ULTRASTRUCTURE OF THE RESTING AND CONTRACTED STRIATED MUSCLE FIBER AT DIFFERENT DEGREES OF STRETCH

Passive stretch, isometric contraction, and shortening were studied in electron micrographs of striated, non-glycerinated frog muscle fibers. The artifacts due to the different steps of preparation were evaluated by comparing sarcomere length and fiber diameter before, during, and after fixation and after sectioning. Tension and length were recorded in the resting and contracted fiber before and during fixation. The I filaments could be traced to enter the A band between the A filaments on both sides of the I band, creating a zone of overlap which decreased linearly with stretch and increased with shortening. This is consistent with a sliding filament model. The decrease in the length of the A and I filaments during isometric contraction and the finding that fibers stretched to a sarcomere length of 3.7 µ still developed 30 per cent of the maximum tetanic tension could not be explained in terms of the sliding filament model. Shortening of the sarcomeres near the myotendinous junctions which still have overlap could account for only one-sixth of this tension, indicating that even those sarcomeres stretched to such a degree that there is a gap between A and I filaments are activated during isometric contraction (increase in stiffness). Shortening, too, was associated with changes in filament length. The diameter of A filaments remained unaltered with stretch and with isometric contraction. Shortening of 50 per cent was associated with a 13 per cent increase in A filament diameter. The area occupied by the fibrils and by the interfibrillar space increased with shortening, indicating a 20 per cent reduction in the volume of the fibrils when shortening amounted to 40 per cent.     

CONTRACTILITY AND ULTRASTRUCTURE IN GLYCEROL-EXTRACTED MUSCLE FIBERS : II. Ultrastructure in Resting and Shortened Fibers

Glycerol-extracted rabbit psoas muscle fibers were examined by electron microscopy both before and after ATP-induced isotonic shortening. Ultrastructural changes were correlated with the initial sarcomere length and the degree of shortening. The ultrastructural appearance of the resting fiber at rest length was identical with that described by H. E. Huxley and Hanson. At sarcomere lengths greater than 3.7 to 3.8 µ, the A and I filaments were detached and separated by a gap. The presence of "gap" filaments was confirmed, and evidence is presented which indicates that these filaments form connections between the ends of the A and I filaments. Shortening from initial sarcomere lengths at which the filaments overlapped took place through sliding of the filaments. If shortening was initiated from sarcomere lengths at which there was a gap, a narrowing of the I band was brought about by a curling of the I filaments at the boundary between the A and I bands. No evidence could be found that the I filaments moved into the A band.     

The Effect of Bathing Solution Tonicity on Resting Tension in Frog Muscle Fibers

Resting tension and short-range elastic properties of isolated twitch muscle fibers of the frog have been studied while bathed by solutions of different tonicities. Resting tension in isotonic solution at 2.3-µm sarcomere spacing averaged 0.46 mN·mm^-2 and was proportional to the fiber cross-section area. Hypertonic solutions, containing 0.1–0.5 mM tetracaine to block contracture tension, caused a small sustained tension increase, which was proportional to the fiber cross-section area and which reached 0.9 mN·mm^-2 at two times normal tonicity (2T). Further increases in tonicity caused little increase in tension. Hypotonic solutions decreased tension. Thus, tension at 2.3 µm is a continuous, direct function of tonicity. The dependence of tension on tonicity lessened at greater sarcomere lengths. At 3.2 µm either a very small rise or, in some fibers, a fall in tension resulted from an increase in tonicity. Hypertonic solutions also decreased the tension of extended sarcolemma preparations. In constant-speed stretch experiments the elastic modulus, calculated from the initial part of the stretch response, rose steeply with tonicity over the whole range investigated (1–2.5T). The results show that tension and stiffness of the short-range elastic component do not increase in parallel in hypertonic solutions.     

ELECTRON MICROSCOPICAL STUDY OF SKELETAL MUSCLE DURING ISOTONIC (AFTERLOAD) AND ISOMETRIC CONTRACTION

Bundles of the curarized semitendinosus muscle of the frog were fixed during isotonic (afterload) and isometric contraction and the length of the A and I bands investigated by electron microscopy. The sarcomere length, during afterload contraction initiated at 25 per cent stretch, varied depending on the afterload applied between 3.0 and 1.2 µ, i.e. the shortening amounted to 5 to 50 per cent. The shortening involved both the A and I bands. Between a sarcomere length of 3.0 to 1.7 µ (shortening 5 to 35 per cent) the A bands remained practically constant at about 1.5 µ (6 to 8 per cent shortening); the length of the I bands decreased from 1.4 to 0.3 µ (80 per cent shortening). Below a sarcomere length of 1.7 to 1.2 µ the A bands shortened from 1.5 to 1.0 µ (from 6 to 8 to 25 per cent). At sarcomere lengths 1.6 to 1.2 µ the I band was replaced by a contraction band. During isometric contraction the A bands shortened by about 8 to 10 per cent; the I bands were correspondingly elongated.     

STRUCTURE OF LIMULUS STRIATED MUSCLE : The Contractile Apparatus at Various Sarcomere Lengths

The musculature of the telson of Limulus polyphemus L. consists of three dorsal muscles: the medial and lateral telson levators and the telson abductor, and one large ventral muscle; the telson depressor, which has three major divisions: the dorsal, medioventral, and lateroventral heads. The telson muscles are composed of one type of striated muscle fiber, which has irregularly shaped myofibrils. The sarcomeres are long, with discrete A and I and discontinuous Z bands. M lines are not present. H zones can be identified easily, only in thick (1.0 µm) longitudinal sections or thin cross sections. In lengthened fibers, the Z bands are irregular and the A bands appear very long due to misalignment of constituent thick filaments. As the sarcomeres shorten, the Z lines straighten somewhat and the thick filaments become more aligned within the A band, leading to apparent decrease in A band length. Further A band shortening, seen at sarcomere lengths below 7.4 µm may be a function of conformational changes of the thick filaments, possibly brought about by alterations in the ordering of their paramyosin cores.     

THE MYOFILAMENT ARRANGEMENT IN THE FEMORAL MUSCLE OF THE COCKROACH, LEUCOPHAEA MADERAE FABRICIUS

The structure of the femoral muscle of the cockroach, Leucophaea maderae, was investigated by light and electron microscopy. The several hundred fibers of either the extensor or flexor muscle are 20 to 40 µ in diameter in transverse sections and are subdivided into closely packed myofibrils. In glutaraldehyde-fixed and epoxy resin-embedded material of stretched fibers, the A band is about 4.5 µ long, the thin filaments are about 2.3 µ in length, the H zone and I band vary with the amount of stretch, and the M band is absent. The transverse sections of the filaments reveal in the area of a single overlap of thick and thin filaments an array of 10 to 12 thin filaments encircling each thick filament; whereas, in the area of double overlap in which the thin filaments interdigitate from opposite ends of the A band, the thin filaments show a twofold increase in number. The thick filament is approximately 205 to 185 A in diameter along most of its length, but at about 0.2 µ from the end it tapers to a point. Furthermore, some well oriented, very thin transverse sections show these filaments to have electron-transparent cores. The diameter of the thin filament is about 70 A. Transverse sections exhibit the sarcolemma invaginating clearly at regular intervals into the lateral regions of the A band. Three distinct types of mitochondria are associated with the muscle: an oval, an elongate, and a type with three processes. It is evident, in this muscle, that the sliding filament hypothesis is valid, and that perhaps the function of the extra thin filaments is to increase the tensile strength of the fiber and to create additional reactive sites between the thick and thin filaments. These sites are probably required for the functioning of the long sarcomeres.     

Resting Sarcomere Length-Tension Relation in Living Frog Heart

The sarcomere pattern and tension of isolated resting frog atrial trabeculae were continuously monitored. In the absence of any resting tension the sarcomere lengths varied with the diameter of the trabeculae. In over 75 % of the trabeculae the value exceeded 2.05 µm, the estimated in vivo length of the thin filaments, and it was never less than 1.89 µm. When the trabeculae were stretched the increase in length of the central undamaged portion could be completely accounted for by an increase in sarcomere length. The width of the A band was constant only at sarcomere lengths between 2.3 and 2.6 µm it decreased at smaller and increased at larger sarcomere lengths. A group of spontaneously active cells stretched the sarcomeres in cells in series to longer lengths than could be produced by passive tension applied to the ends of the trabeculae, but they did not influence the sarcomeres of adjacent cells. It is proposed that the connective tissue is a major factor in determining sarcomere length and that there are interactions between thick and thin filaments in resting muscles.     

The length–tension curve in muscle depends on lattice spacing

Classic interpretations of the striated muscle length–tension curve focus on how force varies with overlap of thin (actin) and thick (myosin) filaments. New models of sarcomere geometry and experiments with skinned synchronous insect flight muscle suggest that changes in the radial distance between the actin and myosin filaments, the filament lattice spacing, are responsible for between 20% and 50% of the change in force seen between sarcomere lengths of 1.4 and 3.4 µm. Thus, lattice spacing is a significant force regulator, increasing the slope of muscle''s force–length dependence.     

Sarcomere lengthening and tension drop in the latent period of isolated frog skeletal muscle fibers

A laser diffraction technique has been developed for registering small changes in sarcomere length. The technique is capable of resolving changes as small as 0.2 A in isolated frog skeletal muscle fibers. The small sarcomere lengthening that accompanies the drop in tension in the latent period of contraction was investigated. We suggest this lengthening be named latency elongation (LE). The LE is present in a completely slack fiber and must, therefore, be caused by a forcible lengthening process. Furthermore, the LE is dependent on the existence of an overlap between thin and tick filaments. The rate of elongation and the time interval between stimulation and maximum elongation may vary along the fiber. The maximum elongation was 3-5 A per sarcomere. At any instant the drop in tension is a product of the sum of sarcomere lengthenings along the fiber and the slope stiffness of the series elasticity. The latency relaxation (LR) could be registered in the sarcomere length range from 2.2 mum to 3.6-3.7 mum. The amplitude went through a sharp maximum at 3.0-3.1 mum. In the sarcomere length range from 2.2 to 2.8 mum the delay from onset to maximum LR was nearly proportional to the distance from the Z-line to the overlap zone. A working hypothesis is presented. It is suggested that the LE is caused by a lengthening of the thin filaments.     

Tension relaxation after stretch in resting mammalian muscle fibers: stretch activation at physiological temperatures.

Tension responses to ramp stretches of 1-3% Lo (fiber length) in amplitude were examined in resting muscle fibers of the rat at temperatures ranging from 10 degrees C to 36 degrees C. Experiments were done using bundles of approximately 10 intact fibers isolated from the extensor digitorum longus (a fast muscle) and the soleus (a slow muscle). At low temperatures (below approximately 20 degrees C), the tension response consisted of an initial rise to a peak during the ramp followed by a complex tension decay to a plateau level; the tension decay occurred at approximately constant sarcomere length. The tension decay after a standard stretch at approximately 3-4.Lo/s contained a fast, an intermediate, and a (small amplitude) slow component, which at 10 degrees C (sarcomere length approximately 2.5 microns) were approximately 2000.s-1, approximately 150.s-1, and approximately 25.s-1 for fast fibers and approximately 2000.s-1, approximately 70.s-1 and approximately 8.s-1 for slow fibers, respectively. The fast component may represent the decay of interfilamentary viscous resistance, and the intermediate component may be due to viscoelasticity in the gap (titin, connectin) filament. The two- to threefold fast-slow muscle difference in the rate of passive tension relaxation (in the intermediate and the slow components) compares with previously reported differences in the speed of their active contractions; this suggests that "passive viscoelasticity" is appropriately matched to contraction speed in different muscle fiber types. At approximately 35 degrees C, the fast and intermediate components of tension relaxation were followed by a delayed tension rise at approximately 10.s-1 (fast fibers) and 2.5.s-1 (slow fibers); the delayed tension rise was accompanied by sarcomere shortening. BDM (5-10 mM) reduced the active twitch and tetanic tension responses and the delayed tension rise at 35 degrees C; the results indicate stretch sensitive activation in mammalian sarcomeres at physiological temperatures.     

Fast Skeletal Muscle Troponin Activation Increases Force of Mouse Fast Skeletal Muscle and Ameliorates Weakness Due to Nebulin-Deficiency

The effect of the fast skeletal muscle troponin activator, CK-2066260, on calcium-induced force development was studied in skinned fast skeletal muscle fibers from wildtype (WT) and nebulin deficient (NEB KO) mice. Nebulin is a sarcomeric protein that when absent (NEB KO mouse) or present at low levels (nemaline myopathy (NM) patients with NEB mutations) causes muscle weakness. We studied the effect of fast skeletal troponin activation on WT muscle and tested whether it might be a therapeutic mechanism to increase muscle strength in nebulin deficient muscle. We measured tension–pCa relations with and without added CK-2066260. Maximal active tension in NEB KO tibialis cranialis fibers in the absence of CK-2066260 was ∼60% less than in WT fibers, consistent with earlier work. CK-2066260 shifted the tension-calcium relationship leftwards, with the largest relative increase (up to 8-fold) at low to intermediate calcium levels. This was a general effect that was present in both WT and NEB KO fiber bundles. At pCa levels above ∼6.0 (i.e., calcium concentrations <1 µM), CK-2066260 increased tension of NEB KO fibers to beyond that of WT fibers. Crossbridge cycling kinetics were studied by measuring k_tr (rate constant of force redevelopment following a rapid shortening/restretch). CK-2066260 greatly increased k_tr at submaximal activation levels in both WT and NEB KO fiber bundles. We also studied the sarcomere length (SL) dependence of the CK-2066260 effect (SL 2.1 µm and 2.6 µm) and found that in the NEB KO fibers, CK-2066260 had a larger effect on calcium sensitivity at the long SL. We conclude that fast skeletal muscle troponin activation increases force at submaximal activation in both wildtype and NEB KO fiber bundles and, importantly, that this troponin activation is a potential therapeutic mechanism for increasing force in NM and other skeletal muscle diseases with loss of muscle strength.     

Passive interaction between sliding filaments in the osmotically compressed skinned muscle fibers of the frog.

Shortening and lengthening velocities, instantaneous stiffness, and tension transients after stretch were measured in compressed muscle fibers from the frog in the presence or absence of polyvinylpyrrolidone (PVP K30) or Dextran T70. Both shortening and lengthening velocities clearly decreased with the concentration of polymer. In the presence of polymer, "passive" stiffness was observed in relaxing solution depending on fiber diameter, and stiffness increased further by activation. This increase by activation above "passive" stiffness was nearly constant in the wide range of polymer concentrations. These active and "passive" stiffnesses were found to be dependent on sarcomere length. The stiffness of a compressed rigor fiber was indicated to be composed of constant rigor stiffness and a variable "passive" one. The tension transient after stretch in a compressed active or rigor fiber was also indicated to be composed of two kinds of transients. The above results suggest that (a) there exist two kinds of interactions in parallel in a compressed active or rigor fiber: one active or rigor and another "passive" between sliding filaments, and (b) the decrease in shortening velocity in a compressed fiber may be brought about by this "passive" interaction.     

Optical Diffraction Studies of Muscle Fibers

A new technique to monitor light diffraction patterns electrically is applied to frog semitendinosus muscle fibers at various levels of stretch. The intensity of the diffraction lines, sarcomere length change, and the length-dispersion (line width) were calculated by fast analogue circuits and displayed in real time. A heliumneon laser (wavelength 6328 Å) was used as a light source. It was found that the intensity of the first-order diffraction line drops significantly (30-50%) at an optimal sarcomere length of 2.8 μm on isometric tetanic stimulation. Such stimulation produced contraction of half-sarcomeres by about 22 nm presumably by stretching inactive elements such as tendons. The dispersion of the sarcomere lengths is extremely small, and it is proportional to the sarcomere length (less than 4%). The dispersion increases on stimulation. These changes on isometric tetanic stimulation were dependent on sarcomere length. No vibration or oscillation in the averaged length of the sarcomeres was found during isometric tetanus within a resolution of 3 nm; however, our observation of increased length dispersion of the sarcomeres together with detection of the averaged shortening of the sarcomere lengths suggests the presence of asynchronous cyclic motions between thick and thin filaments. An alternative explanation is simply an increase of the length dispersion of sarcomeres without cyclic motions.     

Compliance of thin filaments in skinned fibers of rabbit skeletal muscle.

The mechanical compliance (reciprocal of stiffness) of thin filaments was estimated from the relative compliance of single, skinned muscle fibers in rigor at sarcomere lengths between 1.8 and 2.4 micron. The compliance of the fibers was calculated as the ratio of sarcomere length change to tension change during imposition of repetitive cycles of small stretches and releases. Fiber compliance decreased as the sarcomere length was decreased below 2.4 micron. The compliance of the thin filaments could be estimated from this decrement because in this range of lengths overlap between the thick and thin filaments is complete and all of the myosin heads bind to the thin filament in rigor. Thus, the compliance of the overlap region of the sarcomere is constant as length is changed and the decrease in fiber compliance is due to decrease of the nonoverlap length of the thin filaments (the I band). The compliance value obtained for the thin filaments implies that at 2.4-microns sarcomere length, the thin filaments contribute approximately 55% of the total sarcomere compliance. Considering that the sarcomeres are approximately 1.25-fold more compliant in active isometric contractions than in rigor, the thin filaments contribute approximately 44% to sarcomere compliance during isometric contraction.     

CORRELATED MORPHOLOGICAL AND PHYSIOLOGICAL STUDIES ON ISOLATED SINGLE MUSCLE FIBERS : I. Fine Structure of the Crayfish Muscle Fiber

Single fibers isolated from walking leg muscles of crayfish have 8- to 10-µ sarcomeres which are divided into A, I, and Z bands. The H zone is poorly defined and no M band is distinguishable. Changes in the width of the I band, accompanied by change in the overlap between thick and thin myofilaments, occur when the length of the sarcomere is changed by stretching or by shortening the fiber. The thick myofilaments (ca. 200 A in diameter) are confined to the A band. The thin myofilaments (ca. 50 A in diameter) are difficult to resolve except in swollen fibers, when they clearly lie between the thick filaments and run to the Z disc. The sarcolemma invaginates at 50 to 200 sites in each sarcomere. The sarcolemmal invaginations (SI) form tubes about 0.2 µ in diameter which run radially into the fiber and have longitudinal side branches. Tubules about 150 A in diameter arise from the SI and from the sarcolemma. The invaginations and tubules are all derived from and are continuous with the plasma membrane, forming the transverse tubular system (TTS), which is analogous with the T system of vertebrate muscle. In the A band region each myofibril is enveloped by a fenestrated membranous covering of sarcoplasmic reticulum (SR). Sacculations of the SR extend over the A-I junctions of the myofibrils, where they make specialized contacts (diads) with the TTS. At the diads the opposing membranes of the TTS and SR are spaced 150 A apart, with a 35-A plate centrally located in the gap. It appears likely that the anion-permselective membrane of the TTS which was described previously is located at the diads, and that this property of the diadic structures therefore may function in excitation-contraction coupling.     

Sarcomere length dependence of the force-velocity relation in single frog muscle fibers.

The force-velocity relation of single frog fibers was measured at sarcomere lengths of 2.15, 2.65, and 3.15 microns. Sarcomere length was obtained on-line with a system that measures the distance between two markers attached to the surface of the fiber, approximately 800 microns apart. Maximal shortening velocity, determined by extrapolating the Hill equation, was similar at the three sarcomere lengths: 6.5, 6.0, and 5.7 microns/s at sarcomere lengths of 2.15, 2.65, and 3.15 microns, respectively. For loads not close to zero the shortening velocity decreased with increasing sarcomere length. This was the case when force was expressed as a percentage of the maximal force at optimal fiber length or as a percentage of the sarcomere-isometric force at the respective sarcomere lengths. The force-velocity relation was discontinuous around zero velocity: load clamps above the level that kept sarcomeres isometric resulted in stretch that was much slower than when the load was decreased below isometric by a similar amount. We fitted the force-velocity relation for slow shortening (less than 600 nm/s) and for slow stretch (less than 200 nm/s) with linear regression lines. At a sarcomere length of 2.15 microns the slopes of these lines was 8.6 times higher for shortening than for stretch. At 2.65 and 3.15 microns the values were 21.8 and 14.1, respectively. At a sarcomere length of 2.15 microm, the velocity of stretch abruptly increased at loads that were 160-170% of the sarcomere isometric load, i.e., the muscle yielded. However, at a sarcomere length of 2.65 and 3.15 microm yield was absent at such loads. Even the highest loads tested (260%) resulted in only slow stretch.(ABSTRACT TRUNCATED AT 250 WORDS)     

Stiffness of glycerinated rabbit psoas fibers in the rigor state. Filament-overlap relation

The stiffness of glycerinated rabbit psoas fibers in the rigor state was measured at various sarcomere lengths in order to determine the distribution of the sarcomere compliance between the cross-bridge and other structures. The stiffness was determined by measuring the tension increment at one end of a fiber segment while stretching the other end of the fiber. The contribution of the end compliance to the rigor segments was checked both by laser diffractometry of the sarcomere length change and by measuring the length dependence of the Young's modulus; the contribution was found to be small. The stiffness in the rigor state was constant at sarcomere lengths of 2.4 microns or less; at greater sarcomere lengths the stiffness, when corrected for the contribution of resting stiffness, scaled with the amount of overlap between the thick and thin filaments. These results suggest that the source of the sarcomere compliance of the rigor fiber at the full overlapping of filaments is mostly the cross-bridge compliance.     

THE ULTRASTRUCTURE OF THE M LINE IN SKELETAL MUSCLE

By electron microscopy, the ultrastructure of the M line was investigated in fibers from frog nonglycerinated semitendinosus muscles at body length and at different degrees of shortening and stretch. The M line appeared as a line of high electron opacity in the middle of the A band. Its framework consists of: (i) three (four or five) arrays of transverse M bridges, 200 A apart, connecting each A filament with its six neighbors; (ii) M filaments, parallel to the A filaments, passing through the M line and linking each set of M bridges together. In the shortened fiber the M line remained distinct. At high degrees of stretch, the M line became fainter or indiscernible. This appearance reflects a misalignment of the M components caused by a staggering of the A filaments. The M line reappeared after release of fibers stretched 70–80% above equilibrium length. On the basis of the structural analysis, the possible function of the M line is compared with that of the Z line, and a model is suggested for the M line.     

New insights into the behavior of muscle during active lengthening.

A muscle fiber was modeled as a series-connected string of sarcomeres, using an A. V. Hill type model for each sarcomere and allowing for some random variation in the properties of the sarcomeres. Applying stretches to this model led to the prediction that lengthening of active muscle on or beyond the plateau of the length tension curve will take place very nonuniformly, essentially by rapid, uncontrolled elongation of individual sarcomeres, one at a time, in order from the weakest toward the strongest. Such a "popped" sarcomere, at least in a single fiber, will be stretched to a length where there is no overlap between thick and thin filaments, and the tension is borne by passive components. This prediction allows modeling of many results that have previously been inexplicable, notably the permanent extra tension after stretch on the descending limb of the length tension curve, and the continued rise of tension during a continued stretch.     

CORRELATION BETWEEN FIBER LENGTH, ULTRASTRUCTURE, AND THE LENGTH-TENSION RELATIONSHIP OF MAMMALIAN SMOOTH MUSCLE

The length-tension relationship was determined for strips of guinea pig taenia coli and correlated with the length and ultrastructural organization of the component fibers. The mean fiber length in "stretched" strips (passive ≥ active tension) was 30% greater than that for fibers in "unstretched" strips (active >> passive tension). In stretched fibers the dense bodies and 100 A diameter myofilaments were consolidated into a mass near the center of fibers in cross-sectional profile. The thick myofilaments were segregated into the periphery of the fiber profiles. In unstretched fibers the dense bodies-100 A diameter filaments and the thick myofilaments were uniformly distributed throughout cross-sectional profiles. A tentative model is proposed to account for the change in fiber length and ultrastructural organization that accompanies stretch. The basic features of the model require the dense bodies to be linked together into a network by the 100 A diameter filaments. The functional consequences of stretching the fibers are discussed in relation to the model proposed for this network.