Equatorial x-ray diffraction from single skinned rabbit psoas fibers at various degrees of activation. Changes in intensities and lattice spacing.

Equatorial x-ray diffraction patterns were obtained from single skinned rabbit psoas fibers during various degrees of activation under isometric conditions at ionic strength 170 mM and 6-9 degrees C. By direct calcium activation, contraction was homogeneous throughout the preparation, and by using a cycling technique (Brenner, 1983) integrity of the fiber was maintained even during prolonged steady activation. The intensity ratio of the two innermost reflections I11/I10, and the normalized intensities I*10 and I*11 varied linearly with increasing force. Thus the result agreed qualitatively with an earlier finding, obtained from the whole sartorius muscle, that intensity changes in 10 and 11 are directly correlated with isometric force level (Yu et al., 1979). Spacing of the myofilament lattice (d10) was found to decrease with increasing isometric tension. With the filaments in full overlap, maximum shrinkage was 14%. The lattice spacing started to level off when the degree of calcium activation was greater than or equal to 50%, approaching a limit approximately at 380-360 A. This decrease of the lattice spacing indicates that there is a radial force produced by force generating cross-bridges, but the net radial force appears to become insignificant as lattice spacing approaches 380-360 A.