The Journal of General Physiology, Vol 76, 609-629, Copyright © 1980 by The Rockefeller University Press
Chemical energetics of force development, force maintenance, and relaxation in mammalian smooth muscle
MJ Siegman, TM Butler, SU Mooers and RE Davies
High-energy phosphate utilization (delta approximately P) associated with
force development, force maintenance, and relaxation has been determined
during single isometric tetani in the rabbit taenia coli. ATP resynthesis
from glycolysis and respiration was stopped without deleterious effects on
the muscle. At 18 degrees C and a muscle length of 95% l0, the resting rate
of energy utilization is 1.8 +/- 0.2 nmol/g . s-1, or 0.85 +/- 0.2 mmol
approximately P/mol of total creatine (Ct) . s-1, where Ct = 2.7 mumol/g
wet wt. During the initial 25 s of stimulation when force is developed, the
average rate of delta approximately P was -8.2 +/- 0.8 mmol/mol Ct . s-1,
some four times greater than during the subsequent 35 s of force
maintenance, when the rate was -2.0 +/- 0.6 mmol approximately P/mol Ct .
s-1. The energy cost of force redevelopment (0 to 95% P0) after a quick
release from the peak of a tetanus is very low compared with the initial
force development. Therefore, the high rate of energy utilization during
force development is not due only to internal work done against the series
elasticity nor to any high rate of cross-bridge cycling inherently
associated with force development. The high economy of force maintenance
compared with other muscle types is undoubtedly due to a slower
cross-bridge cycle time. The energy utilization during 45 s of relaxation
was not statistically significant, and integral of Pdt/delta approximately
P was higher during relaxation than during force maintenance in the
stimulated muscle.
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