Force-length relations of glycerinated rabbit psoas.

Force-length curves of glycerinated rabbit psoas were determined in media of physiological interest. In one set of experiments the free [Ca++] was constant at 10(-5) M and the [MgATP] varied. At low [MgATP] the elastic properties are those of a highly ordered, inextensible fiber; at high [MgATP] they are representative of a much more elastic body. At intermediate concentrations sigmoidal shaped curves were observed as were also found when [MgATP] was constant and the free [Ca++] varied. These curves closely resemble coexistence curves that have been reported for fibrous proteins.