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
Papers

Summary

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.

Keywords

Steady State Muscle Fibre ATPase Activity Simultaneous Measurement Isometric Force 

The abbreviations used are

TnC

troponin C

DANZ

5-dimethylaminonapthalene-2-sulphonyl aziridine

TnCDANZ

DANZ-labelled TnC

a

the number of half sarcomeres

A

the cross-sectional area of the fibre

AP5A

P1,P5-di(adenosine-5′)pentaphosphate

EGTA

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

F

force a muscle fibre develops

fapp

apparent rate of formation of force generating myosin crossbridges

Fav

the average force per myosin head

Fs

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

gapp

apparent rate of dissociation of force generating myosin crossbridges

L1/2s′

the length of a half sarcomere

LDH

lactate dehydrogenase

[M]

is the concentration of myosin per fibre volume

NAD

nicotinamide adenine dinucleotide

NADH

reduced form of NAD

pCa

-log10 of the free Ca2+ concentration

PEP

phosphenol pyruvate

PK

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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