Tension Development Research Articles

Taurine protects cardiac contractility in killifish, Fundulus heteroclitus, by enhancing sarcoplasmic reticular Ca2+ cycling.

Intracellular taurine is abundant in many animals and it influences an array of physiological processes, including osmoregulation, metabolism, and cardiac contractility. Taurine is an important osmolyte in teleost hearts, but its role in stress tolerance, cardiac metabolism, and contractility has not been assessed. The goal of this study was to determine if ventricular taurine concentration changes in response to environmental stress and to characterize its influence on contractility. Cardiac taurine concentrations varied in killifish (Fundulus heteroclitus) but were generally maintained following acute environmental challenges. In isometrically contracting ventricular strips, supplemental taurine (40mmol L-1) protected peak tension development (F max) at high stimulation frequencies, an effect abolished by treatment with ryanodine, a blocker of sarcoplasmic reticulum Ca2+ release. In the presence of ryanodine, taurine-treated preparations were also better able to maintain F max at supraphysiological extracellular Ca2+ levels, but a prior anoxia exposure abolished this effect. Taurine had no impact on basal F max during or after anoxia, but it provided additive protection to high-frequency contractility post-anoxia. Tissue oxygen consumption and extracellular glucose utilization were unaffected by taurine in non-contracting preparations, indicating that it does not impact energy metabolism. Overall, the results suggest that cardiac taurine levels are well maintained on acute time scales in this highly stress-tolerant species. Supplemental taurine has no effect on aerobic metabolism in vitro, but it significantly improved cardiac contractility in a manner dependent upon sarcoplasmic reticulum Ca2+ cycling. The data indicate that taurine likely plays an important role in the regulation of cardiac performance in teleosts.

Cardiac hyporesponsiveness in severe sepsis is associated with nitric oxide-dependent activation of G protein receptor kinase.

G protein-coupled receptor kinase isoform 2 (GRK2) has a critical role in physiological and pharmacological responses to endogenous and exogenous substances. Sepsis causes an important cardiovascular dysfunction in which nitric oxide (NO) has a relevant role. The present study aimed to assess the putative effect of inducible NO synthase (NOS2)-derived NO on the activity of GRK2 in the context of septic cardiac dysfunction. C57BL/6 mice were submitted to severe septic injury by cecal ligation and puncture (CLP). Heart function was assessed by isolated and perfused heart, echocardiography, and β-adrenergic receptor binding. GRK2 was determined by immunofluorescence and Western blot analysis in the heart and isolated cardiac myocytes. Sepsis increased NOS2 expression in the heart, increased plasma nitrite + nitrate levels, and reduced isoproterenol-induced isolated ventricle contraction, whole heart tension development, and β-adrenergic receptor density. Treatment with 1400W or with GRK2 inhibitor prevented CLP-induced cardiac hyporesponsiveness 12 and 24 h after CLP. Increased labeling of total and phosphorylated GRK2 was detected in hearts after CLP. With treatment of 1400W or in hearts taken from septic NOS2 knockout mice, the activation of GRK2 was reduced. 1400W or GRK2 inhibitor reduced mortality, improved echocardiographic cardiac parameters, and prevented organ damage. Therefore, during sepsis, NOS2-derived NO increases GRK2, which leads to a reduction in β-adrenergic receptor density, contributing to the heart dysfunction. Isolated cardiac myocyte data indicate that NO acts through the soluble guanylyl cyclase/cGMP/PKG pathway. GRK2 inhibition may be a potential therapeutic target in sepsis-induced cardiac dysfunction.NEW & NOTEWORTHY The main novelty presented here is to show that septic shock induces cardiac hyporesponsiveness to isoproterenol by a mechanism dependent on nitric oxide and mediated by G protein-coupled receptor kinase isoform 2. Therefore, G protein-coupled receptor kinase isoform 2 inhibition may be a potential therapeutic target in sepsis-induced cardiac dysfunction.

Non-muscle (NM) myosin heavy chain phosphorylation regulates the formation of NM myosin filaments, adhesome assembly and smooth muscle contraction.

Non-muscle (NM) and smooth muscle (SM) myosin II are both expressed in smooth muscle tissues, however the role of NM myosin in SM contraction is unknown. Contractile stimulation of tracheal smooth muscle tissues stimulates phosphorylation of the NM myosin heavy chain on Ser1943 and causes NM myosin filament assembly at the SM cell cortex. Expression of a non-phosphorylatable NM myosin mutant, NM myosin S1943A, in SM tissues inhibits ACh-induced NM myosin filament assembly and SM contraction, and also inhibits the assembly of membrane adhesome complexes during contractile stimulation. NM myosin regulatory light chain (RLC) phosphorylation but not SM myosin RLC phosphorylation is regulated by RhoA GTPase during ACh stimulation, and NM RLC phosphorylation is required for NM myosin filament assembly and SM contraction. NM myosin II plays a critical role in airway SM contraction that is independent and distinct from the function of SM myosin. The molecular function of non-muscle (NM) isoforms of myosin II in smooth muscle (SM) tissues and their possible role in contraction are largely unknown. We evaluated the function of NM myosin during contractile stimulation of canine tracheal SM tissues. Stimulation with ACh caused NM myosin filament assembly, as assessed by a Triton solubility assay and a proximity ligation assay aiming to measure interactions between NM myosin monomers. ACh stimulated the phosphorylation of NM myosin heavy chain on Ser1943 in tracheal SM tissues, which can regulate NM myosin IIA filament assembly in vitro. Expression of the non-phosphorylatable mutant NM myosin S1943A in SM tissues inhibited ACh-induced endogenous NM myosin Ser1943 phosphorylation, NM myosin filament formation, the assembly of membrane adhesome complexes and tension development. The NM myosin cross-bridge cycling inhibitor blebbistatin suppressed adhesome complex assembly and SM contraction without inhibiting NM myosin Ser1943 phosphorylation or NM myosin filament assembly. RhoA inactivation selectively inhibited phosphorylation of the NM myosin regulatory light chain (RLC), NM myosin filament assembly and contraction, although it did not inhibit SM RLC phosphorylation. We conclude that the assembly and activation of NM myosin II is regulated during contractile stimulation of airway SM tissues by RhoA-mediated NM myosin RLC phosphorylation and by NM myosin heavy chain Ser1943 phosphorylation. NM myosin II actomyosin cross-bridge cycling regulates the assembly of membrane adhesome complexes that mediate the cytoskeletal processes required for tension generation. NM myosin II plays a critical role in airway SM contraction that is independent and distinct from the function of SM myosin.

Axon tension regulates fasciculation/defasciculation through the control of axon shaft zippering.

While axon fasciculation plays a key role in the development of neural networks, very little is known about its dynamics and the underlying biophysical mechanisms. In a model system composed of neurons grown ex vivo from explants of embryonic mouse olfactory epithelia, we observed that axons dynamically interact with each other through their shafts, leading to zippering and unzippering behavior that regulates their fasciculation. Taking advantage of this new preparation suitable for studying such interactions, we carried out a detailed biophysical analysis of zippering, occurring either spontaneously or induced by micromanipulations and pharmacological treatments. We show that zippering arises from the competition of axon-axon adhesion and mechanical tension in the axons, and provide the first quantification of the force of axon-axon adhesion. Furthermore, we introduce a biophysical model of the zippering dynamics, and we quantitatively relate the individual zipper properties to global characteristics of the developing axon network. Our study uncovers a new role of mechanical tension in neural development: the regulation of axon fasciculation.

A model of cardiac contraction based on novel measurements of tension development in human cardiomyocytes

Experimental data from human cardiac myocytes at body temperature is crucial for a quantitative understanding of clinically relevant cardiac function and development of whole-organ computational models. However, such experimental data is currently very limited. Specifically, important measurements to characterize changes in tension development in human cardiomyocytes that occur with perturbations in cell length are not available. To address this deficiency, in this study we present an experimental data set collected from skinned human cardiac myocytes, including the passive and viscoelastic properties of isolated myocytes, the steady-state force calcium relationship at different sarcomere lengths, and changes in tension following a rapid increase or decrease in length, and after constant velocity shortening. This data set is, to our knowledge, the first characterization of length and velocity-dependence of tension generation in human skinned cardiac myocytes at body temperature.We use this data to develop a computational model of contraction and passive viscoelasticity in human myocytes. Our model includes troponin C kinetics, tropomyosin kinetics, a three-state crossbridge model that accounts for the distortion of crossbridges, and the cellular viscoelastic response. Each component is parametrized using our experimental data collected in human cardiomyocytes at body temperature. Furthermore we are able to confirm that properties of length-dependent activation at 37°C are similar to other species, with a shift in calcium sensitivity and increase in maximum tension. We revise our model of tension generation in the skinned isolated myocyte to replicate reported tension traces generated in intact muscle during isometric tension, to provide a model of human tension generation for multi-scale simulations. This process requires changes to calcium sensitivity, cooperativity, and crossbridge transition rates. We apply this model within multi-scale simulations of biventricular cardiac function and further refine the parametrization within the whole organ context, based on obtaining a healthy ejection fraction. This process reveals that crossbridge cycling rates differ between skinned myocytes and intact myocytes.

Supporting Sensemaking to Promote a Systemic View of Organizational Change – Contributions from Activity Theory

ABSTRACTIn this case study, the potential of intentional arenas and tools to promote collective sensemaking during major structural and technical transformations in a specialist organization in the Finnish forestry sector is examined. The results suggest that sensemaking processes in complex change situations are intimately linked to the temporal development of tensions concerning new ways and structures of working. The article demonstrates how Change Workshops, based on activity theoretical methods and tools, shaped sensemaking by offering means to collectively investigate developmental tensions and their potential resolutions and promoted the creation of a systemic view of changing work. This research thus provides an insight into ways in which to foster systemic and future-orientated perspectives in collective sensemaking processes, which in turn improve the management of organizational change.

SIMULATION MODELING OF PROGNOSTICATION OF SOCIAL AND ECONOMIC DEVELOPMENT OF UKRAINE' REGIONS IN THE NON-STATIONARY CONDITIONS

The character of dynamics, pace and proportions of socio-economic indicators in Ukraine are largely under the influence of non-stationary conditions of operation. Some signs of non-stationary structure are unstable macroeconomic processes in time, the difficulty of formalizing their dynamics, the uncertain future etc. Ignoring these features along with the lack of statistical data makes impossible the evaluation, analysis and forecasting of dynamics of macroeconomic processes by classical methods. Therefore the study aims at reviewing the nature and characteristics of simulation for analyzing and forecasting the socio-economic development of the country. The results of the study have shown that the basis of such simulations are diagrams causality – cognitive map and study the problem on the basis of a cognitive map is through the distribution of the pulsed process. The authors have also analyzed a number of modern simulation (cognitive) models of regional development proposed in the scientific literature that demonstrate the use of this tool as the modeling of the dynamics of macroeconomic indicators and indicators of local economic and social development of the region. Finally, the basic advantages and disadvantages of the use of cognitive models in the modeling of macroeconomic and regional dynamics were defined. Having completed the study the authors made the following conclusions. The experiments made for several scenarios of development of social tension in different regions and implementing various managerial influences helped identify priority directions for reducing social tension, a high effect of increasing employment and subsidies to the poor in particular. Applying pulse models can predict the development of specific situations in certain regions and make substatntiated recommendations on measures of economic and social policy.

A simple model of cardiac muscle for multiscale simulation: Passive mechanics, crossbridge kinetics and calcium regulation

A simple model of cardiac muscle was designed for multiscale simulation of heart mechanics. Relaxed cardiac muscle was described as a transversally isotropic hyperelastic material. Active tension caused by actin-myosin crossbridges depends on the ensemble averaged strain of myosin heads bound to actin. Calcium activation was modeled by Ca2+ binding to troponin-C. To account for the dependence of troponin affinity for Ca2+ on myosin heads strongly bound to actin, the kinetics of troponin binding to Ca2+ in the overlap zone of the thin and thick filaments and outside it were separated. The changes in the length of these zones during muscle shortening or lengthening were accounted for explicitly. Simplified version of the model contains only 5 ordinary differential equations (ODE). Model parameters were estimated from a limited set of experiments with skeletal and cardiac muscle. Simulations have shown that model reproduces qualitatively a number of experimental observations: steady-state force-velocity and stiffness-velocity relations; mechanical responses to step changes in muscle length or load; steep Ca2+-tension relationship and its dependence on sarcomere length tension (the Frank-Starling mechanism); tension, shortening and Ca2+-transients in twitch isometric and isotonic contractions, tension development and redevelopment upon fast change in Ca2+ concentration or muscle release followed by re-stretch. We believe that the model can be effectively used for modeling contraction and relaxation of the heart.

The Skeletal Muscle Molecular Clock Regulates Titin Isoform Expression

The circadian clock transcription factors, BMAL1 and CLOCK, are fundamental transcriptional regulators of cell time keeping and critical cell specific genes important for homeostasis. To determine the specific role of the molecular clock in adult skeletal muscle, our lab developed the inducible, skeletal muscle specific Bmal1 knockout (iMSBmal1-/-) mouse. Notably, skeletal muscles from these mice exhibit decreased specific tension and reduced unstimulated tension developed during a fatigability test. To begin to discern the molecular mechanisms that link the changes in the molecular clock with changes in muscle function, we tested whether iMSBmal1-/- muscle would also exhibit changes in sarcomeric protein expression. We determined that the tibialis anterior muscle of the iMSBmal1-/- mice exhibits a significant increase (38% in iMSBmal1-/- mice vs. 19% in iMSBmal1+/+ mice) in expression of a longer isoform of titin. It is established that titin, the giant filamentous protein that maintains sarcomeric structure, underlies passive tension, and contributes to active tension development. To determine if this increased heterogeneity of titin isoforms within the TA had an effect on sarcomere length, we performed immunohistochemistry. An antibody to α-actinin was used to demarcate Z-lines in longitudinal sections of the tibialis anterior muscle and sarcomere length was measured as the distance between Z-lines. While average sarcomere length was not different between iMSBmal1+/+ and iMSBmal1-/- muscle, variation in sarcomere length was increased following Bmal1 knockout in skeletal muscle. Further studies with our model are currently ongoing and will lead to increased knowledge of the importance of titin isoform maintenance in a variety of pathological conditions.

Functional Significance of C-Terminal Mobile Domain of Cardiac Troponin I

C-terminal mobile domain of cTnI (cTnI-MD, residues 168-210) is a highly conserved region which plays important roles in regulating myofilament function. Due to the unresolved structure of the region, it is still unclear how cTnI-MD plays a role in modulating the sarcomere length (SL)-dependent and Ca2+-induced effects on cardiac contractility. In this study, the functional significance of the MD in contractile behavior of skinned rat papillary muscle fibers and SL dependent activation was studied by truncation of the entire MD (cTnI(1-167)) and the C-terminus of MD (cTnI(1-193)). We hypothesized that different truncations may have different impacts on SL-dependent Ca2+-regulation of tension development and cross-bridge cycling. To test this hypothesis, the tension-pCa relationships and myosin cross-bridge kinetics were measured as cTnI(1-167) and cTnI(1-193) were exchanged into skinned rat papillary muscle fibers at short (SL=1.9µm) and long SLs (SL=2.2µm). Our results indicate that the SL induced enhancement of Ca2+-sensitivity and cooperativity observed in wild-type fibers is preserved in cTnI(1-193) and is diminished in cTnI(1-167) fibers. cTnI(1-167) truncation resulted in enhanced maximum tension and cooperativity of Ca2+-activation compared to cTnI(WT) and cTnI(1-193) and enhanced Ca2+-sensitivity compared to cTnI(WT) at both short and long SLs. cTnI(1-193) truncation resulted in enhanced myofilament Ca2+-sensitivity at both SLs and a depressed maximum tension just at long SL. Both cTnI-MD truncations showed a faster cross-bridge attachment rate at both SLs. Compared to cTnI(WT), cTnI(1-167) showed a slower cross-bridge detachment rate while cTnI(1-193) showed a faster cross-bridge detachment rate at long SL. The results of our study indicate that the truncation of cTnI-MD at different regions causes destabilization of the closed-state position of tropomyosin suggested by the observed increase in the cross-bridge attachment rate to thin filament as well as Ca2+-sensitivity of tension development at both sarcomere lengths.

Sarcomere Lengthdependent Effects on the Ca2+-Troponin Regulation in Skinned Myocardial Fiber from Titin RBM20 Deletion Mice

The length dependent activation of the myofilaments has been considered the cellular basis underlying the Frank-Starling law of the heart trough dynamic and complex interplays between a multitude of thick- and thin-filament components. Among these components, both troponin and giant protein titin are likely key players, but the mechanism, by which the length dependent myofilament Ca2+ regulation is linked functionally to titin, is elusive. In this study we investigate the link using in situ FRET technique to monitor how the length dependent Ca2+-induced conformational change of the N-domain of cTnC within myocardium is modulated by alteration of titin compliance that directly determines the passive tension of myocardium. To achieve the objective, FRET donor and acceptor modified cTnC(13C/51C)AEDANS-DDPM was reconstituted into chemically-skinned myocardial fibers from wild-type mice and RBM20 deficient mice that expresses more compliant N2BA titin. The Ca2+-induced conformational change in cTnC under different conditions were quantitatively characterized using time-resolved FRET measurements. Our results from wild type myocardium showed that sarcomere length dependent enhancement of Ca2+-troponin regulation was found to be directly associated with strong crossbridge and it was diminished when the strong crossbridge was inhibited. When measurements were performed with skinned myocardial fibers from RBM20 deletion animals, the length dependent change was blunted. These results support that titin based passive tension development of myocardium is indeed involved in sarcomere length modulated Ca2+-troponin regulation and that the strong crossbridge plays an important role in linking the modulatory effect of titin compliance to Ca2+-troponin regulation.

Incompressible fluid plays a mechanical role in the development of passive muscle tension.

Over short time scales, muscle fibres maintain a nearly constant volume of intracellular fluid. This fluid is essential to normal biochemical function, but its role in determining the mechanical properties of muscle has been considered in only a few theoretical analyses. Here we investigate the mechanical role of fluid in a fundamental property of muscle, its development of passive tension in response to stretch. We test a model of muscle structure in which incompressible fluid directly influences passive tension by constraining the geometry of intramuscular connective tissues. This interaction is demonstrated using a simple physical model of muscle morphology comprising a fluid-filled bladder wrapped by helical fibres. The behaviour of the model is compared with that of isolated bullfrog muscle subjected to an osmotic perturbation of intracellular fluid volume. Increasing muscle volume by 40% resulted in 69% increased passive tension, occurring in a manner consistent with the behaviour of the model. These observations support the notion that the interaction of connective tissues with the muscle fibres they surround influences the mechanical behaviour of whole muscles, and highlight the role of fluid as a mechanical component of muscle.

The effects of pH and Pi on tension and Ca2+ sensitivity of ventricular myofilaments from the anoxia-tolerant painted turtle

We aimed to determine how increases in intracellular H+ and inorganic phosphate (Pi) to levels observed during anoxic submergence affect contractility in ventricular muscle of the anoxia-tolerant Western painted turtle, Chrysemys picta bellii Skinned multicellular preparations were exposed to six treatments with physiologically relevant levels of pH (7.4, 7.0, 6.6) and Pi (3 and 8 mmol l-1). Each preparation was tested in a range of calcium concentrations (pCa 9.0-4.5) to determine the pCa-tension relationship for each treatment. Acidosis significantly decreased contractility by decreasing Ca2+ sensitivity (pCa50) and tension development (P<0.001). Increasing [Pi] also decreased contractility by decreasing tension development at every pH level (P<0.001) but, alone, did not affect Ca2+ sensitivity (P=0.689). Simultaneous increases in [H+] and [Pi] interacted to attenuate the decreased tension development and Ca2+ sensitivity (P<0.001), possibly reflecting a decreased sensitivity to Pi when it is present as the dihydrogen phosphate form, which increases as pH decreases. Compared with that of mammals, the ventricle of turtles exhibits higher Ca2+ sensitivity, which is consistent with previous studies of ectothermic vertebrates.

Importance of SERCA2a on early isolated diastolic dysfunction induced by supravalvular aortic stenosis in rats

Cardiac remodeling is defined as changes in shape and function of the heart in response to aggression (pressure overload). The sarcoplasmic reticulum calcium ATPase cardiac isoform 2a (SERCA2a) is a known factor that influences function. A wide spectrum of studies report a decrease in SERCA2a in heart failure, but none evaluate it's the role in early isolated diastolic dysfunction in supravalvular aortic stenosis (AoS). Our hypothesis was that SERCA2a participates in such dysfunction. Thirty-day-old male Wistar rats (60-80 g) were divided into AoS and Sham groups, which were submitted to surgery with or without aorta clipping, respectively. After 6 weeks, the animals were submitted to echocardiogram and functional analysis by isolated papillary muscle (IPM) in basal condition, hypoxia, and SERCA2a blockage with cyclopiazonic acid at calcium concentrations of 0.5, 1.5, and 2.5 mM. Western-blot analyses were used for SERCA2a and phospholamban detection. Data analysis was carried out with Student's t-test and ANOVA. AoS enhanced left atrium and E and A wave ratio, with preserved ejection fraction. Basal condition in IPM showed similar increases in developed tension (DT) and resting tension (RT) in AoS, and hypoxia was similar between groups. After cyclopiazonic acid blockage, final DT was equally decreased and RT was similar between groups, but the speed of relaxation was decreased in the AoS group. Western-blot was uniform in all evaluations. The hypothesis was confirmed, since functional parameters regarding SERCA2a were changed in the AoS group.

COMPLEX MULTI-LEVEL ASSESSMENT OF THE SUSTAINABILITY OF REGIONAL DEVELOPMENT AND ITS VISUALIZATION

This article describes a method of complex assessment of sustainable development of territories on the basis of social development index, index of economic development and index of the ecological tension. The developed method allows one to comprehensively analyze sustainable development of territories, and at the same time enables one to detail the elements of the problem detected by successive hierarchical decomposition of each of aggregated indices. The methodology takes into account the peculiarities of inhomogeneous and difficult comparable regions, the evaluation of which by using conventional methods of identification and typology does not provide the most rapid, effective and objective results. Approbation of methodology was carried out on the example of the Russian Federation. The possibilities for the analysis of sustainable development are supplemented with a map, built in the color triangle method, which allows one to visualize the balance of components at each analyzed level. Considered theoretical principles and their practical use helped to complete an assessment of basic parameters of sustainable development of Russian regions. On their basis it is possible to form a unified list of criteria, which could become a national standard for the assessment of sustainable development at the local, regional and federal levels, and be recommended for practical use in the development and adoption of new regional solutions.

Contradicción y crisis en el capitalismo cognitivo

En la reestructuración del capitalismo que se produjo en los años 70, las nuevas tecnologías de la información y la comunicación tuvieron un papel fundamental permitiendo una utilización intensiva del conocimiento, papel que podemos rastrear en su continuidad hasta el día de hoy. En las últimas décadas asistimos al desarrollo de una tensión contradictoria entre la tendencia a la privatización propia del capitalismo cognitivo y la tendencia democratizadora de la economía social del conocimiento. El presente artículo trata de analizar dicha tensión apuntando a las consecuencias para los modelos de desarrollo que entran en disputa en el actual contexto global.

Effect of flange restraints on shear Tension Field Action in cold-formed C-sections

The recently incorporated Direct Strength Method (DSM) rules for shear in the North American Specification for Cold-Formed Steel Structural Members (NAS S100:2012) consist of design equations for unperforated sections with and without Tension Field Action (TFA). The TFA is mobilised as a result of the development of diagonal tension due to the full depth bolt restraints. The design shear equations based on a reduction factor α to account for reduced TFA were proposed by Pham and Hancock in cases of partly bolted connections with lateral flange restraints. This paper presents an experimental program to further investigate the TFA of cold-formed C-sections in shear. A total of twelve tests were carried out at the University of Sydney. Three different bolted connection configurations were chosen with and without lateral restraints of the top flanges at the supports. Numerical simulations using the Finite Element Method (FEM) were also performed to validate test results and extend data range. For tests without lateral flange restraints, combined twisting and shear failure modes were observed. The lack of lateral flange restraint leads to a significant reduction of the ultimate load especially when more bolts were removed. Based on these experimental and FEM modelling data, a further reduction factor β dependent upon the slenderness of the sections is introduced in this paper to account for the lack of lateral flange restraint along with the previously proposed reduction factor α which accounts for bolt reduction.

Muscle dysfunction in a zebrafish model of Duchenne muscular dystrophy.

Sapje zebrafish lack the protein dystrophin and are the smallest vertebrate model of Duchenne muscular dystrophy (DMD). Their small size makes them ideal for large-scale drug discovery screens. However, the extent that sapje mimic the muscle dysfunction of higher vertebrate models of DMD is unclear. We used an optical birefringence assay to differentiate affected dystrophic sapje larvae from their unaffected siblings and then studied trunk muscle contractility at 4-7 days postfertilization. Preparation cross-sectional area (CSA) was similar for affected and unaffected larvae, yet tetanic forces of affected preparations were only 30-60% of normal. ANCOVA indicated that the linear relationship observed between tetanic force and CSA for unaffected preparations was absent in the affected population. Consequently, the average force/CSA of affected larvae was depressed 30-70%. Disproportionate reductions in twitch vs. tetanic force, and a slowing of twitch tension development and relaxation, indicated that the myofibrillar disorganization evident in the birefringence assay could not explain the entire force loss. Single eccentric contractions, in which activated preparations were lengthened 5-10%, resulted in tetanic force deficits in both groups of larvae. However, deficits of affected preparations were three- to fivefold greater at all strains and ages, even after accounting for any recovery. Based on these functional assessments, we conclude that the sapje mutant zebrafish is a phenotypically severe model of DMD. The severe contractile deficits of sapje larvae represent novel physiological endpoints for therapeutic drug screening.

Limitation of Infarct Size and the Open Artery Hypothesis: A Conversation With Eugene Braunwald, MD.

Eugene Braunwald received an MD from the New York University School of Medicine. He completed an internal medicine residency at Johns Hopkins Hospital and cardiology fellowships at Mount Sinai Hospital, Columbia University, and the National Heart Institute. He served as chief of cardiology and subsequently clinical director of the institute. From 1968 to 1972, he was the founding chair of medicine at the University of California, San Diego. From 1972 to 1996, he served as chair of medicine at the Brigham and Women’s Hospital. In 1975, he was elected to the National Academy of Sciences. He is now the Distinguished Hersey Professor of Medicine at Harvard Medical School and a senior investigator of the TIMI Study Group, which he founded in 1984. Dr Braunwald replies: I became aware of the importance of AMI in 1951 during an elective in cardiology during my senior year at the New York University School of Medicine. At the time, AMI was by far the most common cause of death in adults in the United States. Of the patients who survived to be admitted to hospitals (no one knew how many AMI patients failed to reach hospital emergency departments), about one third died before discharge, either suddenly, presumably from an arrhythmia, or secondary to pump failure. Of the patients who were discharged alive, about one quarter died within a year, most frequently as a consequence of heart failure or recurrent MI. Pathologists had taught that AMI was usually associated with coronary thrombosis. Indeed, during my cardiology elective, Herrick’s classic article describing AMI was required reading. Its title, “Clinical Features of Sudden Obstruction of the Coronary Arteries,” showed how these 2 conditions (coronary occlusion and AMI) were intertwined.1 My direct involvement in the care of several patients who did not survive the acute event left …

A structure-function analysis of the left ventricle.

This study presents a structure-function analysis of the mammalian left ventricle and examines the performance of the cardiac capillary network, mitochondria, and myofibrils at rest and during simulated heavy exercise. Left ventricular external mechanical work rate was calculated from cardiac output and systemic mean arterial blood pressure in resting sheep (Ovis aries; n = 4) and goats (Capra hircus; n = 4) under mild sedation, followed by perfusion-fixation of the left ventricle and quantification of the cardiac capillary-tissue geometry and cardiomyocyte ultrastructure. The investigation was then extended to heavy exercise by increasing cardiac work according to published hemodynamics of sheep and goats performing sustained treadmill exercise. Left ventricular work rate averaged 0.017 W/cm3 of tissue at rest and was estimated to increase to ∼0.060 W/cm3 during heavy exercise. According to an oxygen transport model we applied to the left ventricular tissue, we predicted that oxygen consumption increases from 195 nmol O2·s-1·cm-3 of tissue at rest to ∼600 nmol O2·s-1·cm-3 during heavy exercise, which is within 90% of the oxygen demand rate and consistent with work remaining predominantly aerobic. Mitochondria represent 21-22% of cardiomyocyte volume and consume oxygen at a rate of 1,150 nmol O2·s-1·cm-3 of mitochondria at rest and ∼3,600 nmol O2·s-1·cm-3 during heavy exercise, which is within 80% of maximum in vitro rates and consistent with mitochondria operating near their functional limits. Myofibrils represent 65-66% of cardiomyocyte volume, and according to a Laplacian model of the left ventricular chamber, generate peak fiber tensions in the range of 50 to 70 kPa at rest and during heavy exercise, which is less than maximum tension of isolated cardiac tissue (120-140 kPa) and is explained by an apparent reserve capacity for tension development built into the left ventricle.