Time-resolved x-ray diffraction by skinned skeletal muscle fibers during activation and shortening

Force, sarcomere length, and equatorial x-ray reflections (using synchrotro n radiation) were studied in chemically skinned bundles of fibers from Rana temporaria sartorius muscle, activated by UV flash photolysis of a new pho tolabile calcium chelator, NP-EGTA, Experiments were performed with or with out compression by 3% dextran at 4 degreesC. Isometric tension developed at a similar rate (t(1/2) = 40 +/- 5 ms) to the development of tetanic tensio n measured in other studies (Cecchi et al., 1991). Changes in intensity of equatorial reflections (I-11 t(1/2), 15-19 ms; I-10 t(1/2), 24-26 ms) led i sometric tension development and were faster than for tetanus. During short ening at 0.14P(o), I-10 and I-11 changes were partially reversed (18% and 3 0%, respectively, compressed lattice), in agreement with intact cell data. In zero dextran, activation caused a compression of A-band lattice spacing by 0.7 nm. In 3% dextran, activation caused an expansion of 1.4 nm, consist ent with an equilibrium spacing of 45 nm. But, in both cases, discharge of isometric tension by shortening caused a rapid lattice expansion of 1.0-1.1 nm, suggesting discharge of a compressive cross-bridge force, with or with out compression by dextran, and the development of an additional expansive force during activation. In contrast to I-10 and I-11 data, these findings for lattice spacing did not resemble intact fiber data.