Effect of 50% external sodium in solutions of normal and twice normal tonicity on internal sodium activity in frog skeletal muscle.

Neutral carrier based sodium-selective microelectrodes were used to monitor intracellular sodium activity in single frog skeletal muscle fibres during exposure to 50% external sodium solutions at normal and twice normal tonicity. Intracellular sodium activity in normal Ringer was 12.3 +/- 0.7 mM and was increased to 34.4 +/- 1.3 mM in hypertonic solution. Exposure to normotonic or hypertonic solutions containing only 50% sodium (NaCl) replaced by sucrose to maintain tonicity) did not affect the intracellular sodium activity during at least 20 min. Thus, in frog skeletal muscle, external sodium appears not to play a major role in regulating internal sodium, e.g. through ion exchange mechanisms as postulated for other excitable tissues.