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1 Imperial College London, Division of Biomedical Sciences, Biological Nanoscience Section, SAF Building, London SW7 2AZ, UK
We measured the effects of ionic strength (IS), 200 (standard) and 400 mmol l–1 (high), on force and ATP hydrolysis during isometric contractions of permeabilized white fibres from dogfish myotomal muscle at their physiological temperature, 12°C. One goal was to test the validity of our kinetic scheme that accounts for energy release, work production and ATP hydrolysis. Fibres were activated by flash photolysis of the P3-1-(2 nitrophenyl) ethyl ester of ATP (NPE-caged ATP), and time-resolved phosphate (Pi) release was detected with the fluorescent protein MDCC-PBP, N-(2[1-maleimidyl]ethyl)-7-diethylamino-coumarin-3-carboxamide phosphate binding protein. High IS slowed the transition from rest to contraction, but as the fibres approached the isometric force plateau they showed little IS sensitivity. By 0.5 s of contraction, the force and the rate of Pi release at standard and high IS values were not significantly different. A five-step reaction mechanism was used to account for the observed time courses of force and Pi release in all conditions explored here. Only the rate constants for reactions of ATP, ADP and Pi with the contractile proteins varied with IS, thus suggesting that the actin–myosin interactions are largely non-ionic. Our reaction scheme also fits previous results for intact fibres.
(Received 22 March 2005;
accepted after revision 14 July 2005;
first published online 21 July 2005)
Corresponding author T. West: Division of Biomedical Sciences, Biological Nanoscience, Sir Alexander Fleming Building, Imperial College London, London SW7 2AZ, UK. Email: t.west{at}imperial.ac.uk
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