KINETICS OF FORCE GENERATION AND PHOSPHATE RELEASE IN SKINNED RABBIT SOLEUS MUSCLE-FIBERS

The kinetics of the force generating and P(i) release steps of the actomyosin-adenosinetriphosphatase (ATPase) cycle have been compared in Ca2+-activated skinned fibers of rabbit soleus (slow twitch) and psoas (fast twitch) muscle. P(i) was rapidly photogenerated within the fiber lattice by laser flash photolysis of caged P(i) [1-(2-nitro)phenylethyl phosphate]. P(i) reduces isometric tension in the steady state but is less effective in slow-twitch muscle than in fast-twitch muscle (e.g., 14 mM P(i) reduces tension by 29 +/- 4.6% in slow and by 47 +/- 5.3% in fast). The tension response to a sudden increase in P(i) 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 P(i) binding to cross bridges, whereas the other three phases are probably indirect effects caused by caged P(i) photolysis. The amplitude of phase II is consistent with the steady-state reduction in force by P(i). The rate of phase II (kP(i)) is 3.9 +/- 0.33 s-1 at 20-degrees-C and 0.28 +/- 0.02 s-1 at 10-degrees-C (1 mM P(i)). kP(i) 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 kP(i) is sufficiently slow to partially limit the ATPase turnover rate.