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Received September 9, 2003
Revised October 1, 2003
Accepted after revision October 14, 2003
1 Imperial College London
2 National Institute for Medical Research
3 King's College London
4 University College London
* To whom correspondence should be addressed. E-mail: n.curtin{at}imperial.ac.uk.
Energy turnover was measured during isometric contractions of intact and Triton-permeabilised white fibres from dogfish (Scyliorhinus canicula) at 12oC. Heat+work from actomyosin in intact fibres was determined from the dependence of heat+work output on filament overlap. Inorganic phosphate (Pi) release by permeabilised fibres was recorded using the fluorescent phosphate binding protein MDCC-PBP. The steady state ADP release rate was measured using a linked enzyme assay. The rates decreased five-fold during contraction in both intact and permeabilised fibres. In intact fibres the rate of heat+work output by actomyosin decreased from 134 ± + SEM 28 µ W.mg -1 (n=17) at 0.055 s to 42% of this value at 0.25 s, and to 20% at 3.5 s. The force remained constant between 0.25 and 3.5 s. Similarly in permeabilised fibres the Pi release rate decreased from 5.00 ± + 0.39 mmol.l - 1.s -1 at 0.055 s to 39% of this value at 0.25 s and to 19% at 0.5 s. The steady state ADP release rate at 15 s was 21% of the Pi rate at 0.055 s. Using a single set of rate constants, the time course of force, heat+work, and Pi release were described by an actomyosin model that took account of the transition from the initial state (rest or rigor) to the contracting state, shortening and the consequent work against series elasticity, and reaction heats. The model suggests that increasing Pi concentration slows the cycle in intact fibres, and that changes in ATP and ADP slow the cycle in permeabilised fibres.
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