Abstract
Although evidence has been presented that, at low ionic strength, myosin heads in relaxed skeletal muscle fibers form linkages with actin filaments, the effect of low ionic strength on contraction characteristics of Ca2+-activated muscle fibers has not yet been studied in detail. To give information about the mechanism of muscle contraction, we have examined the effect of low ionic strength on the mechanical properties and the contraction characteristics of skinned rabbit psoas muscle fibers in both relaxed and maximally Ca2+-activated states. By progressively decreasing KCl concentration from 125 mM to 0 mM (corresponding to a decrease in ionic strength μ from 170 mM to 50 mM), relaxed fibers showed changes in mechanical response to sinusoidal length changes and ramp stretches, which are consistent with the idea of actin-myosin linkage formation at low ionic strength. In maximally Ca2+-activated fibers, on the other hand, the maximum isometric force increased about twofold by reducing KCl concentration from 125 to 0 mM. Unexpectedly, determination of the force-velocity curves indicated that, the maximum unloaded shortening velocity Vmax, remained unchanged at low ionic strength. This finding indicates that the actin-myosin linkages, which has been detected in relaxed fibers at low ionic strength, are broken quickly on Ca2+ activation, so that the linkages in relaxed fibers no longer provide any internal resistance against fiber shortening. The force-velocity curves, obtained at various levels of steady Ca2+-activated isometric force, were found to be identical if they are normalized with respect to the maximum isometric force. The MgATPase activity of muscle fibers during isometric force generation was found not to change appreciably at low ionic strength despite the two-fold increase in Ca2+-activated isometric force. These results can be explained in terms of enhancement of force generated by individual myosin heads, but not by any changes in kinetic properties of cyclic actin-myosin interaction.
Highlights
It is generally believed that the mechanical activity of vertebrate skeletal muscle is regulated by binding of Ca2+ to troponin on the thin filaments, which produces a shift in the position of tropomyosin on the thin filaments
The present experiments were undertaken to give information about the mechanism of muscle contraction by thoroughly examining the effect of low ionic strength on mechanical properties and contraction characteristics in skinned rabbit psoas muscle fibers at rest and during Ca2+activated mechanical activity.Here we report that, when the KCl concentration was reduced progressively from 125 mM to 0 mM, (1) relaxed fibers show mechanical responses to sinusoidal length changes and ramp stretches consistent with possible formation of actin-myosin linkages, (2) the magnitude of steady Ca2+-activated isometric force increases about twofold, (3)
The stiffness in relaxed muscle fibers was measured when KCl concentration of relaxing solution was progressively decreased from the standard value of 125 mM to 0 mM
Summary
It is generally believed that the mechanical activity of vertebrate skeletal muscle is regulated by binding of Ca2+ to troponin on the thin filaments, which produces a shift in the position of tropomyosin on the thin filaments. In accordance with the above in vitro biochemical results, relaxed rabbit psoas muscle fibers at low ionic strength show substantial stiffness in response to rapid stretches [4], as well as Xray diffraction patterns analogous to that of rigor state [5,6]. These results indicate that, in relaxed fibers, myosin heads form linkages with actin at low ionic strength. The present experiments were undertaken to give information about the mechanism of muscle contraction by thoroughly examining the effect of low ionic strength on mechanical properties and contraction characteristics in skinned rabbit psoas muscle fibers at rest and during Ca2+activated mechanical activity.Here we report that, when the KCl concentration was reduced progressively from 125 mM to 0 mM (corresponding to a decrease in ionic strength mfrom 170 mM to 50 mM), (1) relaxed fibers show mechanical responses to sinusoidal length changes and ramp stretches consistent with possible formation of actin-myosin linkages, (2) the magnitude of steady Ca2+-activated isometric force increases about twofold, (3)
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