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Smooth
muscle. Cross-bridge cycle and ATP breakdown
Smooth-muscle
contraction requires the release of chemical energy stored
in ATP molecules. The release of this chemical energy by
the myosin cross bridge and the resultant mechanical work
is commonly referred to as the cross-bridge cycle, which
in smooth muscle is believed to be a multistep process similar
to that in striated muscle. Therefore, the mechanical properties
of smooth muscle, as of striated muscles, are intimately
linked to this multistate cross-bridge cycle. For instance,
there is a correlation between the rate at which the cross
bridges cycle and the maximum shortening velocity of the
muscle. Since the actomyosin ATPase cross-bridge cycle in
smooth muscle is considerably slower than that in striated
muscle, the slower shortening velocity in smooth muscle
must be due, in part, to the reduced turnover rate of the
cross bridge. The slower cycling rate could also account
for the high economy of ATP utilization that characterizes
smooth-muscle force production, since fewer cycles are required
and less energy is consumed in the generation of force.
