Accumulated strain mechanism for length determination of thick filaments in skeletal muscle. I. Experimental bases

The kinetics of dissociation of myosin from both ends of thick filaments in glycerinated skeletal muscle fibres and myofibrils was studied in the presence of MgATP by use of an optical diffraction method and phase-contrast microscopy. The dissociation velocity, v (=-d L/d t where L is the length of thick filaments at time t), increased with increasing KCl concentration (0.225 to 0.5 m), or increasing pH (6.5 to 8.0) but hardly changed with temperature (5 and 25° C), micromolar concentrations of Ca²⁺ or sarcomere length (2.4 and 2.75 μm). Over a wide range of filament length, the dissociation velocity could be expressed by v₀exp(αL), where v₀ and α are positive constants depending upon the dissociation condition. When the effects of crossbridge formation are minimized it was thus shown that the structural stability of thick filaments in a muscle fibre and a myofibril gradually decreases from the central part to the tips of the filaments. On the basis of these results we propose that the length of thick filaments is largely regulated by an accumulated strain mechanism in which the free energy of association of myosin molecules increases with filament length.