The ATP hydrolysis and phosphate release steps control the time course of force development in rabbit skeletal muscle

Abstract

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• 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• C than at 20 • 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.

Effect of pH on the rate of myosin head detachment in molluscan catch muscle: are myosin heads involved in the catch state?

“…This is because an appreciable elasticity of the contractile apparatus is expanded during the force rise (Huxley et al, 1994;Wakabayashi et al, 1994), and this is most likely associated with more than one cross-bridge cycle. This is suggested by a recent study (Sleep et al, 2005) showing that the force increase in an ATP step experiment has the same kinetics as the force redevelopment following a period of isotonic shortening. The latter was shown to be correlated with ATPase activity (Brenner and Eisenberg, 1986).…”

Temperature change as a probe of muscle crossbridge kinetics: a review and discussion

“…An explanation for the relatively slow response in the phosphate-step experiments has been proposed by Sleep et al (2005;Smith & Sleep 2004). They suggest that the rapid 'force-generating event' (reaction 3 in model 1) is followed by a slower phosphate release step (reaction 4).…”

“…In lengthstep experiments, the length of the muscle fibre is changed suddenly (within approx. 100 ms) so that the myosin Sleep et al (2005) and Caremani et al (2008). Reaction 3 is the force-generating event.…”

Coupling between phosphate release and force generation in muscle actomyosin

“…More complex schemes could, of course, be invoked, and integration of pressure-and temperature-jump and fibre transient data led Ranatunga (1999) to postulate yet another state in this part of the actomyosin cycle. If the 75 s À1 value for P i dissociation measured with isolated acto-S1 applies to experiments in which muscle fibres are activated by photolysis of caged ATP (Goldman et al 1984b;Sleep et al 2004), the rate of force development is too fast for it to follow P i release (Sleep et al 2004). However, the moderate rate of P i release does call into question the fibre kinetic analysis listed in x 4.…”