Structural Features of Force-Generating Cross-Bridges
In several concepts of muscle contraction1–3 it was proposed that during force generation cross-bridges assume a rigor-like conformation. However, so far 2D-X-ray diffraction patterns recorded during isometric contraction did not reveal structural features of rigor-like cross-bridges. It was therefore supposed that the number of force generating cross-bridges in isometric steady state contraction is too low to be detected in 2D-X-ray diffraction patterns.
To test this concept we studied the features of 2D-X-ray diffraction patterns of different fractions of rigor-like cross-bridges by varying the concentration of the nucleotide analog ATPγS at high [Ca++]. Although we had a fully activated system, i.e., that weak binding cross-bridges were attached to the activated thin filament, none of the patterns was like the active pattern. Analysis of the intensity change of the actin layer lines at 1/370 Å−1 and at 1/59 Å−1 in diffraction patterns recorded during isometric contraction vs. relaxed and rigor conditions as well as mechanical experiments revealed that the fraction of force generating cross-bridges apparently is as high as 55–70%. Yet, in agreement with previous studies, in spite of this large fraction of force generating cross-bridges, no rigorlike features were detected in diffraction patterns recorded during isometric contraction. Moreover, the intensity distribution along the actin layer lines was clearly different in rigor compared to isometric contraction. Taken together these results provide evidence that in contrast to the above mentioned models the majority of the force generating cross-bridges seems to be in a conformation which is different from rigor.
KeywordsIsometric Contraction Intensity Profile Thin Filament Layer Line High Calcium Concentration
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