title: Underestimation of Na permeability in muscle cells: implications for the theory of cell potential and for energy requirement of the Na pump.
abstract: 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.
mesh_terms: D000818:Animals; D001693:Biological Transport, Active; D002463:Cell Membrane Permeability; D004734:Energy Metabolism; D008564:Membrane Potentials; D008954:Models, Biological; D009132:Muscles / Q000378:metabolism*; D011894:Rana pipiens; D012964:Sodium / Q000378:metabolism*
chemical_list: D012964:Sodium