Abstract
During a very low load shortening, the intensity ratio of (1,0)/(1,1) equatorial x-ray diffraction (BL45XU at SPring 8), which indicates the mass transfer from myosin to actin filaments, does not decrease so much as during isometric contraction, that is, the amount of mass change is less during low load shortening. The (1,0) lattice spacing (d10) is one of the factors which affects the mass transfer. To examine the reduced mass transfer, the d10 during shortening is carefully studied in rat papillary muscle. The intensity of (1,0) equatorial diffraction is fitted by Gaussian equation, and its standard deviation (σ10) is calculated. Once σ10 is normalized by its mean value (ε10), it varies uniquely during isometric contraction compared with ε10 during low load shortening. The d10 increases more during low load shortening than during isometric contraction as is the results of sarcomere shortening. However the ε10 increases during isometric contraction, it flattens during low load shortening. Therefore, even if the d10 spread widely during low load shortening, the d10 disorder is less pronounced than during isometric contraction. It is suggested that the d10 disorder can be affected by the interaction between actin and myosin filaments caused by crossbridge formation and the amount of disorder may be associated with the strength of force generated by corssbridges. [J Physiol Sci. 2007;57 Suppl:S94]
