Variations in Muscle Electrolyte Composition Due to Sampling and to Aging. ∗

Summary

1) A difference in muscle potassium was demonstrated in samples from 2 different sites of the same animal when potassium was expressed in terms of fat free dry muscle solids. This difference did not exist when potassium was expressed in terms of its concentration in calculated intracellular water. We suggest that the discrepancy was related to differences in connective tissue or extracellular solids between the 2 samples, and that potassium expressed in terms of its concentration in intracellular water was the more meaningful expression. 2) A small but significant decrease in potassium associated with growth and aging was also noted. This difference was demonstrated expressing potassium in terms of muscle solids or intracellular water concentration. A reciprocal increase in calculated intracellular sodium was noted such that the sum of sodium and potassium remained constant.