1. If glycerol-extracted muscle fibers are alternately immersed in solutions of inorganic pyrophosphate (PP) or ethylenediamme tetraacetate (EDTA) and KCl two to three times, PP no longer increases extensibility. The original condition is restored by solutions containing Mg. EDTA prevents completely the softening action of PP, but has no effect in the presence of an excess of Mg. These observations are explained by assuming that PP and EDTA remove bound Mg. Evidently PP has no softening action without Mg.

2. If PP is washed out from muscle fibers by solutions containing Mg, the softening action of PP persists for many minutes, but washing out with KCl solution promptly abolishes the effect of PP. Also, if fibers which have been made refractory to PP are immersed into a solution of PP and then into a KCl solution free of PP for several minutes, the addition of MgCl2 alone increases extensibility. It is concluded that PP forms a complex with protein. The formation of this complex, however, has no influence on the mechanical properties, unless Mg also is present.

3. In the presence of PP and Mg the viscous resistance of muscle fibers drops with diminishing temperatures.

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