Lateral shrinkage of the myofilament lattice in chemically skinned muscles during contraction

A toe muscle was isolated from a hind limb of the mouse and treated with saponin to make the sarcolemma more permeable to the solutes of the bathing medium. The equatorial X-ray diffraction pattern was recorded to determine the 1,0 spacing of the hexagonal myofilament lattice. The spacing in relaxed muscle at a sarcomere length of 2.5 microns was 408 A. When the muscle was maximally activated at pCa 4.4, a steady isometric tension of 1.3 kg/cm2 was produced and the spacing decreased to 384 A. A decrease in spacing of the same magnitude was observed when a relaxed muscle went into rigor, although the rigor tension was only 0.1 kg/cm2, 8% of the maximum contractile tension. From the intensity ratio of the 1,0 and 1,1 reflections the number of myosin heads transferred radially to the vicinity of the thin filament was calculated. During the maximum activation at pCa 4.4 the amount of the radial transfer was 96% of that in rigor. When the muscle was activated at a lower calcium concentration, the lattice shrinkage was smaller and the radial transfer was also smaller. These findings suggest that the lateral force underlying the lattice shrinkage may be due to lateral elasticity of cross-bridges.