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

Osmosensation in TRPV2 dominant negative expressing skeletal muscle fibres

Increased plasma osmolarity induces intracellular water depletion and cell shrinkage (CS) followed by activation of a regulatory volume increase (RVI). In skeletal muscle, the hyperosmotic shock-induced CS is accompanied by a small membrane depolarization responsible for a release of Ca(2+) from intracellular pools. Hyperosmotic shock also induces phosphorylation of STE20/SPS1-related proline/alanine-rich kinase (SPAK). TRPV2 dominant negative expressing fibres challenged with hyperosmotic shock present a slower membrane depolarization, a diminished Ca(2+) response, a smaller RVI response, a decrease in SPAK phosphorylation and defective muscle function. We suggest that hyperosmotic shock induces TRPV2 activation, which accelerates muscle cell depolarization and allows the subsequent Ca(2+) release from the sarcoplasmic reticulum, activation of the Na(+) -K(+) -Cl(-) cotransporter by SPAK, and the RVI response. Increased plasma osmolarity induces intracellular water depletion and cell shrinkage followed by activation of a regulatory volume increase (RVI). In skeletal muscle, this is accompanied by transverse tubule (TT) dilatation and by a membrane depolarization responsible for a release of Ca(2+) from intracellular pools. We observed that both hyperosmotic shock-induced Ca(2+) transients and RVI were inhibited by Gd(3+) , ruthenium red and GsMTx4 toxin, three inhibitors of mechanosensitive ion channels. The response was also completely absent in muscle fibres overexpressing a non-permeant, dominant negative (DN) mutant of the transient receptor potential, V2 isoform (TRPV2) ion channel, suggesting the involvement of TRPV2 or of a TRP isoform susceptible to heterotetramerization with TRPV2. The release of Ca(2+) induced by hyperosmotic shock was increased by cannabidiol, an activator of TRPV2, and decreased by tranilast, an inhibitor of TRPV2, suggesting a role for the TRPV2 channel itself. Hyperosmotic shock-induced membrane depolarization was impaired in TRPV2-DN fibres, suggesting that TRPV2 activation triggers the release of Ca(2+) from the sarcoplasmic reticulum by depolarizing TTs. RVI requires the sequential activation of STE20/SPS1-related proline/alanine-rich kinase (SPAK) and NKCC1, a Na(+) -K(+) -Cl(-) cotransporter, allowing ion entry and driving osmotic water flow. In fibres overexpressing TRPV2-DN as well as in fibres in which Ca(2+) transients were abolished by the Ca(2+) chelator BAPTA, the level of P-SPAK(Ser373) in response to hyperosmotic shock was reduced, suggesting a modulation of SPAK phosphorylation by intracellular Ca(2+) . We conclude that TRPV2 is involved in osmosensation in skeletal muscle fibres, acting in concert with P-SPAK-activated NKCC1.

Kinetics of force recovery following length changes in active skinned single fibres from rabbit psoas muscle: analysis and modelling of the late recovery phase

Redevelopment of isometric force following shortening of skeletal muscle is thought to result from a redistribution of cross-bridge states. We varied the initial force and cross-bridge distribution by applying various length-change protocols to active skinned single fibres from rabbit psoas muscle, and observed the effect on the slowest phase of recovery ('late recovery') that follows transient changes. In response to step releases that reduced force to near zero ( approximately 8 nm (half sarcomere)(-1)) or prolonged shortening at high velocity, late recovery was well described by two exponentials of approximately equal amplitude and rate constants of approximately 2 s(-1) and approximately 9 s(-1) at 5 degrees C. When a large restretch was applied at the end of rapid shortening, recovery was accelerated by (1) the introduction of a slow falling component that truncated the rise in force, and (2) a relative increase in the contribution of the fast exponential component. The rate of the slow fall was similar to that observed after a small isometric step stretch, with a rate of 0.4-0.8 s(-1), and its effects could be reversed by reducing force to near zero immediately after the stretch. Force at the start of late recovery was varied in a series of shortening steps or ramps in order to probe the effect of cross-bridge strain on force redevelopment. The rate constants of the two components fell by 40-50% as initial force was raised to 75-80% of steady isometric force. As initial force increased, the relative contribution of the fast component decreased, and this was associated with a length constant of about 2 nm. The results are consistent with a two-state strain-dependent cross-bridge model. In the model there is a continuous distribution of recovery rate constants, but two-exponential fits show that the fast component results from cross-bridges initially at moderate positive strain and the slow component from cross-bridges at high positive strain.

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.

The role of troponin C in the length dependence of Ca(2+)-sensitive force of mammalian skeletal and cardiac muscles

1. Skinned fibre preparations of right ventricular trabeculae, psoas and soleus muscles from hamster and rabbit were activated by Ca2+ and the length dependencies of their pCa (-log [Ca2+])-force relationships were compared. 2. Ca2+ sensitivity of the myocardium was higher at 2.2-2.4 microns than that at 1.7-1.9 microns. The length dependence was at least twofold greater in cardiac muscle than in fast skeletal fibres at identical temperatures and salt concentrations. Slow-twitch fibres gave a response similar to that in the myocardium. 3. The effect of the troponin C (TnC) phenotype on the length dependence of Ca2+ sensitivity was measured on both fast skeletal fibres and cardiac muscle with TnC exchange in situ. The length-induced increase in Ca2+ sensitivity was found to be greater in the presence of cardiac TnC than with fast skeletal TnC. Thus the results indicate that a certain domain of TnC is specialized in this length function, and that this domain is different in the two phenotypes. 4. The possibility that the enhanced length dependence of Ca2+ sensitivity after cardiac TnC reconstitution was attributable to reduced TnC binding was excluded when the length dependence of partially extracted fast fibres was reduced to one-half the normal value after a 50% deletion of the native TnC. 5. Two recombinant forms of cardiac TnC (kindly provided by Dr John Putkey, Houston, TX, USA) were used next, to investigate the roles of two specific domains in TnC in the control of length dependence of Ca2+ sensitivity and in the contraction-relaxation switching of cardiac muscle: 6. Using mutant CBM1 [corrected], in which site 1 was modified such as to bind the 4th Ca2+ ion, as in skeletal TnC, the length-induced Ca2+ sensitivity in cardiac muscle was suppressed. The effect was intermediate between cardiac and skeletal TnCs under the same conditions. The pSr (-log [Sr2+])-force relationship of cardiac muscle was also measured. In the presence of the mutant, skinned trabeculae manifest pSr-activation curves identical to those of fast fibres. This indicates that the metal ion binding properties of site 1 in TnC modulate the regulatory action of site 2. 7. Using mutant CBM2A, in which site 2 was inactivated, the activation of cardiac muscle by both Ca2+ and Sr2+ ions was completely blocked. This is the expected result, since both regulatory sites were now inactive, regulatory site 1 being normally inactive in cardiac muscle.(ABSTRACT TRUNCATED AT 400 WORDS)

Hydrostatic compression in glycerinated rabbit muscle fibers

Glycerinated muscle fibers isolated from rabbit psoas muscle, and a number of other nonmuscle elastic fibers including glass, rubber, and collagen, were exposed to hydrostatic pressures of up to 10 MPa (100 Atm) to determine the pressure sensitivity of their isometric tension. The isometric tension of muscle fibers in the relaxed state (passive tension) was insensitive to increased pressure, whereas the muscle fiber tension in rigor state increased linearly with pressure. The tension of all other fiber types (except rubber) also increased with pressure; the rubber tension was pressure insensitive. The pressure sensitivity of rigor tension was 2.3 kN/m2/MPa and, in comparison with force/extension relation determined at atmospheric pressure, the hydrostatic compression in rigor muscle fibers was estimated to be 0.03% Lo/MPa. As reported previously, the active muscle fiber tension is depressed by increased pressure. The possible underlying basis of the different pressure-dependent tension behavior in relaxed, rigor, and active muscle is discussed.

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.