Aim: 

Structural changes undergone by the myosin motors upon activation of muscle are investigated during the transition from the resting state to isometric contraction by X-ray interference from single muscle fibres (Linari et al., 2000, PNAS, 97:7226).

Methods: 

Intact fibres from frog muscle (2.1 mm sarcomere length, temperature 4°C) were mounted vertically at the ID2 beamline of the European Synchrotron Radiation Facility (Grenoble, France) between a loudspeaker motor and a capacitance force transducer. Fibres were electrically stimulated to produce isometric tetani. During the rise of force, 2D diffraction patterns were collected on a CCD detector with 5 ms time resolution.

Results: 

During activation and force development, the intensity of the M3 reflection, originating from the 14.5 nm axial repeat of the myosin motors, first decreases to 30% of its resting value, then increases to a steady value that is 70% of that at rest. The interference fine structure of the reflection has one main peak at rest and two main peaks during isometric contraction. The observed changes are reproduced with a structural model in which the M3 reflection originates from coherent scattering of active motors with a constant conformation, the number of which increases in proportion to the force (Brunello et al., 2006, J. Physiol., 577:971), and a decreasing number of resting motors.

Conclusion: 

Changes in the interference fine structure of the M3 reflection during the development of isometric force can be explained to a first approximation by an increasing number of active myosin motors with a constant force and conformation.

Supported by MiUR, CNISM, MRC, EMBO, ESRF, EMBL.