Orientation of spin labels attached to cross-bridges in contracting muscle fibres

Electron micrographs showing different cross-bridge orientations in different states of muscle fibres, and X-ray diffraction patterns indicating axial cross-bridge disorder in contracting muscle first suggested that force generation in the contracting muscle involved a change in orientation of the myosin heads that form cross-bridges between thick and thin filaments ^1,2. This has been supported by subsequent work; the myosin molecule has the required flexibility for changes in orientation^3,4. The orientation of muscle tryptophans and of probes attached to the myosin heads of permeable muscle fibres depends on the state of the muscle^5-9. Recently, fluorescence polarization fluctuations and time-resolved X-ray diffraction patterns have suggested that cross-bridges of a contracting muscle can rotate^10,11. We have used electron paramagnetic resonance (EPR) spectroscopy to monitor the orientation of spin labels attached specifically to a reactive sulphydryl on the myosin heads in glycerinated rabbit psoas skeletal muscle. Previously, it has been shown that the paramagnetic probes are highly ordered in rigor muscle, with a nearly random angular distribution in relaxed muscle¹². We show here that during the generation of isometric tension, ∼80% of the probes display a random angular distribution as in relaxed muscle while the remaining 20% are highly oriented at the same angle as found in rigor muscle. These findings indicate that a domain of the myosin head does not change orientation during the power stroke of the contractile interaction.