Underestimation of Na permeability in muscle cells: implications for the theory of cell potential and for energy requirement of the Na pump.

A new method permits, in a complex efflux curve from living cells, identification of the fraction of labeled solute rate-limited by intracellular-extracellular exchange. Using this method, it was found that in Na+ efflux from frog voluntary muscle, the slow fraction conventionally described as rate-limited by membrane permeability is wrongly assigned. Instead, a much faster fraction, usually camouflaged by the efflux of labeled Na+ in the extracellular space, represents the exchange of intra- and extracellular Na+ rate-limited by cell surface permeation. Quantitatively, the result shows that the Na+ permeability constant has up to now been underestimated by a factor of 10. Implications of that finding for the theory of the resting potential, and the minimum energy required of resting frog muscles, are discussed.