title: Skeletal muscle performance determined by modulation of number of myosin motors rather than motor force or stroke size.
abstract: Skeletal muscle can bear a high load at constant length, or shorten rapidly when the load is low. This force-velocity relationship is the primary determinant of muscle performance in vivo. Here we exploited the quasi-crystalline order of myosin II motors in muscle filaments to determine the molecular basis of this relationship by X-ray interference and mechanical measurements on intact single cells. We found that, during muscle shortening at a wide range of velocities, individual myosin motors maintain a force of about 6 pN while pulling an actin filament through a 6 nm stroke, then quickly detach when the motor reaches a critical conformation. Thus we show that the force-velocity relationship is primarily a result of a reduction in the number of motors attached to actin in each filament in proportion to the filament load. These results explain muscle performance and efficiency in terms of the molecular mechanism of the myosin motor.
authors.affiliation: Laboratorio di Fisiologia, Dipartimento di Biologia Animale e Genetica, Università degli Studi di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Italy.
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: Pasquale
authors.initials: P
authors.lastname: Bianco
authors.forename: Elisabetta
authors.initials: E
authors.lastname: Brunello
authors.forename: Valérie
authors.initials: V
authors.lastname: Decostre
authors.forename: Alex
authors.initials: A
authors.lastname: Stewart
authors.forename: David B
authors.initials: DB
authors.lastname: Gore
authors.forename: Thomas C
authors.initials: TC
authors.lastname: Irving
authors.forename: Malcolm
authors.initials: M
authors.lastname: Irving
authors.forename: Vincenzo
authors.initials: V
authors.lastname: Lombardi
mesh_terms: D000818:Animals; D004548:Elasticity; D020409:Molecular Motor Proteins / Q000378:metabolism*; D009119:Muscle Contraction / Q000502:physiology*; D018482:Muscle, Skeletal / Q000378:metabolism* / Q000648:ultrastructure; D024462:Myosin Type II / Q000378:metabolism*; D011896:Rana temporaria; D013314:Stress, Mechanical; D014961:X-Ray Diffraction
chemical_list: D020409:Molecular Motor Proteins; D024462:Myosin Type II