Active sodium transport by the colon of Bufo marinus: stimulation by aldosterone and antidiuretic hormone.

1. The isolated colon of Bufo marinus transports sodium actively from the mucosal (lumen) to the serosal side, and this transport is expressed quantitatively by the short-circuit current.2. Upon dilution of sodium in Ringer solution on the mucosal side of the preparation, short-circuit current remained a fair expression of sodium transport from mucosa to serosa.3. In view of this, the relation between short-circuit current and dilution of sodium of the luminal side was examined. This relation was curvilinear, which suggests the intervention of a saturable step in the transfer of sodium from lumen to serosal surface of colon.4. The relation between short-circuit current on the one hand, and the amount of sodium drawn from the luminal side and recovered in the membrane (;active sodium transport pool') on the other hand, appeared (almost) linear instead. This is meant to indicate that the ;pump' operates far from capacity. Hence, the observed saturation of sodium transport, when concentration of sodium on the mucosal side was increased, probably occurs at the mucosal border of the preparation.5. After treatment with aldosterone, the ;active sodium transport pool' and short-circuit current increased to the same extent, from which it is inferred that the hormone merely allows sodium easier access to the ;pump' which would react in proportion. Consequently, no direct influence of aldosterone on the ;pump' proper need be postulated.6. Upon exposure of the colon to antidiuretic hormone, there were (modest) increases of short-circuit current and of osmotic water flow across the wall of the organ.