Chemical energy usage during stimulation and stretch of mammalian smooth muscle

High energy phosphate usage was measured in the rabbit taenia coli subjected to stimulation and stretch (0.12 Lo/min) and was compared to that observed previously under isometric conditions. When the muscle was bathed in a medium containing 1.9 mM Ca2+, stretch during the period of initial force development substantially decreased the rate of chemical energy usage compared to that under isometric conditions. When crossbridge cycling rate under isometric conditions was increased by incubation of the muscle in a medium containing 4.5 mM Ca2+, there was a greater decrease in rate of high energy phosphate usage during stretch compared to isometric conditions. The low energy usage during stretch occurs even though average active force output was approximately 40% higher than that under isometric conditions. During the period of subsequent force maintenance when both energy usage and crossbridge cycling rate under isometric conditions were low, there was no significant effect of stretch on the average rate of energy usage at either Ca2+ level. These results are consistent with the hypothesis that during stimulation and stretch in smooth muscle, crossbridge attachment and force production can occur even though the actin-activated myosin ATPase activity normally associated with isometric force development is greatly suppressed.