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1 Imperial College London, Division of Biomedical Sciences, Biological Structure & Function Section, SAF-Building, London SW7 2AZ2 National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA3 King's College London, School of Biomedical Science, New Hunt's House, Guy's Campus, London SE1 1UL4 UCL Institute of Human Performance, Royal National Orthopaedic Hospital Trust, Brockley Hill, Stanmore HA7 4LP, UK
Energy turnover was measured during isometric contractions of intact and Triton-permeabilized white fibres from dogfish (Scyliorhinus canicula) at 12°C. Heat + work from actomyosin in intact fibres was determined from the dependence of heat + work output on filament overlap. Inorganic phosphate (Pi) release by permeabilized fibres was recorded using the fluorescent protein MDCC-PBP, N-(2-[1-maleimidyl]ethyl)-7-diethylamino-coumarin-3 carboxamide phosphate binding protein. The steady-state ADP release rate was measured using a linked enzyme assay. The rates decreased five-fold during contraction in both intact and permeabilized fibres. In intact fibres the rate of heat + work output by actomyosin decreased from 134 ±S.E.M. 28 µW 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 permeabilized 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 courses 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 permeabilized fibres.
(Received 9 September 2003;
accepted after revision 14 October 2003;
first published online 17 October 2003)
Corresponding author N. A. Curtin: Biological Structure & Function Section, Division of Biomedical Sciences, Sir Alexander Fleming Building, Imperial College London, South Kensington Campus, London SW7 2AZ. Email: n.curtin{at}imperial.ac.uk
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