Amphotericin B and K+ transport across excised toad urinary bladder

The transmural electric PD of bladders bathed by Na2SO4 Ringer was not affected by amphotericin (5 x 10(-6) M, mucosal) but the PD followed the direction for K+ diffusion in the presence of a transmural K+ gradient. Increases in bathing solution K+ increased conductance. Ouabain pretreatment did not affect drug-induced changes in PD or conductance. Unidirectional fluxes of radiolabeled Na+ and K+ but not SO42- across the short-circuited bladder were increased by amphotericin. Ninety percent of the rise in the serosal-to-mucosal flow of Na+ disappeared when mucosal Na+ was replaced by choline. Amphotericin induced a 20-fold increase in mucosal-to-serosal K+ flux but K+ serosal-to-mucosal flow increased 200-fold. This flux asymmetry persisted for 110 min, was abolished by pre- or posttreatment with ouabain, and was immeasurable when bathing solution K+ was increased from 2.4 to 59 meq/liter. With 2.4 meq K+/liter the ratio of active Na+ reabsorption to K+ secretion was 8 to 1, but K+ secretion was not closely linked to Na+ transport. The results suggest that amphotericin induces a paracellular K+-selective path, Na+ isotope exchange, and K+ secretion.