Kinetics of the actomyosin ATPase in muscle fibers

Early steps of the Mg(2+)-ATPase of relaxed myofibrils. A comparison with Ca(2+)-activated myofibrils and myosin subfragment 1

The early steps of the Mg(2+)-ATPase activity of relaxed rabbit psoas myofibrils were studied in a buffer of near-physiological ionic strength at 4 degrees C by the rapid flow quench technique. The initial ATP binding steps were studied by the ATP chase, and the cleavage and release of product steps by the Pi burst method. The data obtained were interpreted by [formula: see text] where M represents the myosin heads with or without actin interaction. This work is a continuation of our study on Ca(2+)-activated myofibrils [Houadjeto, M., Travers, F., & Barman, T. (1992) Biochemistry 31, 1564-1569]. Here the constants obtained with relaxed myofibrils were compared with those with activated myofibrils and myosin subfragment 1 (S1). We find that whereas Ca2+ increases 80X the release of products (k4), it has little effect upon the kinetics of the initial binding and cleavage steps. As with activated myofibrils and S1, the second-order binding constant for ATP (k2/K1) was about 1 microM-1 s-1 and the ATP was bound very tightly. With activated myofibrils, it was difficult to obtain an estimate for the koff for ATP(k-2) but it is much less than kcat. Here with relaxed myofibrils we estimate k-2 less than 8 x 10(-4) s-1, which is considerably smaller than kcat (0.019 s-1) and also previous estimates for this constant. The overall Kd for ATP to relaxed myofibrils is less than 8 x 10(-10) M. With S1 this Kd is about 10(-11) M.(ABSTRACT TRUNCATED AT 250 WORDS)

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

The kinetics of the force generating and Pi release steps of the actomyosin-adenosinetriphosphatase (ATPase) cycle have been compared in Ca(2+)-activated skinned fibers of rabbit soleus (slow twitch) and psoas (fast twitch) muscle. Pi was rapidly photogenerated within the fiber lattice by laser flash photolysis of caged Pi [1-(2-nitro)phenylethyl phosphate]. Pi reduces isometric tension in the steady state but is less effective in slow-twitch muscle than in fast-twitch muscle (e.g., 14 mM Pi reduces tension by 29 +/- 4.6% in slow and by 47 +/- 5.3% in fast). The tension response to a sudden increase in Pi concentration in slow-twitch muscle has four phases, but as in fast-twitch muscle, only phase II (an exponential decline in force) appears to be caused by Pi binding to cross bridges, whereas the other three phases are probably indirect effects caused by caged Pi photolysis. The amplitude of phase II is consistent with the steady-state reduction in force by Pi. The rate of phase II (kappa Pi) is 3.9 +/- 0.33 s-1 at 20 degrees C and 0.28 +/- 0.02 s-1 at 10 degrees C (1 mM Pi). kappa Pi is thus 33 times slower in slow-twitch muscle than in fast at 20 degrees C and 84 times slower at 10 degrees C. In contrast to fast-twitch muscle, in slow muscle kappa Pi is sufficiently slow to partially limit the ATPase turnover rate.

Regulation of tension development by MgADP and Pi without Ca2+. Role in spontaneous tension oscillation of skeletal muscle

The length of sarcomeres in isolated myofibrils fixed at both ends spontaneously oscillates when MgADP and Pi coexist with MgATP in the absence of Ca2+ (Okamura, N., and S. Ishiwata, 1988. J. Muscle Res. Cell. Motil. 9:111-119). Here, we report that MgADP and Pi function as an activator and an inhibitor, respectively, of tension development of single skeletal muscle fibers in the absence of Ca2+ and the coexistence of MgADP and Pi with MgATP induces spontaneous tension oscillation. First, the isometric tension sharply increased when the concentration of MgADP became higher than approximately 3x that of MgATP and saturated at approximately 90% of the tension obtained under full Ca2+ activation; in parallel with this sigmoidal increase of tension, MgATPase activity appeared. The inhibition of contraction by the regulatory system seems to be desuppressed by the allosteric effect of actomyosin-ADP complex, similarly to so-called rigor complex. The ADP-induced tension was decreased along a reversed sigmoidal curve by the addition of Pi; actomyosin-ADP-Pi complex, which has no desuppression function, may be formed by exogenous Pi; accompanying the decline of tension, spontaneous oscillations of tension and sarcomere length appeared. It is suggested that the length oscillation of each (half) sarcomere would occur through the transition of cross-bridges between force-generating (on) and non-force-generating (off) states, which may be regulated by the mechanical states (strain) of cross-bridges and/or thin filaments.

Spontaneous oscillation of tension and sarcomere length in skeletal myofibrils. Microscopic measurement and analysis

We have devised a simple method for measuring tension development of single myofibrils by micromanipulation with a pair of glass micro-needles. The tension was estimated from the deflection of a flexible needle under an inverted phase-contrast microscope equipped with an image processor, so that the tension development is always accompanied by the shortening of the myofibril (auxotonic condition) in the present setup. The advantage of this method is that the measurement of tension (1/30 s for time resolution and about 0.05 micrograms for accuracy of tension measurement; 0.05 microns as a spatial resolution for displacement of the micro-needle) and the observation of sarcomere structure are possible at the same time, and the technique to hold myofibrils, even single myofibrils, is very simple. This method has been applied to study the tension development of glycerinated skeletal myofibrils under the condition where spontaneous oscillation of sarcomeres is induced, i.e., the coexistence of MgATP, MgADP and inorganic phosphate without free Ca2+. Under this condition, we found that the tension of myofibrils spontaneously oscillates accompanied by the oscillation of sarcomere length with a main period of a few seconds; the period was lengthened and shortened with stretch and release of myofibrils. A possible mechanism of the oscillation is discussed.

Contraction of glycerinated rabbit slow-twitch muscle fibers as a function of MgATP concentration

We have measured the isometric tension and force-velocity relationships of glycerinated rabbit slow-twitch semimembranosus muscle as a function of MgATP concentration ([MgATP]) and have compared the results with those obtained previously from fast-twitch psoas muscle. We find that isometric tension decreases as [MgATP] increases. The magnitude of the decrease is not as great as observed in psoas. Maximum shortening velocity (Vmax) exhibits classical Michaelian saturation behavior with respect to [MgATP] with a Michaelis constant (Km) for half-maximal velocity of 18 microM and a value at saturating [MgATP] of 0.6 muscle lengths/s. Similar values were observed in fibers from soleus, another slow-twitch muscle. The corresponding values in rabbit psoas muscle are 150 microM and 1.6 lengths/s. Compared with psoas, in semimembranosus muscle Km decreases by a factor of approximately 10, whereas Vmax decreases by about a factor of 3. Thus, although in a nonphysiological regime, at low [MgATP], a "fast" muscle actually has a lower shortening velocity than a "slow" muscle.

Effects of ethylene glycol on the kinetics of contraction on flash photolysis of caged ATP in rat psoas muscle fibres

ATP (1-1.2 mM) was photoreleased from caged ATP (5 mM) in skinned fibres from rat psoas muscle at 15-17 degrees C, to examine the effects of ethylene glycol (EG; 20% in solvent) on the kinetics of isometric contraction. Muscle fibres were stretched by 0.5-2% before photolysis, so that force just before photolysis was almost equal to the steady-state force after photolysis. At the phase of steady-state contraction, force and 500 Hz-stiffness in the presence of EG were 50% and 70% of the controls, respectively, resulting in a higher stiffness-to-force with EG, as reported previously. Following photolysis, force fell before rising to a steady-state plateau. The estimated rate constant of the force decay was approximately 90 s-1, and in the presence of EG was 80-85% of the control. This suggested a small effect of EG on the crossbridge detachment induced by ATP. The rate of force redevelopment was approximately 70 s-1, and EG decreased this rate to 50% of the control. This suggested that EG greatly slows the transition of the crossbridges from the detached state to the reattached force-producing state. The time course of the stiffness signals was consistent with this interpretation. The high stiffness-to-force ratio with EG indicated that EG not only reduces the rate constants which were directly examined in this study but also modifies other aspects of the crossbridge reaction.

Reversal of the cross-bridge force-generating transition by photogeneration of phosphate in rabbit psoas muscle fibres

1. Orthophosphate (P(i), 0.1-2.0 mM) was photogenerated within the filament lattice of isometrically contracting glycerinated fibres of rabbit psoas muscle at 10 and 20 degrees C. The P(i) was produced by laser flash photolysis of the photolabile compound 1-(2-nitrophenyl)ethylphosphate (caged P(i)). Caged P(i) caused a depression of tension that was much smaller than that caused by P(i). 2. Photolysis of caged P(i) produced a decline in isometric force composed of four phases: phase I, a lag phase (e.g. 1-4 ms at 10 degrees C) during which force did not change; phase II, an exponential decline by as much as 20% of the pre-pulse force; phase III, a partial force recovery (0-3% of the pre-pulse force); and phase IV, a further slow (0.5-3 s) decline to the steady value. Phases I, III and IV were largely independent of [P(i)] and are likely to be indirect effects caused by the caged P(i) photolysis. 3. Both the rate and amplitude of phase II depended markedly on [P(i)]. The amplitude of phase II was similar to the reduction of steady-state force by P(i). The rate of phase II increased with increasing temperature and [P(i)]. At high [P(i)] the rate began to saturate, and approached limits of 123 s-1 at 10 degrees C and 194 s-1 at 20 degrees C. 4. The rate of phase II was independent of sarcomere overlap, while the amplitude was proportional to tension at partial filament overlap. A control experiment using caged ATP showed that phase II was not produced by the photolytic by-products or the light pulse. The results suggest that phase II is associated with the force-generating transition of the cross-bridge cycle. 5. Sinusoidal length oscillations at 0.5 and 2 kHz were used to measure muscle stiffness during phase II. Stiffness declined in a single exponential phase, with the same time course as phase II of the tension transient. The change in stiffness was 83 +/- 6% (mean +/- S.E.M., n = 10, 0.5 kHz) of the change in tension when both signals were normalized to their pre-flash values. 6. Analysis of the data shows that two steps are involved in force generation and P(i) release. The non-force exerting AM-ADP-P(i) cross-bridge state first isomerizes to form a force-exerting cross-bridge state (AM'-ADP-P(i)). P(i) is then released to form a second force-generating state, AM'-ADP.(ABSTRACT TRUNCATED AT 400 WORDS)

Kinetics of adenosine triphosphate hydrolysis by shortening myofibrils from rabbit psoas muscle

1. Using a solenoid-operated mixing device, time-resolved measurements were made of shortening and accompanying ATP hydrolysis at 20 degrees C by myofibrils prepared from rabbit psoas muscle. 2. The extent of ATP hydrolysis was determined by an improved Malachite Green method for determination of inorganic phosphate (Pi) in the presence of a large excess of ATP. For the measurement of the change in sarcomere length by phase contrast microscopy, shortening was terminated without delay and artifact by a mixture of 0.2 M-acetate (pH 4.6) and 1.25% (v/v) glutaraldehyde. 3. The shortening velocity per half-sarcomere was 10 microns s-1 in 25 mM-KCl for sarcomere lengths above 1.4 microns, and at least 12 microns s-1 in 150 mM-KCl for sarcomere lengths above 1.7 microns. During this rapid shortening, there was no significant ATP turnover by cross-bridges (upper 95% confidence limit: 0.14 mol (mol of myosin head)-1 in 25 mM-KCl; 0.12 mol mol-1 in KCl solutions greater than or equal to 100 mM). 4. When the sarcomeres shortened below 1.7 microns in KCl concentrations greater than 100 mM or below 1.4 microns in 25 mM-KCl, there was a transient acceleration of ATP hydrolysis (delayed ATP hydrolysis), which was then followed by a steady slow hydrolysis. 5. The magnitudes (+/- estimated standard deviation) of delayed ATP hydrolysis by myofibrils of initial sarcomere length 2.4 microns were 0.42 +/- 0.19, 0.31 +/- 0.10 and 0.17 +/- 0.09 mol (mol myosin head)-1 in 25 mM, 100 mM and 150 mM-KCl, respectively. For myofibrils of sarcomere length 2.0 microns, however, it decreased to 0.24 +/- 0.10 mol mol-1 in 25 mM-KCl or to an insignificant level in 150 mM-KCl. 6. These results indicate that most of the ATP hydrolysis products remain bound to cross-bridges during rapid shortening, and that when the force opposing shortening increases, a proportion of cross-bridges rapidly dissociate the products and enter the next ATP cycle, which diminishes with the decrease in shortening distance as well as the increase in ionic strength. Such behaviour of the cross-bridge is probably a manifestation of its energetic and kinetic properties in the state with bound ADP and Pi interacting with actin filaments at zero load and at a transition from zero to non-zero loads.

Simulation of stochastic processes in motile crossbridge systems

The underlying stochastic nature of many models of the actomyosin interaction should result in fluctuations in both force and shortening velocity. In classical experimental approaches involving intact or glycerinated muscle preparations these fluctuations are too small to resolve owing to the large numbers of crossbridges involved. However, new experimental techniques allow mechanical measurements to be made in systems in which small numbers of myosin heads act on a single actin filament, or small numbers of kinesin molecules act on a single tubulin filament. In these systems, stochastic effects should be evident. To understand better the nature of the expected stochastic effects, we have used computer simulation to investigate the fluctuations predicted by the original model for muscle crossbridge mechanics proposed by A.F. Huxley. We consider three situations: (1) the translation of actin or tubulin filaments by myosin or kinesin motors immobilized on a fixed substrate, (2) the production of tension by ensembles of immobilized myosin which involve the displacement of an elastic load, and (3) the fluctuations in axial displacement of a single, bipolar myosin thick filament interacting with actin filaments as in a sarcomere. In all three cases, fluctuations are clearly evident in simulations involving small numbers of motors. For case (1), we show that translation velocities can vary with crossbridge density. Whether one motor translates a filament faster, slower or at the same speed as many motors depends on the relative magnitudes of the attachment and detachment rate functions. Analytical expressions are provided to quantitate this relationship. For case (2), we show that fluctuations predicted assuming perfectly isometric conditions differ form those observed when the 'isometric state' is achieved against an elastic load. 'Elastic damping' of the fluctuations in the system results from the presence of many attached motors. In case (3) we show that in spite of the presence of stochastic fluctuations which can destabilize the uniformity of filament overlap in a sarcomere, the magnitude of thick filament displacement is less than might be anticipated over time periods of in vivo contraction. Taken together, these simulations allow one to better interpret experimental data in terms of current models of motor function.

Discrimination of Ca(2+)-ATPase activity of the sarcoplasmic reticulum from actomyosin-type ATPase activity of myofibrils in skinned mammalian skeletal muscle fibres: distinct effects of cyclopiazonic acid on the two ATPase activities

We have developed a procedure to discriminate actomyosin-type ATPase activity from Ca(2+)-ATPase activity of sarcoplasmic reticulum (SR) in mechanically skinned fibres, determining simultaneously their Ca(2+)-induced tension and accompanying ATPase activity. When they were treated with an alkaline CyDTA-containing solution of low ionic strength which was reported to remove troponin C, the fibres showed a considerable amount of Ca(2+)-dependent ATPase activity, in spite of having little or no Ca(2+)-induced isometric tension. The residual ATPase activity is ascribed to the Ca(2+)-ATPase activity of SR, because it is completely abolished by 1% CHAPS treatment for 10 min. This conclusion is also supported by the finding that the Ca(2+)-dependence of the ATPase activity is very similar to that of Ca(2+)-ATPase of SR isolated from rabbit skeletal muscle, and that the estimated activity is consistent with the reported values of direct determinations. On the other hand, treatment with a detergent such as CHAPS or Triton X-100 removes SR activities (ATPase and Ca-uptake), leaving Ca(2+)-induced tension and actomyosin-type ATPase activity unchanged. This procedure indicated that the contribution of Ca(2+)-ATPase activity of SR may be minimal in total steady-state ATPase activity of mechanically skinned mammalian skeletal muscle fibres. Successive CyDTA and CHAPS treatments eliminated both Ca(2+)-induced tension and ATPase activity, which were recovered by the addition of troponin C. Using these procedures, we also examined the effect of cyclopiazonic acid (CPA) which was reported to be a specific inhibitor of Ca(2+)-ATPase of SR. Ca(2+)-ATPase activity of SR in skinned fibres was inhibited completely by 10 microM CPA and held to one-half by about 0.2 microM. This effect was only partially reversible. CPA at 10 microM or higher concentrations showed Ca(2+)-sensitizing action on myofibrils, which was readily reversible. CPA at 3 microM inhibited almost completely the Ca(2+)-ATPase activity of SR, while it had no effect on either actomyosin-type ATPase or isometric tension of myofibrils.

Sub-piconewton force fluctuations of actomyosin in vitro

A new system has been developed for measuring the forces produced by a small number (less than 5-150) of myosin molecules interacting with a single actin filament in vitro. The technique can resolve forces of less than a piconewton and has a time resolution in the submillisecond range. It can thus detect fluctuations of force caused by individual molecular interactions. From analysis of these force fluctuations, the coupling between the enzymatic ATPase activity of actomyosin and the resulting mechanical impulses can be elucidated.

Changes in sarcoplasmic metabolite concentrations and pH associated with the catch contraction and relaxation of the anterior byssus retractor muscle of Mytilus edulis measured by phosphorus-31 nuclear magnetic resonance

The sarcoplasmic concentrations of phosphorus metabolites and pH (pHin) were measured in the anterior byssus retractor muscle (ABRM) of Mytilus edulis by 31P nuclear magnetic resonance spectroscopy. During an active contraction induced by 10(-3) acetylcholine, the concentration of arginine phosphate ([Arg-P]in) decreased from the resting value of 7.47 +/- 0.26 (mean +/- SE, n = 8) to 6.67 +/- 0.29 (n = 6) mumol g-1, and that of inorganic phosphate (Pi) consistently increased from 0.84 +/- 0.06 (n = 7) to 1.61 +/- 0.12 (n = 5) mumol g-1. In the 'catch' state following the active contraction, these concentrations were close to their resting levels, indicating that the catch is an inactive state. 5-hydroxytryptamine caused a rapid relaxation of the catch, which was associated with a slight decrease in [Arg-P]in and an increase in pHin by ca 0.2 units. The sarcoplasmic concentration of ATP (mean, 1.6 mumol g-1) did not change throughout the contraction-relaxation cycle.

Comparison of crossbridge dynamics between intact and skinned myocardium from ferret right ventricles

This study compares the crossbridge kinetics of intact and skinned preparations from ferret cardiac muscles at 20 degrees C to determine whether skinning causes any alteration in the crossbridge response to an imposed length change. A papillary or trabecular muscle was isolated from the right ventricle, the muscle length adjusted to give the maximum twitch tension (Lmax), and the preparation was subjected to Ba2+ contracture. When steady tension developed, the length of the preparation was perturbed sinusoidally in 19 discrete frequencies, ranging from 0.13 to 135 Hz, and at a small peak-to-peak amplitude (0.25% Lmax). We identified three exponential processes in the sinusodial force-response to the imposed length oscillation, and these were labeled processes B, C, and D in order of increasing speed. A slow process, A, normally present in fast-twitch skeletal muscles, is very small or absent in cardiac muscles. Process B is an exponential delay, and the muscle produces oscillatory work on the forcing apparatus; processes C and D are exponential advances in which the muscle absorbs work. The preparation was chemically skinned and activated in the presence of (mM) CaEGTA 6 (pCa 4.55), MgATP 5, magnesium propionate 1, and phosphate 1, pH 7.0, with ionic strength adjusted to 200 mM with potassium propionate. We found that the crossbridge kinetics were not altered by the skinning procedure. The apparent rate constants extracted from the sinusoidal analysis were nearly identical in Ba2+ contracture (intact preparation) and in Ca2+ activation (skinned preparation), and the Nyquist plots were similar. Because the rate constants changed sensitively with the substrate (MgATP) concentrations, we concluded that the substrate is adequately supplied during Ba2+ contracture in the intact preparation. Our study demonstrates the compatibility of results obtained from an intact and from a skinned preparation.

Two step mechanism of phosphate release and the mechanism of force generation in chemically skinned fibers of rabbit psoas muscle

The elementary steps of contraction in rabbit fast twitch muscle fibers were investigated with particular emphasis on the mechanism of phosphate (Pi) binding/release, the mechanism of force generation, and the relation between them. We monitor the rate constant 2 pi b of a macroscopic exponential process (B) by imposing sinusoidal length oscillations. We find that the plot of 2 pi b vs. Pi concentration is curved. From this observation we infer that Pi released is a two step phenomenon: an isomerization followed by the actual Pi release. Our results fit well to the kinetic scheme: [formula: see text] where A = actin, M = myosin, S = MgATP (substrate), D = MgADP, P = phosphate, and Det is a composite of all the detached and weakly attached states. For our data to be consistent with this scheme, it is also necessary that step 4 (isomerization) is observed in process (B). By fitting this scheme to our data, we obtained the following kinetic constants: k4 = 56 s-1, k-4 = 129 s-1, and K5 = 0.069 mM-1, assuming that K2 = 4.9. Experiments were performed at pCa 4.82, pH 7.00, MgATP 5 mM, free ATP 5 mM, ionic strength 200 mM in K propionate medium, and at 20 degrees C. Based on these kinetic constants, we calculated the probability of each cross-bridge state as a function of Pi, and correlated this with the isometric tension. Our results indicate that all attached cross-bridges support equal amount of tension. From this, we infer that the force is generated at step 4. Detailed balance indicates that 50-65% of the free energy available from ATP hydrolysis is transformed to work at this step. For our data to be consistent with the above scheme, step 6 must be the slowest step of the cross-bridge cycle (the rate limiting step). Further, AM*D is a distinctly different state from the AMD state that is formed by adding D to the bathing solution. From our earlier ATP hydrolysis data, we estimated k6 to be 9 s-1.

Cross-bridge kinetics in the presence of MgADP investigated by photolysis of caged ATP in rabbit psoas muscle fibres

1. The interaction between MgADP and rigor cross-bridges in glycerol-extracted single fibres from rabbit psoas muscle has been investigated using laser pulse photolysis of caged ATP (P3-1(2-nitrophenyl)ethyladenosine 5'-triphosphate) in the presence of MgADP and following small length changes applied to the rigor fibre. 2. Addition of 465 microM-MgADP to a rigor fibre caused rigor tension to decrease by 15.3 +/- 0.7% (S.E.M., n = 24 trials in thirteen fibres). The half-saturation value for this tension reduction was 18 +/- 4 microM (n = 23, thirteen fibres). 3. Relaxation from rigor by photolysis of caged ATP in the absence of Ca2+ was markedly slowed by inclusion of 20 microM-2 mM-MgADP in the photolysis medium. 4. Four phases of tension relaxation occurred with MgADP in the medium: at, a quick partial relaxation (in pre-stretch fibres); bt, a slowing of relaxation or a rise in tension for 50-100 ms; ct, a sudden acceleration of relaxation; and dt, a final, nearly exponential relaxation. 5. Experiments at varied MgATP and MgADP concentrations suggested that phase at is due to MgATP binding to nucleotide-free cross-bridges. 6. Phase bt was abbreviated by including 1-20 mM-orthophosphate (Pi) in the photolysis medium, or by applying quick stretches before photolysis or during phase bt. These results suggest that phases bt and ct are complex processes involving ADP dissociation, cross-bridge reattachment and co-operative detachment involving filament sliding and the Ca(2+)-regulatory system. 7. Stretching relaxed muscle fibres to 3.2-3.4 microns striation spacing followed by ATP removal and release of the rigor fibre until tension fell below the relaxed level allowed investigation of the strain dependence of relaxation in the regions of negative cross-bridge strain. In the presence of 50 microM-2 mM-MgADP and either 10 mM-Pi or 20 mM-2,3-butanedione monoxime, relaxation following photolysis of caged ATP was 6- to 8-fold faster for negatively strained cross-bridges than for positively strained ones. This marked strain dependence of cross-bridge detachment is predicted from the model of A. F. Huxley (1957). 8. In the presence of Ca2+, activation of contraction following photolysis of caged ATP was slowed by inclusion of 20-500 microM-MgADP in the medium. An initial decrease in tension related to cross-bridge detachment by MgATP was markedly suppressed in the presence of MgADP. 9. Ten millimolar Pi partly suppressed active tension generation in the presence of MgADP.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Mechanochemical coupling in actomyosin energy transduction studied by in vitro movement assay

In order to study the mechanochemical coupling in actomyosin energy transduction, the sliding distance of an actin filament induced by one ATP hydrolysis cycle was obtained by using an in vitro movement assay that permitted quantitative and simultaneous measurements of (1) the movements of single fluorescently labeled actin filaments on myosin bound to coverslip surfaces and (2) the ATPase rates. The sliding distance was determined as (the working stroke time in one ATPase cycle, tws) x (the filament velocity, v). tws was obtained from the ATPase turnover rate of myosin during the sliding (kt), the ATP hydrolysis time (delta t) and the ON-rate at which myosin heads enter into the working stroke state when they encounter actin (kON); tws approximately 1/kt-delta t-1/kON. kt was estimated from the ATPase rates of the myosin-coated surface during the sliding of actin filaments. delta t has been determined as less than 1/100 per second, kON was estimated by analyzing the movements of very short (40 nm) filaments. The resulting sliding distance during one ATP hydrolysis cycle near zero load was greater than 100 nm, which is about ten times longer than that expected for a single attachment-detachment cycle between an actin and a myosin head. This leads to the conclusion that the coupling between the ATPase and attachment-detachment cycles is not determined rigidly in a one-to-one fashion.

A purely mechanical theory of muscular contraction

A simple mathematic model is constructed for the cross-bridge dynamics that govern muscular contraction, based on ideas introduced by Huxley and Simmons. The model differs from earlier ones in that it is based on distributions over time rather than displacement and uses a sharp disengagement rule. The constitutive functions of the model are set using classical experimental results and its predictions for the length-step experiment of Ford, Huxley, and Simmons are examined.

Myosin-product complex in the resting state and during relaxation of smooth muscle

Previous findings suggested that in resting smooth muscle ADP is bound to myosin and that phosphorylation of the myosin, and its subsequent interaction with actin, increases the rate of ADP release. We have now extended these studies to include measurements of bound Pi as well as bound ADP in permeabilized rabbit portal vein. We report that in resting smooth muscle that has been exposed to [3H]ATP and [gamma-32P]ATP, followed by a chase in an unlabeled relaxing solution, the ratio of bound [3H]ADP to bound [32P]Pi is close to unity, and both are released at approximately the same rate. This suggests that myosin exists predominantly with both ADP and Pi bound under resting conditions and that the release of one is quickly followed by the release of the other. In contrast, there is a significant 30% excess of bound Pi over ADP in a muscle during relaxation from an isometric contraction. Under these conditions, while force output is slowly decreasing, both light chain phosphorylation and adenosinetriphosphatase (ATPase) activity have decreased to near-resting values. The time course of relaxation is similar to the time course of Pi release from both the resting and relaxing muscle. We propose that during relaxation the dephosphorylated cross bridges which are bearing force have Pi but not ADP bound and that detachment of the cross bridge (and thus force decay) is limited by Pi release from myosin which occurs at the same rate as in the resting muscle.

Mechanisms relating force and high-frequency stiffness in skeletal muscle

Muscle stiffness increases faster than muscle force during the rising phase of a tetanic contraction, and decreases more slowly during the falling phase. Different models of the stiffness arising from series, parallels, and crossbridge elasticity were compared to determine whether they could account quantitatively for the observed time course of force and stiffness. Data for slow and fast twitch mouse muscles at temperatures from 6 to 37 degrees C (Stein and Gordon, Can. J. Physiol. Pharmacol. 64, 1236-1244, 1986) and for single frog muscle fibers (Cecchi et al., Contractile Mechanisms in Muscle, pp. 641-655. Plenum, New York, 1984) were compared. The results showed that a good fit to the data for mouse muscles could be obtained with a model in which: (1) a nonlinear series elasticity contributed significantly to stiffness; (2) the attached crossbridges went from a stiff, force-generating state to a stiff, non-force-generating state; and (3) the rate of transition between these two states increased abruptly at the onset of relaxation. The increased transition rate probably arises from the internal rearrangement in which some sarcomeres shorten at the expense of other sarcomeres, once the muscle begins to relax. A significant series elasticity was not required for the frog data, but a pre-tension state was then needed to obtain a good fit.

Role of MgATP and MgADP in the cross-bridge kinetics in chemically skinned rabbit psoas fibers. Study of a fast exponential process (C)

The role of the substrate (MgATP) and product (MgADP) molecules in cross-bridge kinetics is investigated by small amplitude length oscillations (peak to peak: 3 nm/cross-bridge) and by following amplitude change and phase shift in tension time courses. The range of discrete frequencies used for this investigation is 0.25-250 Hz, which corresponds to 0.6-600 ms in time domain. This report investigates the identity of the high frequency exponential advance (process C), which is equivalent to "phase 2" of step analysis. The experiments are performed in maximally activated (pCa 4.5-5.0) single fibers from chemically skinned rabbit psoas fibers at 20 degrees C and at the ionic strength 195 mM. The rate constant 2 pi c deduced from process (C) increases and saturates hyperbolically with an increase in MgATP concentration, whereas the same rate constant decreases monotonically with an increase in MgADP concentration. The effects of MgATP and MgADP are opposite in all respects we have studied. These observations are consistent with a cross-bridge scheme in which MgATP and MgADP are in rapid equilibria with rigorlike cross-bridges, and they compete for the substrate site on myosin heads. From our measurements, the association constants are found to be 1.4 mM-1 for MgATP and 2.8 mM-1 for MgADP. We further deduced that the composite second order rate constant of MgATP binding to cross-bridges and subsequent isomerization/dissociation reaction to be 0.57 x 10(6)M-1s-1.

Heat changes during transient tension responses to small releases in active frog muscle

Tension and heat production were measured in frog sartorius muscles in response to small shortening ramps (releases) at high and moderate speed. Transient tension responses to fast releases (0.1 to 0.4 mm in 1 or 4 ms) were similar to the tension transients length-clamped single fibers. Tension time courses during releases at 25 mm/s were like fiber responses calculated from the first two phases of the step responses (Ford et al., 1977). We conclude that similar crossbridge transitions produce tension transients observed in whole muscles and single fibers. Heat was absorbed during rapid tension recovery after fast releases and during the later part of releases at 25 mm/s. Variation of heat absorption with release size was compared with that of crossbridge movement predicted by the Huxley-Simmons hypothesis of force generation (Huxley and Simmons, 1971). Agreement between the two supports the conclusion that heat is absorbed by the crossbridge transitions responsible for rapid tension recovery after release. The results indicate that the entropy change of these transitions is positive.

Relaxation of muscle fibers with adenosine 5'-[gamma-thio]triphosphate (ATP[gamma S]) and by laser photolysis of caged ATP[gamma S]: evidence for Ca2+-dependent affinity of rapidly detaching zero-force cross-bridges

The relationship between the mechanical and biochemical states of the muscle cross-bridge cycle and the control of contraction were investigated by using the nucleotide analogs adenosine 5'-[gamma-thio]triphosphate (ATP[gamma S]) and caged ATP[gamma S] [the O-1(2-nitrophenyl)ethyl P3-ester of ATP[gamma S]]. ATP[gamma S] interacts with actomyosin in a manner similar to ATP but is hydrolyzed (by a factor of 500) more slowly. Generation of ATP[gamma S] by photolysis of caged ATP[gamma S] within a permeabilized fiber in rigor in the absence of Ca2+ relaxed tension and stiffness as occurs with ATP. The transient rise in tension prior to final relaxation observed with photolysis of caged ATP was absent with caged ATP[gamma S]. This result suggests that following detachment of a cross-bridge, ATP is normally hydrolyzed before force generation. In the presence of Ca2+, photolysis of caged ATP[gamma S] within rigor fibers caused tension to relax fully but significant stiffness remained. Stiffness also developed without concomitant tension when Ca2+ concentration was raised from less than 1 nM to 30 microM in the presence of ATP[gamma S]. The amplitude of the tension response to ramp stretches in the presence of Ca2+ and ATP[gamma S] increased with ramp stretch velocity, suggesting that the cross-bridges have detachment rate constants extending into the 10(3) s-1 range. The results provide evidence that the Ca2+-regulatory system can directly control attachment of cross-bridges into states before the power stroke.

Cross-bridge kinetics, cooperativity, and negatively strained cross-bridges in vertebrate smooth muscle. A laser-flash photolysis study

The effects of laser-flash photolytic release of ATP from caged ATP [P3-1(2-nitrophenyl)ethyladenosine-5'-triphosphate] on stiffness and tension transients were studied in permeabilized guinea pig protal vein smooth muscle. During rigor, induced by removing ATP from the relaxed or contracting muscles, stiffness was greater than in relaxed muscle, and electron microscopy showed cross-bridges attached to actin filaments at an approximately 45 degree angle. In the absence of Ca2+, liberation of ATP (0.1-1 mM) into muscles in rigor caused relaxation, with kinetics indicating cooperative reattachment of some cross-bridges. Inorganic phosphate (Pi; 20 mM) accelerated relaxation. A rapid phase of force development, accompanied by a decline in stiffness and unaffected by 20 mM Pi, was observed upon liberation of ATP in muscles that were released by 0.5-1.0% just before the laser pulse. This force increment observed upon detachment suggests that the cross-bridges can bear a negative tension. The second-order rate constant for detachment of rigor cross-bridges by ATP, in the absence of Ca2+, was estimated to be 0.1-2.5 X 10(5) M-1s-1, which indicates that this reaction is too fast to limit the rate of ATP hydrolysis during physiological contractions. In the presence of Ca2+, force development occurred at a rate (0.4 s-1) similar to that of intact, electrically stimulated tissue. The rate of force development was an order of magnitude faster in muscles that had been thiophosphorylated with ATP gamma S before the photochemical liberation of ATP, which indicates that under physiological conditions, in non-thiophosphorylated muscles, light-chain phosphorylation, rather than intrinsic properties of the actomyosin cross-bridges, limits the rate of force development. The release of micromolar ATP or CTP from caged ATP or caged CTP caused force development of up to 40% of maximal active tension in the absence of Ca2+, consistent with cooperative attachment of cross-bridges. Cooperative reattachment of dephosphorylated cross-bridges may contribute to force maintenance at low energy cost and low cross-bridge cycling rates in smooth muscle.