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

We studied the effect of myosin inhibitors, N-benzyl-p-toluenesulfonamide (BTS), blebbistatin, and butanedione monoxime (BDM) on X-ray diffraction patterns from rabbit psoas fibers under relaxing and contracting conditions. The first two inhibitors suppressed the contractile force almost completely at a 100 μM concentration, and a similar effect was obtained at 50 mM for BDM. However, still substantial changes were observed in the diffraction patterns upon calcium-activation of inhibited muscle fibers. (1) The 2nd actin layer-line reflection was enhanced normally, indicating that calcium binding to troponin and the subsequent movement of tropomyosin are not inhibited, (2) the myosin layer-line reflections became much weaker, and (3) the 1,1/1,0 intensity ratio of the equatorial reflections was increased. The observations (2) and (3) indicate that, even in the presence of the inhibitors at a saturating concentration, myosin heads leave the helix on the thick filaments and approach the thin filaments. Interestingly, the d1,0 spacing of the filament lattice remained unchanged upon activation of inhibited fibers, in contrast to the case of normal activation in which the spacing is decreased. This suggests that the normal activated myosin heads exert a pull in both axial and radial directions, but in the presence of the inhibitors, the pull is suppressed, and as a result, the heads simply bind to actin without exerting any force. The results support the idea that the inhibitors do not block the myosin binding to actin, but block the step of force-producing transition of the bound actomyosin complex.