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

We examined the equatorial X-ray diffraction of skinned fibers from rat psoas muscle in isometric contraction induced by photorelease of 1.1 mM ATP at 24 degrees C in the absence of Ca2+. At 15-20 ms after release of ATP, tension peaked at 21% of the maximum active tension. From the equatorial intensity ratio (I1,0/I1,1), we estimated the amount of myosin heads associated with actin filaments (A.M) at the tension peak. It was 32% of that in rigor and about half of that in the maximum contraction. ADP (0.7 mM) enhanced the contraction: tension peaked at 47% of the maximum at 25-30 ms. The estimated A.M at this peak was 50% of that in rigor, which was close to that in the maximum contraction. These results indicate that the association of myosin heads with the actin filaments in the Ca(2+)-free contraction is more than expected from the tension size, both in the absence and presence of ADP.