Disassembly kinetics of thick filaments in rabbit skeletal muscle fibers. Effects of ionic strength, Ca²⁺ concentration, pH, temperature, and cross-bridges on the stability of thick filament structure

The kinetics of dissociation from both ends of thick filaments in a muscle fiber was investigated by an optical diffraction method. The dissociation velocity of thick filaments at a sarcomere length of 2.75 μm increased with increasing the KCl concentration (from 60 mM to 0.5 M), increasing the pH value (from 6.2 to 8.0) or decreasing the temperature (from 25 to 5°C) in the presence of 10 mM pyrophosphate and 5 mM MgCl₂. Micromolar concentrations of Ca²⁺ suppressed the dissociation velocity markedly at shorter sarcomere lengths. The dissociation velocity, v, decreased as thick filaments became shorter, and v = -db/dt = v(o) exp(αb), where b is the length of the thick filament at time t and v(o) and α are constants. The v(o) value was largely dependent on the KCl concentration but the α value was not. The stiffness of a muscle fiber decreased nearly in proportion to the decrease of overlap between thick and thin filaments induced by the dissociation of thick filaments. This indicates that cross-bridges are uniformly distributed and contribute independently to the stiffness of a muscle fiber during the dissociation of thick filaments.