Sarcomere length-tension relationship of rat cardiac myocytes at lengths greater than optimum

Differences in Effects of Length-Dependent Regulation of Force and Ca2+ Transient in the Myocardial Trabeculae of the Rat Right Atrium and Ventricle

The comparative differences in the fundamental mechanisms of contractility regulation and calcium handling of atrial and ventricular myocardium remain poorly studied. An isometric force-length protocol was performed for the entire range of preloads in isolated rat right atrial (RA) and ventricular (RV) trabeculae with simultaneous measurements of force (Frank-Starling mechanism) and Ca²⁺ transients (CaT). Differences were found between length-dependent effects in RA and RV muscles: (a) the RA muscles were stiffer, faster, and presented with weaker active force than the RV muscles throughout the preload range; (b) the active/passive force-length relationships were almost linear for the RA and RV muscles; (c) the value of the relative length-dependent growth of passive/active mechanical tension did not differ between the RA and RV muscles; (d) the time-to-peak and amplitude of CaT did not differ between the RA and RV muscles; (e) the CaT decay phase was essentially monotonic and almost independent of preload in the RA muscles, but not in the RV muscles. Higher peak tension, prolonged isometric twitch, and CaT in the RV muscle may be the result of higher Ca²⁺ buffering by myofilaments. The molecular mechanisms that constitute the Frank-Starling mechanism are common in the rat RA and RV myocardium.

Synchrony of sarcomeric movement regulates left ventricular pump function in the in vivo beating mouse heart

Sarcomeric contraction in cardiomyocytes serves as the basis for the heart's pump functions. It has generally been considered that in cardiac muscle as well as in skeletal muscle, sarcomeres equally contribute to myofibrillar dynamics in myocytes at varying loads by producing similar levels of active and passive force. In the present study, we expressed α-actinin-AcGFP in Z-disks to analyze dynamic behaviors of sequentially connected individual sarcomeres along a myofibril in a left ventricular (LV) myocyte of the in vivo beating mouse heart. To quantify the magnitude of the contribution of individual sarcomeres to myofibrillar dynamics, we introduced the novel parameter "contribution index" (CI) to measure the synchrony in movements between a sarcomere and a myofibril (from -1 [complete asynchrony] to 1 [complete synchrony]). First, CI varied markedly between sarcomeres, with an average value of ∼0.3 during normal systole. Second, when the movements between adjacent sarcomeres were asynchronous (CI < 0), a sarcomere and the ones next to the adjacent sarcomeres and farther away moved in synchrony (CI > 0) along a myofibril. Third, when difference in LV pressure in diastole and systole (ΔLVP) was lowered to <10 mm Hg, diastolic sarcomere length increased. Under depressed conditions, the movements between adjacent sarcomeres were in marked asynchrony (CI, -0.3 to -0.4), and, as a result, average CI was linearly decreased in association with a decrease in ΔLVP. These findings suggest that in the left ventricle of the in vivo beating mouse heart, (1) sarcomeres heterogeneously contribute to myofibrillar dynamics due to an imbalance of active and passive force between neighboring sarcomeres, (2) the force imbalance is pronounced under depressed conditions coupled with a marked increase in passive force and the ensuing tug-of-war between sarcomeres, and (3) sarcomere synchrony via the distal intersarcomere interaction regulates the heart's pump function in coordination with myofibrillar contractility.

Increased Expression of N2BA Titin Corresponds to More Compliant Myofibrils in Athlete's Heart

Long-term exercise induces physiological cardiac adaptation, a condition referred to as athlete’s heart. Exercise tolerance is known to be associated with decreased cardiac passive stiffness. Passive stiffness of the heart muscle is determined by the giant elastic protein titin. The adult cardiac muscle contains two titin isoforms: the more compliant N2BA and the stiffer N2B. Titin-based passive stiffness may be controlled by altering the expression of the different isoforms or via post-translational modifications such as phosphorylation. Currently, there is very limited knowledge about titin’s role in cardiac adaptation during long-term exercise. Our aim was to determine the N2BA/N2B ratio and post-translational phosphorylation of titin in the left ventricle and to correlate the changes with the structure and transverse stiffness of cardiac sarcomeres in a rat model of an athlete’s heart. The athlete’s heart was induced by a 12-week-long swim-based training. In the exercised myocardium the N2BA/N2B ratio was significantly increased, Ser11878 of the PEVK domain was hypophosphorlyated, and the sarcomeric transverse elastic modulus was reduced. Thus, the reduced passive stiffness in the athlete’s heart is likely caused by a shift towards the expression of the longer cardiac titin isoform and a phosphorylation-induced softening of the PEVK domain which is manifested in a mechanical rearrangement locally, within the cardiac sarcomere.

Engineering skeletal muscle tissues with advanced maturity improves synapse formation with human induced pluripotent stem cell-derived motor neurons

To develop effective cures for neuromuscular diseases, human-relevant in vitro models of neuromuscular tissues are critically needed to probe disease mechanisms on a cellular and molecular level. However, previous attempts to co-culture motor neurons and skeletal muscle have resulted in relatively immature neuromuscular junctions (NMJs). In this study, NMJs formed by human induced pluripotent stem cell (hiPSC)-derived motor neurons were improved by optimizing the maturity of the co-cultured muscle tissue. First, muscle tissues engineered from the C2C12 mouse myoblast cell line, cryopreserved primary human myoblasts, and freshly isolated primary chick myoblasts on micromolded gelatin hydrogels were compared. After three weeks, only chick muscle tissues remained stably adhered to hydrogels and exhibited progressive increases in myogenic index and stress generation, approaching values generated by native muscle tissue. After three weeks of co-culture with hiPSC-derived motor neurons, engineered chick muscle tissues formed NMJs with increasing co-localization of pre- and postsynaptic markers as well as increased frequency and magnitude of synaptic activity, surpassing structural and functional maturity of previous in vitro models. Engineered chick muscle tissues also demonstrated increased expression of genes related to sarcomere maturation and innervation over time, revealing new insights into the molecular pathways that likely contribute to enhanced NMJ formation. These approaches for engineering advanced neuromuscular tissues with relatively mature NMJs and interrogating their structure and function have many applications in neuromuscular disease modeling and drug development.

Regulation of calcium dynamics and propagation velocity by tissue microstructure in engineered strands of cardiac tissue

Disruptions to cardiac tissue microstructure are common in diseased or injured myocardium and are known substrates for arrhythmias. However, we have a relatively coarse understanding of the relationships between myocardial tissue microstructure, propagation velocity and calcium cycling, due largely to the limitations of conventional experimental tools. To address this, we used microcontact printing to engineer strands of cardiac tissue with eight different widths, quantified several structural and functional parameters and established correlation coefficients. As strand width increased, actin alignment, nuclei density, sarcomere index and cell aspect ratio decreased with unique trends. The propagation velocity of calcium waves decreased and the rise time of calcium transients increased with increasing strand width. The decay time constant of calcium transients decreased and then slightly increased with increasing strand width. Based on correlation coefficients, actin alignment was the strongest predictor of propagation velocity and calcium transient rise time. Sarcomere index and cell aspect ratio were also strongly correlated with propagation velocity. Actin alignment, sarcomere index and cell aspect ratio were all weak predictors of the calcium transient decay time constant. We also measured the expression of several genes relevant to propagation and calcium cycling and found higher expression of the genes that encode for connexin 43 (Cx43) and a subunit of L-type calcium channels in thin strands compared to isotropic tissues. Together, these results suggest that thinner strands have higher values of propagation velocity and calcium transient rise time due to a combination of favorable tissue microstructure and enhanced expression of genes for Cx43 and L-type calcium channels. These data are important for defining how microstructural features regulate intercellular and intracellular calcium handling, which is needed to understand mechanisms of propagation in physiological situations and arrhythmogenesis in pathological situations.

Residual and passive force enhancement in skinned cardiac fibre bundles

In skeletal muscle, steady-state force is consistently greater following active stretch than during a purely isometric contraction at the same length (residual force enhancement; RFE). Similarly, when deactivated, the force remains higher following active stretch than following an isometric condition (passive force enhancement; PFE). RFE and PFE have been associated with the sarcomere protein titin, but skeletal and cardiac titin have different structures, and results regarding RFE in cardiac muscle have been inconsistent and contradictory. Therefore, the purpose of this study was to determine if cardiac muscle exhibits RFE and PFE. Skinned fibre bundles (n = 10) were activated isometrically at a sarcomere length of 2.2 μm and actively stretched by 15% of their length. The resultant active and passive forces were compared to the corresponding forces obtained for purely isometric contractions at the long length. RFE was observed in all fibre bundles, averaging 5.5 ± 2.5% (ranging from 2.3 to 9.4%). PFE was observed in nine of the ten bundles, averaging 11.1 ± 6.5% (ranging from -2.1 to 18.7%). Stiffness was not different between the active isometric and the force enhanced conditions, but was higher following deactivation from the force-enhanced compared to the isometric reference state. We conclude that there is RFE and PFE in cardiac muscle. We speculate that cardiac muscle has the same RFE capability as skeletal muscle, and that the most likely mechanism for the RFE and PFE is the engagement of a passive structural element during active stretching.

Mechanical function of cardiac fibre bundles is partly protected by exercise in response to diet-induced obesity in rats

Decrements in contractile function resulting from obesity are thought to be major reasons for the link between obesity and cardiovascular disease, while exercise has been shown to improve cardiac muscle contractile function. The purpose of this study was to evaluate cardiac contractile properties following obesity induction and the potential protective effect of exercise. Twelve-week-old rats (n = 30) were organized into either a chow diet or a high-fat, high-sucrose (HFHS) diet group. Following 12 weeks of obesity induction the HFHS group animals were stratified and grouped into sedentary (HFHS+Sed) and exercise (HFHS+Ex) groups for an additional 12 weeks. Following 24 weeks of diet intervention, with 12 weeks of aerobic exercise (25 m/min, 30 min/day, 5 days/week) for the HFHS+Ex group, skinned cardiac fibre bundle testing was used to evaluate cardiac contractile properties. Body fat and mass were significantly greater in the HFHS-fed animals compared with the chow controls (p < 0.043). Hearts from rats in the HFHS+Sed group had significantly greater mass (p < 0.03), significantly slower maximum shortening velocity (p = 0.001), and tended to have lower calcium sensitivity (p = 0.077) and a lower proportion of α-myosin heavy chain composition (p = 0.074) than the sedentary chow animals. However, 12 weeks of moderate aerobic exercise partially prevented these decrements in contractile properties. Novelty Cardiac muscle from animals exposed to an obesogenic diet for 24 weeks had impaired contractile properties compared with controls. Obesity-induced impairment of contractile properties of the heart were partially prevented by a 12-week aerobic exercise regime.

Mechanical adaptations of skinned cardiac muscle in response to dietary-induced obesity during adolescence in rats

Childhood obesity is a major risk factor for heart disease during adulthood, independent of adulthood behaviours. Therefore, it seems that childhood obesity leads to partly irreversible decrements in cardiac function. Little is known about how obesity during maturation affects the mechanical properties of the heart. The purpose of this study was to evaluate contractile properties in developing hearts from animals with dietary-induced obesity (high-fat high-sucrose diet). We hypothesized that obesity induced during adolescence results in decrements in cardiac contractile function. Three-week-old rats (n = 16) were randomized into control (chow) or dietary-induced obesity (high-fat high-sucrose diet) groups. Following 14 weeks on the diet, skinned cardiac trabeculae fibre bundle testing was performed to evaluate active and passive force, maximum shortening velocity, and calcium sensitivity. Rats in the high-fat high-sucrose diet group had significantly larger body mass and total body fat percentage. There were no differences in maximal active or passive properties of hearts between groups. Hearts from the high-fat high-sucrose diet rats had significantly slower maximum shortening velocities and lower calcium sensitivity than controls. Decreased shortening velocity and calcium sensitivity in hearts of obese animals may constitute increased risk of cardiac disease in adulthood. Novelty Cardiac muscle from animals exposed to an obesogenic diet during development had lower shortening velocity and calcium sensitivity than those from animals fed a chow diet. These alterations in mechanical function may be a mechanism for the increased risk of cardiac disease observed in adulthood.

Fatty acid-based monolayer culture to promote in vitro neonatal rat cardiomyocyte maturation

The development of functional and reliable in vitro cardiac models composed of fully mature cardiomyocytes is essential for improving drug screening test quality, therefore, the success of clinical trial outcomes. In their lifespan, cardiomyocytes undergo a dynamic maturation process from the fetal to adult stage, radically changing their metabolism, morphology, contractility and electrical properties. Before employing cells of human origin, in vitro models often use neonatal rat cardiomyocytes (NRCM) to obtain key proof-of-principles. Nevertheless, NRCM monolayers are prone to de-differentiate when maintained in culture. Supplementation of free fatty acids (FFA), the main energy source for mature cardiomyocytes, and co-culture with fibroblasts are each by itself known to promote the shift from fetal to adult cardiomyocytes. Using a co-culture system, our study investigates the effects of FFA on the cardiomyocyte phenotype in comparison to glucose as typical fetal energy source, and to 10% serum used as standard control condition. NRCM decreased their differentiation status and fibroblasts increased in number after 7days of culture in the control condition. On the contrary, both glucose- and FFA-supplementation better preserved protein expression of myosin-light-chain-2v, a marker of mature cardiomyocytes, and the fibroblast number at levels similar to those found in freshly isolated NRCM. Nevertheless, compared to glucose, FFA resulted in a significant increase in sarcomere striation and organization. Our findings constitute an important step forward towards the definition of the optimal culture conditions, highlighting the possible benefits of a further supplementation of specific FFA to promote CM maturation in a co-culture system with FB.

Force properties of skinned cardiac muscle following increasing volumes of aerobic exercise in rats

The positive effects of chronic endurance exercise training on health and performance have been well documented. These positive effects have been evaluated primarily at the structural level, and work has begun to evaluate mechanical adaptations of the myocardium. However, it remains poorly understood how the volume of exercise training affects cardiac adaptation. To gain some understanding, we subjected 3-mo-old Sprague-Dawley rats ( n = 23) to treadmill running for 11 wk at one of three exercise volumes (moderate, high, and extra high). Following training, hearts were excised and mechanical testing was completed on skinned trabecular fiber bundles. Performance on a maximal fitness test was dose dependent on training volume, where greater levels of training led to greater performance. No differences were observed between animals from any group for maximal active stress and passive stress at a sarcomere length of 2.2 µm. Heart mass and passive stress at sarcomere lengths beyond 2.4 µm increased in a dose-dependent manner for animals in the control and moderate- and high-duration groups. However, hearts from animals in the extra high-duration group presented with inhibited responses for heart mass and passive stress, despite performing greatest on a graded treadmill fitness test. These results suggest that heart mass and passive stress adapt in a dose-dependent manner, until exercise becomes excessive and adaptation is inhibited. Our findings are in agreement with the beneficial role exercise has in cardiac adaptation. However, excessive exercise comes with risks of maladaptation, which must be weighed against the desire to increase performance. NEW & NOTEWORTHY For the first time, we present findings on cardiac trabecular muscle passive stiffness and show the effect of excessive exercise on the heart. We demonstrated that heart mass increases with exercise until a maximum, after which greater exercise volume results in inhibited adaptation. At paraphysiological lengths, passive stiffness increases with exercise but to a lesser degree with excessive training. Despite greater performance on graded exercise tests, animals in the highest trained group exhibited possible maladaptation.

Rediscovery of Otto Frank's contribution to science

In the late 19th century, German physiologist Otto Frank (1865-1944) embarked on a near life-long research program of laying down the mathematical, methodological, and theoretical foundations in order to understand and define the performance of the heart and circulatory system in all their complexity. The existence of the "Frank-Starling law" testifies to this. Two of his seminal publications have been translated into English previously, introducing Frank's research on the dynamics of the heart and the arterial pulse to a wider audience. It is likely that there are a host of other comparable achievements and publications of Frank that are still unknown to the international scientific (cardiological and physiological) community. However, their influence can still be felt and seen in modern cardiology and cardio-physiology, such as in the development of modern interactive simulating and teaching programs. We have translated and commented on ten of these papers, which can be read in parallel with the German originals. These publications show a wealth of theoretical assumptions and projections regarding the importance of the sarcomere, the development of models of contraction, thermo-dynamical considerations for muscular activity, differences between cardiac and skeletal muscles, problems related to methodology and measurement, and the first pressure-volume diagram (published 120 years ago). These topics were envisioned by Frank long before they became a focus of subsequent modern research. Nowadays, frequent measurements of pressure-volume relationships are made in research using the pressure-volume conductance catheter technique. In commenting Frank's scientific topics, we try to show how interconnected his thinking was, and thus how it enabled him to cover such a wide range of subjects.

Severe DCM phenotype of patient harboring RBM20 mutation S635A can be modeled by patient-specific induced pluripotent stem cell-derived cardiomyocytes

The ability to generate patient-specific induced pluripotent stem cells (iPSCs) provides a unique opportunity for modeling heart disease in vitro. In this study, we generated iPSCs from a patient with dilated cardiomyopathy (DCM) caused by a missense mutation S635A in RNA-binding motif protein 20 (RBM20) and investigated the functionality and cell biology of cardiomyocytes (CMs) derived from patient-specific iPSCs (RBM20-iPSCs). The RBM20-iPSC-CMs showed abnormal distribution of sarcomeric α-actinin and defective calcium handling compared to control-iPSC-CMs, suggesting disorganized myofilament structure and altered calcium machinery in CMs of the RBM20 patient. Engineered heart muscles (EHMs) from RBM20-iPSC-CMs showed that not only active force generation was impaired in RBM20-EHMs but also passive stress of the tissue was decreased, suggesting a higher visco-elasticity of RBM20-EHMs. Furthermore, we observed a reduced titin (TTN) N2B-isoform expression in RBM20-iPSC-CMs by demonstrating a reduction of exon skipping in the PEVK region of TTN and an inhibition of TTN isoform switch. In contrast, in control-iPSC-CMs both TTN isoforms N2B and N2BA were expressed, indicating that the TTN isoform switch occurs already during early cardiogenesis. Using next generation RNA sequencing, we mapped transcriptome and splicing target profiles of RBM20-iPSC-CMs and identified different cardiac gene networks in response to the analyzed RBM20 mutation in cardiac-specific processes. These findings shed the first light on molecular mechanisms of RBM20-dependent pathological cardiac remodeling leading to DCM. Our data demonstrate that iPSC-CMs coupled with EHMs provide a powerful tool for evaluating disease-relevant functional defects and for a deeper mechanistic understanding of alternative splicing-related cardiac diseases.

A two dimensional electromechanical model of a cardiomyocyte to assess intra-cellular regional mechanical heterogeneities

Experimental studies on isolated cardiomyocytes from different animal species and human hearts have demonstrated that there are regional differences in the Ca2+ release, Ca2+ decay and sarcomere deformation. Local deformation heterogeneities can occur due to a combination of factors: regional/local differences in Ca2+ release and/or re-uptake, intra-cellular material properties, sarcomere proteins and distribution of the intracellular organelles. To investigate the possible causes of these heterogeneities, we developed a two-dimensional finite-element electromechanical model of a cardiomyocyte that takes into account the experimentally measured local deformation and cytosolic [Ca2+] to locally define the different variables of the constitutive equations describing the electro/mechanical behaviour of the cell. Then, the model was individualised to three different rat cardiac cells. The local [Ca2+] transients were used to define the [Ca2+]-dependent activation functions. The cell-specific local Young's moduli were estimated by solving an inverse problem, minimizing the error between the measured and simulated local deformations along the longitudinal axis of the cell. We found that heterogeneities in the deformation during contraction were determined mainly by the local elasticity rather than the local amount of Ca2+, while in the relaxation phase deformation was mainly influenced by Ca2+ re-uptake. Our electromechanical model was able to successfully estimate the local elasticity along the longitudinal direction in three different cells. In conclusion, our proposed model seems to be a good approximation to assess the heterogeneous intracellular mechanical properties to help in the understanding of the underlying mechanisms of cardiomyocyte dysfunction.

Mathematical modelling of the maternal cardiovascular system in the three stages of pregnancy

In this study, a mathematical model of the female circulation during pregnancy is presented in order to investigate the hemodynamic response to the cardiovascular changes associated with each trimester of pregnancy. First, a preliminary lumped parameter model of the circulation of a non-pregnant female was developed, including the heart, the systemic circulation with a specific block for the uterine district and the pulmonary circulation. The model was first tested at rest; then heart rate and vascular resistances were individually varied to verify the correct response to parameter alterations characterising pregnancy. In order to simulate hemodynamics during pregnancy at each trimester, the main changes applied to the model consisted in reducing vascular resistances, and simultaneously increasing heart rate and ventricular wall volumes. Overall, reasonable agreement was found between model outputs and in vivo data, with the trends of the cardiac hemodynamic quantities suggesting correct response of the heart model throughout pregnancy. Results were reported for uterine hemodynamics, with flow tracings resembling typical Doppler velocity waveforms at each stage, including pulsatility indexes. Such a model may be used to explore the changes that happen during pregnancy in female with cardiovascular diseases.

Impact of TIEG1 Deletion on the Passive Mechanical Properties of Fast and Slow Twitch Skeletal Muscles in Female Mice

As transforming growth factor (TGF)-β inducible early gene-1 is highly expressed in skeletal muscle, the effect of TIEG1 gene deletion on the passive mechanical properties of slow and fast twitch muscle fibers was analyzed. Twenty five muscle fibers were harvested from soleus (Sol) and extensor digitorum longus (EDL) muscles from TIEG1^-/^- (N = 5) and control (N = 5) mice. Mechanical tests were performed on fibers and the dynamic and static stresses were measured. A viscoelastic Hill model of 3rd order was used to fit the experimental relaxation test data. In parallel, immunohistochemical analyses were performed on three serial transverse sections to detect the myosin isoforms within the slow and fast muscles. The percentage and the mean cross sectional area of each fiber type were calculated. These tests revealed a significant increase in the mechanical stress properties for the TIEG1^-/^- Sol fibers while a significant decrease appeared for the TIEG1^-/^- EDL fibers. Hill model tracked the shape of the experimental relaxation curve for both genotypes and both fiber types. Immunohistochemical results showed hypertrophy of all fiber types for TIEG1^-/^- muscles with an increase in the percentage of glycolytic fibers (IIX, and IIB) and a decrease of oxidative fibers (I, and IIA). This study has provided new insights into the role of TIEG1, known as KLF10, in the functional (Sol_typeI: more resistant, EDL_typeIIB: less resistant) and morphological (glycolytic hypertrophy) properties of fast and slow twitch skeletal muscles. Further investigation at the cellular level will better reveal the role of the TIEG1 gene in skeletal muscle tissue.

Disordered Myocardial Ca2+ Homeostasis Results in Substructural Alterations That May Promote Occurrence of Malignant Arrhythmias

We aimed to determine the impact of Ca2+-related disorders induced in intact animal hearts on ultrastructure of the cardiomyocytes prior to occurrence of severe arrhythmias. Three types of acute experiments were performed that are known to be accompanied by disturbances in Ca2+ handling. Langedorff-perfused rat or guinea pig hearts subjected to K+-deficient perfusion to induce ventricular fibrillation (VF), burst atrial pacing to induce atrial fibrillation (AF) and open chest pig heart exposed to intramyocardial noradrenaline infusion to induce ventricular tachycardia (VT). Tissue samples for electron microscopic examination were taken during basal condition, prior and during occurrence of malignant arrhythmias. Cardiomyocyte alterations preceding occurrence of arrhythmias consisted of non-uniform sarcomere shortening, disruption of myofilaments and injury of mitochondria that most likely reflected cytosolic Ca2+ disturbances and Ca2+ overload. These disorders were linked with non-uniform pattern of neighboring cardiomyocytes and dissociation of adhesive junctions suggesting defects in cardiac cell-to-cell coupling. Our findings identified heterogeneously distributed high [Ca2+]i-induced subcellular injury of the cardiomyocytes and their junctions as a common feature prior occurrence of VT, VF or AF. In conclusion, there is a link between Ca2+-related disorders in contractility and coupling of the cardiomyocytes pointing out a novel paradigm implicated in development of severe arrhythmias.

Quantification of sarcomere length distribution in whole muscle frozen sections

Laser diffraction (LD) is a valuable tool for measuring sarcomere length (Ls), a major determinant of muscle function. However, this method relies on few measurements per sample that are often extrapolated to whole muscle properties. Currently it is not possible to measure Ls throughout an entire muscle and determine how Ls varies at this scale. To address this issue, we developed an actuated LD scanner for sampling large numbers of sarcomeres in thick whole muscle longitudinal sections. Sections of high optical quality and fixation were produced from tibialis anterior and extensor digitorum longus muscles of Sprague-Dawley rats (N=6). Scans produced two-dimensional Ls maps, capturing >85% of the muscle area per section. Individual Ls measures generated by automatic LD and bright-field microscopy showed excellent agreement over a large Ls range (ICC>0.93). Two-dimensional maps also revealed prominent regional Ls variations across muscles.

Cardiac-specific succinate dehydrogenase deficiency in Barth syndrome

Barth syndrome (BTHS) is a cardiomyopathy caused by the loss of tafazzin, a mitochondrial acyltransferase involved in the maturation of the glycerophospholipid cardiolipin. It has remained enigmatic as to why a systemic loss of cardiolipin leads to cardiomyopathy. Using a genetic ablation of tafazzin function in the BTHS mouse model, we identified severe structural changes in respiratory chain supercomplexes at a pre-onset stage of the disease. This reorganization of supercomplexes was specific to cardiac tissue and could be recapitulated in cardiomyocytes derived from BTHS patients. Moreover, our analyses demonstrate a cardiac-specific loss of succinate dehydrogenase (SDH), an enzyme linking the respiratory chain with the tricarboxylic acid cycle. As a similar defect of SDH is apparent in patient cell-derived cardiomyocytes, we conclude that these defects represent a molecular basis for the cardiac pathology in Barth syndrome.

Fibrous scaffolds for building hearts and heart parts

Extracellular matrix (ECM) structure and biochemistry provide cell-instructive cues that promote and regulate tissue growth, function, and repair. From a structural perspective, the ECM is a scaffold that guides the self-assembly of cells into distinct functional tissues. The ECM promotes the interaction between individual cells and between different cell types, and increases the strength and resilience of the tissue in mechanically dynamic environments. From a biochemical perspective, factors regulating cell-ECM adhesion have been described and diverse aspects of cell-ECM interactions in health and disease continue to be clarified. Natural ECMs therefore provide excellent design rules for tissue engineering scaffolds. The design of regenerative three-dimensional (3D) engineered scaffolds is informed by the target ECM structure, chemistry, and mechanics, to encourage cell infiltration and tissue genesis. This can be achieved using nanofibrous scaffolds composed of polymers that simultaneously recapitulate 3D ECM architecture, high-fidelity nanoscale topography, and bio-activity. Their high porosity, structural anisotropy, and bio-activity present unique advantages for engineering 3D anisotropic tissues. Here, we use the heart as a case study and examine the potential of ECM-inspired nanofibrous scaffolds for cardiac tissue engineering. We asked: Do we know enough to build a heart? To answer this question, we tabulated structural and functional properties of myocardial and valvular tissues for use as design criteria, reviewed nanofiber manufacturing platforms and assessed their capabilities to produce scaffolds that meet our design criteria. Our knowledge of the anatomy and physiology of the heart, as well as our ability to create synthetic ECM scaffolds have advanced to the point that valve replacement with nanofibrous scaffolds may be achieved in the short term, while myocardial repair requires further study in vitro and in vivo.

In vitro maturation of large-scale cardiac patches based on a perfusable starter matrix by cyclic mechanical stimulation

The ultimate goal of tissue engineering is the generation of implants similar to native tissue. Thus, it is essential to utilize physiological stimuli to improve the quality of engineered constructs. Numerous publications reported that mechanical stimulation of small-sized, non-perfusable, tissue engineered cardiac constructs leads to a maturation of immature cardiomyocytes like neonatal rat cardiomyocytes or induced pluripotent stem cells/embryonic stem cells derived self-contracting cells. The aim of this study was to investigate the impact of mechanical stimulation and perfusion on the maturation process of large-scale (2.5×4.5cm), implantable cardiac patches based on decellularized porcine small intestinal submucosa (SIS) or Biological Vascularized Matrix (BioVaM) and a 3-dimensional construct containing neonatal rat heart cells. Application of cyclic mechanical stretch improved contractile function, cardiomyocyte alignment along the stretch axis and gene expression of cardiomyocyte markers. The development of a complex network formed by endothelial cells within the cardiac construct was enhanced by cyclic stretch. Finally, the utilization of BioVaM enabled the perfusion of the matrix during stimulation, augmenting the beneficial influence of cyclic stretch. Thus, this study demonstrates the maturation of cardiac constructs with clinically relevant dimensions by the application of cyclic mechanical stretch and perfusion of the starter matrix.

STATEMENT OF SIGNIFICANCE

Considering the poor endogenous regeneration of the heart, engineering of bioartificial cardiac tissue for the replacement of infarcted myocardium is an exciting strategy. Most techniques for the generation of cardiac tissue result in relative small-sized constructs insufficient for clinical applications. Another issue is to achieve cardiomyocytes and tissue maturation in culture. Here we report, for the first time, the effect of mechanical stimulation and simultaneous perfusion on the maturation of cardiac constructs of clinical relevant dimensions, which are based on a perfusable starter matrix derived from porcine small intestine. In response to these stimuli superior organization of cardiomyocytes and vascular networks was observed in contrast to untreated controls. The study provides substantial progress towards the generation of implantable cardiac patches.

Maternal Nutrient Restriction Alters Ca2+ Handling Properties and Contractile Function of Isolated Left Ventricle Bundles in Male But Not Female Juvenile Rats

Intrauterine growth restriction (IUGR), defined as a birth weight below the 10th centile, may be caused by maternal undernutrition, with evidence that IUGR offspring have an increased risk of cardiovascular disease (CVD) in adulthood. Calcium ions (Ca²⁺) are an integral messenger for several steps associated with excitation-contraction coupling (ECC); the cascade of events from the initiation of an action potential at the surface membrane, to contraction of the cardiomyocyte. Any changes in Ca²⁺ storage and release from the sarcoplasmic reticulum (SR), or sensitivity of the contractile apparatus to Ca²⁺ may underlie the mechanism linking IUGR to an increased risk of CVD. This study aimed to explore the effects of maternal nutrient restriction on cardiac function, including Ca²⁺ handling by the SR and force development by the contractile apparatus. Juvenile Long Evans hooded rats born to Control (C) and nutrient restricted (NR) dams were anaesthetized for collection of the heart at 10–12 weeks of age. Left ventricular bundles from male NR offspring displayed increased maximum Ca²⁺-activated force, and decreased protein content of troponin I (cTnI) compared to C males. Furthermore, male NR offspring showed a reduction in rate of rise of the caffeine-induced Ca²⁺ force response and a decrease in the protein content of ryanodine receptor (RYR2). These physiological and biochemical findings observed in males were not evident in female offspring. These findings illustrate a sex-specific effect of maternal NR on cardiac development, and also highlight a possible mechanism for the development of hypertension and hypertrophy in male NR offspring.

Cardiac myocytes' dynamic contractile behavior differs depending on heart segment

Cardiac myocytes originating from different parts of the heart exhibit varying morphology and ultrastructure. However, the difference in their dynamic behavior is unclear. We examined the contraction of cardiac myocytes originating from the apex, ventricle, and atrium, and found that their dynamic behavior, such as amplitude and frequency of contraction, differs depending on the heart segment of origin. Using video microscopy and high-precision image correlation, we found that: (1) apex myocytes exhibited the highest contraction rate (∼17 beats/min); (2) ventricular myocytes exhibited the highest contraction amplitude (∼5.2 micron); and (3) as myocyte contraction synchronized, their frequency did not change significantly, but the amplitude of contraction increased in apex and ventricular myocytes. In addition, as myocyte cultures mature they formed contractile filaments, further emphasizing the difference in myocyte dynamics is persistent. These results suggest that the dynamic behavior (in addition to static properties) of myocytes is dependent on their segment of origin.

The use of neural networks and texture analysis for rapid objective selection of regions of interest in cytoskeletal images

Understanding cytoskeletal dynamics in living tissue is prerequisite to understanding mechanisms of injury, mechanotransduction, and mechanical signaling. Real-time visualization is now possible using transfection with plasmids that encode fluorescent cytoskeletal proteins. Using this approach with the muscle-specific intermediate filament protein desmin, we found that a green fluorescent protein-desmin chimeric protein was unevenly distributed throughout the muscle fiber, resulting in some image areas that were saturated as well as others that lacked any signal. Our goal was to analyze the muscle fiber cytoskeletal network quantitatively in an unbiased fashion. To objectively select areas of the muscle fiber that are suitable for analysis, we devised a method that provides objective classification of regions of images of striated cytoskeletal structures into "usable" and "unusable" categories. This method consists of a combination of spatial analysis of the image using Fourier methods along with a boosted neural network that "decides" on the quality of the image based on previous training. We trained the neural network using the expert opinion of three scientists familiar with these types of images. We found that this method was over 300 times faster than manual classification and that it permitted objective and accurate classification of image regions.

Modeling of cardiac muscle thin films: pre-stretch, passive and active behavior

Recent progress in tissue engineering has made it possible to build contractile bio-hybrid materials that undergo conformational changes by growing a layer of cardiac muscle on elastic polymeric membranes. Further development of such muscular thin films for building actuators and powering devices requires exploring several design parameters, which include the alignment of the cardiac myocytes and the thickness/Young's modulus of elastomeric film. To more efficiently explore these design parameters, we propose a 3-D phenomenological constitutive model, which accounts for both the passive deformation including pre-stretch and the active behavior of the cardiomyocytes. The proposed 3-D constitutive model is implemented within a finite element framework, and can be used to improve the current design of bio-hybrid thin films and help developing bio-hybrid constructs capable of complex conformational changes.

Is titin a 'winding filament'? A new twist on muscle contraction

Recent studies have demonstrated a role for the elastic protein titin in active muscle, but the mechanisms by which titin plays this role remain to be elucidated. In active muscle, Ca(2+)-binding has been shown to increase titin stiffness, but the observed increase is too small to explain the increased stiffness of parallel elastic elements upon muscle activation. We propose a 'winding filament' mechanism for titin's role in active muscle. First, we hypothesize that Ca(2+)-dependent binding of titin's N2A region to thin filaments increases titin stiffness by preventing low-force straightening of proximal immunoglobulin domains that occurs during passive stretch. This mechanism explains the difference in length dependence of force between skeletal myofibrils and cardiac myocytes. Second, we hypothesize that cross-bridges serve not only as motors that pull thin filaments towards the M-line, but also as rotors that wind titin on the thin filaments, storing elastic potential energy in PEVK during force development and active stretch. Energy stored during force development can be recovered during active shortening. The winding filament hypothesis accounts for force enhancement during stretch and force depression during shortening, and provides testable predictions that will encourage new directions for research on mechanisms of muscle contraction.

Cardiac electromechanical models: from cell to organ

The heart is a multiphysics and multiscale system that has driven the development of the most sophisticated mathematical models at the frontiers of computational physiology and medicine. This review focuses on electromechanical (EM) models of the heart from the molecular level of myofilaments to anatomical models of the organ. Because of the coupling in terms of function and emergent behaviors at each level of biological hierarchy, separation of behaviors at a given scale is difficult. Here, a separation is drawn at the cell level so that the first half addresses subcellular/single-cell models and the second half addresses organ models. At the subcellular level, myofilament models represent actin–myosin interaction and Ca-based activation. The discussion of specific models emphasizes the roles of cooperative mechanisms and sarcomere length dependence of contraction force, considered to be the cellular basis of the Frank–Starling law. A model of electrophysiology and Ca handling can be coupled to a myofilament model to produce an EM cell model, and representative examples are summarized to provide an overview of the progression of the field. The second half of the review covers organ-level models that require solution of the electrical component as a reaction–diffusion system and the mechanical component, in which active tension generated by the myocytes produces deformation of the organ as described by the equations of continuum mechanics. As outlined in the review, different organ-level models have chosen to use different ionic and myofilament models depending on the specific application; this choice has been largely dictated by compromises between model complexity and computational tractability. The review also addresses application areas of EM models such as cardiac resynchronization therapy and the role of mechano-electric coupling in arrhythmias and defibrillation.

Enhanced length-dependent Ca2+ activation in fish cardiomyocytes permits a large operating range of sarcomere lengths

Fish myocytes continue to develop active tension when stretched to sarcomere lengths (SLs) on the descending limb of the mammalian length-tension relationship. A greater length-dependent activation in fish than mammals could account for this because the increase in Ca(2+) sensitivity may overcome the tendency for force to fall due to reduced cross-bridge availability at SLs above optimal myofilament overlap. We stretched skinned fish and rat ventricular myocytes over a wide range of SLs, including those on the descending limb of the mammalian length-tension relationship. We found that fish myocytes developed greater active tension than rat myocytes at physiological Ca(2+) concentrations at long SLs as a result of a higher Ca(2+) sensitivity and a steeper relationship between Ca(2+) sensitivity and SL. We also investigated the diastolic properties of fish and rat myocytes at long SLs by measuring titin-based passive tension, titin isoform expression and titin phosphorylation. Fish myocytes produced higher titin-based passive tension despite expressing a higher proportion of a long N2BA-like isoform (38.0+/-2% of total vs 0% in rat). However, titin phosphorylation in fish myocytes was lower than in rat, which may explain some of the difference in passive tension between species. The high level of titin-based passive tension and the differential phosphorylation of sarcomeric proteins in fish myocytes may contribute to the enhanced length-dependent activation and underlie the extended range of in vivo stroke volumes found in fish compared with mammals.

An integrated formulation of anisotropic force-calcium relations driving spatio-temporal contractions of cardiac myocytes

Isolated cardiac myocytes exhibit spontaneous patterns of rhythmic contraction, driven by intracellular calcium waves. In order to study the coupling between spatio-temporal calcium dynamics and cell contraction in large deformation regimes, a new strain-energy function, describing the influence of sarcomere length on the calcium-dependent generation of active intracellular stresses, is proposed. This strain-energy function includes anisotropic passive and active contributions that were first validated separately from experimental stress-strain curves and stress-sarcomere length curves, respectively. An extended validation of this formulation was then conducted by considering this strain-energy function as the core of an integrated mechano-chemical three-dimensional model of cardiac myocyte contraction, where autocatalytic intracellular calcium dynamics were described by a representative two-variable model able to generate realistic intracellular calcium waves similar to those observed experimentally. Finite-element simulations of the three-dimensional cell model, conducted for different intracellular locations of triggering calcium sparks, explained very satisfactorily, both qualitatively and quantitatively, the contraction patterns of cardiac myocytes observed by time-lapse videomicroscopy. This integrative approach of the mechano-chemical couplings driving cardiac myocyte contraction provides a comprehensive framework for analysing active stress regulation and associated mechano-transduction processes that contribute to the efficiency of cardiac cell contractility in both physiological and pathological contexts.

Synchronous in situ ATPase activity, mechanics, and Ca2+ sensitivity of human and porcine myocardium

Flash-frozen myocardium samples provide a valuable means of correlating clinical cardiomyopathies with abnormalities in sarcomeric contractile and biochemical parameters. We examined flash-frozen left-ventricle human cardiomyocyte bundles from healthy donors to determine control parameters for isometric tension (P(o)) development and Ca(2+) sensitivity, while simultaneously measuring actomyosin ATPase activity in situ by a fluorimetric technique. P(o) was 17 kN m(-2) and pCa(50%) was 5.99 (28 degrees C, I = 130 mM). ATPase activity increased linearly with tension to 132 muM s(-1). To determine the influence of flash-freezing, we compared the same parameters in both glycerinated and flash-frozen porcine left-ventricle trabeculae. P(o) in glycerinated porcine myocardium was 25 kN m(-2), and maximum ATPase activity was 183 microM s(-1). In flash-frozen porcine myocardium, P(o) was 16 kN m(-2) and maximum ATPase activity was 207 microM s(-1). pCa(50%) was 5.77 in the glycerinated and 5.83 in the flash-frozen sample. Both passive and active stiffness of flash-frozen porcine myocardium were lower than for glycerinated tissue and similar to the human samples. Although lower stiffness and isometric tension development may indicate flash-freezing impairment of axial force transmission, we cannot exclude variability between samples as the cause. ATPase activity and pCa(50%) were unaffected by flash-freezing. The lower ATPase activity measured in human tissue suggests a slower actomyosin turnover by the contractile proteins.

Approximate model of cooperative activation and crossbridge cycling in cardiac muscle using ordinary differential equations

We develop a point model of the cardiac myofilament (MF) to simulate a wide variety of experimental muscle characterizations including Force-Ca relations and twitches under isometric, isosarcometric, isotonic, and auxotonic conditions. Complex MF behaviors are difficult to model because spatial interactions cannot be directly implemented as ordinary differential equations. We therefore allow phenomenological approximations with careful consideration to the relationships with the underlying biophysical mechanisms. We describe new formulations that avoid mean-field approximations found in most existing MF models. To increase the scope and applicability of the model, we include length- and temperature-dependent effects that play important roles in MF responses. We have also included a representation of passive restoring forces to simulate isolated cell shortening protocols. Possessing both computational efficiency and the ability to simulate a wide variety of muscle responses, the MF representation is well suited for coupling to existing cardiac cell models of electrophysiology and Ca-handling mechanisms. To illustrate this suitability, the MF model is coupled to the Chicago rabbit cardiomyocyte model. The combined model generates realistic appearing action potentials, intracellular Ca transients, and cell shortening signals. The combined model also demonstrates that the feedback effects of force on Ca binding to troponin can modify the cytosolic Ca transient.

Visualization of cardiac muscle thin filaments and measurement of their lengths by electron tomography

Aims

An intriguing difference between vertebrate skeletal and cardiac muscles is that the lengths of the thin filaments are constant in the former but variable in the latter. The thick filaments have constant lengths in both types of muscles. The contractile behaviour of a muscle is affected by the lengths of both types of filaments as the tension generated during contraction depends on the amount of filament overlap. To understand the behaviour of cardiac muscle, it is important to know the distribution of the thin filament lengths. The previous detailed analysis by Robinson and Winegrad used serial transverse sections to determine the lengths of the thin filaments. However, the precision, set by the 100 nm section thickness, was low. Here, we have used electron tomography to produce 3D images of rat and mouse cardiac muscles in which we can actually see individual thin filaments up to the free ends and see that these free ends have variable locations. For comparison, we also measure the thin filament lengths in skeletal muscle (frog sartorius).

Methods and results

Cardiac papillary muscles were obtained from a rat (Sprague–Dawley) and a mouse (C57/B6). Skeletal muscle (sartorius) was obtained from a frog (Rana pipiens). Longitudinal sections (100 nm thick) were used to produce tilt series and tomograms from which the thin filament paths were traced. Cardiac papillary muscle thin filaments in rat and mouse range from 0.94 to 1.10 µm, with a mean length of 1.04 µm and standard deviation of 0.03 µm. For frog sartorius muscle, the thin filament length was 0.94 µm with standard deviation of 0.01 µm.

Conclusion

Electron tomography of cardiac and skeletal muscles allows direct visualization and high precision measurement of the lengths of thin filaments.

Mechanism of the Frank–Starling law—A simulation study with a novel cardiac muscle contraction model that includes titin and troponin I

A stretch-induced increase of active tension is one of the most important properties of the heart, known as the Frank-Starling law. Although a variation of myofilament Ca(2+) sensitivity with sarcomere length (SL) change was found to be involved, the underlying molecular mechanisms are not fully clarified. Some recent experimental studies indicate that a reduction of the lattice spacing between thin and thick filaments, through the increase of passive tension caused by the sarcomeric protein titin with an increase in SL within the physiological range, promotes formation of force-generating crossbridges (Xbs). However, the mechanism by which the Xb concentration determines the degree of cooperativity for a given SL has so far evaded experimental elucidation. In this simulation study, a novel, rather simple molecular-based cardiac contraction model, appropriate for integration into a ventricular cell model, was designed, being the first model to introduce experimental data on titin-based radial tension to account for the SL-dependent modulation of the interfilament lattice spacing and to include a conformational change of troponin I (TnI). Simulation results for the isometric twitch contraction time course, the length-tension and the force-[Ca(2+)] relationships are comparable to experimental data. A complete potential Frank-Starling mechanism was analyzed by this simulation study. The SL-dependent modulation of the myosin binding rate through titin's passive tension determines the Xb concentration which then alters the degree of positive cooperativity affecting the rate of the TnI conformation change and causing the Hill coefficient to be SL-dependent.

Reconstitution of the Frank-Starling mechanism in engineered heart tissues

According to the Frank-Starling mechanism, as the heart is stretched, it increases its contraction force. Reconstitution of the Frank-Starling mechanism is an important milestone for producing functional heart tissue constructs. Spontaneously contracting engineered heart tissues (EHTs) were reconstituted by growing dissociated chicken embryo cardiomyocytes in collagen matrices. Twitch and baseline tensions were recorded at precisely controlled levels of tissue strain. The EHTs showed a steep increase in twitch tension from 0.47 +/- 0.02 to 0.91 +/- 0.02 mN/mm2 as they were stretched at a constant rate (2.67% per min) from 86% to 100% of the length at which maximum twitch force was exerted. In response to a sudden stretch (3.3%), the twitch tension increased gradually (approximately 60 s) in a Gd3+-sensitive manner, suggesting the presence of stretch-activated Ca2+ channels. A large difference in baseline tension between lengthening (loading) and shortening (unloading) was also recorded. Disruption of nonsarcomeric actin filaments by cytochalasin D and latrunculin B decreased this difference. A simple mechanical model interprets these results in terms of mechanical connections between myocytes and nonmuscle cells. The experimental results strongly suggest that regulation of twitch tension in EHTs is similar to that of natural myocardium.

Cardiac titin: molecular basis of elasticity and cellular contribution to elastic and viscous stiffness components in myocardium

Myocardium resists the inflow of blood during diastole through stretch-dependent generation of passive tension. Earlier we proposed that this tension is mainly due to collagen stiffness at degrees of stretch corresponding to sarcomere lengths (SLS) > or = 2.2 microns, but at shorter lengths, is principally determined by the giant sarcomere protein titin. Myocardial passive force consists of stretch-velocity-sensitive (viscous/viscoelastic) and velocity-insensitive (elastic) components; these force components are seen also in isolated cardiac myofibrils or skinned cells devoid of collagen. Here we examine the cellular/myofibrillar origins of passive force and describe the contribution of titin, or interactions involving titin, to individual passive-force components. We construct force-extension relationships for the four distinct elastic regions of cardiac titin, using results of in situ titin segment-extension studies and force measurements on isolated cardiac myofibrils. Then, we compare these relationships with those calculated for each region with the wormlike-chain (WLC) model of entropic polymer elasticity. Parameters used in the WLC calculations were determined experimentally by single-molecule atomic force-microscopy measurements on engineered titin domains. The WLC modelling faithfully predicts the steady-state-force vs. extension behavior of all cardiac-titin segments over much of the physiological SL range. Thus, the elastic-force component of cardiac myofibrils can be described in terms of the entropic-spring properties of titin segments. In contrast, entropic elasticity cannot account for the passive-force decay of cardiac myofibrils following quick stretch (stress relaxation). Instead, slower (viscoelastic) components of stress relaxation could be simulated by using a Monte-Carlo approach, in which unfolding of a few immunoglobulin domains per titin molecule explains the force decay. Fast components of stress relaxation (viscous drag) result mainly from interaction between actin and titin filaments; actin extraction of cardiac sarcomeres by gelsolin immediately suppressed the quickly decaying force transients. The combined results reveal the sources of velocity sensitive and insensitive force components of cardiomyofibrils stretched in diastole.

Simultaneous imaging and functional assessment of cytoskeletal protein connections in passively loaded single muscle cells

We describe a novel system that permits simultaneous confocal imaging of protein interactions and measurement of cell mechanical properties during passive loading. A mechanical apparatus was designed to replace the stage of a confocal microscope, enabling cell manipulation, force transduction, and imaging. In addition, image processing algorithms were developed to quantify the degree of connectivity between subcellular structures. Using this system, we examined the interactions among three cellular structures thought to be linked by the muscle's intermediate filament system: Z-disks, nuclei, and the costamere protein complexes located at the muscle cell surface. Fast Fourier transforms (FFTs) and autocorrelations (ACs) were implemented to quantify image periodicity and relative phase shifts among structures. We demonstrated in sample wild-type muscle cells that there was significant connectivity among Z-disks in the same fiber at various sarcomere lengths, as well as between Z-disks and the costamere complexes. This approach can be applied to any cell system in which structural periodicity and mechanical connectivity are of interest.

Nonuniformity of sarcomere shortenings in the isolated rat ventricular myocyte

By projecting the image of a single ventricular myocyte upon a linear image sensor, the striation pattern was analyzed during resting and contracting states. During rest, the individual cycle length (ICL) of the striation pattern varied from 1.8 to 2.0 microm within a given cell. ICL measured every 2 ms fluctuated approximately 0.05 microm around the mean. The variance of temporal fluctuations was decreased by chelating the extracellular Ca(2+) and increased by the Ca(2+) overload. Blocking the Ca(2+) release channels with 10 micromol/l ryanodine reversed this increase. In the power spectral density, an increase in the power occurred in the frequency range below 10 Hz. This increase should reflect overall kinetics of both intracellular Ca(2+) handling and responses of contractile filaments, because the same pattern was observed when spontaneous contractions occurred as well as when contractions were evoked by activating the L-type Ca(2+) channels. It is suggested that the temporal fluctuations of ICL in the resting state are caused by spontaneous Ca(2+) release from ryanodine receptors on the sarcoplasmic reticulum. Furthermore, under evoked contractions the shortenings of ICL were spatially inhomogeneous. These findings of nonuniform sarcomere shortenings are consistent with the temporal and spatial inhomogeneity of Ca(2+) transients.

Single muscle fiber contraction is dictated by inter-sarcomere dynamics

This paper presents first results from a study where we developed a generic framework for analysing inter-sarcomere dynamics. Our objective is to find an accurate description of a muscle as a linear motor composed of parallel and series coupled subunits. The quality of theoretical models can be tested through their ability to predict experimental observations. With the current method we have found rigorous mathematical explanations for mechanisms such as sarcomere popping, extra tension and homogenization. These phenomena have been observed for many years in single fibers experiments, yet have never been completely understood in terms of a mechanical model. Now they can be explained on a theoretical basis. Interestingly, rather simplistic descriptions of each of the various molecular components in the sarcomere (actin-myosin cross-bridges, titin and contributions from passive elastic components) are sufficient to predict these behaviors. The complexity of a real muscle fiber is addressed through rigorous coupling of the single component models in a system of differential equations. We examine the properties of the differential equations, based on a down-stripped model, which permits the derivation of analytical solutions. They suggest that the contraction characteristics of inter-connected sarcomeres are essentially dictated by the initial distribution of the sarcomeres on the force-length curve and their starting velocities. The complete model is applied to show the complexity of inter-sarcomere dynamics of activated fibers in stretch-release experiments with either external force or length control. Seemingly contradictory and unexpected observations from single fiber experiments, which have hitherto been discussed with the argument of uncontrollable biological variability, turn out to be a consistent set of possible fiber responses. They result from a convolution of multiple relatively simple rules each of them defining a certain characteristics of the single sarcomere.