Influence of a strong-binding myosin analogue on calcium-sensitive mechanical properties of skinned skeletal muscle fibers.
The ability of strong-binding myosin heads to activate the thin filament was investigated by incubating skinned single muscle fibers with N-ethylmaleimide-(NEM) modified myosin subfragment-1 (S1). Isometric force was influenced in a complex manner: during maximal calcium activation, NEM-S1 inhibited force with half-maximal inhibition at 20 microM while at submaximal calcium, NEM-S1 potentiated force with greatest effect at 6 microM. When fibers were treated with NEM-S1 (4-8 microM), the tension-pCa (-log [Ca2+]) relationship became less steep (i.e. the Hill coefficient decreased from 5.4 to 3.0 upon treatment with NEM-S1), but the midpoint was unchanged. These results support the idea that strong binding of intrinsic heads contributes to the cooperativity observed in Ca2+ activation of force. The NEM-S1-induced increase in force at low Ca2+ was associated with an acceleration of a kinetic transition, and this transition was activated to near maximum while force was not. The rate of force redevelopment following restretch (ktr) at submaximal calcium was increased by NEM-S1 in a concentration-dependent manner, yielding a maximum rate at low [Ca2+] which was similar to that observed during full activation. The effects of NEM-S1 on force and ktr indicate that strong-binding myosin cross-bridges are involved in activation of the thin filament.
Swartz
D R
DR
Department of Physiology, University of Wisconsin, Madison 53706.
Moss
R L
RL
D000818:Animals; D002118:Calcium / Q000502:physiology*; D005033:Ethylmaleimide / Q000494:pharmacology; D007537:Isometric Contraction; D008856:Microscopy, Fluorescence; D009132:Muscles / Q000187:drug effects* / Q000502:physiology; D009218:Myosins / Q000031:analogs & derivatives* / Q000494:pharmacology; D011817:Rabbits
D009218:Myosins; D005033:Ethylmaleimide; D002118:Calcium