X-ray diffraction patterns from intact muscle fibres of Rana esculenta were collected at rest and during isometric contraction at sarcomere length 2.1–3.5 mm (temperature 4°C). For each exposure (2–5 ms) the change in the number of myofilaments in the X-ray beam at different positions along each fibre and between different fibres was taken into account by normalizing the intensity of the reflection of interest by the intensity of the equatorial 1,0 reflection at rest (I1,0). The very low-angle diffracted intensity along the meridian had an almost sinusoidal intensity distribution with a periodicity correlated with the sarcomere length up to 2.8 mm, while for longer sarcomere lengths it became weaker, indicating the loss of sarcomeric order. In the range 2.1–2.8 mm the intensity during active contraction of the meridional M3 reflection (I_M3), originating from the axial repeat of myosin heads, was linearly related to sarcomere length, with an abscissa intercept at 3.5 mm (Linari et al. PNAS 97:7226, 2000). The fine structure of the M3 reflection, defined by the intensity ratio R_M3, decreased from 0.93 at 2.1 mm to 0.84 at 2.7 mm. These results are better explained by structural models that assume that the ~40% contribution to I_M3 of the detached heads is mainly due to the partner heads of dimers with the first head attached to actin (Brunello et al. PNAS 104:20114, 2007).

Supported by MiUR, FIRB Futuro in ricerca (Italy), MRC (UK), ESRF.