Letter | Published:

Active sodium transport by turtle colon via an electrogenic Na–K exchange pump

Naturevolume 287pages237239 (1980) | Download Citation



Active sodium absorption by a variety of epithelia is abolished by ouabain, but the obligatory coupling between the movement of sodium and potassium expected from a basolateral (Na+ + K+) ATPase has not been convincingly demonstrated1. According to the model of Koefoed-Johnsen and Ussing2, the asymmetric cation selectivities of the apical and basolateral membranes prevent basolateral Na–K exchange from being expressed as opposing transmural ion flows. An additional consequence of this asymmetry is that the short-circuit current (ISC) cannot be identified with the current through the sodium–potassium pump. We used the polyene antibiotic, amphotericin-B, to reduce the resistance and the cation selectivity of the apical membrane of isolated turtle colon so that the basolateral membrane current could be dissected into two components: one through a barium-sensitive potassium channel and another which represents the current associated with ouabain-sensitive, electrogenic, Na–K exchange. Comparison of cation fluxes and short circuit current indicates that in these conditions active sodium absorption is entirely attributable to an electrogenic Na–K pump with a stoichiometry of approximately 3Na:2K.

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  1. Department of Physiology and Biophysics, University of Iowa College of Medicine, Iowa City, Iowa, 52240

    • Kevin L. Kirk
    • , Dan R. Halm
    •  & David C. Dawson


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