Peak power output is maintained in rabbit psoas and rat soleus single muscle fibers when CTP replaces ATP

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.