Optical depolarization changes in single, skinned muscle fibers. Evidence for cross-bridge involvement

Optical ellipsometry studies of single, skinned muscle fibers conducted on the diffraction orders have yielded spectra that are sensitive to the state of the fiber. The linearly polarized light field vector becomes elliptically polarized as it passes through the fiber and may be collected at the diffraction orders. Fibers that have been subjected to extraction of myosin (0.6 M KCl) retain a weak diffraction pattern and exhibit a substantially decreased depolarization of incident linearly polarized light. A significant decrease in polarization is seen in skinned fibers that are subject to an increase in pH from 7.0 to 8.0. This increase in pH results in a decrease of approximately 30% in the depolarization angle of single fibers. The major decrease in depolarization angle that we observe at pH 8.0 is consistent with the notion that as cross-bridges move out from the shaft of the thick filament, their ability to cause depolarization of the incident linearly polarized light decreases. This interpretation is also consistent with the work of Ueno and Harrington where the decrease in the ability to cross-link S-1 and S-2 to the thick filament at pH 8.2 suggests cross-bridge movement away from the thick filament. A large decrease in birefringence, seen after treatment of skinned fibers with alpha-chymotrypsin, appears to be related to the breakdown of myosin into rod, S-1, heavy meromyosin, and light meromyosin.