論文

The capacity to concentrate urine develops progressively during postnatal life in most mammalian species. Here we have examined whether low expression of the arginine vasopressin (AVP)-activated water channel aquaporin-2 (AQP-2) may be a limiting factor for the concentrating capacity in the infant rats. Urine osmolality in response to 24-h dehydration increased significantly from 10 to 40 days of age. The most rapid increase occurred during the weaning period, i.e., days 15-20. A similar developmental pattern was observed for AQP-2 mRNA levels in the renal medulla. AQP-2 protein levels also increased markedly from day 10 to 40. Immunohistochemistry revealed that AQP-2 was exclusively located in collecting duct principal cells both in infant and adult rats but that the signal was much weaker in infants. To further examine the relationship between urinary concentrating capacity and AQP-2 expression, we treated rats with a single injection of betamethasone, which is known to accelerate maturation in several organs. Twenty-four hours after treatment, there was an increase in urine osmolality, renal medullary AQP-2 mRNA, and AQP-2 protein levels in infant but not in adult rats. A single injection of a specific V2 agonist caused within 6 h significant increase of AQP-2 mRNA in both infant arid adult. The expression of the mRNA of three other transporters involved in the concentrating process, medullary Na+-K+ATPase a-subunit, Na-K-2Cl cotransporter, and epithelial chloride channel also increased during the weaning period and were upregulated by glucocorticoids. We conclude that there is a well-synchronized development of the many of the components that determine the concentrating capacity and that the low expression of AQP-2 is one of the limiting factors for low concentrating capacity in infants. Copyright ©1996 the American Physiological Society.

The development of gastric H,K-ATPase from fetal to adult life was studied in the rat. The α and β H,K-ATPase mRNA abundance, the protein abundance, and the enzyme activity increased postnatally. The sharpest increase in mRNA and enzyme activity was observed in the weaning period. Several intestinal enzymes are known to be stimulated by glucocorticoids at the time of weaning. To study the role of glucocorticoids in the maturation of gastric H,K-ATPase, we treated 10-d-old rats with a single injection of betamethasone. Twenty-four hours after betamethasone injection, the enzyme activity was significantly higher than in the control animals (2.6-fold, p < 0.05). The abundance of catalytic α H,K-ATPase protein was also increased (2.5-fold, p < 0.01). The time-dependent effect of betamethasone on α H,K- ATPase mRNA was determined from 6 to 24 h after treatment. Glucocorticoids did not significantly alter the mRNA abundance within 18 h. Twenty four hours after injection, the gastric H,K-ATPase mRNA was significantly increased compared with controls (2.8- and 2.2-fold increase for α and β subunits, respectively, p < 0.01 for both). In conclusion this study indicates that glucocorticoids may regulate the long term maturation of gastric H,K-ATPase by indirectly stimulating enzyme synthesis.

Several epithelial ion transporters are developmentally regulated in the preweaning period, at the time when the circulating levels of glucocorticoid and mineralocorticoid hormones increase. The specific role of glucocorticoids and mineralocorticoids in the maturation of epithelial ion transport is still disputed. In this study, we investigated the effect of corticosteroids on the mRNA expression of ion transporters in the infant rat colon, a glucocorticoid- and mineralocorticoid-sensitive organ. The expression of the Na,K-ATPase, the H,K-ATPase and the amiloride-sensitive Na+ channel mRNA was investigated in control rats from fetal to adult life. We found that the mRNA of the three transporters is temporally up-regulated in the preweaning period. Rats were then injected with a single dose of betamethasone or aldosterone at 10 d of age. The main effect was the glucocorticoid stimulation of the Na,K-ATPase mRNA within 6 h (4-fold). Glucocorticoids did not alter H,K-ATPase nor Na+ channel mRNA within 6 h. Aldosterone moderately (1.7-fold) stimulated Na+ channel within 6 h, but did not alter Na,K-ATPase nor H,K-ATPase mRNA. Twenty-four hours after injection, both glucocorticoids and mineralocorticoids had less pronounced and distinct effects. In tissue with lower aldosterone receptor abundance (renal cortex) or with no aldosterone receptor (stomach), glucocorticoids induce a similarly rapid increases in Na,K-ATPase mRNA (4-fold within 6 h), whereas aldosterone had no effect within 6 h. However, glucocorticoids did not stimulate Na,K-ATPase mRNA in the brain, a tissue rich in glucocorticoid receptors. This study indicates that Na,K-ATPase is a primary target for glucocorticoids in the preweaning period, and suggests that glucocorticoid induction of Na,K-ATPase mRNA may play an important role in the maturation of epithelial ion transport capacity. The effect is probably mediated by glucocorticoids and not by mineralocorticoid receptors. However, it seems that an auxiliary factor is required for glucocorticoid-dependent stimulation of Na,K-ATPase mRNA.

Glucocorticoids modulate the maturation of Na+-K+-ATPase mRNA in a tissue- and age-dependent manner. In this study, we report the effect of glucocorticoids on Na+-K+-ATPase gene transcription in the infant rat kidney. Ten-day-old rats were treated with one intraperitoneal injection of betamethasone. In glucocorticoid-treated rats, there was a significant increase in renal cortical α1- and β1-mRNAs (3.08 ± 0.34- and 4.06 ± 0.10-fold). Pretreatment with cycloheximide, an inhibitor of protein synthesis, did not abolish the increase in α1- and β1-mRNA after glucocorticoids. The α1- and β1-gene transcription rates were significantly increased in nuclei isolated from kidneys of glucocorticoid- treated rat (2.16 ± 0.05- and 3.12 ± 0.50-fold). Interaction between nuclear proteins and Na+-K+-ATPase α1-promoter was studied by gel retardation assay. Nuclear protein from glucocorticoid-treated rats retarded a fragment of α1-promoter that includes a half-consensus glucocorticoid response element (GRE) at position -750 bp but did not retard a fragment including a half-consensus GRE at position -481. Retardation of α1- promoter was inhibited by incubation with molar excess of GRE or with a monoclonal antibody against glucocorticoid receptor. We conclude that in the infant kidney, glucocorticoids directly stimulate the transcription of α1- and β1-Na+-K+-ATPase subunits. It is likely that the binding of glucocorticoid receptor to α1-Na+-K+-ATPase promoter requires the presence of an auxiliary factor.

It is suitable to examine the utilization of carbohydrates and fats using stable isotope-labelled substrates in neonates because of their non-radioactivity. Administering medium chain triglycerides (MCT) and oligosaccharides is of use in enteral nutrition for a patient with a limited water intake such as a neonate. In this study, the oxidation of MCT and maltose administered orally as an energy supplement in neonates has been examined using a stable isotope-labelled breath test. Five normal term neonates and five growing preterm infants were given [13C]-trioctanoin orally and three growing preterm infants were given [13C]-glucose and [13C]-maltose orally. The [13C] enrichment in carbon dioxide was analyzed by isotope ratio mass spectrometry, and oxidation rates over 6 hr and 12 hr respectively, were calculated. The oxidation rates for [13C]-octanoin after 6 hr were 46.2 ± 3.6% in preterm infants and 53.5 ± 13.8% in normal neonates, respectively (no significant difference), and 58.4 ± 9.4% and 52.8 ± 6.0% for [13C]-glucose and [13C]-maltose, respectively (not significant). The results demonstrate that orally administered MCT and maltose are oxidized sufficiently in preterm infants.