Changes in thick filament structure during compression of the filament lattice in relaxed frog sartorius muscle

Equatorial X-ray diffraction patterns from relaxed, chemically-skinned frog sartorius muscles under a range of external osmotic pressures from 0 to 290 torr have been analysed and compared to the pattern from the relaxed intact muscle. Lattice spacings and electron density diagrams were determined as a function of external pressure. Reflection intensities, averaged over small ranges of pressures, were determined out to the 4,0 reflection; phases for the first five orders were established as ++--+ over the whole pressure range. As external pressure was increased, lattice spacing decreased, as did full width at half maximum density for both thick and thin filaments. Most of the lattice spacing and thick filament compression occurred at low pressure, whereas thin filaments were compressed proportionally to pressure over the whole pressure range. These conclusions were confirmed by fitting cylindrical models for filament density to the X-ray diffraction patterns. Axially-projected electron density across the A-band filament lattice showed that in relaxed muscle the thick filament projections (myosin heads) are concentrated in regions between adjacent thick filaments, as far as possible from the thin filaments, and they tend to become pushed against the thick filament backbone as the lattice is compressed. Both thick and thin filament axes can be displaced randomly from their lattice positions; on average this displacement is about twice as great for the thin filaments, accounting for their larger projected size as compared to isolated thin filaments and for their apparent decrease in diameter as the lattice is compressed.