Estimation of cross-bridge stiffness from maximum thermodynamic efficiency

In muscle, work is performed by myosin cross-bridges during interactions wi th actin filaments. The amount of work performed during each interaction ca n be related to the mechanical properties of the cross-bridge; work is the integral of the force produced with respect to the distance that the cross- bridge moves the actin filament, anal force is determined by the stiffness of the attached cross-bridge. In this paper, cross-bridge stiffness in frog sartorius muscle was estimated from thermodynamic efficiency (work/free en ergy change) using a two-state cross-bridge model, assuming constant stiffn ess over the working range and tight-coupling between cross-bridge cycles a nd ATP use. This model accurately predicts mechanical efficiency (work/enth alpy output). A critical review of the literature indicates that a realisti c value for maximum thermodynamic efficiency of frog sartorius is 0.45 unde r conditions commonly used in experiments on isolated muscle. Cross-bridge stiffness was estimated for a range of power stroke amplitudes. For realist ic amplitudes (10-15 nm), estimated cross-bridge stiffness was between 1 an d 2.2 pN nm(-1). These values are similar to those estimated from quick-rel ease experiments, taking into account compliance arising from structures ot her than cross-bridges, but are substantially higher than those from isolat ed protein studies. The effects on stiffness estimates of relaxing the tigh t-coupling requirement and of incorporating more force-producing cross-brid ge states are also considered. (C) Kluwer Academic Publishers.