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

Lattice arrangement of myosin filaments correlates with fiber type in rat skeletal muscle

The thick (myosin-containing) filaments of vertebrate skeletal muscle are arranged in a hexagonal lattice, interleaved with an array of thin (actin-containing) filaments with which they interact to produce contraction. X-ray diffraction and EM have shown that there are two types of thick filament lattice. In the simple lattice, all filaments have the same orientation about their long axis, while in the superlattice, nearest neighbors have rotations differing by 0° or 60°. Tetrapods (amphibians, reptiles, birds, and mammals) typically have only a superlattice, while the simple lattice is confined to fish. We have performed x-ray diffraction and electron microscopy of the soleus (SOL) and extensor digitorum longus (EDL) muscles of the rat and found that while the EDL has a superlattice as expected, the SOL has a simple lattice. The EDL and SOL of the rat are unusual in being essentially pure fast and slow muscles, respectively. The mixed fiber content of most tetrapod muscles and/or lattice disorder may explain why the simple lattice has not been apparent in these vertebrates before. This is supported by only weak simple lattice diffraction in the x-ray pattern of mouse SOL, which has a greater mix of fiber types than rat SOL. We conclude that the simple lattice might be common in tetrapods. The correlation between fiber type and filament lattice arrangement suggests that the lattice arrangement may contribute to the functional properties of a muscle.

Molecular basis of the catch state in molluscan smooth muscles: a catchy challenge

The catch state (or 'catch') of molluscan smooth muscles is a passive holding state that occurs after cessation of stimulation. During catch, force and, in particular, resistance to stretch are maintained for long time periods with low (or no) energy consumption at basal intracellular free [Ca2+]. The catch state is initiated by Ca2+-stimulated dephosphorylation of the titin-like protein twitchin and is inhibited by cAMP-dependent phosphorylation of twitchin. In addition, catch is pH sensitive, but the reason for this is unknown. According to a traditional model, catch is due to slower cross-bridge cycles where myosin heads remain longer attached to the actin filaments after force generation, possibly caused by a hindered release of ADP from the myosin heads. However, this model was disproved by recent findings which showed that (i) inhibitors of myosin function, such as vanadate, do not affect catch force; (ii) factors which terminate the catch state do not accelerate myosin head detachment kinetics and (iii) a catch-like high resistance to stretch is still inducible when force development is prevented. Thus, catch probably involves passive linkage structures interconnecting the myofilaments (catch linkages). For example twitchin could (i) tie myosin heads to the thin filaments, (ii) mechanically lock them in a stretch resistant state or (iii) interconnect thick and thin filaments directly. However, it is questionable if these mechanisms are sufficient since twitchin seems to be about 15-times less abundant than myosin. Therefore, in addition, interconnections between thick filaments could exist, which could involve e.g. paramyosin or twitchin. Catch could even involve changes in the compliance of thick filaments. The function of myorod, found specifically in catch muscles in equal abundance with myosin, is not known. The suggestion is made here that catch linkages are present already during active contraction either as ratchet-like elements resisting stretch and not opposing shortening or in some kind of 'standby' mode ready to transform suddenly into the working mode by stretches or after Ca2+ removal following cessation of stimulation.

Effect of strain on actomyosin kinetics in isometric muscle fibers

Investigations were conducted into the biochemical and mechanical states of cross-bridges during isometric muscle contraction. Rapid length steps (3 or 6 nm hs(-1)) were applied to rabbit psoas fibers, permeabilized and isometric, at either 12 degrees C or 20 degrees C. Fibers were activated by photolysis of P(3)-1-(2-nitrophenyl)-ethyl ester of ATP infused into rigor fibers at saturating Ca(2+). Sarcomere length, tension, and phosphate release were recorded-the latter using the MDCC-PBP fluorescent probe. A reduction in strain, induced by a rapid release step, produced a short-lived acceleration of phosphate release. Rates of the phosphate transient and that of phases 3 and 4 of tension recovery were unaffected by step size but were elevated at higher temperatures. In contrast the amplitude of the phosphate transient was smaller at 20 degrees C than 12 degrees C. The presence of 0.5 or 1.0 mM added ADP during a release step reduced both the rate of tension recovery and the poststep isometric tension. A kinetic scheme is presented to simulate the observed data and to precisely determine the rate constants for the elementary steps of the ATPase cycle.

Structural transients of contractile proteins upon sudden ATP liberation in skeletal muscle fibers

Structural changes of contractile proteins were examined by millisecond time-resolved two-dimensional x-ray diffraction recordings during relaxation of skinned skeletal muscle fibers from rigor after caged ATP photolysis. It is known that the initial dissociation of the rigor actomyosin complex is followed by a period of transient active contraction, which is markedly prolonged in the presence of ADP by a mechanism yet to be clarified. Both single-headed (overstretched muscle fibers with exogenous myosin subfragment-1) and two-headed (fibers with full filament overlap) preparations were used. Analyses of various actin-based layer line reflections from both specimens showed the following: 1), The dissociation of the rigor actomyosin complex was fast and only modestly decelerated by ADP and occurred in a single exponential manner without passing through any detectable transitory state. Its ADP sensitivity was greater in the two-headed preparation but fell short of explaining the large ADP effect on the transient active contraction. 2), The decay of the activated state of the thin filament followed the time course of tension more closely in an ADP-dependent manner. These results suggest that the interplay between the reattached active myosin heads and the thin filament is responsible for the prolonged active contraction in the presence of ADP.

The “Roll and Lock” Mechanism of Force Generation in Muscle

Muscle force results from the interaction of the globular heads of myosin-II with actin filaments. We studied the structure-function relationship in the myosin motor in contracting muscle fibers by using temperature jumps or length steps combined with time-resolved, low-angle X-ray diffraction. Both perturbations induced simultaneous changes in the active muscle force and in the extent of labeling of the actin helix by stereo-specifically bound myosin heads at a constant total number of attached heads. The generally accepted hypothesis assumes that muscle force is generated solely by tilting of the lever arm, or the light chain domain of the myosin head, about its catalytic domain firmly bound to actin. Data obtained suggest an additional force-generating step: the "roll and lock" transition of catalytic domains of non-stereo-specifically attached heads to a stereo-specifically bound state. A model based on this scheme is described to quantitatively explain the data.

Single turnover of cross-bridge ATPase in rat muscle fibers studied by photolysis of caged ATP

A mechanical study on skinned rat psoas muscle fibers was performed at about 16 degrees C with X-ray diffraction and caged-ATP photolysis. The amount of photoreleased ATP was set < 0.2 mM for analysis of a 'single turnover' of the cross-bridge ATPase. With regard to the phase of activation, the results under the single turn-over condition were generally consistent with previous results obtained with larger amount of photoreleased ATP. Formation of the ADP-rigor state was mechanically monitored by the 90 degrees out-of-phase component of stiffness at 500 Hz, which was elevated on activation and then decreased to zero with a half-time of 0.2-0.3 s. Intensity changes of the X-ray reflections (e.g. equatorial reflections, actin layer lines and a myosin meridional reflection) indicated that a large number of cross-bridges returned to the rigor structure with a half-time of 0.5-0.7 s. During this phase, tension did not increase but slowly decreased with a half-time of about 1.0 s. The in-phase stiffness increased only 20-30% at the most. These results indicate that, even if the number of cross-bridges formed at any moment during full contraction is small, they can interact with actin and form rigor bonds with a rate of 1 s(-1). The force developed in the rigor formation is probably lost due to the presence of rigor bridges and compliance in the preparation.

Activating efficiency of Ca2+ and cross-bridges as measured by phosphate analog release

To assess the activating efficiency of Ca2+ and cross-bridges, the release rates of phosphate analogs from skinned fibers were estimated from the recovery of contractility and that of stiffness. Estimations were performed based on the assumptions that contractility was indicative of the population of analog-free myosin heads and that stiffness reflected the population of formed cross-bridges. Aluminofluoride (AlFx) and orthovanadate (Vi) were used as phosphate analogs with mechanically skinned fibers from rabbit psoas muscle. The use of the analogs enabled the functional assessment of activation level in the total absence of ATP. Fibers loaded with the analogs gradually recovered contractility and stiffness in normal plain rigor solution. The addition of Ca2+ to the plain rigor solution significantly accelerated their recovery, whereas ADP had no appreciable effect. ATP plus Ca2+(contracting condition) accelerated the recovery by several tens of times. These results indicate that the cross-bridges formed during contraction have prominent activating efficiency, which is indispensable to attain full activation. A comparison between the activating efficiency evaluated from stiffness and that from contractility suggested that Ca2+ is more potent in accelerating the binding of actin to analog-bound myosin heads whereas cross-bridges mainly accelerate the subsequent analog-releasing step.

ATP consumption and efficiency of human single muscle fibers with different myosin isoform composition

Chemomechanical transduction was studied in single fibers isolated from human skeletal muscle containing different myosin isoforms. Permeabilized fibers were activated by laser-pulse photolytic release of 1.5 mM ATP from p(3)-1-(2-nitrophenyl)ethylester of ATP. The ATP hydrolysis rate in the muscle fibers was determined with a fluorescently labeled phosphate-binding protein. The effects of varying load and shortening velocity during contraction were investigated. The myosin isoform composition was determined in each fiber by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. At 12 degrees C large variations (three- to fourfold) were found between slow and fast (2A and 2A-2B) fibers in their maximum shortening velocity, peak power output, velocity at which peak power is produced, isometric ATPase activity, and tension cost. Isometric tension was similar in all fiber groups. The ATP consumption rate increased during shortening in proportion to shortening velocity. At 12 degrees C the maximum efficiency was similar (0.21-0.27) for all fiber types and was reached at a higher speed of shortening for the faster fibers. In all fibers, peak efficiency increased to approximately 0.4 when the temperature was raised from 12 degrees C to 20 degrees C. The results were simulated with a kinetic scheme describing the ATPase cycle, in which the rate constant controlling ADP release is sensitive to the load on the muscle. The main difference between slow and fast fibers was reproduced by increasing the rate constant for the hydrolysis step, which was rate limiting at low loads. Simulation of the effect of increasing temperature required an increase in the force per cross-bridge and an acceleration of the rate constants in the reaction pathway.

Influence of ADP on cross-bridge-dependent activation of myofibrillar thin filaments

Contraction of skeletal muscle is regulated by calcium at the level of the thin filament via troponin and tropomyosin. Studies have indicated that strong cross-bridge binding is also involved in activation of the thin filament. To further test this, myofibrils were incubated with a wide range of fluorescent myosin subfragment 1(fS1) at pCa 9 or pCa 4 with or without ADP. Sarcomere fluorescence intensity and the fluorescence intensity ratio (non-overlap region/overlap region) were measured to determine the amount and location of bound fS1 in the myofibril. There was lower sarcomere fluorescence intensity with ADP compared to without ADP for both calcium levels. Similar data were obtained from biochemical measures of bound fS1, validating the fluorescence microscopy measurements. The intensity ratio, which is related to activation of the thin filament, increased with increasing [fS1] with or without ADP. At pCa 9, the fluorescence intensity ratio was constant until 80-160 nM fS1 without ADP conditions, then it went up dramatically and finally attained saturation. The dramatic shift of the ratio demonstrated the cooperative character of strong cross-bridge binding, and this was not observed at high calcium. A similar pattern was observed with ADP in that the ratio was right-shifted with respect to total [fS1]. Saturation was obtained with both the fluorescence intensity and ratio data. Plots of intensity ratio as a function of normalized sarcomere intensity (bound fS1) showed little difference between with and without ADP. This suggests that the amount of strongly bound fS1, not fS1 state (with or without ADP) is related to activation of the thin filament.

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.

Structural responses to the photolytic release of ATP in frog muscle fibres, observed by time-resolved X-ray diffraction

1. Structural changes following the photolytic release of ATP were observed in single, permeabilised fibres of frog skeletal muscle at 5-6 C, using time-resolved, low-angle X-ray diffraction. The structural order in the fibres and their isometric function were preserved by cross-linking 10-20 % of the myosin cross-bridges to the thin filaments. 2. The time courses of the change in force, stiffness and in intensity of the main equatorial reflections (1,0) and (1,1), of the third myosin layer line (M3) at a reciprocal spacing of (14.5 nm)-1 on the meridian and of the first myosin-actin layer line (LL1) were measured with 1 ms time resolution. 3. In the absence of Ca2+, photolytic release of ATP in muscle fibres initially in the rigor state caused the force and stiffness to decrease monotonically towards their values in relaxed muscle fibres. 4. In the presence of Ca2+, photolytic release of ATP resulted in an initial rapid decrease in force, followed by a slower increase to the isometric plateau. Muscle fibre stiffness decreased rapidly to approximately 65 % of its value in rigor. 5. In the absence of Ca2+, changes on the equator, in LL1 and in M3 occurred with a time scale comparable to that of the changes in tension and stiffness. 6. In the presence of Ca2+, the changes on the equator and LL1 occurred simultaneously with the early phase of tension decrease. The changes in the intensity of M3 (IM3) occurred on the time scale of the subsequent increase in force. The time courses of the changes in tension and IM3 were similar following the photolytic release of 0. 33 or 1.1 mM ATP. However the gradual return towards the rigor state began earlier when only 0.33 mM ATP was released. 7. In the presence of Ca2+, the time course of changes in IM3 closely mimicked that of force development following photolytic release of ATP. This is consistent with models that propose that force development results from a change in the average orientation of cross-bridges, although other factors, such as their redistribution, may also be involved.

Modulating factors of calcium-free contraction at low [MgATP]: a physiological study on the steady states of skinned fibres of frog skeletal muscle

Factors that modulate Ca(2+)-free contraction at low [MgATP] were examined by analysing steady tension development in skinned fibres of frog skeletal muscle. The commonly accepted bell-shaped relationship between steady tension and log (1/[MgATP]) was found to be highly susceptible to subtle experimental conditions at the higher [MgATP] side (right limb). The limb shifted to the right with increased fibre thickness, interrupted stirring of the bathing solution, increased temperature and fibre extension, although the effects of temperature and extension were marked only in thick fibres (cross-sectional area > 6000 microns 2). The shift of the right limb was reproduced by an addition of ADP to the bathing solution. These results, together with the extreme steepness of the right limb in thick fibres, suggest that a diffusion-dependent self-regenerative activation occurs in thick fibres in which ADP accumulation and ATP depletion positively feed back through further activation of the myofibrillar ATPase. Numerical simulation supported the hypothesis of the self-regenerative activation under poor diffusion conditions, and suggested that a small rise in temperature and fibre extension can trigger the self-regenerative process at the right limb. Consequently, ADP, temperature and fibre extension are deduced to be the primary potentiators of the activation at low [MgATP]. The high efficiency of ADP in shifting the limb suggests that the activating efficiency of the MgADP-bound actomyosin complex is higher than the nucleotide-free actomyosin complex.

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.

Effects of ADP and low ATP on the Ca(2+)-sensitive transient contraction upon photolysis of caged ATP in rat muscle fibres: a study on the Bremel-Weber type cooperation

Caged ATP was photolysed in rat psoas fibres under various conditions to examine whether ADP plays a special role in the 'Bremel-Weber type cooperation', viz. the Ca(2+)-like action of the rigor cross-bridges. Various concentrations of ATP (0.25-1.6 mM) were photoreleased in the presence of various concentrations of ADP (0-0.4 mM) at approximately 8 or 20 degrees C. The Ca2+ concentration was set at around 0.2 microM in order that the ATP-induced contraction was significant but short. Both lower [ATP] and higher [ADP] resulted in a slower detachment of the rigor cross-bridges and a larger contraction after the detachment. ADP did not seem to affect the relationship between the rate of detachment and the size of the Ca(2+)-sensitive component of the subsequent contraction. It is concluded that there is little evidence that the ADP-bound rigor cross-bridges are more potent than the nucleotide-free ones in the Bremel-Weber type cooperation that is seen in the ATP-induced transient contraction. The mechanism by which the ADP-specific, Ca(2+)-independent contraction occurs immediately after the release of ATP remains to be clarified.