Cellular lithium and transepithelial transport across toad urinary bladder

Toad urinary bladders were exposed on either their mucosal or serosal surfaces, or on both surfaces, to medium in which sodium was replaced completely by lithium. With mucosal lithium Ringer's, serosal sodium Ringer's, short-circuit current (SCC) declined by about 50 percent over the first 60 min and was then maintained over a further 180 min. Cellular lithium content was comparable to the sodium transport pool. With lithium Ringer's serosa, SCC was abolished over 60 to 120 min whether the mucosal cation was sodium or lithium. Measurements of cellular ionic composition revealed that the epithelial cells gained lithium from both the mucosal and serosal media. With lithium Ringer's mucosa and serosa, cells lost potassium and gained lithium and a little chloride and water, but these changes in cellular ions could not account for the current flow across the tissue under these conditions, which must, therefore, have been carried by a transepithelial movement of lithium itself. The inhibition by serosal lithium of SCC was overcome by exposure of the mucosal surface of the bladders to amphotericin B. Thus it reflected, predominantly, an inhibition of lithium entry to the cells across the apical membrane. It is suggested that this inhibition is a consequence of cellular lithium accumulation.