AJP - Cell Physiology, Vol 251, Issue 6 945-C950, Copyright © 1986 by American Physiological Society
Slowing of cross-bridge cycling in smooth muscle without evidence of an internal load
T. M. Butler, M. J. Siegman and S. U. Mooers
We have tested whether the slowing in velocity of shortening that occurs in
mammalian smooth muscle under conditions of high force output and
dephosphorylated myosin light chains is due to an internal load presented
by non- or slowly cycling cross bridges. The experimental design included
stimulation of the rabbit taenia coli for 25 s under isometric conditions
followed by 30 s of relaxation prior to the test stimulus. There was a
large decrease in maximum velocity of shortening and chemical energy usage
as well as a significantly decreased myosin light-chain phosphorylation
response during the test stimulus even though active force developed to 90
+/- 4% of the original. The chemical energy cost of active external work
production was not significantly different from that of the initial
stimulation. There is, therefore, no energetic evidence for the dissipation
of work against an internal load under conditions during which most of the
force would be expected to be generated by latch bridges. We conclude that
force maintenance with a low energy input and a slow velocity of shortening
in mammalian smooth muscle represents a general slowing of the cross-bridge
cycling rate rather than the appearance of a class of latch bridges that
provide an internal load against which other cross bridges must operate.
