title: X-ray diffraction studies of the contractile mechanism in single muscle fibres.
abstract: The molecular mechanism of muscle contraction was investigated in intact muscle fibres by X-ray diffraction. Changes in the intensities of the axial X-ray reflections produced by imposing rapid changes in fibre length establish the average conformation of the myosin heads during active isometric contraction, and show that the heads tilt during the elastic response to a change in fibre length and during the elementary force generating process: the working stroke. X-ray interference between the two arrays of myosin heads in each filament allows the axial motions of the heads following a sudden drop in force from the isometric level to be measured in situ with unprecedented precision. At low load, the average working stroke is 12 nm, which is consistent with crystallographic studies. The working stroke is smaller and slower at a higher load. The compliance of the actin and myosin filaments was also determined from the change in the axial spacings of the X-ray reflections following a force step, and shown to be responsible for most of the sarcomere compliance. The mechanical properties of the sarcomere depend on both the motor actions of the myosin heads and the compliance of the myosin and actin filaments.
authors.affiliation: Laboratorio di Fisiologia, DBAG, Universitá di Firenze, I-50134 Firenze, Italy.
authors.forename: Vincenzo
authors.initials: V
authors.lastname: Lombardi
authors.forename: Gabriella
authors.initials: G
authors.lastname: Piazzesi
authors.forename: Massimo
authors.initials: M
authors.lastname: Reconditi
authors.forename: Marco
authors.initials: M
authors.lastname: Linari
authors.forename: Leonardo
authors.initials: L
authors.lastname: Lucii
authors.forename: Alex
authors.initials: A
authors.lastname: Stewart
authors.forename: Yin-Biao
authors.initials: YB
authors.lastname: Sun
authors.forename: Peter
authors.initials: P
authors.lastname: Boesecke
authors.forename: Theyencheri
authors.initials: T
authors.lastname: Narayanan
authors.forename: Tom
authors.initials: T
authors.lastname: Irving
authors.forename: Malcolm
authors.initials: M
authors.lastname: Irving
mesh_terms: D000199:Actins / Q000378:metabolism; D000818:Animals; D001696:Biomechanical Phenomena; D008958:Models, Molecular; D009119:Muscle Contraction / Q000502:physiology*; D018485:Muscle Fibers, Skeletal / Q000000981:diagnostic imaging / Q000502:physiology*; D009218:Myosins / Q000378:metabolism / Q000502:physiology*; D011487:Protein Conformation; D011859:Radiography; D012518:Sarcomeres / Q000502:physiology*; D014961:X-Ray Diffraction
chemical_list: D000199:Actins; D009218:Myosins