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Received October 18, 2001
Accepted after revision February 18, 2002
1 Dipartimento di Scienze Fisiologiche, Università degli studi di Firenze, viale GB, Morgagni 63, I-50134 Firenze, Italy
2 Dipartimento di Scienze Fisiologiche, Università degli Studi di Firenze, Viale GB, Morgagni 63, I-50134 Firenze, Italy
* To whom correspondence should be addressed. E-mail: chiara.tesi{at}unifi.it.
The inhibitory effects of inorganic phosphate (Pi) on isometric force in striated muscle suggest that in the ATPase reaction Pi release is coupled to force generation. Whether Pi 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.Pi) prior to the following release of Pi is still controversial. Examination of the dependence of isometric force on [Pi] in rabbit fast (psoas; 5-15 °C) and slow (soleus; 15-20 °C) myofibrils was used to test the two-step hypothesis of force generation and Pi release. Hyperbolic fits of force-[Pi] relations obtained in fast and slow myofibrils at 15 °C produced an apparent asymptote as [Pi]->[infinity] of 0.07 and 0.44 maximal isometric force (i.e. force in the absence of Pi) in psoas and soleus myofibrils, respectively, with an apparent Kd of 4.3 mM in both. In each muscle type, the force-[Pi] 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.Pi complex immediately preceding the Pi release step.
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