Structural Changes in the Actin–Myosin Cross-Bridges Associated with Force Generation Induced by Temperature Jump in Permeabilized Frog Muscle Fibers

Abstract

Structural changes induced by Joule temperature jumps (T-jumps) in frog muscle fibers were monitored using time-resolved x-ray diffraction. Experiments made use of single, permeabilized fibers that were fully activated after slight cross-linking with 1-ethyl-3-[3-dimethylamino)propyl]carbodiimide to preserve their structural order. After T-jumps from 5–6 to ∼17°C and then on to ∼30°C, tension increased by a factor of 1.51 and 1.84, respectively, whereas fiber stiffness did not change with temperature. The tension rise was accompanied by a decrease in the intensity of the (1, 0) equatorial x-ray reflection by 15 and 26% (at ∼17 and ∼30°C) and by an increase in the intensity of the M3 myosin reflection by 20% and 41%, respectively. The intensity of the (1, 1) equatorial reflection increased slightly. The peak of the intensity on the 6th actin layer line shifted toward the meridian with temperature. The intensity of the 1st actin layer line increased from 12% (of its rigor value) at 5–6°C to 36% at ∼30°C, so that the fraction of the cross-bridges labeling the actin helix estimated from this intensity increased proportionally to tension from ∼35% at 5–6°C to ∼60% at ∼30°C. This suggests that force is generated during a transition of nonstereo-specifically attached myosin cross-bridges to a stereo-specific binding state.