Decellularization: Leveraging a Tissue Engineering Technique for Food Production

The edible nature of many plants makes leaves particularly useful as scaffolds for the development of cultured meat, where animal tissue is grown in the laboratory setting. Recently, we demonstrated that decellularized spinach leaves can serve as scaffolds to grow and differentiate cells for cultured meat products. However, conventional decellularization methods use solutions that are not considered safe for use in food, such as organic solvents (hexanes) and detergents (triton X-100 (TX100)). This study modified decellularization protocols to incorporate detergents that are regulated (REG) by the United States Food and Drug Administration (FDA) for use in food, such as Polysorbate 20 (PS20), and eliminates the use of hexanes for cuticle removal. Spinach leaves were decellularized with sodium dodecyl sulfate and then with either TX100 (control) or PS20. The average DNA content for TX100 samples and PS20 samples was similar (1.3 ± 0.07 vs 1.3 ± 0.05 ng/mg; TX100 vs PS20, p = ns). The importance of cuticle removal was tested by removing hexanes from the protocol. Groups that included the cuticle removal step exhibited an average reduction in DNA content of approximately 91.7%, and groups that omitted the cuticle removal step exhibited an average reduction of approximately 90.3% (p = ns), suggesting that the omission of the cuticle removal step did not impede decellularization. Lastly, primary bovine satellite cells (PBSCs) were cultured for 7 days (d) on the surface of spinach leaves decellularized using the REG protocol. After the 7 d incubation period, PBSCs grown on the surface of REG scaffolds had an average viability of approximately 97.4%. These observations suggest that the REG protocol described in this study is an effective decellularization method, more closely adhering to food safety guidelines, that could be implemented in lab grown meat and alternative protein products.

Micropatterned fibrin scaffolds increase cardiomyocyte alignment and contractility for the fabrication of engineered myocardial tissue

Cardiovascular disease is the leading cause of death in the United States, which can result in blockage of a coronary artery, triggering a myocardial infarction (MI), scar tissue formation in the myocardium, and ultimately heart failure. Currently, the gold-standard solution for total heart failure is a heart transplantation. An alternative to total-organ transplantation is surgically remodeling the ventricle with the implantation of a cardiac patch. Acellular cardiac patches have previously been investigated using synthetic or decellularized native materials to improve cardiac function. However, a limitation of this strategy is that acellular cardiac patches only reshape the ventricle and do not increase cardiac contractile function. Toward the development of a cardiac patch, our laboratory previously developed a cell-populated composite fibrin scaffold and aligned microthreads to recapitulate the mechanical properties of native myocardium. In this study, we explore micropatterning the surfaces of fibrin gels to mimic anisotropic native tissue architecture and promote cellular alignment of human induced pluripotent stem cell cardiomyocytes (hiPS-CM), which is crucial for increasing scaffold contractile properties. hiPS-CMs seeded on micropatterned surfaces exhibit cellular elongation, distinct sarcomere alignment, and circumferential connexin-43 staining at 14 days of culture, which are necessary for mature contractile properties. Constructs were also subject to electrical stimulation during culture to promote increased contractile properties. After 7 days of stimulation, contractile strains of micropatterned constructs were significantly higher than unpatterned controls. These results suggest that the use of micropatterned topographic cues on fibrin scaffolds may be a promising strategy for creating engineered cardiac tissue.

Abstract P1008: Bioelectric Sutures Generated From HiPSC-derived Cardiomyocytes Enable In Vitro Electrical Coupling Between Engineered Cardiac Tissues

Heart attack survivors live with permanent damage in their hearts and are at higher risk of developing heart failure. Current treatments do not directly repair or restore function in the damaged tissue. Implanting engineered cardiac tissues (ECTs) to restore contractility to the damaged myocardium is a promising therapy. However, this approach is challenged by poor integration between the implanted ECT and the host heart. To address this issue, we developed a bioelectric suture capable of propagating electrical signals to improve coupling between the two interfaces. We differentiated ventricular cardiomyocytes (CMs) from human induced pluripotent stem cells (hiPSCs) following a well-adopted small molecule modulation of the Wnt signaling pathway. Metabolic-based lactate purification was performed to increase CM purity to > 80%, evaluated by flow cytometry analysis of cardiac troponin-T. We constructed fibrin threads by coextruding fibrinogen and thrombin through a blending tip connector into a HEPES bath. Polymerized, stretched, and dried threads were coated in Matrigel, clamped to suture heads, and seeded with hiPSC-CMs mixed with 5% human ventricular cardiac fibroblasts at a density of 5x10 5 cells/cm for 72 hours. We cultured the conductive and contractile sutures for 2 weeks under 1 Hz electrical stimulation. Optical mapping was performed to characterize electrical properties of bioelectric sutures, including action potential duration (APD) and conduction velocity (CV). Bioelectric sutures were electromechanically active with APD 100 = 547 +/- 29 ms and CV = 25.1 +/- 5.9 mm/s. a-actinin and connexin-43 staining revealed that hiPSC-CMs preferentially aligned with the thread and formed significant gap junctions, suggesting electrical syncytium formation. Preliminary results and ongoing work showed that bioelectric sutures sutured to two separate ECTs electrically coupled within 3 days, enabling directed electrical propagation from one tissue to the other. Collectively, these results demonstrate that bioelectric sutures establish a CM bridge for electrical propagation, which may improve the integration of implanted ECTs with the host heart and enable the development of therapies to treat diseases of cardiac electrical conduction.

Creation of a contractile biomaterial from a decellularized spinach leaf without ECM protein coating: An in vitro study

Myocardial infarction (MI) results in the death of cardiac tissue, decreases regional contraction, and can lead to heart failure. Tissue engineered cardiac patches containing human induced pluripotent stem cell-derived cardiomyocytes (hiPS-CMs) can restore contractile function. However, cells within thick patches require vasculature for blood flow. Recently, we demonstrated fibronectin coated decellularized leaves provide a suitable scaffold for hiPS-CMs. Yet, the necessity of this additional coating step is unclear. Therefore, we compared hiPS-CM behavior on decellularized leaves coated with collagen IV or fibronectin extracellular matrix (ECM) proteins to noncoated leaves for up to 21 days. Successful coating was verified by immunofluorescence. Similar numbers of hiPS-CMs adhered to coated and noncoated decellularized leaves for 21 days. At Day 14, collagen IV coated leaves contracted more than noncoated leaves (3.25 ± 0.39% vs. 1.54 ± 0.60%; p < .05). However, no differences in contraction were found between coated leaves, coated tissue culture plastic (TCP), noncoated leaves, or noncoated TCP at other time points. No significant differences were observed in hiPS-CM spreading or sarcomere lengths on leaves with or without coating. This study demonstrates that cardiac scaffolds can be created from decellularized leaves without ECM coatings. Noncoated decellularized leaf surfaces facilitate robust cell attachment for an engineered tissue patch.

After 50 Years of Heart Transplants: What Does the Next 50 Years Hold for Cardiovascular Medicine? A Perspective From the International Society for Applied Cardiovascular Biology

The first successful heart transplant 50 years ago by Dr.Christiaan Barnard in Cape Town, South Africa revolutionized cardiovascular medicine and research. Following this procedure, numerous other advances have reduced many contributors to cardiovascular morbidity and mortality; yet, cardiovascular disease remains the leading cause of death globally. Various unmet needs in cardiovascular medicine affect developing and underserved communities, where access to state-of-the-art advances remain out of reach. Addressing the remaining challenges in cardiovascular medicine in both developed and developing nations will require collaborative efforts from basic science researchers, engineers, industry, and clinicians. In this perspective, we discuss the advancements made in cardiovascular medicine since Dr. Barnard's groundbreaking procedure and ongoing research efforts to address these medical issues. Particular focus is given to the mission of the International Society for Applied Cardiovascular Biology (ISACB), which was founded in Cape Town during the 20th celebration of the first heart transplant in order to promote collaborative and translational research in the field of cardiovascular medicine.

Development of a Contractile Cardiac Fiber From Pluripotent Stem Cell Derived Cardiomyocytes

Stem cell therapy has the potential to regenerate cardiac function after myocardial infarction. In this study, we sought to examine if fibrin microthread technology could be leveraged to develop a contractile fiber from human pluripotent stem cell derived cardiomyocytes (hPS-CM). hPS-CM seeded onto fibrin microthreads were able to adhere to the microthread and began to contract seven days after initial seeding. A digital speckle tracking algorithm was applied to high speed video data (>60 fps) to determine contraction behaviour including beat frequency, average and maximum contractile strain, and the principal angle of contraction of hPS-CM contracting on the microthreads over 21 days. At day 7, cells seeded on tissue culture plastic beat at 0.83 ± 0.25 beats/sec with an average contractile strain of 4.23±0.23%, which was significantly different from a beat frequency of 1.11 ± 0.45 beats/sec and an average contractile strain of 3.08±0.19% at day 21 (n = 18, p < 0.05). hPS-CM seeded on microthreads beat at 0.84 ± 0.15 beats/sec with an average contractile strain of 3.56±0.22%, which significantly increased to 1.03 ± 0.19 beats/sec and 4.47±0.29%, respectively, at 21 days (n = 18, p < 0.05). At day 7, 27% of the cells had a principle angle of contraction within 20 degrees of the microthread, whereas at day 21, 65% of hPS-CM were contracting within 20 degrees of the microthread (n = 17). Utilizing high speed calcium transient data (>300 fps) of Fluo-4AM loaded hPS-CM seeded microthreads, conduction velocities significantly increased from 3.69 ± 1.76 cm/s at day 7 to 24.26 ± 8.42 cm/s at day 21 (n = 5-6, p < 0.05). hPS-CM seeded microthreads exhibited positive expression for connexin 43, a gap junction protein, between cells. These data suggest that the fibrin microthread is a suitable scaffold for hPS-CM attachment and contraction. In addition, extended culture allows cells to contract in the direction of the thread, suggesting alignment of the cells in the microthread direction.

Two Methods for Decellularization of Plant Tissues for Tissue Engineering Applications

The autologous, synthetic, and animal-derived grafts currently used as scaffolds for tissue replacement have limitations due to low availability, poor biocompatibility, and cost. Plant tissues have favorable characteristics that make them uniquely suited for use as scaffolds, such as high surface area, excellent water transport and retention, interconnected porosity, preexisting vascular networks, and a wide range of mechanical properties. Two successful methods of plant decellularization for tissue engineering applications are described here. The first method is based on detergent baths to remove cellular matter, which is similar to previously established methods used to clear mammalian tissues. The second is a detergent-free method adapted from a protocol that isolates leaf vasculature and involves the use of a heated bleach and salt bath to clear the leaves and stems. Both methods yield scaffolds with comparable mechanical properties and low cellular metabolic impact, thus allowing the user to select the protocol which better suits their intended application.

Optical Method to Quantify Mechanical Contraction and Calcium Transients of Human Pluripotent Stem Cell-Derived Cardiomyocytes

Differentiation of human pluripotent stem cells into cardiomyocytes (hPS-CMs) holds promise for myocardial regeneration therapies, drug discovery, and models of cardiac disease. Potential cardiotoxicities may affect hPS-CM mechanical contraction independent of calcium signaling. Herein, a method using an image capture system is described to measure hPS-CM contractility and intracellular calcium concurrently, with high spatial and temporal resolution. The image capture system rapidly alternates between brightfield and epifluorescent illumination of contracting cells. Mechanical contraction is quantified by a speckle tracking algorithm applied to brightfield image pairs, whereas calcium transients are measured by a fluorescent calcium reporter. This technique captured changes in contractile strain, calcium transients, and beat frequency of hPS-CMs over 21 days in culture, as well as acute responses to isoproterenol and Cytochalasin D. The technique described above can be applied without the need to alter the culture platform, allowing for determination of hPS-CM behavior over weeks in culture for drug discovery and myocardial regeneration applications.

Creation of a Bioengineered Skin Flap Scaffold with a Perfusable Vascular Pedicle

Full-thickness skin loss is a challenging problem due to limited reconstructive options, demanding 75 million surgical procedures annually in the United States. Autologous skin grafting is the gold standard treatment, but results in donor-site morbidity and poor aesthetics. Numerous skin substitutes are available on the market to date, however, none truly functions as full-thickness skin due to lack of a vascular network. The creation of an autologous full-thickness skin analogue with a vascular pedicle would result in a paradigm shift in the management of wounds and in reconstruction of full-thickness skin defects. To create a clinically relevant foundation, we generated an acellular skin flap scaffold (SFS) with a perfusable vascular pedicle of clinically relevant size by perfusion decellularization of porcine fasciocutaneous flaps. We then analyzed the yielded SFS for mechanical properties, biocompatibility, and regenerative potential in vitro and in vivo. Furthermore, we assessed the immunological response using an in vivo model. Finally, we recellularized the vascular compartment of an SFS and reconnected it to a recipient's blood supply to test for perfusability. Perfusion decellularization removed all cellular components with preservation of native extracellular matrix composition and architecture. Biaxial testing revealed preserved mechanical properties. Immunologic response and biocompatibility assessed via implantation and compared with native xenogenic skin and commercially available dermal substitutes revealed rapid neovascularization and complete tissue integration. Composition of infiltrating immune cells showed no evidence of allorejection and resembled the inflammatory phase of wound healing. Implantation into full-thickness skin defects demonstrated good tissue integration and skin regeneration without cicatrization. We have developed a protocol for the generation of an SFS of clinically relevant size, containing a vascular pedicle, which can be utilized for perfusion decellularization and, ultimately, anastomosis to the recipient vascular system after precellularization. The observed favorable immunological response and good tissue integration indicate the substantial regenerative potential of this platform.

Crossing kingdoms: Using decellularized plants as perfusable tissue engineering scaffolds

Despite significant advances in the fabrication of bioengineered scaffolds for tissue engineering, delivery of nutrients in complex engineered human tissues remains a challenge. By taking advantage of the similarities in the vascular structure of plant and animal tissues, we developed decellularized plant tissue as a prevascularized scaffold for tissue engineering applications. Perfusion-based decellularization was modified for different plant species, providing different geometries of scaffolding. After decellularization, plant scaffolds remained patent and able to transport microparticles. Plant scaffolds were recellularized with human endothelial cells that colonized the inner surfaces of plant vasculature. Human mesenchymal stem cells and human pluripotent stem cell derived cardiomyocytes adhered to the outer surfaces of plant scaffolds. Cardiomyocytes demonstrated contractile function and calcium handling capabilities over the course of 21 days. These data demonstrate the potential of decellularized plants as scaffolds for tissue engineering, which could ultimately provide a cost-efficient, "green" technology for regenerating large volume vascularized tissue mass.

Delivering stem cells to the healthy heart on biological sutures: effects on regional mechanical function

Current cardiac cell therapies cannot effectively target and retain cells in a specific area of the heart. Cell-seeded biological sutures were previously developed to overcome this limitation, demonstrating targeted delivery with > 60% cell retention. In this study, both cell-seeded and non-seeded fibrin-based biological sutures were implanted into normal functioning rat hearts to determine the effects on mechanical function and fibrotic response. Human mesenchymal stem cells (hMSCs) were used based on previous work and established cardioprotective effects. Non-seeded or hMSC-seeded sutures were implanted into healthy athymic rat hearts. Before cell seeding, hMSCs were passively loaded with quantum dot nanoparticles. One week after implantation, regional stroke work index and systolic area of contraction (SAC) were evaluated on the epicardial surface above the suture. Cell delivery and retention were confirmed by quantum dot tracking, and the fibrotic tissue area was evaluated. Non-seeded biological sutures decreased SAC near the suture from 0.20 ± 0.01 measured in sham hearts to 0.08 ± 0.02, whereas hMSC-seeded biological sutures dampened the decrease in SAC (0.15 ± 0.02). Non-seeded sutures also displayed a small amount of fibrosis around the sutures (1.0 ± 0.1 mm2 ). Sutures seeded with hMSCs displayed a significant reduction in fibrosis (0.5 ± 0.1 mm2 , p < 0.001), with quantum dot-labelled hMSCs found along the suture track. These results show that the addition of hMSCs attenuates the fibrotic response observed with non-seeded sutures, leading to improved regional mechanics of the implantation region. Copyright © 2014 John Wiley & Sons, Ltd.

Functional Effects of Delivering Human Mesenchymal Stem Cell-Seeded Biological Sutures to an Infarcted Heart

Stem cell therapy has the potential to improve cardiac function after myocardial infarction (MI); however, existing methods to deliver cells to the myocardium, including intramyocardial injection, suffer from low engraftment rates. In this study, we used a rat model of acute MI to assess the effects of human mesenchymal stem cell (hMSC)-seeded fibrin biological sutures on cardiac function at 1 week after implant. Biological sutures were seeded with quantum dot (Qdot)-loaded hMSCs for 24 h before implantation. At 1 week postinfarct, the heart was imaged to assess mechanical function in the infarct region. Regional parameters assessed were regional stroke work (RSW) and systolic area of contraction (SAC) and global parameters derived from the pressure waveform. MI (n = 6) significantly decreased RSW (0.026 ± 0.011) and SAC (0.022 ± 0.015) when compared with sham operation (RSW: 0.141 ± 0.009; SAC: 0.166 ± 0.005, n = 6) (p < 0.05). The delivery of unseeded biological sutures to the infarcted hearts did not change regional mechanical function compared with the infarcted hearts (RSW: 0.032 ± 0.004, SAC: 0.037 ± 0.008, n = 6). The delivery of hMSC-seeded sutures exerted a trend toward increase of regional mechanical function compared with the infarcted heart (RSW: 0.057 ± 0.011; SAC: 0.051 ± 0.014, n = 6). Global function showed no significant differences between any group (p > 0.05); however, there was a trend toward improved function with the addition of either unseeded or seeded biological suture. Histology demonstrated that Qdot-loaded hMSCs remained present in the infarcted myocardium after 1 week. Analysis of serial sections of Masson's trichrome staining revealed that the greatest infarct size was in the infarct group (7.0% ± 2.2%), where unseeded (3.8% ± 0.6%) and hMSC-seeded (3.7% ± 0.8%) suture groups maintained similar infarct sizes. Furthermore, the remaining suture area was significantly decreased in the unseeded group compared with that in the hMSC-seeded group (p < 0.05). This study demonstrated that hMSC-seeded biological sutures are a method to deliver cells to the infarcted myocardium and have treatment potential.

Aprotinin extends mechanical integrity time of cell-seeded fibrin sutures

Cell therapy has the potential to treat different pathologies, including myocardial infarctions (heart attacks), although cell engraftment remains elusive with most delivery methods. Biological sutures composed of fibrin have been shown to effectively deliver human mesenchymal stem cell (MSC) to infarcted hearts. However, human MSCs rapidly degrade fibrin making cell seeding and delivery time sensitive. To delay the degradation process, we propose using Aprotinin, a proteolytic enzyme inhibitor that has been shown to slow fibrinolysis. Human MSCs seeded on fibrin sutures and incubated with Aprotinin demonstrated similar cell viability, examined using a LIVE/DEAD stain, to controls. No differences in proliferation, as determined by Ki-67 presence, were observed. Human MSCs incubated in Aprotinin differentiated into adipocytes, osteocytes, and chondrocytes, confirming multipotency. The number of cells adhered to fibrin sutures increased through Aprotinin supplementation at 2, 3, and 5 day time points. Uniaxial tensile testing was used to examine the effect of Aprotinin on suture integrity. Sutures exposed to Aprotinin had higher ultimate tensile strength and modulus when compared to sutures exposed to standard growth media. Fibrin sutures incubated in Aprotinin had larger diameters and less fibrin degradation products compared to the controls, confirming decreased fibrinolysis. These data suggest that Aprotinin can reduce degradation of fibrin sutures without significant effects on MSC function, providing a novel method for extending the implantation window and increasing the number of cells delivered via fibrin sutures. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 104A: 2271-2279, 2016.

Designing Biopolymer Microthreads for Tissue Engineering and Regenerative Medicine

Native tissue structures possess elaborate extracellular matrix (ECM) architectures that inspire the design of fibrous structures in the field of regenerative medicine. We review the literature with respect to the successes and failures, as well as the future promise of biopolymer microthreads as scaffolds to promote endogenous and exogenous tissue regeneration. Biomimetic microthread tissue constructs have been proposed for the functional regeneration of tendon, ligament, skeletal muscle, and ventricular myocardial tissues. To date, biopolymer microthreads have demonstrated promising results as materials to recapitulate the hierarchical structure of simple and complex tissues and well as biochemical signaling cues to direct cell-mediated tissue regeneration. Biopolymer microthreads have also demonstrated exciting potential as a platform technology for the targeted delivery of stem cells and therapeutic molecules. Future studies will focus on the design of microthread-based tissue analogs that strategically integrate growth factors and progenitor cells to temporally direct cell-mediated processes that promote enhanced functional tissue regeneration.

Bioengineering Human Myocardium on Native Extracellular Matrix

RATIONALE

More than 25 million individuals have heart failure worldwide, with ≈4000 patients currently awaiting heart transplantation in the United States. Donor organ shortage and allograft rejection remain major limitations with only ≈2500 hearts transplanted each year. As a theoretical alternative to allotransplantation, patient-derived bioartificial myocardium could provide functional support and ultimately impact the treatment of heart failure.

OBJECTIVE

The objective of this study is to translate previous work to human scale and clinically relevant cells for the bioengineering of functional myocardial tissue based on the combination of human cardiac matrix and human induced pluripotent stem cell-derived cardiomyocytes.

METHODS AND RESULTS

To provide a clinically relevant tissue scaffold, we translated perfusion-decellularization to human scale and obtained biocompatible human acellular cardiac scaffolds with preserved extracellular matrix composition, architecture, and perfusable coronary vasculature. We then repopulated this native human cardiac matrix with cardiomyocytes derived from nontransgenic human induced pluripotent stem cells and generated tissues of increasing 3-dimensional complexity. We maintained such cardiac tissue constructs in culture for 120 days to demonstrate definitive sarcomeric structure, cell and matrix deformation, contractile force, and electrical conduction. To show that functional myocardial tissue of human scale can be built on this platform, we then partially recellularized human whole-heart scaffolds with human induced pluripotent stem cell-derived cardiomyocytes. Under biomimetic culture, the seeded constructs developed force-generating human myocardial tissue and showed electrical conductivity, left ventricular pressure development, and metabolic function.

CONCLUSIONS

Native cardiac extracellular matrix scaffolds maintain matrix components and structure to support the seeding and engraftment of human induced pluripotent stem cell-derived cardiomyocytes and enable the bioengineering of functional human myocardial-like tissue of multiple complexities.

Acute reductions in mechanical wall strain precede the formation of intimal hyperplasia in a murine model of arterial occlusive disease

OBJECTIVE

Intimal hyperplasia (IH) continues to plague the durability of vascular interventions. Employing a validated murine model, ultrasound biomicroscopy, and speckle-tracking algorithms, we tested the hypothesis that reduced cyclic arterial wall strain results in accentuated arterial wall IH.

METHODS

A 9-0 suture was tied around the left mouse (n = 10) common carotid artery and a 35-gauge (outer diameter = 0.14 mm) blunt mandrel. We previously showed that mandrel removal results in a ∼78% reduction in diameter and ∼85% reduction in flow, with subsequent delayed induction of IH by day 28. Preoperative, postoperative day-4 (before measurable IH), and postoperative day-27 circumferential wall strains were measured in locations 1, 2, and 3 mm proximal to the stenosis and in the same locations on the contralateral (nonstenosed) carotid. At postoperative day 28, arteries were perfusion fixed and arterial wall morphology was assessed microscopically in the same regions.

RESULTS

Strains were the same in all locations preoperatively. Wall strain was decreased in all regions proximal to the stenosis by day 4 (0.26 ± 0.01 to 0.11 ± 0.02; P < .001), while strains remained unchanged for the contralateral artery (P = .45). No statistical regional differences in mean strain or IH were noted at any time point for the experimental or contralateral artery. Based on the median, regions were divided into those with low strain (≤0.1) and high strain (>0.1). Average preoperative strains in both groups were the same (0.27 ± 0.09 and 0.27 ± 0.08). All segments in the low-strain group (n = 13) demonstrated significant IH formation by day 28, while only 31% of the high strain group demonstrated any detectable IH at day 28. (Mean low-strain intimal thickness = 32 ± 20 μm, high strain = 8.0 ± 16 μm; P < .01). Changes in cross-sectional area at diastole drove the reduction in strain in the low-strain group, increasing significantly from preoperatively to day 4 (P = .04), while lumen cross-section at systole remained unchanged (P = .46). Cross-sectional area at diastole and systole in the high-strain group remained unchanged from preoperatively to day 4 (P = .67).

CONCLUSIONS

Early reduction in arterial wall strain is associated with subsequent development of hemodynamically induced IH.

Abstract 270: Acute Reductions in Wall Strain Precede Formation of Intimal Hyperplasia

Intimal hyperplasia (IH) remains the major culprit in revascularization failures. We aimed to unravel relationships between acute changes in circumferential arterial wall strain and genesis of IH.

Methods

To induce IH, we employed a validated model using a 9-0 nylon suture tie around the distal mouse common carotid artery (n=10) and an external 35-gauge needle mandrel (OD=0.14mm), with subsequent removal of the mandrel to create a distal common carotid focal stenosis (~78% lumen diameter/~85% flow reduction). Wall strains were measured in three, 1 mm wide regions along the vessel proximal to the focal stenosis at pre-op day 1 and at post-op day 4 (before detectable IH) using Vevo 2100 ultrasonography with VevoVasc software. At post-op day 28, arteries were perfusion fixed and IH was assessed in the same regions as those where strain was analyzed. Strain and morphology were also assessed in the contralateral control artery.

Results

Decreased wall strain was noted in all regions proximal to the focal stenosis from 0.26 ± 0.01 to 0.11 ± 0.02 (p<0.001) with no change in the control artery from pre-op to post-op day 4 (p=0.45). Based on a strain level histogram, vessels were divided into groups with strain ≤0.1 and >0.1. All segments (n = 13) with wall strain ≤0.1 at post-op day 4 had significant IH at day 28. In regions with strains >0.1 at day 4, only 30% had IH at day 28. The average pre-op strains were identical in >0.1 and ≤0.1 strain groups (0.27 ± 0.09 and 0.27 ± 0.08). Mean intimal thickness in vessels with strain ≤0.1 was 32 ± 20 μm, significantly greater than 8.0 ± 16 μm in the group with strain >0.1 (p<0.01). To further understand the mechanisms underlying changes in strain, systolic and diastolic lumen areas were assessed. Although systolic lumen areas in both >0.1 and ≤0.1 groups remained unchanged from pre-op to post-op day 4 (p=0.46), diastolic area was significantly increased in regions with post-op day 4 strain ≤0.1 (p=0.04) but remained unchanged in mice with post-op day 4 strain >0.1 (p=0.67).

Conclusions

Acute reduction in wall strain precedes the formation of IH in this murine model and this change is primarily caused by an increase in diastolic lumen area. Manipulations of wall strain offer a strategy to prevent and attenuate occlusive IH lesions after revascularizations.

A simplified murine intimal hyperplasia model founded on a focal carotid stenosis

Murine models offer a powerful tool for unraveling the mechanisms of intimal hyperplasia and vascular remodeling, although their technical complexity increases experimental variability and limits widespread application. We describe a simple and clinically relevant mouse model of arterial intimal hyperplasia and remodeling. Focal left carotid artery (LCA) stenosis was created by placing 9-0 nylon suture around the artery using an external 35-gauge mandrel needle (middle or distal location), which was then removed. The effect of adjunctive diet-induced obesity was defined. Flowmetry, wall strain analyses, biomicroscopy, and histology were completed. LCA blood flow sharply decreased by ∼85%, followed by a responsive right carotid artery increase of ∼71%. Circumferential strain decreased by ∼2.1% proximal to the stenosis in both dietary groups. At 28 days, morphologic adaptations included proximal LCA intimal hyperplasia, which was exacerbated by diet-induced obesity. The proximal and distal LCA underwent outward and negative inward remodeling, respectively, in the mid-focal stenosis (remodeling indexes, 1.10 and 0.53). A simple, defined common carotid focal stenosis yields reproducible murine intimal hyperplasia and substantial differentials in arterial wall adaptations. This model offers a tool for investigating mechanisms of hemodynamically driven intimal hyperplasia and arterial wall remodeling.

Development and characterization of a 3D multicell microtissue culture model of airway smooth muscle

Airway smooth muscle (ASM) cellular and molecular biology is typically studied with single-cell cultures grown on flat 2D substrates. However, cells in vivo exist as part of complex 3D structures, and it is well established in other cell types that altering substrate geometry exerts potent effects on phenotype and function. These factors may be especially relevant to asthma, a disease characterized by structural remodeling of the airway wall, and highlights a need for more physiologically relevant models of ASM function. We utilized a tissue engineering platform known as microfabricated tissue gauges to develop a 3D culture model of ASM featuring arrays of ∼0.4 mm long, ∼350 cell "microtissues" capable of simultaneous contractile force measurement and cell-level microscopy. ASM-only microtissues generated baseline tension, exhibited strong cellular organization, and developed actin stress fibers, but lost structural integrity and dissociated from the cantilevers within 3 days. Addition of 3T3-fibroblasts dramatically improved survival times without affecting tension development or morphology. ASM-3T3 microtissues contracted similarly to ex vivo ASM, exhibiting reproducible responses to a range of contractile and relaxant agents. Compared with 2D cultures, microtissues demonstrated identical responses to acetylcholine and KCl, but not histamine, forskolin, or cytochalasin D, suggesting that contractility is regulated by substrate geometry. Microtissues represent a novel model for studying ASM, incorporating a physiological 3D structure, realistic mechanical environment, coculture of multiple cells types, and comparable contractile properties to existing models. This new model allows for rapid screening of biochemical and mechanical factors to provide insight into ASM dysfunction in asthma.

A novel suture-based method for efficient transplantation of stem cells

Advances in regenerative medicine have improved the potential of using cellular therapy for treating several diseases. However, the effectiveness of new cellular therapies is largely limited by low cell engraftment and inadequate localization. To improve on these limitations, we developed a novel delivery mechanism using cell-seeded biological sutures. We demonstrate the ability of cell-seeded biological sutures to efficiently implant human mesenchymal stem cells (hMSCs) to specific regions within the beating heart; a tissue known to have low cell retention and engraftment shortly after delivery. Cell-seeded biological sutures were developed by bundling discrete microthreads extruded from extracellular matrix proteins, attaching a surgical needle to the bundle and seeding the bundle with hMSCs. During cell preparation, hMSCs were loaded with quantum dot nanoparticles for cell tracking within the myocardium. Each biological suture contained an average of 5903 ± 1966 hMSCs/cm suture length. Delivery efficiency was evaluated by comparing cell-seeded biological suture implantation with intramyocardial (IM) cell injections (10,000 hMSCs in 35 μL) into the left ventricle of normal, noninfarcted rat hearts after 1 h. Delivery efficiency of hMSCs by biological sutures (63.6 ± 10.6%) was significantly higher than IM injection (11.8 ± 6.2%; p < 0.05). Cell-tracking analysis indicated suture-delivered hMSCs were found throughout the thickness of the ventricular myocardium: along the entire length of the biological suture track, localizing closely with native myocardium. These results suggest cell-seeded biological sutures can deliver cells to the heart more efficiently than conventional methods, demonstrating an effective delivery method for implanting cells in soft tissue.

Fibrin microthreads support mesenchymal stem cell growth while maintaining differentiation potential

We developed a method to produce discrete fibrin microthreads, which can be seeded with human mesenchymal stem cells (hMSCs) and used as a suture to enhance the efficiency and localization of cell delivery. To assess the efficacy of fibrin microthreads to support hMSC attachment, proliferation, and survival, microthreads (100 μm diameter per microthread) were bundled together, seeded with 50,000 hMSCs for 2 h, and cultured for 5 days. Cell density on microthread bundles increased over time in culture to a maximum average density of 731 ± 101 cells/mm(2) after 5 days. A LIVE/DEAD assay confirmed that the cells were viable, and Ki-67 staining verified hMSC proliferation. In addition, functional differentiation assays demonstrated that hMSCs cultured on microthreads retained their ability to differentiate into adipocytes and osteocytes. The results of this study demonstrate that fibrin microthreads support hMSC viability and proliferation, while maintaining their multipotency. We anticipate that these cell-seeded fibrin microthreads will serve as a platform technology to improve localized delivery and engraftment of viable cells to damaged tissue.

Increased myocyte content and mechanical function within a tissue-engineered myocardial patch following implantation

During the past few years, studies involving the implantation of stem cells, chemical factors, and scaffolds have demonstrated the ability to augment the mammalian heart's native regenerative capacity. Scaffolds comprised of extracellular matrix (ECM) have been used to repair myocardial defects. These scaffolds become populated with myocytes and provide regional contractile function, but quantification of the myocyte population has not yet been conducted. The purpose of this study was to quantitate the myocyte content within the ECM bioscaffold and to correlate this cell population with the regional mechanical function over time. Xenogenic ECM scaffolds derived from porcine urinary bladder were implanted into a full-thickness, surgically induced, right ventricular-free wall defect in a dog model. Zero, 2, and 8 weeks following implantation, regional function and myocyte content were determined in each patch region. Regional function did not significantly increase from 0 to 2 weeks. At 8 weeks, however, regional stroke work increased to 3.7 +/- 0.7% and systolic contraction increased to 4.4 +/- 1.2%. The myocyte content also significantly increased during that period generating a linear relationship between regional function and myocyte content. In conclusion, ECM used as a myocardial patch increases both the regional function and the myocyte content over time. The mechanical function generated in the patch region is correlated with the quantity of local tissue myocytes.

Enhanced recovery of mechanical function in the canine heart by seeding an extracellular matrix patch with mesenchymal stem cells committed to a cardiac lineage

The need to regenerate tissue is paramount, especially for the heart that lacks the ability to regenerate after injury. The urinary bladder extracellular matrix (ECM), when used to repair a right ventricular defect, successfully regenerated some mechanical function. The objective of the current study was to determine whether the regenerative effect of ECM could be improved by seeding the patch with human mesenchymal stem cells (hMSCs) enhanced to differentiate down a cardiac linage. hMSCs were used to form three-dimensional spheroids. The expression of cardiac proteins was determined in cells exposed to the spheroid formation and compared with nonmanipulated hMSCs. To determine whether functional calcium channels were present, the cells were patch clamped. To evaluate the ability of these cells to regenerate mechanical function, the spheroids were seeded on ECM and then implanted into the canine heart to repair a full-thickness right ventricular defect. As a result, many of the cells spreading from the spheroids expressed cardiac-specific proteins, including sarcomeric alpha-actinin, cardiotin, and atrial natriuretic peptide, as well as the cell cycle markers cyclin D1 and proliferating cell nuclear antigen. A calcium current similar in amplitude to that of ventricular myocytes was present in 16% of the cells. The cardiogenic cell-seeded scaffolds increased the regional mechanical function in the canine heart compared with the unmanipulated hMSC-seeded scaffolds. In addition, the cells prelabeled with fluorescent markers demonstrated myocyte-specific actinin staining with sarcomere spacing similar to that of normal myocytes. In conclusion, the spheroid-derived cells express cardiac-specific proteins and demonstrate a calcium current similar to adult ventricular myocytes. When these cells are implanted into the canine heart, some of these cells appear striated and mechanical function is improved compared with the unmanipulated hMSCs. Further investigation will be required to determine whether the increased mechanical function is due to a differentiation of the cardiogenic cells to myocytes or to other effects.

Replacing damaged myocardium

Heart failure survival after diagnosis has barely changed for more than half a century. Recently, investigation has focused on differentiation of stem cells in vitro and their delivery for use in vivo as replacement cardiac contractile elements. Here we report preliminary results using mesenchymal stem cells partially differentiated to a cardiac lineage in vitro. When delivered to the canine heart on an extracellular matrix patch to replace a full-thickness ventricular defect in vivo, they improve regional mechanical function. The delivered cells were also tracked, and some became myocytes with mature sarcomeres.

Finding fluorescent needles in the cardiac haystack: tracking human mesenchymal stem cells labeled with quantum dots for quantitative in vivo three-dimensional fluorescence analysis

Stem cells show promise for repair of damaged cardiac tissue. Little is known with certainty, however, about the distribution of these cells once introduced in vivo. Previous attempts at tracking delivered stem cells have been hampered by the autofluorescence of host tissue and limitations of existing labeling techniques. We have developed a novel loading approach to stably label human mesenchymal stem cells with quantum dot (QD) nanoparticles. We report the optimization and validation of this long-term tracking technique and highlight several important biological applications by delivering labeled cells to the mammalian heart. The bright QD crystals illuminate exogenous stem cells in histologic sections for at least 8 weeks following delivery and permit, for the first time, the complete three-dimensional reconstruction of the locations of all stem cells following injection into the heart. Disclosure of potential conflicts of interest is found at the end of this article.

Mesenchymal stem cells support migration, extracellular matrix invasion, proliferation, and survival of endothelial cells in vitro

We investigated effects of the paracrine factors secreted by human mesenchymal stem cells (hMSCs) on endothelial cell migration, extracellular matrix invasion, proliferation, and survival in vitro. Human mesenchymal stem cells were cultured as a monolayer or as three-dimensional aggregates in hanging drops (hMSC spheroids). We performed analysis of paracrine factors in medium conditioned by a monolayer of hMSCs and hMSC spheroids. Concentrations of vascular endothelial growth factor (VEGF), basic fibroblast growth factor, angiogenin, procathepsin B, interleukin (IL)-11, and bone morphogenic protein 2 were increased 5-20 times in medium conditioned by hMSC spheroids, whereas concentrations of IL-6, IL-8, and monocyte hemoattractant protein-1 were not increased. Concentrations of VEGF and angiogenin in medium conditioned by hMSC spheroids showed a weak dependence on the presence of serum, which allows serum-free conditioned medium with elevated concentrations of angiogenic cytokines to be obtained. Medium conditioned by hMSC spheroids was more effective in stimulation of umbilical vein endothelial cell proliferation, migration, and basement membrane invasion than medium conditioned by a monolayer of hMSCs. This medium also promotes endothelial cell survival in vitro. We suggest that culturing of hMSCs as three-dimensional cellular aggregates provides a method to concentrate proangiogenic factors secreted by hMSCs and allows for reduction of serum concentration in conditioned medium. Our data support the hypothesis that hMSCs serve as trophic mediators for endothelial cells. Disclosure of potential conflicts of interest is found at the end of this article.

Accuracy and reproducibility of a subpixel extended phase correlation method to determine micron level displacements in the heart

Future treatment of heart disease may involve local perturbations of mechanical function, such as intramyocardial injections of angiogenic growth factors or progenitor cells. This necessitates an accurate measurement technique to determine regional heart function. We have previously developed a method to determine regional heart function using a phase correlation algorithm. However, in determining regional function over a single heartbeat it is necessary to sum displacements between many images. We have therefore incorporated a subpixel algorithm that models the result of phase correlation as a sinc function in order to increase the accuracy of our technique. This method, which we have named high density mapping (HDM), determines the subpixel displacement of 64 x 64 pixel regions from images of the heart. To determine the accuracy and precision of the technique, a high contrast image of a heart was digitally shifted 1, 2 or 3 pixels. The original and shifted images were then downsampled four times resulting in 0.25, 0.50 or 0.75 pixel shifts between the original and shifted images. The average accuracy of HDM in the digitally shifted images was 0.06 pixels, with a precision of 0.08 pixels. Effectiveness of HDM in characterization of deformation was also assessed in digitally stretched images. Error in quantification of strain was found to be less than 3.5% of the calculated strain. In an additional set of experiments, in which accuracy was determined using physical motion instead of digital shifting and downsampling, a speckle pattern was displaced by known distances using a micromanipulator, such that the displacement between the captured images was 0.5 pixels. These data demonstrated an accuracy of 0.09 pixels and a precision of 0.02 pixels. Finally, as HDM is used to determine the regional stroke work index (RSW) in beating hearts, the repeatability of using this method to compute RSW was assessed. RSW, the integral of intraventricular pressure with respect to unitless regional area, where end diastolic area was normalized to unity, was assessed in consecutive beats from four different hearts. The average standard deviation of RSW was 0.098 mmHg. Uncertainty analysis determined the maximum error of RSW to be +/-0.41 mmHg, approximately two-thirds of the measured biologic variability. These data demonstrate the ability of HDM to accurately and reproducibly measure displacement and regional function in the beating heart.

The use of extracellular matrix as an inductive scaffold for the partial replacement of functional myocardium

Regenerative medicine approaches for the treatment of damaged or missing myocardial tissue include cell-based therapies, scaffold-based therapies, and/or the use of specific growth factors and cytokines. The present study evaluated the ability of extracellular matrix (ECM) derived from porcine urinary bladder to serve as an inductive scaffold for myocardial repair. ECM scaffolds have been shown to support constructive remodeling of other tissue types including the lower urinary tract, the dermis, the esophagus, and dura mater by mechanisms that include the recruitment of bone marrow-derived progenitor cells, angiogenesis, and the generation of bioactive molecules that result from degradation of the ECM. ECM derived from the urinary bladder matrix, identified as UBM, was configured as a single layer sheet and used as a biologic scaffold for a surgically created 2 cm2 full-thickness defect in the right ventricular free wall. Sixteen dogs were divided into two equal groups of eight each. The defect in one group was repaired with a UBM scaffold and the defect in the second group was repaired with a Dacron patch. Each group was divided into two equal subgroups (n = 4), one of which was sacrificed 15 min after surgical repair and the other of which was sacrificed after 8 weeks. Global right ventricular contractility was similar in all four subgroups groups at the time of sacrifice. However, 8 weeks after implantation the UBM-treated defect area showed significantly greater (p < 0.05) regional systolic contraction compared to the myocardial defects repaired with by Dacron (3.3 +/- 1.3% vs. -1.8 +/- 1.1%; respectively). Unlike the Dacron-repaired region, the UBM-repaired region showed an increase in systolic contraction over the 8-week implantation period (-4.2 +/- 1.7% at the time of implantation vs. 3.3 +/- 1.3% at 8 weeks). Histological analysis showed the expected fibrotic reaction surrounding the embedded Dacron material with no evidence for myocardial regeneration. Histologic examination of the UBM scaffold site showed cardiomyocytes accounting for approximately 30% of the remodeled tissue. The cardiomyocytes were arranged in an apparently randomly dispersed pattern throughout the entire tissue specimen and stained positive for alpha- sarcomeric actinin and Connexin 43. The thickness of the UBM graft site increased greatly from the time of implantation to the 8-week sacrifice time point when it was approximately the thickness of the normal right ventricular wall. Histologic examination suggested complete degradation of the originally implanted ECM scaffold and replacement by host tissues. We conclude that UBM facilitates a constructive remodeling of myocardial tissue when used as replacement scaffold for excisional defects.

High resolution mechanical function in the intact porcine heart: mechanical effects of pacemaker location

The necessity to quantify the mechanical function with high spatial resolution stemmed from the advancement of myocardial salvaging techniques. Since these therapies are localized interventions, a whole field technique with high spatial resolution was needed to differentiate the normal, diseased, and treated myocardium. We developed a phase correlation algorithm for measuring myocardial displacement at high spatial resolution and to determine the regional mechanical function in the intact heart. Porcine hearts were exposed and high contrast microparticles were placed on the myocardium. A pressure transducer, inserted into the left ventricle, synchronized the pressure (LVP) with image acquisition using a charge-coupled device camera. The deformation of the myocardium was measured with a resolution of 0.58+/-0.04 mm. Within the region of interest (ROI), regional stroke work (RSW), defined as the integral of LVP with respect to regional area, was determined on average at 21 locations with a resolution of 27.1+/-2.7 mm2. To alter regional mechanical function, the heart was paced at three different locations around the ROI. Independent of the pacemaker location, RSW decreased in the ROI. In addition, a gradient of increasing RSW in the outward direction radiating from the pacemaker was observed in all pacing protocols. These data demonstrated the ability to determine regional whole field mechanical function with high spatial resolution, and the significant alterations induced by electrical pacing.

Inflammation effects on the electrical properties of atrial tissue and inducibility of postoperative atrial fibrillation

BACKGROUND

Atrial fibrillation is the most common complication after cardiac surgery. Postoperative atrial fibrillation (PAF) has been shown to increase length of stay, morbidity, and mortality. Because the clinical behavior of PAF parallels that of inflammation following surgery, we investigated the effect of the inflammatory mediator arachidonic acid on the electrical behavior of normal atrial tissue in vitro and assessed the efficacy of the topical application of anti-inflammatory drugs at suppressing PAF in an animal model.

METHODS

To study changes in electrical behavior from inflammation, the conduction properties of six normal canine right atrial appendages were quantified as a function of the direction of impulse propagation with and without 80 mum arachidonic acid. To study the effect of topical anti-inflammatory drugs, 24 adult mongrel dogs were prepared according to the model of sterile talc pericarditis. Nine dogs received talc alone (T), seven received talc combined with 600 mg ibuprofen (T + I), and eight received talc combined with 10 mg methylprednisolone (T + M). Three days following preparation, programmed electrical stimulation was performed to quantify conduction characteristics and to attempt the induction of atrial fibrillation (AF).

RESULTS

In vitro, arachidonic acid produced an anisotropic and rapidly reversible 36.1 +/- 3.4% (P = 0.01) decrease in conduction velocity transverse to the long axis only. In vivo, both ibuprofen and methylprednisolone significantly reduced the incidence of sustained AF (from 56 to 0% T + I and 12% T + M, respectively, P = 0.02). No differences in conduction velocities or refractory periods were seen during sinus rhythm among the groups.

CONCLUSIONS

Acute inflammation as mimicked by arachidonic acid slows conduction anisotropically, mainly transverse to the long axis of the atrial myocardial fibers. This may set the stage for reentry. Preventing inflammation in vivo by the topical application of anti-inflammatory drugs supports this hypothesis, suggesting a possible role for inflammation in the genesis of postoperative atrial fibrillation and shedding light on the mechanism underlying PAF.

Tissue-engineered myocardial patch derived from extracellular matrix provides regional mechanical function

BACKGROUND

Extracellular matrix (ECM), a tissue-engineered scaffold, recently demonstrated cardiomyocyte population after myocardial implantation. Surgical restoration of myocardium frequently uses Dacron as a myocardial patch. We hypothesized that an ECM-derived myocardial patch would provide a mechanical benefit not seen with Dacron.

METHODS AND RESULTS

Using a canine model, a full thickness defect in the right ventricle was repaired with either Dacron or ECM. A third group had no surgery and determined baseline RV function. Eight weeks later, global systolic function was assessed by the preload recruitable stroke work relationship. Regional systolic function was measured by systolic area contraction (SAC), calculated by high density mechanical mapping. Tau was used to assess global diastolic function. Recoil rate and diastolic shear were used as measures of regional diastolic function. After functional data acquisition, tissue was fixed for histological evaluation. Global systolic and diastolic functions were similar at baseline and after ECM and Dacron implantation. Regional systolic function was greater in the ECM group compared with the Dacron group (SAC: 4.1+/-0.9% versus -1.8+/-1.1, P<0.05). Regional diastolic function was also greater in the ECM group (recoil rate (degrees sec(-1)): -44+/-7 versus -17+/-2, ECM versus Dacron; P<0.05). Immunohistochemical analysis revealed cardiomyocytes in the ECM implant region, a finding not seen with Dacron.

CONCLUSIONS

At 8 weeks, an ECM-derived tissue-engineered myocardial patch provides regional mechanical function, likely related to cardiomyocyte population. These results are in sharp contrast to Dacron, a commonly used myocardial patch.

Effects of Ischemia on Epicardial Deformation in the Passive Rabbit Heart

BACKGROUND

Surgically induced ischemia in the arrested heart can result in changes in the mechanical properties of the myocardium. Regions of ischemia may be characterized based on the amount of epicardial deformation for a given load. Computer aided speckle interferometry (CASI), which tracks the movement of clusters of particles, is developed as a technique for measuring epicardial deformation, thereby determining the perfusion status of the passive heart.

MATERIALS AND METHODS

Silicone carbide particles and retroreflective beads were dispersed randomly onto the epicardial surface of 11 isolated rabbit hearts to form speckle images. The hearts were arrested with hyperkalemic Krebs-Henseleit buffered solution. Each heart was then exposed to a series of intracavitary pressures, and at each pressure speckle images were acquired with a charge-coupled device (CCD) camera. Nine hearts were exposed to global ischemia, and two hearts were exposed to regional ischemia by occluding the second diagonal branch of the left anterior descending artery (LAD). The hearts were again loaded and the speckle images were acquired. CASI was used to determine the distribution of deformation field.

RESULTS

CASI was able to determine displacements with a spatial resolution of about 50 microns. Global ischemia resulted in a significant increase in the maximum principle strain and the first invariant of the 2-D strain tensor. In the regionally ischemic heart, a large difference in deformation between the ischemic and perfused regions was clearly observed.

CONCLUSION

Based on epicardial deformation, CASI is able to distinguish between perfused and ischemic myocardium, with a spatial resolution of 50 microns.

Both metabolic inhibition and mitochondrial K(ATP) channel opening are myoprotective and initiate a compensatory sarcolemmal outward membrane current

BACKGROUND

Blockade of oxidative phosphorylation may activate ATP sensitive mitochondrial potassium (mitoK(ATP)) channels. We examined whether both metabolic inhibition and mitoK(ATP) channel openers protect both the whole organ and isolated cells from ischemia.

METHODS AND RESULTS

Using a Langendorff preparation, one group of isolated rabbit hearts were exposed to ischemic preconditioning (IPC) via 2 episodes of flow interruption. The second group of hearts was preconditioned with 2 episodes of either the metabolic inhibitor, sodium cyanide (NaCN), or the mitoK(ATP) channel opener, diazoxide. The third group of hearts was exposed to the mitoK(ATP) channel inhibitor, 5-hydroxydecanoic acid (5-HD) prior to preconditioning with NaCN, diazoxide or IPC. Controls had no drug infused. Then, ischemia was induced in all hearts by left anterior descending coronary artery occlusion and infarct size was determined. Compared with controls (40+/-3%), infarct size was significantly reduced in hearts preconditioned with NaCN, diazoxide or IPC (18+/-3%, 26+/-3%, 21+/-2%, respectively; P<0.05 versus control). These reductions were reversed by 5-HD (36+/-3%, 33+/-2%, 37+/-2%; NaCN, diazoxide, IPC, respectively). Secondly, whole cell patch clamped isolated guinea pig ventricular myocytes were preconditioned with 2 episodes of either NaCN or diazoxide followed by Tyrodes perfusion with membrane potential set to -70 mV. Control cells were exposed to Tyrodes solution. All cells were then clamped to -20 mV and exposed to NaCN, which caused induction of an outward potassium current. Compared with controls, the average time to induction of the outward current was significantly reduced in cells preconditioned with either brief application of NaCN (11.6+/-1.8 versus 5.1+/-1.0 minutes, control versus NaCN, P<0.05) or diazoxide (5.5+/-1.4 versus 2.0+/-0.8 minutes, control versus diazoxide, P<0.05).

CONCLUSIONS

Preconditioning protects the heart through mitoK(ATP). This protection also alters a surface membrane current, which may be important in myocardial protection.

Myocardial recovery from ischemia is impaired in CD36-null mice and restored by myocyte CD36 expression or medium-chain fatty acids

Long-chain fatty acid uptake, which provides a large part of myocardial energy, is impaired in human and murine hearts deficient in the membrane fatty acid translocase, FAT/CD36. We examined myocardial function in CD36-null mice using the working heart. Fatty acid oxidation and stores of glycogen, triglycerides, and ATP were reduced in CD36-deficient hearts and were restored to WT levels by rescue of myocyte CD36. Under normal perfusion conditions, CD36-null hearts had similar cardiac outputs and end-diastolic pressures as WT or transgenic hearts. After 6 min of ischemia, cardiac output decreased by 41% and end diastolic pressure tripled for CD36-null hearts, with no significant changes in WT or transgenic hearts. Null hearts also failed more frequently after ischemia as compared with WT or transgenics. To dissect out contribution of fatty acid uptake, a perfusate-lacking fatty acids was used. This decreased cardiac output after ischemia by 30% in WT hearts as compared with 50% for CD36-deficient hearts. End diastolic pressure, a negative index of myocardial performance, increased after ischemia in all heart types. Addition to the perfusate of a medium-chain fatty acid (caprylic acid) that does not require CD36 for uptake alleviated poor ischemic tolerance of CD36-null hearts. In summary, recovery from ischemia is compromised in CD36-deficient hearts and can be restored by CD36 rescue or by supplying medium-chain fatty acids. It would be important to determine whether the findings apply to the human situation where polymorphisms of the CD36 gene are relatively common.

Gap junction uncoupling protects the heart against ischemia

BACKGROUND

Many stimuli can successfully protect the heart against ischemia. We investigated whether gap junction uncoupling before ischemia was myoprotective. We also studied the function of the adenosine triphosphate-dependent potassium channel, which has been implicated in the mechanism of pharmacologic preconditioning, with respect to gap junction physiology.

METHODS

Twenty-eight rabbit hearts were placed on a Langendorff perfusion apparatus. Five were given a 5-minute infusion of 1 mmol/L heptanol (a gap junction uncoupler), 5 were given 10 micromol/L 2,3-butanedione monoxime (an electromechanical uncoupler), and 6 were given no drug. The left anterior descending coronary artery was then occluded for 1 hour and reperfused for 2 hours. Six hearts received 10 micromol/L glybenclamide before heptanol to evaluate the role of the adenosine triphosphate-dependent potassium channel. Six hearts underwent ischemic preconditioning with 2 cycles of 5 minutes of global ischemia and reperfusion. Action-potential duration of the ischemic zone, left ventricular developed pressure, and coronary flow were measured continuously. Infarct size was determined at the end of reperfusion.

RESULTS

Heptanol significantly reduced infarct size (from 46% +/- 2% to 22% +/- 5%, P <.01), an effect that was not prevented by glybenclamide. Butanedione monoxime decreased developed pressure but did not significantly reduce infarct size (46% +/- 5% vs 46% +/- 2%, P = not significant). There were no differences among groups with regard to developed pressure or action-potential duration.

CONCLUSION

Directly blocking gap junctions preconditions the heart. This protection is not a direct result of a decrease in developed pressure before a prolonged ischemic period nor is it achieved through a mechanism involving the adenosine triphosphate-dependent potassium channel.

Isoform-specific stimulation of cardiac Na/K pumps by nanomolar concentrations of glycosides

It is well-known that micromolar to millimolar concentrations of cardiac glycosides inhibit Na/K pump activity, however, some early reports suggested nanomolar concentrations of these glycosides stimulate activity. These early reports were based on indirect measurements in multicellular preparations, hence, there was some uncertainty whether ion accumulation/depletion rather than pump stimulation caused the observations. Here, we utilize the whole-cell patch-clamp technique on isolated cardiac myocytes to directly measure Na/K pump current (I(P)) in conditions that minimize the possibility of ion accumulation/depletion causing the observed effects. In guinea pig ventricular myocytes, nanomolar concentrations of dihydro-ouabain (DHO) caused an outward current that appeared to be due to stimulation of I(P) because of the following: (1) it was absent in 0 mM [K(+)](o), as was I(P); (2) it was absent in 0 mM [Na(+)](i), as was I(P); (3) at reduced [Na(+)](i), the outward current was reduced in proportion to the reduction in I(P); (4) it was eliminated by intracellular vanadate, as was I(P). Our previous work suggested guinea pig ventricular myocytes coexpress the alpha(1)- and alpha(2)-isoforms of the Na/K pumps. The stimulation of I(P) appears to be through stimulation of the high glycoside affinity alpha(2)-isoform and not the alpha(1)-isoform because of the following: (1) regulatory signals that specifically increased activity of the alpha(2)-isoform increased the amplitude of the stimulation; (2) regulatory signals that specifically altered the activity of the alpha(1)-isoform did not affect the stimulation; (3) changes in [K(+)](o) that affected activity of the alpha(1)-isoform, but not the alpha(2)-isoform, did not affect the stimulation; (4) myocytes from one group of guinea pigs expressed the alpha(1)-isoform but not the alpha(2)-isoform, and these myocytes did not show the stimulation. At 10 nM DHO, total I(P) increased by 35 +/- 10% (mean +/- SD, n = 18). If one accepts the hypothesis that this increase is due to stimulation of just the alpha(2)-isoform, then activity of the alpha(2)-isoform increased by 107 +/- 30%. In the guinea pig myocytes, nanomolar ouabain as well as DHO stimulated the alpha(2)-isoform, but both the stimulatory and inhibitory concentrations of ouabain were approximately 10-fold lower than those for DHO. Stimulation of I(P) by nanomolar DHO was observed in canine atrial and ventricular myocytes, which express the alpha(1)- and alpha(3)-isoforms of the Na/K pumps, suggesting the other high glycoside affinity isoform (the alpha(3)-isoform) also was stimulated by nanomolar concentrations of DHO. Human atrial and ventricular myocytes express all three isoforms, but isoform affinity for glycosides is too similar to separate their activity. Nevertheless, nanomolar DHO caused a stimulation of I(P) that was very similar to that seen in other species. Thus, in all species studied, nanomolar DHO caused stimulation of I(P), and where the contributions of the high glycoside affinity alpha(2)- and alpha(3)-isoforms could be separated from that of the alpha(1)-isoform, it was only the high glycoside affinity isoform that was stimulated. These observations support early reports that nanomolar concentrations of glycosides stimulate Na/K pump activity, and suggest a novel mechanism of isoform-specific regulation of I(P) in heart by nanomolar concentrations of endogenous ouabain-like molecules.

Computer aided speckle interferometry: a technique for measuring deformation of the surface of the heart

An investigation of the inhomogeneous and anisotropic properties of myocardium necessitates a whole field measurement technique with high spatial resolution. Computer aided speckle interferometry (CASI) may be applied to measuring deformation on the epicardial surface of the heart. Silicone carbide particles (approximately 40 microm in diameter) were sprinkled randomly onto the epicardial surface of isolated rabbit hearts. When illuminated with white light, speckles may be observed with a charge coupled device (CCD) camera. A balloon was placed in the left ventricle to control the intracavitary load on the arrested heart. To compare CASI to the "gold" standard technique of sonomicrometry, two ultrasonic transducers were implanted into the wall of the myocardium. Three hearts were exposed to various loading conditions, and at each condition speckle images were recorded. CASI was used to determine the distribution of displacement vectors (both direction and magnitude) in the region imaged by the CCD camera. Strain along the axis of the implanted transducers was determined with CASI and compared to that obtained with sonomicrometry. Strain determined from CASI and sonomicrometry produced equivalent results. Unlike sonomicrometry, whereby the displacement between two points with a relatively large gauge length is obtained, CASI is able to determine displacement vectors for hundreds of "points" within the same region. In conclusion, CASI produced equivalent results to those obtained from sonomicrometry (although not with the same temporal resolution), but it is a whole field deformation mapping technique that has a spatial resolution three orders of magnitude higher than that of sonomicrometry.

Ischemic but not pharmacological preconditioning requires protein synthesis

BACKGROUND

Ischemic preconditioning (IPC) and pharmacological preconditioning (PPC) have both been shown to confer cardioprotective effects. However, the role of protein synthesis in preconditioning is unclear.

METHODS AND RESULTS

Isolated rabbit hearts were treated with cycloheximide (CHx, 10 micromol/L), a protein synthesis inhibitor at the translational level, before 2 cycles of IPC (5 minutes of global ischemia/5 minutes of reperfusion, n=6) or PPC by pinacidil (PIN, 10 micromol/L; n=6), an ATP-sensitive potassium channel opener. Six rabbit hearts received actinomycin D (Act D, 20 micromol/L; n=6), a protein synthesis inhibitor at the transcriptional level, before IPC. The left anterior descending coronary artery was then occluded for 60 minutes and reperfused for 120 minutes. Control hearts received no treatment before prolonged ischemia (n=6). Left ventricular pressure, action potential duration, and coronary flow were measured. Infarct size is expressed as a percentage of the area at risk. IPC (n=6) and PIN (n=8) hearts experienced reduced infarct size compared with control hearts (22+/-3% and 27+/-2% versus 46+/-3%, IPC and PIN versus control; P:<0.01). Translational blockade (CHx) reversed the IPC infarct size reduction effect (22+/-3% versus 48+/-4%, IPC versus CHx+IPC; P:<0.01) but not the effects of pinacidil (27+/-2% versus 29+/-3%, PIN versus CHx+PIN; P:=NS). Transcriptional blockade (Act D) did not abolish the IPC effect (23+/-5% versus 22+/-3%, Act D+IPC versus IPC; P:=NS). There were no significant differences in electromechanical function consequent to CHx and Act D treatment.

CONCLUSIONS

These findings suggest an important role for protein synthesis in the mechanism for IPC-mediated protection at the translational level, which may be different from PPC.

Preconditioning with PKC and the ATP-sensitive potassium channels: a codependent relationship

BACKGROUND

Both potassium channel openers and protein kinase C have been shown to independently elicit the myoprotective preconditioning response. However, the in vivo dependency between the two is unknown.

METHODS

Thirty-seven sheep were divided into seven groups; animals received no pretreatment, pinacidil, pinacidil and potassium channel opener blocker glibenclamide, protein kinase C activator 4beta-phorbol-12,13-dibutyrate (PDBu), or PDBu and protein kinase C blocker chelerythrine. The last two groups underwent opposite blockade, chelerythrine + pinacidil, or glibenclamide + PDBu. All groups underwent 60 minutes of regional ischemia followed by 180 minutes of reperfusion. Regional function was assessed throughout the experiment, and at the conclusion of the study the infarct size (as a percentage of the area at risk) was determined.

RESULTS

Infarct size decreased in the groups receiving only pinacidil or PDBu (control: 54%+/-3%, pinacidil: 25% +/-2%, PDBu: 21%+/-3%; p<0.05 pinacidil or PDBu versus control). This preconditioning protection was lost when the direct blocker was given (58%+/-5%, glibenclamide + pinacidil; 70%+/-6%, chelerythrine + PDBu; p = not significant versus control). The preconditioning response was again attenuated when the opposite blockers were given (64%+/-5%, chelerythrine + pinacidil; 63%+/-1%, glibenclamide + PDBu; p = not significant versus control). There was no significant difference in regional function.

CONCLUSIONS

This study shows that both protein kinase C and potassium channels are necessary and codependent for preconditioning in the in vivo heart.

Pharmacological preconditioning with the adenosine triphosphate-sensitive potassium channel opener pinacidil

BACKGROUND

Ischemic preconditioning (IPC) decreases infarct size after global or regional ischemia. Potassium channel openers also precondition but are subject to dose-limiting vasodilation. We compared the mechanical and electrophysiological effects of ischemic and pharmacological preconditioning in an isolated rabbit heart model.

METHODS

Rabbit hearts were preconditioned with either 10 micromol/L pinacidil alone (P-), 10 micromol/L pinacidil with 10 micromol/L phenylephrine (P+), or two cycles of global ischemia and reperfusion (IPC) before 1 hour of LAD occlusion. Left ventricular pressure, epicardial monophasic action potential duration (APD) and coronary flow were monitored throughout. Infarct size was determined at the end of reperfusion.

RESULTS

Regional ischemia uniformly decreased APD (p<0.05). During reperfusion, APDs were prolonged beyond preischemic values in all preconditioned groups (p<0.05). P- and P+ reduced the incidence of fibrillation. P- significantly increased coronary flow (+15%, p = 0.001), whereas IPC and P+ did not. However, IPC and P- significantly decreased systolic function (p<0.05) but P+ did not. In addition, IPC depressed diastolic function (p<0.05) but P- and P+ did not. Infarct size was reduced by all methods (p<0.05).

CONCLUSIONS

Pinacidil presents a safe and effective alternative to IPC for preserving the heart during regional ischemia. Its coronary vasodilatory effects are safely and effectively reversed by the addition of phenylephrine.

Amrinone preconditioning in the isolated perfused rabbit heart

BACKGROUND

Ischemic preconditioning (IPC) reduces infarct size in experimental preparations. IPC, however, is not without detrimental effects. We studied amrinone as a possible alternative to IPC.

METHODS

Isolated perfused rabbit hearts were given a 5-minute infusion of 10 micromol/L amrinone followed by a 5-minute washout (n = 6). The anterior descending artery was then occluded for 1 hour and reperfused for 1 hour. Six hearts underwent IPC, with two episodes of 5-minute global ischemia followed by 5-minute reperfusion before LAD occlusion; eight control hearts received no preconditioning. Left ventricular pressure and ischemic zone epicardial monophasic action potentials were continuously monitored.

RESULTS

IPC but not amrinone reduced peak pressure before anterior descending artery occlusion. Peak pressure fell significantly during ischemia and reperfusion in all hearts. End diastolic pressure rose significantly during reperfusion in control and IPC hearts but not in amrinone hearts. Action potentials shortened during ischemia in all hearts. They returned to preocclusion values in control hearts but lasted beyond preocclusion values in IPC and amrinone hearts. Both the incidences of ventricular fibrillation and infarct size were significantly reduced in amrinone hearts but not in IPC hearts.

CONCLUSIONS

Amrinone is not only a useful inotropic agent but is also a superior preconditioning agent when compared to IPC.

The role of nitric oxide, K(+)(ATP) channels, and cGMP in the preconditioning response of the rabbit

BACKGROUND

The role of nitric oxide (NO), K(+)(ATP) channels, and cyclic GMP (cGMP) in preconditioning is unknown.

MATERIAL AND METHODS

Isolated rabbit hearts were pretreated with the NO precursor L-arginine (L-Arg), both alone and after infusion of the NO synthetase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME). Guanylate cyclase inhibitor methylene blue (MB) was infused prior to L-Arg in a separate group of hearts. To contrast the mechanisms of NO preconditioning and potassium channel opener (PCO) preconditioning, we infused the PCO pinacidil after L-NAME and the PCO blocker glibenclamide before L-Arg. Control hearts had no drug infused. The LAD coronary artery was occluded for 1 h and reperfused for 1 h in all hearts. Action potential duration (APD(50)), coronary flow (CF), and left ventricular developed pressure (DP) were measured, and infarct size (IS) was determined and expressed as a percentage of the area at risk.

RESULTS

L-Arg prolonged APD(50) at 60 min of reperfusion (94 +/- 6 ms vs 69 +/- 2 ms (control) vs 70 +/- 2 ms (L-NAME) vs 74 +/- 3 ms (MB), P < 0.05). L-Arg reduced IS compared with control (24 +/- 2% vs 49 +/- 3%, P < 0.05); this was reversed by either L-NAME (53 +/- 4%, P < 0.05) or MB (43 +/- 3%, P < 0.05), but not by glibenclamide (20 +/- 4%), unlike the increase in CF during L-Arg infusion, which was blocked by glibenclamide. Pinacidil infusion decreased IS (26 +/- 2%), but this effect was blocked by L-NAME (53 +/- 7%, P < 0.05 vs pinacidil), although L-NAME did not blunt the increase in CF. There were no significant differences in DP among groups.

CONCLUSION

L-Arginine preconditions the heart through NO generation, and this response is mediated through a cGMP-dependent mechanism, but is independent of the K(+)(ATP) channels. Coronary vasodilation is mediated through a mechanism different from that responsible for cardiomyocyte preconditioning.