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Strong binding of myosin increases shortening velocity of rabbit skinned skeletal muscle fibres at low levels of Ca²⁺

Darl R. Swartz * and Richard L. Moss

1. At low levels of activation, unloaded shortening of skinned skeletal muscle fibres takes place in two phases: an initial phase of high-velocity shortening followed by a phase of low-velocity shortening. The basis for Ca²⁺ dependence of unloaded shortening velocity (V_o) in the low-velocity phase was investigated by varying the level of thin filament activation with Ca²⁺ and N-ethyl-maleimide myosin subfragment-1 (NEM-S1), a non-tension-generating, strong binding derivative of subfragment-1. V_o was measured with the slack-test method.
2. Treatment of skinned fibres with 5 µM NEM-S1 eliminated the low-velocity phase of shortening but had no effect on the high-velocity phase of shortening during submaximal activation with Ca²⁺, or on V_o during maximal activation with Ca²⁺.
3. Extensive washout of NEM-S1 from the treated fibres restored the low-velocity phase of shortening and returned low-velocity V_o to pre-treatment values.
4. The effect of NEM-S1 to increase low-velocity V_o can be explained in terms of a model in which strong binding myosin cross-bridges activate the thin filament to a state in which the rate of ADP release from the actin-myosin-ADP complex and the rate of cross-bridge detachment from actin are accelerated during unloaded shortening.

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