X-Ray Equatorial Diffraction during ATP-Induced Ca²⁺-Free Muscle Contraction and the Effect of ADP

Abstract

We examined the equatorial X-ray diffraction of skinned fibers from rat psoas muscle in isometric contraction induced by photorelease of 1.1mM ATP at 24°C in the absence of Call. At 15-20 ms after release of ATP, tension peaked at 21% of the maximum active tension. From the equatorial intensity ratio (I_{1, 0}/I_{1, 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 (0.7mM) 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 Ca²⁺-free contraction is more than expected from the tension size, both in the absence and presence of ADP.