Time‐resolved X‐ray diffraction studies on the effect of slow length changes on tetanized frog skeletal muscle.

Abstract

SUMMARY1. The mechanism of the enhancement and the deficit of isometric force by slow length changes in frog skeletal muscle was studied with the time-resolved X-ray diffraction technique, using intense X-rays of synchrotron radiation.2. When a tetanized muscle was slowly stretched by 4 % from sarcomere lengths 2-3-2-4 ,um, the force rose to a peak during stretch and then decreased to a steady level 10-15 % higher than that immediately before stretch.3. The intensity of the 1,1 equatorial reflection decreased nearly linearly during stretch and then again increased after the completion of stretch, reaching a steady level 12+5% (mean + S.D., n = 11) lower than that immediately before stretch. The above 1,1 intensity change was roughly a mirror image of the force change.4. The intensity of the 1,0 equatorial reflection showed no marked changes in response to a slow stretch, except for an initial transient increase observed occasionally.5. If a tetanized muscle was slowly released by 4 % from sarcomere lengths 2-3-2'4 ,sm, the steady force attained after the completion of release was lower than that immediately before release. 6. The 1,1 intensity increased slightly during release, while the 1,0 intensity did not change significantly.7. The half-width of both the 1,0 and the 1,1 reflections did not change appreciably in response to slow length changes.8. Slow length changes always produced changes in the spacing between the reflections as expected from the constant-volume behaviour of the myofilament lattice.9. These results indicate that a slow stretch produces disordering of the myofilament lattice in such a way that the thin filaments are displaced from trigonal positions in the thick filament lattice. The resulting increase in the overall repulsion * Where this work was carried out. t To whom correspondence should be addressed.

Does the speed of shortening affect steady-state force depression in cat soleus muscle?

“…Mare´chal and Plaghki (1979) suggested that force depression might be caused by a stress-induced inhibition of cross-bridge attachments in the overlap zone that is formed between actin and myosin filaments as the muscle shortens. Herzog (1998) refined this idea by speculating that the compliant actin filaments (Amemiya et al, 1988;Goldman and Huxley, 1994;Kojima et al, 1994;Wakabayashi et al, 1994) might be stretched in the isometric contraction prior to shortening, and that this stretching would cause a reorientation of the actin attachment sites (Daniel et al, 1998) which in turn would decrease the probability of crossbridge attachments. This theory would require that force depression is dependent on the force and work performed in the shortening phase and is independent of the speed of shortening.…”

Muscle residual force enhancement: a brief review

“…There are several possible mechanisms at work: a stretch-induced phosphorylation of myosin light chains (52), a disordering of the myofilament lattice (53), or an increase in the duration of attachment (dwell time) (3). …”

“…Stretch after activation may induce cross-bridge phosphorylation (57), myofilament-lattice disordering (53), and/or repartitioning of the cross-bridge population between the “force-bearing” and “non-force-bearing” molecular states (58-61). In the first case, the phosphorylated myosin light chains would bend myosin heads towards the actin filament (62,63).…”

“…In the first case, the phosphorylated myosin light chains would bend myosin heads towards the actin filament (62,63). In the second case, myofilament disordering would cause the thin filaments to move closer to the thick filaments (53). In the third case, changes in cross-bridge distribution between the two aforementioned molecular states could induce an increase in the “non-force-bearing” state that would indirectly slow down the detachment rate after stretch because there would be more bridges in the “non-force-bearing” state available to move forward to the “force-bearing” state (59,60).…”

Time-resolved two-dimensional X-ray diffraction study of the effect of shortening on activation of contracting skeletal muscle

“…The molecular mechanism of this phenomenon was pursued in a time-resolved X-ray diffraction study, in which equatorial reflection intensities were measured to monitor the number of myosin heads that attach to or stay in the vicinity of actin [5]. In twitch contractions, a shortening step induces the intensities to approach their resting values [5], while only a small, if any, effect was observed in tetanus [1,4]. These results suggest that at low [Ca 2+ ] i the detached myosin heads do not reattach to actin, but move away from it towards their resting positions on the myosin filament backbone.…”