title: Cross-bridge duty cycle in isometric contraction of skeletal myofibrils.
abstract: During interaction of actin with myosin, cross-bridges impart mechanical impulses to thin filaments resulting in rotations of actin monomers. Impulses are delivered on the average every tc seconds. A cross-bridge spends a fraction of this time (ts) strongly attached to actin, during which it generates force. The "duty cycle" (DC), defined as the fraction of the total cross-bridge cycle that myosin spends attached to actin in a force generating state (ts/ tc), is small for cross-bridges acting against zero load, like freely shortening muscle, and increases as the load rises. Here we report, for the first time, an attempt to measure DC of a single cross-bridge in muscle. A single actin molecule in a half-sarcomere was labeled with fluorescent phalloidin. Its orientation was measured by monitoring intensity of the polarized TIRF images. Actin changed orientation when a cross-bridge bound to it. During isometric contraction, but not during rigor, actin orientation oscillated between two values, corresponding to the actin-bound and actin-free state of the cross-bridge. The average ts and tc were 3.4 and 6 s, respectively. These results suggest that, in isometrically working muscle, cross-bridges spend about half of the cycle time attached to actin. The fact that 1/ tc was much smaller than the ATPase rate suggests that the bulk of the energy of ATP hydrolysis is used for purposes other than performance of mechanical work.
authors.affiliation: Department of Molecular Biology & Immunology and Center for Commercialization of Fluorescence Technology, the University of North Texas HSC, Fort Worth, Texas 76107, USA.
authors.forename: P
authors.initials: P
authors.lastname: Muthu
authors.forename: J M
authors.initials: JM
authors.lastname: Talent
authors.forename: I
authors.initials: I
authors.lastname: Gryczynski
authors.forename: J
authors.initials: J
authors.lastname: Borejdo
mesh_terms: D003432:Cross-Linking Reagents / Q000737:chemistry*; D009119:Muscle Contraction; D009210:Myofibrils / Q000737:chemistry* / Q000378:metabolism*; D013816:Thermodynamics; D013997:Time Factors
chemical_list: D003432:Cross-Linking Reagents