The intensity of light scattered by chemically skinned rabbit psoas fibers in relaxed, rigor, and activated states was monitored at 90 degrees to the incident beam. In the relaxed state, scattering varied in proportion to the volume of muscle in the beam. Scattering increased to 2.3 times the resting value when rigor was induced by withdrawal of MgATP or when the myofibrils were activated by the caffeine-induced release of Ca from the sarcoplasmic reticulum. The rigor-induced increase in scattering decreased monotonically when MgATP was reintroduced stepwise (0-100 microM). This decrease in scattering was accompanied by an increase in tension up to an optimum MgATP level of approximately 10 microM, and then tension decreased at higher concentrations (10-100 microM). The increase in scattering during both rigor and activation was dependent upon fiber length. At lengths when thick-thin filament overlap was near zero, the light signal due to rigor and activation fell to within 10% of the signal for the relaxed fiber at that length. The signal during rigor increased only minimally (approximately 10%) when stretch (approximately 1%) was applied. This increase in signal was small despite a measured 5- to 10-fold increase in tension and an estimated twofold increase in stiffness. Thus, the increased light scattering caused by rigor and activation depends on filament overlap and not tension, stiffness, or substrate binding.

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