Some proposals in cardiac muscle mechanics and energetics

Based on A. V. Hill's three-component model, mechanical properties of the contractile element (CE), such as velocity and tension, were determined as muscle shortening and loads in quick-release or afterloaded isotonic contraction. The method is applicable for studying cardiac mechanics, to obtain force-velocity data of the same CE length at varous afterloads. Analysis of the energetics of cardiac muscle was based on simulation studies of cardiac mechanics (Wong 1971, 1972). By proper derivation, the conventional contractile element work (CEW) was found to be a minor energy determinant. The tension-time integral and tension-independent heat (Ricchiuti and Gibbs, 1965) represent energy utilization for activation and maintenance of tension, the primary energy determinant.     

A fuzzy-genetic model for estimating forces from electromyographical activity of antagonistic muscles due to planar lower arm movements: the effect of nonlinear muscle properties

The aim of this paper is to create a model for mapping the surface electromyogram (EMG) signals to the force that generated by human arm muscles. Because the parameters of each person's muscle are individual, the model of the muscle must have two characteristics: (1) The model must be adjustable for each subject. (2) The relationship between the input and output of model must be affected by the force-length and the force-velocity behaviors are proven through Hill's experiments. Hill's model is a kinematic mechanistic model with three elements, i.e. one contractile component and two nonlinear spring elements.In this research, fuzzy systems are applied to improve the muscle model. The advantages of using fuzzy system are as follows: they are robust to noise, they prove an adjustable nonlinear mapping, and are able to model the uncertainties of the muscle.Three fuzzy coefficients have been added to the relationships of force-length (active and passive) and force-velocity existing in Hill's model. Then, a genetic algorithm (GA) has been used as a biological search method that can adjust the parameters of the model in order to achieve the optimal possible fit.Finally, the accuracy of the fuzzy genetic implementation Hill-based muscle model (FGIHM) is invested as following: the FGIHM results have 12.4% RMS error (in worse case) in comparison to the experimental data recorded from three healthy male subjects. Moreover, the FGIHM active force-length relationship which is the key characteristics of muscles has been compared to virtual muscle (VM) and Zajac muscle model. The sensitivity of the FGIHM has been evaluated by adding a white noise with zero mean to the input and FGIHM has proved to have lower sensitivity to input noise than the traditional Hill's muscle model.     

Optimal length of smooth muscle assessed by a microstructurally and statistically based constitutive model

Smooth muscle exhibits an optimal length at which it is able to generate a maximum amount of force. In this study, the optimal length is assessed by use of a microstructurally and statistically based constitutive model for smooth muscle. The model is based on the sliding filament theory, and a modified version of Hill's mechanical model was adopted. It was conjectured, that a variation in the overlap in the actomyosin contractile units together with a statistical dispersion in the size of the dense bodies are responsible for the optimal length characteristics. The influence of contractile unit length, dense body size and dense body compliance was investigated, and the model was fully able to predict experimental data. The results indicate that the compliance of the dense bodies does not contribute significantly to the total compliance of the contractile apparatus.     

Mechanics of cardiac muscle, based on Huxley's model: Mathematical simulation of isometric contraction

Hill's three-component model (Maxwell model) is used to represent the mechanical property of cardiac muscle. The parallel and series elastic elements of the fibres are described according to their non-linear exponential function; and Huxley's sliding-filaments model, together with the activating role of calcium, is applied to the contractile element.

With this composite model, the following responses can be simulated mathematically: isometric twitch at various muscle lengths, tension-length relationships; isometric contraction during quick stretch; and the Bowditch Treppe and tension velocity relationships of the contractile element.     

Contractility Measurements on Isolated Papillary Muscles for the Investigation of Cardiac Inotropy in Mice

Papillary muscle isolated from adult mouse hearts can be used to study cardiac contractility during different physiological/pathological conditions. The contractile characteristics can be evaluated independently of external influences such as vascular tonus or neurohumoral status. It depicts a scientific approach between single cell measurements with isolated cardiac myocytes and in vivo studies like echocardiography. Thus, papillary muscle preparations serve as an excellent model to study cardiac physiology/pathophysiology and can be used for investigations like the modulation by pharmacological agents or the exploration of transgenic animal models. Here, we describe a method of isolating the murine left anterior papillary muscle to investigate cardiac contractility in an organ bath setup. In contrast to a muscle strip preparation isolated from the ventricular wall, the papillary muscle can be prepared in toto without damaging the muscle tissue severely. The organ bath setup consists of several temperature-controlled, gassed and electrode-equipped organ bath chambers. The isolated papillary muscle is fixed in the organ bath chamber and electrically stimulated. The evoked twitch force is recorded using a pressure transducer and parameters such as twitch force amplitude and twitch kinetics are analyzed. Different experimental protocols can be performed to investigate the calcium- and frequency-dependent contractility as well as dose-response curves of contractile agents such as catecholamines or other pharmaceuticals. Additionally, pathologic conditions like acute ischemia can be simulated.     

A model of multicomponent cardiac fibre

In order to simulate the contraction of a cardiac myofibre, a multicomponent fibre model has been developed. This model is composed of a series of segments which are activated in succession. Each segment is represented by the Hill's three component model of the sarcomere. The contractile element behaviour is described by the Huxley's theory and the time dependence agrees with the activation factor proposed by Julian for skeletal muscle, and modified by Wong for cardiac muscle. The two elastic elements have non-linear exponential characteristics. The isometric contraction of the multicomponent fibre has been simulated by means of a computer program. The results show the tension generated by the fibre, the propagation of the contraction along the fibre and the different contribution of each segment depending on its position inside the fibre.     

Investigations of Molecular Mechanisms of Actin–Myosin Interactions in Cardiac Muscle

The functional characteristics of cardiac muscle depend on the composition of protein isoforms in the cardiomyocyte contractile machinery. In the ventricular myocardium of mammals, several isoforms of contractile and regulatory proteins are expressed–two isoforms of myosin (V1 and V3) and three isoforms of tropomyosin chains (α, β, and κ). Expression of protein isoforms depends on the animal species, its age and hormonal status, and this can change with pathologies of the myocardium. Mutations in these proteins can lead to cardiomyopathies. The functional significance of the protein isoform composition has been studied mainly on intact hearts or on isolated preparations of myocardium, which could not provide a clear comprehension of the role of each particular isoform. Present-day experimental techniques such as an optical trap and in vitro motility assay make it possible to investigate the phenomena of interactions of contractile and regulatory proteins on the molecular level, thus avoiding effects associated with properties of a whole muscle or muscle tissue. These methods enable free combining of the isoforms to test the molecular mechanisms of their participation in the actin–myosin interaction. Using the optical trap and the in vitro motility assay, we have studied functional characteristics of the cardiac myosin isoforms, molecular mechanisms of the calcium-dependent regulation of actin–myosin interaction, and the role of myosin and tropomyosin isoforms in the cooperativity mechanisms in myocardium. The knowledge of molecular mechanisms underlying myocardial contractility and its regulation is necessary for comprehension of cardiac muscle functioning, its disorders in pathologies, and for development of approaches for their correction.     

Hypoxia differentially regulates muscle oxidative fiber type and metabolism in a HIF-1α-dependent manner

Loss of skeletal muscle oxidative fiber types and mitochondrial capacity is a hallmark of chronic obstructive pulmonary disease and chronic heart failure. Based on in vivo human and animal studies, tissue hypoxia has been hypothesized as determinant, but the direct effect of hypoxia on muscle oxidative phenotype remains to be established. Hence, we determined the effect of hypoxia on in vitro cultured muscle cells, including gene and protein expression levels of mitochondrial components, myosin isoforms (reflecting slow-oxidative versus fast-glycolytic fibers), and the involvement of the regulatory PPAR/PGC-1α pathway. We found that hypoxia inhibits the PPAR/PGC-1α pathway and the expression of mitochondrial components through HIF-1α. However, in contrast to our hypothesis, hypoxia stimulated the expression of slow-oxidative type I myosin via HIF-1α. Collectively, this study shows that hypoxia differentially regulates contractile and metabolic components of muscle oxidative phenotype in a HIF-1α-dependent manner.     

Muscle architecture and force-velocity relationships in humans

The in vivo torque-velocity relationships of the knee extensors (KE), knee flexors (KF), ankle plantarflexors (PF), and ankle dorsiflexors (DF) were determined in 12 untrained subjects using an isokinetic testing device (Cybex II). These data were then matched to the predicted maximum forces and shortening velocities derived from muscle architectural determinations made on three hemipelvectomies (36). The torque-velocity curves of all muscle groups resembled that predicted by Hill's (19, 20) equation except at the higher forces and lower velocities. The peak torques occurred at mean velocities ranging from 41-62 rad X s-1 for the KE, KF, and PF. Although the peak torque of the DF occurred at the isometric loading condition, it was also lower than that predicted by Hill's equation. The muscle fiber length and physiological cross-sectional area measurements indicate that the architecture of the human leg musculature has a major influence on the torque-velocity characteristics. These data corroborate previous findings (24) that some neural inhibitory mechanism exists in the control of the leg musculature, which limits the maximum forces that could be produced under optimal stimulating conditions.     

A Phenomenological Theory of Muscular Contraction: II. Generalized Length Variations

In part I of this series, the theory of irreversible thermodynamics was applied to the sliding filament model to obtain rate equations for a contracting muscle at the in situ length l_o. In this paper we extend the theory to include length variations derived from the sliding filament model of contracting muscle using the work of Gordon, Huxley, and Julian (1). Accepting the validity of Hill's forcevelocity relation (2) at the in situ length, we show that Hill's equation is valid for any length provided that the values of the parameters, a, b, and V_m vary with length as derived herein. The predicted variation with length of the velocity for a lightly loaded isotonic contraction is shown to agree well with that measured by Gordon, Huxley, and Julian (1). Chemical rates are derived as functions of length using parameters that can be obtained experimentally.     

Electron microscopy and three-dimensional reconstruction of native thin filaments reveal species-specific differences in regulatory strand densities

Throughout the animal kingdom striated muscle contraction is regulated by the thin filament troponin-tropomyosin complex. Homologous regulatory components are shared among vertebrate and arthropod muscles; however, unique protein extensions and/or components characterize the latter. The Troponin T (TnT) isoforms of Drosophila indirect flight and tarantula femur muscle for example contain distinct C-terminal extensions and are ∼20% larger overall than their vertebrate counterpart. Using electron microscopy and three-dimensional helical reconstruction of native Drosophila, tarantula and frog muscle thin filaments we have identified species-specific differences in tropomyosin regulatory strand densities. The strands on the arthropod thin filaments were significantly larger in diameter than those from vertebrates, although not significantly different from each other. These findings reflect differences in the regulatory troponin-tropomyosin complex, which are likely due to the larger TnT molecules aligning and extending along much of the tropomyosin strands’ length. Such an arrangement potentially alters the physical properties of the regulatory strands and may help establish contractile characteristics unique to certain arthropod muscles.     

Is there an effect of ischemic conditioning on myocardial contractile function following acute myocardial ischemia/reperfusion injury?

Ischemic conditioning induces cardioprotection; the final infarct size following a myocardial ischemic event is reduced. However, whether ischemic conditioning has long-term beneficial effects on myocardial contractile function following such an ischemic event needs further elucidation. To date, ex vivo studies have shown that ischemic conditioning improves the contractile recovery of isolated ventricular papillary muscle or atrial trabeculae following simulated ischemia. However, in vivo animal studies and studies in patients undergoing elective cardiac surgery show conflicting results. At the subcellular level, it is known that ischemic conditioning improved energy metabolism, preserved mitochondrial respiration, ATP production, and Ca²⁺ homeostasis in isolated mitochondria from the myocardium. Ischemic conditioning also presents with post-translational modifications of proteins in the contractile machinery of the myocardium. The beneficial effects on myocardial contractile function need further elucidation. This article is part of a Special Issue entitled: The power of metabolism: Linking energy supply and demand to contractile function edited by Torsten Doenst, Michael Schwarzer and Christine Des Rosiers.     

Tension generation by threads of contractile proteins

Threads of contractile proteins were formed via extrusion and their isometric tensions and isotonic contraction velocities were measured. We obtained reproducible data by using a new and sensitive tensiometer. The force-velocity curves of actomyosin threads were similar to those of muscle, with isometric tensions of the order of 10g/cm2 and maximum contraction velocites of the order of 10(-2) lengths/s. The data could be fitted by Hill's equation. Addition of tropomyosin and troponin to the threads increased isometric tension and maximum contraction velocity. Threads which contained troponin and tropomyosin required Ca++ for contraction and the dependence of their isometric tension on the level of free Ca++ was like that of muscle. The dependence of tension or of contraction velocity upon temperature or upon ionic strength is similar for actomyosin threads and muscle fibers. In contrast, the dependence of most parameters which are characteristic of the actomyosin interaction in solution (or suspension) upon these variables is not similar to the dependence of the muscle fiber parameters. The conclusion we have drawn from these results is that the mechanism of tension generation in the threads is similar to the mechanism that exists in muscle. Because the protein composition of the thread system can be manipulated readily and because the tensions and velocities of the threads can be related directly to the physiological parameters of muscle fibers, the threads provide a powerful method for studying contractile proteins.     

Accuracy of gastrocnemius muscles forces in walking and running goats predicted by one-element and two-element Hill-type models

Hill-type models are commonly used to estimate muscle forces during human and animal movement—yet the accuracy of the forces estimated during walking, running, and other tasks remains largely unknown. Further, most Hill-type models assume a single contractile element, despite evidence that faster and slower motor units, which have different activation–deactivation dynamics, may be independently or collectively excited. This study evaluated a novel, two-element Hill-type model with “differential” activation of fast and slow contractile elements. Model performance was assessed using a comprehensive data set (including measures of EMG intensity, fascicle length, and tendon force) collected from the gastrocnemius muscles of goats during locomotor experiments. Muscle forces predicted by the new two-element model were compared to the forces estimated using traditional one-element models and to the forces measured in vivo using tendon buckle transducers. Overall, the two-element model resulted in the best predictions of in vivo gastrocnemius force. The coefficient of determination, r², was up to 26.9% higher and the root mean square error, RMSE, was up to 37.4% lower for the two-element model than for the one-element models tested. All models captured salient features of the measured muscle force during walking, trotting, and galloping (r²=0.26–0.51), and all exhibited some errors (RMSE=9.63–32.2% of the maximum in vivo force). These comparisons provide important insight into the accuracy of Hill-type models. The results also show that incorporation of fast and slow contractile elements within muscle models can improve estimates of time-varying, whole muscle force during locomotor tasks.     

Optimal systems: III. Muscle

The optimal systems approach to the muscular system leads to difficulties since the properties of the muscular system are determined to a great extent by the nature of the contractile unit or molecule. This unit has determined the morphology and dynamic characteristics of muscle, and only smaller order alterations are then possible to adapt muscle to its several functions. A model of the contractile unit is developed that shows agreement with experimental findings with respect to the velocity-load relation, heat effects, and several aspects of knowledge of the structure of the contractile proteins.     

Development of the muscle system and contractile activity in the mussel <Emphasis Type="Italic">Mytilus trossulus</Emphasis> (Mollusca,Bivalvia)

The development of contractile apparatus was subjected to comparative analysis during ontogenesis of the mussel Mytilus trossulus. Indirect immunofluorescence with the polyclonal antibody against mussel twitchin, a protein of thick filaments, and fluorescent phalloidin as a marker of filamentous cell actin were used to monitor changes in the developing muscle system at different larval stages. The first definitive muscle structures were found at the late trochophore stage (36 h after fertilization) and starting from the midveliger stage (96h), striated muscles, which are never present in adult mussels, were distinctly seen. The striated muscle periodicity was 1.25 μm in both mussel larvae and adult scallop. The contractile activities of veliger and adult muscles were measured using an electronic signal-processing video workstation. This work is the first complex study of morphological, biochemical, and physiological characteristics of the muscle system in the larvae and adult molluscs.     

Comparative proteomic analysis of the contractile-protein-depleted fraction from normal versus dystrophic skeletal muscle

In basic and applied myology, gel-based proteomics is routinely used for studying global changes in the protein constellation of contractile fibers during myogenesis, physiological adaptations, neuromuscular degeneration, and the natural aging process. Since the main proteins of the actomyosin apparatus and its auxiliary sarcomeric components often negate weak signals from minor muscle proteins during proteomic investigations, we have here evaluated whether a simple prefractionation step can be employed to eliminate certain aspects of this analytical obstacle. To remove a large portion of highly abundant contractile proteins from skeletal muscle homogenates without the usage of major manipulative steps, differential centrifugation was used to decisively reduce the sample complexity of crude muscle tissue extracts. The resulting protein fraction was separated by two-dimensional gel electrophoresis, and 2D-landmark proteins were identified by mass spectrometry. To evaluate the suitability of the contractile-protein-depleted fraction for comparative proteomics, normal versus dystrophic muscle preparations were examined. The mass spectrometric analysis of differentially expressed proteins, as determined by fluorescence difference in-gel electrophoresis, identified 10 protein species in dystrophic mdx hindlimb muscles. Interesting new biomarker candidates included Hsp70, transferrin, and ferritin, whereby their altered concentration levels in dystrophin-deficient muscle were confirmed by immunoblotting.     

Cholinergic regulation of body-wall muscle contraction of the ascidian Styela rustica (Linnaeus, 1767)

The regulation of body-wall muscle contraction in the ascidian Styela rustica was studied. Acetylcholine (ACh, 1?C10 ??M) induced a significant contraction of isolated muscle strips. The ACh-induced contractile response was potentiated and prolonged in the presence of proserine (15 ??M), which confirms acetylcholinesterase activity in the S. rustica body-wall muscle. Atropine (1?C100 ??M, M-cholinoreceptor blocker) did not prevent the ACh-induced contractile response, while d-tubocurarine (1?C100 ??M, N-cholinoreceptor blocker) progressively reduced muscle contraction induced by 10 ??M ACh. Thus, neuromuscular transmission in the S. rustica body-wall muscle is mediated by nicotinic-like ACh-receptors. Procaine reduced ACh-induced (10 ??M) muscle contraction. As well, our experiments showed spontaneous rhythmic contractile activity in isolated muscle strips of S. rustica. Atropine, d-tubocurarine, procaine, and proserine did not alter rhythmic activity. Myogenic automaticity is suggested as a possible cause of the rhythmic contraction of the ascidian body-wall muscle.