Origin of unusual interaction between thick and thin filaments in shrunk skinned muscle fibers of frog.

Isometric tension, shortening velocity and instantaneous stiffness as a function of relative fiber width were measured in skinned muscle fibers from the frog. Fiber width was reduced by adding the long chain polymer PVP (polyvinylpyrrolidone) K30 or dextran T70 to the bathing solution. Both shortening velocity and isometric tension clearly decreased with the concentration of PVP. Instantaneous stiffness measured by a quick length step was detected in a relaxed, reduced-width fiber and it increased with degree of compression. In the presence of polymer, higher stiffness was observed always in activating solution than in relaxing one. The excess stiffness induced by activation above that in relaxing solution was nearly constant in the wide range of polymer concentration. The stiffness in a relaxed, reduced-width fiber was shown to decrease with the increase of sarcomere length. In rigor fibers also, stiffness increased as the fiber width was reduced. These facts suggest that two kinds of stiffnesses exist in a compressed fiber, one is active or rigor stiffness and another is the parallel "unusual" stiffness which may result from sliding filaments and that the latter has passive natures.