Effect of Active Shortening and Stretching on Lattice Spacing and Cross-Bridge Binding in Skinned Muscle Fibres

Abstract

When a tetanized skeletal muscle is actively shortened or stretched the resulting steady-state force is different from the purely isometric force at the corresponding length. Active shortening reduces steady-state force (termed force depression-FD), while active lengthening increases steady-state force (termed force enhancement-FE). According to the cross-bridge theory, FD and FE can occur if myofilament lattice spacing was modified after active shortening and stretching leading to a change in the proportion of attached cross-bridges, or if the kinetics of cross-bridges were modified leading to a change in the proportion of attached cross-bridges and/or the average force per cross-bridge. Here, we determined the effect of active shortening and stretching on the myofilament lattice spacing and the proximity of the myosin heads to the thin filaments using small angle X-ray diffraction.Lattice spacing (d1,0) was calculated from the spacing between the 1,0 and 1,1 equatorial reflections. The ratio of the intensities of the 1,1 and 1,0 equatorial reflections (I1,1:I1,0) was used as an estimate of the degree of association of myosin heads with the thin filaments.d1,0 increased (38.9±0.1 versus 38.1±0.2nm, p 0.05) between the FE state and its reference state, and I1,1:I1,0 was increased (2.5±0.2 versus 1.3±0.1, p<0.05) in the FE state. These results suggest that the proportion of attached cross-bridges might be increased after active stretch and that this increase is not due to a decrease in lattice spacing but rather a change in cross-bridge kinetics.