Key result
In rabbit myofibrils, force-[Pi] relations produced an apparent asymptote of 0.07 and 0.44 maximal isometric force, supporting models where force is produced by an isomerization preceding Pi release.
The findings support models of cross-bridge action in which force is produced by an isomerization of the AM.ADP.P(i) complex immediately preceding the P(i) release step.
Supports cross-bridge models in rabbit myofibrils; leaves open translation to human cardiac mechanics.
The inhibitory effects of inorganic phosphate (P(i)) on isometric force in striated muscle suggest that in the ATPase reaction P(i) release is coupled to force generation. Whether P(i) release and the power stroke are synchronous events or force is generated by an isomerization of the quaternary complex of actomyosin and ATPase products (AM.ADP.P(i)) prior to the following release of P(i) is still controversial. Examination of the dependence of isometric force on [P(i)] in rabbit fast (psoas; 5-15 degrees C) and slow (soleus; 15-20 degrees C) myofibrils was used to test the two-step hypothesis of force generation and P(i) release. Hyperbolic fits of force-[P(i)] relations obtained in fast and slow myofibrils at 15 degrees C produced an apparent asymptote as [P(i)]-->infinity of 0.07 and 0.44 maximal isometric force (i.e. force in the absence of P(i)) in psoas and soleus myofibrils, respectively, with an apparent K(d) of 4.3 mM in both. In each muscle type, the force-[P(i)] relation was independent of temperature. However, 2,3-butanedione 2-monoxime (BDM) decreased the apparent asymptote of force in both muscle types, as expected from its inhibition of the force-generating isomerization. These data lend strong support to models of cross-bridge action in which force is produced by an isomerization of the AM.ADP.P(i) complex immediately preceding the P(i) release step.
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Tesi et al. (2002) studied this question. Inorganic phosphate (Pi) and 2,3-butanedione 2-monoxime (BDM) vs. Absence of Pi was evaluated on Dependence of isometric force on inorganic phosphate concentration. In rabbit myofibrils, force-[Pi] relations produced an apparent asymptote of 0.07 and 0.44 maximal isometric force, supporting models where force is produced by an isomerization preceding Pi release.
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