T H E Effect of Polyphosphates and Magnesium on the Mechanical Properties of Extracted Muscle Fibers by Emil Bozler

Living muscle fibers at rest have high extensibility, similar to that of rubber. However, no final conclusions can be drawn from this observation because supporting structures may determine the mechanical properties of muscle when examined with static methods. In the glycerol-extracted muscle fibers introduced by Szent-Gy~rgyi (1) mechanical prope-ties can be studied under a great variety of conditions, because the composition of the medium surrounding the myofibrils can be readily controlled owing to the breakdown of the cell surface. This makes it possible to modify the physical properties of these fibers within wide limits. Because contractility remains intact, such studies may throw light on some of the essential characteristics of muscle. The observations on extracted muscle fibers reported here indicate that, all appearances to the contrary, myofibrils, even at rest, have short range elasticity, like muscle in rigor, and that extension is largely a process of plastic deformation. These conclusions are based on an analysis of the softening action of polyphosphates and on the observation that under certain conditions contraction may be induced by ATP x without an appreciable change in extensibility and elasticity. Additional evidence confirmed that PP and ATP cause softening and partial relaxation only in the presence of Mg and that these effects do not require a relaxation factor (2, 3).