Effect of a saline environment on sodium transport by the toad colon.

1. Colons isolated from saline-adapted or aldosterone-injected toads maintained transmural potential differences with the serosal side positive to the mucosa. The short-circuit currents of colons taken from aldosterone-injected toads could be expressed quantitatively by the net flux of sodium measured isotopically. This did not apply to saline-adapted colons where the net sodium flux was from serosa to mucosa.2. The short-circuit currents of colons taken from aldosterone-injected animals increased as the sodium concentration was raised from 10 to 50 mM, then decreased as the sodium concentration was further increased to 115 mM. Adaptation to saline changed this relationship, the short-circuit current becoming directly dependent on the sodium concentration.3. Faecal sodium was higher than serum sodium in saline-adapted toads. There was little or no change in the level of serum sodium or potassium. The urine of saline-adapted toads also contained high concentrations of sodium.4. The total and ouabain-sensitive ATPase activities of mucosal scrapings taken from saline-adapted colons were about half those found in the aldosterone-injected animal. Ten per cent of the total ATPase activity could be inhibited by ouabain.5. Microsomal fractions of mucosal scrapings taken from saline-adapted toads contained 4 times less Na(+) + K(+)-activated ATPase than did corresponding fractions from aldosterone-injected animals. High concentrations of sodium inhibited the microsomal ATPase activity irrespective of the previous conditions of adaptation.6. Regulation of sodium movements across the toad colon appears to be a complex process with the mucosal cells changing their properties so that they either absorb or secrete sodium ions depending on the physiological state of the animal.