Action of ouabain on sodium transport in toad urinary bladder, Evidence for two pathways for sodium entry

The cardiac glycoside ouabain inhibits transepithelial sodium transport in the toad urinary bladder. It is shown that this drug reduces the rate coefficient for sodium exit at the serosal pump site. In addition, ouabain inhibits entry across the mucosal border whenever the electrochemical potential gradient for sodium is made less favorable. The data are interpreted as indicating the existence of two separate pathways for sodium entry, one of which is ouabain inhibitable.