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 degrees C, using time-resolved, low-angle X-ray diffraction. The structural order in t he fibres and their isometric function were preserved by cross-linking 10-2 0 % of the myosin cross-bridges to the thin filaments. 2. The time courses of the change in force, stiffness and in intensity of t he main equatorial reflections (1,0) and (1,1), of the third myrosin layer line (M3) at a reciprocal spacing of (14.5 nm)(-1) on the meridian and of t he first myosin-actin layer line (LL1) were measured with 1 ms time resolut ion. 3. In the absence of Ca2+, photolytic release of ATP in muscle fibres initi ally in the rigor state caused the force and stiffness to decrease monotoni cally towards their values in relaxed muscle fibres. 4. In the presence of Ca2+, photolytic release of ATP resulted in an initia l rapid decrease in force, followed by a slower increase to the isometric p lateau. Muscle fibre stiffness decreased rapidly to similar to 65 % of its value in rigor. 5. In the absence of Ca2+, changes on the equator, in LL1 and in M3 occurre d with a time scale comparable to that of the changes in tension and stiffn ess. 6. In the presence of Ca2+, the changes on the equator and LL1 occurred sim ultaneously with the early phase of tension decrease. The changes in the in tensity of M3 (I-M3) occurred on the time scale of the subsequent increase in force. The time courses of the changes in tension and I-M3 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 rel eased. 7. In the presence of Ca2+, the time course of changes in I-M3 closely mimi cked 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.