Submillisecond changes in myosin lattice spacing resulting from rapid length changes

Area of Science:

• Muscle physiology
• Biophysics
• X-ray diffraction studies

Background:

• Understanding the mechanics of muscle contraction requires knowledge of how the myofilament lattice structure changes.
• The speed of these lattice spacing changes is crucial for understanding the kinetics of force generation.

Purpose of the Study:

• To investigate the time course of myofilament lattice spacing changes in response to rapid mechanical perturbations in frog muscle fibers.
• To determine if lattice spacing dynamics are fast enough to be associated with the crossbridge power stroke.

Main Methods:

• Used synchrotron X-ray diffraction with 250-microsecond time resolution to measure equatorial lattice spacing.
• Performed rapid ramp releases and step length changes on single, intact frog muscle fibers during isometric tetani.
• Analyzed spacing changes using exponential functions and elastic components related to axial tension recovery.

Main Results:

• Ramp releases caused lattice spacing increases (exponential + elastic components).
• Step releases showed faster lattice expansion kinetics compared to ramp releases.
• Step stretches resulted in lattice compression with exponential and elastic components.
• The time course of spacing changes correlated with axial tension recovery and 14.5 nm reflection intensity changes.

Conclusions:

• Myofilament lattice spacing changes occur rapidly, with time courses consistent with the kinetics of the molecular power stroke.
• Radial forces exerted by attached crossbridges are responsible for these fast lattice spacing dynamics.
• Lattice spacing adjustments are integral to the muscle's force generation mechanism during contraction.