title: Peak power output is maintained in rabbit psoas and rat soleus single muscle fibers when CTP replaces ATP.
abstract: 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.
authors.affiliation: Department of Physiology, University of Michigan, Ann Arbor, Michigan 48109-0622, USA.
authors.forename: P A
authors.initials: PA
authors.lastname: Wahr
authors.forename: J M
authors.initials: JM
authors.lastname: Metzger
mesh_terms: 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
chemical_list: D003570:Cytidine Triphosphate; D000255:Adenosine Triphosphate; D002118:Calcium