Kinetics of force generation and phosphate release in skinned rabbit soleus muscle fibers

Creatine Kinase Equilibration and ΔGATP over an Extended Range of Physiological Conditions: Implications for Cellular Energetics, Signaling, and Muscle Performance

In this report, we establish a straightforward method for estimating the equilibrium constant for the creatine kinase reaction (CK Keq″) over wide but physiologically and experimentally relevant ranges of pH, Mg2+ and temperature. Our empirical formula for CK Keq″ is based on experimental measurements. It can be used to estimate [ADP] when [ADP] is below the resolution of experimental measurements, a typical situation because [ADP] is on the order of micromolar concentrations in living cells and may be much lower in many in vitro experiments. Accurate prediction of [ADP] is essential for in vivo studies of cellular energetics and metabolism and for in vitro studies of ATP-dependent enzyme function under near-physiological conditions. With [ADP], we were able to obtain improved estimates of ΔGATP, necessitating the reinvestigation of previously reported ADP- and ΔGATP-dependent processes. Application to actomyosin force generation in muscle provides support for the hypothesis that, when [Pi] varies and pH is not altered, the maximum Ca2+-activated isometric force depends on ΔGATP in both living and permeabilized muscle preparations. Further analysis of the pH studies introduces a novel hypothesis around the role of submicromolar ADP in force generation.

Orthophosphate increases the efficiency of slow muscle-myosin isoform in the presence of omecamtiv mecarbil

Omecamtiv mecarbil (OM) is a putative positive inotropic tool for treatment of systolic heart dysfunction, based on the finding that in vivo it increases the ejection fraction and in vitro it prolongs the actin-bond life time of the cardiac and slow-skeletal muscle isoforms of myosin. OM action in situ, however, is still poorly understood as the enhanced Ca²⁺-sensitivity of the myofilaments is at odds with the reduction of force and rate of force development observed at saturating Ca²⁺. Here we show, by combining fast sarcomere-level mechanics and ATPase measurements in single slow demembranated fibres from rabbit soleus, that the depressant effect of OM on the force per attached motor is reversed, without effect on the ATPase rate, by physiological concentrations of inorganic phosphate (Pi) (1-10 mM). This mechanism could underpin an energetically efficient reduction of systolic tension cost in OM-treated patients, whenever [Pi] increases with heart-beat frequency.

Omecamtiv mecarbil is a small molecule effector under clinical trial for the treatment of systolic heart failure. Here the authors define the molecular mechanisms of its inotropic action and find it can increase the efficiency of contraction in muscle fibres when the orthophosphate concentration rises with the beat frequency.

A myosin-based mechanism for stretch activation and its possible role revealed by varying phosphate concentration in fast and slow mouse skeletal muscle fibers

Stretch activation (SA) is a delayed increase in force following a rapid muscle length increase. SA is best known for its role in asynchronous insect flight muscle, where it has replaced calcium's typical role of modulating muscle force levels during a contraction cycle. SA also occurs in mammalian skeletal muscle but has previously been thought to be too low in magnitude, relative to calcium-activated (CA) force, to be a significant contributor to force generation during locomotion. To test this supposition, we compared SA and CA force at different P_i concentrations (0-16 mM) in skinned mouse soleus (slow-twitch) and extensor digitorum longus (EDL; fast-twitch) muscle fibers. CA isometric force decreased similarly in both muscles with increasing P_i, as expected. SA force decreased with P_i in EDL (40%), leaving the SA to CA force ratio relatively constant across P_i concentrations (17-25%). In contrast, SA force increased in soleus (42%), causing a quadrupling of the SA to CA force ratio, from 11% at 0 mM P_i to 43% at 16 mM P_i, showing that SA is a significant force modulator in slow-twitch mammalian fibers. This modulation would be most prominent during prolonged muscle use, which increases P_i concentration and impairs calcium cycling. Based upon our previous Drosophila myosin isoform studies and this work, we propose that in slow-twitch fibers a rapid stretch in the presence of P_i reverses myosin's power stroke, enabling quick rebinding to actin and enhanced force production, while in fast-twitch fibers, stretch and P_i cause myosin to detach from actin.

Thick Filament Protein Network, Functions, and Disease Association

Sarcomeres consist of highly ordered arrays of thick myosin and thin actin filaments along with accessory proteins. Thick filaments occupy the center of sarcomeres where they partially overlap with thin filaments. The sliding of thick filaments past thin filaments is a highly regulated process that occurs in an ATP-dependent manner driving muscle contraction. In addition to myosin that makes up the backbone of the thick filament, four other proteins which are intimately bound to the thick filament, myosin binding protein-C, titin, myomesin, and obscurin play important structural and regulatory roles. Consistent with this, mutations in the respective genes have been associated with idiopathic and congenital forms of skeletal and cardiac myopathies. In this review, we aim to summarize our current knowledge on the molecular structure, subcellular localization, interacting partners, function, modulation via posttranslational modifications, and disease involvement of these five major proteins that comprise the thick filament of striated muscle cells. © 2018 American Physiological Society. Compr Physiol 8:631-709, 2018.

Mechanical parameters of the molecular motor myosin II determined in permeabilised fibres from slow and fast skeletal muscles of the rabbit

KEY POINTS

The different performance of slow and fast muscles is mainly attributed to diversity of the myosin heavy chain (MHC) isoform expressed within them. In this study fast sarcomere-level mechanics has been applied to Ca²⁺ -activated single permeabilised fibres isolated from soleus (containing the slow myosin isoform) and psoas (containing the fast myosin isoform) muscles of rabbit for a comparative definition of the mechano-kinetics of force generation by slow and fast myosin isoforms in situ. The stiffness and the force of the slow myosin isoform are three times smaller than those of the fast isoform, suggesting that the stiffness of the myosin motor is a determinant of the isoform-dependent functional diversity between skeletal muscles. These results open the question of the mechanism that can reconcile the reduced performance of the slow MHC with the higher efficiency of the slow muscle.

ABSTRACT

The skeletal muscle exhibits large functional differences depending on the myosin heavy chain (MHC) isoform expressed in its molecular motor, myosin II. The differences in the mechanical features of force generation by myosin isoforms were investigated in situ by using fast sarcomere-level mechanical methods in permeabilised fibres (sarcomere length 2.4 μm, temperature 12°C, 4% dextran T-500) from slow (soleus, containing the MHC-1 isoform) and fast (psoas, containing the MHC-2X isoform) skeletal muscle of the rabbit. The stiffness of the half-sarcomere was determined at the plateau of Ca²⁺ -activated isometric contractions and in rigor and analysed with a model that accounted for the filament compliance to estimate the stiffness of the myosin motor (ε). ε was 0.56 ± 0.04 and 1.70 ± 0.37 pN nm^-1 for the slow and fast isoform, respectively, while the average strain per attached motor (s₀ ) was similar (∼3.3 nm) in both isoforms. Consequently the force per motor (F₀  = εs₀ ) was three times smaller in the slow isoform than in the fast isoform (1.89 ± 0.43 versus 5.35 ± 1.51 pN). The fraction of actin-attached motors responsible for maximum isometric force at saturating Ca²⁺ (T_0,4.5 ) was 0.47 ± 0.09 in soleus fibres, 70% larger than that in psoas fibres (0.29 ± 0.08), so that F₀ in slow fibres was decreased by only 53%. The lower stiffness and force of the slow myosin isoform open the question of the molecular basis of the higher efficiency of slow muscle with respect to fast muscle.

Kinetic coupling of phosphate release, force generation and rate-limiting steps in the cross-bridge cycle

A basic goal in muscle research is to understand how the cyclic ATPase activity of cross-bridges is converted into mechanical force. A direct approach to study the chemo-mechanical coupling between P_i release and the force-generating step is provided by the kinetics of force response induced by a rapid change in [P_i]. Classical studies on fibres using caged-P_i discovered that rapid increases in [P_i] induce fast force decays dependent on final [P_i] whose kinetics were interpreted to probe a fast force-generating step prior to P_i release. However, this hypothesis was called into question by studies on skeletal and cardiac myofibrils subjected to P_i jumps in both directions (increases and decreases in [P_i]) which revealed that rapid decreases in [P_i] trigger force rises with slow kinetics, similar to those of calcium-induced force development and mechanically-induced force redevelopment at the same [P_i]. A possible explanation for this discrepancy came from imaging of individual sarcomeres in cardiac myofibrils, showing that the fast force decay upon increase in [P_i] results from so-called sarcomere 'give'. The slow force rise upon decrease in [P_i] was found to better reflect overall sarcomeres cross-bridge kinetics and its [P_i] dependence, suggesting that the force generation coupled to P_i release cannot be separated from the rate-limiting transition. The reasons for the different conclusions achieved in fibre and myofibril studies are re-examined as the recent findings on cardiac myofibrils have fundamental consequences for the coupling between P_i release, rate-limiting steps and force generation. The implications from P_i-induced force kinetics of myofibrils are discussed in combination with historical and recent models of the cross-bridge cycle.

Force Responses and Sarcomere Dynamics of Cardiac Myofibrils Induced by Rapid Changes in [Pi]

The second phase of the biphasic force decay upon release of phosphate from caged phosphate was previously interpreted as a signature of kinetics of the force-generating step in the cross-bridge cycle. To test this hypothesis without using caged compounds, force responses and individual sarcomere dynamics upon rapid increases or decreases in concentration of inorganic phosphate [P_i] were investigated in calcium-activated cardiac myofibrils. Rapid increases in [P_i] induced a biphasic force decay with an initial slow decline (phase 1) and a subsequent 3-5-fold faster major decay (phase 2). Phase 2 started with the distinct elongation of a single sarcomere, the so-called sarcomere "give". "Give" then propagated from sarcomere to sarcomere along the myofibril. Propagation speed and rate constant of phase 2 (k_+Pi(2)) had a similar [P_i]-dependence, indicating that the kinetics of the major force decay (phase 2) upon rapid increase in [P_i] is determined by sarcomere dynamics. In contrast, no "give" was observed during phase 1 after rapid [P_i]-increase (rate constant k_+Pi(1)) and during the single-exponential force rise (rate constant k_-Pi) after rapid [P_i]-decrease. The values of k_+Pi(1) and k_-Pi were similar to the rate constant of mechanically induced force redevelopment (k_TR) and Ca²⁺-induced force development (k_ACT) measured at same [P_i]. These results indicate that the major phase 2 of force decay upon a P_i-jump does not reflect kinetics of the force-generating step but results from sarcomere "give". The other phases of P_i-induced force kinetics that occur in the absence of "give" yield the same information as mechanically and Ca²⁺-induced force kinetics (k_+Pi(1) ∼ k_-Pi ∼ k_TR ∼ k_ACT). Model simulations indicate that P_i-induced force kinetics neither enable the separation of P_i-release from the rate-limiting transition f into force states nor differentiate whether the "force-generating step" occurs before, along, or after the P_i-release.

Both the elongation of attached crossbridges and residual force enhancement contribute to joint torque enhancement by the stretch-shortening cycle

This study examined the influence of the elongation of attached crossbridges and residual force enhancement on joint torque enhancement by the stretch-shortening cycle (SSC). Electrically evoked submaximal tetanic plantar flexions were adopted. Concentric contractions were evoked in the following three conditions: after 2 s isometric preactivation (ISO condition), after 1 s isometric then 1 s eccentric preactivation (ECC condition), and after 1 s eccentric then 1 s isometric preactivation (TRAN condition). Joint torque and fascicle length were measured during the concentric contraction phase. While no differences in fascicle length were observed among conditions at any time points, joint torque was significantly higher in the ECC than TRAN condition at the onset of concentric contraction. This difference would be caused by the dissipation of the elastic energy stored in the attached crossbridges induced by eccentric preactivation in TRAN condition due to 1 s transition phase. Furthermore, joint torques observed 0.3 and 0.6 s after concentric contraction were significantly larger in the ECC and TRAN conditions than in the ISO condition while no difference was observed between the ECC and TRAN conditions. Since the elastic energy stored in the attached crossbridges would have dissipated over this time frame, this result suggests that residual force enhancement induced by eccentric preactivation also contributes to joint torque enhancement by the SSC.

Modulation of Skeletal Muscle Contraction by Myosin Phosphorylation

The striated muscle sarcomere is a highly organized and complex enzymatic and structural organelle. Evolutionary pressures have played a vital role in determining the structure-function relationship of each protein within the sarcomere. A key part of this multimeric assembly is the light chain-binding domain (LCBD) of the myosin II motor molecule. This elongated "beam" functions as a biological lever, amplifying small interdomain movements within the myosin head into piconewton forces and nanometer displacements against the thin filament during the cross-bridge cycle. The LCBD contains two subunits known as the essential and regulatory myosin light chains (ELC and RLC, respectively). Isoformic differences in these respective species provide molecular diversity and, in addition, sites for phosphorylation of serine residues, a highly conserved feature of striated muscle systems. Work on permeabilized skeletal fibers and thick filament systems shows that the skeletal myosin light chain kinase catalyzed phosphorylation of the RLC alters the "interacting head motif" of myosin motor heads on the thick filament surface, with myriad consequences for muscle biology. At rest, structure-function changes may upregulate actomyosin ATPase activity of phosphorylated cross-bridges. During activation, these same changes may increase the Ca2+ sensitivity of force development to enhance force, work, and power output, outcomes known as "potentiation." Thus, although other mechanisms may contribute, RLC phosphorylation may represent a form of thick filament activation that provides a "molecular memory" of contraction. The clinical significance of these RLC phosphorylation mediated alterations to contractile performance of various striated muscle systems are just beginning to be understood. © 2017 American Physiological Society. Compr Physiol 7:171-212, 2017.

Metabolic dysfunction and altered mitochondrial dynamics in the utrophin-dystrophin deficient mouse model of duchenne muscular dystrophy

The utrophin-dystrophin deficient (DKO) mouse model has been widely used to understand the progression of Duchenne muscular dystrophy (DMD). However, it is unclear as to what extent muscle pathology affects metabolism. Therefore, the present study was focused on understanding energy expenditure in the whole animal and in isolated extensor digitorum longus (EDL) muscle and to determine changes in metabolic enzymes. Our results show that the 8 week-old DKO mice consume higher oxygen relative to activity levels. Interestingly the EDL muscle from DKO mouse consumes higher oxygen per unit integral force, generates less force and performs better in the presence of pyruvate thus mimicking a slow twitch muscle. We also found that the expression of hexokinase 1 and pyruvate kinase M2 was upregulated several fold suggesting increased glycolytic flux. Additionally, there is a dramatic increase in dynamin-related protein 1 (Drp 1) and mitofusin 2 protein levels suggesting increased mitochondrial fission and fusion, a feature associated with increased energy demand and altered mitochondrial dynamics. Collectively our studies point out that the dystrophic disease has caused significant changes in muscle metabolism. To meet the increased energetic demand, upregulation of metabolic enzymes and regulators of mitochondrial fusion and fission is observed in the dystrophic muscle. A better understanding of the metabolic demands and the accompanied alterations in the dystrophic muscle can help us design improved intervention therapies along with existing drug treatments for the DMD patients.

Transgenic overexpression of ribonucleotide reductase improves cardiac performance

We previously demonstrated that cardiac myosin can use 2-deoxy-ATP (dATP) as an energy substrate, that it enhances contraction and relaxation with minimal effect on calcium-handling properties in vitro, and that contractile enhancement occurs with only minor elevation of cellular [dATP]. Here, we report the effect of chronically enhanced dATP concentration on cardiac function using a transgenic mouse that overexpresses the enzyme ribonucleotide reductase (TgRR), which catalyzes the rate-limiting step in de novo deoxyribonucleotide biosynthesis. Hearts from TgRR mice had elevated left ventricular systolic function compared with wild-type (WT) mice, both in vivo and in vitro, without signs of hypertrophy or altered diastolic function. Isolated cardiomyocytes from TgRR mice had enhanced contraction and relaxation, with no change in Ca(2+) transients, suggesting targeted improvement of myofilament function. TgRR hearts had normal ATP and only slightly decreased phosphocreatine levels by (31)P NMR spectroscopy, and they maintained rate responsiveness to dobutamine challenge. These data demonstrate long-term (at least 5-mo) elevation of cardiac [dATP] results in sustained elevation of basal left ventricular performance, with maintained β-adrenergic responsiveness and energetic reserves. Combined with results from previous studies, we conclude that this occurs primarily via enhanced myofilament activation and contraction, with similar or faster ability to relax. The data are sufficiently compelling to consider elevated cardiac [dATP] as a therapeutic option to treat systolic dysfunction.

The effects of phosphate and acidosis on regulated thin-filament velocity in an in vitro motility assay

Muscle fatigue from intense contractile activity is thought to result, in large part, from the accumulation of inorganic phosphate (Pi) and hydrogen ions (H+) acting to directly inhibit the function of the contractile proteins; however, the molecular basis of this process remain unclear. We used an in vitro motility assay and determined the effects of elevated H+ and Pi on the ability of myosin to bind to and translocate regulated actin filaments (RTF) to gain novel insights into the molecular basis of fatigue. At saturating Ca++, acidosis depressed regulated filament velocity ( VRTF) by ∼90% (6.2 ± 0.3 vs. 0.5 ± 0.2 μm/s at pH 7.4 and 6.5, respectively). However, the addition of 30 mM Pi caused VRTF to increase fivefold, from 0.5 ± 0.2 to 2.6 ± 0.3 μm/s at pH 6.5. Similarly, at all subsaturating Ca++ levels, acidosis slowed VRTF, but the addition of Pi significantly attenuated this effect. We also manipulated the [ADP] in addition to the [Pi] to probe which specific step(s) of cross-bridge cycle of myosin is affected by elevated H+. The findings are consistent with acidosis slowing the isomerization step between two actomyosin ADP-bound states. Because the state before this isomerization is most vulnerable to Pi rebinding, and the associated detachment from actin, this finding may also explain the Pi-induced enhancement of VRTF at low pH. These results therefore may provide a molecular basis for a significant portion of the loss of shortening velocity and possibly muscular power during fatigue.

Recent insights into muscle fatigue at the cross-bridge level

The depression in force and/or velocity associated with muscular fatigue can be the result of a failure at any level, from the initial events in the motor cortex of the brain to the formation of an actomyosin cross-bridge in the muscle cell. Since all the force and motion generated by muscle ultimately derives from the cyclical interaction of actin and myosin, researchers have focused heavily on the impact of the accumulation of intracellular metabolites [e.g., P(i), H(+) and adenosine diphoshphate (ADP)] on the function these contractile proteins. At saturating Ca(++) levels, elevated P(i) appears to be the primary cause for the loss in maximal isometric force, while increased [H(+)] and possibly ADP act to slow unloaded shortening velocity in single muscle fibers, suggesting a causative role in muscular fatigue. However the precise mechanisms through which these metabolites might affect the individual function of the contractile proteins remain unclear because intact muscle is a highly complex structure. To simplify problem isolated actin and myosin have been studied in the in vitro motility assay and more recently the single molecule laser trap assay with the findings showing that both P(i) and H(+) alter single actomyosin function in unique ways. In addition to these new insights, we are also gaining important information about the roles played by the muscle regulatory proteins troponin (Tn) and tropomyosin (Tm) in the fatigue process. In vitro studies, suggest that both the acidosis and elevated levels of P(i) can inhibit velocity and force at sub-saturating levels of Ca(++) in the presence of Tn and Tm and that this inhibition can be greater than that observed in the absence of regulation. To understand the molecular basis of the role of regulatory proteins in the fatigue process researchers are taking advantage of modern molecular biological techniques to manipulate the structure and function of Tn/Tm. These efforts are beginning to reveal the relevant structures and how their functions might be altered during fatigue. Thus, it is a very exciting time to study muscle fatigue because the technological advances occurring in the fields of biophysics and molecular biology are providing researchers with the ability to directly test long held hypotheses and consequently reshaping our understanding of this age-old question.

Regulation of fibre contraction in a rat model of myocardial ischemia

Background

The changes in the actomyosin crossbridge cycle underlying altered contractility of the heart are not well described, despite their importance to devising rational treatment approaches.

Methodology/Principal Findings

A rat ischemia–reperfusion model was used to determine the transitions of the crossbridge cycle impacted during ischemia. Compared to perfused hearts, the maximum force per cross-sectional area and Ca²⁺ sensitivity of fibers from ischemic hearts were both reduced. Muscle activation by photolytic release of Ca²⁺ and ATP suggested that the altered contractility was best described as a reduction in the rate of activation of noncycling actomyosin crossbridges to activated, cycling states. More specifically, the apparent forward rate constant of the transition between the nonforce bearing A-M.ADP.Pi state and the bound, force bearing AM*.ADP.Pi state was reduced in ischemic fibers, suggesting that this transition is commensurate with initial crossbridge activation. These results suggested an alteration in the relationship between the activation of thin filament regulatory units and initial crossbridge attachment, prompting an examination of the post-translational state of troponin (Tn) T and I. These analyses indicated a reduction in the diphosphorylated form of TnT during ischemia, along with lower Ser23/24 phosphorylation of TnI. Treatment of perfused fibers by 8-Br-cAMP increased Ser23/24 phosphorylation of TnI, altering the reverse rate constant of the Pi isomerization in a manner consistent with the lusitropic effect of β-adrenergic stimulation. However, similar treatment of ischemic fibers did not change TnI phosphorylation or the kinetics of the Pi isomerization.

Conclusions

Ischemia reduces the isomerization from A-M.ADP.Pi to AM*.ADP.Pi, altering the kinetics of crossbridge activation through a mechanism that may be mediated by altered TnT and TnI phosphorylation.

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.

Alterations in inorganic phosphate in mouse hindlimb muscles during limb disuse

Muscle disuse induces a wide array of structural, biochemical, and neural adaptations in skeletal muscle, which can affect its function. We recently demonstrated in patients with an orthopedic injury that cast immobilization alters the resting P(i) content of skeletal muscle, which may contribute to loss of specific force. The goal of this study was to determine the direct effect of disuse on the basal phosphate content in skeletal muscle in an animal model, avoiding the confounding effects of injury/surgery. (31)P and (1)H MRS data were acquired from the gastrocnemius muscle of young adult mice (C57BL6 female, n = 8), at rest and during a reversible ischemia experiment, before and after 2 weeks of cast immobilization. Cast immobilization resulted in an increase in resting P(i) content (75%; p < 0.001) and the P(i) to phosphocreatine (PCr) ratio (P(i)/PCr; 80%, p < 0.001). The resting concentrations of ATP, PCr and total creatine (PCr + creatine) and the intracellular pH were not significantly different after immobilization. During ischemia (30 min), PCr concentrations decreased to 54 +/- 2% and 52 +/- 6% of the resting values in pre-immobilized and immobilized muscles, respectively, but there were no detectable differences in the rates of P(i) increase or PCr depletion (0.55 +/- 0.01 mM min(-1) and 0.52 +/- 0.03 mM min(-1) before and after immobilization, respectively; p = 0.78). At the end of ischemia, immobilized muscles had a twofold higher phosphorylation potential ([ADP][P(i)]/[ATP]) and intracellular buffering capacity (3.38 +/- 0.54 slykes vs 6.18 +/- 0.57 slykes). However, the rate of PCr resynthesis (k(PCr)) after ischemia, a measure of in vivo mitochondrial function, was significantly lower in the immobilized muscles (0.31 +/- 0.04 min(-1)) than in pre-immobilized muscles (0.43 +/- 0.04 min(-1)). In conclusion, our findings indicate that 2 weeks of cast immobilization, independent of injury-related alterations, leads to a significant increase in the resting P(i) content of mouse skeletal muscle. The increase in P(i) with muscle disuse has a significant effect on the cytosolic phosphorylation potential during transient ischemia and increases the intracellular buffering capacity of skeletal muscle.

The Slow Skeletal Muscle Isoform of Myosin Shows Kinetic Features Common to Smooth and Non-muscle Myosins

Fast and slow mammalian muscle myosins differ in the heavy chain sequences (MHC-2, MHC-1) and muscles expressing the two isoforms contract at markedly different velocities. One role of slow skeletal muscles is to maintain posture with low ATP turnover, and MHC-1 expressed in these muscles is identical to heavy chain of the beta-myosin of cardiac muscle. Few studies have addressed the biochemical kinetic properties of the slow MHC-1 isoform. We report here a detailed analysis of the MHC-1 isoform of the rabbit compared with MHC-2 and focus on the mechanism of ADP release. We show that MHC-1, like some non-muscle myosins, shows a biphasic dissociation of actin-myosin by ATP. Most of the actin-myosin dissociates at up to approximately 1000 s(-1), a very similar rate constant to MHC-2, but 10-15% of the complex must go through a slow isomerization (approximately 20 s(-1)) before ATP can dissociate it. Similar slow isomerizations were seen in the displacement of ADP from actin-myosin.ADP and provide evidence of three closely related actin-myosin.ADP complexes, a complex in rapid equilibrium with free ADP, a complex from which ADP is released at the rate required to define the maximum shortening velocity of slow muscle fibers (approximately 20 s(-1)), and a third complex that releases ADP too slowly (approximately 6 s(-1)) to be on the main ATPase pathway. The role of these actin-myosin.ADP complexes in the mechanochemistry of slow muscle contraction is discussed in relation to the load dependence of ADP release.

Regulation of contraction kinetics in skinned skeletal muscle fibers by calcium and troponin C

The influences of [Ca(2+)] and Ca(2+) dissociation rate from troponin C (TnC) on the kinetics of contraction (k(Ca)) activated by photolysis of a caged Ca(2+) compound in skinned fast-twitch psoas and slow-twitch soleus fibers from rabbits were investigated at 15 degrees C. Increasing the amount of Ca(2+) released increased the amount of force in psoas and soleus fibers and increased k(Ca) in a curvilinear manner in psoas fibers approximately 5-fold but did not alter k(Ca) in soleus fibers. Reconstituting psoas fibers with mutants of TnC that in solution exhibited increased Ca(2+) affinity and approximately 2- to 5-fold decreased Ca(2+) dissociation rate (M82Q TnC) or decreased Ca(2+) affinity and approximately 2-fold increased Ca(2+) dissociation rate (NHdel TnC) did not affect maximal k(Ca). Thus the influence of [Ca(2+)] on k(Ca) is fiber type dependent and the maximum k(Ca) in psoas fibers is dominated by kinetics of cross-bridge cycling over kinetics of Ca(2+) exchange with TnC.

The depressive effect of Pi on the force-pCa relationship in skinned single muscle fibers is temperature dependent

Increases in P(i) combined with decreases in myoplasmic Ca(2+) are believed to cause a significant portion of the decrease in muscular force during fatigue. To investigate this further, we determined the effect of 30 mM P(i) on the force-Ca(2+) relationship of chemically skinned single muscle fibers at near-physiological temperature (30 degrees C). Fibers isolated from rat soleus (slow) and gastrocnemius (fast) muscle were subjected to a series of solutions with an increasing free Ca(2+) concentration in the presence and absence of 30 mM P(i) at both low (15 degrees C) and high (30 degrees C) temperature. In slow fibers, 30 mM P(i) significantly increased the Ca(2+) required to elicit measurable force, referred to as the activation threshold at both low and high temperatures; however, the effect was twofold greater at the higher temperature. In fast fibers, the activation threshold was unaffected by elevating P(i) at 15 degrees C but was significantly increased at 30 degrees C. At both low and high temperatures, 30 mM P(i) increased the Ca(2+) required to elicit half-maximal force (pCa(50)) in both slow and fast fibers, with the effect of P(i) twofold greater at the higher temperature. These data suggest that during fatigue, reductions in the myoplasmic Ca(2+) and increases in P(i) act synergistically to reduce muscular force. Consequently, the combined changes in these ions likely account for a greater portion of fatigue than previously predicted based on studies at lower temperatures or high temperatures at saturating Ca(2+) levels.

Elementary steps of the cross-bridge cycle in fast-twitch fiber types from rabbit skeletal muscles

To understand the molecular mechanism underlying the diversity of mammalian skeletal muscle fibers, the elementary steps of the cross-bridge cycle were investigated in three fast-twitch fiber types from rabbit limb muscles. Skinned fibers were maximally Ca(2+)-activated at 20 degrees C and the effects of MgATP, phosphate (P, P(i)), and MgADP were studied on three exponential processes by sinusoidal analysis. The fiber types (IIA, IID, and IIB) were determined by analyzing the myosin heavy-chain isoforms after mechanical experiments using high-resolution SDS-PAGE. The results were consistent with the following cross-bridge scheme: where A is actin, M is myosin, D is MgADP, and S is MgATP. All states except for those in brackets are strongly bound states. All rate constants of elementary steps (k(2), 198-526 s(-1); k(-2), 51-328 s(-1); k(4), 13.6-143 s(-1); k(-4), 13.6-81 s(-1)) were progressively larger in the order of type IIA, type IID, and type IIB fibers. The rate constants of a transition from a weakly bound state to a strongly bound state (k(-2), k(4)) varied more among fiber types than their reversals (k(2), k(-4)). The equilibrium constants K(1) (MgATP affinity) and K(2) (=k(2)/k(-2), ATP isomerization) were progressively less in the order IIA, IID, and IIB. K(4) (=k(4)/k(-4), force generation) and K(5) (P(i) affinity) were larger in IIB than IIA and IID fibers. K(1) showed the largest variation indicating that the myosin head binds MgATP more tightly in the order IIA (8.7 mM(-1)), IID (4.9 mM(-1)), and IIB (0.84 mM(-1)). Similarly, the MgADP affinity (K(0)) was larger in type IID fibers than in type IIB fibers.

Inorganic phosphate affects the pCa-force relationship more than the pCa-ATPase by increasing the rate of dissociation of force generating cross-bridges in skinned fibers from both EDL and soleus muscles of the rat

The effect of inorganic phosphate (Pi) on Ca2+ -activation of actomyosin ATPase activity and force in permeabilized (skinned) single extensor digitorum longus (EDL) and soleus muscle fibers of the rat were investigated. Increasing concentrations of Pi decreased force more than ATPase rate at all Ca2+ concentrations and this effect was more pronounced at submaximal Ca2+ -activation. Increasing Pi caused both the normalized pCa-ATPase and pCa-force relationship to be shifted to a higher Ca2+ concentration. At all Ca2+ concentrations ATPase was activated at a lower concentration of Ca2+ than force and this difference in Ca2+ concentration required for the activation of ATPase and force was greater in fast-twitch (EDL) than in slow twitch (soleus) muscle. Soleus muscle pCa-ATPase and pCa-force curves were more sensitive to Ca2+ (pCa50 = 5.97 and 5.89, respectively) than EDL (pCa50 = 5.68 and 5.54, respectively). Finally the shape of the pCa-ATPase and pCa-force curves was similar and not affected by Pi. Analysis shows that Pi increases the rate of dissociation of force generating myosin cross-bridges (ratio of ATPase/force (g(app at all Ca2+ concentration, especially at submaximal Ca2+ -activation levels. Pi effects on g(app) are discussed in terms Pi interacting with the isomerization high force AM*ADP states to form high force transitional AM*ADP*Pi* states which facilitate the dissociation of ADP from AM*ADP. Increasing Ca2+ during Ca2+ -activation of the fibers is associated with a progressive decrease in rate of dissociation of force generating myosin cross-bridges g(app).

Mechanokinetics of rapid tension recovery in muscle: the Myosin working stroke is followed by a slower release of phosphate

Crystallographic and biochemical evidence suggests that the myosin working stroke that generates force in muscle is accompanied by the release of inorganic phosphate (Pi), but the order and relative speed of these transitions is not firmly established. To address this problem, the theory of A. F. Huxley and R. M. Simmons for the length-step response is averaged over elastic strains imposed by filament structure and extended to include a Pi-release transition. Models of this kind are applied to existing tension-recovery data from length steps at different phosphate concentrations, and from phosphate jumps upon release of caged phosphate. This body of data is simulated by the model in which the force-generating event is followed by Pi release. A version in which the Pi-release transition is slow provides a better fit than a version with rapid Pi release and a slow transition preceding force generation. If Pi is released before force generation, the predicted rate of slow recovery increases with the size of the step, which is not observed. Some implications for theories of muscle contraction are discussed.

Kinetics of muscle contraction and actomyosin NTP hydrolysis from rabbit using a series of metal-nucleotide substrates

Mechanical properties of skinned single fibres from rabbit psoas muscle have been correlated with biochemical steps in the cross-bridge cycle using a series of metal-nucleotide (Me.NTP) substrates (Mn(2+) or Ni(2+) substituted for Mg(2+); CTP or ITP for ATP) and inorganic phosphate. Measurements were made of the rate of force redevelopment following (1) slack tests in which force recovery followed a period of unloaded shortening, or (2) ramp shortening at low load terminated by a rapid restretch. The form and rate of force recovery were described as the sum of two exponential functions. Actomyosin-Subfragment 1 (acto-S1) Me.NTPase activity and Me.NDP release were monitored under the same conditions as the fibre experiments. Mn.ATP and Mg.CTP both supported contraction well and maintained good striation order. Relative to Mg.ATP, they increased the rates and Me.NTPase activity of cross-linked acto-S1 and the fast component of a double-exponential fit to force recovery by approximately 50% and 10-35%, respectively, while shortening velocity was moderately reduced (by 20-30%). Phosphate also increased the rate of the fast component of force recovery. In contrast to Mn(2+) and CTP, Ni.ATP and Mg.ITP did not support contraction well and caused striations to become disordered. The rates of force recovery and Me.NTPase activity were less than for Mg.ATP (by 40-80% and 50-85%, respectively), while shortening velocity was greatly reduced (by approximately 80%). Dissociation of ADP from acto-S1 was little affected by Ni(2+), suggesting that Ni.ADP dissociation does not account for the large reduction in shortening velocity. The different effects of Ni(2+) and Mn(2+) were also observed during brief activations elicited by photolytic release of ATP. These results confirm that at least one rate-limiting step is shared by acto-S1 ATPase activity and force development. Our results are consistent with a dual rate-limitation model in which the rate of force recovery is limited by both NTP cleavage and phosphate release, with their relative contributions and apparent rate constants influenced by an intervening rapid force-generating transition.

Activation kinetics of skinned cardiac muscle by laser photolysis of nitrophenyl-EGTA

The kinetics of Ca(2+)-induced contractions of chemically skinned guinea pig trabeculae was studied using laser photolysis of NP-EGTA. The amount of free Ca(2+) released was altered by varying the output from a frequency-doubled ruby laser focused on the trabeculae, while maintaining constant total [NP-EGTA] and [Ca(2+)]. The time courses of the rise in stiffness and tension were biexponential at 23 degrees C, pH 7.1, and 200 mM ionic strength. At full activation (pCa < 5.0), the rates of the rapid phase of the stiffness and tension rise were 56 +/- 7 s(-1) (n = 7) and 48 +/- 6 s(-1) (n = 11) while the amplitudes were 21 +/- 2 and 23 +/- 3%, respectively. These rates had similar dependencies on final [Ca(2+)] achieved by photolysis: 43 and 50 s(-1) per pCa unit, respectively, over a range of [Ca(2+)] producing from 15% to 90% of maximal isometric tension. At all [Ca(2+)], the rise in stiffness initially was faster than that of tension. The maximal rates for the slower components of the rise in stiffness and tension were 4.1 +/- 0.8 and 6.2 +/- 1.0 s(-1). The rate of this slower phase exhibited significantly less Ca(2+) sensitivity, 1 and 4 s(-1) per pCa unit for stiffness and tension, respectively. These data, along with previous studies indicating that the force-generating step in the cross-bridge cycle of cardiac muscle is marginally sensitive to [Ca(2+)], suggest a mechanism of regulation in which Ca(2+) controls the attachment step in the cross-bridge cycle via a rapid equilibrium with the thin filament activation state. Myosin kinetics sets the time course for the rise in stiffness and force generation with the biexponential nature of the mechanical responses to steps in [Ca(2+)] arising from a shift to slower cross-bridge kinetics as the number of strongly bound cross-bridges increases.

Changes in inorganic phosphate and force production in human skeletal muscle after cast immobilization

Cast immobilization is associated with decreases in muscle contractile area, specific force, and functional ability. The pathophysiological processes underlying the loss of specific force production as well as the role of metabolic alterations are not well understood. The aim of this study was to quantify changes in the resting energy-rich phosphate content and specific force production after immobilization. (31)P-magnetic resonance spectroscopy, three-dimensional magnetic resonance imaging, and isometric strength testing were performed in healthy subjects and patients with an ankle fracture after 7 wk of immobilization and during rehabilitation. Muscle biopsies were obtained in a subset of patients. After immobilization, there was a significant decrease in the specific plantar flexor torque and a significant increase in the inorganic phosphate (P(i)) concentration (P < 0.001) and the P(i)-to-phosphocreatine (PCr) ratio (P < 0.001). No significant change in the PCr content or basal pH was noted. During rehabilitation, both the P(i) content and the P(i)-to-PCr ratio decreased and specific torque increased, approaching control values after 10 wk of rehabilitation. Regression analysis showed an inverse relationship between the in vivo P(i) concentration and specific torque (r = 0.65, P < 0.01). In vitro force mechanics performed on skinned human muscle fibers demonstrated that varying the P(i) levels within the ranges observed across individuals in vivo (4-10 mM) changed force production by approximately 16%. In summary, our findings clearly depict a change in the resting energy-rich phosphate content of skeletal muscle with immobilization, which may negatively impact its force generation.

Cross-bridge mechanisms underlying the history-dependent properties of muscle spindles and stretch reflexes

The effects of prior movement on the force responses of skeletal muscle are compared with the effects of movement history on the changes in firing rate of muscle spindle receptors. Prior release results in the linearization of the mechanical properties of skeletal muscles, which can be provisionally explained by cross-bridge models of muscular contraction. The history-dependence of responses of muscle spindle receptors in unanesthetized decerebrate preparations appears to result from the kinetics of cycling and noncycling cross-bridges. The results of this comparison indicate that the integration of mechanical properties of muscle and spindle receptor promotes stiffness regulation.Key words: predictive control, muscular stiffness, muscle receptors, reflex compensation, cross-bridge cycling, nonlinear mechanical properties, feline motor control.

Fiber type and temperature dependence of inorganic phosphate: implications for fatigue

Elevated levels of P(i) are thought to cause a substantial proportion of the loss in muscular force and power output during fatigue from intense contractile activity. However, support for this hypothesis is based, in part, on data from skinned single fibers obtained at low temperatures (< or =15 degrees C). The effect of high (30 mM) P(i) concentration on the contractile function of chemically skinned single fibers was examined at both low (15 degrees C) and high (30 degrees C) temperatures using fibers isolated from rat soleus (type I fibers) and gastrocnemius (type II fibers) muscles. Elevating P(i) from 0 to 30 mM at saturating free Ca(2+) levels depressed maximum isometric force (P(o)) by 54% at 15 degrees C and by 19% at 30 degrees C (P < 0.05; significant interaction) in type I fibers. Similarly, the P(o) of type II fibers was significantly more sensitive to high levels of P(i) at the lower (50% decrease) vs. higher temperature (5% decrease). The maximal shortening velocity of both type I and type II fibers was not significantly affected by elevated P(i) at either temperature. However, peak fiber power was depressed by 49% at 15 degrees C but by only 16% at 30 degrees C in type I fibers. Similarly, in type II fibers, peak power was depressed by 40 and 18% at 15 and 30 degrees C, respectively. These data suggest that near physiological temperatures and at saturating levels of intracellular Ca(2+), elevated levels of P(i) contribute less to fatigue than might be inferred from data obtained at lower temperatures.

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.

Kinetic effects of fiber type on the two subcomponents of the Huxley-Simmons phase 2 in muscle

The Huxley-Simmons phase 2 controls the kinetics of the first stages of tension recovery after a step-change in fiber length and is considered intimately associated with tension generation. It had been shown that phase 2 is comprised of two distinct unrelated phases. This is confirmed here by showing that the properties of phase 2(fast) are independent of fiber type, whereas those of phase 2(slow) are fiber type dependent. Phase 2(fast) has a rate of 1000-2000 s(-1) and is temperature insensitive (Q(10) approximately 1.16) in fast, medium, and slow speed fibers. Regardless of fiber type and temperature, the amplitude of phase 2(fast) is half (approximately 0.46) that of phase 1 (fiber instantaneous stiffness). Consequently, fiber compliance (cross-bridge and thick/thin filament) appears to be the common source of both phase 1 elasticity and phase 2(fast) viscoelasticity. In fast fibers, stiffness increases in direct proportion to tension from an extrapolated positive origin at zero tension. The simplest explanation is that tension generation can be approximated by two-state transition from attached preforce generating (moderate stiffness) to attached force generating (high stiffness) states. Phase 2(slow) is quite different, progressively slowing in concert with fiber type. An interesting interpretation of the amplitude and rate data is that reverse coupling of phase 2(slow) back to P(i) release and ATP hydrolysis appears absent in fast fibers, detectable in medium speed fibers, and marked in slow fibers contracting isometrically. Contracting slow and heart muscles stretched under load could employ this enhanced reversibility of the cross-bridge cycle as a mechanism to conserve energy.

Activation of striated muscle: nearest-neighbor regulatory-unit and cross-bridge influence on myofilament kinetics

We have formulated a three-compartment model of muscle activation that includes both strong cross-bridge (XB) and Ca(2+)-activated regulatory-unit (RU) mediated nearest-neighbor cooperative influences. The model is based on the tight coupling premise--that XB retain activating Ca(2+) on the thin filament. Using global non-linear least-squares, the model produced excellent fits to experimental steady-state force-pCa and ATPase-pCa data from skinned rat soleus fibers. In terms of the model, nearest-neighbor influences over the range of Ca(2+) required for activation cause the Ca(2+) dissociation rate from regulatory-units (k(off)) to decrease and the cross-bridge association rate (f) to increase each more than ten-fold. Moreover, the rate variations occur in separate Ca(2+) regimes. The energy of activation governing f is strongly influenced by both neighboring RU and XB. In contrast, the energy of activation governing k(off) is less affected by neighboring XB than by neighboring RU. Nearest-neighbor cooperative influences provide both an overall sensitization to Ca(2+) and the well-known steep response of force to free Ca(2+). The apparent sensitivity for Ca(2+)-activation of force and ATPase is a function of cross-bridge kinetic rates. The model and derived parameter set produce simulated behavior in qualitative agreement with steady-state experiments reported in the literature for partial TnC replacement, increased [P(i)], increased [ADP], and MalNEt-S1 addition. The model is an initial attempt to construct a general theory of striated muscle activation-one that can be consistently used to interpret data from various types of muscle manipulation experiments.

Relaxation kinetics following sudden Ca(2+) reduction in single myofibrils from skeletal muscle

To investigate the roles of cross-bridge dissociation and cross-bridge-induced thin filament activation in the time course of muscle relaxation, we initiated force relaxation in single myofibrils from skeletal muscles by rapidly (approximately 10 ms) switching from high to low [Ca(2+)] solutions. Full force decay from maximal activation occurs in two phases: a slow one followed by a rapid one. The latter is initiated by sarcomere "give" and dominated by inter-sarcomere dynamics (see the companion paper, Stehle, R., M. Krueger, and G. Pfitzer. 2002. Biophys. J. 83:2152-2161), while the former occurs under nearly isometric conditions and is sensitive to mechanical perturbations. Decreasing the Ca(2+)-activated force preceding the start of relaxation does not increase the rate of the slow isometric phase, suggesting that cycling force-generating cross-bridges do not significantly sustain activation during relaxation. This conclusion is strengthened by the finding that the rate of isometric relaxation from maximum force to any given Ca(2+)-activated force level is similar to that of Ca(2+)-activation from rest to that given force. It is likely, therefore, that the slow rate of force decay in full relaxation simply reflects the rate at which cross-bridges leave force-generating states. Because increasing [P(i)] accelerates relaxation while increasing [MgADP] slows relaxation, both forward and backward transitions of cross-bridges from force-generating to non-force-generating states contribute to muscle relaxation.

Kinetic effects of myosin regulatory light chain phosphorylation on skeletal muscle contraction

Kinetic analysis of contracting fast and slow rabbit muscle fibers in the presence of the tension inhibitor 2,3-butanedione monoxime suggests that regulatory light chain (RLC) phosphorylation up-regulates the flux of weakly attached cross-bridges entering the contractile cycle by increasing the actin-catalyzed release of phosphate from myosin. This step appears to be separate from earlier Ca(2+) regulated steps. Small step-stretches of single skinned fibers were used to study the effect of phosphorylation on fiber mechanics. Subdivision of the resultant tension transients into the Huxley-Simmons phases 1, 2(fast), 2(slow), 3, and 4 reveals that phosphorylation reduces the normalized amplitude of the delayed rise in tension (stretch activation response) by decreasing the amplitudes of phase 3 and, to a lesser extent, phase 2(slow). In slow fibers, the RLC P1 isoform phosphorylates at least 4-fold faster than the P2 isoform, complicating the role of RLC phosphorylation in heart and slow muscle. We discuss the functional relevance of the regulation of stretch activation by RLC phosphorylation for cardiac and other oscillating muscles and speculate how the interaction of the two heads of myosin could account for the inverse effect of Ca(2+) levels on isometric tension and rate of force redevelopment (k(TR)).

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 P(i) on unloaded shortening velocity of slow and fast mammalian muscle fibers

Chemically skinned muscle fibers, prepared from the rat medial gastrocnemius and soleus, were subjected to four sequential slack tests in Ca(2+)-activating solutions containing 0, 15, 30, and 0 mM added P(i). P(i) (15 and 30 mM) had no effect on the unloaded shortening velocity (V(o)) of fibers expressing type IIb myosin heavy chain (MHC). For fibers expressing type I MHC, 15 mM P(i) did not alter V(o), whereas 30 mM P(i) reduced V(o) to 81 plus minus 1% of the original 0 mM P(i) value. This effect was readily reversible when P(i) was lowered back to 0 mM. These results are not compatible with current cross-bridge models, developed exclusively from data obtained from fast fibers, in which V(o) is independent of P(i). The response of the type I fibers at 30 mM P(i) is most likely the result of increased internal drag opposing fiber shortening resulting from fiber type-specific effects of P(i) on cross bridges, the thin filament, or the rate-limiting step of the cross-bridge cycle.

Elementary steps of the cross-bridge cycle in bovine myocardium with and without regulatory proteins

The role of regulatory proteins in the elementary steps of the cross-bridge cycle in bovine myocardium was investigated. The thin filament was selectively removed by gelsolin and the actin filament was reconstituted without tropomyosin or troponin. Further reconstitution was achieved by adding tropomyosin and troponin. The effects of MgATP and phosphate (Pi) on the rate constants of exponential processes were studied in control, actin filament-reconstituted, and thin filament-reconstituted myocardium at pCa < or = 4.66, pH 7.00, 25 degrees C. In control myocardium, the MgATP association constant was 9.1 +/- 1.3 mM(-1), and the Pi association constant 0.14 +/- 0.04 mM(-1). The equilibrium constant of the cross-bridge detachment step was 2.6 +/- 0.4, and the equilibrium constant of the force generation step was 0.59 +/- 0.04. In actin filament-reconstituted myocardium without regulatory proteins, the MgATP association constant was approximately the same, and the Pi association constant increased to 2.8x. The equilibrium constant of cross-bridge detachment decreased to 0.2x, but the equilibrium constant of the force generation step increased to 4x. These kinetic constants regained control values after reconstitution of the thin filament. These results indicate that tension/cross-bridge in the presence of regulatory proteins is approximately 1.5-1.7x of that in the absence of regulatory proteins. These results further indicate that regulatory proteins promote detachment of cross-bridges.

Temperature dependence of active tension in mammalian (rabbit psoas) muscle fibres: effect of inorganic phosphate

1. The effect of added inorganic phosphate (P(i), range 3-25 mM) on active tension was examined at a range of temperatures (5-30 degrees C) in chemically skinned (0.5 % Brij) rabbit psoas muscle fibres. Three types of experiments were carried out. 2. In one type of experiment, a muscle fibre was maximally activated at low temperature (5 degrees C) and its tension change was recorded during stepwise heating to high temperature in approximately 60 s. As found in previous studies, the tension increased with temperature and the normalised tension-(reciprocal) temperature relation was sigmoidal, with a half-maximal tension at 8 degrees C. In the presence of 25 mM added P(i), the temperature for half-maximal tension of the normalised curve was approximately 5 degrees C higher than in the control. The difference in the slope was small. 3. In a second type of experiment, the tension increment during a large temperature jump (from 5 to 30 degrees C) was examined during an active contraction. The relative increase of active tension on heating was significantly higher in the presence of 25 mM added P(i) (30/5 degrees C tension ratio of 6-7) than in the control with no added P(i) (tension ratio of approximately 3). 4. In a third type of experiment, the effect on the maximal Ca(2+)-activated tension of different levels of added P(i) (3-25 mM) (and P(i) mop adequate to reduce contaminating P(i) to micromolar levels) was examined at 5, 10, 20 and 30 degrees C. The tension was depressed with increased [P(i)] in a concentration-dependent manner at all temperatures, and the data could be fitted with a hyperbolic relation. The calculated maximal tension depression in excess [P(i)] was approximately 65 % of the control at 5-10 degrees C, in contrast to a maximal depression of 40 % at 20 degrees C and 30 % at 30 degrees C. 5. These experiments indicate that the active tension depression induced by P(i) in psoas fibres is temperature sensitive, the depression becoming less marked at high temperatures. A reduced P(i)-induced tension depression is qualitatively predicted by a simplified actomyosin ATPase cycle where a pre-phosphate release, force-generation step is enhanced by temperature.

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.

Endothermic force generation in skinned cardiac muscle from rat

Isometric tension responses to rapid temperature jumps (T-jumps) of 2-6 degrees C were examined in skinned muscle fibre bundles isolated from papillary muscles of the rat heart. T-jumps were induced by an infra-red laser pulse (wave length 1.32 microm, pulse duration 0.2 ms) obtained from a Nd-YAG laser, which heated the fibres and bathing buffer solution in a 50 microl trough; the increased temperature by laser pulse was clamped at the high temperature by a Peltier system (see Ranatunga, 1996). In maximally Ca2+ -activated (pCa ca. 4.5) fibres, the relationship between tension and temperature was non-linear, the increase of active tension with temperature being more pronounced at lower temperatures (below ca. 20 degrees C). A T-jump at any temperature (range 3-35 degrees C) induced an initial step decrease of tension of variable amplitude (Phase 1), probably due to thermal expansion, and it was followed by a tension transient which resulted in a net rise of tension above the pre-T-jump level. The rate of net rise of tension (Phase 2b or endothermic force generation) was 7-10/s at ca. 12 degrees C and its Q10 was 6.3 (below 25 degrees C). In cases where the step decrease of tension in Phase 1 was prominent, an initial quick tension recovery phase (Phase 2a, 70-100/s at 12 degrees C) that did not contribute to a rise of tension above the pre-T-jump level, was also seen. This phase (Phase 2a) appeared to be similar to the quick tension recovery induced by a small length release and its rate increased with temperature with a Q10 of 1.8. In some cases where Phase 2a was present, a slower tension rise (Phase 3) was seen; its rate (ca. 5/s) was temperature-insensitive. The results show that the rate of endothermic force generation in cardiac fibres is clearly different from that of either fast-twitch or slow-twitch mammalian skeletal muscle fibres; implication of such fibre type-specific differences is discussed in relation to the difficulty in identifying the biochemical step underlying endothermic cross-bridge force generation.

Effects of inorganic phosphate on endothermic force generation in muscle

Using a rapid (ca. 0.2 ms) laser temperature jump technique, the rate of endothermic force generation was examined in single-skinned (rabbit psoas) muscle fibres when they were exposed to different levels of inorganic phosphate (a product released during ATP hydrolysis in active muscle). The steady force is reduced by increased phosphate but the apparent rate constant of force generation induced by a standard temperature jump (from ca. 9 degrees C to ca. 12 degrees C) increases two- to threefold when the phosphate added is increased from zero to ca. 25 mM. The increase in the apparent rate constant also exhibits saturation at higher phosphate levels and the relation is hyperbolic. Detailed examination of the data, particularly in relation to our pressure release experiments, leads to a scheme for the molecular steps involved in phosphate release and force generation in active muscle fibres, where phosphate release from attached cross-bridges involves three reversible and sequentially faster molecular steps. Step one is a moderately slow, pre-force generation step that probably represents a transition of cross-bridges from non-specific to stereospecific attached states. Step two is moderately fast and represents endothermic cross-bridge force generation (temperature sensitive) and step three is a very rapid phosphate release. Such a scheme accommodates findings from a variety of different studies, including pressure perturbation experiments and other studies where the effect of phosphate on muscle force was studied.

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.

Rate of phosphate release after photoliberation of adenosine 5'-triphosphate in slow and fast skeletal muscle fibers

Inorganic phosphate (Pi) release was determined by means of a fluorescent Pi-probe in single permeabilized rabbit soleus and psoas muscle fibers. Measurements of Pi release followed photoliberation of approximately 1.5 mM ATP by flash photolysis of NPE-caged ATP in the absence and presence of Ca2+ at 15 degrees C. In the absence of Ca2+, Pi release occurred with a slow rate of 11 +/- 3 microM . s-1 (n = 3) in soleus fibers and 23 +/- 1 microM . s-1 (n = 10) in psoas fibers. At saturating Ca2+ concentrations (pCa 4.5), photoliberation of ATP was followed by rapid force development. The initial rate of Pi release was 0.57 +/- 0.05 mM . s-1 in soleus (n = 13) and 4.7 +/- 0.2 mM . s-1 in psoas (n = 23), corresponding to a rate of Pi release per myosin head of 3.8 s-1 in soleus and 31.5 s-1 in psoas. Pi release declined at a rate of 0.48 s-1 in soleus and of 5.2 s-1 in psoas. Pi release in soleus was slightly faster in the presence of an ATP regenerating system but slower when 0.5 mM ADP was added. The reduction in the rate of Pi release results from an initial redistribution of cross-bridges over different states and a subsequent ADP-sensitive slowing of cross-bridge detachment.

ATP analogs and muscle contraction: mechanics and kinetics of nucleoside triphosphate binding and hydrolysis

The mechanical behavior of skinned rabbit psoas muscle fiber contractions and in vitro motility of F-actin (Vf) have been examined using ATP, CTP, UTP, or their 2-deoxy forms (collectively designated as nucleotide triphosphates or NTPs) as contractile substrates. Measurements of actin-activated heavy meromyosin (HMM) NTPase, the rates of NTP binding to myosin and actomyosin, NTP-mediated acto-HMM dissociation, and NTP hydrolysis by acto-HMM were made for comparison to the mechanical results. The data suggest a very similar mechanism of acto-HMM NTP hydrolysis. Whereas all NTPs studied support force production and stiffness that vary by a factor 2 or less, the unloaded shortening velocity (Vu) of muscle fibers varies by almost 10-fold. 2-Deoxy ATP (dATP) was unique in that Vu was 30% greater than with ATP. Parallel behavior was observed between Vf and the steady-state maximum actin-activated HMM ATPase rate. Further comparisons suggest that the variation in force correlates with the rate and equilibrium constant for NTP cleavage; the variations in Vu or Vf are related to the rate of cross-bridge dissociation caused by NTP binding or to the rate(s) of product release.

Temperature sensitivity of force and shortening velocity in maximally activated skinned smooth muscle

We have studied the temperature dependence of isometric force, rate of force development and maximal shortening velocity (Vmax) in skinned guinea-pig taenia coli smooth muscle. To eliminate the influence of temperature on activation mechanisms, maximally thiophosphorylated preparations were used. Isometric force in the range 2-35 degrees C was maximal at 22 degrees C with a decrease of 25% at 2 degrees C and 10% at 35 degrees C. Rate of tension development from rigor after photolytic release of ATP increased four-fold between 5 degrees C and 30 degrees C. Vmax increased with a Q10 of about 2 (1.6, range 5-15 degrees C, and 2.2, range 22-30 degrees C). The temperature dependence of the rate of tension development indicates rate-limitation by transitions into force-generating states or by the hydrolysis reaction. The temperature dependence of Vmax reflects effects of temperature on reactions (e.g. the ADP-release) associated with cross-bridge detachment. The small temperature dependence of steady-state force in smooth compared with skeletal muscle suggests differences in the cross-bridge reactions controlling the number of attached force-generating states in the two muscle types.

Brush border myosin-I structure and ADP-dependent conformational changes revealed by cryoelectron microscopy and image analysis

Brush border myosin-I (BBM-I) is a single-headed myosin found in the microvilli of intestinal epithelial cells, where it forms lateral bridges connecting the core bundle of actin filaments to the plasma membrane. Extending previous observations (Jontes, J.D., E.M. Wilson-Kubalek, and R.A. Milligan. 1995. Nature [Lond.]. 378:751-753), we have used cryoelectron microscopy and helical image analysis to generate three-dimensional (3D) maps of actin filaments decorated with BBM-I in both the presence and absence of 1 mM MgADP. In the improved 3D maps, we are able to see the entire light chain-binding domain, containing density for all three calmodulin light chains. This has enabled us to model a high resolution structure of BBM-I using the crystal structures of the chicken skeletal muscle myosin catalytic domain and essential light chain. Thus, we are able to directly measure the full magnitude of the ADP-dependent tail swing. The approximately 31 degrees swing corresponds to approximately 63 A at the end of the rigid light chain-binding domain. Comparison of the behavior of BBM-I with skeletal and smooth muscle subfragments-1 suggests that there are substantial differences in the structure and energetics of the biochemical transitions in the actomyosin ATPase cycle.

Force generation and phosphate release steps in skinned rabbit soleus slow-twitch muscle fibers

The force-generation and phosphate-release steps of the cross-bridge cycle in rabbit soleus slow-twitch muscle fibers (STF) were investigated using sinusoidal analysis, and the results were compared with those of rabbit psoas fast-twitch fibers (FTF). Single fiber preparations were activated at pCa 4.40 and ionic strength 180 mM at 20 degrees C. The effects of inorganic phosphate (Pi) concentrations on three exponential processes, B, C, and D, were studied. Results are consistent with the following cross-bridge scheme: [formula: see text] where A is actin, M is myosin, D is MgADP, and P is inorganic phosphate. The values determined are k4 = 5.7 +/- 0.5 s-1 (rate constant of isomerization step, N = 9, mean +/- SE), k-4 = 4.5 +/- 0.5 s-1 (rate constant of reverse isomerization), K4 = 1.37 +/- 0.13 (equilibrium constant of the isomerization), and K5 = 0.18 +/- 0.01 mM-1 (Pi association constant). The isomerization step (k4) in soleus STF is 20 times slower, and its reversal (k-4) is 20 times slower than psoas fibers. Consequently, the equilibrium constant of the isomerization step (K4) is the same in these two types of fibers. The Pi association constant (K5) is slightly higher in STF than in FTF, indicating that Pi binds to cross-bridges slightly more tightly in STF than FTF. By correlating the cross-bridge distribution with isometric tension, it was confirmed that force is generated during the isomerization (step 4) of the AMDP state and before Pi release in soleus STF.

Mechanical study of rat soleus muscle using caged ATP and X-ray diffraction: high ADP affinity of slow cross-bridges

1. The cross-bridges in slow- and fast-twitch fibres (taken, respectively, from soleus and psoas muscles of rats) were examined in mechanical experiments using caged ATP and X-ray diffraction, to compare their binding of ATP and ADP. 2. Caged ATP was photolysed in rigor fibres. When ADP was removed from pre-photolysis fibres, the initial relaxation (+/- Ca2+) in soleus was as fast as that in psoas fibres, whereas the subsequent contraction (+Ca2+) was slower in soleus than in psoas. The ATPase rate during the steady-state contraction was also slower in soleus fibres. 3. When ADP was added to pre-photolysis fibres (+/- Ca2+), tension developed even in the initial phase, the overall tension development being biphasic. Both initial and late components of the Ca(2+)-free contraction were enhanced when ADP was added before photolysis, although pre-photolysis ADP was not a prerequisite for the late component. The effect of ADP was greater in soleus than in psoas fibres. Static experiments on rigor fibres revealed a higher ADP affinity in soleus fibres. 4. The intensity of the actin layer-line from ADP rigor soleus fibres decreased rapidly on photorelease of ATP. We conclude that, despite the tight ADP binding of the soleus cross-bridge, its isometric reaction is not rate limited by the 'off' rate of ADP.