The Frank-Starling mechanism is not mediated by changes in rate of cross-bridge detachment

We tested the hypothesis that the Frank-Starling relationship is mediated by changes in the rate of cross-bridge detachment in cardiac muscle. We simultaneously measured isometric force development and the rate of ATP consumption at various levels of Ca2+ activation in skinned rat cardiac trabecular muscles at three sarcomere lengths (2.0, 2.1, and 2.2 mu m). The maximum rate of ATP consumption was 1.5 nmol.s(-1).mu l fiber vol(-1), which represents an estimated adenosinetriphosphatase (ATPase) rate of similar to 10 s(-1) per myosin head at 24 degrees C. The rate of ATP consumption was tightly and linearly coupled to the level of isometric force development, and changes in sarcomere length had no effect on the slope of the force-ATPase relationships. The average slope of the force-ATPase relationships was 15.5 pmol.mN(-1).mm(-1). These results suggest that the mechanisms that underlie the Frank-Starling relationship in cardiac muscle do not involve changes in the kinetics of the apparent detachment step in the cross-bridge cycle.