Rigor contraction and the effect of various phosphate compounds on glycerinated insect flight and vertebrate muscle

A novel kinetic model to demonstrate the independent effects of ATP and ADP/Pi concentrations on sarcomere function

Understanding muscle contraction mechanisms is a standing challenge, and one of the approaches has been to create models of the sarcomere-the basic contractile unit of striated muscle. While these models have been successful in elucidating many aspects of muscle contraction, they fall short in explaining the energetics of functional phenomena, such as rigor, and in particular, their dependence on the concentrations of the biomolecules involved in the cross-bridge cycle. Our hypothesis posits that the stochastic time delay between ATP adsorption and ADP/Pi release in the cross-bridge cycle necessitates a modeling approach where the rates of these two reaction steps are controlled by two independent parts of the total free energy change of the hydrolysis reaction. To test this hypothesis, we built a two-filament, stochastic-mechanical half-sarcomere model that separates the energetic roles of ATP and ADP/Pi in the cross-bridge cycle's free energy landscape. Our results clearly demonstrate that there is a nontrivial dependence of the cross-bridge cycle's kinetics on the independent concentrations of ATP, ADP, and Pi. The simplicity of the proposed model allows for analytical solutions of the more basic systems, which provide novel insight into the dominant mechanisms driving some of the experimentally observed contractile phenomena.

Structure of the Drosophila melanogaster Flight Muscle Myosin Filament at 4.7 Å Resolution Reveals New Details of Non-Myosin Proteins

Striated muscle thick filaments are composed of myosin II and several non-myosin proteins which define the filament length and modify its function. Myosin II has a globular N-terminal motor domain comprising its catalytic and actin-binding activities and a long α-helical, coiled tail that forms the dense filament backbone. Myosin alone polymerizes into filaments of irregular length, but striated muscle thick filaments have defined lengths that, with thin filaments, define the sarcomere structure. The motor domain structure and function are well understood, but the myosin filament backbone is not. Here we report on the structure of the flight muscle thick filaments from Drosophila melanogaster at 4.7 Å resolution, which eliminates previous ambiguities in non-myosin densities. The full proximal S2 region is resolved, as are the connecting densities between the Ig domains of stretchin-klp. The proteins, flightin, and myofilin are resolved in sufficient detail to build an atomic model based on an AlphaFold prediction. Our results suggest a method by which flightin and myofilin cooperate to define the structure of the thick filament and explains a key myosin mutation that affects flightin incorporation. Drosophila is a genetic model organism for which our results can define strategies for functional testing.

Novel γ-sarcoglycan interactors in murine muscle membranes

BACKGROUND

The sarcoglycan complex (SC) is part of a network that links the striated muscle cytoskeleton to the basal lamina across the sarcolemma. The SC coordinates changes in phosphorylation and Ca⁺⁺-flux during mechanical deformation, and these processes are disrupted with loss-of-function mutations in gamma-sarcoglycan (Sgcg) that cause Limb girdle muscular dystrophy 2C/R5.

METHODS

To gain insight into how the SC mediates mechano-signaling in muscle, we utilized LC-MS/MS proteomics of SC-associated proteins in immunoprecipitates from enriched sarcolemmal fractions. Criteria for inclusion were co-immunoprecipitation with anti-Sgcg from C57BL/6 control muscle and under-representation in parallel experiments with Sgcg-null muscle and with non-specific IgG. Validation of interaction was performed in co-expression experiments in human RH30 rhabdomyosarcoma cells.

RESULTS

We identified 19 candidates as direct or indirect interactors for Sgcg, including the other 3 SC proteins. Novel potential interactors included protein-phosphatase-1-catalytic-subunit-beta (Ppp1cb, PP1b) and Na⁺-K⁺-Cl^--co-transporter NKCC1 (SLC12A2). NKCC1 co-localized with Sgcg after co-expression in human RH30 rhabdomyosarcoma cells, and its cytosolic domains depleted Sgcg from cell lysates upon immunoprecipitation and co-localized with Sgcg after detergent permeabilization. NKCC1 localized in proximity to the dystrophin complex at costameres in vivo. Bumetanide inhibition of NKCC1 cotransporter activity in isolated muscles reduced SC-dependent, strain-induced increases in phosphorylation of extracellular signal-regulated kinases 1 and 2 (ERK1/2). In silico analysis suggests that candidate SC interactors may cross-talk with survival signaling pathways, including p53, estrogen receptor, and TRIM25.

CONCLUSIONS

Results support that NKCC1 is a new SC-associated signaling protein. Moreover, the identities of other candidate SC interactors suggest ways by which the SC and NKCC1, along with other Sgcg interactors such as the membrane-cytoskeleton linker archvillin, may regulate kinase- and Ca⁺⁺-mediated survival signaling in skeletal muscle.

Longitudinal vibration interferes with cross-bridge attachment and prevents muscle fibre shrinkage under PSE-like conditions

The impact of longitudinal vibration on cross-bridge attachments between myofilaments was investigated in single fibres and intact muscle. Sinusoidal length vibration (frequency 50 Hz, amplitude 5% of fibre length) reduced active force by 40% when fibres were activated by elevation of [Ca²⁺], but did not alter the force when fibres were in rigor state. When vibrated for 30 min in rigor at pH 5.5 and 38 °C (PSE conditions), the lateral shrinkage of the fibres was significantly reduced, suggesting a potential positive influence of vibration on water-holding capacity. In whole muscle incubated at 38 °C until 8 h post mortem, the progress of rigor onset was accessed by measuring the increase in muscle stiffness. Vibration applied 3-5 h post mortem postponed rigor development, but did not have significant influence on water-holding capacity compared with non-vibrated conditions. In conclusion, the results suggest that muscle vibration can be a future technique to delay rigor development and prevent muscle fibre shrinkage and PSE development after slaughter.

Nanomolar ATP binding to single myosin cross-bridges in rigor: a molecular approach to studying myosin ATP kinetics using single human cardiomyocytes

Our knowledge in the field of cardiac muscle and associated cardiomyopathies has been evolving incrementally over the past 60 years and all was possible due to the parallel progress in techniques and methods allowing to take a fresh glimpse at an old problem. Here, we describe an exciting tool used to examine the various states of the human cardiac myosin at the single molecule level. By imaging single Alexa647-ATP binding to permeabilised cardiomyocytes using total internal reflection fluorescence (TIRF) microscopy, we are able to acquire large populations of events in a short timeframe (~ 5000 sites in ~ 10 min) and measure each binding event with high spatio-temporal resolution. The applied algorithm decomposes the point pattern of single molecule binding events into individually distinct binding sites that enables us to recover kinetic parameters, such as bound or free time per site. This single molecule binding approach is a useful tool used to examine muscle contractility. Of particular importance is its application to probing the dynamic lifetimes and proportion of myosins in the super-relaxed state in human cardiomyopathies.

The concentrations of free magnesium and free calcium ions both increase in skeletal muscle fibres entering Rigor mortis

Metabolic inhibitors have been used to induce rigor in mammalian muscle fibres previously injected with the Ca(2+)- and Mg(2+)-binding dye arsenazo III. The spectral changes which ensue, after the onset of rigor, indicate that a large increase in intracellular free Ca(2+) occurs (to more than 10(-4)m) but that this is preceded and accompanied by a substantial increase in free Mg(2+) concentration (to 2 mm or more) presumably as a result of the decline in the ATP concentration within the cell. Fibres in rigor have been treated with the divalent cation ionophore A23187 to enable Ca(2+) and Mg(2+) to be extracted selectively from the cell by extracelluar chelating agents. Optical measurements performed during this extraction, confirm that both Ca(2+)- and Mg(2+)-binding to arsenazo III contribute to the absorption change which is recorded, whilst the treatment of relaxed fibres with the ionophore together with EDTA, produces an absorption change which, in combination with other measurements, indicates that in the sarcoplasm of the resting relaxed cell, approximately one third of the dye is complexed with Mg(2+). This indicates that the cytoplasmic free Mg(2+) concentration, prior to rigor onset, is about 0·5 mm.

Invertebrate muscles: thin and thick filament structure; molecular basis of contraction and its regulation, catch and asynchronous muscle

This is the second in a series of canonical reviews on invertebrate muscle. We cover here thin and thick filament structure, the molecular basis of force generation and its regulation, and two special properties of some invertebrate muscle, catch and asynchronous muscle. Invertebrate thin filaments resemble vertebrate thin filaments, although helix structure and tropomyosin arrangement show small differences. Invertebrate thick filaments, alternatively, are very different from vertebrate striated thick filaments and show great variation within invertebrates. Part of this diversity stems from variation in paramyosin content, which is greatly increased in very large diameter invertebrate thick filaments. Other of it arises from relatively small changes in filament backbone structure, which results in filaments with grossly similar myosin head placements (rotating crowns of heads every 14.5 nm) but large changes in detail (distances between heads in azimuthal registration varying from three to thousands of crowns). The lever arm basis of force generation is common to both vertebrates and invertebrates, and in some invertebrates this process is understood on the near atomic level. Invertebrate actomyosin is both thin (tropomyosin:troponin) and thick (primarily via direct Ca(++) binding to myosin) filament regulated, and most invertebrate muscles are dually regulated. These mechanisms are well understood on the molecular level, but the behavioral utility of dual regulation is less so. The phosphorylation state of the thick filament associated giant protein, twitchin, has been recently shown to be the molecular basis of catch. The molecular basis of the stretch activation underlying asynchronous muscle activity, however, remains unresolved.

Regulation of force development studied by photolysis of caged ADP in rabbit skinned psoas fibers

The present study examined the effects of Ca(2+) and strongly bound cross-bridges on tension development induced by changes in the concentration of MgADP. Addition of MgADP to the bath increased isometric tension over a wide range of [Ca(2+)] in skinned fibers from rabbit psoas muscle. Tension-pCa (pCa is -log [Ca(2+)]) relationships and stiffness measurements indicated that MgADP increased mean force per cross-bridge at maximal Ca(2+) and increased recruitment of cross-bridges at submaximal Ca(2+). Photolysis of caged ADP to cause a 0.5 mM MgADP jump initiated an increase in isometric tension under all conditions examined, even at pCa 6.4 where there was no active tension before ADP release. Tension increased monophasically with an observed rate constant, k(ADP), which was similar in rate and Ca(2+) sensitivity to the rate constant of tension re-development, k(tr), measured in the same fibers by a release-re-stretch protocol. The amplitude of the caged ADP tension transient had a bell-shaped dependence on Ca(2+), reaching a maximum at intermediate Ca(2+) (pCa 6). The role of strong binding cross-bridges in the ADP response was tested by treatment of fibers with a strong binding derivative of myosin subfragment 1 (NEM-S1). In the presence of NEM-S1, the rate and amplitude of the caged ADP response were no longer sensitive to variations in the level of activator Ca(2+). The results are consistent with a model in which ADP-bound cross-bridges cooperatively activate the thin filament regulatory system at submaximal Ca(2+). This cooperative interaction influences both the magnitude and kinetics of force generation in skeletal muscle.

Effects of magnesium pyrophosphate on mechanical properties of skinned smooth muscle from the guinea pig taenia coli

Effects of the non-hydrolyzable nucleotide analogue magnesium pyrophosphate (MgPPi) on cross-bridge properties were investigated in skinned smooth muscle of the guinea pig Taenia coli. A "high" rigor state was obtained by removing MgATP at the plateau of an active contraction. Rigor force decayed slowly towards an apparent plateau of approximately 25-35% of maximal active force. MgPPi markedly increased the rate of force decay. The initial rate of the force decay depended on [MgPPi] and could be described by the Michaelis-Menten equation with a dissociation constant of 1.6 mM. The decay was irreversible amounting to approximately 50% of the rigor force. Stiffness decreased by 20%, suggesting that the major part of the cross-bridges were still attached. The results can be interpreted as "slippage" of PPi-cross-bridges to positions of lower strain. The initial rate of MgPPi-induced force decay decreased with decreasing ionic strength in the range 45-150 mM and was approximately 25% lower in thiophosphorylated fibers. MgADP inhibited the MgPPi-induced force decay with an apparent Ki of 2 microM. The apparent Km of MgATP for the maximal shortening velocity in thiophosphorylated fibers was 32 microM. This low Km of MgATP suggests that steps other than MgATP-induced detachment are responsible for the low shortening velocity in smooth muscle. No effects were observed of 4 mM MgPPi on the force-velocity relation, suggesting that cross-bridges with bound MgPPi do not constitute an internal load or that binding of MgPPi is weaker in negatively strained cross-bridges during shortening.

The molecular basis of chemomechanical coupling in muscle and in other biological engines

It is argued that the force driving muscular shortening (psi) differs from that (phi) responsible for rigor tension generation. psi is associated with ATP-induced dissociation of actomyosin (a.m.), whereas phi is due to an isomerization reaction of a.m., following the hydrolysis of ATP. Both forces are intimately coupled with appreciable changes in the structure of the hydration shell of a.m., mainly at the interface between the two proteins, which involve the release of stored energy. When an active muscle is allowed to shorten freely, psi gives rise to a sliding distance (s.d.) delta l1 which differs in character and in magnitude from the s.d. (delta l2) observed when a muscle which had developed rigor tension isometrically is released. The maximal values of the two forces (psi 0 and phi 0) as well as delta l2 are calculated on the basis of experimental data. The forces and their corresponding s.d.'s are related through the standard free energies of the chemical reactions which are responsible for them. It is claimed that the same mechanochemical (m.c.) mechanisms operate also in all microtube-based locomotion and force-generation systems and, furthermore, that practically the same values of psi 0, phi 0, delta l1, and delta l2 are shared by the two types of biological m.c. convertors.

Regulation of binding of subfragment 1 in isolated rigor myofibrils

A steric-hindrance model has been used to explain the regulation of muscle contraction by tropomyosin-troponin complex. The regulation of binding was studied by microscopic observation of mixtures of fluorescent subfragment 1 (S1) with rigor myofibrils at different actin-to-S1 ratios and in the presence and absence of calcium. Procedures were adapted to protect the critical thiols of S1 before conjugation to thiol-specific fluorochromes, this giving fluorescent S1 with unaltered enzyme activity. S1 binding was greatest in the I band (except at the Z-lines) in the presence of calcium regardless of the [S1]. The patterns in the absence of calcium depended on the actin-to-S1 ratios: low [S1], binding in the myosin-actin overlap region; intermediate [S1], highest binding at the A-I junction; high [S1], greatest binding in the I-band. The two distinct binding patterns observed at low [S1] were demonstrated by dual-channel fluorescence microscopy when myofibrils were sequentially incubated with fluorescent S1 without calcium followed by a different fluorescent S1 with calcium. These observations support the concept of rigor activation of actin sites. The change in the pattern upon increasing [S1] without calcium demonstrate cooperative interactions along the thin filament. However, these interactions (under the conditions used without calcium) do not appear to extend over greater than 2-3 tropomyosin-troponin-7 actin functional units.

Orientation of spin-labeled light chain 2 of myosin heads in muscle fibers

Electron paramagnetic resonance (e.p.r.) spectroscopy has been used to monitor the orientation of spin labels attached rigidly to a reactive SH residue on the light chain 2 (LC2) of myosin heads in muscle fibers. e.p.r. spectra from spin-labeled myosin subfragment-1 (S1), allowed to diffuse into unlabeled rigor (ATP-free) fibers, were roughly approximated by a narrow angular distribution of spin labels centered at 66 degrees relative to the fiber axis, indicating a uniform orientation of S1 bound to actin. On the other hand, spectra from spin-labeled heavy meromyosin (HMM) were roughly approximated by two narrow angular distributions centered at 42 degrees and 66 degrees, suggesting that the LC2 domains of the two HMM heads have different orientations. In contrast to S1 or HMM, the spectra from rigor fibers, in which LC2 of endogenous myosin heads was labeled, showed a random orientation which may be due to distortion imposed by the structure of the filament lattice and the mismatch of the helical periodicities of the thick and thin filaments. However, spectra from the fibers in the presence of ATP analog 5'-adenylyl imidodiphosphate (AMPPNP) were approximated by two narrow angular distributions similar to those obtained with HMM. Thus, AMPPNP may cause the LC2 domain to be less flexible and/or the S2 portion to be more flexible, so as to release the distortion of the LC2 domain and make it return to its natural position. At high ionic strength, AMPPNP disoriented the spin labels as ATP did under relaxing conditions, suggesting that the myosin head is detached from and/or weakly (flexibly) attached to a thin filament.

Three-dimensional image reconstruction of insect flight muscle. II. The rigor actin layer

The averaged structure of rigor cross-bridges in insect flight muscle is further revealed by three-dimensional reconstruction from 25-nm sections containing a single layer of thin filaments. These exhibit two thin filament orientations that differ by 60 degrees from each other and from myac layer filaments. Data from multiple tilt views (to +/- 60 degrees) was supplemented by data from thick sections (equivalent to 90 degrees tilts). In combination with the reconstruction from the myac layer (Taylor et al., 1989), the entire unit cell is reconstructed, giving the most complete view of in situ cross-bridges yet obtained. All our reconstructions show two classes of averaged rigor cross-bridges. Lead bridges have a triangular shape with leading edge angled at approximately 45 degrees and trailing edge angled at approximately 90 degrees to the filament axis. We propose that the lead bridge contains two myosin heads of differing conformation bound along one strand of F-actin. The lead bridge is associated with a region of the thin filament that is apparently untwisted. We suggest that the untwisting may reflect the distribution of strain between myosin and actin resulting from two-headed, single filament binding in the lead bridge. Rear bridges are oriented at approximately 90 degrees to the filament axis, and are smaller and more cylindrical, suggesting that they consist of single myosin heads. The rear bridge is associated with a region of apparently normal thin filament twist. We propose that differing myosin head angles and conformations consistently observed in rigor embody different stages of the power stroke which have been trapped by a temporal sequence of rigor cross-bridge formation under the constraints of the intact filament lattice.

Effect of adenosine triphosphate analogues on skeletal muscle fibers in rigor

It is commonly believed, for both vertebrate striated and insect flight muscle, that when the ATP analogue adenyl-5'-yl imidodiphosphate (AMPPNP) is added to the muscle fiber in rigor, it causes the fiber to lengthen by 0.15%. This has been interpretated (Marston S.B., C.D. Roger, and R.T. Tregear. 1976. J. Mol. Biol. 104:263-267) as suggesting (a) that in rigor the crossbridge is fixed to, i.e., almost never detaches from the actin filament; (b), that the crossbridge remains fixed to the actin filament after AMPPNP addition; and (c) that the ability of AMPPNP to cause apparent lengthening of a muscle fiber is due to its ability to cause a conformational change in the myosin crossbridge that has an axial component of approximately 1.6 nm/half-sarcomere. The present study, done only on chemically-skinned rabbit psoas fibers, confirms that AMPPNP can cause muscle fibers to lengthen by 0.15% but only for a narrow set of experimental conditions. When experimental conditions are varied over a wider range, it becomes apparent that the extent of lengthening of a rigor muscle fiber upon AMPPNP addition depends almost entirely on the strain present in the rigor fiber before AMPPNP addition. Addition of AMPPNP to an unstrained rigor fiber (one supporting zero tension), induces zero length change while addition of AMPPNP to very highly strained rigor fibers induces length changes greater than 0.15%. The data thus do not support the hypotheses that the crossbridges remain fixed to the actin filament after AMPPNP addition and that the size of the apparent length change induced by AMPPNP is related to the size of the axial component of a conformational change. Instead, the data support the idea that the ability of AMPPNP to cause lengthening of a rigor muscle fiber is related to its ability to accelerate the rate at which strained crossbridges detach from actin and reattach in positions in lesser strain. The data do not rule out a conformational change upon AMPPNP binding, they simply make clear that any attempt to measure a force response conceivably due to a conformational change, would be more than obscured by the force changes due to crossbridges detaching and reattaching in positions of lesser strain.

Two attached non-rigor crossbridge forms in insect flight muscle

We have performed thin-section electron microscopy on muscle fibers fixed in different mechanically monitored states, in order to identify structural changes in myosin crossbridges associated with force production and maintenance. Tension and stiffness of fibers from glycerinated Lethocerus flight muscle were monitored during a sequence of conditions using AMPPNP and then AMPPNP plus increasing concentrations of ethylene glycol, which brought fibers through a graded sequence from rigor relaxation. Two intermediate crossbridge forms distinct from the rigor or relaxed forms were observed. The first was produced by AMPPNP at 20 degrees C, which reduced isometric tension 60 to 70% below rigor level without reducing rigor stiffness. Electron microscopy of these fibers showed that, in spite of the drop in tension, no obvious change from the 45 degrees crossbridge angle characteristic of rigor occurred. However, the thick filament ends of the crossbridges were altered from their rigor positions, so that they now marked a 14.5 nm repeat, and formed four separate origins at each crossbridge level. The bridges were also less slewed and bent than rigor bridges, as seen in transverse sections. The second crossbridge form was seen in glycol-AMPPNP at 4 degrees C, just below the glycol concentration that produced mechanical relaxation. These fibers retained 90% of rigor stiffness at 40 Hz oscillation, but would not bear sustained tension. Stiffness was also high in the presence of calcium at room temperature under similar conditions. Electron microscopy showed crossbridges projecting from the thick filaments at an angle that centered around 90 degrees, rather than the 45 degree angle familiar from rigor. This coupling of relaxed appearance with persistent stiffness suggests that the 90 degree form may represent a weakly attached crossbridge state like that proposed to precede force development in current models of the crossbridge power stroke.

The range of sarcomere lengths in the muscles of the human lower limb

The lengths of the sarcomeres of some muscles of the human leg were determined for the anatomical position, using a method based on diffraction. Measurements were made of the muscle lengths and angles of pennation from cadavers, and these were used to predict sarcomere lengths at other limb positions. The measured and predicted sarcomere lengths were compared with the length-tension curve for human muscle, which showed the range of sarcomere length from both extremes of muscle length to cover the entire range of the length-tension curve.

Energetics of the actomyosin bond in the filament array of muscle fibers

The interaction between actin and myosin in the filament array of glycerinated muscle fibers has been monitored using paramagnetic probes and mechanical measurements. Both fiber stiffness and the spectra of probes bound to a reactive sulfydral on the myosin head were measured as the actomyosin bond was weakened by addition of magnesium pyrophosphate (MgPPi) and glycerol. In the absence of MgPPi, all myosin heads are attached to actin with oriented probes. When fibers were incubated in buffers containing MgPPi, a fraction of the probes became disordered, and this effect was greater in the presence of glycerol. To determine whether the heads with disordered probes were detached from actin, spin-labeled myosin subfragment-1 (MSL-S1) was diffused into unlabeled fibers, and the fractions bound to actin and free in the medium were correlated with the oriented and disordered spectral components. These experiments showed that the label was oriented when MSL-S1 was attached to actin in a ternary complex with the ligand and that all heads with disordered probes were detached from actin. Thus the fraction of oriented labels could be used to determine the fraction of heads attached to actin in a fiber in the presence of ligand. The fraction of myosin heads attached to actin decreased with increasing [MgPPi], and in the absence of glycerol approximately 50% of the myosin heads were dissociated at 3.3 mM ligand with little change in fiber stiffness. In the presence of 37% glycerol plus ligand, up to 80% of the heads could be detached with a 50% decrease in fiber stiffness. The data indicate that there are two populations of myosin heads in the fiber. All the data could be fit with a model in which one population of myosin heads (comprising approximately 50% of the total) sees an apparent actin concentration of 0.1 mM and can be released from actin with little change in fiber stiffness. A second population of myosin heads (approximately 50%) sees a higher actin concentration (5 mM) and is only released in the presence of both glycerol and ligand.

A myosin site involved in energy transduction during muscle contraction

An antibody was developed against a 23-kDa fragment of myosin which contains a part of the ATP-binding site, and applied to skinned muscle fibers. The antibody abolished active tension generation of the fibers, but did not block their assumption of a rigor state nor their release from this state by ATP. The primary amino acid sequence of the antigenic site on the fragment was found to be the region containing residues 77-80. This sequence region is predicted to have interesting secondary structures, and is distinct from the proposed ATP-binding site. We discuss the possibility that the region is responsible for the energy-transducing step in muscle contraction.

The influence of doubly attached crossbridges on the mechanical behavior of skeletal muscle fibers under equilibrium conditions

A simple model of a double-headed crossbridge is introduced to explain the retardation of force decay after an imposed stretch in skeletal muscle fibers under equilibrium conditions. The critical assumption in the model is that once one of the heads of a crossbridge is attached to one of the available actin sites, the attachment of the second head will be restricted to a level of strain determined by the attachment of the first head. The crossbridge structure, namely the connection of both heads of a crossbridge to the same tail region, is assumed to impose this constraint on the spatial configurations of crossbridge heads. The unique feature of the model is the prediction that, in the presence of a ligand (PPi, ADP, AMP-PNP) and absence of Ca2+, the halftime of force decay is many times larger than the inverse rate of detachment of a crossbridge head measured in solution. This prediction is in agreement with measured values of half-times of force decay in fibers under similar conditions (Schoenberg, M., and E. Eisenberg. 1985. Biophys. J. 48:863-871f). It is predicted that a crossbridge head is more likely to re-attach to its previously strained position than remain unattached while the other head is attached, leading to the slow decay of force. Our computations also show that the apparent cooperativity in crossbridge binding observed in experiments (Brenner, B., L. C. Yu, L. E. Greene, E. Eisenberg, and M. Schoenberg. 1986. Biophys. J. 50:1101-1108) can be partially accounted by the double-headed crossbridge attachment. Our model predictions fit the aforementioned data best when the crossbridge stiffness does not change significantly with the dissociation of one of the two attached heads. This observation suggests that crossbridge stiffness is determined either by the extensibility (flexibility) of the double helical tail region or its junction to the thick filament backbone.

Rigor crossbridges are double-headed in fast muscle from crayfish

The structure of rigor crossbridges was examined by comparing rigor crossbridges in fast muscle fibers from glycerol-extracted abdominal flexor muscle of crayfish with those in "natively decorated" thin filaments from the same muscle. Natively decorated thin filaments were obtained by dissociating the backbone of the myosin filaments of rigor myofibrils in 0.6 M KCl. Intact fibers were freeze-fractured, deep-etched, and rotary shadowed; isolated filaments were either negatively stained or freeze dried and rotary shadowed. The crossbridges on the natively decorated actin maintain the original spacing and the disposition in chevrons and double chevrons for several hours, indicating that no rearrangement of the actomyosin interactions occurs. Thus the crossbridges of the natively decorated filaments were formed within the geometrical constraints of the intact myofibril. The majority of crossbridges in the intact muscle have a triangular shape indicative of double-headed crossbridge. The triangular shape is maintained in the isolated filaments and negative staining resolves two heads in a single crossbridge. In the isolated filaments, crossbridges are attached at uniform acute angles. Unlike those in insect flight muscle (Taylor et al., 1984), lead and rear elements of the double chevron may be both double-headed. Deep-etched images reveal a twisted arrangement of subfilaments in the backbone of the thick filament.

An in vitro method for the measurement of sarcomere length in human muscle

A possible means of quantifying the surface electromyographic (EMG) signal is to try to relate it to the sarcomere length of the muscle, which is a function of muscle tension. The present work deals with the measurement of sarcomere length in human muscles. Sarcomere length is measured by using a thin sheet of muscle as a diffraction grating and obtaining and photographing its diffraction pattern using a modified light microscope. Samples of human muscle are available only from cadavers in the anatomical position, so a procedure for prediction of sarcomere length at other positions is described. The limitations of working with cadavaric tissue are outlined and discussed, and, for the readers unfamiliar with the structure and physiology of striated muscle, a brief description of this is given.

Relaxation of chemically skinned guinea pig taenia coli smooth muscle from rigor by photolytic release of adenosine-5'-triphosphate

The mechanical events following release of ATP from P3-1-(2-nitro)phenylethyladenosine-5'-triphosphate (caged-ATP) in skinned guinea pig taenia coli smooth muscle in rigor were investigated. A rigor force of about 25-35% of the maximal active force was obtained by removing ATP at the plateau of a maximal active contraction. In the rigor solution free-Mg2+ was 2 mM, ionic strength 90 mM and pH 7.0. When caged-ATP (12.5 mM) was diffused into the preparation there was no change in the rigor force. Photolytic production of about 2 mM ATP was achieved with a xenon flash lamp. Following illumination, force decreased with an approximate initial rate constant of 0.7 s-1. The rate of relaxation was increased in the presence of inorganic phosphate (at 3 mM: 1.3 s-1; 10 mM: 2.2 s-1). At higher Mg2+ concentrations the rate of relaxation was slower (5 mM: 0.2 s-1) and at lower concentrations the rate was faster (0.5 mM: 1.2 s-1). An increased rate of relaxation was observed when ionic strength was increased to 150 mM (2.2 s-1). Phosphate increased the rate of relaxation at the different levels of Mg2+ (0.5-10 mM) and ionic strength (90, 150 mM). In preparations shortened (by 1-3%) to give reduced rigor force, a small transient increase in tension was recorded after ATP release. In comparison to the rates of ATP-induced dissociation of actomyosin in solution, reported in the literature, the rate of relaxation from rigor is slower. This may reflect a slow rigor cross-bridge dissociation or mechanical interactions not associated with cross-bridges in the muscle fibre.(ABSTRACT TRUNCATED AT 250 WORDS)

Crossbridges in insect flight muscles of the blowfly (Sarcophaga bullata)

Crossbridges in quick-frozen deep-etched blowfly flight muscles (from Sarcophaga bullata) were compared with those observed in the traditional waterbug preparation (Lethocerus) and found to be indistinguishable. Hence, blowfly was chosen as a fresher more accessible tissue for determining the effect of various fixatives and nucleotides on crossbridge structure. In rigor control, crossbridges were most regular in muscles that were stabilized before freezing by prefixation in glutaraldehyde followed by 'hardening' with neutralized tannic acid, so all nucleotide treatments were terminated by such fixation. MgATP (5 mM) converted the rigor pattern of crossbridges into a random array of disconnected thick filament projections. Lower levels of ATP (0.1 mM) caused a variable but generally lesser degree of crossbridge disconnection, as did 5 mM ADP (probably because it slowly converted to ATP inside the muscle fibres). Vanadate (1-2 mM) potentiated muscle relaxation in the latter two nucleotide treatments (i.e. produced a greater degree of crossbridge disconnection). Thus, differences in overall crossbridge abundance were readily apparent in chemically fixed muscles. Structural details within individual crossbridges were less well preserved, however. Chemical prefixation tended to collapse the muscle lattice, add a surface film to the filaments and thus obscure crossbridge details. Rigorous control of fixative pH largely prevented these problems and permitted recognition of the fact that in Sarcophaga flight muscle, as in Lethocerus muscle in rigor, the S1 'heads' of crossbridges attach to the thin filaments in the expected 'arrowhead' configuration.

ADP binding to myosin cross-bridges and its effect on the cross-bridge detachment rate constants

We have studied the binding of adenosine diphosphate (ADP) to attached cross-bridges in chemically skinned rabbit psoas muscle fibers and the effect of that binding on the cross-bridge detachment rate constants. Cross-bridges with ADP bound to the active site behave very similarly to cross-bridges without any nucleotide at the active site. First, fiber stiffness is the same as in rigor, which presumably implies that, as in rigor, all the cross-bridges are attached. Second, the cross-bridge detachment rate constants in the presence of ADP, measured from the rate of decay of the force induced by a small stretch, are, over a time scale of minutes, similar to those seen in rigor. Because ADP binding to the active site does not cause an increase in the cross-bridge detachment rate constants, whereas binding of nucleotide analogues such as adenyl-5'-yl imidodiphosphate (AMP-PNP) and pyrophosphate (PPi) do, it was possible, by using ADP as a competitive inhibitor of PPi or AMP-PNP, to measure the competitive inhibition constant and thereby the dissociation constant for ADP binding to attached cross-bridges. We found that adding 175 microM ADP to 4 mM PPi or 4 mM AMP-PNP produces as much of a decrease in the apparent cross-bridge detachment rate constants as reducing the analogue concentration from 4 to 1 mM. This suggests that ADP is binding to attached cross-bridges with a dissociation constant of approximately 60 microM. This value is quite similar to that reported for ADP binding to actomyosin subfragment-1 (acto-S1) in solution, which provides further support for the idea that nucleotides and nucleotide analogues seem to bind about as strongly to attached cross-bridges in fibers as to acto-S1 in solution (Johnson, R.E., and P. H. Adams. 1984. FEBS Letters. 174:11-14; Schoenberg, M., and E. Eisenberg. 1985. Biophysical Journal. 48:863-871; Biosca, J.A., L.E. Greene, and E. Eisenberg. 1986. Journal of Biological Chemistry. 261:9793-9800).

Regulation of actomyosin ATPase activity by troponin-tropomyosin: effect of the binding of the myosin subfragment 1 (S-1).ATP complex

In our model of regulation, the observed lack of cooperativity in the binding of myosin subfragment 1 (S-1) with bound ATP to the troponin-tropomyosin-actin complex (regulated actin) is explained by S-1.ATP having about the same affinity for the conformation of the regulated actin that activates the myosin ATPase activity (turned-on form) and the conformation that does not activate the myosin ATPase activity (turned-off form). This predicts that, in the absence of Ca2+, S-1.ATP should not turn on the regulated actin filament. In the present study, we tested this prediction by using either unmodified S-1 or S-1 chemically modified with N,N'-p-phenylenedimaleimide (pPDM X S-1) so that functionally it acts like S-1.ATP, although it does not hydrolyze ATP. We found that, in the absence of Ca2+, neither S-1.ATP nor pPDM X S-1.ATP significantly turns on the ATPase activity of the regulated complex of actin and S-1 (acto X S-1). In contrast, in the presence of Ca2+, pPDM X S-1.ATP binding almost completely turns on the regulated acto.S-1 ATPase activity. These results can be explained by our original cooperativity model, with pPDM X S-1.ATP binding only approximately equal to 2-fold more strongly to the turned-on form than to the turned-off form of regulated actin. However, our results are not consistent with our alternative model, which predicts that if pPDM X S-1.ATP binds to actin in the absence of Ca2+ but does not turn on the ATPase activity, then it should also not turn on the ATPase activity in the presence of Ca2+.

Force-velocity relation and rate of ATP hydrolysis in osmotically compressed skinned smooth muscle of the guinea pig

Chemically skinned guinea pig taenia coli fibre bundles showed a concentration-dependent decrease in width when incubated in media containing Dextran T500 (0-0.2 g ml-1). The maximal reduction in width, observed at 0.2 g ml-1 dextran, was 32%. The effect was reversible upon removal of dextran. Isometric force was slightly increased (about 10%) at the lowest dextran concentration (0.025 g ml-1) but decreased at higher concentrations (40% decrease at 0.2 g/ml-1). The energetic tension cost (ATP turnover/force) was decreased by about 40% after dextran addition. Force development and relaxation were markedly slower in 0.1 g ml-1 and absent in 0.2 g ml-1 dextran. In isotonic quick-release experiments 0.025 g ml-1 dextran did not influence maximal shortening velocity (Vmax) and relative stiffness, whereas 0.1 g ml-1 markedly increased stiffness and decreased Vmax to about 27%. Vanadate induced relaxation in the activated muscle (pCa 4.5) both in the absence and presence (0.1 g ml-1) of dextran and increased the rate of relaxation (pCa 9) at 0.1 g ml-1 dextran. The isometric rate of crossbridge turnover, as reflected by the energetic tension cost and the rate of relaxation, was decreased at all degrees of osmotic compression. Crossbridge turnover rate during shortening (Vmax) was unaffected at an osmotic compression of 12% (width) but was decreased at higher compression (32%).

Ca2+-sensitive cross-bridge dissociation in the presence of magnesium pyrophosphate in skinned rabbit psoas fibers

We find that at 6 degrees C in the presence of 4 mM MgPPi, at low or moderate ionic strength, skinned rabbit psoas fibers exhibit a stiffness and an equatorial x-ray diffraction pattern similar to that of rigor fibers. As the ionic strength is increased in the absence of Ca2+, both the stiffness and the equatorial x-ray diffraction pattern approach those of the relaxed state. This suggests that, as in solution, increasing ionic strength weakens the affinity of myosin cross-bridges for actin, which results in a decrease in the number of cross-bridges attached. The effect is Ca2+-sensitive. Assuming that stiffness is a measure of the number of cross-bridge heads attached, in the absence of Ca2+, the fraction of attached cross-bridge heads varies from approximately 75% to approximately 25% over an ionic strength range where ionic strength in solution weakens the binding constant for myosin subfragment-1 binding to unregulated actin by less than a factor of 3. Therefore, this phenomenon appears similar to the cooperative Ca2+-sensitive binding of S1 to regulated actin in solution (Greene, L. E., and E. Eisenberg, 1980, Proc. Natl. Acad. Sci. USA, 77:2616). By comparing the binding constants in solution and in the fiber under similar conditions, we find that the "effective actin concentration," that is, the concentration that gives the same fraction of S1 molecules bound to actin in solution as cross-bridge heads are bound to actin in a fiber, is in the millimolar range. An effective actin concentration in the millimolar range suggests that the strength of actin binding to cross-bridges in fibers may be several orders of magnitude weaker than the strength of ATP binding. Previously, it has been assumed that these two quantities were equal, as this gives the minimum energy loss when ATP dissociates the cross-bridge from actin (Morales, 1980, J. Supramol. Struct., 3:105:1975; Eisenberg, E.,Hill, T. L. and Y. Chen, 1980, Biophys. J., 29:195).

Actin-attached and detached crossbridges in myofibrils: segregation into two populations according to their sensitivity to proteolytic digestion of myosin heavy chain

Tryptic digestion of myofibrils was used to assess the interaction of crossbridges with thin filaments in the presence of ATP analogues. The relative amounts of 200 kDa fragment produced by trypsin from myosin heavy chain when the crossbridge is attached to actin, and of 160 kDa fragment produced when the crossbridge is detached from actin, served as a measure of crossbridge-actin interaction. In rigor only the 200 kDa fragment was produced suggesting that a great majority of the crossbridges were strongly attached to actin; in the presence of MgPPi at 0 degrees C only the 160 kDa fragment was finally produced suggesting that eventually all crossbridges detached from actin. In the presence of MgPPi or MgAMPPNP at 25 degrees C both 200 and 160 kDa fragments were present for several minutes after myosin heavy chain had been completely digested, suggesting that two populations of crossbridges (attached and detached) co-existed at the same time within the myofibril. It is concluded that the addition of ATP analogues to muscle does not simply affect the chemical equilibrium of binding of myosin heads to actin but that it causes rapid dissociation of one crossbridge population without significant effect on binding to actin of the remaining crossbridge population.

A model for the interaction of muscle cross-bridges with ligands which compete with ATP

A model is presented to describe the inhibition of muscle fiber contraction by ligands that compete with MgATP. Two ligands, adenosine 5' (beta, gamma-imido) triphosphate (AMPPNP) and pyrophosphate (PPi), decrease the force developed in isometric contractions and act as weak competitive inhibitors of the maximum velocity of contraction (Pate & Cooke, 1985). These observations provide information on the energetics of actomyosin ligand states at the end of the power-stroke where MgATP dissociates the myosin cross-bridge from actin, and they are analysed in terms of a seven state model of cross-bridge kinetics. The model can reconcile the observations that these ligands bind tightly to fibers, Kd = 10(-4) M, while they are only weak inhibitors of fiber velocity, Ki = 2 X 10(-3) M. It provides a reasonable fit to the data and leads to several conclusions concerning the properties of the cross-bridge states. The states with bound ligand are shifted axially so that they occur earlier in the power-stroke than the nucleotide-free rigor state. This shift also explains the axial lengthening seen upon addition of ligands to rigor fibers. We can conclude that these ligands cause small perturbations in the cross-bridge configuration rather than large shifts. A second conclusion is that cross-bridges do not detach from actin during their power-strokes. Instead they traverse the entire length of the power stroke and are detached only at the end, leading to the suggestion that the cycling of bridges in isometric fibers is due to fluctuations in the relative positions of thick and thin filaments. With some further assumptions, the model also explains many of the rate constants and equilibrium constants of the actin-myosin-ligand interaction that have been measured in solution.

Muscle cross-bridge kinetics in rigor and in the presence of ATP analogues

Recently we reported preliminary mechanical experiments on freshly skinned rabbit psoas fibers that suggested that while almost all of the cross-bridges are attached to actin in the presence of 4 mM adenyl-5'-yl-imidodiphosphate (AMP-PNP) (ionic strength, 0.13 M), there is an equilibrium between the attached and detached states, so that, in the presence of 4 mM AMP-PNP, fibers should not be able to maintain tension (Schoenberg, et al., 1984, in Contractile Mechanisms in Muscle, Pollack and Sugi, editors., Plenum Publishing Corp., NY). Since this suggestion was at variance with published results of Clarke and Tregear (1982, FEBS [Fed. Eur. Biochem. Soc.] Lett, 143:217), we reinvestigated the ability of rabbit psoas fibers to support tension following a 2-nm stretch in rigor and in the presence of the nucleotide analogues, PPi and AMP-PNP, for analogue concentrations ranging from 0.25 to 4 mM. We find that, whereas in rigor there is very little tension decay following a stretch, in 4 mM nucleotide analogue solution, the force generated by stretch quickly decays to zero. The force decay is not exponential; rather, it can be described by rate constants that range from approximately 0.1 to 100 s-1 in 4 mM PPi, and 0.01 to 10 s-1 in 4 mM AMP-PNP. This large range of decay rate constants may be partially related to the dependence of either analogue binding or cross-bridge dissociation upon strain, since we find that the rate constants for force decay decrease with decreasing size of stretch or with decrease of analogue concentration below the maximum studied (4 mM). In general the results are consistent with an equilibrium model for cross-bridge binding to actin, where the rate constants for cross-bridge detachment determine the rate of tension decay.

The effects of ADP and phosphate on the contraction of muscle fibers

The products of MgATP hydrolysis bind to the nucleotide site of myosin and thus may be expected to inhibit the contraction of muscle fibers. We measured the effects of phosphate and MgADP on the isometric tensions and isotonic contraction velocities of glycerinated rabbit psoas muscle at 10 degrees C. Addition of phosphate decreased isometric force but did not affect the maximum velocity of shortening. To characterize the effects of ADP on fiber contractions, force-velocity curves were measured for fibers bathed in media containing various concentrations of MgATP (1.5-4 mM) and various concentrations of MgADP (1-4 mM). As the [MgADP]/[MgATP] ratio in the fiber increases, the maximum velocity achieved by the fiber decreases while the isometric tension increases. The inhibition of fiber velocities and the potentiation of fiber tension by MgADP is not altered by the presence of 12 mM phosphate. The concentration of both MgADP and MgATP within the fiber was calculated from the diffusion coefficient for nucleotides within the fiber, and the rate of MgADP production within the fiber. Using the calculated values for the nucleotide concentration inside the fiber, observed values of the maximum contraction velocity could be described, within experimental accuracy, by a model in which MgADP competed with MgATP and inhibited fiber velocity with an effective Ki of 0.2-0.3 mM. The average MgADP level generated by the fiber ATPase activity within the fiber was approximately 0.9 mM. In fatigued fibers MgADP and phosphate levels are known to be elevated, and tension and the maximum velocity of contraction are depressed. The results obtained here suggest that levels of MgADP in fatigued fibers play no role in these decreases in function, but the elevation of both phosphate and H+ is sufficient to account for much of the decrease in tension.

Rigor crossbridge structure in tilted single filament layers and flared-X formations from insect flight muscle

The averaged structure of rigor crossbridges in insect flight muscle has been studied in filtered images. Their three-dimensional structure has been deduced by relating tilt views of single filament layers in 25 nm longitudinal sections (myac layers and actin layers) to the flared-X appearance in 15 nm cross-sections showing single crossbridge levels. Tilting myac or actin layers around the filament axis makes crossbridges show one of two patterns. Beadlike densities appear either singly over thin filaments ("center-beading") or doubled and flanking thin filaments ("straddle-beading"). These express two different projections from the crossbridge-actin complexes as seen end-on in flared-X formations. Tannic acid/glutaraldehyde fixation gave improved actin preservation, showing, in 15 nm cross-sections, the long-pitch helical strands as "two-dot" profiles of consistent azimuth in the gaps between double chevrons. The azimuth in the flared-X arms was then inferred from lattice relationships, since it was not seen directly. The tangential attachment of comma-shaped crossbridges to the inferred actin dyad fits the binding geometry in recent actin-subfragment 1 complex reconstructions. However, averaged crossbridge structure differs between lead and rear members of double chevrons, unlike the uniform heads on decorated actin. In filtered images of myac layers, the lead bridges are dense and steeply angled; the rear chevron is seen as a dense bead over the thin filament with faint, less angled bars extending laterally. Actin layer images also suggest that rear and lead bridges differ in angle. Left and right flared-X arms are end-on views of lead and rear chevron bridges, respectively, and differ in shape. Improved fixation with tannic acid/glutaraldehyde allows us to distinguish three crossbridge domains in flared-X arms: (1) a dense bulb-like head merged into the thin filament; (2) a dense but thinner neck tangential to actin; and (3) a faint thin stem joining the necks to myosin filaments. Shape differences in lead and rear members between the head-neck-actin complexes are indicated by the names "L sigmoid" and "R dogleg". Within crossbridges, internal angles between the head-neck axis and the head-actin-head axis differ between sigmoid and dogleg by about 30 degrees, implying a flexible junction between bridge-head and bridge-neck. Lead and rear bridges are axially at least 13 nm apart on actin; the expected 60 degrees difference in azimuth is expressed by head-neck portions, but the head-actin-head axis rotates by only 30 degrees.(ABSTRACT TRUNCATED AT 400 WORDS)

The inhibition of muscle contraction by adenosine 5' (beta, gamma-imido) triphosphate and by pyrophosphate

We have studied the inhibition of the contraction of glycerinated rabbit psoas muscle caused by ligands that bind to the ATPase site of myosin. Two ligands, adenosine 5' (beta, gamma-imido) triphosphate (AMPPNP) and pyrophosphate (PPi), decreased the force and stiffness developed in isometric contractions and the velocity of shortening of isotonic contractions. The force exerted by isometric fibers was measured as a function of MgATP in the presence and absence of a constant concentration of the ligands. As the MgATP concentration decreased, the inhibition of tension caused by the ligand increased, reaching approximately 50% at 25 microM MgATP and either 2 mM MgPPi or 2 mM MgAMPPNP. The maximum velocity of shortening was also measured as a function of MgATP concentration in the presence of 1 and 2 mM MgPPi and 2.5 and 5 mM MgAMPPNP. Both ligands acted as pure competitive inhibitors with Ki = 3.0 mM for PPi and 5.1 mM for MgAMPPNP. These data show that both ligands are weak inhibitors of the contraction of fibers. The results provided information on the energetics of actin-myosin-ligand states that occur in the portion of the cross-bridge cycle where MgATP binds to myosin. A simple analysis of the inhibition of velocity suggests that MgAMPPNP binds to the actomyosin complex at this step of the cycle with an effective affinity constant of approximately 2 X 10(2) M-1.

Binding of myosin subfragment 1 to glycerinated insect flight muscle in the rigor state

The binding of rabbit muscle myosin subfragment 1 (S1) to glycerinated insect flight muscle fibers has been studied by low-angle x-ray diffraction, quantitative sodium dodecyl sulfate gel electrophoresis, quantitative interference microscopy, and electron microscopy. Changes induced in the rigor x-ray diffraction pattern are consistent with the idea that vacant myosin-binding sites on thin filaments are filled by exogenous S1. Electron microscopy indicates that S1 permeates and labels fibers and fibrils completely. Electron micrographs also show that cross-bridges are not displaced by exogenous S1 under the conditions used, and this is supported by the unchanged mechanical stiffness of the S1-labeled fibers. The amount of bound S1, as measured by gel electrophoresis and interference microscopy, together with the magnitude of the intensity changes in the x-ray diffraction pattern, is consistent with a thick filament structure that contains four molecules of endogenous myosin per 14.5 nm of its length, but does not agree well with earlier estimates of six myosins per crown. Lack of information on possible inhibition of S1-binding by factors other than the presence of cross-bridges, e.g., troponin, render uncertain calculations of the number of attached cross-bridges in the rigor state. However, it appears that at least 75% of the endogenous myosin heads are attached. Occupancy of binding sites on thin filaments after incubation with S1 is high, probably greater than 85%, so that x-ray scattering from those parts of the structure that adhere to the symmetry of the thin filaments can be treated as diffraction from S1-decorated thin filaments. In addition, we show in thin flared X cross sections that exo-S1 heads bind to actin with the geometry described in decorated actin by Taylor, K.A., and L.A. Amos.

Three-dimensional reconstruction of rigor insect flight muscle from tilted thin sections

Rigor cross-bridges show two conformations paired within each 38.7-nm axial repeat. The two forms may express two stages of the cross-bridge cycle during contraction. Differing numbers of myosin heads per cross-bridge and associated helical changes in the thin filament distinguish the two forms and suggest that both myosin heads usually bind to a single thin filament.

Cross-bridge interaction with oppositely polarized actin filaments in double-overlap zones of insect flight muscle

An unresolved problem in understanding muscular contraction is why the internal resistance to sarcomere shortening increases progressively during contraction. We have addressed this problem here by investigating the movement of detached acting filaments in the sarcomeres of insect flight muscle. The final position of the detached actin filaments shows that they were able to slide freely into regions where they have the wrong polarity to interact actively with myosin (double-overlap zones) but where they prevent the exertion of force by cross-bridges between myosin and the correctly polarized acting filaments. These observations indicate that the isometric tension at all sarcomere lengths is directly proportional to the number of cross-bridges in the region of single-overlap of correctly polarized actin and myosin filaments. The decrease in tension as sarcomeres shorten is thus the result of the decrease in the number of effective cross-bridges as actin filaments slide into regions where they are of the wrong polarity to form cross-bridges, and where they inhibit the existing cross-bridges.

Calcium independent contraction induced by iodoacetic acid in isolated cerebral arteries

In helically-cut strips of cerebral arteries isolated from dogs and monkeys, the addition of 1 mM iodoacetic acid (IAA) produced contractions during an early period (5 to 10 min) and also a prolonged exposure (50 to 70 min). The early contraction was abolished by exposure to Ca++-free media containing EGTA, and significantly attenuated by treatment with procaine or dantrolene. Verapamil, lidocaine, ATP and pyruvate did not inhibit the contraction. On the other hand, the late contraction was not prevented by exposure to Ca++-free, EGTA-containing media and by treatment with procaine, dantrolene, lidocaine, ATP or pyruvate. Nitroglycerin and papaverine did not relax the IAA-contracted arteries. In dog and monkey mesenteric arteries and dog coronary, renal and femoral arteries, IAA elicited contractions after a prolonged exposure, which were not inhibited by soaking the preparations in Ca++-free, EGTA-containing media. Passive tensions developed by rapid stretch in Ca++-free media did not differ in IAA-treated and control arteries. During an early period of IAA actions, Ca++ appears to be released from intracellularly stored sites in the amount sufficient to produce significant contractions in cerebral, but not in peripheral arteries. It is concluded that the involvement of Ca++ in the late contraction induced by IAA is if any minimal, and such a contraction may be associated with functional alterations induced by the metabolic inhibitor in arterial tissues other than smooth muscle.

Creatine kinase in regulation of heart function and metabolism. II. The effect of phosphocreatine on the rigor tension of EGTA-treated rat myocardial fibers

Bundles of rat cardiac fibers were treated with EGTA to increase the permeability of the sarcolemma to ions and small molecules. In the medium without calcium, the EGTA-treated fibers developed rigor tension dependent on the concentration of MgATP in the bathing solution: half-maximal tension was recorded at 2.5 mM MgATP and maximal tension at 0.1 mM MgATP in the medium. However, in the presence of 15 mM phosphocreatine without added creatine kinase a decrease of MgATP concentration to 0.1 mM did not result in any development of rigor tension. Phosphocreatine prevented rigor tension development in the absence of added MgATP when MgADP was added. In the presence of MgADP, phosphocreatine decreased rigor tension more rapidly and to a higher extent than added MgATP. At 5 mM MgADP, half-maximal rigor tension was observed in the presence of 2 mM phosphocreatine which is close to the Km value for phosphocreatine in the creatine-kinase reaction. These results demonstrate that the intact creatine kinase in the EGTA-treated fibers with increased sarcolemmal permeability is able to ensure rapid replenishment of MgATP in the myofibrillar compartment at the expense of phosphocreatine. The data obtained conform completely to the concept of adenine-nucleotide compartmentation in cardiac cells and of energy channelling by the phosphocreatine-creatine shuttle mechanism.

Contraction of rabbit skinned skeletal muscle fibers at low levels of magnesium adenosine triphosphate

The contractile properties of skinned single fibers from rabbit psoas muscle were investigated under conditions of low MgATP and no Ca2+ (i.e., less than 10(-8) M). At 1 microM MgATP, fibers shortened at a maximum velocity of 660 +/- 420 A/half sarcomere/s (n = 9), compared with 34,000 A/half sarcomere/s measured during maximum Ca2+-activation at 1 mM MgATP (Moss, R. L., 1982. J. Muscle Res. Cell. Motil ., 3:295-311). The observed dependence of Vmax on pMgATP between 7.0 and 5.3 was similar to that of actomyosin ATPase measured previously by Weber, A., R. Herz , and I. Reiss (1969, Biochemistry, 8:2266-2270). Isometric tension was found to vary with pMgATP in a manner much like that reported by Reuben , J. P., P. W. Brandt, M. Berman , and H. Grundfest (J. Gen. Physiol. 1971. 57:385-407). A simple cross-bridge model was developed to simulate contractile behaviour at both high and low levels of MgATP. It was found that the pMgATP dependence of Vmax and ATPase could be successfully modeled if the rate of detachment of the cross-bridge was made proportional to the concentration of MgATP. In the model, the similar dependence of Vmax and ATPase on pMgATP was derived from the fact that in this range of pMgATP every pass of a cross-bridge by an actin site resulted in an attachment-detachment cycle, and every such cycle caused hydrolysis of one molecule of ATP.

Radial stiffness of frog skinned muscle fibers in relaxed and rigor conditions

Radial stiffness in various conditions of mechanically skinned fibers of semitendinosus muscle of Rana catesbeiana was determined by compressing the fiber with polyvinylpyrrolidone (PVP K-30, Mr = 40,000) in incubating solution. The change in width (D) of fibers with increasing and decreasing PVP concentrations was highly reproducible at a range 0-6% PVP. Radial stiffness of relaxed fibers was almost independent of the sarcomere length. On the other hand, radial stiffness of rigor fibers showed a linear relation against the sarcomere length. These results indicate that cross-bridge attachment would be a major factor in the increase of the radial stiffness. Radial stiffness of relaxed and rigor fibers was (2.14 +/- 0.52) X 10(4) N/m2 (mean +/- SD) and (8.76 +/- 2.04) X 10(4) N/m2, respectively, at the relative fiber width (D/D0) of 0.92, where D0 denotes the fiber width in the rigor solution at 0% PVP. Radial stiffness of a fiber in a rigor solution containing pyrophosphate (PPi) was between those of relaxed and rigor fibers, i.e., (4.76 +/- 0.86) X 10(4) N/m2 at D/Do of 0.92. In PPi and rigor solutions, radial stiffness reversibly increased to around 150 and 130%, respectively, in the presence of 10(-6) M Ca2+. To explain these results, especially the Ca2+-induced change in the radial stiffness, some factor in addition to the number of attached cross-bridges has to be taken into account. The variation of radial stiffness under various conditions will be discussed in relation to the possible manner of cross-bridge attachment.

The elasticity of relaxed insect fibrillar flight muscle

The mechanical properties of glycerol-extracted fibres from the dorsal longitudinal muscle of Lethocerus have been determined by sinusoidal and transient analysis in the time range 1 ms-1000 s, and from rest length to 10% strain for fibres in relaxing and rigor solutions. The fibres behave reversibly up to strains of about 5%, but reach an elastic limit in the range 5-9% strain, depending upon the rate of strain. Electron micrographs of fibres at different degrees of stretch, and after partial extraction of the contractile proteins, suggest that a connexion between the end of the A filament and the Z line, named a C filament, is responsible for the high stiffness of the relaxed muscle. Estimates are made of the compliance of the A, I and C filaments. The mechanical response of the relaxed muscle, over the entire frequency range studied, is assignable to the C filaments. An analysis of the stiffness of the fibres at different tensions in activating and relaxing solutions, and in fibres relaxed by orthovanadate, shows that the C filaments still exert their mechanical effect in the active muscle. That is, the response of the active muscle consists of the contribution from the cross-bridges plus that of the C filaments, acting mechanically in parallel. This situation is incompatible with earlier explanations of the fully activated mechanical dynamics of fibrillar muscle. Alternative explanations at the cross-bridge level are described in the paper that follows this one.