Peak power output is maintained in rabbit psoas and rat soleus single muscle fibers when CTP replaces ATP |
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Authors: | Wahr Philip A; Metzger Joseph M |
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Abstract: | The chemomechanicalcoupling mechanism in striated muscle contraction was examined bychanging the nucleotide substrate from ATP to CTP. Maximum shorteningvelocity extrapolation to zero force from force-velocity relation(Vmax) andslope of slack test plots (V0)], maximumisometric force (Po), power, andthe curvature of the force-velocity curvea/Po(dimensionless parameter inversely related to the curvature)] weredetermined during maximumCa2+-activated isotoniccontractions of fibers from fast rabbit psoas and slow rat soleusmuscles by using 0.2 mM MgATP, 4 mM MgATP, 4 mM MgCTP, or 10 mM MgCTPas the nucleotide substrate. In addition to a decrease in the maximumCa2+-activated force in both fibertypes, a change from 4 mM ATP to 10 mM CTP resulted in a decrease inVmax in psoasfibers 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 thechange in nucleotide as the result of a three- to fourfold decrease inthe curvature of the force-velocity relationship. The results areinterpreted in terms of the Huxley model of muscle contraction as anincrease in f1and g1 coupled toa decrease in g2(where f1 is therate of cross-bridge attachment and g1 andg2 are rates ofdetachment) when CTP replaces ATP. This adequately accounts for theobserved changes in Po,a/Po,and Vmax.However, the two-state Huxley model does not explicitly reveal thecross-bridge transitions that determine curvature of the force-velocityrelationship. We hypothesize that a nucleotide-sensitive transitionamong strong-binding cross-bridge states followingPi release, but before the release of the nucleotide diphosphate, underlies the alterations ina/Po reported here. |
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