Ion flux changes induced by voltage clamping or by amphotericin B in the isolated urinary bladder of the trout.

1. When bathed with Ringer solutions on both sides in Ussing chambers, isolated urinary bladders of trout carry out an electrically silent transport of Na+ and Cl- ions from mucosa to serosa. In the present experiments, an electrical gradient was set up across the epithelium mounted under these conditions either by using a voltage-clamp technique or by submitting the mucosal side to the polyene antibiotic Amphotericin B. 2. With stable potential differences between -40 and +40 mV applied to the tissue, both unidirectional fluxes (Jms and Jsm) for Na+ and for Cl- varied as a function of the electrical gradient. This indicates that a sizeable fraction of these ionic exchanges is due to passive diffusion across a low-resistance pathway. 3. Amphotericin B (40 micrograms/ml.) applied to the mucosal side induced an immediate and large (up to 50 mV, serosa positive) potential difference and a sustained diminution in tissue resistance. These effects were strictly Na+-dependent and were reduced in the presence of ouabain. Therefore they are due to increased passive entry of Na+ along its electrochemical gradient across the cellular apical membrane. 4. The effect of Amphotericin B was enhanced when Cl- was replaced by an impermeant anion such as gluconate. Therefore the shunting effect of Cl- on the induced potential observed in the control conditions appears to be due to an increase in Cl- permeability across an anion pathway. Flux data indicate that this pathway is transcellular.