title: The ATP hydrolysis and phosphate release steps control the time course of force development in rabbit skeletal muscle.
abstract: The time course of isometric force development following photolytic release of ATP in the presence of Ca(2+) was characterized in single skinned fibres from rabbit psoas muscle. Pre-photolysis force was minimized using apyrase to remove contaminating ATP and ADP. After the initial force rise induced by ATP release, a rapid shortening ramp terminated by a step stretch to the original length was imposed, and the time course of the subsequent force redevelopment was again characterized. Force development after ATP release was accurately described by a lag phase followed by one or two exponential components. At 20 degrees C, the lag was 5.6 +/- 0.4 ms (s.e.m., n = 11), and the force rise was well fitted by a single exponential with rate constant 71 +/- 4 s(-1). Force redevelopment after shortening-restretch began from about half the plateau force level, and its single-exponential rate constant was 68 +/- 3 s(-1), very similar to that following ATP release. When fibres were activated by the addition of Ca(2+) in ATP-containing solution, force developed more slowly, and the rate constant for force redevelopment following shortening-restretch reached a maximum value of 38 +/- 4 s(-1) (n = 6) after about 6 s of activation. This lower value may be associated with progressive sarcomere disorder at elevated temperature. Force development following ATP release was much slower at 5 degrees C than at 20 degrees C. The rate constant of a single-exponential fit to the force rise was 4.3 +/- 0.4 s(-1) (n = 22), and this was again similar to that after shortening-restretch in the same activation at this temperature, 3.8 +/- 0.2 s(-1). We conclude that force development after ATP release and shortening-restretch are controlled by the same steps in the actin-myosin ATPase cycle. The present results and much previous work on mechanical-chemical coupling in muscle can be explained by a kinetic scheme in which force is generated by a rapid conformational change bracketed by two biochemical steps with similar rate constants -- ATP hydrolysis and the release of inorganic phosphate -- both of which combine to control the rate of force development.
authors.affiliation: Randall Division of Cell and Molecular Biophysics, New Hunt's House, Guy's Campus, King's College London, London SE1 1UL, UK. john.sleep@kcl.ac.uk
authors.forename: John
authors.initials: J
authors.lastname: Sleep
authors.forename: Malcolm
authors.initials: M
authors.lastname: Irving
authors.forename: Kevin
authors.initials: K
authors.lastname: Burton
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism*; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002478:Cells, Cultured; D003198:Computer Simulation; D004548:Elasticity; D004735:Energy Transfer / Q000502:physiology; D006868:Hydrolysis; D007537:Isometric Contraction / Q000502:physiology*; D007700:Kinetics; D008297:Male; D008954:Models, Biological; D032342:Muscle Cells / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D010710:Phosphates / Q000378:metabolism*; D011817:Rabbits; D013314:Stress, Mechanical
chemical_list: D010710:Phosphates; D000255:Adenosine Triphosphate; D002118:Calcium