Disassembly from both ends of thick filaments in rabbit skeletal muscle fibers. An optical diffraction study

We show in this paper that the change of the internal structure of a sarcomere in a rabbit glycerinated psoas muscle fiber can be examined by analyzing the intensity change of the first- and second-order optical diffraction lines. A unit-cell (sarcomere)-structure model has been applied to the estimation of the length of thick filaments in a muscle fiber while they undergo dissociation. The optical factors, except for the unit-cell-structure factor, hardly changed during the dissociation of the filaments. Our results show that thick filaments dissociate from both ends on increasing the KCl concentration in the presence of 10 mM pyrophosphate and 5 mM MgCl₂. Micromolar concentrations of Ca²⁺ suppressed to some extent the dissociation of thick filaments. The disassembly of thick filaments occurred at higher KCl concentrations in the absence of pyrophosphate. There was a correlation between the stability of the thick filament structure and cross-bridge formation, which was induced either by the addition of micromolar concentrations of Ca²⁺ in the presence of Mg-pyrophosphate or by removal of Mg-pyrophosphate.