Filament interaction monitored by light scattering in skinned fibers.

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Abstract
The intensity of light scattered by chemically skinned rabbit psoas fibers in relaxed, rigor and activated states was monitored at 90.degree. 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 .mu.M). This decrease in scattering was accompanied by an increase in tension up to an optimum MgATP level of .apprx. 10 .mu.M, and then tension decreased at higher concentrations (10-100 .mu.M). The increase in scattering during both rigor and activation was dependent upon fiber length. At lengths when thick-thin filament overlap was near 0, the light signal due to rigor and activation fell to within 10% of the single for the relaxed fiber at that length. The singal during rigor increased only minimally (.apprx. 10%) when stretch (.apprx. 1%) was applied. This increase in signal was small despite a measured 5-10-fold increase in tension and an estimated 2-fold increase in stiffness. The increased light scattering caused by rigor and activation depends on filament overlap and not tension, stiffness or substrate binding.