A model relating protein osmotic pressure to the stiffness of the cross-bridge components and the contractile force of skeletal muscle.

We have modeled the effect of protein osmotic pressure on the orientation of the monomer in F-actin, in tropomyosin-F-actin, in the myosin subfragment-1 decorated F-actin and in the myosin subfragment-1 decorated tropomyosin-F-actin. According to the model, at the physiological protein osmotic pressure (18 kPa), the elastic moduli by bending of the monomer in F-actin and in tropomyosin-F-actin are calculated to be 4.74 MPa and 5.8 MPa, respectively. The elastic moduli by bending of the monomer in the myosin subfragment-1 decorated F-actin and in the myosin subfragment-1 decorated tropomyosin-F-actin are calculated to be 22MPa and 22.3MPa, respectively. These latter values are in excellent agreement with the values of the elastic moduli by stretching found for the fibres of frog and rabbit muscle. We have also calculated that, at the physiological protein osmotic pressure, the myosin subfragment-1 decorated F-actin rigor complex can develop a force of 3.96 pN, a force correctly oriented to promote the sliding of the actin filament toward the center of the sarcomere. The magnitude of this force is comparable to that reported for intact skeletal muscle. In contrast, the myosin subfragment-1 decorated tropomyosin-F-actin rigor complex develops a much smaller driving force, that favours relaxation. Apparently tropomyosin uncouples the osmotic and the mechanical event. It is proposed that the elastic energy for muscle contraction is provided by protein osmotic pressure.