Aldosterone induced changes in colonic sodium transport occurring naturally during development in the neonatal pig.

1. Serum concentrations of aldosterone in later fetal, 3-6 week old and adult pigs are of the order of 300 pg ml.-1. This increases to about 2000 pg ml.-1 in the period immediately after birth. 2. Canrenoate injected into pigs from birth onwards stops the increase in colonic short-circuit current, seen to take place normally during early postnatal development. Amiloride has little or no effect on the short-circuit current of colons taken from canrenoate injected pigs. 3. Canrenoate stops the post-natal increase in colonic Na influx (and therefore net transport) seen to occur under normal conditions. 4. There is in the neonatal pig distal colon a portion of Na transport which appears to be resistant to inhibition by amiloride or canrenoate. 5. There is a second portion of Na transport, increasing in importance as the piglets become older, which is electrogenic and which is electrogenic and which is inhibited by prior injection of canrenoate. It is assumed that this fraction of Na transport is influenced by aldosterone. 6. There is a third part of Na transport, maximal in colons taken from one day old animals, which appears to be non-electrogenic. This is also blocked by prior injection of canrenoate. 7. The physiological relevance of these findings is discussed.