Chemical energy usage during shortening and work production in mammalian smooth muscle

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Abstract
The chemical energy usage associated with shortening and external work production was determined in the rabbit taenia coli at 18.degree. C. High-energy phosphate utilizations during isovelocity shortenings [17-50% of the maximum shortening velocity, (Vmax)] initiated at the beginning of tetanic stimulation were not different from those measured during isometric contractions of the same durations. The average rates of chemical energy usage during isovelocity shortenings (7-50% Vmax) initiated after development of isometric tension were 2.5 times that observed during force maintenance under isometric conditions. The average suprabasal rates of chemical energy usage for identical shortenings starting from the beginning and from the plateau of a tetanus were not different, but rather there was a 4-fold decrease in rate of energy usage during the isometric tetanus. Each mole of phosphagen breakdown was associated with 3-7 kJ of active external work production at velocities of 7-50% Vmax. The degree of phosphorylation of the 20,000-dalton L chain of myosin was also determined under mechanical conditions similar to those in the energetics experiments. Phosphorylation increased from 11 .+-. 1% at rest at 31 .+-. 3% following 5 s of stimulation under isometric conditions at the optimal length for active force development. During continued stimulation, there was a slow but significant net dephosphorylation. The average degree of phosphorylation was not directly proportional to the average rates of energy usage at different times during an isometric tetanus. During relaxation, degree of phosphorylation and the ability of the muscle to redevelop force following a quick release decreased more rapidly than did active force output from the muscle.