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

Abstract

Active Na absorption by a variety of epithelia is abolished by ouabain, but the obligatory coupling between the movement of Na and K expected from a basolateral (Na+ + K+) ATPase was not 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 Na-K pump. The polyene antibiotic, amphotericin-B was used to reduce the resistance and the cation selectivity of the apical membrane of isolated turtle colon so the basolateral membrane current could apparently be dissected into 2 components: one through a Ba-sensitive K channel and another which represents the current associated with ouabain-sensitive, electrogenic, Na-K exchange. Comparison of cation fluxes and short-circuit current indicates in these conditions active Na absorption is apparently entirely attributable to an electrogenic Na-K pump, with a stoichiometry of .apprx. 3Na:2K.