Modulation Of Cardiac Thin Filament Azimuthal Rigidity By Calcium And Cross-Bridges

The cardiac actin-containing filament periodicity within the myofilament lattice remain poorly understood mainly due to the challenges of obtaining thin filament meridional reflections and layer lines in heart muscle tissue. Small-angle X-ray diffraction experiments conducted on permeabilized fresh porcine cardiac papillary muscle reveal for the first time to our knowledge that cardiac muscle exhibits distinctive thin filament properties compared to fast skeletal muscle. Furthermore, the use of permeabilized muscle allows us to determine the individual roles of Ca2+ and myosin cross-bridges with respect to cardiac thin filament azimuthal rigidity during muscle contraction. Our results reveal that 1) cardiac actin-containing filaments display reduced stiffness compared to fast skeletal muscle; 2) Ca2+ and strong-binding cross-bridges both contribute to thin filament azimuthal rigidity during contraction and in rigor; and 3) actin first layer line reflections were observed under rigor conditions, however, no tropomyosin reflections (on the second actin layer line) were detected. These data suggest that both Ca2+ alone and myosin cross-bridges contribute to the thin filament length changes. Furthermore, compared to fast skeletal muscle, cardiac muscle shows an overall reduction in the long-pitch helix periodicity of the actin filament spacing, similar to that seen in nebulin-knockout soleus muscle, suggesting that while nebulin is important to pre-stretch the skeletal muscle, this mechanism may be lacking in the cardiac muscle. NIH-R01HL128683&NIH-P41GM103622.