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

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Abstract
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 constantg app for the dissociation of force generating myosin crossbridges is proportional to the actomyosin ATPase/isometric force ratio. This measure ofg app shows approximately a fivefold decrease during Ca2+ activation of isometric force. This change ing app 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 ing app is reduced and consequently the difference in Ca2+ sensitivity between normalized steady state ATPase and force is also reduced.