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Muscle cross-bridges: do they rotate?

R Cooke, M S Crowder, C H Wendt

Advances in Experimental Medicine and Biology

|January 1, 1984

Summary

Electron paramagnetic resonance (EPR) spectroscopy reveals myosin head orientation in muscle fibers. During contraction, most myosin heads are disordered, but a rigid, ordered population remains attached to actin, independent of force generation.

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Area of Science:

Muscle Physiology
Biophysics
Skeletal Muscle Mechanics

Background:

Myosin heads play a crucial role in muscle contraction by interacting with actin filaments.
Previous studies using electron paramagnetic resonance (EPR) showed distinct myosin head probe orientations in rigor versus relaxed muscle states.

Purpose of the Study:

To investigate the orientation and dynamics of myosin heads during muscle contraction using EPR spectroscopy.
To determine the relationship between myosin head attachment, orientation, and force generation in skeletal muscle.

Main Methods:

Utilized electron paramagnetic resonance (EPR) spectroscopy to monitor spin labels attached to myosin heads in glycerinated rabbit psoas skeletal muscle fibers.
Analyzed spectral changes under various conditions including rigor, relaxation, and during isometric tension generation with different nucleotide analogs (ADP, AMPPNP, PPi).
Employed saturation transfer EPR to assess the mobility of different myosin head populations.

Main Results:

Muscle contraction under isometric tension showed a mixed population of myosin heads: ~80% disordered (random distribution) and ~20% ordered (fixed angle).
The ordered component exhibited the same orientation as observed in rigor muscle, irrespective of applied stress or nucleotide conditions.
Saturation transfer EPR confirmed the ordered component is rigid, while the disordered component is mobile on the microsecond timescale.

Conclusions:

The disordered EPR spectral component corresponds to detached, mobile myosin heads, while the ordered component represents actin-attached, rigid myosin heads.
Myosin head orientation, specifically the ordered fraction, is not directly coupled to force generation during muscle contraction.
The fixed orientation of the ordered myosin heads suggests a structural constraint independent of the contractile force.

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