X-ray equatorial diffraction during ATP-induced Ca(2+)-free muscle contraction and the effect of ADP

Structural transients of contractile proteins upon sudden ATP liberation in skeletal muscle fibers

Structural changes of contractile proteins were examined by millisecond time-resolved two-dimensional x-ray diffraction recordings during relaxation of skinned skeletal muscle fibers from rigor after caged ATP photolysis. It is known that the initial dissociation of the rigor actomyosin complex is followed by a period of transient active contraction, which is markedly prolonged in the presence of ADP by a mechanism yet to be clarified. Both single-headed (overstretched muscle fibers with exogenous myosin subfragment-1) and two-headed (fibers with full filament overlap) preparations were used. Analyses of various actin-based layer line reflections from both specimens showed the following: 1), The dissociation of the rigor actomyosin complex was fast and only modestly decelerated by ADP and occurred in a single exponential manner without passing through any detectable transitory state. Its ADP sensitivity was greater in the two-headed preparation but fell short of explaining the large ADP effect on the transient active contraction. 2), The decay of the activated state of the thin filament followed the time course of tension more closely in an ADP-dependent manner. These results suggest that the interplay between the reattached active myosin heads and the thin filament is responsible for the prolonged active contraction in the presence of ADP.

Single turnover of cross-bridge ATPase in rat muscle fibers studied by photolysis of caged ATP

A mechanical study on skinned rat psoas muscle fibers was performed at about 16 degrees C with X-ray diffraction and caged-ATP photolysis. The amount of photoreleased ATP was set < 0.2 mM for analysis of a 'single turnover' of the cross-bridge ATPase. With regard to the phase of activation, the results under the single turn-over condition were generally consistent with previous results obtained with larger amount of photoreleased ATP. Formation of the ADP-rigor state was mechanically monitored by the 90 degrees out-of-phase component of stiffness at 500 Hz, which was elevated on activation and then decreased to zero with a half-time of 0.2-0.3 s. Intensity changes of the X-ray reflections (e.g. equatorial reflections, actin layer lines and a myosin meridional reflection) indicated that a large number of cross-bridges returned to the rigor structure with a half-time of 0.5-0.7 s. During this phase, tension did not increase but slowly decreased with a half-time of about 1.0 s. The in-phase stiffness increased only 20-30% at the most. These results indicate that, even if the number of cross-bridges formed at any moment during full contraction is small, they can interact with actin and form rigor bonds with a rate of 1 s(-1). The force developed in the rigor formation is probably lost due to the presence of rigor bridges and compliance in the preparation.

Structural responses to the photolytic release of ATP in frog muscle fibres, observed by time-resolved X-ray diffraction

1. Structural changes following the photolytic release of ATP were observed in single, permeabilised fibres of frog skeletal muscle at 5-6 C, using time-resolved, low-angle X-ray diffraction. The structural order in the fibres and their isometric function were preserved by cross-linking 10-20 % of the myosin cross-bridges to the thin filaments. 2. The time courses of the change in force, stiffness and in intensity of the main equatorial reflections (1,0) and (1,1), of the third myosin layer line (M3) at a reciprocal spacing of (14.5 nm)-1 on the meridian and of the first myosin-actin layer line (LL1) were measured with 1 ms time resolution. 3. In the absence of Ca2+, photolytic release of ATP in muscle fibres initially in the rigor state caused the force and stiffness to decrease monotonically towards their values in relaxed muscle fibres. 4. In the presence of Ca2+, photolytic release of ATP resulted in an initial rapid decrease in force, followed by a slower increase to the isometric plateau. Muscle fibre stiffness decreased rapidly to approximately 65 % of its value in rigor. 5. In the absence of Ca2+, changes on the equator, in LL1 and in M3 occurred with a time scale comparable to that of the changes in tension and stiffness. 6. In the presence of Ca2+, the changes on the equator and LL1 occurred simultaneously with the early phase of tension decrease. The changes in the intensity of M3 (IM3) occurred on the time scale of the subsequent increase in force. The time courses of the changes in tension and IM3 were similar following the photolytic release of 0. 33 or 1.1 mM ATP. However the gradual return towards the rigor state began earlier when only 0.33 mM ATP was released. 7. In the presence of Ca2+, the time course of changes in IM3 closely mimicked that of force development following photolytic release of ATP. This is consistent with models that propose that force development results from a change in the average orientation of cross-bridges, although other factors, such as their redistribution, may also be involved.

Mechanical study of rat soleus muscle using caged ATP and X-ray diffraction: high ADP affinity of slow cross-bridges

1. The cross-bridges in slow- and fast-twitch fibres (taken, respectively, from soleus and psoas muscles of rats) were examined in mechanical experiments using caged ATP and X-ray diffraction, to compare their binding of ATP and ADP. 2. Caged ATP was photolysed in rigor fibres. When ADP was removed from pre-photolysis fibres, the initial relaxation (+/- Ca2+) in soleus was as fast as that in psoas fibres, whereas the subsequent contraction (+Ca2+) was slower in soleus than in psoas. The ATPase rate during the steady-state contraction was also slower in soleus fibres. 3. When ADP was added to pre-photolysis fibres (+/- Ca2+), tension developed even in the initial phase, the overall tension development being biphasic. Both initial and late components of the Ca(2+)-free contraction were enhanced when ADP was added before photolysis, although pre-photolysis ADP was not a prerequisite for the late component. The effect of ADP was greater in soleus than in psoas fibres. Static experiments on rigor fibres revealed a higher ADP affinity in soleus fibres. 4. The intensity of the actin layer-line from ADP rigor soleus fibres decreased rapidly on photorelease of ATP. We conclude that, despite the tight ADP binding of the soleus cross-bridge, its isometric reaction is not rate limited by the 'off' rate of ADP.