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 (Vmax) and slope of slack test plots (V0)], maximum isometric force (Po), power, and the curvature of the force-velocity curve [a/Po (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 mM ATP to 10 mM CTP resulted in a decrease in Vmax in psoas fibers from 3.26 to 1.87 muscle length/s. In soleus fibers, Vmax 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 Po, a/Po, and Vmax. 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 Pi release, but before the release of the nucleotide diphosphate, underlies the alterations in a/Po reported here.
Wahr
P A
PA
Department of Physiology, University of Michigan, Ann Arbor, Michigan 48109-0622, USA.
Metzger
J M
JM
D000255:Adenosine Triphosphate / Q000502:physiology*; D000818:Animals; D002118:Calcium / Q000502:physiology; D003570:Cytidine Triphosphate / Q000502:physiology*; D004734:Energy Metabolism / Q000502:physiology*; D005260:Female; D066298:In Vitro Techniques; D007700:Kinetics; D008954:Models, Biological; D009119:Muscle Contraction / Q000502:physiology; D018485:Muscle Fibers, Skeletal / Q000502:physiology*; D018482:Muscle, Skeletal / Q000166:cytology / Q000502:physiology*; D011817:Rabbits; D051381:Rats; D017207:Rats, Sprague-Dawley
D003570:Cytidine Triphosphate; D000255:Adenosine Triphosphate; D002118:Calcium