

CONCLUSION

Autologous porcine heart cell transplantation improved regional perfusion and global ventricular function after a myocardial infarction.

Autologous heart cell transplantation improves cardiac function after myocardial injury

BACKGROUND

Fetal ventricular cardiomyocyte transplantation into a cardiac scar improved ventricular function, but these cells were eventually eliminated by rejection. We therefore examined the feasibility of autologous adult heart cell transplantation.

METHODS

A transmural scar was produced in the left ventricular free wall of adult rats by cryoinjury. The left atrial appendage was harvested, and the atrial heart cells were cultured and their number expanded ex vivo. Three weeks after cryoinjury, either a cell suspension (2 x 10(6) cells, n = 12 rats, transplant group) or culture medium (n = 10 rats, control group) was injected into the scar. Rats having a sham operation (n = 5) did not undergo cryoinjury or transplantation with cells or culture medium.

RESULTS

Five weeks after injection, ventricular function was evaluated in a Langendorff preparation, measuring systolic, diastolic, and developed pressures over a range of intraventricular balloon volumes. Systolic and developed pressures were greater in the transplant group than in the control group (p = 0.0001). Rats with a sham operation had the greatest systolic, diastolic, and developed pressures (p = 0.0001). Histologic studies demonstrated survival of the transplanted heart cells within the scar. The area of the scar was smaller (p = 0.0003) and its thickness greater (p = 0.0003) in rats in the transplant group. Left ventricular chamber volume was smaller in the transplant group (p = 0.043).

CONCLUSIONS

Transplantation of autologous cultured adult atrial heart cells limited scar thinning and dilatation and improved myocardial function compared with results in control hearts. This technique may lead to a novel therapy to prevent scar expansion after a myocardial infarction and prevent the development of congestive heart failure.

Fetal cell transplantation: a comparison of three cell types

OBJECTIVE

We have previously reported that fetal cardiomyocyte transplantation into myocardial scar improves heart function. The mechanism by which this occurs, however, has not been elucidated. To investigate possible mechanisms by which cell transplantation may improve heart function, we compared cardiac function after transplantation of 3 different fetal cell types: cardiomyocytes, smooth muscle cells (nonstriated muscle cells), and fibroblasts (noncontractile cells).

METHODS

A left ventricular scar was created by cryoinjury in adult rats. Four weeks after injury, cultured fetal ventricular cardiomyocytes (n = 13), enteric smooth muscle cells (n = 10), skin fibroblasts (n = 10), or culture medium (control, n = 15 total) were injected into the myocardial scar. All rats received cyclosporine A (INN: ciclosporin). Four weeks after transplantation, left ventricular function was evaluated in a Langendorff preparation.

RESULTS

The implanted cells were identified histologically. All transplanted cell types formed tissue within the myocardial scar. At an end-diastolic volume of 0.2 mL, developed pressures in cardiomyocytes group were significantly greater than smooth muscle cells and skin fibroblasts groups (cardiomyocytes, 134% +/- 22% of control; smooth muscle cells, 108% +/- 14% of control; skin fibroblasts, 106% +/- 17% of control; P =.0001), as were +dP/dt(max) (cardiomyocytes, 119% +/- 37% of control; smooth muscle cells, 98% +/- 18% of control; skin fibroblasts, 92% +/- 11% of control; P =. 0001) and -dP/dt(max) (cardiomyocytes, 126% +/- 29% of control; smooth muscle cells, 108% +/- 19% of control; skin fibroblasts, 99% +/- 16% control; P =.0001).

CONCLUSIONS

Fetal cardiomyocytes transplanted into myocardial scar provided greater contractility and relaxation than fetal smooth muscle cells or fetal fibroblasts. The contractile and elastic properties of transplanted cells determine the degree of improvement in ventricular function achievable with cell transplantation.

Myocardial aerobic metabolism is impaired in a cell culture model of cyanotic heart disease

A human pediatric cardiomyocyte cell culture model of chronic cyanosis was used to assess the effects of low oxygen tension on mitochondrial enzyme activity to address the postoperative increase in lactate and decreased ATP in the myocardium and the high incidence of low-output failure with restoration of normal oxygen tension, after technically successful corrective cardiac surgery. Chronically hypoxic cells (PO2 = 40 mmHg for 7 days) exhibited significantly reduced activities for pyruvate dehydrogenase, cytochrome-c oxidase, succinate cytochrome c reductase, succinate dehydrogenase, and citrate synthase. The activity of NADH-cytochrome c reductase was unaffected. Lactate production and the lactate-to-pyruvate ratio were significantly greater in hypoxic cardiomyocytes. Western and Northern analysis demonstrated a decrease in the levels of various mRNA and corresponding polypeptides in hypoxic cells. Thus hypoxia influences mitochondrial metabolism through acute and chronic adaptive mechanisms, reflecting allosteric (posttranscriptional) and transcriptional modulation. Transcriptional downregulation of key mitochondrial enzyme systems can explain the insufficient myocardial aerobic metabolism and low-output failure in children with cyanotic heart disease after cardiac surgery.

Insulin stimulates pyruvate dehydrogenase and protects human ventricular cardiomyocytes from simulated ischemia

Impaired myocardial metabolism after cardioplegic arrest results in persistent anaerobic lactate production. Insulin may protect the heart from ischemia and reperfusion by enhancing myocardial metabolic recovery. However, the stimulation of glycolysis during ischemia may be detrimental because of an accumulation of metabolic end-products. We examined the effect of insulin on quiescent human ventricular cardiomyocytes subjected to simulated cardioplegic ischemia and reperfusion.

METHODS

Primary cardiomyocyte cultures were established from patients undergoing corrective repair of tetralogy of Fallot. Cells were exposed to varying concentrations of glucose and insulin during 30 minutes of stabilization in 10 mL of phosphate-buffered saline solution. Ischemia was simulated by exposing the cells to a low volume (1.5 mL) of deoxygenated phosphate-buffered saline solution for 90 minutes followed by 30 minutes of simulated reperfusion in 10 mL of normoxic phosphate-buffered saline solution. Cell viability was assessed by trypan blue exclusion. The activity of mitochondrial pyruvate dehydrogenase was measured in 3 states: stabilization, ischemia, and reperfusion. In addition intracellular lactate, adenine nucleotides, extracellular lactate, pyruvate, and acid release were measured.

RESULTS

Higher ambient glucose concentrations resulted in greater cellular injury although insulin-treated cells displayed less injury after ischemia and reperfusion. Insulin increased the pyruvate dehydrogenase activity by 31% in cardiomyocytes and reduced extracellular lactate production by 40%. Intracellular adenosine triphosphate was improved by 75% in cells exposed to high glucose concentrations in the presence of insulin.

CONCLUSIONS

Insulin protected human ventricular cardiomyocytes from ischemia and reperfusion. This protection may be due to a stimulation of pyruvate dehydrogenase activity which resulted in improved aerobic metabolism.

Preconditioning human cardiomyocytes and endothelial cells

BACKGROUND

The effects of simulated "ischemia" and "reperfusion" were evaluated in cell cultures of human ventricular cardiomyocytes and human saphenous vein endothelial cells.

METHODS

Myocyte and endothelial cell cultures were exposed to a low volume (1.5 ml) of either hypoxic (oxygen tension = 16 mm Hg) or anoxic (oxygen tension = 0 mm Hg) phosphate-buffered saline solution for 90 minutes ("ischemia") followed by 30 minutes of simulated "reperfusion." Cell injury was evaluated by trypan blue exclusion. Next, the effects of a preconditioning stimulus were evaluated by a brief (10 minute) exposure to hypoxic or anoxic ischemia and 10 minutes of reperfusion before prolonged (90 minutes) anoxic ischemia. Finally, the effects of anoxic preconditioning on intracellular lactate accumulation and extracellular lactate and acid release were assessed.

RESULTS

"Ischemia" and "reperfusion" resulted in greater injury to endothelial cells than to cardiomyocytes. In both cell types, anoxic ischemia resulted in greater injury than hypoxic ischemia. Preconditioning reduced cell injury in myocytes but not in endothelial cells. Endothelial cells produced more lactate than cardiomyocytes under normoxic conditions. Ischemia increased lactate accumulation and release in cardiomyocytes but not endothelial cells. Preconditioning reduced lactate accumulation and release in cardiomyocytes but not endothelial cells.

CONCLUSIONS

Endothelial cells were more susceptible to the same period of simulated ischemia than cardiomyocytes. Preconditioning protected cardiomyocytes but not endothelial cells from a subsequent prolonged period of ischemia and reperfusion.