Journal of Muscle Research & Cell Motility

, Volume 12, Issue 1, pp 53–60 | Cite as

The apparent rate constant for the dissociation of force generating myosin crossbridges from actin decreases during Ca2+ activation of skinned muscle fibres

  • W. Glenn L. Kerrick
  • James D. Potter
  • Phyllis E. Hoar


The effect of Ca2+ activation on the apparent rate constant governing the dissociation of force generating myosin cross-bridges was studied in skinned rabbit adductor magnus fibres (fast-twitch) at 21±1 °C. Simultaneous measurements of Ca2+-activated isometric force and ATPase activity were conducted in parallel with simultaneous measurements of DANZ-labelled troponin C (TnCDANZ) fluorescence and isometric force in fibres whose endogenous troponin C had been partially replaced with TnCDANZ. The Ca2+ activation of isometric force occurred at approximately two times higher Ca2+ concentration than did actomyosin ATPase activity at 2.0 mM MgATP. Since increases in both TnCDANZ fluorescence and ATPase activity occurred over approximately the same Ca2+ concentration range at substantially lower concentrations of Ca2+ than did force, this data suggests that the TnCDANZ fluorescence is associated with the Ca2+ activation of myosin crossbridge turnover (ATPase) rather than force. According to the model of Huxley (1957) and assuming the hydrolysis of one molecule of ATP per cycle of the crossbridge, the apparent rate constantgapp for the dissociation of force generating myosin crossbridges is proportional to the actomyosin ATPase/isometric force ratio. This measure ofgapp shows approximately a fivefold decrease during Ca2+ activation of isometric force. This change ingapp is responsible for separation of the Ca2+ sensitivity of the normalized ATPase activity and isometric force curves. If the MgATP concentration is reduced to 0.5 mM, the change ingapp is reduced and consequently the difference in Ca2+ sensitivity between normalized steady state ATPase and force is also reduced.


Steady State Muscle Fibre ATPase Activity Simultaneous Measurement Isometric Force 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

The abbreviations used are


troponin C


5-dimethylaminonapthalene-2-sulphonyl aziridine


DANZ-labelled TnC


the number of half sarcomeres


the cross-sectional area of the fibre




ethyleneglycolbis-(betaaminoethyl ether)-N,N,N′,N′-tetraacetic acid


force a muscle fibre develops


apparent rate of formation of force generating myosin crossbridges


the average force per myosin head


steady-state fraction of cycling myosin crossbridges in the force generating state


apparent rate of dissociation of force generating myosin crossbridges


the length of a half sarcomere


lactate dehydrogenase


is the concentration of myosin per fibre volume


nicotinamide adenine dinucleotide


reduced form of NAD


-log10 of the free Ca2+ concentration


phosphenol pyruvate


pyruvate kinase


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Copyright information

© Chapman and Hall Ltd 1991

Authors and Affiliations

  • W. Glenn L. Kerrick
    • 1
    • 2
  • James D. Potter
    • 2
  • Phyllis E. Hoar
    • 1
  1. 1.Departments of Physiology & BiophysicsUniversity of Miami School of MedicineMiami
  2. 2.Departments of Molecular & Cellular PharmacologyUniversity of Miami School of MedicineMiami

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