Kinetics of force generation and Pi release in rabbit soleus muscle fibers

Recent studies have shown that the release of Pi from the AM.ADP.Pi crossbridge occurs in two steps: an initial force-generating isomerization which is followed by the release per se of Pi. These two steps have been shown to be little affected by the presence or absence of Ca2+ but both steps are significantly altered by temperature. In this study, the kinetics of the force generating and Pi release steps of the actomyosin 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 muscle than in fast (eg. 14 mM Pi reduces tension by 29 +/- 4.6% in slow muscle and by 47 +/- 5.3% in fast). As in fast muscle, the tension response to a sudden increase in [Pi] in slow muscle has four phases. As in fast muscle, only phase II (an exponential decline in force) appears to be caused by Pi binding to cross-bridges, while 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 (kpi) 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).(ABSTRACT TRUNCATED AT 250 WORDS)