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 approximately 17 degrees C and then on to approximately 30 degrees 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 approximately 17 and approximately 30 degrees 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 degrees C to 36% at approximately 30 degrees C, so that the fraction of the cross-bridges labeling the actin helix estimated from this intensity increased proportionally to tension from approximately 35% at 5-6 degrees C to approximately 60% at approximately 30 degrees C. This suggests that force is generated during a transition of nonstereo-specifically attached myosin cross-bridges to a stereo-specific binding state.

Changes in myosin S1 orientation and force induced by a temperature increase

“…This increase probably is absent because of increased disordering of radial alignment of myofilaments during tetani at higher temperature. In temperaturejump experiments, where such disordering may not have time to develop, an increase in I M3 has been observed (25). By determining tail disposition relative to a reference point (I M3 max) using sinusoidal oscillations, we avoid such difficulties in defining S1 structure purely from static intensity and provide an alternative and powerful means of determination of tail orientation in different contractile states.…”

Very low force-generating ability and unusually high temperature-dependency in hummingbird flight muscle fibers

“…An approximate fivefold increase in force generated in rabbit psoas fibers, following a rapidly induced temperature jump of more than 30°C (from ~5 tõ 36°C), has been reported (Bershitsky and Tsaturyan, 2002). Q 10-force values for force generation in frog fibers are even lower [~1.2-1.4 (reviewed in Rall and Woledge, 1990;Tsaturyan et al, 1999)]. The mean P o /CSA of adult chicken pectoralis fibers at 15°C (Reiser et al, 1992;Reiser et al, 1996) was approximately three times greater than at 10°C (P.J.R., unpublished).…”

Mechanics and models of the myosin motor

“…Bershitsky et al (1997), however, found that during the rise of tension following a temperature jump, this actin layer line becomes stronger, implying that the azimuthal Mechanics and models of the myosin motor A. F. Huxley 437 orientation of the myosin heads attached to actin comes to follow the long helix of the thin ¢lament. Furthermore, Tsaturyan et al (1999) estimate that the fraction of crossbridges labelling the actin helix increases from about 35% at 5^6 8C to about 60% at 30 8C (permeabilized frog ¢bres). The absence of an increase of sti¡ness following the temperature jump shows that the heads must have been already attached to actin but disorientated in azimuth.…”