THE EFFECT OF ACTIN FILAMENT COMPLIANCE ON THE INTERPRETATION OF THE ELASTIC PROPERTIES OF SKELETAL-MUSCLE FIBERS

Recently, X-ray diffraction studies provided direct evidence for an ap preciable length change in the actin filament upon activation. This fi nding has profound implications on the interpretation of the elastic p roperties of skeletal muscle fibre. In this study we determined the co mpliance of the actin filament during activation, using the data obtai ned previously from quick stretch and release experiments on skeletal muscle fibres of the frog. The effects of filament compliance are demo nstrated clearly in the elastic properties of partially activated fibr es. The low-frequency elasticity increases linearly with tension, refl ecting an increase in the number of force-producing cross-bridges. At higher frequencies, this linearity is lost. Ln this study we describe the data consistently in terms of a cross-bridge stiffness increasing linearly with tension and a constant Young's modulus for the actin fil ament of 44 MN m(-2). This corresponds to a compliance of 23 pm mu m(- 1) per kN m(-2) tension developed. Using this value for the actin fila ment Young's modulus, its contribution to the elastic properties of sk eletal muscle fibre of the frog is considered in rigor and relaxation. The filament compliance hardly affects the overall elasticity of the musle fibre in relaxation. In contrast, it contributes to a large exte nt to the overall elasticity in rigor. Taking account of the filament compliance, we find that the Youngs modulus in rigor exhibits an incre ase from 14 MN m(-2) at frequencies below 500 Hz to 55 MN m(-2) above 40 kHz.