Strong binding of myosin increases shortening velocity of rabbit skinned skeletal muscle fibres at low levels of Ca(2+).
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 Ca(2+) dependence of unloaded shortening velocity (V(o)) in the low-velocity phase was investigated by varying the level of thin filament activation with Ca(2+) 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 microM NEM-S1 eliminated the low-velocity phase of shortening but had no effect on the high-velocity phase of shortening during submaximal activation with Ca(2+), or on V(o) during maximal activation with Ca(2+). 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.
Swartz
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DR
Department of Anatomy, Indiana University School of Medicine, Indianapolis, IN 46202, USA.
Moss
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RL
D008841:Actin Cytoskeleton / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D003432:Cross-Linking Reagents / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005033:Ethylmaleimide / Q000494:pharmacology; D066298:In Vitro Techniques; D009119:Muscle Contraction / Q000187:drug effects / Q000502:physiology*; D018485:Muscle Fibers, Skeletal / Q000502:physiology*; D015879:Myosin Subfragments / Q000378:metabolism* / Q000494:pharmacology; D011485:Protein Binding / Q000502:physiology; D016658:Psoas Muscles / Q000166:cytology; D011817:Rabbits
D003432:Cross-Linking Reagents; D004791:Enzyme Inhibitors; D015879:Myosin Subfragments; D005033:Ethylmaleimide; D002118:Calcium