Kinetics of force generation and phosphate release in skinned rabbit soleus muscle fibers

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Abstract
The kinetics of the force generating and Pi release steps of the actomyosin-adenosinetriphosphatase (ATPase) cycle have been compared in Ca(2+)-activated skinned fibers of rabbit soleus (slow twitch) and psoas (fast twitch) muscle. Pi was rapidly photogenerated within the fiber lattice by laser flash photolysis of caged Pi [1-(2-nitro)phenylethyl phosphate]. Pi reduces isometric tension in the steady state but is less effective in slow-twitch muscle than in fast-twitch muscle (e.g., 14 mM Pi reduces tension by 29 +/- 4.6% in slow and by 47 +/- 5.3% in fast). The tension response to a sudden increase in Pi concentration in slow-twitch muscle has four phases, but as in fast-twitch muscle, only phase II (an exponential decline in force) appears to be caused by Pi binding to cross bridges, whereas the other three phases are probably indirect effects caused by caged Pi photolysis. The amplitude of phase II is consistent with the steady-state reduction in force by Pi. The rate of phase II (kappa Pi) is 3.9 +/- 0.33 s-1 at 20 degrees C and 0.28 +/- 0.02 s-1 at 10 degrees C (1 mM Pi). kappa Pi is thus 33 times slower in slow-twitch muscle than in fast at 20 degrees C and 84 times slower at 10 degrees C. In contrast to fast-twitch muscle, in slow muscle kappa Pi is sufficiently slow to partially limit the ATPase turnover rate.