Sodium and Potassium Activities in Normal and "Sodium-Rich" Frog Skeletal Muscle

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Abstract
In frog sartorius muscles immersed for 2 hours at 26°C in normal Ringer solution, the intrafiber potassium concentration, CK(in millimoles per liter), was 123 ± 2 (mean value plus or minus standard error), and the potassium activity, aK (in millimoles per liter), was 90 ± 1.0. The corresponding sodium concentration and activity were 20 ± 1 and 6.5 ± 0.4, respectively. After overnight immersion in K+-free Ringer solution the values were: CK, 97 ± 2; aK, 81.5 ± 1.6; CNa, 47 ± 2; and aNa, 11.2 ± 0.6. The changes in aK and aNa during storage were not consistent with an exchange between predominantly "free" fiber K+ and external Na+. These results suggest that the Na+ taken up during overnight immersion largely replaced adsorbed or sequestered K+ in the fibers.
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