Rotation of myosin lever arms during isometric contraction of skeletal myofibrils

Muscle contraction is brought about by the coupling of chemical energy of ATP hydrolysis to conformational changes in myosin. The chemical changes in myosin are well understood, but the corresponding conformational changes occurring ex-vivo are not. In order to obtain information about conformation it is essential to observe only a few cross-bridges. Observing a large number averages out individual contributions making it impossible to extract kinetic information under steady-state conditions. To minimize the number of observed cross-bridges, only one in 60,000 lever arms in myofibrils were labeled with fluorescent myosin essential light chain 1 (LC1). When such sparsely labeled myofibril is observed through a small aperture of a confocal microscope, ~8 fluorescent lever arms are observed in a volume smaller than a single half-sarcomere. Conformation was measured by recording normalized differences between parallel and perpendicular components of the fluorescence of LC1 (polarized fluorescence, PF). We measured in a single half-sarcomere during isometric contraction ex-vivo, the rate of rotational change of the lever arms and the probability distributions of their orientations. Isometric contraction involved extrusion of massive amount of solvent from the myofilament space, necessitating normalization of PF data. The results indicated that during isometric contraction lever arms rotated at the rate of ~10 s-1 and that the probability distributions of PF were narrower during contraction than relaxation suggesting that the lever arms in contracting muscle were not completely disorganized.