Optical density changes of single muscle fibers in sodium-free solutions

The optical density, O.D., of single striated muscle fibers from the giant barnacle, Balanus nubilus, was measured at 50-mμ intervals between 450 and 850 mμ. At all wavelengths, the O.D. decreased markedly when the normal Ringer bathing solution was replaced by sodium-free sucrose Ringer solution. For example, at 850 mμ the O.D. of the fibers, relative to the initial value in normal Ringer solution, decreased from 1 to 0.21 ± 0.06 in 25 min. The corresponding increase in the transmittance, T, (O.D. = −log T) was from 5% to 55%. This change in O.D. could be reversed by returning the normal Ringer bathing solution to the bath. Large, reversible decreases in O.D. were also observed when potassium and Tris were used as substitutes for sodium. These changes in O.D. are explained by the theory of light scattering if it is assumed that sodium is bound to macromolecules in the myoplasm. This assumption is supported by experiments with cation-sensitive microelectrodes, which indicate that most of the sodium in the muscle fibers is not free in the myoplasm. When the fibers were bathed in sodium-free, lithium-substituted Ringer solution, a small reversible increase in the O.D. was observed, which may indicate that lithium is complexed more strongly than sodium by macromolecules in the myoplasm. This conclusion is compatible with the known affinities of carboxylate ion exchange resins for the alkali metal cations.