Active force as a function of filament spacing in crayfish skinned muscle fibers

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

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

Effects of solution tonicity on crossbridge properties and myosin lever arm disposition in intact frog muscle fibres

The aims of this study were to investigate the effects of solution tonicity on muscle properties, and to verify their consistence with the lever arm theory of force generation. Experiments were made in single muscle fibres and in fibre bundles from the frog, using both fast stretches and time-resolved X-ray diffraction, in isotonic Ringer solution (1T), hypertonic (1.4T) and hypotonic (0.8T) solutions. Fast stretches (0.4-0.6 ms duration and 16-25 nm per half-sarcomere (nm hs(-1)) amplitude) were applied at various tensions during the force development in isometric tetani. Force increased during the stretch up to a peak (critical tension, Pc) at which it started to fall, in spite of continued stretching. In all solutions, Pc was proportional to the initial isometric tension developed. For a given isometric tension, Pc increased with solution tonicity and occurred at a precise sarcomere elongation (critical length, Lc) which also increased with tonicity. M3 meridional layer line intensity (I M3) was measured during the application of sinusoidal length oscillations (1 kHz frequency, and about 2% fibre length amplitude) at tetanus plateau. I M3 changed during the length oscillations in a sinusoidal manner in phase opposition to length changes, but a double peak distortion occurred at the peak of the release phase. The presence of the distortion, which decreased with tonicity, allowed calculation of the mean position of the myosin head (S1) during the oscillation cycle. In agreement with the lever arm theory, both X-ray diffraction and mechanical data show that solution tonicity affects S1 mean position and consequently crossbridge individual extension and force, with no effect on crossbridge number. The force needed to break the single crossbridge was insensitive to solution tonicity suggesting a non-ionic nature of the actomyosin bond.

Interfilament spacing, Ca2+ sensitivity, and Ca2+ binding in skinned bovine cardiac muscle

The length-dependence of myofilament Ca2+ sensitivity in cardiac muscle appears to be a function of length-dependent variation in the lateral separation of actin and myosin filaments. The goal of this study was to determine how force, Ca2+ sensitivity, and Ca2+ binding to troponin C are correlated in skinned bovine ventricular muscle bundles set at sarcomere length 1.9 microm and subjected to varying degrees of osmotic compression with Dextran T-500. With 5, 10, and 15% Dextran T-500 the muscle diameter was reduced by 13, 21, and 25%, respectively. Addition of 5% Dextran T-500 caused increases in developed force, Ca2+ sensitivity, and in the affinity of Ca2+ for the regulatory binding site on troponin C. All of these parameters were reversed back toward control levels with 10% Dextran T-500. With 15% Dextran T-500 all parameters were decreased to below control levels. These data indicate that (1) there is an optimal filament separation at which both Ca2+ sensitivity and Ca2+ binding are maximized, and (2) Ca2+-troponin C affinity is linked to changes in Ca2+ sensitivity rather than to changes in interfilament spacing.

Quantitative electrophoretic analysis of myosin heavy chains in single muscle fibers

To better understand the molecular basis of the large variation in mechanical properties of different fiber types, there has been an intense effort to relate the mechanical and energetic properties measured in skinned single fibers to those of their constituent cross bridges. There is a significant technical obstacle, however, in estimating the number of cross bridges in a single fiber. In this study, we have developed a procedure for extraction and quantification of myosin heavy chains (MHCs) that permits the routine and direct measurement of the myosin content in single muscle fibers. To validate this method, we also compared MHC concentration measured in single fibers with the MHC concentration in whole fast-twitch (psoas and gracilis) and slow-twitch (soleus) muscles of rabbit. We found that the MHC concentration in intact psoas (184 microM) was larger than that in soleus (144 microM), as would be expected from their differing mitochondrial content and volume of myofibrils. We obtained excellent agreement between MHC concentration measured at the single fiber level with that measured at the whole muscle level. This not only verifies the efficacy of our procedure but also shows that the difference in concentration at the whole muscle level simply reflects the concentration differences in the constituent fiber types. This new procedure should be of considerable help in future attempts to determine kinetic differences in cross bridges from different fiber types.

Length-dependent effects of osmotic compression on skinned rabbit psoas muscle fibers

The goal of this study was to characterize the interrelationship between sarcomere length and interfilament spacing in the control of Ca2+ sensitivity in skinned rabbit psoas muscle fibers. Measurements were made at sarcomere lengths 2.0, 2.7 and 3.4 microm. At 2.7 microm the fiber width was reduced by 17% relative to that at 2.0 microm and the pCa50 for force development was increased by approximately 0.3 pCa units. In the presence of 5% Dextran T-500 the fiber width at sarcomere length 2.0 microm was also decreased by 17% and the Ca2+ sensitivity was increased to the same value as at 2.7 microm. In contrast, at sarcomere length 2.7 microm the addition of as much as 10% Dextran T-500 had no effect on Ca2+ sensitivity. At sarcomere length 3.4 microm there was an additional 7% compression and the Ca2+ sensitivity was increased slightly (approximately 0.1 pCa units) relative to that at 2.7 microm. However at 3.4 microm the addition of 5% Dextran T-500 caused the Ca2+ sensitivity to decrease to the level seen at 2.0 microm. Given that the skinning process causes a swelling of the filament lattice it is evident that the relationship between sarcomere length and Ca2+ sensitivity observed in skinned fibers may not always be applicable to intact fibers. These data are consistent with measurements of Ca2+ in intact fibers which indicate that there might be a decline in Ca2+ sensitivity at long sarcomere lengths.

Effects of MgADP on length dependence of tension generation in skinned rat cardiac muscle

The effect of MgADP on the sarcomere length (SL) dependence of tension generation was investigated using skinned rat ventricular trabeculae. Increasing SL from 1.9 to 2.3 microm decreased the muscle width by approximately 11% and shifted the midpoint of the pCa-tension relationship (pCa(50)) leftward by about 0.2 pCa units. MgADP (0.1, 1, and 5 mmol/L) augmented maximal and submaximal Ca(2+)-activated tension and concomitantly diminished the SL-dependent shift of pCa(50) in a concentration-dependent manner. In contrast, pimobendan, a Ca(2+) sensitizer, which promotes Ca(2+) binding to troponin C (TnC), exhibited no effect on the SL-dependent shift of pCa(50), suggesting that TnC does not participate in the modulation of SL-dependent tension generation by MgADP. At a SL of 1. 9 microm, osmotic compression, produced by 5% wt/vol dextran (molecular weight approximately 464 000), reduced the muscle width by approximately 13% and shifted pCa(50) leftward to a similar degree as that observed when increasing SL to 2.3 microm. This favors the idea that a decrease in the interfilament lattice spacing is the primary mechanism for SL-dependent tension generation. MgADP (5 mmol/L) markedly attenuated the dextran-induced shift of pCa(50), and the degree of attenuation was similar to that observed in a study of varying SL. The actomyosin-ADP complex (AM.ADP) induced by exogenous MgADP has been reported to cooperatively promote myosin attachment to the thin filament. We hereby conclude that the increase in the number of force-generating crossbridges on a decrease in the lattice spacing is masked by the cooperative effect of AM.ADP, resulting in depressed SL-dependent tension generation.

The filament lattice of striated muscle

The filament lattice of striated muscle is an overlapping hexagonal array of thick and thin filaments within which muscle contraction takes place. Its structure can be studied by electron microscopy or X-ray diffraction. With the latter technique, structural changes can be monitored during contraction and other physiological conditions. The lattice of intact muscle fibers can change size through osmotic swelling or shrinking or by changing the sarcomere length of the muscle. Similarly, muscle fibers that have been chemically or mechanically skinned can be compressed with bathing solutions containing very large inert polymeric molecules. The effects of lattice change on muscle contraction in vertebrate skeletal and cardiac muscle and in invertebrate striated muscle are reviewed. The force developed, the speed of shortening, and stiffness are compared with structural changes occurring within the lattice. Radial forces between the filaments in the lattice, which can include electrostatic, Van der Waals, entropic, structural, and cross bridge, are assessed for their contributions to lattice stability and to the contraction process.

The tension response to stretch of intact skeletal muscle fibres of the frog at varied tonicity of the extracellular medium

Ramp stretches (2-3% of the fibre length; velocity < 0.3 fibre lengths per s) were imposed on tetanically stimulated skeletal muscle fibres from the frog (1.4-3.0 degrees C; sarcomere length 2.1-2.2 microns). The fibre was immersed in normal Ringer solution (osmotic strength 1.00 R) or in solutions made hypotonic by reduction of the sodium concentration (0.81 R) or hypertonic by addition of different amounts of sucrose (1.22 R and 1.44 R). The shape of the force response to stretch was similar at the different tonicity levels but the force enhancement (E) during stretch was significantly increased by raised tonicity and reduced by lowered tonicity. Since the steady state isometric force (T0), in contrast to E, is markedly reduced by raised tonicity and increased by lowered tonicity the total force during stretch (T0 + E) was little affected by changes in tonicity. After the end of stretch tension decayed towards the isometric level with a time course that could be approximated by a double exponential function. The rate constant of both the fast (t1/2 approximately 10 ms) and the slow (t1/2 approximately 300 ms) exponential process was reduced by increased tonicity suggesting reduced rate of crossbridge detachment. The different effects of varied tonicity on T0 and on E would be consistent with the idea that varied tonicity affects the distribution between high-force and low-force crossbridge states during isometric contraction but not during stretch. The effect may be simulated by assuming that increased tonicity reduces the difference in the strength of binding (energy of binding) between the two attached crossbridge states in the model of Huxley and Simmons.

Simple harmonic motion of tropomyosin: proposed mechanism for length-dependent regulation of muscle active tension

A simple harmonic theory is proposed to describe the regulatory mechanism of tropomyosin in the activation of muscle contraction. The theory proposes that activation-associated displacement of tropomyosin is inherent to tropomyosin, a consequence of the molecule's large-scale vibrational motion (5-10 A root mean square displacement). In association with thin filament the vibrational motion may become less complex, approaching the ideal case of simple harmonic motion. The degree of activation increases as the amplitude of the simple harmonic motion increases, causing tropomyosin to shorten lengthwise, shiftings its position from the periphery of thin filament (OFF) to the actin groove (ON). The amplitude may be regulated in a rectilinear manner by the thick filament electrostatic force, the thin filament hydrophobic force, and the Ca(2+)-dependent force of the troponin complex. The radial and tangenital components of the resultant force may vary as the muscle is stretched, regulating maximum active tension and Ca2+ sensitivity, respectively. This may represent the molecular basis for Starling's law of the heart. The mechanism may be important for describing the regulatory mechanism of tropomyosin in smooth muscle and nonmuscle cells and may facilitate a clinically relevant understanding of the effects of pH, Mg2+ concentration, ionic strength, and ethanol on the regulation of active tension.

Myofilament spacing and force generation in intact frog muscle fibres

1. The relation between sarcomere length and steady tetanic tension was determined at 10-12 degrees C for 70-80 microns long length-clamped segments of single fibres isolated from the tibialis anterior muscle of the frog, in normal and hypertonic or hypotonic Ringer solutions. 2. The tension depression and potentiation observed in hypertonic and hypotonic Ringers solutions varied with sarcomere length, so that, as opposed to myofilament overlap predictions, the optimum length for tension development was shorter in hypertonic Ringer solution and longer in hypotonic Ringer solution than in normal Ringer solution. As the fibres were stretched from 1.96 to 2.24 microns sarcomere length, both tension depression in hypertonic Ringer solution and tension potentiation in hypotonic Ringer solution increased by 9 and 5%, respectively. 3. Within this range of sarcomere lengths the length-stiffness relation in hypotonic and in hypertonic Ringer solutions exhibit little or no change relative to that in normal Ringer solution. 4. The results indicate that separation between the thick and the thin myofilaments influences the mechanism of force generation. There is an optimum interfilament distance (10-12 nm surface to surface between the thick and the thin filaments) for tension production. In isotonic Ringer solution, this corresponds to the interfilament distance at sarcomere lengths around 2.10 microns. The force per attached cross-bridge, rather than their number, appears to decrease as the interfilament distance is brought above or below the optimum length. Even if this effect is moderate in isotonic Ringer solution, it should be taken into account in models of the force-generation mechanism.

Diffraction ellipsometry studies of osmotically compressed muscle fibers

Microstructural features of relaxed, skinned muscle fibers compressed with polyvinylpyrrolidone were examined by optical diffraction ellipsometry. This technique is sensitive to the optical anisotropy within the muscle, including that due to intrinsic properties of the protein molecules as well as that due to the regular arrangement of proteins in the surrounding medium. The change in polarization state of light after interacting with the muscle is described by the differential field ratio (DFR) and birefringence (delta n). Compression of single fibers (sarcomere length = 2.6 microns) with 0%-21% polyvinylpyrrolidone caused an increase of up to 23% and 31% for DFR and delta n, respectively. The largest increase in both parameters occurred at intermediate sarcomere lengths. Theoretical modelling of the results suggest that the average S-1 tilt angle may be reduced upon compression of the filament lattice. This is supported by experiments in which S-1 was enzymatically cleaved with alpha-chymotrypsin. Separate experiments comparing fibers with intact membranes and skinned fibers compressed to an equivalent lattice spacing showed little difference in DFR or delta n.

The effects of tonicity on tension and stiffness of tetanized skeletal muscle fibres of the frog

Tension and stiffness of tetanically activated skeletal muscle fibres of the frog were studied at varied tonicity of the extracellular medium (1.7-3.2 degrees C; sarcomere length, 2.13-2.22 microns). The stiffness was measured from the change in peak tension in response to fast (0.2 ms) stretches and releases of small amplitude (0.11-0.15% of the fibre length). The bathing solution was made hypotonic by reduction of NaCl and hypertonic by addition of sucrose. The osmotic strength of the solutions tested varied from 81 to 168% of the isotonic value. Maximum tetanic tension decreased markedly with increased tonicity. The active stiffness, on the other hand, increased as the tonicity was raised, and the tension/stiffness ratio (the total extension of the undamped fibre elasticity) was thus greatly reduced under these conditions. Evidence is presented to show that the change in the tension/stiffness ratio is due neither to the development of rigor cross-bridges nor to the recruitment of passive parallel-elastic elements in response to increased tonicity. Neither are viscous-like components important for explaining the effect. A change in the tension/stiffness ratio, similar to that seen in response to increased tonicity, did not occur as fibre width was reduced by increasing the sarcomere length. This suggests that the changes in the fibre volume affect this ratio mainly by mechanisms that are unrelated to changes in lateral spacing between the myofilaments.

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. It is concluded that properties of the force generators change with sarcomere length. This is not anticipated by the cross-bridge model of muscle contraction.

Filament lattice of frog striated muscle. Radial forces, lattice stability, and filament compression in the A-band of relaxed and rigor muscle

Repulsive pressure in the A-band filament lattice of relaxed frog skeletal muscle has been measured as a function of interfilament spacing using an osmotic shrinking technique. Much improved chemical skinning was obtained when the muscles were equilibrated in the presence of EGTA before skinning. The lattice shrank with increasing external osmotic pressure. At any specific pressure, the lattice spacing in relaxed muscle was smaller than that of muscle in rigor, except at low pressures where the reverse was found. The lattice spacing was the same in the two states at a spacing close to that found in vivo. The data were consistent with an electrostatic repulsion over most of the pressure range. For relaxed muscle, the data lay close to electrostatic pressure curves for a thick filament charge diameter of approximately 26 nm, suggesting that charges stabilizing the lattice are situated about midway along the thick filament projections (HMM-S1). At low pressures, observed spacings were larger than calculated, consistent with the idea that thick filament projections move away from the filament backbone. Under all conditions studied, relaxed and rigor, at short and very long sarcomere lengths, the filament lattice could be modeled by assuming a repulsive electrostatic pressure, a weak attractive pressure, and a radial stiffness of the thick filaments (projections) that differed between relaxed and rigor conditions. Each thick filament projection could be compressed by approximately 5 or 2.6 nm requiring a force of 1.3 or 80 pN for relaxed and rigor conditions respectively.

Thin filament regulation of shortening velocity in rat skinned skeletal muscle: effects of osmotic compression

1. Maximum shortening velocity (Vmax) was examined in skinned single fibres from rat slow-twitch soleus muscles at various degrees of Ca2+ activation of the thin filament, in the presence and absence of osmotic compression induced by 5% dextran. 2. At maximal levels of Ca2+ activation, Vmax remained constant as the extent of shortening was varied, with values averaging 1.43 +/- 0.05 muscle lengths/s (mean + S.E.M., n = 13). When thin filament activation was reduced by lowering the concentration of Ca2+, unloaded shortening consisted of an initial high-velocity phase for extents of shortening in the range 20-80 nm/half-sarcomere, and a subsequent low-velocity phase for greater extents of shortening. 3. In the absence of dextran, Vmax in the high-velocity phase of shortening was relatively invariant over a wide range of activation; however, at very low levels of activation, yielding tensions less than 5% of the peak value. Vmax declined precipitously. In contrast, fibres compressed radially with dextran to diameters comparable to those of intact fibres demonstrated a marked decrease in Vmax in the high-velocity phase when thin filament activation was varied over a wide range. These findings are consistent with the idea that cross-bridges in the expanded filament lattice of skinned fibres do not bear as great an axial compressive force as in intact fibres (Goldman & Simmons, 1986; Goldman, 1987) but instead buckle as the fibre shortens. 4. The value of Vmax in the low-velocity phase of shortening decreased as thin filament activation was reduced in both control and osmotically compressed fibres. The low-velocity phase, which occurred only at reduced levels of thin filament activation, may be a manifestation of a population of slowly dissociating crossbridges which with shortening become negatively strained and oppose contraction.

Shortening velocity in skinned single muscle fibers. Influence of filament lattice spacing

In this study maximum shortening velocity (Vmax) and isometric tension (P0) in skinned single fibers from rat slow soleus (SOL) and fast superficial vastus lateralis (SVL) muscles were examined after varying degrees of filament lattice compression with dextran. In both fiber types Vmax was greatest in the absence of dextran and decreased as the concentration of dextran was increased between 2.5 and 10 g/100 ml. At 10% dextran, which compressed fiber width by 31-38%, Vmax relative to the initial 0% dextran value was 0.28 +/- 0.03 (mean +/- SE) and 0.26 +/- 0.02 in SVL and SOL fibers, respectively. The effect of compression to depress Vmax was reversed completely by returning the fiber to 0% dextran. The force-generating capability of skinned fibers was not as sensitive to variations in cell width. In both the SOL and SVL fibers P0 increased by 3-7% when the concentration of dextran was increased from 0 to 5%. Further compression of lattice volume with 10% dextran resulted in a 8-13% decline in P0 relative to the initial value. While the precise mechanism by which filament lattice spacing modulates contractile function is not known, our results suggest that the major effect is upon the rate constant for cross-bridge detachment.

Donnan potentials from striated muscle liquid crystals. Lattice spacing dependence

Electrochemical potentials were measured as a function of myofilament packing density in crayfish striated muscle. The A-band striations are supramolecular smectic B1 lattice assemblies of myosin filaments and the I-band striations are nematic liquid crystals of actin filaments. Both A- and I-bands generate potentials derived from the fixed charge that is associated with structural proteins. In the reported experiments, filament packing density was varied by osmotically reducing lattice volume. The electrochemical potentials were measured from the A- and I-bands in the relaxed condition over a range of lattice volumes. From the measurements of relative cross-sectional area, unit-cell volume (obtained by low-angle x-ray diffraction) and previously determined effective linear charge densities (Aldoroty, R.A., N.B. Garty, and E.W. April, 1985, Biophys. J., 47:89-96), Donnan potentials can be predicted for any amount of compression. In the relaxed condition, the predicted Donnan potentials correspond to the measured electrochemical potentials. In the rigor condition, however, a net increase in negative charge associated with the myosin filament is observed. The predictability of the data demonstrates the applicability of Donnan equilibrium theory to the measurement of electrochemical potentials from liquid-crystalline systems. Moreover, the relationship between filament spacing and the Donnan potential is consistent with the concept that surface charge provides the necessary electrostatic force to stabilize the myofilament lattice.