EQUATORIAL X-RAY-INTENSITIES AND ISOMETRIC FORCE LEVELS IN FROG SARTORIUS MUSCLE

Isometric force levels, ranging between 0 and 100% of maximal force P0 at 2 to 3 °C, were elicited in frog sartorius muscle by means of rapidly cooling a Ringer solution containing 1·25 to 2·0 mm-caffeine. Equatorial X-ray diffraction patterns were obtained in the resting state and during contraction. The ratio of the intensities I11 I10 increased with force almost linearly, with a slight upward curvature. The individual intensities for the contracting state were normalized relative to both the intensity of the undiffracted beam and the intensity of each reflection in the resting state. These normalized intensities were found to vary in a reciprocal way: I10 decreased while I11 increased throughout the range of forces studied. The gradual change in I11 I10 with force level indicates that this ratio is a sensitive measure of the number of cross-bridges in the isometric state. A two-state model in which myosin projections are either in a resting or attached state and in which force is proportional to the fraction of projections in the attached state was applied to the experimental data of the individual reflections. I10 deviates from this model in a way that suggests that formation of the first few cross-bridges may decrease the regularity of the remaining unattached myosin projections. © 1979.