ATPase and shortening rates in frog fast skeletal myofibrils by time-resolved measurements of protein-bound and free Pi

Ex vivo Methods for Measuring Cardiac Muscle Mechanical Properties

Cardiovascular disease continues to be the leading cause of morbidity and mortality in the United States and thousands of manuscripts each year are aimed at elucidating mechanisms underlying cardiac disease. The methods for quantifying cardiac performance are quite varied, with each technique assessing unique features of cardiac muscle mechanical properties. Accordingly, in this review, we discuss current ex vivo methods for quantifying cardiac muscle performance, highlighting what can be learned from each method, and how each technique can be used in conjunction to complement others for a more comprehensive understanding of cardiac function. Importantly, cardiac function can be assessed at several different levels, from the whole organ down to individual protein-protein interactions. Here, we take a reductionist view of methods that are commonly used to measure the distinct aspects of cardiac mechanical function, beginning with whole heart preparations and finishing with the in vitro motility assay. While each of the techniques are individually well-documented in the literature, there is a significant need for a comparison of the techniques, delineating the mechanical parameters that can are best measured with each technique, as well as the strengths and weaknesses inherent to each method. Additionally, we will consider complementary techniques and how these methods can be used in combination to improve our understanding of cardiac mechanical function. By presenting each of these methods, with their strengths and limitations, in a single manuscript, this review will assist cardiovascular biologists in understanding the existing literature on cardiac mechanical function, as well as designing future experiments.

No Difference in Myosin Kinetics and Spatial Distribution of the Lever Arm in the Left and Right Ventricles of Human Hearts

The systemic circulation offers larger resistance to the blood flow than the pulmonary system. Consequently, the left ventricle (LV) must pump blood with more force than the right ventricle (RV). The question arises whether the stronger pumping action of the LV is due to a more efficient action of left ventricular myosin, or whether it is due to the morphological differences between ventricles. Such a question cannot be answered by studying the entire ventricles or myocytes because any observed differences would be wiped out by averaging the information obtained from trillions of myosin molecules present in a ventricle or myocyte. We therefore searched for the differences between single myosin molecules of the LV and RV of failing hearts In-situ. We show that the parameters that define the mechanical characteristics of working myosin (kinetic rates and the distribution of spatial orientation of myosin lever arm) were the same in both ventricles. These results suggest that there is no difference in the way myosin interacts with thin filaments in myocytes of failing hearts, and suggests that the difference in pumping efficiencies are caused by interactions between muscle proteins other than myosin or that they are purely morphological.

Effect of a myosin regulatory light chain mutation K104E on actin-myosin interactions

Familial hypertrophic cardiomyopathy (FHC) is the most common cause of sudden cardiac death in young individuals. Molecular mechanisms underlying this disorder are largely unknown; this study aims at revealing how disruptions in actin-myosin interactions can play a role in this disorder. Cross-bridge (XB) kinetics and the degree of order were examined in contracting myofibrils from the ex vivo left ventricles of transgenic (Tg) mice expressing FHC regulatory light chain (RLC) mutation K104E. Because the degree of order and the kinetics are best studied when an individual XB makes a significant contribution to the overall signal, the number of observed XBs in an ex vivo ventricle was minimized to ∼20. Autofluorescence and photobleaching were minimized by labeling the myosin lever arm with a relatively long-lived red-emitting dye containing a chromophore system encapsulated in a cyclic macromolecule. Mutated XBs were significantly better ordered during steady-state contraction and during rigor, but the mutation had no effect on the degree of order in relaxed myofibrils. The K104E mutation increased the rate of XB binding to thin filaments and the rate of execution of the power stroke. The stopped-flow experiments revealed a significantly faster observed dissociation rate in Tg-K104E vs. Tg-wild-type (WT) myosin and a smaller second-order ATP-binding rate for the K104E compared with WT myosin. Collectively, our data indicate that the mutation-induced changes in the interaction of myosin with actin during the contraction-relaxation cycle may contribute to altered contractility and the development of FHC.

The spatial distribution of actin and mechanical cycle of myosin are different in right and left ventricles of healthy mouse hearts

The contraction of the right ventricle (RV) expels blood into the pulmonary circulation, and the contraction of the left ventricle (LV) pumps blood into the systemic circulation through the aorta. The respective afterloads imposed on the LV and RV by aortic and pulmonary artery pressures create very different mechanical requirements for the two ventricles. Indeed, differences have been observed in the contractile performance between left and right ventricular myocytes in dilated cardiomyopathy, in congestive heart failure, and in energy usage and speed of contraction at light loads in healthy hearts. In spite of these functional differences, it is commonly believed that the right and left ventricular muscles are identical because there were no differences in stress development, twitch duration, work performance, or power among the RV and LV in dogs. This report shows that on a mesoscopic scale [when only a few molecules are studied (here three to six molecules of actin) in ex vivo ventricular myofibrils], the two ventricles in rigor differ in the degree of orientational disorder of actin within in filaments and during contraction in the kinetics of the cross-bridge cycle.

Comparison of orientation and rotational motion of skeletal muscle cross-bridges containing phosphorylated and dephosphorylated myosin regulatory light chain

Calcium binding to thin filaments is a major element controlling active force generation in striated muscles. Recent evidence suggests that processes other than Ca(2+) binding, such as phosphorylation of myosin regulatory light chain (RLC) also controls contraction of vertebrate striated muscle (Cooke, R. (2011) Biophys. Rev. 3, 33-45). Electron paramagnetic resonance (EPR) studies using nucleotide analog spin label probes showed that dephosphorylated myosin heads are highly ordered in the relaxed fibers and have very low ATPase activity. This ordered structure of myosin cross-bridges disappears with the phosphorylation of RLC (Stewart, M. (2010) Proc. Natl. Acad. Sci. U.S.A. 107, 430-435). The slower ATPase activity in the dephosporylated moiety has been defined as a new super-relaxed state (SRX). It can be observed in both skeletal and cardiac muscle fibers (Hooijman, P., Stewart, M. A., and Cooke, R. (2011) Biophys. J. 100, 1969-1976). Given the importance of the finding that suggests a novel pathway of regulation of skeletal muscle, we aim to examine the effects of phosphorylation on cross-bridge orientation and rotational motion. We find that: (i) relaxed cross-bridges, but not active ones, are statistically better ordered in muscle where the RLC is dephosporylated compared with phosphorylated RLC; (ii) relaxed phosphorylated and dephosphorylated cross-bridges rotate equally slowly; and (iii) active phosphorylated cross-bridges rotate considerably faster than dephosphorylated ones during isometric contraction but the duty cycle remained the same, suggesting that both phosphorylated and dephosphorylated muscles develop the same isometric tension at full Ca(2+) saturation. A simple theory was developed to account for this fact.

Along the road not taken: how many myosin heads act on a single actin filament at any instant in working muscle?

We reconsider the use of stiffness measurements to estimate N, the number of myosin heads acting (working at any instant to produce tension) on a single actin filament in vertebrate striated muscle, and give reasons for our rejection of numbers produced from such measurements. We go on to present a different approach to the problem, citing and extending a model bearing on the value of N which is derived from other physiological and biochemical data and which offers insight into the fundamental actin-myosin contractile event as an impulsive force. New experimental data accumulating over the past decade support this model, in which the myosin heads act sequentially along the actin filament (this is an example of Conformational Spread). In this model only a single myosin head acts on a single actin filament to produce an impulse at any given instant in normally-contracting muscle, either in the isometric or the isotonic mode, so N = 1. However, extra impulses occur within the same time frame after quick release of length or tension. The predictions of this sequential model are in striking agreement with a large body of recent detailed biophysical and biochemical evidence. We suggest that this warrants further in-depth experimental work, specifically to explore and test the sequential model and its implications.

A multisegmental cross-bridge kinetics model of the myofibril

Striated muscle is a mechanical system that develops force and generates power in serving vital activities in the body. Striated muscle is a complex biological system; a single mammalian muscle fibre contains up to hundred or even more myofibrils in parallel connected via an inter-myofibril filament network. In one single myofibril thousands of sarcomeres are lined up as a series of linear motors. We recently demonstrated that half-sarcomeres (hS) in a single myofibril operate non-uniformly. We outline a mathematical framework based on cross-bridge kinetics for the simulation of the force response and length change of individual hS in a myofibril. The model describes the muscle myofibril in contraction experiments under various conditions. The myofibril is modeled as a multisegmental mechanical system of hS models, which have active and viscoelastic properties. In the first approach, a two-state cross-bridge formalism relates the hS force to the chemical kinetics of ATP hydrolysis, as first described by Huxley [1957. Muscle structure and theories of contraction. Prog. Biophys. Mol. Biol. 7, 255-318]. Two possible types of biological variability are introduced and modeled. Numerical simulations of a myofibril composed of four to eight hS show a non-uniform hS length distribution and complex internal dynamics upon activation. We demonstrate that the steady-state approximation holds only in restricted time zones during activation. Simulations of myofibril contraction experiments that reproduce the classic steady-state force-length and force-velocity relationships, strictly constrained or "clamped" in either end-held isometric or isotonic contraction conditions, reveal a small but conspicuous effect of hS dynamics on force.

Insights into the kinetics of Ca2+-regulated contraction and relaxation from myofibril studies

Muscle contraction results from force-generating interactions between myosin cross-bridges on the thick filament and actin on the thin filament. The force-generating interactions are regulated by Ca(2+) via specialised proteins of the thin filament. It is controversial how the contractile and regulatory systems dynamically interact to determine the time course of muscle contraction and relaxation. Whereas kinetics of Ca(2+)-induced thin-filament regulation is often investigated with isolated proteins, force kinetics is usually studied in muscle fibres. The gap between studies on isolated proteins and structured fibres is now bridged by recent techniques that analyse the chemical and mechanical kinetics of small components of a muscle fibre, subcellular myofibrils isolated from skeletal and cardiac muscle. Formed of serially arranged repeating units called sarcomeres, myofibrils have a complete fully structured ensemble of contractile and Ca(2+) regulatory proteins. The small diameter of myofibrils (few micrometres) facilitates analysis of the kinetics of sarcomere contraction and relaxation induced by rapid changes of [ATP] or [Ca(2+)]. Among the processes studied on myofibrils are: (1) the Ca(2+)-regulated switch on/off of the troponin complex, (2) the chemical steps in the cross-bridge adenosine triphosphatase cycle, (3) the mechanics of force generation and (4) the length dynamics of individual sarcomeres. These studies give new insights into the kinetics of thin-filament regulation and of cross-bridge turnover, how cross-bridges transform chemical energy into mechanical work, and suggest that the cross-bridge ensembles of each half-sarcomere cooperate with each other across the half-sarcomere borders. Additionally, we now have a better understanding of muscle relaxation and its impairment in certain muscle diseases.

Does phosphate release limit the ATPases of soleus myofibrils? Evidence that (A)M. ADP.Pi states predominate on the cross-bridge cycle

The ATPases (+/-Ca2+) of myofibrils from rabbit soleus (a slow muscle) and psoas (a fast muscle) have different Ea: -Ca2+, 78 and 60 kJ/mol and +Ca2+, 155 and 71 kJ/mol, respectively. At physiological temperatures, the two types of myofibrillar ATPase are very similar and yet the mechanical properties of the muscles are different (Candau et al. (2003) Biophys J 85: 3132-3141). Muscle contraction relies on specific interactions of the different chemical states on the myosin head ATPase pathway with the thin filament. An explanation for the Ea data is that different states populate the pathways of the two types of myofibril because the rate limiting steps are different. Here, we put this to the test by a comparison of the transient kinetics of the initial steps of the ATPases of the two types of myofibril at 4 degrees C. We used two methods: rapid flow quench ('cold ATP chase': titration of active sites, ATP binding kinetics, k(cat); 'Pi burst': ATP cleavage kinetics) and fluorescence stopped-flow (MDCC-phosphate binding protein for free Pi; myofibrillar tryptophan fluorescence for myosin head-thin filament detachment and ATP cleavage kinetics). We find that, as with psoas myofibrils, the most populated state on the cross-bridge cycle of soleus myofibrils, whether relaxed or activated, is (A)M.ADP.Pi. We propose a reaction pathway that includes several (A)M.ADP.Pi sub-states that are either 'weak' or 'strong', depending on the mechanical condition.

Repriming the actomyosin crossbridge cycle

The central features of the mechanical cycle that drives the contraction of muscle are two translational steps: the working stroke, whereby an attached myosin crossbridge moves relative to the actin filament, and the repriming step, in which the crossbridge returns to its original orientation. Although the mechanism of the first of these is understood in some detail, that of the second has received less attention. Here, we show that repriming occurs after detachment of the crossbridge from the actin, rather than intervening between two actomyosin states with ATP bound [Eisenberg, E. & Greene, L. E. (1980) Annu. Rev. Physiol. 42, 293-309]. To discriminate between these two models we investigated the single-molecule mechanics of the myosin-actin interaction in the presence of ATP analogues such as GTP, for which the hydrolytic step itself limits the actomyosin GTPase rate to a much lower rate than for ATP. The lifetimes of bound states was proportional to 1/[GTP], indicating that during the bound period myosin was in the actomyosin rigor configuration. Moreover, despite the very low actomyosin GTPase, the rate of actin binding and formation of the rigor state was higher than with ATP; it follows that most interactions with actin result in the release of GTP and not of the products, GDP and phosphate. There was no significant movement of the actin during this interaction, so repriming must occur while myosin is dissociated, as in the original Lymn-Taylor scheme [Lymn, R. W. & Taylor, E. W. (1971) Biochemistry 10, 4617-4624].

In vitro actomyosin motility in deuterium oxide

Actin filament velocities in an in vitro motility assay system were measured both in heavy water (deuterium oxide, D2O) and water (H2O) to examine the effect of D2O on the actomyosin interaction. The dependence of the sliding velocity on pD of the D2O assay solution showed a broad pD optimum of around pD 8.5 which resembled the broad pH optimum (pH 8.5) of the H2O assay solution, but the maximum velocity (4.1 +/- 0.5 microm/sec, n=11) at pD 8.5 in D2O was about 60% of that (7.1 +/- 1.1 microm/sec, n=11) at pH 8.5 in H2O. The Km values of 95 and 80 microM and Vmax values of 3.2 and 5.1 microm/sec for the D2O and H2O assay were obtained by fitting the ATP concentration dependence of the velocity (at pD and pH 7.5) to the Michaelis-Menten equation. The Km value of actin-activated Mg-ATPase activity of myosin subfragment 1(S1) was decreased from 50 microM[actin] in H2O to 33 microM[actin] in D2O without any significant changes in Vmax (9.4 s(-1) in D2O and 9.3 s(-1) in H2O). The rate constants of ADP release from the acto-S1-ADP complex measured by the stopped flow method were 361 +/- 26 s(-1) (n=27) in D2O and 512 +/- 39 s(-1) (n=27) in H2O at 6 degrees C. These results suggest that the decrease in the in vitro actin-myosin sliding velocity in D2O results from a slowing of the release of ADP from the actomyosin-ADP complex and the increase in the affinity of actin for myosin in the presence of ATP in D2O.

At physiological temperatures the ATPase rates of shortening soleus and psoas myofibrils are similar

We obtained the temperature dependences of the adenosine triphosphatase (ATPase) activities (calcium-activated and relaxed) of myofibrils from a slow muscle, which we compared with those from a fast muscle. We chose rabbit soleus and psoas because their myosin heavy chains are almost pure: isoforms I and IIX, respectively. The Arrhenius plots of the ATPases are linear (4-35 degrees C) with energies of activation for soleus myofibrils 155 kJ mol(-1) (activated) and 78 kJ mol(-1) (relaxed). With psoas myofibrils, the energies of activation were 71 kJ mol(-1) (activated) and 60 kJ mol(-1) (relaxed). When extrapolated to 42 degrees C the ATPase rates of the two types of myofibril were identical: 50 s(-1) (activated) and 0.23 s(-1) (relaxed). Whereas with psoas myofibrils the K(m) for adenosine triphosphate (activated ATPase) is relatively insensitive to temperature, that for soleus myofibrils increased from 0.3 microM at 4 degrees C to 66.5 microM at 35 degrees C. Our results illustrate the importance of temperature when comparing the mechanochemical coupling in different types of muscle. We discuss the problem of how to reconcile the similarity of the myofibrillar ATPase rates at physiological temperatures with their different mechanical properties.

Evidence that phosphate release is the rate-limiting step on the overall ATPase of psoas myofibrils prevented from shortening by chemical cross-linking

It has been suggested that the mechanical condition determines the rate-limiting step of the ATPase of the myosin heads in fibers: when fibers are isometrically contracting, the ADP release kinetics are rate-limiting, but as the strain is reduced and the fibers are allowed to shorten, the ADP release kinetics accelerate and P(i) release becomes rate-limiting. We have put this idea to the test with myofibrils as a model because with these both mechanical and chemical kinetic measurements are possible. With relaxed or rapidly shortening myofibrils, P(i) release is rate-limiting and (A)M.ADP.P(i) states accumulate in the steady state [Lionne, C., et al. (1995) FEBS Lett. 364, 59]. We have now studied the kinetics of P(i) release with chemically cross-linked myofibrils that, when adequately cross-linked, appear to be a good model for isometric contraction. By using a method that is specific for free P(i) and rapid quench flow that measures the amount of (A)M.ADP.P(i) states and free P(i), we show that (A)M.ADP.P(i) states predominate which suggests that the overall ATPase is limited by P(i) release kinetics. Therefore, under our experimental conditions with myofibrils prevented from shortening, the concentration of (A)M.ADP states is low, as with rapidly shortening and relaxed myofibrils. This result is difficult to reconcile with the sensitivity of force development in fibers and myofibrils to P(i) which implies interaction of P(i) with an (A)M.ADP state. We discuss two models for accommodating the mechanical and chemical kinetics with reference to the duty cycle in skeletal muscle.

Characterization of the cross-bridge force-generating step using inorganic phosphate and BDM in myofibrils from rabbit skeletal muscles

The inhibitory effects of inorganic phosphate (P(i)) on isometric force in striated muscle suggest that in the ATPase reaction P(i) release is coupled to force generation. Whether P(i) release and the power stroke are synchronous events or force is generated by an isomerization of the quaternary complex of actomyosin and ATPase products (AM.ADP.P(i)) prior to the following release of P(i) is still controversial. Examination of the dependence of isometric force on [P(i)] in rabbit fast (psoas; 5-15 degrees C) and slow (soleus; 15-20 degrees C) myofibrils was used to test the two-step hypothesis of force generation and P(i) release. Hyperbolic fits of force-[P(i)] relations obtained in fast and slow myofibrils at 15 degrees C produced an apparent asymptote as [P(i)]-->infinity of 0.07 and 0.44 maximal isometric force (i.e. force in the absence of P(i)) in psoas and soleus myofibrils, respectively, with an apparent K(d) of 4.3 mM in both. In each muscle type, the force-[P(i)] relation was independent of temperature. However, 2,3-butanedione 2-monoxime (BDM) decreased the apparent asymptote of force in both muscle types, as expected from its inhibition of the force-generating isomerization. These data lend strong support to models of cross-bridge action in which force is produced by an isomerization of the AM.ADP.P(i) complex immediately preceding the P(i) release step.

Effect of deuterium oxide on actomyosin motility in vitro

Actin filament velocities in an in vitro motility assay system were measured both in heavy water (deuterium oxide, D(2)O) and water (H(2)O) to examine the effect of D(2)O on the actomyosin interaction. The dependence of the sliding velocity on pD of the D(2)O assay solution showed a broad pD optimum of around pD 8.5 which resembled the broad pH optimum (pH 8.5) of the H(2)O assay solution, but the maximum velocity (4.1+/-0.5 microm/s, n=11) at pD 8.5 in D(2)O was about 60% of that (7.1+/-1.1 microm/s, n=11) at pH 8.5 in H(2)O. The K(m) values of 95 and 80 microM and V(max) values of 3.2 and 5.1 microm/s for the D(2)O and H(2)O assay were obtained by fitting the ATP concentration dependence of the velocity (at pD and pH 7.5) to the Michaelis-Menten equation. The K(m) value of actin-activated Mg-ATPase activity of myosin subfragment 1 (S1) was decreased from 50 microM [actin] in H(2)O to 33 microM [actin] in D(2)O without any significant changes in V(max) (9.4 s(-1) in D(2)O and 9.3 s(-1) in H(2)O). The rate constants of ADP release from the acto-S1-ADP complex measured by the stopped flow method were 361+/-26 s(-1) (n=27) in D(2)O and 512+/-39 s(-1) (n=27) in H(2)O at 6 degrees C. These results suggest that the decrease in the in vitro actin-myosin sliding velocity in D(2)O results from a slowing of the release of ADP from the actomyosin-ADP complex and the increase in the affinity of actin for myosin in the presence of ATP in D(2)O.

A fluorescent sensor of the phosphorylation state of nucleoside diphosphate kinase and its use to monitor nucleoside diphosphate concentrations in real time

A sensor for purine nucleoside diphosphates in solution based on nucleoside diphosphate kinase (NDPK) has been developed. A single cysteine was introduced into the protein and labeled with the environmentally sensitive fluorophore, N-[2-(iodoacetamido)ethyl]-7-diethylaminocoumarin-3-carboxamide. The resultant molecule shows a 4-fold fluorescence increase when phosphorylated on His117; this phosphorylation is on the normal reaction pathway of the enzyme. The emission maximum of the phosphoenzyme is at 475 nm, with maximum excitation at 430 nm. The fluorescent phosphorylated NDPK is used to measure the amount of ADP and the unphosphorylated to measure ATP. The labeled protein is phosphorylated to > 90%, and the resultant molecule is stable on ice or can be stored at -80 degrees C. The fluorescence responds to the fraction of protein phosphorylated and so to the equilibrium between ADP plus NDPK approximately P and ATP plus NDPK. In effect, the sensor measures the ADP/ATP concentration ratio. The enzyme has a broad specificity for the purine of the nucleotides, so the sensor also can measure GDP/GTP ratios. The fluorescence and kinetic properties of the labeled protein are described. The binding rate constants of nucleotides are approximately 10(5) M(-1) s(-1), and the fluorescence change is at >200 s(-1) when the ADP concentration is >1 mM. Results are presented with two well-defined systems, namely, the kinetics of ADP release from myosin subfragment 1 and GDP release from the small G protein, human rho. The results obtained with this novel sensor agree with those from alternate methods and demonstrate the applicability for following micromolar changes in nucleoside diphosphate in real time.

Optical determination of glutamine using a genetically engineered protein

We have developed a reagentless optical biosensor for glutamine based on the Escherichia coli glutamine binding protein (GlnBP). Site-directed mutagenesis was performed to engineer single cysteine mutants which were covalently modified with environmentally sensitive sulfhydryl-reactive probes. The fluorescence emission of acrylodan and 2-(4'-(iodoacetamido)anilino)naphthalene-6-sulfonic acid (IAANS) attached to GlnBP mutant S179C was shown to decrease 65 and 35%, respectively, upon titration with increasing amounts of glutamine (0 to 6.4 microM; K(Dapp) 160 nM). No significant changes in the fluorescence intensity were observed for the structurally similar amino acids glutamate, asparagine, and arginine. Time-resolved intensity decays showed a 2.4-fold decrease in mean lifetime for GlnBP S179C-acrylodan upon the addition of glutamine, indicating the possibility of a lifetime-based assay. Anisotropy decay measurements for GlnBPS179C-acrylodan showed a 13-ns rotational correlation time in the ligand-free state, whereas multiple correlation times were assigned in the glutamine-bound conformation. The decrease in fluorescence intensity of S179C-acrylodan was adapted to polarization sensing of glutamine. The engineered GlnBP is a first step toward the development of a nonenzymatic biosensor capable of determining glutamine concentrations in cell cultures.

Link between the enzymatic kinetics and mechanical behavior in an actomyosin motor

We have attempted to link the solution actomyosin ATPase with the mechanical properties of in vitro actin filament sliding over heavy meromyosin. To accomplish this we perturbed the system by altering the substrate with various NTPs and divalent cations, and by altering ionic strength. A wide variety of enzymatic and mechanical measurements were made under very similar solution conditions. Excellent correlations between the mechanical and enzymatic quantities were revealed. Analysis of these correlations based on a force-balance model led us to two fundamental equations, which can be described approximately as follows: the maximum sliding velocity is proportional to square root of V(max)K(m)(A), where K(m)(A) is the actin concentration at which the substrate turnover rate is half of its maximum (V(max)). The active force generated by a cross-bridge under no external load or under a small external load is proportional to square root of V(max)/K(m)(A). The equations successfully accounted for the correlations observed in the present study and observations in other laboratories.

Kinetics of the initial steps of rabbit psoas myofibrillar ATPases studied by tryptophan and pyrene fluorescence stopped-flow and rapid flow-quench. Evidence that cross-bridge detachment is slower than ATP binding

The kinetics of the tryptophan fluorescence enhancement that occurs when myofibrils (rabbit psoas) are mixed with Mg-ATP were studied by stopped-flow in different solvents (water, 40% ethylene glycol, 20% methanol) at 4 degrees C. Under relaxing conditions (low Ca(2+)) in water (mu = 0.16 M, pH 7.4) and at high ATP concentrations, the transient was biphasic, giving a k(fast)(max) of 230 s(-)(1) and a k(slow)(max) of 15 s(-)(1). The kinetics of the two phases were compared with those obtained by chemical sampling using [gamma-(32)P]ATP and quenching in acid (P(i) burst experiments: these give unambiguously the ATP cleavage kinetics), or cold Mg-ATP (cold ATP chase: ATP binding kinetics). k(slow) is due to ATP cleavage, as with S1. Interestingly, k(fast) is slower than the ATP binding kinetics. Instead, this constant appears to report ATP-induced cross-bridge detachment from actin because (1) it was identical to the fluorescence transient obtained on addition of ATP to pyrene-labeled myofibrils; (2) when the initial filament overlap in the myofibrils was decreased, the amplitude of the fast phase decreased; (3) there was no fluorescent enhancement upon the addition of ADP to myofibrils. This is different from the situation with S1 or actoS1 where there was also a fast fluorescent ATP-induced transient but whose kinetics were identical to those of the tight ATP binding. To increase the time resolution and to confirm our results, we also carried out transient kinetics in ethylene glycol and methanol. We interpret our results by a scheme in which a rapid equilibrium between attached (AM.ATP) and detached (M.ATP) states is modulated by the fraction of myosin heads in rigor (AM) during the time of experiment.

Viscosity and solute dependence of F-actin translocation by rabbit skeletal heavy meromyosin

We tested the hypothesis that solvent viscosity affects translocation of rhodamine phalloidin-labeled F-actin by rabbit skeletal heavy meromyosin (HMM). When viscosity was increased using either glycerol, fructose, sucrose, or dextran (1.5, 6.0, or 15-20 kDa mol mass), there was little or no effect on the fraction of moving filaments, whereas sliding speed decreased in inverse proportion to viscosity. The results could be explained neither by an effect of osmotic pressure at high solute concentrations nor by altered solvent drag on the actin filament. Elevated viscosity inhibited HMM ATPase activity in solution, but only at much higher viscosities than were needed to reduce sliding speed. Polyethylene glycols (300, 1,000, or 3,000 mol wt) also inhibited speed via elevated viscosity but secondarily inhibited by enhancing electrostatic interactions. These results demonstrate that a diffusion-controlled process intrinsic to cross-bridge cycling can be limiting to actomyosin function.

The effect of inorganic phosphate on force generation in single myofibrils from rabbit skeletal muscle

In striated muscle, force generation and phosphate (P(i)) release are closely related. Alterations in the [P(i)] bathing skinned fibers have been used to probe key transitions of the mechanochemical coupling. Accuracy in this kind of studies is reduced, however, by diffusional barriers. A new perfusion technique is used to study the effect of [P(i)] in single or very thin bundles (1-3 microM in diameter; 5 degrees C) of rabbit psoas myofibrils. With this technique, it is possible to rapidly jump [P(i)] during contraction and observe the transient and steady-state effects on force of both an increase and a decrease in [P(i)]. Steady-state isometric force decreases linearly with an increase in log[P(i)] in the range 500 microM to 10 mM (slope -0.4/decade). Between 5 and 200 microM P(i), the slope of the relation is smaller ( approximately -0.07/decade). The rate constant of force development (k(TR)) increases with an increase in [P(i)] over the same concentration range. After rapid jumps in [P(i)], the kinetics of both the force decrease with an increase in [P(i)] (k(Pi(+))) and the force increase with a decrease in [P(i)] (k(Pi(-))) were measured. As observed in skinned fibers with caged P(i), k(Pi(+)) is about three to four times higher than k(TR), strongly dependent on final [P(i)], and scarcely modulated by the activation level. Unexpectedly, the kinetics of force increase after jumps from high to low [P(i)] is slower: k(Pi(-)) is indistinguishable from k(TR) measured at the same [P(i)] and has the same calcium sensitivity.

Regulation of contraction in striated muscle

Ca(2+) regulation of contraction in vertebrate striated muscle is exerted primarily through effects on the thin filament, which regulate strong cross-bridge binding to actin. Structural and biochemical studies suggest that the position of tropomyosin (Tm) and troponin (Tn) on the thin filament determines the interaction of myosin with the binding sites on actin. These binding sites can be characterized as blocked (unable to bind to cross bridges), closed (able to weakly bind cross bridges), or open (able to bind cross bridges so that they subsequently isomerize to become strongly bound and release ATP hydrolysis products). Flexibility of the Tm may allow variability in actin (A) affinity for myosin along the thin filament other than through a single 7 actin:1 tropomyosin:1 troponin (A(7)TmTn) regulatory unit. Tm position on the actin filament is regulated by the occupancy of NH-terminal Ca(2+) binding sites on TnC, conformational changes resulting from Ca(2+) binding, and changes in the interactions among Tn, Tm, and actin and as well as by strong S1 binding to actin. Ca(2+) binding to TnC enhances TnC-TnI interaction, weakens TnI attachment to its binding sites on 1-2 actins of the regulatory unit, increases Tm movement over the actin surface, and exposes myosin-binding sites on actin previously blocked by Tm. Adjacent Tm are coupled in their overlap regions where Tm movement is also controlled by interactions with TnT. TnT also interacts with TnC-TnI in a Ca(2+)-dependent manner. All these interactions may vary with the different protein isoforms. The movement of Tm over the actin surface increases the "open" probability of myosin binding sites on actins so that some are in the open configuration available for myosin binding and cross-bridge isomerization to strong binding, force-producing states. In skeletal muscle, strong binding of cycling cross bridges promotes additional Tm movement. This movement effectively stabilizes Tm in the open position and allows cooperative activation of additional actins in that and possibly neighboring A(7)TmTn regulatory units. The structural and biochemical findings support the physiological observations of steady-state and transient mechanical behavior. Physiological studies suggest the following. 1) Ca(2+) binding to Tn/Tm exposes sites on actin to which myosin can bind. 2) Ca(2+) regulates the strong binding of M.ADP.P(i) to actin, which precedes the production of force (and/or shortening) and release of hydrolysis products. 3) The initial rate of force development depends mostly on the extent of Ca(2+) activation of the thin filament and myosin kinetic properties but depends little on the initial force level. 4) A small number of strongly attached cross bridges within an A(7)TmTn regulatory unit can activate the actins in one unit and perhaps those in neighboring units. This results in additional myosin binding and isomerization to strongly bound states and force production. 5) The rates of the product release steps per se (as indicated by the unloaded shortening velocity) early in shortening are largely independent of the extent of thin filament activation ([Ca(2+)]) beyond a given baseline level. However, with a greater extent of shortening, the rates depend on the activation level. 6) The cooperativity between neighboring regulatory units contributes to the activation by strong cross bridges of steady-state force but does not affect the rate of force development. 7) Strongly attached, cycling cross bridges can delay relaxation in skeletal muscle in a cooperative manner. 8) Strongly attached and cycling cross bridges can enhance Ca(2+) binding to cardiac TnC, but influence skeletal TnC to a lesser extent. 9) Different Tn subunit isoforms can modulate the cross-bridge detachment rate as shown by studies with mutant regulatory proteins in myotubes and in in vitro motility assays. (ABSTRACT TRUNCATED)

Measurement of nucleotide exchange rate constants in single rabbit soleus myofibrils during shortening and lengthening using a fluorescent ATP analog

The kinetics of displacement of a fluorescent nucleotide, 2'(3')-O-[N[2-[[Cy3]amido]ethyl]carbamoyl]-adenosine 5'-triphosphate (Cy3-EDA-ATP), bound to rabbit soleus muscle myofibrils were studied using flash photolysis of caged ATP. Use of myofibrils from this slow twitch muscle allowed better resolution of the kinetics of nucleotide exchange than previous studies with psoas muscle myofibrils (, Biophys. J. 73:2033-2042). Soleus myofibrils in the presence of Cy3-EDA-nucleotides (Cy3-EDA-ATP or Cy3-EDA-ADP) showed selective fluorescence staining of the A-band. The K(m) for Cy3-EDA-ATP and the K(d) for Cy3-EDA-ADP binding to the myofibril A-band were 1.9 microM and 3.8 microM, respectively, indicating stronger binding of nucleotide to soleus cross-bridges compared to psoas cross-bridges (2.6 microM and 50 microM, respectively). After flash photolysis of caged ATP, the A-band fluorescence of the myofibril in the Cy3-EDA-ATP solution under isometric conditions decayed exponentially with a rate constant of 0.045 +/- 0.007 s(-1) (n = 32) at 10 degrees C, which was about seven times slower than that for psoas myofibrils. When a myofibril was allowed to shorten with a constant velocity, the nucleotide displacement rate constant increased from 0.066 s(-1) (isometric) to 0.14 s(-1) at 20 degrees C with increasing shortening velocity up to 0.1 myofibril length/s (V(max), the shortening velocity under no load was approximately 0. 2 myofibril lengths/s). The rate constant was not significantly affected by an isovelocity stretch of up to 0.1 myofibril lengths/s. These results suggest that the cross-bridge kinetics are not significantly affected at higher strain during lengthening but depend on the lower strain during shortening. These data also indicate that the interaction distance between a cross-bridge and the actin filament is at least 16 nm for a single cycle of the ATPase.

Cryoenzymic studies on an organized system: myofibrillar ATPases and shortening

We have exploited cryoenzymology, first, to probe the product release steps of myofibrillar ATPase under relaxing conditions and, second, to define the conditions for studying the contractile process in slow motion. Cryoenzymology implies perturbation by temperature and by the antifreeze added to allow for work at subzero temperatures. Here, we studied myofibrillar shortening and ATPases by the rapid quench flow method over a wide temperature range (-15 to 30 degrees C) in two antifreezes, 40% ethylene glycol and 20% methanol. The choice of solvent and temperature was dictated by the purpose of the experiment. Ethylene glycol (40%) is suitable for investigating the kinetics of the products release steps which is difficult in water. In this cryosolvent, the myofibrillar ATPase is not activated by Ca2+ nor is there shortening, except under special conditions, i.e., Ca2+ plus strong rigor bridges [Stehle, R., Lionne, C., Travers, F., and Barman, T. (1998) J. Muscl. Res. Cell Motil. 19, 381-392]. By the use of the glycol, we show that at low Ca2+ the kinetics of the ADP release are much faster with myofibrils than with S1. On the other hand, the kinetics of the Pi release were very similar for the two materials. Therefore, we suggest that, upon Ca2+ activation, only the Pi release kinetics are accelerated. In 20% methanol, in the presence of Ca2+, myofibrils shortened at temperatures above -2 degrees C but not below. At a given temperature above -2 degrees C, both the shortening and ATPase rates were reduced by the methanol. The temperature dependences of the myofibrillar ATPases (+/-Ca2+) converged with a decrease in temperature: at 20 degrees C, Ca2+ activated 30-fold, but at -15 degrees C, only about 5-fold. We suggest that studies in methanol may open the way for an investigation of muscle contraction in slow motion and, further, to obtain thermodynamic information on the internal forces involved in the shortening process.

Modulation by substrate concentration of maximal shortening velocity and isometric force in single myofibrils from frog and rabbit fast skeletal muscle

1. The effects of magnesium adenosine triphosphate (MgATP; also referred to as 'substrate') concentration on maximal force and shortening velocity have been studied at 5 C in single and thin bundles of striated muscle myofibrils. The minute diameters of the preparations promote rapid diffusional equilibrium between the bathing medium and lattice space so that during contraction fine control of substrate and product concentrations is achieved. 2. Myofibrils from frog tibialis anterior and rabbit psoas fast skeletal muscles were activated maximally by rapidly (10 ms) exchanging a continuous flux of pCa 8.0 for one at pCa 4.75 at a range of substrate concentrations from 10 microM to 5 mM. At high substrate concentrations maximal isometric tension and shortening velocity of both frog and rabbit myofibrils were very close to those determined in whole fibre preparations from the same muscle types. 3. As in frog and rabbit skinned whole fibres, the maximal isometric force of the myofibril preparations decreases as MgATP concentration is increased. The maximal velocity of unloaded shortening (V0) depends hyperbolically on substrate concentration. V0 extrapolated to infinite MgATP (3.6 +/- 0.2 and 0.8 +/- 0.03 l0 s-1 in frog and rabbit myofibrils, respectively) is very close to that determined directly at high substrate concentration. The Km is 210 +/- 20 microM for frog tibialis anterior and 120 +/- 10 microM for rabbit psoas myofibrils, values about half those found in larger whole fibre preparations of the same muscle types. This implies that measurements in whole skinned fibres are perturbed by diffusional delays, even in the presence of MgATP regenerating systems. 4. In both frog and rabbit myofibrils, the Km for V0 is about one order of magnitude higher than the Km for myofibrillar MgATPase determined biochemically in the same experimental conditions. This confirms that the difference between the Km values for MgATPase and shortening velocity is a basic feature of the mechanism of chemomechanical transduction in muscle contraction.

A weakly coupled version of the Huxley crossbridge model can simulate energetics of amphibian and mammalian skeletal muscle

This study aimed to establish whether quantitatively accurate predictions of the rate of crossbridge-dependent energy output from shortening muscle could be made on the basis of a 2-state model of crossbridge kinetics incorporating weak coupling between mechanical cycles and ATP hydrolysis. The model was based on Huxley's (1957) model but included rapid detachment, without ATP hydrolysis, of crossbridges when their strain energy increased sufficiently that crossbridge free energy exceeded that of the unbound state (Cooke et al., 1994). An expression was derived relating force to steady-state velocity in terms of the model's rate constants. The values of the rate constants that both provided the best fit through force-velocity data and correctly predicted crossbridge-dependent rate of energy output during an isometric contraction were found and used to predict the variation in rate of energy liberation with shortening velocity. The model predictions closely matched the estimated crossbridge energetics of frog sartorius muscle, including the decline in rate of enthalpy output at high shortening velocities. Data from fast- and slow-twitch muscles of the mouse were also simulated. The velocity-dependence of rate of energy liberation from fast-twitch EDL muscle was well described by the model. The model overestimated crossbridge-dependent energy output from slow-twitch soleus at low shortening velocities but provided accurate predictions of energy output at high velocities. In terms of this model, the distinctive energetics of fast and slow muscles cannot be explained exclusively by differences in cross-bridge detachment rate; differences in the relative rates of crossbridge attachment must also be considered to explain the different relations between energy output and shortening velocity.