Chemical Energetics of contraction in mammalian smooth muscle.

Measurements of high-energy phosphate utilization during isometric tetanic contractions were made on a rabbit taenia coli preparation that had been treated so that respiration and glycolysis were blocked without altering the mechanical response to the muscle. At 18 C the first sign of high-energy phosphate usage after electrical stimulation was a net breakdown of ATP with a subsequent net resynthesis when the rate of phosphocreatine breakdown was high. The average rate of chemical energy usage was more than four times as high during the period of initial force development compared to that during maximum force maintenance. This initial high rate of energy usage was not due only to the internal work done against the series elasticity during force development. Comparison of energetics and mechanics data for the rabbit taenia coli and frog sartorius showed that smooth muscle was 100-fold more economical in maintaining isometric force and probably had a cross-bridge cycle time that was about 150-fold slower. The rate of energy usage during relaxation from an isometric tetanus in smooth muscle was very low, and force was exerted with a lower expenditure of energy during relaxation than during maximum force maintenance. Phosphorylation of the 20,000-dalton light chain of myosin occurred to the extent of 32% during a maximally activated isometric tetanus and was not proportional to the rate of energy usage during different stages of the tetanus. During isometric relaxation, the degree of phosphorylation, rate of energy usage, and active state all decreased more quickly than did active force output.