77 research outputs found
3D ultrastructural organisation of calcium release units in the avian sarcoplasmic reticulum
Excitation-contraction coupling in vertebrate hearts is underpinned by calcium (Ca2+) release from Ca2+ release units (CRUs). CRUs are formed by clusters of channels called ryanodine receptors on the sarcoplasmic reticulum (SR) within the cardiomyocyte. Distances between CRUs influence the diffusion of Ca2+, thus influencing the rate and strength of excitation-contraction coupling. Avian myocytes lack T-tubules, thus Ca2+ from surface CRUs (peripheral couplings, PCs), must diffuse to internal CRU sites of the corbular SR (cSR) during centripetal propagation. Despite this, avian hearts achieve higher contractile rates and develop greater contractile strength than many mammalian hearts, which have T-tubules to provide simultaneous activation of the Ca2+ signal through the myocyte. We used 3D electron tomography to test the hypothesis that the intracellular distribution of CRUs in the avian heart permits faster and stronger contractions despite the absence T-tubules. Nearest edge-edge distances between PCs and cSR, and geometric information including surface area and volumes of individual cSR, were obtained for each cardiac chamber of the White Leghorn chicken. Computational modelling was then used to establish a relationship between CRUs distances and cell activation time in the avian heart. Our data suggest that cSR clustered close together along the Z-line is vital for rapid propagation of the Ca2+ signal from the cell periphery to the cell centre which would aid in the strong and fast contractions of the avian heart
Mouse intact cardiac myocyte mechanics: cross-bridge and titin-based stress in unactivated cells
A carbon fiberâbased cell attachment and force measurement system was used to measure the diastolic stressâsarcomere length (SL) relation of mouse intact cardiomyocytes, before and after the addition of actomyosin inhibitors (2,3-butanedione monoxime [BDM] or blebbistatin). Stress was measured during the diastolic interval of twitching myocytes that were stretched at 100% base length/second. Diastolic stress increased close to linear from 0 at SL 1.85 ”m to 4.2 mN/mm2 at SL 2.1 ”m. The actomyosin inhibitors BDM and blebbistatin significantly lowered diastolic stress by âŒ1.5 mN/mm2 (at SL 2.1 ”m, âŒ30% of total), suggesting that during diastole actomyosin interaction is not fully switched off. To test this further, calcium sensitivity of skinned myocytes was studied under conditions that simulate diastole: 37°C, presence of Dextran T500 to compress the myofilament lattice to the physiological level, and [Ca2+] from below to above 100 nM. Mean active stress was significantly increased at [Ca2+] > 55 nM (pCa 7.25) and was âŒ0.7 mN/mm2 at 100 nM [Ca2+] (pCa 7.0) and âŒ1.3 mN/mm2 at 175 nM Ca2+ (pCa 6.75). Inhibiting active stress in intact cells attached to carbon fibers at their resting SL and stretching the cells while first measuring restoring stress (pushing outward) and then passive stress (pulling inward) made it possible to determine the passive cellâs mechanical slack SL as âŒ1.95 ”m and the restoring stiffness and passive stiffness of the cells around the slack SL each as âŒ17 mN/mm2/”m/SL. Comparison between the results of intact and skinned cells shows that titin is the main contributor to restoring stress and passive stress of intact cells, but that under physiological conditions, calcium sensitivity is sufficiently high for actomyosin interaction to contribute to diastolic stress. These findings are relevant for understanding diastolic function and for future studies of diastolic heart failure
History-Dependent Mechanical Properties of Permeabilized Rat Soleus Muscle Fibers
AbstractPermeabilized rat soleus muscle fibers were subjected to repeated triangular length changes (paired ramp stretches/releases, 0.03 l0,±0.1 l0 sâ1 imposed under sarcomere length control) to investigate whether the rate of stiffness recovery after movement increased with the level of Ca2+ activation. Actively contracting fibers exhibited a characteristic tension response to stretch: tension rose sharply during the initial phase of the movement before dropping slightly to a plateau, which was maintained during the remainder of the stretch. When the fibers were stretched twice, the initial phase of the response was reduced by an amount that depended on both the level of Ca2+ activation and the elapsed time since the first movement. Detailed analysis revealed three new and important findings. 1) The rates of stiffness and tension recovery and 2) the relative height of the tension plateau each increased with the level of Ca2+ activation. 3) The tension plateau developed more quickly during the second stretch at high free Ca2+ concentrations than at low. These findings are consistent with a cross-bridge mechanism but suggest that the rate of the force-generating power-stroke increases with the intracellular Ca2+ concentration and cross-bridge strain
Assessment of contractility in intact ventricular cardiomyocytes using the dimensionless âFrankâStarling Gainâ index
This paper briefly recapitulates the FrankâStarling law of the heart, reviews approaches to establishing diastolic and systolic forceâlength behaviour in intact isolated cardiomyocytes, and introduces a dimensionless index called âFrankâStarling Gainâ, calculated as the ratio of slopes of end-systolic and end-diastolic forceâlength relations. The benefits and limitations of this index are illustrated on the example of regional differences in Guinea pig intact ventricular cardiomyocyte mechanics. Potential applicability of the FrankâStarling Gain for the comparison of cell contractility changes upon stretch will be discussed in the context of intra- and inter-individual variability of cardiomyocyte properties
L'intégration linguistique et sociale des élÚves allophones par le tutorat
In 2017, in France, the allophone children represented 0,56% of the total school population. Their integration into the school system is essential to their educational and professional success. This is made possible thanks to different measures implemented by the teaching staff which allow the students to integrate themselves socially and linguistically. It is with that objective in mind that peer tutoring is analyzed here through defining its advantages and limitations. To that end, the first part addresses allophone studentsâ integration from a theoretical perspective. The second part introduces the survey used in this study. Lastly, we will analyze the results in order to get a better understanding of the effects of peer tutoring among the allophone students. * An immigrant child whose parentsâ mother tongue is not French.En France, en 2017, lâensemble des enfants allophones reprĂ©sentait 0.56 % des effectifs par rapport Ă la population scolaire totale. Leur intĂ©gration dans le systĂšme scolaire est essentielle Ă leur rĂ©ussite scolaire et professionnelle. Celle-ci passe par les dispositifs mis en place par les enseignants afin quâils puissent sâintĂ©grer aussi bien dâun point de vue linguistique que sociale. Câest dans cette perspective que le tutorat entre pairs est ici Ă©tudiĂ© afin dâen connaĂźtre les avantages et les limites. Pour ce faire, une premiĂšre partie aborde lâintĂ©gration des enfants allophones dâun point de vue thĂ©orique. La deuxiĂšme partie prĂ©sente le questionnaire ayant permis cette recherche. Enfin, la derniĂšre partie analyse ses rĂ©sultats afin de comprendre lâeffet du tutorat entre pairs chez lâĂ©lĂšve allophone
Modeling our understanding of the His-Purkinje system
The His-Purkinje System (HPS) is responsible for the rapid electric conduction in the ventricles. It relays electrical impulses from the atrioventricular node to the muscle cells and, thus, coordinates the contraction of ventricles in order to ensure proper cardiac pump function. The HPS has been implicated in the genesis of ventricular tachycardia and fibrillation as a source of ectopic beats, as well as forming distinct portions of reentry circuitry. Despite its importance, it remains much less well characterized, structurally and functionally, than the myocardium. Notably, important differences exist with regard to cell structure and electrophysiology, including ion channels, intracellular calcium handling, and gap junctions. Very few computational models address the HPS, and the majority of organ level modeling studies omit it. This review will provide an overview of our current knowledge of structure and function (including electrophysiology) of the HPS. We will review the most recent advances in modeling of the system from the single cell to the organ level, with considerations for relevant interspecies distinctions
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