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

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 (IM3), M3 spacing (dM3), and the equatorial I11/I10 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, IM3 was 78% of its relaxed value. BDM distorted dynamic IM3 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 IM3 at low overlap was similar to that in high [BDM] (79% of relaxed IM3). Tetanic dM3 dependence on active tension in overlap experiments differed from that observed with BDM. At high BDM, tetanic dM3 approached its relaxed value (14.34 nm), whereas tetanic dM3 at low overlap was 14.50 nm, close to its value at full overlap (14.56 nm). This difference in tetanic dM3 behavior was explicable by a nonlinear thick filament compliance which is extended by both active and passive tension.