Fig 1.
Computed Changes of ATP concentration gradient across the cross-section of a single muscle fiber.
(A) Change of ATP concentration C gradient at various times after transfer of a single muscle fiber (diameter, 50μ m) from contracting solution to ATP-free rigor solution. (B) Change of ATP concentration gradient at various times after transfer of the fiber from rigor solution to relaxing solution containing 4mM ATP. In both A and B, ATP concentrations (mM) in logarithmic scale are plotted against distances r (μm) from the center of the fiber. Calculations were performed by the equation, dC(r) / dt = D [d2C(r) / dr2 + 1/r • dC(r) / dt, where C(r) is ATP concentration as a function of r.
Fig 2.
Stiffness and tension changes when a skinned fiber is put into high-Ca2+ rigor state.
The fiber was first activated maximally in contracting solution, and then transferred into high-Ca rigor solution. Arrows indicate timing of solution exchanges. In this and Fig 3, upper traces show changes in in-phase stiffness and quadrature stiffness, while lower trace shows tension. The tension increment on application of rigor solution is an artefact accompanying solution exchange procedure.
Fig 3.
Stiffness and tension changes when a skinned fiber was put into low-Ca rigor state.
The fiber was transferred from relaxing solution to low-Ca rigor solution.
Fig 4.
Tension responses of skinned fibers in high-Ca rigor state to repeated application of ramp-shaped releases.
Ramp-shaped releases (amplitude, 0.5% of Lo; duration, 5ms) were repeatedly applied to the fibers in high-Ca rigor state at appropriate intervals. Note that, following each release, initial elastic drop of tension is followed by distinct tension redevelopment, i.e. tension recovery to a steady level. Downward and upward arrows indicate times of application of contracting and rigor solutions, respectively. In this and Fig 7, downward and upward arrowheads at the bottom of tension records indicate times of application of release and restretch, respectively.
Fig 5.
Diagram illustrating method of estimating amplitude of tension recovery.
Upper and lower traces show length and tension changes of the fiber, respectively. The fiber is activated maximally in contracting solution, and transferred to high-Ca rigor solution. On application of release, the initial elastic tension drop from To to T1 is followed by the subsequent tension recovery from T1 to T2. The amplitude of tension recovery relative to To is expressed as Trec = (T2–s1) / To.
Fig 6.
Changes in amplitude of tension recovery and tension immediately before release in high-Ca rigor solution.
The fiber was subjected to repeated releases. Values of Trec (filled circles) and the tension immediately before release To (open circles) are plotted against time in high-Ca rigor solution. Tension changes in the fiber in response to the first to the ninth release-restretch cycles are shown in the inset. Note that Trec decreases by ~65% for the first 10min in rigor solution, and then remains almost unchanged over many minutes until it eventually disappears, while To decreases continuously with time in rigor solution.
Fig 7.
Tension response of skinned fibers in low-Ca rigor state.
(A) Tension recovery following repeated releases (amplitude, 0.5% of Lo; duration, 5ms). Note that tension recovery takes place from negative tension below zero tension baseline. (B) Disappearance of tension recovery following repeated releases (amplitude, 0.5% of Lo; duration, 5ms) in the presence of 10mM EDTA. Note that the tension drop coincident with applied release is not followed by tension recovery. (C) Disappearance of tension recovery following repeated releases (amplitude, 0.3% of Lo; duration, 5ms) in the presence of 10mM EDTA. In A, B and C, downward arrow indicates time of application of Low-Ca rigor solution (pCa, > 9), while horizontal broken line indicates zero tension baseline.
Fig 8.
Diagrams showing possible mechanism of tension recovery following release in high-Ca rigor fibers.
(A) A myosin head, consisting of catalytic (CAD), converter (CD), and lever arm (LD) domains, extends from myosin filament to attach an actin monomer (shaded) in actin filament, while preserving its configuration at the end of power stroke. Arrow indicated direction of power stroke. (B) On application of release, myosin head is displaced in direction indicated by arrow, producing elastic drop in rigor tension. (C) After completion of release, limited degree of elastic restoration takes place in distorted myosin head, to show up as tension recovery.