Delayed development of amiloride-sensitive sodium transport in lamb distal colon

1. Na transport has been measured in vitro across distal colons taken from fetal and early post-natal lambs. 2. The mucosal to serosal Na flux fell by half during the first three weeks of post-natal life. Similar falls were recorded for measured short-circuit current and open-circuit voltage. The net transport of Na, measured isotopically at all stages of development, corresponded approximately to that calculated from measurements of short-circuit current. 3. The serosal to mucosal flux of Na was approximately half the mucosal to serosal flux at all stages in development. 4. Amiloride affected neither the short-circuit current nor the mucosal to serosal flux of Na measured during the first five days of post-natal life. The short-circuit current of an increasing proportion of distal colons (30-70%) began to show some sensitivity to amiloride during the second to third week of post-natal life. Sensitivity of the short-circuit to inhibition by amiloride appeared to be fully developed in the 8-week-old lamb colon. 5. The serum concentration of aldosterone was high at birth. It then fell rapidly to levels approaching those found in fetuses in the last week of pregnancy. The serum concentration of cortisol was two orders of magnitude higher than for aldosterone. Changes in cortisol concentration during development paralleled those for aldosterone. 6. There appears to be no aldosterone-induced change in Na transport across lamb distal colon during the first week of post-natal development. Comparisons are made between this situation and that found in the neonatal pig, where aldosterone acts to increase Na reabsorption immediately after birth. Possible reasons for this species difference are discussed.