SODIUM AND WATER BINDING IN SINGLE STRIATED MUSCLE FIBERS OF THE GIANT BARNACLE

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Abstract
Cation-sensitive glass microelectrodes were inserted into single striated muscle fibers of the giant barnacle, Balanus nubilus, to measure the activity of sodium and potassium in the myoplasm. These measurements, combined with a knowledge of the total cellular water and sodium and potassium content (flame photometry), permitted the minimal percentage of bound sodium and water to be calculated. These values were 84% and 42% respectively. When muscle fibers were soaked in sucrose Ringer solution, about 30% of the total sodium was removed proportionally from the bound and the free fractions. Potassium replaced sodium in the cellular bound fraction. In some experiments the chloride content of muscle fibers was determined. Assuming no binding and a passive distribution of this ion, the results predict 65% binding of the fiber water. This value for water binding was used with the previous data to calculate that 91% of the intracellular sodium and 38% of the intracellular potassium were bound.