Time-Resolved X-Ray Diffraction by Skinned Skeletal Muscle Fibers during Activation and Shortening

Abstract

Force, sarcomere length, and equatorial x-ray reflections (using synchrotron radiation) were studied in chemically skinned bundles of fibers from Rana temporaria sartorius muscle, activated by UV flash photolysis of a new photolabile calcium chelator, NP-EGTA. Experiments were performed with or without compression by 3% dextran at 4°C. Isometric tension developed at a similar rate ( t 1/2 = 40 ± 5 ms) to the development of tetanic tension 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 isometric tension development and were faster than for tetanus. During shortening at 0.14 P o, I 10 and I 11 changes were partially reversed (18% and 30%, 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, consistent 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 without 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.