The Location of Sodium in the Transverse Tubules of Skeletal Muscle

Recent studies on the sarcoplasmic reticulum and transverse system of skeletal muscle have led to the conclusion that both systems are independent ones and that the transverse tubules (T system) are continuous with the external membrane of the muscle fiber (1). Ferritin penetrates into the lumen of the transverse tubule but is not found inside the sarcoplasmic reticulum (2). Thus, the fluid contained in the transverse tubules could easily be identical in composition with the extracellular fluid. The distribution of sodium in skeletal muscle assessed with analytical methods indicates that the actual free sodium inside the limits of the fiber is very small, of the order of 1 mmole/kg of cell water, if a special region with high concentration of sodium at the level of the sarcolemma is taken into consideration (3). More knowledge on the detailed location of different ions inside the membranous system of skeletal muscle is desired in view of their role in the process of excitation as well as in the coupling of excitation to contraction (4). In an attempt to localize sodium by means of electron microscopy in skeletal muscle, the method utilized by Komnick and Komnick (5) and by Kaye et al. (6, 7) was applied with slight modifications. This method permits the observation of a fine precipitate of sodium pyroantimonate in sections of tissues fixed in OsO4. The principle of this method, used in the past to determine sodium in biological fluids as a microtitration method (8), consists in the precipitation of sodium in solution as its pyroantimonate salt when in contact with potassium pyroantimonate. The latter salt is soluble in water whereas the sodium one is not. The preservation of the muscles fixed in the presence of the pyroantimonate reagent is not very good when compared to the preservation of muscles treated with the fixatives alone and used as controls. Nevertheless, the images were clear enough to detect the precipitate and to recognize with certainty the spatial relationships of the precipitate to the fine structure of the muscles. The precipitate of sodium pyroantimonate was observed inside the transverse tubules of the frog sartorius muscles which confirms the assumption that the composition of the fluid filling these tubules could resemble that of the extracellular fluid. No precipitate was found in the sarcoplasmic reticulum in any of its portions, and a considerable amount was seen on the outside of the sarcolemma of the fibers. …