Fig. 1.

Structural organization and X-ray diffraction from myosin filaments in beating cardiac trabeculae. (A) Confocal micrograph of a rat cardiac trabecula stained with anti–α-actinin (magenta) and anti–MyBP-C (green); cardiomyocyte boundary outlined in yellow. (Scale bar 10 μm; Inset, 2 μm.) (B) Organization of actin (black) and myosin filaments in one sarcomere repeat, indicating the MyBP-C–containing C-zone (green) and the proximal P- and distal D-zones (white); myosin filament midpoint, M; Z-band, Z (magenta). (C) Small-angle X-ray diffraction pattern from demembranated trabeculae in relaxing solution at SL 2.15 µm, 27 °C, showing meridional myosin-based reflections M1 to M6, the first myosin layer line (ML1), and the 1,0 and 1,1 equatorial reflections (digitally attenuated). Data added from four trabeculae; total exposure time, 160 ms; detector distance, 1.6 m. (Inset) Ultrasmall angle X-ray pattern showing the second-fifth order reflections from the sarcomere repeat; total exposure time, 60 ms; detector distance, 31 m. (D) Time course of force and length change as percentage of initial length (% L₀). Trabeculae were stimulated (vertical line, St) continuously at 1 Hz at SL 1.95 µm, 27 °C. Once per minute trabeculae were stretched by 10% L₀ in 5 ms, starting 40 ms before the stimulus. The original length was restored 600 ms after the stimulus.