Mechanisms for colonic sodium transport during ontogeny: loss of an amiloride-sensitive sodium pathway

Net colonic sodium and fluid absorption is higher in suckling and weanling rats than in adult rats. This study was undertaken to investigate the mechanisms behind these differences. In vivo perfusion of the colon was performed in 14- to 80-day-old rats. Net Na and water uptake decreased exponentially from 14 to 80 days of age. Na uptake was 402 +/- 73 micrograms Eq/min/g DT in 20-day-old rats and 116 +/- 6 mu Eq/min/g DT in 40-day-old rats. After amiloride treatment, net Na transport was reversed to net secretion (-79 +/- 36 mu Eq/min/g DT) in 20-day-old rats. Amiloride had no effect on the net Na transport in 40-day-old rats. cAMP stimulation significantly increased the net Na uptake in 20-day-old rats and significantly reduced the net Na uptake in 80-day-old rats. cAMP did not increase the net uptake in amiloride-treated 20-day-old rats. We conclude that there are both quantitative and regulatory changes in the pathways for colonic Na transport during development, and we speculate that the large amiloride-sensitive sodium uptake in the young colon is adaptive and contributes to the sodium accretion necessary for rapid growth during late postnatal development.