Effects of substituting uridine triphosphate for ATP on the crossbridge cycle of rabbit muscle.

1. Substituting uridine triphosphate (UTP) for ATP as a substrate for rabbit skeletal myosin and actin at 4 degrees C slowed the dissociation of myosin-S1 from actin by threefold, and hydrolysis of the nucleotide by sevenfold, without a decrease in the rates of phosphate or uridine diphosphate dissociation from actomyosin. 2. The same substitution in skinned rabbit psoas fibres at 2-3 degrees C reduced the maximum shortening velocity by 56 % and increased the force asymptote of the force-velocity curve relative to force (alpha/P(o)) by 112 % without altering the velocity asymptote, beta. It also decreased isometric force by 35 % and isometric stiffness by 20 %, so that the stiffness/force ratio was increased by 23 %. 3. Tension transient experiments showed that the stiffness/force increase was associated with a 10 % reduction in the amplitude of the rapid, partial (phase 2) recovery relative to the isometric force, and the addition of two new components, one that recovered at a step-size-independent rate of 100 s(-1) and another that did not recover following the length change. 4. The increased alpha/P(o) with constant beta suggests an internal load, as expected of attached crossbridges detained in their movement. An increased stiffness/force ratio suggests a greater fraction of attached bridges in low-force states, as expected of bridges with unhydrolyzed UTP detained in low-force states. Decreased phase 2 recovery suggests the detention of high-force bridges, as expected of slowed actomyosin dissociation by nucleotide. 5. These results suggest that the separation of hydrolysed phosphates from nucleotides occurs early in the attached phase of the crossbridge cycle, near and possibly identical to a transition to a firmly attached, low-force state from an initial state where bridges with hydrolysed nucleotides are easily detached by shortening.