Non-isovolumic behavior of the unit cell of skinned striated muscle fibers.

X-ray diffraction studies indicate that the sarcomeric unit cell of the thick filament lattice of crayfish muscle fibers stripped of the sarcolemma does not shorten isovolumically. This particular behavior can be explained in terms of the variation of negative charge within the A-band with interdigitation of thin filaments. Values for effective charge densities of the filaments are determined and used to calculate the total effective charge within the A-band which is then related to separation between myosin filaments. Correlation between empirical and theoretical values illustrates that interfilament separation in skinned fibers is a linear function of A-band charge.