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1 The Randall Centre, Guy's Campus, King's College London, London SE1 1UL, UK
2 Department of Physiological Sciences, Eastern Virginia Medical School, Norfolk, VA 23501, USA
Mechanical properties of skinned single fibres from rabbit psoas muscle have been correlated with biochemical steps in the cross-bridge cycle using a series of metal–nucleotide (Me·NTP) substrates (Mn2+ or Ni2+ substituted for Mg2+; CTP or ITP for ATP) and inorganic phosphate. Measurements were made of the rate of force redevelopment following (1) slack tests in which force recovery followed a period of unloaded shortening, or (2) ramp shortening at low load terminated by a rapid restretch. The form and rate of force recovery were described as the sum of two exponential functions. Actomyosin-Subfragment 1 (acto-S1) Me·NTPase activity and Me·NDP release were monitored under the same conditions as the fibre experiments. Mn·ATP and Mg·CTP both supported contraction well and maintained good striation order. Relative to Mg·ATP, they increased the rates and Me·NTPase activity of cross-linked acto-S1 and the fast component of a double-exponential fit to force recovery by 
50% and 10–35%, respectively, while shortening velocity was moderately reduced (by 20–30%). Phosphate also increased the rate of the fast component of force recovery. In contrast to Mn2+ and CTP, Ni·ATP and Mg·ITP did not support contraction well and caused striations to become disordered. The rates of force recovery and Me·NTPase activity were less than for Mg·ATP (by 40–80% and 50–85%, respectively), while shortening velocity was greatly reduced (by 80%). Dissociation of ADP from acto-S1 was little affected by Ni2+, suggesting that Ni·ADP dissociation does not account for the large reduction in shortening velocity. The different effects of Ni2+ and Mn2+ were also observed during brief activations elicited by photolytic release of ATP. These results confirm that at least one rate-limiting step is shared by acto-S1 ATPase activity and force development. Our results are consistent with a dual rate-limitation model in which the rate of force recovery is limited by both NTP cleavage and phosphate release, with their relative contributions and apparent rate constants influenced by an intervening rapid force-generating transition.
(Received 8 November 2004;
accepted after revision 9 December 2004;
first published online 20 December 2004)
Corresponding author J. Sleep: The Randall Centre, New Hunt's House, Guy's Campus, King's College London, London SE1 1UL, UK. Email: john.sleep{at}kcl.ac.uk
This article has been cited by other articles:
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  K. Burton, R. M. Simmons, J. Sleep, R. M. Simmons, K. Burton, and D. A. Smith Kinetics of force recovery following length changes in active skinned single fibres from rabbit psoas muscle: with an Appendix: Analysis and modelling of the late recovery phase J. Physiol., June 1, 2006; 573(2): 305 - 328. [Abstract] [Full Text] [PDF]
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 K. S. Campbell Tension Recovery in Permeabilized Rat Soleus Muscle Fibers after Rapid Shortening and Restretch Biophys. J., February 15, 2006; 90(4): 1288 - 1294. [Abstract] [Full Text] [PDF]
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J. Sleep, M. Irving, and K. Burton The ATP hydrolysis and phosphate release steps control the time course of force development in rabbit skeletal muscle J. Physiol., March 15, 2005; 563(3): 671 - 687. [Abstract] [Full Text] [PDF]
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