Intrinsic shortening speed of temperature-jump-activated intact muscle fibers. Effects of varying osmotic pressure with sucrose and KCl

Osmolality- and Na+-dependent effects of hyperosmotic NaCl solution on contractile activity and Ca2+ cycling in rat ventricular myocytes

Hypertonic NaCl solutions have been used for small-volume resuscitation from hypovolemic shock. We sought to identify osmolality- and Na(+)-dependent components of the effects of the hyperosmotic NaCl solution (85 mOsm/kg increment) on contraction and cytosolic Ca(2+) concentration ([Ca(2+)](i)) in isolated rat ventricular myocytes. The biphasic change in contraction and Ca(2+) transient amplitude (decrease followed by recovery) was accompanied by qualitatively similar changes in sarcoplasmic reticulum (SR) Ca(2+) content and fractional release and was mimicked by isosmotic, equimolar increase in extracellular [Na(+)] ([Na(+)](o)). Raising osmolality with sucrose, however, augmented systolic [Ca(2+)](i) monotonically without change in SR parameters and markedly decreased contraction amplitude and diastolic cell length. Functional SR inhibition with thapsigargin abolished hyperosmolality effects on [Ca(2+)](i). After 15-min perfusion, both hyperosmotic solutions slowed mechanical relaxation during twitches and [Ca(2+)](i) decline during caffeine-evoked transients, raised diastolic and systolic [Ca(2+)](i), and depressed systolic contractile activity. These effects were greater with sucrose solution, and were not observed after isosmotic [Na(+)](o) increase. We conclude that under the present experimental conditions, transmembrane Na(+) redistribution apparently plays an important role in determining changes in SR Ca(2+) mobilization, which markedly affect contractile response to hyperosmotic NaCl solutions and attenuate the osmotically induced depression of contractile activity.

Z/I and A-band lattice spacings in frog skeletal muscle: effects of contraction and osmolarity

A-band and Z-line/I-band lattice spacings were measured by small-angle X-ray diffraction from relaxed and isometrically-contracting whole frog sartorius muscles with lattice spacings reduced or swollen by changing the osmolarity of the bathing solution. A-band spacing increased by approximately 3% upon isometric contraction at reduced lattice spacings (245-356 mOsm) and decreased by approximately 1% at swollen spacings (172 mOsm), similarly to the behaviour of skinned muscles upon changing from the relaxed state to rigor. The Z/I lattice underwent a significant lattice expansion (3-8%) upon isometric contraction at all osmolarities, in qualitative agreement (but quantitative disagreement) with results from electron microscopy on mammalian skeletal muscle. Lattice areas calculated for the Z/I and A-band lattices indicate a barrel-shaped sarcomere in the resting state, which may provide a partial explanation for how longitudinal forces produced in the A-band can produce a radial expansive force in the Z-line during contraction. The radial component of cross-bridge stiffness was calculated from the A-band data for contracting muscle, using a lattice stability model incorporating structural, osmotic and electrostatic forces. The calculations gave a radial cross-bridge stiffness during contraction of about 9 x 10(5) N m-2, and outward radial force per thick filament in normal Ringer's solution of 6 x 10(-9) N, corresponding to a radial force per cross-bridge of 10(-11) N.

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.

Calcium regulation of skeletal muscle thin filament motility in vitro

Using an in vitro motility assay, we have investigated Ca2+ regulation of individual, regulated thin filaments reconstituted from rabbit fast skeletal actin, troponin, and tropomyosin. Rhodamine-phalloidin labeling was used to visualize the filaments by epifluorescence, and assays were conducted at 30 degrees C and at ionic strengths near the physiological range. Regulated thin filaments exhibited well-regulated behavior when tropomyosin and troponin were added to the motility solutions because there was no directed motion in the absence of Ca2+. Unlike F-actin, the speed increased in a graded manner with increasing [Ca2+], whereas the number of regulated thin filaments moving was more steeply regulated. With increased ionic strength, Ca2+ sensitivity of both the number of filaments moving and their speed was shifted toward higher [Ca2+] and was steepest at the highest ionic strength studied (0.14 M gamma/2). Methylcellulose concentration (0.4% versus 0.7%) had no effect on the Ca2+ dependence of speed or number of filaments moving. These conclusions hold for five different methods used to analyze the data, indicating that the conclusions are robust. The force-pCa relationship (pCa = -log10[Ca2+]) for rabbit psoas skinned fibers taken under similar conditions of temperature and solution composition (0.14 M gamma/2) paralleled the speed-pCa relationship for the regulated filaments in the in vitro motility assay. Comparison of motility results with the force-pCa relationship in fibers suggests that relatively few cross-bridges are needed to make filaments move, but many have to be cycling to make the regulated filament move at maximum speed.

The effect of hypertonicity on force generation in tetanized single fibres from frog skeletal muscle

1. We compared the tension transient that follows a step change in sarcomere length in normal Ringer solution with that in Ringer solution made hypertonic by the addition of 98 mM sucrose. Steps were applied on tetanized single muscle fibres during either the isometric plateau or the steady force response to lengthening at low speed. Sarcomere length was controlled on selected fibre segments by a striation follower. Analysis is limited to phase 1 (the tension change simultaneous with the length step, mainly due to cross-bridge elasticity) and phase 2 (the quick phase of tension recovery, a manifestation of the cross-bridge elementary force-generating process). 2. At the isometric tetanus plateau the steady force is reduced by 19% in hypertonic solution, and the stiffness is slightly increased. During slow lengthening both steady force and stiffness are similar in normal solution and in hypertonic solution. In hypertonic solution the tension-to-stiffness ratio, a measure of the mean cross-bridge extension before the step, is markedly reduced in isometric conditions (-23%), but not during lengthening (-2%). 3. The plots of instantaneous tension versus the length change during the step show that in hypertonic medium the elasticity of the fibre is almost undamped. Thus the increase in stiffness cannot be attributed to an increase in viscosity. 4. In isometric conditions (T2-T1)/(Ti-T1), the proportion of the initial tension drop recovered at the end of phase 2, is not affected by hypertonicity for releases of moderate and large size (> 2 nm) and is reduced for small releases (< 2 nm) and for stretches. The abscissa intercept of the relation (T2-T1)/(Ti-T1) versus step amplitude is the same in both media. During lengthening, for releases of small and moderate size, (T2-T1)/(Ti-T1) is 20% lower in hypertonic solution. For large releases the slope of the relation is lower so that the abscissa intercept is not changed. 5. The speed of quick tension recovery following a step length change imposed in isometric conditions is slightly depressed in hypertonic solution. The relation between speed of recovery and step amplitude maintains its shape and is shifted downwards. During lengthening, the speed of quick tension recovery in hypertonic solution is less dependent on step amplitude than in normal solution, as if a more linear viscoelasticity is responsible for a large fraction of residual recovery.(ABSTRACT TRUNCATED AT 400 WORDS)

Z-line/I-band and A-band lattices of intact frog sartorius muscle at altered interfilament spacing

Muscle contraction has long been known to be affected by the osmolarity of the bathing solution. Part of this effect is caused by changes in interfilament spacing in the A-band. We have investigated the variation in spacing of the square lattice of thin filaments within and near the Z-line (the Z-line/I-band or Z-I lattice) in intact frog sartorius muscle over a wide range of osmolarities and compared it with the corresponding changes in the A-band lattice. Both lattices have a lower limit for compression and an upper limit for swelling. The spacing of the Z-I lattice is nearly proportional to that of the A-band, but shows a 2-3% variation at extreme shrinkage or swelling. In normal intact muscle, the osmotically-inactive volume of both lattices is between 20 and 30%. These in vivo measurements of lattice spacing differ significantly from those observed in electron micrographs. With moderate variations in osmolarity, lattice spacing and muscle fibre width show similar behaviour, but at extreme osmolarities, the lattice spacing changes less than the fibre width. An equatorial reflection was observed in intact muscle, previously identified in skinned muscle, which does not index on the A-band and which changes with osmolarity in a manner different from that observed for the A-band and Z-I lattices. This reflection may arise from changes in the ordering of the Z-I lattice or may involve components additional to the thick and thin filaments.

Membrane-related thermo-osmotic effect as measured by medium conductivity in isotonic cell suspensions

Changes in temperature result in changes in cell-membrane permeability for water and solutes. At isotonic conditions this thermo-osmotic effect is relatively small and remains indistinguishable by the ordinary impedance methods. The thermo-osmotic effect can be investigated in cell suspensions in the beta-dispersion region (high-frequency electric field with an intensity of 100-300 V/cm, peak-to-peak). The current through the cell suspension of Nicotiana tabacum protoplasts (or human erythrocytes) is a nonlinear function of temperature jump and/or cooling rate. In experiments carried out in a microchamber with thermostatted electrodes at supra-zero temperatures quantitative data were obtained for the thermo-osmotic phenomenon in cell suspension as well as for individual cells.

Behaviour of the crossbridges in stretched or compressed muscle fibres

The maximum chord of the myosin heads is comparable to the closest surface-to-surface spacing between the myofilaments in a muscle at the slack length. Therefore, when the sarcomere length increases or when the fibre is compressed, the surface-to-surface myofilament spacing becomes lower than the head long axis. We conclude that, in stretched or compressed fibres, some crossbridges cannot attach, owing to steric hindrance. When the amount of compression is limited, this hindrance may be overcome by a tilting of the heads in the plane perpendicular to the filament axes; in this case, there is no consequence as concerns the crossbridge properties. In highly compressed fibres, the crossbridges become progressively hindered and all the crossbridges are hindered for an axis-to-axis spacing representing about 60% of the spacing observed under zero external osmotic pressure. In this case, both the isometric tension and the ATPase activity of the fibre are zero. In fibres stretched up to 3.77 microns (sarcomere length corresponding to the disappearance of the overlap between the thick and the thin filaments), the ratio of hindered crossbridges over the functional crossbridges may be estimated at about 55%. In stretched fibres, a noticeable proportion of crossbridges are sterically hindered and the crossbridges performance (e.g. constants of attachment and detachment) depends on filament spacing, i.e. on sarcomere length. Therefore, we think it is probably impossible to consider the crossbridges as independent force converters, since this idea requires that the crossbridge properties are independent of sarcomere length. In this connection, all the experiments performed on osmotically compressed fibres are of major importance for the understanding of the true mechanisms of muscle contraction.

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.

Measurement of sarcomere shortening in skinned fibers from frog muscle by white light diffraction

A new optical-electronic method has been developed to detect striation spacing of single muscle fibers. The technique avoids Bragg-angle and interference-fringe effects associated with laser light diffraction by using polychromatic (white) light. The light is diffracted once by an acousto-optical device and then diffracted again by the muscle fiber. The double diffraction reverses the chromatic dispersion normally obtained with polychromatic light. In frog skinned muscle fibers, active and passive sarcomere shortening were smooth when observed by white light diffraction, whereas steps and pauses occurred in the striation spacing signals obtained with laser illumination. During active contractions skinned fibers shortened at high rates (3-5 microns/s per half sarcomere, 0-5 degrees C) at loads below 5% of isometric tension. Compression of the myofibrillar lateral filament spacing using osmotic agents reduced the shortening velocity at low loads. A hypothesis is presented that high shortening velocities are observed with skinned muscle fibers because the cross-bridges cannot support compressive loads when the filament lattice is swollen.

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.

Ca2+ activation of troponin C-extracted vertebrate striated fast-twitch muscle fibers

To characterize the tension control in vertebrate striated muscle fibers, and to obtain insights into the cross-bridge mechanisms, Ca2+ activation on troponin C (TnC)-extracted skinned fibers was studied in standard (180 mM, physiological) and low (20-41 mM) ionic strength solutions. By tension measurement, TnC-extracted fibers had nearly lost their Ca2+ sensitivity in the standard ionic strength solutions, but surprisingly the fiber still exhibited significant tension on activation with Ca2+ in low ionic strength. Also, the presence of weak bridges (zero-force bridges) was inferred by stiffness measurements in Ca2+-free low ionic strength solution, and were found even after TnC extraction. The possibility is discussed that dual regulation by Ca2+ is present in the vertebrate muscle. One mechanism activates the thin filaments. The second may directly control the kinetic step for the transition between the weak and strong bridges, in the cross-bridge cycle in the fiber, and in this way may act as an additional Ca2+ switch.

Kinetics of force redevelopment in isolated intact frog fibers in solutions of varied osmolarity

Isolated intact frog muscle fibers, while shortening with the intrinsic maximal speed, were stretched back to the original length to measure the kinetics of force redevelopment. These kinetics give information on the attachment rate constant in the cross-bridge cycle in vivo, and a value of approximately 25.6 s-1 (0 degree C) is found in the present study. We find that these kinetics were slightly less sensitive to temperature than was the unloaded shortening speed. The effect of hyperosmolarity on force redevelopment was also measured in solutions with added sucrose or KCl. The rate constant was nearly halved with 120 mM sucrose, but there was practically no effect with isosmotic (60 mM) KCl. These results indicate that the rate constant of force redevelopment is insensitive to raised intracellular ionic strength. In sucrose, the fiber width was also compressed, and the attenuation of the rate constant of force redevelopment in this case is consequently attributed to the decrease in interfilament space. The order of magnitude of the rate constant found in this study suggests that tension transduction by a cross-bridge, during each turnover cycle, requires a series of elementary steps following the attachment.

Critical dependence of calcium-activated force on width in highly compressed skinned fibers of the frog

Force development by skinned frog semitendinosus fibers was studied at various levels of lateral compression to compare the results with intact fibers and to evaluate the limits on cross-bridge movements during isometric contraction. The skinned fibers were compressed osmotically using a high molecular weight polymer, dextran T500. Ca-activated force remained constant down to 58% of the fiber width (w0) after skinning, corresponding to a nearly twofold change in separation between the thin and thick filaments in the myofilament lattice. This agrees with the earlier result on intact fibers, and gives additional evidence that the cross-bridge mechanism for force generation is relatively insensitive to large changes in interfilament separation. Further compression, below 0.58 w0, produced a sharp drop in force, and the force was practically zero at a fiber width of 50%. The effect at high compression was the same at all pCa's, which indicates that the Ca sensitivity of the myofilaments is unaffected by radial compression. The stiffness of the fiber remained high in rigor in the presence of dextran, which indicates that the rigor cross-bridge attachment is not inhibited, and actually may be improved, with decreases in the interfilament space. Also, the drop in active force with the highest compression was similar when the compressed fibers were put in rigor before contraction, which suggests that the force drop also was not due to a hindrance to cross-bridge attachment. The results appear to exclude large motions such as tilting and rocking of the bridge as a rigid molecule, but suggest that at least some molecular movement is essential for force development; they also raise the possibility that there is a critical interfilament separation in the fiber, below which the cross-bridge cannot function.

Shortening velocity extrapolated to zero load and unloaded shortening velocity of whole rat skeletal muscle

The shortening velocity at zero load (Vmax) extrapolated from velocities measured during isotonic releases was compared with unloaded shortening velocity (V0) determined by the slack test. Experiments were performed in vitro at 20 degrees C on soleus muscles from rats. The Vmax was 3.2 +/- 0.1 (mean +/- S.E. of mean, n = 10) fibre lengths/s while the V0 determined for the same muscles was 5.0 +/- 0.1 fibre lengths/s. The ratio of V0/Vmax was 1.6 +/- 0.1. Soleus muscles of the rat are heterogeneous with respect to the intrinsic shortening velocities of their fibres. The results suggest that V0 is a measure of the unloaded shortening velocity of the fastest fibres whereas Vmax is a function of the force-velocity characteristics of all the fibres within a skeletal muscle preparation.