Electrolytes in Leg and Neck Muscles of Rats during Ontogeny

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Abstract
The contents of sodium, potassium, magnesium, chloride, phosphorus and water were determined in the leg and neck muscle of 3– and 14‐day old rats. There were marked differences in electrolyte content between the two age groups and also between the leg and neck muscle of the 3‐day old rat.The results indicate that there is a decrease in the amount of extracellular fluid during maturation. The composition of the intracellular fluid also changes during development insofar as there is a marked decrease in its sodium content.The results indicated that the decrease in intracellular sodium roughly paralleled the rise in membrane potential. It was therefore suggested that the developmental change in the intracellular sodium concentration was secondary to a decrease in the sodium permeability of the muscle cell membrane or to a change in the active transport of sodium across the membrane.