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Section on Cardiology, Bowman Gray School of Medicine, Wake Forest University, Winston-Salem, North Carolina 27157-1045
We tested the hypothesis that the
Frank-Starling relationship is mediated by changes in the rate of
cross-bridge detachment in cardiac muscle. We simultaneously measured
isometric force development and the rate of ATP consumption at various
levels of Ca2+ activation in
skinned rat cardiac trabecular muscles at three sarcomere lengths (2.0, 2.1, and 2.2 µm). The maximum rate of ATP consumption was 1.5 nmol · s
1 · µl
fiber vol
1, which
represents an estimated adenosinetriphosphatase (ATPase) rate of ~10
s
1 per myosin head at
24°C. The rate of ATP consumption was tightly and linearly coupled
to the level of isometric force development, and changes in sarcomere
length had no effect on the slope of the force-ATPase relationships.
The average slope of the force-ATPase relationships was 15.5 pmol · mN
1 · mm
1.
These results suggest that the mechanisms that underlie the Frank-Starling relationship in cardiac muscle do not involve changes in
the kinetics of the apparent detachment step in the cross-bridge cycle.
contractility; skinned fibers; myofilaments; ATP consumption; myofilament economy
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