Simultaneous efflux of K+ and Na+ from frog sartorius muscle freed of extracellular fluids: evidence for rapidly exchanging Na+ from the cells.

After removal of radioactivity trapped in the extracellular space and correcting for the contribution of connective tissue elements, the K+ -efflux curve of frog sartorius muscles becomes a perfect straight line in a semilogarithmic plot. The simultaneously recorded Na+-efflux curve from the same muscles remains strongly curved, and can be resolved into a slow fraction (which conventionally has been regarded as representing the entire cell Na+) and at least one fast fraction. The fast fraction of Na+ could not have originated from a sarcoplasmic reticulum or any other extracellular space extensions; otherwise a similar fast fraction should exist for K+. The data agree with the interpretation that it is the fast fraction that is rate limiting by cell permeability and the slow fraction by desorption from intracellular adsorption sites.