[Relationship between unidirectional sodium ion currents and their concentration in muscle and medium]

Sodium fluxes were measured as concentrations of external sodium and intracellular sodium were changed under K-free condition. Paired sartorious muscles with high intracellular sodium concentration from frog (Rana ridibunda) were used throughout this investigation. The measured uptake of 22Na (mM/kg . h) is directly proportional to intracellular sodium but when influx of 22Na is calculated per unit surface (pmole/cm2s) taking account of "effect of relation volume/surface" the influx is to be independent of Nain. Sodium influx varied linearly with changing external Na. Sodium efflux is not dependent on external sodium concentration. The results are in favour of the conclusion that the sodium fluxes across the skeletal muscle membrane do not exhibit the mechanism of sodium--sodium exchange.