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Experiments were performed to examine how aldosterone modulates colonic NaKATPase activity during the weaning period. NaKATPase activity was determined in proximal and distal colon in control rats aged 16, 20, and 40 days, in rats aged 20 and 40 days fed low sodium diet for 4 days and in 20-day-old rats adrenalectomized on day 16. In some protocols net sodium and water transport was determined with in vivo perfusion technique. In control rats colonic NaKATPase activity increased significantly between day 16 and 20. This increase was abolished by adrenalectomy but restored by aldosterone substitution, 5 µg/100 g body weight/12 h. No significant increase in NaKATPase activity occurred between day 20 and 40. Serum levels of both aldosterone and of corticosterone were low until day 14 and increased to peak level at day 18-20. In 20- and 40-day-old rats fed a low sodium diet, NaKATPase activity increased significantly in proximal and distal colon in both age groups but the increases were significantly greater in the 20- than the 40-day-old animals. A low sodium diet increased serum aldosterone, but not serum corticosterone levels in both age groups also the low sodium diet significantly increased net sodium and water transport in 20- but not in 40-day-old rats. Aldosterone is of physiological importance for the regulation of NaKATPase activity in the colon at the time of weaning. The immature colon may have an enhanced sensitivity to aldosterone.

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 μgEq/min/g DT in 20-day-old rats and 116 ± 6 μEq/min/g DT in 40-day-old rats. After amiloride treatment, net Na transport was reversed to net secretion (-79 ± 36 μEq/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 (Pediatr Res 24: 46–49, 1988)