Effects of the Number of Actin-Bound S1 and Axial Force on X-Ray Patterns of Intact Skeletal Muscle

Abstract

Effects of the number of actin-bound S1 and of axial tension on x-ray patterns from tetanized, intact skeletal muscle fibers were investigated. The muscle relaxant, BDM, reduced tetanic M3 meridional x-ray reflection intensity (I(M3)), M3 spacing (d(M3)), and the equatorial I(11)/I(10) ratio in a manner consistent with a reduction in the fraction of S1 bound to actin rather than by generation of low-force S1-actin isomers. At complete force suppression, I(M3) was 78% of its relaxed value. BDM distorted dynamic I(M3) responses to sinusoidal length oscillations in a manner consistent with an increased cross-bridge contribution to total sarcomere compliance, rather than a changed S1 lever orientation in BDM. When the number of actin-bound S1 was varied by altering myofilament overlap, tetanic I(M3) at low overlap was similar to that in high [BDM] (79% of relaxed I(M3)). Tetanic d(M3) dependence on active tension in overlap experiments differed from that observed with BDM. At high BDM, tetanic d(M3) approached its relaxed value (14.34 nm), whereas tetanic d(M3) at low overlap was 14.50 nm, close to its value at full overlap (14.56 nm). This difference in tetanic d(M3) behavior was explicable by a nonlinear thick filament compliance which is extended by both active and passive tension.

Probing Muscle Myosin Motor Action: X-Ray (M3 and M6) Interference Measurements Report Motor Domain Not Lever Arm Movement

“…1b and c) for the three main myosin head shapes seen or inferred by X-ray crystallography as in the analysis by Irving et al 4 These are the Dominguez et al 2 pre-powerstroke shape, the Rayment et al 3 rigor or postpowerstroke shape, both shown in (Fig. 1b), and, thirdly, a hypothetical intermediate, so-called '5.4-nm' shape discussed by Irving et al 4 In order for there not to be differences in the head density profiles being used, we have reproduced these published profiles (from Ref.…”

Monitoring the myosin crossbridge cycle in contracting muscle: steps towards ‘Muscle—the Movie’

“…The M3 reflection has been the subject of intense study, including elegant experiments recording M3 interference changes from active muscle (e.g. Huxley et al 1982; Lombardi et al 1995; Irving et al 1992, 2000; Dobbie et al 1998; Linari et al 2000; Bagni et al 2001; Piazzesi et al 2002; Reconditi et al 2004; Ferenczi et al 2005; Brunello et al 2006; Griffiths et al 2006; Huxley et al 2006a, b). These experiments were originally interpreted in terms of the active M3 coming solely from heads attached to actin, with the M3 intensity depending on the position of the lever arm of the actin-attached heads.…”

Effects of myosin inhibitors on the X-ray diffraction patterns of relaxed and calcium-activated rabbit skeletal muscle fibers

“…These studies show that the intensity ratio of the 1,1 and 1,0 equatorial reflections (I1,1/I1,0 ratio, reflecting the number of myosin heads attached to or staying in the vicinity of the thin filaments) is greater than expected from the force level in muscle fibers activated in the presence of BDM. However, a conflicting result was obtained by Griffiths et al [ 23 ], who showed that the I1,1/I1,0 ratio approached linearly to the relaxed level as the BDM concentration is increased. From this result they concluded that BDM simply reduces the number of attached myosin heads.…”