Filament interaction in intact muscle fibers monitored by light scattering

Measurement of changes in optical properties of intact muscle fibers during contraction has proven to be difficult or, in some cases, impossible due to movement of the muscle relative to the incident beam. In this paper we describe a technique for immobilizing single fibers in clear gelatin, which permits measurement of light scattering signals undistorted by movement artifacts. We also describe the phase and amplitude relationship between changes in intensity of light scattering (at 90 degrees to incident beam) and tensions induced by electrically activating single fibers. With tensions that range up to 50% Po (Po = maximal tension measured by exposure of fibers to 200 mM K+), the maximal increase in light scattering is about 25% of that for resting fibers. The scattering increase precedes tension, and at low temperatures the interval between the two peaks can be 50--100 msec. We interpret these data on intact fibers, as we did our earlier data from studied on skinned fibers, as indicating that increases in light scattering power of muscle are primarily due to attachment of myosin cross-bridges to actin filaments.