Connectin filaments in stretched skinned fibers of frog skeletal muscle.

Indirect immunofluorescence microscopy of highly stretched skinned frog semi-tendinous muscle fibers revealed that connectin, an elastic protein of muscle, is located in the gap between actin and myosin filaments and also in the region of myosin filaments except in their centers. Electron microscopic observations showed that there were easily recognizable filaments extending from the myosin filaments to the I band region and to Z lines in the myofibrils treated with antiserum against connectin. In thin sections prepared with tannic acid, very thin filaments connected myosin filaments to actin filaments. These filaments were also observed in myofibrils extracted with a modified Hasselbach-Schneider solution (0.6 M KCl, 0.1 M phosphate buffer, pH 6.5, 2 mM ATP, 2 mM MgCl2, and 1 mM EGTA) and with 0.6 M Kl. SDS PAGE revealed that connectin (also called titin) remained in extracted myofibrils. We suggest that connectin filaments play an important role in the generation of tension upon passive stretch. A scheme of the cytoskeletal structure of myofibrils of vertebrate skeletal muscle is presented on the basis of our present information of connectin and intermediate filaments.

Ca2+ dependence of tension and ADP production in segments of chemically skinned muscle fibers.

Both ADP production and tension have been measured in segments of chemically skinned fibers contracting at different Ca2+ concentrations. Full mechanical activation occurred between pCa 7.00 and pCa 6.50. The total ATPase was due to both actomyosin and non-actomyosin ATPase. Actomyosin ATPase was observed at pCa 7.09 without accompanying tension. The Ca2+ dependence of tension was steeper than actomyosin ATPase. This finding implies some rate constants of the mechano-chemical cycle are Ca2+ dependent. Non-actomyosin ATPase was measured in fibers stretched beyond overlap of the thick and thin filaments. Sarcoplasmic reticulum was isolated and sarcoplasmic reticulum activity was measured in vitro under the same conditions as the single-fiber experiments. Non-actomyosin ATPase in the single fibers was found to be small compared to maximally activated actomyosin ATPase but larger than the ATPase that could be attributed to sarcoplasmic reticulum activity.

Optical diffraction study of muscle fibers. II. Electro-optical properties of muscle fibers.

When an electric field is applied along the fiber axis, the intensities of all observable optical diffraction lines of skeletal muscle fibers increase. This electro-optical effect was extensively studied and it was confirmed that the effect is due to the interaction between electric dipole moments of thin filaments and the applied field. From the present study on the intensity modulation due to applied field in sinusoidal and square forms, we confirmed that (1) the thin filament is a semiflexible rod, (2) the second order mode of the bending motion of thin filaments contributes to the electro-optical effect of muscle fibers at higher frequencies of a sinusodidal field or shorter durations of a square field, (3) the induced moment has no appreciable effect, and (4) the estimated value of the flexural rigidity of thin filaments strongly depends on the concentrations of free calcium ions in the myofibrillar space.

Time-resolved synchrotron X-ray diffraction studies of a single frog skeletal muscle fiber. Time courses of intensity changes of the equatorial reflections and intracellular Ca2+ transients.

Time-resolved X-ray equatorial diffraction studies on a single frog skeletal muscle fiber were performed with a 10 ms time resolution using synchrotron radiation in order to compare the time courses of the molecular changes of contractile proteins and the intracellular Ca2+ transient during an isometric twitch contraction at 2.7 degrees C. Measurements of the Ca2+ transient using aequorin as an intracellular Ca2+ indicator were conducted separately just before and after the X-ray experiments under very similar experimental conditions. The results, which showed a similar time course of tension to that observed in the X-ray experiment, were compared with the aequorin light signal, tension and the intensity changes of the 1,0 and 1,1 equatorial reflections. No appreciable change in both reflection spacings indicated that the effect of internal shortening of the muscle was minimized during contraction. The intensity change of the equatorial reflections generally occurred after the aequorin light signal. In the rising phase, the time course of increase in the 1,1 intensity paralleled that of the rise of the light signal and the intensity peak occurred 20-30 ms after the peak of the light signal. The decrease in the 1,0 intensity showed a time course similar to that of tension and the intensity minimum roughly coincided with the tension peak, coming at 80-90 ms and about 60 ms after the peaks of the light signal and the 1,1 intensity change, respectively. In the relaxation phase, the 1,1 intensity seemed to fall rapidly just before the tension peak and then returned to the original level in parallel with the decay of tension. The 1,0 intensity returned more slowly than the tension relaxation. Thus, the change of the 1,1 intensity was faster than that of the 1,0 intensity in both the rising and relaxation phases. When the measured aequorin light signal was corrected for the kinetic delay of the aequorin reaction with a first-order rate constant of either 50 or 17 s-1, the peak of the corrected light signal preceded that of the measured one by approx. 30 ms. Thus, the peak of the Ca2+ transient appeared earlier than the peaks of the 1,1 and 1,0 intensity changes by 50-60 and 110-120 ms, respectively. The time lag between the extent of structural change and the Ca2+ transient is discussed in relation to the double-headed attachment of a cross-bridge to actin.

Properties of spike potentials detected by a surface electrode in intact human muscle.

A clearly discriminable train of spikes was detected by ordinary silver disc electrodes fastened to the skin surface overlying m. vastus medialis during voluntary contraction in man. Some properties of these surface spikes were obtained. (1) Motor unit potentials detected by the inserted electrode loaded in the muscle closely under the fascia were found to be synchronized with the surface spikes. (2) The conduction velocity was around 3.5 m/s. (3) A starting point of the excitation, i.e., ,end-plate," was located at a point one-third of the observed length from the distal end in this particular case. (4) The conduction velocity showed a linear relationship with muscular temperature and Q10 approximately equal to 2.0 in the range of 17-31 degrees C. (5) The amplitude of the surface spikes decreased monotonously with increasing the distance between the source and electrode. (6) Wave forms and threshold values were highly reproducible. (7) Similar surface spikes have been found in six other muscles in the subject S.M. and in m. vastus medialis in five other subjects.

Effects of Ca2+ and calmodulin on contraction of chemically skinned muscle fibers from pregnant rat uteri.

The effects of Ca2+ and calmodulin on contraction of saponin-treated (chemically skinned) uterine smooth muscle fibers of pregnant rats were examined. Ca2+ sensitivity, defined as the pCa required for half maximum activation of force production, was found to change with the progress of pregnancy; low in the early and middle stages and high in the later stages of pregnancy. The overall change of Ca2+ sensitivity was about pCa 1.5 during the period of pregnancy. The effect of calmodulin on contraction was also found to be dependent on the stages of pregnancy. Calmodulin was effective on the augmentation of the tension rather than the change in Ca2+ sensitivity, and this augmentation was large in the early and middle stages of pregnancy. The amount of calmodulin, which eluted out of uterine muscle cells during saponin treatment, was large in the early and middle stages of pregnancy. The results indicate that the contractile response of the uterine muscle cells during the period of pregnancy seems to be controlled by both the changes in Ca2+ sensitivity and in the amount of free calmodulin in uterine muscle cells.

Lateral shrinkage of the myofilament lattice in chemically skinned muscles during contraction.

A toe muscle was isolated from a hind limb of the mouse and treated with saponin to make the sarcolemma more permeable to the solutes of the bathing medium. The equatorial X-ray diffraction pattern was recorded to determine the 1,0 spacing of the hexagonal myofilament lattice. The spacing in relaxed muscle at a sarcomere length of 2.5 microns was 408 A. When the muscle was maximally activated at pCa 4.4, a steady isometric tension of 1.3 kg/cm2 was produced and the spacing decreased to 384 A. A decrease in spacing of the same magnitude was observed when a relaxed muscle went into rigor, although the rigor tension was only 0.1 kg/cm2, 8% of the maximum contractile tension. From the intensity ratio of the 1,0 and 1,1 reflections the number of myosin heads transferred radially to the vicinity of the thin filament was calculated. During the maximum activation at pCa 4.4 the amount of the radial transfer was 96% of that in rigor. When the muscle was activated at a lower calcium concentration, the lattice shrinkage was smaller and the radial transfer was also smaller. These findings suggest that the lateral force underlying the lattice shrinkage may be due to lateral elasticity of cross-bridges.

Lattice shrinkage with increasing resting tension in stretched, single skinned fibers of frog muscle.

The 1,0 lattice spacing d1,0 in chemically and mechanically skinned single fibers of frog muscle was measured at various sarcomere lengths, L, in the range from L = 2.1 to 6.0 microns by an x-ray diffraction method. In chemically skinned fibers, d1,0 decreased with a similar slope to that of mechanically skinned fibers up to L congruent to 3 microns, but beyond this point d1,0 steeply decreased with further stretching. This steep decrease in d1,0 could be ascribed mainly to an increase in the compressing force associated with the longitudinal extension of a remnant of the sarcolemma. In mechanically skinned fibers, the gradual decrease in d1,0 continued beyond filament overlap (L greater than or equal to 3.5 microns) and was highly proportional to a resting tension. This decrease in d1,0 at L greater than or equal to 3.5 microns could be ascribed to an increase in the force exerted by lateral elastic components, which is proportional to the longitudinal resting tension. A conceptual model is proposed of a network structure of elastic components in a sarcomere.

Localization of the parallel elastic components in frog skinned muscle fibers studied by the dissociation of the A- and I-bands.

Localization of the parallel elastic components (PECs) in skinned muscle fibers was investigated by analyzing the change of the resting tension, which accompanies the dissociation of the A- and I-bands. The A-band was dissociated from both ends by increasing the concentration of KCl under relaxing conditions (0.09-0.54 M KCl, 4.0 mM MgATP, 1.0 mM Mg2+, 4.0 mM EGTA, pH 6.0-9.0, 20 degrees C). At sarcomere lengths greater than or equal to 3.5 microns, the length of the A-band was estimated by comparing the intensity of the first-order optical diffraction line with the results of model calculations. These results were supported by differential-interference microscopy and sodium dodecyl sulfate gel electrophoresis. It was shown that the resting tension decreased nearly in proportion to the residual length of the A-band. At sarcomere lengths less than or equal to 4.0 microns, the resting tension after the dissociation of the A-band was lowered to less than 10% of the initial value. On the other hand, at sarcomere lengths greater than or equal to 5.0 microns the resting tension after the dissociation of the A-band still showed approximately 35% of the initial value and did not change even after the I-band was dissociated by a solution containing KI. From these results, we propose that most of the PECs contributing to resting tension bind almost uniformly to the A-band and there are also PECs connecting Z-lines.

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.

Electro-optical property of extremely stretched skinned muscle fibers.

Skinned fibers of frog semitendinosus muscle could easily be stretched up to 8 mum or more in sarcomere length. Such extremely stretched fibers gave quite sharp optical diffraction patterns. The intensities of all observable diffraction lines were found to increase on application of electric field (10 similar to 100 V/cm) parallel to the fiber axis, provided that there was no overlap between thin and thick filaments. By use of a polarizing microscope, it was concluded that I-bands were mainly responsible for this intensity increase. By application of square pulses, the time course of the intensity increase and decay was followed. The analysis based on a simple model suggests: (a) Each thin filament has a permanent dipole movement and the movement directs from Z-bands to the free end of the thin filament. (b) The flexural rigidity of thin filaments is estimated to be similar to 3 with 10-17 dyn with cm-2. The present fibers will provide various applications in physiochemical studies of in vivo thin and thick filaments.

Deterioration induced by physiological concentration of calcium ions in skinned muscle fibres.

The deteriorating effect of microM order of Ca2+ on skinned frog skeletal muscle fibres was studied from the view point of the digestion of proteins by calcium-activated neutral protease (CANP). Tension developed in solutions containing no MgATP (rigor solution) decreased irreversibly with the addition of Ca2+ in quantities of more than 0.1 microM. Low temperature was seen to suppress (Q10 greater than 4), and neutral pH to maximize, this decrease in tension. In rigor solution containing Ca2+, SDS electrophoresis indicated that a 95 k dalton component (alpha-actinin) was released from the fibre; electron micrography showed the disappearance of Z-lines. These results suggest that one of the causes for decrease in rigor tension is the proteolytic activity of CANP, and its inhibitors were shown to be quite useful in experiments on skinned fibre.

Width and lattice spacing in radially compressed frog skinned muscle fibres at various pH values, magnesium ion concentrations and ionic strengths.

The width (D) and the 1,0 lattice spacing (d1,0) at various ionic compositions of mechanically skinned single fibres (from semitendinosus muscle of Rana catesbeiana) were measured at various concentrations of polyvinyl pyrrolidone (PVP K-30, Mn = 40 000) from 0 to 6% at 20 degrees C. In a standard relaxing solution (4 mM MgATP2-, 1 mM Mg2+, 4 mM EGTA, ionic strength 150 mM and pH 7), d1,0 decreased exponentially as the PVP concentration increased: d1,0 was 41.3 +/- 0.4 (mean +/- S.D.) nm at 0% PVP and 32.9 +/- 0.4 nm at 6% PVP. D was proportional to d1,0 except at very low PVP concentrations, i.e. at 1% PVP, D decreased by 7%, whereas d1,0 decreased by only 3%. At 0% PVP, D and d1,0 decreased when either pH or ionic strength (gamma/2) was lowered. At 6% PVP, D and d1,0 decreased with lowered pH or increased [Mg2+], but was independent of gamma/2. The radial stiffness, or degree of resistance to the changes of D against the compressing force, increased considerably at d1,0 less than or equal to 35 nm in a standard relaxing solution, but not at pH 5.5 or 30 mM [Mg2+]. These effects of pH, [Mg2+] and gamma/2 on D or d1,0 and on the radial stiffness can be explained by the modification of the properties of the elastic element and the hinge between subfragment-1 and -2 and/or the hinge between subfragment-2 and light meromyosin.

Lateral filamentary spacing in chemically skinned murine muscles during contraction.

A mouse toe muscle was chemically skinned with saponin and the 1,0 spacing of the hexagonal myofilament lattice at a sarcomere length of 2.5 micron was measured with the X-ray-diffraction method. In the relaxed state, the 1,0 spacing was 40.8 nm. When the muscle was maximally activated at pCa 4.4, the spacing decreased to 38.4 nm. During contractions at lower calcium concentrations, the spacing decreased less. In rigor, the spacing decreased to almost the same extent as during maximum contraction, although the rigor tension was only 8% of the maximum tension. When the spacing in relaxed muscle had been adjusted osmotically to about 38 nm, activation caused no further decrease in the spacing. The results support the view that the force responsible for the lattice shrinkage during contraction is produced by cross-bridges displaced from their optimum lateral positions.

Myosin heads contact with thin filaments in compressed relaxed skinned fibres of frog skeletal muscle.

When skinned skeletal muscle fibres with rest sarcomere length (L = 2.5 microns) are compressed by the addition of various concentrations ([PVP]) of polyvinylpyrrolidine, the relation between the 1,0 spacing (d) of thick filament lattice and [PVP] has been known to break at d of around 35 nm, resulting in a steeper slope of the relationship at d greater than 35 nm. To clarify the cause of this, X-ray diffraction and crosslinking experiments were carried out. The d versus [PVP] relationship of stretched fibres (L = 3.5 microns) breaks at a d of around 29 nm. The difference between these characteristic d values, 35-29 = 6 nm, is close to the diameter of thin filaments (8 nm). The crosslinking efficiency of formaldehyde between myosin heads and thin filament surface, measured by radial stiffness increase, was found to begin to markedly increase when the relaxed fibre with rest L was compressed to a d of nearly 35 nm. In addition to these results, the d versus [PVP] relationship obtained in rigor and in high [Mg2+] (30 mM) relaxing solutions, and the crosslinking efficiency seen in high [Mg2+] solutions supported our previous hypothesis that in normal relaxing solution (containing 1 mM Mg2+) the probability of myosin heads coming into contact with the thin filament surface abruptly increases at d near 35 nm in fibres with rest L.

Mechanism of action of 2, 3-butanedione 2-monoxime on contraction of frog skeletal muscle fibres.

The mechanism of the inhibitory effect of 2,3-butanedione 2-monoxime (BDM) on contraction of frog skeletal muscles was studied using skinned fibres and aequorin-injected intact fibres. The tension development of skinned fibres directly activated with calcium was strongly inhibited by BDM. This agent also had effects on the sarcoplasmic reticulum in the skinned preparations, suppressing the calcium pump function and enhancing the activity of the 'calcium-induced calcium release' mechanism. In electrically stimulated intact fibres, although BDM slightly suppressed the elevation of the intracellular calcium ion concentration, this effect was so weak that it would not explain the strong inhibitory effect of the agent on the tension development by the intact fibres. It was concluded that the tension reducing effect of BDM on intact fibres was due mainly to its direct action on the contractile system. The mode of this action of BDM was further examined with skinned fibres in view of its effects on the maximum shortening speed and isometric tension in low MgATP environments.