title: Active sodium transport by turtle colon via an electrogenic Na-K exchange pump.
abstract: 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 demonstrated. According to the model of Koefoed-Johnsen and Ussing, 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.
mesh_terms: D000666:Amphotericin B / Q000494:pharmacology; D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects; D002451:Cell Compartmentation; D002462:Cell Membrane / Q000502:physiology; D003106:Colon / Q000378:metabolism*; D004553:Electric Conductivity; D007413:Intestinal Mucosa / Q000502:physiology; D007473:Ion Channels / Q000502:physiology; D010042:Ouabain / Q000494:pharmacology; D011188:Potassium / Q000502:physiology; D012964:Sodium / Q000378:metabolism*; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*; D014426:Turtles / Q000502:physiology*
chemical_list: D007473:Ion Channels; D010042:Ouabain; D000666:Amphotericin B; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium