SERCA2a, phospholamban, sarcolipin, and ryanodine receptors gene expression in children with congenital heart defects

In animal models of conotruncal heart defects, an abnormal calcium sensitivity of the contractile apparatus and a depressed L-type calcium current have been described. Sarcoplasmic reticulum (SR) Ca(2+) ATPase (SERCA) is a membrane protein that catalyzes the ATP-dependent transport of Ca(2+) from the cytosol to the SR. The activity of SERCA is inhibited by phospholamban (PLN) and sarcolipin (SLN), and all these proteins participate in maintaining the normal intracellular calcium handling. Ryanodine receptors (RyRs) are the major SR calcium-release channels required for excitation-contraction coupling in skeletal and cardiac muscle. Our objective was to evaluate SERCA2a (i.e., the SERCA cardiac isoform), PLN, SLN, and RyR2 (i.e., the RyR isoform enriched in the heart) gene expression in myocardial tissue of patients affected by tetralogy of Fallot (TOF), a conotruncal heart defect. The gene expression of target genes was assessed semiquantitatively by RT-PCR using the calsequestrin (CASQ, a housekeeping gene) RNA as internal standard in the atrial myocardium of 23 pediatric patients undergoing surgical correction of TOF, in 10 age-matched patients with ventricular septal defect (VSD) and in 13 age-matched children with atrial septal defect (ASD). We observed a significantly lower expression of PLN and SLN in TOF patients, while there was no difference between the expression of SERCA2a and RyR2 in TOF and VSD. These data suggest a complex mechanism aimed to enhance the intracellular Ca(2+) reserve in children affected by tetralogy of Fallot.     

The possible role of intracellular Ca2+-stores in the rhythm-inotropic relationship of frog heart muscle (a simulation study)

The hypothesis that intracellular calcium stores play an essential role in determining force-frequency relationships of frog myocardium was tested quantitatively. A simplified mathematical model of excitation-contraction coupling in frog heart muscle was developed and its behaviour under various patterns of stimulation was analysed by means of computer simulation. The model represents a system of ordinary differential equations for individual fluxes within the cell Ca2+-recirculation system and includes a one-compartmental intracellular pool as opposed to the two-compartmental structure of the mammalian sarcoplasmic reticulum (Kaufmann et al. 1974). The behaviour of the model is consistent with available experimental data concerning the basic rhythm-inotropic characteristics of amphibian myocardium and offers some evidence in favour of the basic concept. Within the framework of the proposed model the staircase phenomena in amphibia were accounted for and the impact of different intracellular Ca-movements on the resulting contractile response and rhythm-inotropic phenomena was elucidated.     

Tumor necrosis factor-alpha alters calcium handling and increases arrhythmogenesis of pulmonary vein cardiomyocytes

Inflammation and abnormal calcium homeostasis play important roles in atrial fibrillation. Tumor necrosis factor-alpha (TNFalpha), a proinflammatory cytokine, can induce cardiac arrhythmias. Pulmonary veins (PVs) are critical in initiating paroxysmal atrial fibrillation. This study was designed to investigate whether TNFalpha may change the calcium handling and arrhythmogenic activity of PV cardiomyocytes. We used whole-cell patch clamp and indo-1 fluorimetric ratio technique to investigate the action potentials, ionic currents and intracellular calcium in isolated rabbit single PV cardiomyocytes with and without (control) incubation with TNFalpha (25 ng/ml) for 7-10 h. The expression of sarcoplasmic reticulum ATPase in the control and TNFalpha-treated PV cardiomyocytes was evaluated by confocal micrographs and Western blot. We found that the spontaneous beating rates were similar between the control (n=45) and TNFalpha-treated (n=28) PV cardiomyocytes. Compared with the control PV cardiomyocytes, the TNFalpha-treated PV cardiomyocytes had significantly a larger amplitude of the delayed afterdepolarizations (6.0+/-1.7 vs. 2.6+/-0.8 mV, P<0.05), smaller L-type calcium currents, larger transient inward currents, larger Na(+)-Ca(2+) exchanger currents, a smaller intracellular calcium transient, smaller sarcoplasmic reticulum calcium content, larger diastolic intracellular calcium, a longer decay portion of the calcium transient (Tau), and a decreased sarcoplasmic reticulum ATPase expression. In conclusion, TNFalpha can increase the PV arrhythmogenicity and induce an abnormal calcium homeostasis, thereby causing inflammation-related atrial fibrillation.     

Sarcoplasmic Ca2+ release is prolonged in nonfailing myocardium of diabetic patients

Background Asymptomatic diabetic patients have a high incidence of clinically unrecognized left ventricular dysfunction with an abnormal cardiac response to exercise. We, therefore, examined subclinical defects in the contraction–relaxation cycle and intracellular Ca²⁺ regulation in myocardium of asymptomatic type 2 diabetic patients. Methods Alterations in the dynamics of the intracellular Ca²⁺ transient and contractility were recorded in right atrial myocardium of type 2 diabetic patients and non-diabetic control tissue loaded with fura-2. In order to gain an insight into mechanisms underlying the altered Ca²⁺ handling in diabetic myocardium levels of mRNA, protein expression and phosphorylation of key proteins in sarcoplasmic Ca²⁺ handling were determined. Results In isolated atrial trabeculae of diabetic myocardium the rise of systolic Ca²⁺ was significantly prolonged, but relaxation of the Ca²⁺ transient was unaltered compared to control tissue. Accordingly, the levels of expression of mRNA and protein of the Ca²⁺ release channel (RyR2) of the sarcoplasmic reticulum were reduced by 68 and 22%, respectively. Endogenous phosphorylation of RyR2 by protein kinases C, however, was increased by 31% in diabetic myocardium, as assessed by the back-phosphorylation technique. Levels of expression of SERCA2 and phospholamban were unaltered between both groups. Conclusions Intracellular Ca²⁺ release is prolonged in non-failing myocardium of type 2 diabetic patients and this may be primarily due to a decreased expression of RyR2. This defective Ca²⁺ release may represent an early stage of ventricular dysfunction in type 2 diabetes and would favor the abnormal response to exercise frequently observed in asymptomatic diabetic patients.     

Calcium signaling in isolated skeletal muscle fibers investigated under “silicone voltage-clamp” conditions

In skeletal muscle, release of calcium from the sarcoplasmic reticulum (SR) represents the major source of cytoplasmic Ca²⁺ elevation. SR calcium release is under the strict command of the membrane potential, which drives the interaction between the voltage sensors in the t-tubule membrane and the calcium-release channels. Either detection or control of the membrane voltage is thus essential when studying intracellular calcium signaling in an intact muscle fiber preparation. The silicone-clamp technique used in combination with intracellular calcium measurements represents an efficient tool for such studies. This article reviews some properties of the plasma membrane and intracellular signals measured with this methodology in mouse skeletal muscle fibers. Focus is given to the potency of this approach to investigate both fundamental aspects of excitation-contraction coupling and potential alterations of intracellular calcium handling in some muscle diseases.     

Ca(2+) handling in isolated human atrial myocardium

Physiologically, human atrial and ventricular myocardium are coupled by an identical beating rate and rhythm. However, contractile behavior in atrial myocardium may be different from that in ventricular myocardium, and little is known about intracellular Ca(2+) handling in human atrium under physiological conditions. We used rapid cooling contractures (RCCs) to assess sarcoplasmic reticulum (SR) Ca(2+) content and the photoprotein aequorin to assess intracellular Ca(2+) transients in atrial and ventricular muscle strips isolated from nonfailing human hearts. In atrial myocardium (n = 19), isometric twitch force frequency dependently (0. 25-3 Hz) increased by 78 +/- 25% (at 3 Hz; P < 0.05). In parallel, aequorin light signals increased by 111 +/- 57% (P < 0.05) and RCC amplitudes by 49 +/- 13% (P < 0.05). Similar results were obtained in ventricular myocardium (n = 13). SR Ca(2+) uptake (relative to Na(+)/Ca(2+) exchange) frequency dependently increased in atrial and ventricular myocardium (P < 0.05). With increasing rest intervals (1-240 s), atrial myocardium (n = 7) exhibited a parallel decrease in postrest twitch force (at 240 s by 68 +/- 5%, P < 0.05) and RCCs (by 49 +/- 10%, P < 0.05). In contrast, postrest twitch force and RCCs significantly increased in ventricular myocardium (n = 6). We conclude that in human atrial and ventricular myocardium the positive force-frequency relation results from increased SR Ca(2+) turnover. In contrast, rest intervals in atrial myocardium are associated with depressed contractility and intracellular Ca(2+) handling, which may be due to rest-dependent SR Ca(2+) loss (Ca(2+) leak) and subsequent Ca(2+) extrusion via Na(+)/Ca(2+) exchange. Therefore, the influence of rate and rhythm on mechanical performance is not uniform in atrial and ventricular myocardium.     

Cardiac force-interval relationship,adrenaline and sarcoplasmic reticulum in rainbow trout

The force-interval relationship was examined at 20 and 10 °C in electrically paced atrial and ventricular tissue of rainbow trout,Oncorhynchus mykiss, regarding dependence on the sarcoplasmic reticulum and influence of adrenaline. In both tissues, adrenaline (10^-6 mol·l^-1) doubled control force developed at 0.5 Hz. In atrial but not in ventricular tissue it also shortened the diastolic interval needed for recovery of a given fraction of the control force. In atrial tissue and in ventricular tissue at 20 °C, the fraction of force recovered in the presence of adrenaline was diminished by 10 mol·l^-1 of ryanodine, a specific inhibitor of the sarcoplasmic reticulum. In atrial tissue not exposed to adrenaline and in ventricular tissue at 10 °C irrespective of adrenaline, ryanodine did not affect recovery. In atrial but not in ventricular tissue it also diminished control force. In conclusion, the cardiac sarcoplasmic reticulum of trout seems to support force development during adrenaline dependent increases in heart rate, and in atrial tissue also the force at steady state.Abbreviations E-C coupling excitation-contraction coupling - P-R potential - SR sarcoplasmic reticulum - SE standard error of the mean     

Protein kinase C activation in human ventricular myocardium

We investigated the effects of 12-deoxyphorbol 13 isobutyrate 20 acetate (DPBA) on contractile function and intracellular calcium handling in normal human ventricular myocardium. The activation of protein kinase C by DPBA resulted in a decrease in sarcoplasmic reticulum calcium release and a reduction in isometric twitch. Force-Calcium relationships were obtained by tetanizing intact muscles or by chemically skinning muscle fibers. These relationships were fitted to a modified Hill function. In intact preparations, DPBA shifted the force-calcium relationship towards higher intracellular calcium concentrations by 0.12 microM (n = 5) and maximal force production was decreased 45.5 +/- 6.1%. These experiments show that protein kinase C activation affects intracellular calcium availability and myofibrillar calcium responsiveness.     

A dynamic model of excitation-contraction coupling during acidosis in cardiac ventricular myocytes

Acidosis in cardiac myocytes is a major factor in the reduced inotropy that occurs in the ischemic heart. During acidosis, diastolic calcium concentration and the amplitude of the calcium transient increase, while the strength of contraction decreases. This has been attributed to the inhibition by protons of calcium uptake and release by the sarcoplasmic reticulum, to a rise of intracellular sodium caused by activation of sodium-hydrogen exchange, decreased calcium binding affinity to Troponin-C, and direct effects on the contractile machinery. The relative contributions and concerted action of these effects are, however, difficult to establish experimentally. We have developed a mathematical model to examine altered calcium-handling mechanisms during acidosis. Each of the alterations was incorporated into a dynamical model of pH regulation and excitation-contraction coupling to predict the time courses of key ionic species during acidosis, in particular intracellular pH, sodium and the calcium transient, and contraction. This modeling study suggests that the most significant effects are elevated sodium, inhibition of sodium-calcium exchange, and the direct interaction of protons with the contractile machinery; and shows how the experimental data on these contributions can be reconciled to understand the overall effects of acidosis in the beating heart.     

Specific absence of the alpha 1 subunit of the dihydropyridine receptor in mice with muscular dysgenesis

Muscular dysgenesis is a lethal mutation in mice that results in a complete absence of skeletal muscle contraction due to the failure of depolarization of the transverse tubular membrane to trigger calcium release from the sarcoplasmic reticulum. In order to determine whether the defect in muscular dysgenesis leads to a specific loss of one of the components of excitation-contraction coupling or to a generalized loss of all components of excitation-contraction coupling, we have analyzed skeletal muscle from control and dysgenic mice for the sarcoplasmic reticulum and transverse tubular proteins which are believe to function in excitation-contraction coupling. We report that the proteins involved in sarcoplasmic reticulum calcium transport, storage, and release [Ca2+ + Mg2+)-ATPase, calsequestrin, and calcium release channel) are present in dysgenic muscle. Also present in dysgenic muscle is the 175/150-kDa glycoprotein subunit (alpha 2) of the dihydropyridine receptor. However, the 170-kDa dihydropyridine binding subunit (alpha 1) of the dihydropyridine receptor is absent in dysgenic muscle. These results suggest that the specific absence of the alpha 1 subunit of the dihydropyridine receptor is responsible for the defects in muscular dysgenesis and that the alpha 1 subunit of the dihydropyridine receptor is essential for excitation-contraction coupling in skeletal muscle.     

Morphology and molecular composition of sarcoplasmic reticulum surface junctions in the absence of DHPR and RyR in mouse skeletal muscle

Calcium release during excitation-contraction coupling of skeletal muscle cells is initiated by the functional interaction of the exterior membrane and the sarcoplasmic reticulum (SR), mediated by the "mechanical" coupling of ryanodine receptors (RyR) and dihydropyridine receptors (DHPR). RyR is the sarcoplasmic reticulum Ca(2+) release channel and DHPR is an L-type calcium channel of exterior membranes (surface membrane and T tubules), which acts as the voltage sensor of excitation-contraction coupling. The two proteins communicate with each other at junctions between SR and exterior membranes called calcium release units and are associated with several proteins of which triadin and calsequestrin are the best characterized. Calcium release units are present in diaphragm muscles and hind limb derived primary cultures of double knock out mice lacking both DHPR and RyR. The junctions show coupling between exterior membranes and SR, and an apparently normal content and disposition of triadin and calsequestrin. Therefore SR-surface docking, targeting of triadin and calsequestrin to the junctional SR domains and the structural organization of the two latter proteins are not affected by lack of DHPR and RyR. Interestingly, simultaneous lack of the two major excitation-contraction coupling proteins results in decrease of calcium release units frequency in the diaphragm, compared with either single knockout mutation.     

Force-frequency relations in hypertrophic heart muscle: a mathematical model for excitation-contraction coupling.

Static and dynamic chrono-inotropic responses were recorded from both normal and hypertrophic rat auricular myocardium. The slope of the static force-frequency relation for hypertrophic hearts was steeper than that for control hearts. Computer experiments were designed to study the cellular mechanisms underlying the changes in the force-frequency response associated with heart hypertrophy, with the aid of a mathematical model for excitation-contraction coupling in rat heart. A set of equations was derived which permitted to study the effects on the chronoinotropic relations of both the geometrical dimensions of cardiomyocytes and the sarcoplasmic reticulum, and of the variation in activity of mechanisms for Ca movements through the sarcolemma and the sarcoreticular membrane. A comparison of data obtained from simulated and real experiments suggested that the features characteristic of force-frequency relations for hypertrophic heart are a result of an enhanced volume of intracellular Ca-stores rather than of the total volume of the cardiomyocyte.     

Decreased mAKAP, ryanodine receptor, and SERCA2a gene expression in mdx hearts

Duchenne muscular dystrophy (DMD) is a common genetic disease resulting from mutations in the dystrophin gene. The lack of dystrophin function as a cytoskeletal protein leads to abnormal intracellular Ca(2+) homeostasis, the actual source and functional consequences of which remain obscure. The mdx mouse, a mouse model of DMD, revealed alterations in contractile properties that are likely due to defective Ca(2+) handling. However, the exact mechanisms of the Ca(2+) handling defect are unclear. We performed suppressive subtractive hybridization to isolate differentially expressed genes between 5-month-old mdx and control mice. We observed a decrease in muscle A-kinase anchoring protein (mAKAP) in the mdx hearts. We noticed not only down-regulation of mAKAP mRNA but also decreased mRNA level of the molecules involved in Ca(2+) handling and excitation-contraction (E-C) coupling in the sarcoplasmic reticulum (SR), the cardiac ryanodine receptor, and the sarcoplasmic reticulum Ca(2+) ATPase. Therefore, dystrophin deficiency may cause an impairment of SR Ca(2+) homeostasis and E-C coupling in mdx hearts, in part, by decreased gene expression of molecules involved in SR Ca(2+) handling.     

心肌细胞兴奋收缩耦联的分子机制

肌细胞兴奋时,动作电位通过电压门控钙通道激活肌质网钙释放,由此引发的细胞内钙离子的瞬时升高驱动细胞收缩,这个过程叫做兴奋收缩耦联．21世纪以来,随着钙成像技术和分子细胞生物学技术的联合应用,心肌兴奋收缩耦联的分子机制逐步阐明．本文结合本实验室的相关研究,系统总结该领域的前沿进展,包括钙释放通道的分子性质、电压门控钙通道激活肌质网钙释放通道的动力学过程、生理调控以及病理变化．     

S100A1: a novel inotropic regulator of cardiac performance. Transition from molecular physiology to pathophysiological relevance

Here we review the considerable body of evidence that has accumulated to support the notion of S100A1, a cardiac-specific Ca(2+)-sensor protein of the EF-hand type, as a physiological regulator of excitation-contraction coupling and inotropic reserve mechanisms in the mammalian heart. In particular, molecular mechanisms will be discussed conveying the Ca(2+)-dependent inotropic actions of S100A1 protein in cardiomyocytes occurring independently of beta-adrenergic signaling. Moreover, we will shed light on the molecular structure-function relationship of S100A1 with its cardiac target proteins at the sarcoplasmic reticulum, the sarcomere, and the mitochondria. Furthermore, pathophysiological consequences of disturbed S100A1 protein expression on altered Ca(2+) handling and intertwined systems in failing myocardium will be highlighted. Subsequently, therapeutic options by means of genetic manipulation of cardiac S100A1 expression will be discussed, aiming to complete our current understanding of the role of S100A1 in diseased myocardium.     

Impact of sarcoplasmic reticulum calcium release on calcium dynamics and action potential morphology in human atrial myocytes: a computational study

Electrophysiological studies of the human heart face the fundamental challenge that experimental data can be acquired only from patients with underlying heart disease. Regarding human atria, there exist sizable gaps in the understanding of the functional role of cellular Ca2+ dynamics, which differ crucially from that of ventricular cells, in the modulation of excitation-contraction coupling. Accordingly, the objective of this study was to develop a mathematical model of the human atrial myocyte that, in addition to the sarcolemmal (SL) ion currents, accounts for the heterogeneity of intracellular Ca2+ dynamics emerging from a structurally detailed sarcoplasmic reticulum (SR). Based on the simulation results, our model convincingly reproduces the principal characteristics of Ca2+ dynamics: 1) the biphasic increment during the upstroke of the Ca2+ transient resulting from the delay between the peripheral and central SR Ca2+ release, and 2) the relative contribution of SL Ca2+ current and SR Ca2+ release to the Ca2+ transient. In line with experimental findings, the model also replicates the strong impact of intracellular Ca2+ dynamics on the shape of the action potential. The simulation results suggest that the peripheral SR Ca2+ release sites define the interface between Ca2+ and AP, whereas the central release sites are important for the fire-diffuse-fire propagation of Ca2+ diffusion. Furthermore, our analysis predicts that the modulation of the action potential duration due to increasing heart rate is largely mediated by changes in the intracellular Na+ concentration. Finally, the results indicate that the SR Ca2+ release is a strong modulator of AP duration and, consequently, myocyte refractoriness/excitability. We conclude that the developed model is robust and reproduces many fundamental aspects of the tight coupling between SL ion currents and intracellular Ca2+ signaling. Thus, the model provides a useful framework for future studies of excitation-contraction coupling in human atrial myocytes.     

Tail-anchored membrane protein SLMAP is a novel regulator of cardiac function at the sarcoplasmic reticulum

Sarcolemmal membrane-associated proteins (SLMAPs) are components of cardiac membranes involved in excitation-contraction (E-C) coupling. Here, we assessed the role of SLMAP in cardiac structure and function. We generated transgenic (Tg) mice with cardiac-restricted overexpression of SLMAP1 bearing the transmembrane domain 2 (TM2) to potentially interfere with endogenous SLMAP through homodimerization and subcellular targeting. Histological examination revealed vacuolated myocardium; the severity of which correlated with the expression level of SLMAP1-TM2. High resolution microscopy showed dilation of the sarcoplasmic reticulum/endoplasmic reticulum (SR/ER) and confocal imaging combined with biochemical analysis indicated targeting of SLMAP1-TM2 to the SR/ER membranes and inappropriate homodimerization. Older (28 wk of age) Tg mice exhibited reduced contractility with impaired relaxation as assessed by left ventricle pressure monitoring. The ventricular dysfunction was associated with electrophysiological abnormalities (elongated QT interval). Younger (5 wk of age) Tg mice also exhibited an elongated QT interval with minimal functional disturbances associated with the activation of the fetal gene program. They were less responsive to isoproterenol challenge (ΔdP/dt(max)) and developed electrical and left ventricular pressure alternans. The altered electrophysiological and functional disturbances in Tg mice were associated with diminished expression level of calcium cycling proteins of the sarcoplasmic reticulum such as the ryanodine receptor, Ca(2+)-ATPase, calsequestrin, and triadin (but not phospholamban), as well as significantly reduced calcium uptake in microsomal fractions. These data demonstrate that SLMAP is a regulator of E-C coupling at the level of the SR and its perturbation results in progressive deterioration of cardiac electrophysiology and function.     

Effect of temperature and temperature acclimation on the ryanodine sensitivity of the trout myocardium

The influence of acute temperature change and temperature acclimation on the sensitivity of contracture development to ryanodine were examined in the rainbow trout myocardium using two preparations: in vitro isolated ventricular strips and in situ working perfused hearts. Ryanodine effects in vitro were dependent on test temperature (8 and 18 °C), pacing frequency (0.2–1.5 Hz) and acclimation temperature (8 and 18 °C). At a pacing frequency of 0.2 Hz and a test temperature of 18 °C, ryanodine depressed isometric tension development in ventricular strips both from trout acclimated to 8 and 18 °C but the decrease was significantly greater in strips from 8 °C-acclimated trout. No ryanodine effect was observed in either acclimation group at a test temperature of 8°C. The effect of ryanodine in vitro was reduced or lost at pacing frequencies greater than 0.2 Hz and at 0.6 Hz ryanodine depressed tension development at 18 °C only in strips from 8 °C-acclimated trout. Ryanodine did not affect tension development at stimulation rates above 0.6 Hz in any test group. Likewise, ryanodine did not significantly impair cardiac performance of in situ working perfused heart preparations which operated at intrinsic beat frequencies in excess of 0.6 Hz. These results suggest that the sarcoplamic reticulum calcium release channel of the trout myocardium is expressed but is not functionally involved in beat-to-beat regulation of contractility at either (1) low temperature (8 °C), or (2) at routine physiological heart rate (>0.6 Hz). However, under conditions in which involvement of the sarcoplasmic reticulum is observed (18 °C and a heart rate < 0.6 Hz), prior acclimation to low temperature results in either a greater capacity of the sarcoplasmic reticulum to store releasable calcium or an increase in the amount of calcium that is in releasable form.Abbreviations bm body mass - E-C coupling, excitation-contraction coupling - IVS isometric ventricular strip - SR sarcoplasmic reticulum - TES N-tris[hydroxy-methyl]methyl-2-aminoethane sulfonic acid - WPH in situ working perfused heart     

Ryanodine receptors: structure, expression, molecular details, and function in calcium release

Ryanodine receptors (RyRs) are located in the sarcoplasmic/endoplasmic reticulum membrane and are responsible for the release of Ca(2+) from intracellular stores during excitation-contraction coupling in both cardiac and skeletal muscle. RyRs are the largest known ion channels (> 2MDa) and exist as three mammalian isoforms (RyR 1-3), all of which are homotetrameric proteins that interact with and are regulated by phosphorylation, redox modifications, and a variety of small proteins and ions. Most RyR channel modulators interact with the large cytoplasmic domain whereas the carboxy-terminal portion of the protein forms the ion-conducting pore. Mutations in RyR2 are associated with human disorders such as catecholaminergic polymorphic ventricular tachycardia whereas mutations in RyR1 underlie diseases such as central core disease and malignant hyperthermia. This chapter examines the current concepts of the structure, function and regulation of RyRs and assesses the current state of understanding of their roles in associated disorders.     

A model of excitation-contraction coupling of mammalian cardiac muscle

A model is developed for the excitation-contraction coupling of mammalian cardiac muscle. This model assumes that upon depolarization, the calcium current not only raises the sarcoplasmic Ca²⁺ concentration, but also induces the release of Ca from cisternal sarcoplasmic reticulum, whose rate of release depends on the membrane potential. These two main sources of calcium elevate the sarcoplasmic Ca²⁺ concentration so that it activates the interaction of myosin and actin and initiates contraction in accordance with Huxley's sliding filament mechanism. The uptake and recycling of Ca²⁺ to cisternal sarcoplasmic reticulum is accomplished by the longitudinal sarcoplasmic reticulum. Mitochondria are assumed to accumulate mainly Ca²⁺. The uptake of Ca is considered to be an active process, utilizing energy.The proposed model qualitatively predicts the following electrical-mechanical events often observed in living muscle: tension-voltage-duration, staircase phenomenon, frequency-strength relationship, post-extrasystolic potentiation and contractile behavior after a period of rest.