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Effects of substituting uridine triphosphate for ATP on the crossbridge cycle of rabbit muscle
Author(s) -
Seow Chun Y.,
White Howard D.,
Ford Lincoln E.
Publication year - 2001
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1111/j.1469-7793.2001.00907.x
Subject(s) - crossbridge , isometric exercise , chemistry , biophysics , myosin , actin , adenosine triphosphate , muscle contraction , uridine , anatomy , biochemistry , biology , rna , gene , physiology
1 Substituting uridine triphosphate (UTP) for ATP as a substrate for rabbit skeletal myosin and actin at 4°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°C reduced the maximum shortening velocity by 56% and increased the force asymptote of the force‐velocity curve relative to force (α/ P o ) by 112% without altering the velocity asymptote, β. 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 α/ P o with constant β 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 unhydrolysed 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.