Direct measurement of the influx of sodium in frog skeletal muscle

When a frog's sartorius is immersed in sodium-free lithium-substituted solution at 0 degree C, the tissue sodium content declines in two distinct phases. The rate of the slow phase has a temperature dependence expected for a process dependent on metabolism (Q10, ca. 3), and sodium content (51.5 mmol/kg dry weight) equal to that measured by others using electron microprobe microanalysis. The rate of the rapid phase has a temperature dependence (Q10, 0.3-1) expected for a passive process, and a sodium content equal to that in the sorbitol space. It was concluded that incubation of a muscle at 0 degree C for 45 min in sodium-free solution will wash out almost all of the sodium in the extracellular space but will leave almost all the sodium in the intracellular space. The unidirectional sodium influx was measured by incubating a muscle in 22Na-containing Ringer's solution for a timed interval at 23 degrees C, then in sodium-free lithium-substituted solution at 0 degree C for 45 min, before analysis for ion content and radioactivity. The ratio of the specific activity of sodium in the muscle to that in the radioactive bathing solution was calculated, and the time course of its rise was used to calculate an influx rate coefficient. The use of the specific activity minimizes the error due to the loss of intracellular sodium and radiosodium which occurs during the wash in cold solution. It was found that the rate of the radiosodium uptake varied as the uptake proceeded, in a manner similar to that previously shown for the rate of the radiosodium efflux and attributed to the existence of a diversity of cell size in this muscle.