Regional assessment of crossbridge cycling in beating rat hearts

Abstract

We have shown that the dynamics of crossbridge cycling can now be determined in beating hearts in situ and in real time using high flux synchrotron radiation at SPring-8 and x-ray diffraction techniques. Mass transfer of myosin to actin during contraction was inferred from the change in diffraction intensity ratio (intensity of 1,0 reflection / intensity of 1,1 reflection) obtained from the lattice arrangements of myosin (thick) and actin (thin) filaments in myofibers between diastole and systole. Others have suggested that myofilament spacing might influence the probability of crossbridge formation. In this study we examined for the first time, if myosin lattice spacing (d_1,0 spacing between myosin planes) is regulated to maintain constant lattice volume and how mass transfer is related to d_1,0 spacing during the cardiac cycle in different regions of the left ventricle. Our results show that crossbridge formation was essentially complete within the isovolumic contraction phase, and that while mass transfer was proportional to left ventricle pressure in non-ischemic hearts, this relation deteriorated in ischemic regions of the heart after coronary occlusion. This suggests that diffraction intensity ratio change is more sensitive to regional contractility than global contractility indices. As lattice spacing was significantly larger during contraction than ventricular filling (at the same volume) we suggest that constant lattice volume is not maintained during dynamic contractions. [Jpn J Physiol 54 Suppl:S104 (2004)]