The chemomechanical coupling mechanism in striated muscle contraction was examined by changing the nucleotide substrate from ATP to CTP. Maximum shortening velocity [extrapolation to zero force from force-velocity relation (V(max)) and slope of slack test plots (V0)], maximum isometric force (P0); power, and the curvature of the force-velocity curve [α/P(o) (dimensionless parameter inversely related to the curvature)] were determined during maximum Ca2+-activated isotonic contractions of fibers from fast rabbit psoas and slow rat soleus muscles by using 0.2 mM MgATP, 4 mM MgATP, 4 mM MgCTP, or 10 mM MgCTP as the nucleotide substrate. In addition to a decrease in the maximum Ca2+-activated force in both fiber types, a change from 4 mMATP to 10 mM CTP resulted in a decrease in V(max) in psoas fibers from 3.26 to 1.87 muscle length/s. In soleus fibers, V(max), was reduced from 1.94 to 0.90 muscle length/s by this change in nucleotide. Surprisingly, peak power was unaffected in either fiber type by the change in nucleotide as the result of a three- to fourfold decrease in the curvature of the force-velocity relationship. The results are interpreted in terms of the Huxley model of muscle contraction as an increase in f1 and g1 coupled to a decrease in g2 (where f1 is the rate of cross-bridge attachment and g1 and g2 are rates of detachment) when CTP replaces ATP. This adequately accounts for the observed changes in P(o), α/P(o), and V(max). However, the two-state Huxley model does not explicitly reveal the cross-bridge transitions that determine curvature of the force-velocity relationship. We hypothesize that a nucleotide-sensitive transition among strong-binding cross-bridge states following P(i) release, but before the release of the nucleotide diphosphate, underlies the alterations in α/P(o) reported here.
|Original language||English (US)|
|Number of pages||8|
|Journal||Journal of Applied Physiology|
|State||Published - Jul 1 1998|
- Adenosine 5'-triphosphate analogs
- Muscle contraction