Chemical energetics of force development, force maintenance, and relaxation in mammalian smooth muscle.

Abstract

High-energy phosphate utilization (.DELTA. .apprx. 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.degree. C and a muscle length of 95% lo, the resting rate of energy utilization is 1.8 .+-. 0.2 nmol/g .cntdot. s-1, or 0.85 .+-. 0.2 mmol .apprx. P/mol of total creatine (Ct) .cntdot. s-1, where Ct = 2.7 .mu.mol/g wet wt. During the initial 25 s of stimulation when force is developed, the average rate of .DELTA. .apprx. P was -8.2 .+-. 0.8 mmol/mol Ct .cntdot. s-1, some 4 times greater than during the subsequent 35 s of force maintenance, when the rate was -2.0 .+-. 0.6 mmol .apprx. P/mol Ct .cntdot. s-1. The energy cost of force redevelopment (0-95% Po) after a quick release from the peak of a tetanus is very low compared with the initial force development. 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 .intg. Pdt/.DELTA. .apprx. P was higher during relaxation than during force maintenance in the stimulated muscle.