Kinetics of ATP hydrolysis and tension production in skinned cardiac muscle of the guinea pig

Myosin Isoform-Dependent Effect of Omecamtiv Mecarbil on the Regulation of Force Generation in Human Cardiac Muscle

Omecamtiv mecarbil (OM) is a small molecule that has been shown to improve the function of the slow human ventricular myosin (MyHC) motor through a complex perturbation of the thin/thick filament regulatory state of the sarcomere mediated by binding to myosin allosteric sites coupled to inorganic phosphate (Pi) release. Here, myofibrils from samples of human left ventricle (β-slow MyHC-7) and left atrium (α-fast MyHC-6) from healthy donors were used to study the differential effects of μmolar [OM] on isometric force in relaxing conditions (pCa 9.0) and at maximal (pCa 4.5) or half-maximal (pCa 5.75) calcium activation, both under control conditions (15 °C; equimolar DMSO; contaminant inorganic phosphate [Pi] ~170 μM) and in the presence of 5 mM [Pi]. The activation state and OM concentration within the contractile lattice were rapidly altered by fast solution switching, demonstrating that the effect of OM was rapid and fully reversible with dose-dependent and myosin isoform-dependent features. In MyHC-7 ventricular myofibrils, OM increased submaximal and maximal Ca2+-activated isometric force with a complex dose-dependent effect peaking (40% increase) at 0.5 μM, whereas in MyHC-6 atrial myofibrils, it had no effect or-at concentrations above 5 µM-decreased the maximum Ca2+-activated force. In both ventricular and atrial myofibrils, OM strongly depressed the kinetics of force development and relaxation up to 90% at 10 μM [OM] and reduced the inhibition of force by inorganic phosphate. Interestingly, in the ventricle, but not in the atrium, OM induced a large dose-dependent Ca2+-independent force development and an increase in basal ATPase that were abolished by the presence of millimolar inorganic phosphate, consistent with the hypothesis that the widely reported Ca2+-sensitising effect of OM may be coupled to a change in the state of the thick filaments that resembles the on-off regulation of thin filaments by Ca2+. The complexity of this scenario may help to understand the disappointing results of clinical trials testing OM as inotropic support in systolic heart failure compared with currently available inotropic drugs that alter the calcium signalling cascade.

Cardiac Disorders and Pathophysiology of Sarcomeric Proteins

The sarcomeric proteins represent the structural building blocks of heart muscle, which are essential for contraction and relaxation. During recent years, it has become evident that posttranslational modifications of sarcomeric proteins, in particular phosphorylation, tune cardiac pump function at rest and during exercise. This delicate, orchestrated interaction is also influenced by mutations, predominantly in sarcomeric proteins, which cause hypertrophic or dilated cardiomyopathy. In this review, we follow a bottom-up approach starting from a description of the basic components of cardiac muscle at the molecular level up to the various forms of cardiac disorders at the organ level. An overview is given of sarcomere changes in acquired and inherited forms of cardiac disease and the underlying disease mechanisms with particular reference to human tissue. A distinction will be made between the primary defect and maladaptive/adaptive secondary changes. Techniques used to unravel functional consequences of disease-induced protein changes are described, and an overview of current and future treatments targeted at sarcomeric proteins is given. The current evidence presented suggests that sarcomeres not only form the basis of cardiac muscle function but also represent a therapeutic target to combat cardiac disease.

A new myofilament contraction model with ATP consumption for ventricular cell model

A new contraction model of cardiac muscle was developed by combining previously described biochemical and biophysical models. The biochemical component of the new contraction model represents events in the presence of Ca2+-crossbridge attachment and power stroke following inorganic phosphate release, detachment evoked by the replacement of ADP by ATP, ATP hydrolysis, and recovery stroke. The biophysical component focuses on Ca2+ activation and force (F b) development assuming an equivalent crossbridge. The new model faithfully incorporates the major characteristics of the biochemical and biophysical models, such as F b activation by transient Ca2+ ([Ca2+]-F b), [Ca2+]-ATP hydrolysis relations, sarcomere length-F b, and F b recovery after jumps in length under the isometric mode and upon sarcomere shortening after a rapid release of mechanical load under the isotonic mode together with the load-velocity relationship. ATP consumption was obtained for all responses. When incorporated in a ventricular cell model, the contraction model was found to share approximately 60% of the total ATP usage in the cell model.

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.

Pathomechanisms in heart failure: the contractile connection

Heart failure is a multi-factorial progressive disease in which eventually the contractile performance of the heart is insufficient to meet the demands of the body, even at rest. A distinction can be made on the basis of the cause of the disease in genetic and acquired heart failure and at the functional level between systolic and diastolic heart failure. Here the basic determinants of contractile function of myocardial cells will be reviewed and an attempt will be made to elucidate their role in the development of heart failure. The following topics are addressed: the tension generating capacity, passive tension, the rate of tension development, the rate of ATP utilisation, calcium sensitivity of tension development, phosphorylation of contractile proteins, length dependent activation and stretch activation. The reduction in contractile performance during systole can be attributed predominantly to a loss of cardiomyocytes (necrosis), myocyte disarray and a decrease in myofibrillar density all resulting in a reduction in the tension generating capacity and likely also to a mismatch between energy supply and demand of the myocardium. This leads to a decline in the ejection fraction of the heart. Diastolic dysfunction can be attributed to fibrosis and an increase in titin stiffness which result in an increase in stiffness of the ventricular wall and hampers the filling of the heart with blood during diastole. A large number of post translation modifications of regulatory sarcomeric proteins influence myocardial function by altering calcium sensitivity of tension development. It is still unclear whether in concert these influences are adaptive or maladaptive during the disease process.

A new state of cardiac myosin with very slow ATP turnover: a potential cardioprotective mechanism in the heart

The mechanisms that control cardiac contractility are complex. Recent work we conducted in vertebrate skeletal muscle identified a new state of myosin, the super-relaxed state (SRX), which had a very low metabolic rate. To determine whether this state also exists in cardiac muscle we used quantitative epi-fluorescence to measure single nucleotide turnovers by myosin in bundles of relaxed permeable rabbit ventricle cells. We measured two turnover times--one compatible with the normal relaxed state, and one much slower which was shown to arise from myosin heads in the SRX. In both skeletal and cardiac muscle, the SRX appears to play a similar role in relaxed cells, providing a state with a very low metabolic rate. However, in active muscle the properties of the SRX differ dramatically. We observed a rapid transition of myosin heads out of the SRX in active skeletal fibers, whereas the population of the SRX remained constant in active cardiac cells. This property allows the SRX to play a very different role in cardiac muscle than in skeletal muscle. The SRX could provide a mechanism for decreasing the metabolic load on the heart, being cardioprotective, particularly in time of stress such as ischemia.

Kinetics of cardiac sarcomeric processes and rate-limiting steps in contraction and relaxation

The sarcomere is the core structure responsible for active mechanical heart function. It is formed primarily by myosin, actin, and titin filaments. Cyclic interactions occur between the cross-bridges of the myosin filaments and the actin filaments. The forces generated by these cyclic interactions provide the molecular basis for cardiac pressure, while the motion produced by these interactions provides the basis for ejection. The cross-bridge cycle is controlled by upstream mechanisms located in the membrane and by downstream mechanisms inside the sarcomere itself. These downstream mechanisms involve the Ca(2+)-controlled conformational change of the regulatory proteins troponin and tropomyosin and strong cooperative interactions between neighboring troponin-tropomyosin units along the actin filament. The kinetics of upstream and downstream processes have been measured in intact and demembranated myocardial preparations. This review outlines a conceptual model of the timing of these processes during the individual mechanical heart phases. Particular focus is given to kinetic data from studies on contraction-relaxation cycles under mechanical loads. Evidence is discussed that the dynamics of cardiac contraction and relaxation are determined mainly by sarcomeric downstream mechanisms, in particular by the kinetics of the cross-bridge cycle. The rate and extent of ventricular pressure development is essentially subjected to the mechanistic principles of cross-bridge action and its upstream and downstream regulation. Sarcomere relengthening during myocardial relaxation plays a key role in the rapid decay of ventricular pressure and in early diastolic filling.

Force transients and minimum cross-bridge models in muscular contraction

Two- and three-state cross-bridge models are considered and examined with respect to their ability to predict three distinct phases of the force transients that occur in response to step change in muscle fiber length. Particular attention is paid to satisfying the Le Châtelier-Brown Principle. This analysis shows that the two-state model can account for phases 1 and 2 of a force transient, but is barely adequate to account for phase 3 (delayed force) unless a stretch results in a sudden increase in the number of cross-bridges in the detached state. The three-state model (A-->B-->C-->A) makes it possible to account for all three phases if we assume that the A-->B transition is fast (corresponding to phase 2), the B-->A transition is of intermediate speed (corresponding to phase 3), and the C-->A transition is slow; in such a scenario, states A and C can support or generate force (high force states) but state B cannot (detached, or low-force state). This model involves at least one ratchet mechanism. In this model, force can be generated by either of two transitions: B-->A or B-->C. To determine which of these is the major force-generating step that consumes ATP and transduces energy, we examine the effects of ATP, ADP, and phosphate (Pi) on force transients. In doing so, we demonstrate that the fast transition (phase 2) is associated with the nucleotide-binding step, and that the intermediate-speed transition (phase 3) is associated with the Pi-release step. To account for all the effects of ligands, it is necessary to expand the three-state model into a six-state model that includes three ligand-bound states. The slowest phase of a force transient (phase 4) cannot be explained by any of the models described unless an additional mechanism is introduced. Here we suggest a role of series compliance to account for this phase, and propose a model that correlates the slowest step of the cross-bridge cycle (transition C-->A) to: phase 4 of step analysis, the rate constant k(tr) of the quick-release and restretch experiment, and the rate constant k(act) for force development time course following Ca(2+) activation.

Impact of temperature on cross-bridge cycling kinetics in rat myocardium

The dependence of contractile properties on intracellular calcium in cardiac tissue is a highly cooperative process. Here, the temperature and calcium dependence of contractile and energetical properties in permeabilized cardiac trabeculae from rat were studied to provide novel insights into the underlying kinetic processes. Myofilament Ca(2+) sensitivity significantly increased with temperature between 15 and 25 degrees C, whereas its steepness was independent of temperature. A direct proportionality between active tension and Ca(2+)-activated rate of ATP hydrolysis was observed; the slope of this relationship (tension cost) was highly temperature dependent. The rate of tension redevelopment following a quick release-restretch manoeuvre (k(tr)) depended in a complex manner on the level of contractile activation and on temperature. At saturating calcium levels, the temperature dependence (Q(10)) of k(tr) and Ca(2+)-activated ATP hydrolysis rate were similar (Q(10) approximately 3.5), and significantly higher than the Q(10) for maximum tension (T(max); Q(10) approximately 1.3) or tension cost (Q(10) approximately 2.5). In contrast, at a low level of contractile activation ( approximately 5% of T(max)), the Q(10) of k(tr) was similar to that of tension cost, and significantly lower than the Q(10) of Ca(2+)-activated ATP hydrolysis at that level of contractile activation. Our results are consistent with the hypothesis that at high levels of contractile activation, the rates of tension redevelopment and Ca(2+)-activated ATP hydrolysis are determined by both apparent cross-bridge attachment and detachment rates, while at low levels, k(tr) is limited by cross-bridge detachment rate. Tension cost, on the other hand, is determined solely by cross-bridge detachment kinetics at all temperatures and levels of contractile activation.

Instabilities in the transient response of muscle

We investigate the isometric transient response of muscle using a quantitative stochastic model of the actomyosin cycle based on the swinging lever-arm hypothesis. We first consider a single pair of filaments, and show that when values of parameters such as the lever-arm displacement and the cross-bridge elasticity are chosen to provide effective energy transduction, the T(2) curve (the tension recovered immediately after a step displacement) displays a region of negative slope. If filament compliance and the discrete nature of the binding sites are taken into account, the negative slope is diminished, but not eliminated. This implies that there is an instability in the dynamics of individual half sarcomeres. However, when the symmetric nature of whole sarcomeres is taken into account, filament rearrangement becomes important during the transient: as tension is recovered, some half sarcomeres lengthen whereas others shorten. This leads to a flat T(2) curve, as observed experimentally. In addition, we investigate the isotonic transient response and show that for a range of parameter values the model displays damped oscillations, as recently observed in experiments on single muscle fibers. We conclude that it is essential to consider the collective dynamics of many sarcomeres, rather than the dynamics of a single pair of filaments, when interpreting the transient response of muscle.

Ca(2+) activation of myofilaments from transgenic mouse hearts expressing R92Q mutant cardiac troponin T

The functional consequences of the R92Q mutation in cardiac troponin T (cTnT), linked to familial hypertrophic cardiomyopathy in humans, are not well understood. We have studied steady- and pre-steady-state mechanical activity of detergent-skinned fiber bundles from a transgenic (TG) mouse model in which 67% of the total cTnT in the heart was replaced by the R92Q mutant cTnT. TG fibers were more sensitive to Ca(2+) than nontransgenic (NTG) fibers [negative logarithm of half maximally activating molar Ca(2+) (pCa(50)) = 5.84 +/- 0.01 and 6.12 +/- 0.01 for NTG and TG fibers, respectively]. The shift in pCa(50) caused by increasing the sarcomere length from 1.9 to 2.3 microm was significantly higher for TG than for NTG fibers (DeltapCa(50) = 0.13 +/- 0.01 and 0.29 +/- 0.02 for NTG and TG fibers, respectively). The relationships between rate of ATP consumption and steady-state isometric tension were linear, and the slopes were the same in NTG and TG fibers. Rate of tension redevelopment was more sensitive to Ca(2+) in TG than in NTG fibers (pCa(50) = 5.71 +/- 0.02 and 6.07 +/- 0.02 for NTG and TG fibers, respectively). We concluded that overall cross-bridge cycling kinetics are not altered by the R92Q mutation but that altered troponin-tropomyosin interactions could be responsible for the increase in myofilament Ca(2+) sensitivity in TG myofilaments.

The myosin cross-bridge cycle and its control by twitchin phosphorylation in catch muscle

The anterior byssus retractor muscle of Mytilus edulis was used to characterize the myosin cross-bridge during catch, a state of tonic force maintenance with a very low rate of energy utilization. Addition of MgATP to permeabilized muscles in high force rigor at pCa > 8 results in a rapid loss of some force followed by a very slow rate of relaxation that is characteristic of catch. The fast component is slowed 3-4-fold in the presence of 1 mM MgADP, but the distribution between the fast and slow (catch) components is not dependent on [MgADP]. Phosphorylation of twitchin results in loss of the catch component. Fewer than 4% of the myosin heads have ADP bound in rigor, and the time course (0.2-10 s) of ADP formation following release of ATP from caged ATP is similar whether or not twitchin is phosphorylated. This suggests that MgATP binding to the cross-bridge and subsequent splitting are independent of twitchin phosphorylation, but detachment occurs only if twitchin is phosphorylated. A similar dependence of detachment on twitchin phosphorylation is seen with AMP-PNP and ATPgammaS. Single turnover experiments on bound ADP suggest an increase in the rate of release of ADP from the cross-bridge when catch is released by phosphorylation of twitchin. Low [Ca(2+)] and unphosphorylated twitchin appear to cause catch by 1) markedly slowing ADP release from attached cross-bridges and 2) preventing detachment following ATP binding to the rigor cross-bridge.

The smooth muscle cross-bridge cycle studied using sinusoidal length perturbations

The mechanical characteristics of smooth muscle can be broadly defined as either phasic, or fast contracting, and tonic, or slow contracting (, Pharmacol. Rev. 20:197-272). To determine if differences in the cross-bridge cycle and/or distribution of the cross-bridge states could contribute to differences in the mechanical properties of smooth muscle, we determined force and stiffness as a function of frequency in Triton-permeabilized strips of rabbit portal vein (phasic) and aorta (tonic). Permeabilized muscle strips were mounted between a piezoelectric length driver and a piezoresistive force transducer. Muscle length was oscillated from 1 to 100 Hz, and the stiffness was determined as a function of frequency from the resulting force response. During calcium activation (pCa 4, 5 mM MgATP), force and stiffness increased to steady-state levels consistent with the attachment of actively cycling cross-bridges. In smooth muscle, because the cross-bridge states involved in force production have yet to be elucidated, the effects of elevation of inorganic phosphate (P(i)) and MgADP on steady-state force and stiffness were examined. When portal vein strips were transferred from activating solution (pCa 4, 5 mM MgATP) to activating solution with 12 mM P(i), force and stiffness decreased proportionally, suggesting that cross-bridge attachment is associated with P(i) release. For the aorta, elevating P(i) decreased force more than stiffness, suggesting the existence of an attached, low-force actin-myosin-ADP- P(i) state. When portal vein strips were transferred from activating solution (pCa 4, 5 mM MgATP) to activating solution with 5 mM MgADP, force remained relatively constant, while stiffness decreased approximately 50%. For the aorta, elevating MgADP decreased force and stiffness proportionally, suggesting for tonic smooth muscle that a significant portion of force production is associated with ADP release. These data suggest that in the portal vein, force is produced either concurrently with or after P(i) release but before MgADP release, whereas in aorta, MgADP release is associated with a portion of the cross-bridge powerstroke. These differences in cross-bridge properties could contribute to the mechanical differences in properties of phasic and tonic smooth muscle.

Loading of calcium and strontium into the sarcoplasmic reticulum in rat ventricular muscle

Previous work suggests that strontium ions (Sr(2+)) are less effective than calcium ions (Ca(2+)) at supporting excitation-contraction (EC) coupling in cardiac muscle. We therefore tested whether this was due to differences in the uptake and release of Ca(2+)and Sr(2+)by the sarcoplasmic reticulum (SR) of rat ventricular trabeculae and myocytes at 22-24 degrees C. In permeabilized trabeculae, isometric contractions activated by exposure to Ca(2+)- and Sr(2+)-containing solutions produced similar maximal force, but were four times more sensitive to Ca(2+)than to Sr(2+). The rate of loading and maximal SR capacity for caffeine-releasable Ca(2+)and Sr(2+)were similar. In isolated, voltage-clamped ventricular myocytes, the SR content was measured as Na(+)-Ca(2+)exchange current during caffeine-induced SR cation releases. The SR Ca(2+)load reached a steady maximum during a train of voltage clamp depolarizations. A similar maximal Sr(2+)load was not observed, suggesting that the SR capacity for Sr(2+)exceeds that for Ca(2+). Therefore, the relative inability of Sr(2+)to support cardiac EC coupling appears not to be due to failure of the SR to sequester Sr(2+). Instead, increases in cytosolic [Sr(2+)] seem to poorly activate Sr(2+)release from the SR.

Influences of protein kinase A and D-cAMP on actin-myosin interaction and energy consumption of cardiac muscles

To address controversies concerning the effects of beta-adrenergic stimulation on the rate of myocardial cross-bridge cycling, we measured three mechanical variables, isometric tension development, transient tension response to a step stretch in length (< 1% of muscle length), maximum velocity of shortening, and a chemical variable, ATPase activity before and after treatment with the catalytic subunit of protein kinase A (PKA) in demembranated rat right ventricular trabeculae, and also measured three mechanical variables before and after treatment with D-cAMP in intact ryanodine-induced tetanized preparations. PKA treatment (I U/microliter, 40 min) shifted the pCa-tension relation to the right from 5.41 to 5.26 at pCa50 (the [Ca2+] required for half maximal steady tension) without changing the steepness of the pCa-tension relation and the maximum tension. The rate of the transient tension changes was significantly increased after either PKA or D-cAMP treatment (5 mM, 15 min), regardless of the level of isometric tension. Vmax was increased for a given Ca2+ concentration after either the PKA or D-cAMP treatment, despite the reduced level of isometric tension. The PKA treatment also shifted the pCa-ATPase activity to the right slightly from 5.47 to 5.40 at pCa50, but increased the ATPase activity during a given level of steady isometric tension generation, resulting in an increased tension cost (ATPase activity/tension). These results suggest that, in rat right ventricular trabeculae, beta-adrenergic stimulation may increase the rate of cross-bridge cycling by increasing the rate of crossbridge detachment from actin through a PKA-mediated mechanism, although PKA reduces the Ca(2+)-sensitivity of the contractile system.

Cross-bridge dynamics in the contracting heart

The mechanical characteristics of the myosin motor is one of the key determinants of ventricular function. In small mammals there are two myosin isoforms, V1 and V3, with profoundly different performance characteristics. We used myothermal and mechanical analysis of intact papillary muscles from thryoxine (V1) and popylthiouracil (V3) treated rabbit hearts to assess the mechanical attributes of the myosin cross-bridge cycle. The average cross-bridge force time integral for V1 papillary muscles is 0.15 +/- 0.02 pNs for the entire isometric twitch and 0.19 +/- 0.03 pNs for the portion of the isometric twitch between 0.9 peak isometric force for the rising and declining portions of the twitch. The ratio of V1/V3 for the cross-bridge force time integral for the entire twitch and at the peak of the twitch is 0.5 (p < 0.05) and 0.4 (p < 0.05), respectively. Since the peak of the twitch measurements minimize internal shortening only these will be presented below. The average unitary force and attachment time during the peak of the twitch for V1 hearts was 1.55 +/- 0.37 pN and 140 +/- 20 msec, respectively. The ratios of V1/V3 for these parameters were 0.6 (p < 0.05) and 0.8 (ns). The cycling rate and duty cycle for V1 were 4.37 +/- 0.81 cycles per head-second and 0.66 +/- 0.22. The ratios of V1/V3 for cycling rate and duty cycle were 2.8 (p < 0.05) and 2.7 (ns). These measurements are consistent with and help explain the energetic and mechanical function of the intact heart.

Modelling the mechanical properties of cardiac muscle

A model of passive and active cardiac muscle mechanics is presented, suitable for use in continuum mechanics models of the whole heart. The model is based on an extensive review of experimental data from a variety of preparations (intact trabeculae, skinned fibres and myofibrils) and species (mainly rat and ferret) at temperatures from 20 to 27 degrees C. Experimental tests include isometric tension development, isotonic loading, quick-release/restretch, length step and sinusoidal perturbations. We show that all of these experiments can be interpreted with a four state variable model which includes (i) the passive elasticity of myocardial tissue, (ii) the rapid binding of Ca2+ to troponin C and its slower tension-dependent release, (iii) the kinetics of tropomyosin movement and availability of crossbridge binding sites and the length dependence of this process and (iv) the kinetics of crossbridge tension development under perturbations of myofilament length.

Effect of pH, phosphate, and ADP on relaxation of myocardium after photolysis of diazo 2

The aim of this study was to examine the effect of the metabolites H+, ADP, and Pi on the rate of cardiac relaxation. We used guinea pig right ventricular trabeculae that had been chemically skinned, allowing the myofilaments to be studied in isolation. Laser-flash photolysis of the caged Ca2+ chelator diazo 2, causing a rapid fall in intracellular Ca2+, enabled investigation of relaxation independently of the rate of Ca2+ diffusion. On the photolysis of diazo 2, the trabeculae relaxed biphasically with exponential rate constants (k1 and k2) of 10.07 and 4.23 s-1, respectively, at 12 degrees C and 18.35 and 2.52 s-1, respectively, at a nominal 20 degrees C. Increasing the concentration of both protons (pH 7.2-6.8) and MgADP (0.5-3.4 mM) slowed the two phases of the relaxation transients. Raising the concentration of Pi from the control level of 1.36 mM to 15.2 mM increased the rate of both phases, with relaxation becoming monoexponential at 19.4 mM Pi (with a k of 20.31 s-1 at 12 degrees C). Cardiac muscle was compared with skeletal muscle under identical conditions; in cardiac muscle 19.4 mM Pi increased the rate of relaxation, whereas in skeletal muscle this concentration of Pi slowed relaxation. We conclude that the mechanism of relaxation differs between cardiac and skeletal muscle. This study is a direct demonstration of the effects of ATP metabolites on cardiac myofilament processes during relaxation.

Age-dependent changes in myosin composition correlate with enhanced economy of contraction in guinea-pig hearts

1. The composition of myosin heavy chains (MHCs) was investigated in young (1- to 8-week-old) and mature (9- to 26-week-old) guinea-pigs using two monoclonal antibodies directed specifically against alpha-MHC and beta-MHC. In addition, maximum force and the rate of ATP consumption during isometric contraction were measured in chemically skinned trabeculae taken from the same hearts. 2. An age-dependent shift in the MHC composition was found. The alpha-MHC fraction decreased from 0.17 +/- 0.02 (mean +/- S.E.M.; n = 24) in young to 0.04 +/- 0.01 (n = 43) in mature hearts. This shift was correlated with a decrease in tension cost (i.e. ATP consumption per second per trabecula volume/force per cross-sectional area) from 4.1 +/- 0.2 mmol kN-1 m-1 s-1 (n = 23) in young to 2.5 +/- 0.1 mmol kN-1 m-1 s-1 (n = 57) in mature hearts. 3. From the results it follows that the slow beta-MHC isoform, which predominates in hearts of mature guinea-pigs, is about 5 times more economical than the fast alpha-MHC isoform. Calcium sensitivity of force and ATP consumption decreased with age, but stabilized within a few weeks after birth. The pronounced dependence of cardiac energetics on MHC composition should be taken into account in long-term studies of cardiac overload.

The Frank-Starling mechanism is not mediated by changes in rate of cross-bridge detachment

We tested the hypothesis that the Frank-Starling relationship is mediated by changes in the rate of cross-bridge detachment in cardiac muscle. We simultaneously measured isometric force development and the rate of ATP consumption at various levels of Ca2+ activation in skinned rat cardiac trabecular muscles at three sarcomere lengths (2.0, 2.1, and 2.2 microns). The maximum rate of ATP consumption was 1.5 nmol.s-1.microliter fiber vol-1, which represents an estimated adenosinetriphosphatase (ATPase) rate of approximately 10 s-1 per myosin head at 24 degrees C. The rate of ATP consumption was tightly and linearly coupled to the level of isometric force development, and changes in sarcomere length had no effect on the slope of the force-ATPase relationships. The average slope of the force-ATPase relationships was 15.5 pmol.mN-1.mm-1. These results suggest that the mechanisms that underlie the Frank-Starling relationship in cardiac muscle do not involve changes in the kinetics of the apparent detachment step in the cross-bridge cycle.

Molecular motor mechanics in the contracting heart. V1 versus V3 myosin heavy chain

The amount of iron in the low molecular weight pool (LMW) increases during no-flow ischemia and is thought to be essential to oxygen radical-derived damage upon reperfusion. Applying three short ischemic periods (5 min) preconditioning before 15 min ischemia results in an improved contractility compared to a direct 15 min ischemic insult. This raises the question whether preconditioning leads to a decrease in hte LMW iron pool. We therefore investigated the change in in hte LMW iron pool during ischemic insult after applying preconditioning. It is assumed that an increase in LMW iron is dependent on the accumulation of reduction equivalents derived from the anaerobic glycolysis. Therefore the glycogen content was also reduced by administration by anoxia and glucagon administration to study the effect on the LMW iron pool.

Effects of 2,3-butanedione monoxime on cross-bridge kinetics in rat cardiac muscle

The effects of 2,3-butanedione monoxime (BDM) on isometric force and myofibrillar adenosine 5'-triphosphatase (ATPase) activity were studied in skinned cardiac trabeculae from the rat. ATP hydrolysis was enzymatically coupled to the breakdown of reduced nicotinamide adeninedinucleotide (NADH). The NADH concentration was monitored photometrically. Measurements were performed at a sarcomere length of 2.1 microm, 20 degrees C and pH 7.0. Without BDM, isometric force was 45 +/- 3 kN/m2 and the isometric ATPase activity 0.49 +/- 0.04 mM/s (mean +/- SEM, n = 31). Force gradually decreased as a function of [BDM] to 2.8 +/- 0.4% at 100 mM BDM. ATPase activity was also depressed by BDM, but to a lesser extent than force. BDM therefore has a marked effect on myofibrillar tension cost. The rate of tension redevelopment after unloaded shortening decreased from 29 +/- 2 s-1 (n = 10) without BDM to 22 +/- 1 s-1 (n = 5) at 20 mM BDM. These results, modelled in a two- and three-state scheme of cross-bridge interaction, indicate that, in cardiac muscle, BDM not only affects cross-bridge formation but, especially at high concentrations (>/= 20 mM), also causes a marked increase in the apparent rate of cross-bridge detachment.

Rate of active tension development from rigor in skinned atrial and ventricular cardiac fibres from swine following photolytic release of ATP from caged ATP

We investigated the rate of tension development (kappa td) after photolytical release of ATP from P3-1-(2-nitrophenyl)-ethyladenosine-5'-triphosphate ('caged ATP') of atrial and ventricular fibre bundles from pig. Contraction was initiated from high-tension (HT) and low-tension (LT) rigor at maximal Ca2+ activation (pCa 4.5). The kappa td of atrial fibre bundles was 6.8 s-1 from LT and 6.9 s-1 from HT rigor. Rate of tension development of ventricular fibre bundles was significantly lower (P < 0.001) being 1.06 s-1 and 0.94 s-1 from LT and HT rigor, respectively. The kappa td of skinned ventricular fibre bundles incubated in a high [K+], low [Ca2+] (cardioplegic) solution prior to the skinning procedure decreased significantly (P < 0.05) to 0.73 s-1 and 0.63 s-1 from LT and HT rigor, respectively, whereas that of skinned atrial fibre bundles remained at 7.1 s-1 and 6.9 s-1 from LT and HT rigor, respectively. Phosphorylation levels of the myosin light chain 2 isoform in the atrial fibre bundles (ALC-2) was 15.6 +/- 2.7%. The corresponding values for the two ventricular isoforms, VLC-2 and VLC-2*, were 31.2 +/- 0.4% and 25.1 +/- 2.1%, respectively. Phosphorylation levels of fibre bundles incubated in cardioplegic solution prior to skinning were 11.6%, 18.9%, and 15.4% of the ALC-2, VLC-2 and VLC-2*, respectively. The results show that the rate of tension development is more than seven-fold higher in the atrial compared with ventricular fibre bundles. These results correlate with the differences in ATPase activity of the contractile proteins in solution and, most likely, reflect differences in the myosin isoform composition. In ventricular fibre bundles the increased levels of light chain phosphorylation were associated with increased rate of contraction.

Protein kinase A does not alter economy of force maintenance in skinned rat cardiac trabeculae

Recent mechanical, biochemical, and energetic experiments have suggested that catecholamines may increase the cycling rate of cross-bridges independent of changes inn intracellular calcium. An increased rate of cross-bridge cycling is expected to result in decreased economy of force maintenance. The present study tested this hypothesis directly by measuring the rate of ATP consumption in skinned cardiac trabeculae as a function of steady state force. Rat cardiac trabeculae were skinned with Triton X-100. Resting sarcomere length was measured by laser diffraction, and ATP consumption was assessed by an enzyme-coupled optical technique. Force-[Ca2+] relations were fit to a modified Hill equation. Force dependency of the rate of ATP consumption was analyzed by multiple linear regression analysis. beta-Adrenergic stimulation was mimicked by incubation of the skinned muscle preparation with the catalytic subunit of protein kinase A (PKA). Treatment with PKA (3 micrograms/mL, 40 minutes) induced a significant (65 +/- 23%, P = .01) increase in [Ca2+] required for half-maximal steady state force, whereas the steepness of the force-[Ca2+] relation was not affected. The rate of ATP consumption was linearly correlated with steady state force, regardless of PKA treatment status (P < .001). However, neither the slope nor the intercept was affected by PKA treatment. Hence, PKA treatment did not affect either the maximum rate of ATP consumption or the economy of force maintenance. These results suggest that beta-adrenergic stimulation does not alter the rate-limiting step of cross-bridge cycling during isometric contraction in myocardium.

The thiadiazinone EMD 57033 speeds the activation of skinned cardiac muscle produced by the photolysis of nitr-5

EMD 57033 is thought to produce its potentiating effect by increasing the apparent Ca2+ sensitivity of the myofilaments, possibly by altering the kinetics of actomyosin interaction. We have investigated the effect of 10 microM EMD 57033 upon activation speed, induced by flash photolysis of 2mM nitr-5 (caged Ca2+), in chemically skinned trabeculae from the guinea-pig at 12 degrees C. EMD 57033 increases the half time of activation from 238 +/- 18.5 msec (n = 6) to 132.1 +/- 34.0 msec (n = 8) (mean +/- s.e.m.) and suggests that this Ca2+ sensitiser has an important effect upon fapp, that is the transition from the non-force to force generating cross-bridge states.

Myofibrillar creatine kinase and cardiac contraction

This article is a review on the organization and function of myofibrillar creatine kinase in striated muscle. The first part describes myofibrillar creatine kinase as an integral structural part of the complex organization of myofibrils in striated muscle. The second part considers the intrinsic biochemical and mechanical properties of myofibrils and the functional coupling between myofibrillar CK and myosin ATPase. Skinned fiber studies have been developed to evidence this functional coupling and the consequences for cardiac contraction. The data show that creatine kinase in myofibrils is effective enough to sustain normal tension and relaxation, normal Ca sensitivity and kinetic characteristics. Moreover, the results suggest that myofibrillar creatine kinase is essential in maintaining adequate ATP/ADP ratio in the vicinity of myosin ATPase active site to prevent dysfunctioning of this enzyme. Implications for the physiology and physiopathology of cardiac muscle are discussed.

Determinants of loaded shortening velocity in single cardiac myocytes permeabilized with alpha-hemolysin

Force-velocity relations were obtained from single cardiac myocytes isolated by enzymatic digestion of rat myocardium and permeabilized with the pore-forming staphylococcal toxin alpha-hemolysin. Single cardiac myocytes were attached to a force transducer and piezoelectric translator and viewed with an inverted microscope to allow periodic monitoring of sarcomere length during experiments. Permeabilized cells were activated by immersion in a bath of known [Ca2+]. We report that the Ca2+ sensitivity of cells obtained by enzymatic digestion and permeabilized using alpha-hemolysin is similar to that reported previously for mechanically disrupted ventricular myocardium; however, the tension-pCa relation is less steep in the new preparation. During isotonic measurements, force was clamped to various loads using a rapid-response servo system. All recordings of shortening under load were distinctly curvilinear, and analysis of data involved fitting each shortening recording with a single exponential curve and calculating the value of the slope at the initial time of the load clamp. In addition, the presence of significant resting force at initial sarcomere lengths in these cells required that the possibility of alteration of velocity due to the presence of resting force be addressed. The maximum shortening velocity in fully Ca(2+)-activated single ventricular myocytes studied by this method was 2.83 muscle lengths per second on average. The basis for curvilinear shortening is postulated to be multifactorial in cardiac muscle, involving a combination of shortening inactivation and one or more passive elasticities that resist stretch or compression depending on sarcomere length. Shortening velocity shows a dependence on myosin isoform content when cells from a single heart are compared; however, this relation does not hold when cells from different hearts are compared. The behavior of single alpha-hemolysin-permeabilized myocyte shortening under loaded conditions at lower levels of Ca2+ is also described. During submaximal Ca2+ activation, initial shortening velocities are faster than those observed in maximally activated cells. This may be due to contributions of high passive force to increase shortening velocity under conditions of low active force generation, when passive force in the cell is a greater proportion of the total force and there are fewer bound crossbridges.

Calcium modulates the influence of length changes on the myofibrillar adenosine triphosphatase activity in rat skinned cardiac trabeculae

The relationship between adenosine triphosphate (ATP) turnover and muscle performance was investigated in skinned cardiac trabeculae of the rat at different [Ca2+] and two different sarcomere lengths (1.8 microns and 2.2 microns) at 20 degrees C. ATP turnover was measured photometrically by enzymatic coupling of the regeneration of ATP to the oxidation of reduced nicotinamide adenine dinucleotide. The trabeculae were studied under isometric conditions and when the length was altered repetitively at a frequency of 23 Hz, with a square wave, by 5% of the initial length. The isometric ATPase activity amounted to 0.48 mM/s. Isometric ATP turnover and force were proportional at different [Ca2+]. During length changes at maximal activation (pCa 4.27) and 2.2 microns sarcomere length, ATPase activity increased to up to 162% whereas at low [Ca2+], ATPase activity decreased with respect to the isometric value at that pCa. At pCa 5.5, ATPase activity was reduced to 33%. These results indicate that during the length changes the apparent cross-bridge detachment rate is increased and the apparent attachment rate is decreased. The findings suggest that the Fenn effect, i.e. the increase in energy turnover above the isometric value during shortening, is present in cardiac trabeculae at high levels of activation, but is absent or reversed at lower levels of activity.

A computer-based servo system for controlling isotonic contractions of muscle

We have developed a computer-based servo system for controlling isotonic releases in muscle. This system is a composite of commercially available devices: an IBM personal computer, an analog-to-digital (A/D) board, an Akers AE801 force transducer, and a Cambridge Technology motor. The servo loop controlling the force clamp is generated by computer via the A/D board, using a program written in QuickBASIC 4.5. Results are shown that illustrate the ability of the system to clamp the force generated by either skinned cardiac trabeculae or single rabbit psoas fibers down to the resolution of the force transducer within 4 ms. This rate is independent of the level of activation of the tissue and the size of the load imposed during the release. The key to the effectiveness of the system consists of two algorithms that are described in detail. The first is used to calculate the error signal to hold force to the desired level. The second algorithm is used to calculate the appropriate gain of the servo for a particular fiber and the size of the desired load to be imposed. The results show that the described computer-based method for controlling isotonic releases in muscle represents a good compromise between simplicity and performance and is an alternative to the custom-built digital/analog servo devices currently being used in studies of muscle mechanics.

A model of stress relaxation in cross-bridge systems: effect of a series elastic element

Many experimental protocols employed in the study of muscle mechanics use tension transients as a probe of the magnitudes of the kinetic rates in the underlying cross-bridge dynamics. These transients could potentially be modified by the elastic elements that exist both within the fiber and at the points of attachment to the experimental apparatus. To better understand the magnitude of such modifications, we have used computer simulation to investigate the transients that would be expected for cross bridges acting on an actin filament attached to an elastic element. The original model of cross-bridge mechanics by A.F. Huxley was used (Prog. Biophys. 7: 255-318, 1957). After an isometric equilibrium is achieved, a tension transient is produced by changing the dissociation rate constant, g1, while holding the attachment rate constant, f1, fixed. This decreases the number of attached, force-producing cross bridges. We find that the tension transients are markedly slowed by the presence of even a few (> or = 2) nanometers of series elastic strain per half-sarcomere. Thus some rate constants inferred from mechanical transients (e.g., those induced by caged ligands) may underestimate the actual kinetic rates of the cross-bridge processes.

Rapid release of an alpha-adrenergic receptor ligand from photolabile analogues

A series of 2-nitrobenzyl derivatives of the alpha 1-selective adrenergic agonist, L-phenylephrine [(R)-N-[2-(3-hydroxyphenyl)-2-hydroxyethyl]-N-methylammonium chloride], have been synthesized and characterized for the purpose of developing biologically inert compounds that can be rapidly converted to L-phenylephrine by near-UV irradiation. The compounds, derivatized on the phenolic oxygen, were O-(1-(2-nitrophenyl)ethyl)phenylephrine (I), O-(2-nitrobenzyl)phenylephrine (II), O-(4,5-dimethoxy-2-nitrobenzyl)phenylephrine (III), and O-(alpha-carboxyl-2-nitrobenzyl)phenylephrine (IV). All four compounds photolyzed to free phenylephrine following a brief exposure to 300-350-nm light or 347-nm laser light with steady-state quantum yields ranging from 0.05 to 0.28. The rates of phenylephrine formation on photolysis were estimated from the decay rates of aci-nitro intermediates detected by absorbance between 380 and 500 nm. Compound IV displayed the highest quantum yield (0.28) and most rapid photolysis rate (1980 s-1) measured under near physiological conditions, pH 7.0, 22 degrees C. Biological properties of the compounds were examined in smooth muscle from rat caudal artery. Laser pulse photolysis of IV at 347 nm initiated a maximal contraction in Krebs buffer, pH 7.1, 25 degrees C, that mimicked the response to 50 microM phenylephrine but was faster in onset. Photoinitiated contractions were characterized by a delay of 0.93 +/- 0.09 s followed by a rising phase with a 10-90% rise time of 3.56 +/- 0.17 s (n = 7). Responses were fully blocked by the alpha 1-selective antagonist prazosin.(ABSTRACT TRUNCATED AT 250 WORDS)

Relaxation from rigor by photolysis of caged-ATP in different types of muscle fibres from Xenopus laevis

Using chemically skinned fast and slow fibres from the iliofibularis muscle of Xenopus laevis, we measured the force changes following laser pulse photolysis of caged-ATP at 4 degrees C in the presence and absence of added calcium. The time course of the early force change in the absence of calcium was used to derive an apparent second order rate constant for crossbridge detachment. These values were compared with previous model-dependent estimates stemming from force-velocity experiments. For fast muscle fibres, the value obtained here was equal to that obtained in the previous study, namely 4 x 10(5) M-1 S-1. For slow fibres, the value obtained from caged-ATP experiments was 1.5 x 10(4) M-1 S-1, whereas the value from force-velocity experiments was 20 times greater (2.9 x 10(5) M-1 S-1). The different values for slow fibres indicate that the model assumptions inherent in the analysis of the force-velocity experiments may not hold for all muscle types. For example, the process of dissociation of the actomyosin complex of slow myosins may be different from that of fast myosins. All observed or calculated kinetic transitions for the crossbridge cycle were slower in slow muscle fibres than in fast muscle fibres. These include the forward and backward rate constants for crossbridge attachment which were lower by a factor of three in slow fibres compared with fast fibres.

Rapid increase in inositol phosphate levels in norepinephrine-stimulated vascular smooth muscle

We examined the correlation between agonist-stimulated increases in inositol phosphates and force development in vascular smooth muscle. Segments of rat tail artery were preincubated with [3H]inositol and treated with norepinephrine (10(-5) M) for 3-10 s. Tissue levels of inositol monophosphate (IP), inositol bisphosphate (IP2), and inositol trisphosphate (IP3) were measured. IP and IP2 increased significantly after 3 s of norepinephrine treatment. IP3 increased significantly after 5 s of norepinephrine treatment. Analysis of tissue extracts by high-pressure liquid chromatography demonstrated that the only isomer of IP3 present in any tissue extract was the 1,4,5-isomer [Ins(1,4,5)P3]. Contractile response to norepinephrine stimulation showed that the increase in inositol phosphates coincides well with the time course of force development. This is the first report demonstrating such an early increase in Ins(1,4,5)P3 in agonist-stimulated vascular smooth muscle. These results are consistent with the hypothetical role of Ins(1,4,5)P3 as a mediator linking agonist-receptor activation to increased intracellular calcium and force development in norepinephrine-stimulated vascular smooth muscle.

Heat released during relaxation equals force-length area in isometric contractions of rabbit papillary muscle

It has been claimed that the mechanical performance and the related energy turnover of the left ventricle can be reliably predicted on the basis of its time-varying elastance behavior. In its most elementary form, this behavior can be mathematically described by E(t) = P(t)/[V(t)-Vd], where E is ventricular elastance, t is time, P is ventricular pressure, V is ventricular volume, and Vd is the intercept of the end-systolic pressure-volume line on the volume axis. To find out how this behavior of the ventricle as a whole is related to the properties of the myocardium, we tested the energetic prediction for the ventricle that the pressure-volume area of an isovolumic contraction equals the energy released in relaxation in experiments on isolated rabbit papillary muscle at 20 degrees C. To that end, the energy (joules) contained by the force-length area of the muscles, contracting isometrically, was compared with the heat (joules) liberated in relaxation as measured with thermopiles. Mechanical performance of the muscles was varied by altering initial muscle length and external calcium. The slope of the resulting relation between force-length area and heat liberated in relaxation (n = 26) was not significantly different from unity. Thus, the energetic prediction of the time-varying elastance model developed for the whole left ventricle was confirmed by experiments on rabbit papillary muscle at 20 degrees C.

Stretch-induced increase in the Ca2+ sensitivity of myofibrillar ATPase activity in skinned fibres from pig ventricles

The increase in force development in the heart with increase in end-diastolic pressure (Frank-Starling mechanism) has been ascribed to an increase in contractile responsiveness of the myofibrils to calcium. We now show that this calcium sensitization is also associated with an increase in calcium responsiveness of the myofibrillar ATPase. Thus, at submaximal Ca activation (pCa 6.0), the ATPase activity of skinned fibres from pig right ventricles is increased from 57.9 +/- 4.4% to 70.6 +/- 4.4% of the maximal Ca2+ activation of ATPase by stretching (by 15% lo). At maximal Ca2+ activation, ATPase was barely altered by stretching. The relationship between ATPase activity of skinned trabecula of pig right ventricle and ATPase-Ca2+ concentrations is shifted (by 0.1 pCa unit) to higher pCa values after a stretch-induced increase of the sarcomere length from 2.1 microns to 2.4 microns. The relationship between force and pCa was affected in a similar way by extension. This increased calcium sensitivity is, however, not associated with an alteration in the relationship between ATPase activity and force development (tension cost). In accordance with Brenner's hypothesis, we propose therefore that stretch activation of ATPase is associated with an increase in the apparent rate constant of crossbridge attachment rather than with a decrease in the apparent rate constant of crossbridge detachment.