CHARACTERIZATION OF RADIAL FORCE AND RADIAL STIFFNESS IN CA2+-ACTIVATED SKINNED FIBERS OF THE RABBIT PSOAS MUSCLE

1. When chemically skinned muscle fibres are activated by Ca2+ at an ionic strength of 170 mM, the spacing between the filaments has been shown to decrease with increasing force, suggesting that the cross-bridges can generate force not only in the axial but also in the radial direction. In the present study, radial force and radial stiffness of activated single skinned rabbit psoas fibres were studied by X-ray diffraction. The responses of the lattice spacing to changes in osmotic pressure by application of dextran T500, which is equivalent to force applied in the radial direction, was examined. The radial force generated by the attached cross-bridges was calculated, with the approximation that a negligible fraction of cross-bridges was attached in the relaxed muscle at the same ionic strength of 170 mM. 2. The active radial force was found to be a slightly non-linear function of lattice spacing, reaching zero at 34 nm. The radial force was compressive at lattice spacing greater than 34 nm and expansive at less than 34 nm. 3. The active axial force, on the other hand, was found to be much less affected by the application of dextran T500. Active axial force increased by 4% to a plateau at 4% dextran T500 and then decreased by 10% at 8% dextran T500. 4. While not under osmotic pressure, the radial force of the activated fibre was determined to be 400 pN (single thick filament)-1. This is of the same order of magnitude as the axial force. The radial stiffness was also comparable to the axial stiffness at 7 pN (thick filament)-1 (0.1 nm)-1. 5. The radial elasticity of the fully activated fibre differs significantly from that of the fiber in rigor. The radial stiffness exhibited by fibres in rigor was approximately five times higher, at 30 pN (thick filament)-1 (0.1 nm)-1 and the point where the radial force reached zero was 38 nm. 6. In the activated state, the point at which radial force reaches zero is independent of the level of Ca2+ activation, i.e. independent of the number of cross-bridges attached to actin in the force-generating state. We suggest that the zero-force point is equivalent to the equilibrium point of a spring and is an intrinsic property of the radial elasticity of the cross-bridge. 7. It is concluded that activated and rigor cross-bridges exhibit a spring-like property in the radial direction. Furthermore, the equilibrium point of the radial elasticity appears to depend on the physiological state of the cross-bridges. The significance of the large magnitude of radial elasticity and of the different equilibrium points is discussed.