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Neonatal animals show an increased intestinal iron absorption despite being iron replete. We have used the developing guinea pig to determine whether alterations in the expression of iron binding proteins in duodenal enterocytes can account for the difference in iron absorption between neonatal and adult guinea pigs. No mRNA for transferrin was detected in the enterocytes. H ferritin mRNA remained constant from birth into adulthood whilst there was a small increase (< 2-fold) in L ferritin mRNA. Western blotting showed a clear increase in immunoreactive ferritin, levels reaching those of the adult 19 days after birth. We have also demonstrated that in the neonate, 80% of newly absorbed iron in the enterocyte is associated with a low molecular weight fraction (LMW-IBF), whereas in the adult, ferritin and LMW-IBF bind iron to the same extent. We conclude that changes in expression of ferritin during ontogeny are accompanied by a change in distribution of iron binding between ferritin and LMW-IBF implicating these proteins in the regulation of iron absorption.

Neonatal animals are iron replete but in comparison with adults they display increased intestinal iron absorption. In order to examine possible mechanisms for this developmental adaptation we have measured the appearance of iron in peripheral blood following 30 min exposure of duodenal and ileal segments of adult and neonatal guinea pigs in vivo to 59Fe-ascorbate. Parallel experiments have determined the kinetics of 125I-labelled diferric transferrin binding to villus enterocytes isolated from duodenum and ileum. In adult animals the rate of appearance of 59Fe in peripheral blood was 11-fold greater following duodenal, compared to ileal exposure to the radioligand. No such regional difference was detected in the neonate. Isolated cells showed saturable binding of [125I]transferrin which was maximal between 30 and 60 min. The kinetics of specific transferrin binding by adult duodenal and ileal enterocytes were similar and were also not significantly different to respective values in neonatal duodenal and ileal cells. Thus, it is likely that increased iron absorption in the neonate is due in part to enhanced ileal iron transfer. The interaction of transferrin with its receptor, however, is not involved in this developmental change in uptake.

Preparations of villus enterocytes and brush border membrane vesicles have been used to study the effects of streptozotocin-induced diabetes mellitus in rats on sugar transport across the brush border and basolateral membranes of ileal epithelial cells. In isolated cells, diabetes increased Na(+)-dependent galactose transport across the brush border of mid-villus but not upper villus cells. Galactose transport across the basolateral membrane was, however, enhanced by diabetes in both cell populations. Kinetic analysis of vesicle data suggested the presence of two transporters for Na(+)-dependent glucose transport. Diabetes induced a 5-fold increase in both KT and Vmax of the high-affinity/low-capacity system together with a 2-fold increase in the Vmax of the low-affinity/high-capacity transporter. Glucose was almost undetectable in the lumen of the upper and lower ileum in control animals but was present at high levels (26.1 +/- 4.3 mM and 6.5 +/- 1.3 mM) in diabetic rats. The possible significance of these changes in luminal sugar concentration in relation to the adaptation of transport across ileal enterocytes is discussed.