Cross-bridge stiffness in Ca(2+)-activated skinned single muscle fibres

Tension transients, in response to small and rapid length changes (completed within 40 microseconds), were obtained from skinned single frog muscle fibres incubated in activating solutions with varying concentrations of Ca2+. The first 2 ms of these transients were described by a linear model in which the fibre is regarded as a rod composed of infinitesimally small, identical segments containing a mass, one undamped elastic element and in the case of relaxed fibres two damped elastic elements in series, or in the case of activated fibres three such elastic elements in series. The stiffness of activated fibres, expressed in elastic constants or apparent elastic constants, increased with increasing concentrations of Ca2+. All the damped elastic constants that were necessary to describe the tension responses of activated fibres were proportional to isometric tension. However, the undamped elastic constant did not increase linearly with increasing isometric tension. Equatorial X-ray diffraction patterns were obtained from single frog muscle fibres under similar conditions as under which the tension transients were obtained. The filament spacing (d10) of Ca(2+)-activated single frog muscle fibres decreased with increasing isometric force, whereas the intensity ratio (I11/I10) increased linearly with increasing isometric force. From experiments in which dextran (MW 200,000 Da) was added, it followed that such a change in filament spacing would modify passive stiffness. The d10 value of relaxed fibres decreased and stiffness increased with increasing concentrations of the polymer dextran, whereas I11/I10 remained constant. The relation of stiffness and filament spacing with concentration of dextran was used to eliminate the effect of decreased filament spacing on stiffness of activated fibres. After correction for changes in filament spacing the undamped complicance C1, normalized to tension, was not constant, but increased with increasing isometric tension. If we assume that isometric tension is proportional to the number of force generating cross-bridges, this means that only part of the undamped compliance of activated fibres is located in the cross-bridges.