Zinc supplementation reduces iron absorption through age-dependent changes in small intestine iron transporter expression in suckling rat pups

Zinc (Zn) supplementation negatively affects iron (Fe) absorption; however, the molecular mechanisms are not understood. We determined effects of Zn supplementation during mid- and late infancy on intestinal Fe transport mechanisms using a suckling rat model. Suckled rat pups were supplemented with 0 (control), 300 (low), or 750 (high) microg Zn/d until weaning at postnatal day (PN) 20. At mid-(PN10) and late (PN20) infancy, tissue Fe distribution, Fe absorption, intestine DMT1, ferroportin-1 (FPN) and hephaestin expression, and localization and liver hepcidin expression were measured. During early infancy, DMT1 and FPN were localized intracellularly. Negative effects of Zn supplementation on Fe absorption were associated with increased small intestine Fe retention, decreased hephaestin, and increased FPN expression. During late infancy, both DMT1 and FPN were appropriately localized to the apical and basolateral membrane, respectively, and negative effects of Zn supplementation on Fe absorption were absent. Although FPN protein level was lower in Zn-supplemented pups, hephaestin protein level was increased, which may have facilitated enhanced Fe efflux. These results indicate that Zn supplementation reduced Fe absorption during early infancy as a consequence of increased intestinal Fe retention due to reduced hephaestin levels. These effects were age-dependent, further demonstrating that Fe transport regulation is not fully developed until weaning, which may have important implications regarding the safety and efficacy of Zn supplementation programs for infants.     

Dietary protein absorption of the small intestine in human neonates

Background: The intestine plays a key role in the absorption of dietary proteins, which determines growth of human neonates. Bowel resection in the neonatal period brings loss of absorptive and protective surface and may consequently lead to malabsorption of dietary nutrients. However, there are no data on net dietary protein absorption of the small intestine in the period after intestinal surgery in human neonates. We therefore evaluated dietary feeding tolerance and quantified net dietary protein absorption capacity of the small intestine in human neonates in whom a temporary jejunostomy or ileostomy was created. Methods: Seventeen patients were included in the study. We collected small intestinal outflow fluid at the level of the enterostomy weekly for 24–48 hours during weeks 3 through 6 postoperatively. Protein levels in the intestinal outflow fluid were determined by bicinchoninic acid (BCA) assay. Results: In 14 patients, an enteral intake of >100 mL/kg/d was reached at a median of 17 days (range, 8–32 days) postoperatively. Three patients did not reach this level within the study period. Overall, the net dietary protein absorption capacity was 70%–90% of the total enteral protein intake. Conclusions: This study demonstrates that the dietary protein absorption capacity of the small intestine is intact in most human neonates after intestinal surgery in a very critical period of their lives. Furthermore, our results do not support the use of hydrolyzed or elemental formula in newborns with an enterostomy to improve amino acid uptake.     

Kinetic study of acamprosate absorption in rat small intestine

Acamprosate (calcium bis acetyl-homotaurine), a homotaurine derivative, a structural analogue of gamma-aminobutyric acid (GABA) and an upper homologue of taurine, is a relatively new drug used to prevent relapse in weaned alcoholics. When administered orally as enteric-coated tablets at relatively high doses, this drug has a bioavailability of about 11%; however, the intestinal absorption mechanism has not been studied in depth. The present study was therefore planned to characterize the intestinal transport of acamprosate in the rat and the effect of chronic alcohol treatment on this process, quantifying its kinetic parameters and investigating possible inhibitors. Using an in vitro technique, acamprosate absorption was measured in the rat intestine from three different groups: alcohol group [fed a liquid diet containing 5% (w/v) ethanol for 4 weeks], isocaloric pair-fed control, and a solid diet group. Intestinal acamprosate absorption was found to occur mainly by passive diffusion with a diffusive permeability of 0.213+/-0.004 cm/h in control pair-fed animals, 0.206+/-0.001 cm/h in animals receiving chronic alcohol treatment, and 0.193+/-0.001 cm/h in the solid diet group. Inhibition studies showed that at a 10(-3) M acamprosate concentration, some compounds such as GABA, taurine, proline, and glycine at 40 mM each did not affect acamprosate transport. Nevertheless, when a lower concentration of the drug (10(-4) M) was assayed, a significant reduction of acamprosate transport in the presence of taurine or GABA 40 mM was found. These results suggest that acamprosate in the rat jejunum, could be transported, in part, by a carrier system. Further experiments using different concentrations of taurine (10, 20, and 80 mM) showed that the maximum inhibition (32%) is achieved at 20 mM of taurine. These latter results suggest that acamprosate and taurine share, at least, an intestinal carrier system in rat jejunum. From the above results, it can be concluded that there are probably two pathways involved in the intestinal absorption of acamprosate: passive diffusion and mediated transport, with the former being predominant. Moreover, neither chronic ethanol intake nor the type of diet seems to alter the intestinal absorption of the drug.     

Phloridzin improves absorption of genistin in isolated rat small intestine

BACKGROUND & AIMS: Cancer-protective effects of isoflavones like genistin or genistein are well known. High intakes and an adequate absorption rate of isoflavones are necessary for efficient chemoprevention, though other dietary agents might increase absorption efficacy. The aim of this study was to investigate the effect of phloridzin, an inhibitor of the sodium-dependent glucose transporter (SGLT1), on genistin absorption and metabolism. METHODS: Phloridzin and genistin were luminally administered in an isolated preparation of luminally and vascularly perfused rat small intestine. A synthetic perfusate free from blood components was used as vascular medium, with a perfluorocarbon as oxygen carrier. Luminal media consisted of a bicarbonate buffered sodium chloride solution spiked with genistin (24.5 micromol/l) and phloridzin (1 mmol/l). RESULTS: In previous experiments, genistin absorption rate of 17.2% has been observed. In the present study, phloridzin administered simultaneously with genistin, increased genistin uptake 2.5 fold (44.5%). CONCLUSION: The naturally occurring substance phloridzin, present in apples, thus considerably amplify genistin absorption. These effects might offer a promising novel method in designing functional foods for cancer prevention by combining genistin- and phloridzin-containing foods.     

Digestion and absorption of NAD by the small intestine of the rat

A number of preparations of varying complexity have been used in an effort to elucidate the reactions by which NAD is hydrolyzed to nicotinamide during intestinal digestion. NAD labeled with 14C in the adenine or pyridine moiety was the substrate used with perfused rat intestine, live rats, perfused live rats, with collection of portal flow, intestinal contents, mucosal tissue, or pancreatic juice. The conclusions reached are that a pyrophosphatase present in the intestinal juice and to a much lesser extent in the pancreatic juice releases 5'-AMP and nicotinamide ribonucleotide. The 5'-AMP was rapidly converted to adenosine then to inosine by bacteria-free intestinal contents. Perfused or intact intestine rapidly hydrolyzed NMN to nicotinamide riboside, which accumulated, but was not absorbed. It was slowly cleaved by an enzyme associated with the mucosal cells to nicotinamide, which was the major if not the only labeled compound absorbed.     

Determination of the site of zinc absorption in rat small intestine.

We have studied net uptake of zinc from segments of rat duodenum, jejunum, and ileum using in vivo intestinal perfusion. We have also evaluated the effect of pancreatic and bile secretions on duodenal zinc absorption. Segments of duodenum, jejunum, and ileum and duodenum with bile and pancreatic duct obstruction, 10 cm in length, were studied in six rats each. Percentage absorption of zinc as determined by atomic absorption spectrophotomety was greater from the ileum (60.1%) when compared to duodenum (19.1%) or jejunum (20.2%). Exclusion of bile and pancreatic secretions from the lumen increased zinc absorption in the duodenum (32.0%). Postperfusion mucosal zinc content comprised 29.0% of absorbed zinc from jejunum, 7.4% from ileum, 5.2% from duodenum, and 2.7% from duodenum with bile and pancreatic duct obstructed, indicating rapid transport of zinc across epithelial cells after uptake from the duodenum and ileum. Our data indicate that the ileum has the greatest capacity for zinc absorption.     

Digestion and absorption of sugars and sugar substitutes in rat small intestine

The bioavailability of newly developed sugar substitutes was observed by measuring the transmural potential difference (delta PD) evoked by Na+-dependent active transport of glucose, which is supposed to be produced by the hydrolysis of sugar substitutes. delta PD was measured using everted intestinal sac prepared from jejunum of adult rats and compared with the digestibility of sugar substitutes in the mucosal homogenate of everted sac. delta PDs evoked by glucose, maltose or maltosylfructose had almost the same levels, however, the delta PD evoked by sucrose was a little lower. delta PDs evoked by maltitol or palatinose were low, and delta PDs evoked by fructo-oligosaccharides were negligible. The hydrolyzing activities of these sugars and sugar substitutes by the mucosal homogenate were correlated with the delta PDs. A significant positive correlation was observed between delta PDmax of various sugars and sugar substitutes and the Vmax of their corresponding hydrolyzing activities. Also, a significant positive correlation was observed between Kt and Km values of these sugars. These results suggest that the absorption of sugar substitutes is dependent on digestibility by membrane digestive enzymes.     

Comparison of absorption of erythorbic acid and ascorbic acid in guinea pig small intestine.

Both the ascorbic acid (AsA) and erythorbic acid (ErA) absorption in the small intestine of guinea pigs were determined by the perfusion of the small intestine using isotonic phosphate buffer recycled in situ. The absorption rate of AsA in the small intestine of guinea pigs was higher than that of ErA; however, Km of AsA absorption was lower than that of ErA in normal guinea pigs. In AsA-deficient guinea pigs, the absorption rates of both AsA and ErA were higher than those in normal ones. The absorption of AsA and ErA in the small intestine of guinea pigs was inhibited by ouabain. Furthermore, AsA and ErA inhibited each other's absorption. Based on the results, the net amount of the absorbed ErA in the small intestine may be lower than that of AsA, and ErA absorption mechanism seemed to be similar to that of AsA. The absorption rate of both AsA and ErA in the small intestine of guinea pig might be dependent on the AsA level in the tissues.     

Kinetic analysis of the effect of luminal pH on transport of folic acid in the small intestine

Transport of folic acid in the small intestine is markedly affected by luminal pH and is optimal at pH 5.5-6.5. To clarify the nature of this pH sensitivity, we measured transport of [3H] folic acid into rat jejunum at pH 5.5 and 7.4, at folic acid concentrations of 1 to 30 microM, using the influx chamber method. At this range of concentrations, uptake of folic acid exhibited Michaelis-Menten kinetics. In order to determine the kinetic parameter which is pH sensitive, we have fitted a modified Michaelis-Menten equation to the pooled data at the two pH conditions such as they share a common Vmax but distinct Km values. This model fitted closely the experimental data (F = 0.693), yielding Km values (+/- SE) of 24.22 (+/- 5.96) and 36.32 (+/- 8.73) microM for pH 5.5 and 7.4 respectively. The difference in the Km at the two pH conditions was statistically significant (p less than 0.01). The change in affinity for folic acid transport at the two pH conditions may reflect different membrane carriers, or may be due to protonation of a membrane carrier and/or the folic acid molecule, favored at low pH.     

稳定性同位素在大鼠移植小肠吸收功能研究中的应用

作者用改良的Ｍｏｎｃｈｉｋ和Ｒｕｓｓｅｌｌ法对大鼠行异体节段性异位小肠移植，在肠外营养支持１０ｄ后用稳定性同位素测定技术测定大鼠移植小肠对＾１５Ｎ标记甘氨酸的吸收功能。结果示１ｈ的吸为３６．１１％，而在２ｈ和３ｈ的吸收率分别达到５６．４８％和６５．５４％，呈持续吸收状态。作者认为本研究中所采用的方法，在实验及临床小肠收功能研究方面值得借鉴。     

Hydrolyzed guar gum decreases postprandial blood glucose and glucose absorption in the rat small intestine

We hypothesized that infusing partially hydrolyzed guar gum (PHGG) into the duodenum would reduce increases in postprandial plasma glucose by decreasing the rate of glucose diffusion from the small intestine luminal digesta of the rat. The postprandial plasma glucose and apparent glucose disappearance from the small intestine were measured after infusing artificial digesta containing 0 (control), 3.0, or 6.0 g/L PHGG into the duodenum via a cannula under anesthesia in experiments 1 and 2. The diffusion of glucose in the artificial digesta was estimated using dialysis tubing, filled with the same artificial digesta, soaked in a buffer in experiment 3. In experiment 1, the plasma glucose concentration was lower in the digesta containing 3.0 and 6.0 g/L PHGG than in the control digesta at 120 minutes (P < .05). The plasma insulin concentration was lower for the digesta containing 6.0 g/L PHGG than for the control digesta at 60 minutes (P < .05) and lower for the digesta containing 6.0 g/L PHGG than for that containing 3.0 g/L PHGG at 120 minutes (P < .05).The area under the curve of plasma glucose and insulin (experiment 1), apparent disappearance of glucose in the lumen of the small intestine (experiment 2), and net disappearance of glucose in the dialysis tube depended negatively on the viscosity of the artificial digesta (P < .05, .05, .001, and .05), which was increased by adding PHGG. Therefore, PHGG can decrease the postprandial blood glucose by lowering the rate of absorption from the small intestine in the rat by reducing the diffusion of glucose in the lumen.     

Thiamine absorption in the rat. ii. intestinal alkaline phosphatase activity and thiamine absorption from rat small intestine in-vitro and in-vivo.

The correlations between intestinal alkaline phosphatase (IAP) activity and thiamine absorption and glucose absorption were studied in the rat. An everted sac in-vitro technique was used in adult rats whereas in-vitro experiments were performed in young rats 10 days old. All incubation experiments were done with 14-C-labeled thiamine. The patterns of IAP activity along the small intestines differed greatly between young and adult rats but were closely paralleled by the distribution of active thiamine transport in adult rats and thiamine absorption in young rats, respectively. When IAP was specifically inhibited in adult rats by L-phenylalanine active thiamine transport in-vitro was abolished. No correlation was found between IAP activity and active transport or glucose in-vitro, nor did inhibition of the enzyme in any way affect glucose transport capacity. It is suggested that the enzyme intestinal alkalinephosphatase is involved in the process of active thiamine absorption.     

The effect of chronic ethanol intake on leucine absorption from the rat small intestine

Total (active + diffusion) absorption of leucine from the entire small intestine of rats or from segments of upper jejunum and lower ileum was unaffected by chronic ethanol feeding for 4 weeks. However, because of ethanol-induced mucosal atrophy, specific absorption (expressed per g dry weight of mucosa) was almost doubled in ethanol-fed rats. In the upper jejunum, the active component of leucine uptake was significantly greater in ethanol-fed rats (72% vs. 52%), whereas in the lower ileum the relative contributions of active uptake and diffusion were unaltered. We propose that the increase in active uptake in the upper jejunum is the result of a higher concentration of aged enterocytes having a greater transport capacity at the villus surface.     

Site of zinc absorption in dog small intestine

An in vivo intestinal perfusion technique was used to study the absorption of zinc from the duodenum, proximal jejunum and distal ileum of six dogs (group 1). Net absorption of zinc from the duodenum before and after ligation of the common bile duct averaged 596 and 574 ng.min-1.cm-1, respectively. Zinc absorption was greater (P less than 0.01) from the duodenum than from the jejunum (251 ng.min-1.cm-1) or ileum (404 ng.min-1.cm-1). Four other dogs (group 2) experienced perfusion of approximately equal segments of the duodenum (in two animals the common bile duct was ligated, and in another two it was not), proximal jejunum and distal ileum for 4 h. No change in absorption of zinc with time was noted, nor was any difference in absorption by the duodenum with and without ligation of the common bile duct observed. The data indicate that the duodenum has the greatest capacity for zinc absorption, followed by the distal ileum and proximal jejunum, and that pancreatic secretions do not appear to be necessary for adequate zinc absorption in the dog duodenum.     

Retinoid metabolism in the rat small intestine

Vitamin A (retinol) is essential for epithelial cell growth, differentiation and proliferation. The absorption of retinol occurs in the small intestine, and the metabolism of this vitamin is not well studied in this organ. The intestinal epithelium has a high rate of cell proliferation and differentiation, and the present study looked at the level of retinoids and metabolizing enzymes involved in their interconversion along the villus-crypt axis under normal conditions. Intestine was removed from control rats, and enterocytes at various stages of maturation and differentiation were quantified by the metal chelation method. Using HPLC, various retinoid concentrations in the cell homogenate and the metabolizing enzymes in the cytosol were quantified. The proliferating crypt cells were found to have a higher level of retinoic acid as well as of the enzymes involved in its formation, such as retinaldehyde oxidase and retinol dehydrogenase, compared with the villus cells, suggesting a possible role for this compound in intestinal epithelial cell proliferation and differentiation. The high level of retinol and high retinaldehyde reductase activity in the villus cells suggest the important role played by this enzyme in the conversion of dietary beta-carotene to retinol via retinaldehyde. In summary, this study has given for the first time a detailed analysis of the retinoid levels and metabolizing enzymes in different cell populations in the rat small intestinal epithelium.     

The large intestine compensates for insufficient calcium absorption in the small intestine in rats

We previously demonstrated that the large intestine compensated for decreased calcium (Ca) absorption caused by renal failure in rats fed a highly fermentable dietary fiber. In this study, we examined whether the large intestine compensated for insufficient Ca absorption in the rat small intestine without ingestion of a fermentable dietary fiber. Rats were fed one of four test diets containing either insoluble (carbonate) or soluble (gluconate, lactate, or citrate) Ca sources. The dietary Ca level was 2.0 g/kg, which is lower than the minimum requirement for rats (3.0 g/kg), to conduct the present study under a condition in which rats can maximally absorb Ca. To prevent Ca absorption in the small intestine, we replaced a primary phosphate (KH2PO4) with secondary phosphates (K2HPO4 and Na2HPO4) in diets. The apparent Ca absorption in the small intestine was estimated by adding chromic oxide (Cr2O3) as an insoluble and an unabsorbed marker to test diets and by measuring the ratio of Ca:Cr in the cecal content. The apparent Ca absorption in the whole intestine was estimated by the intake and fecal excretion of Ca. The apparent Ca absorption in the small intestine was significantly lower from the Ca carbonate diet than from the Ca gluconate, lactate, or citrate diets. The apparent Ca absorption in the whole intestine was not significantly different among the four groups, and the values were similar to the absorption rates in the small intestines of rats fed diets containing soluble Ca sources. These results show the following: (a) In rats fed 0.2% Ca diets containing soluble Ca salts, Ca is mostly absorbed in the small intestine, even in secondary phosphate intakes; (b) In contrast, in rats fed a 0.2% Ca diet containing an insoluble Ca salt (carbonate), Ca is not sufficiently absorbed in the small intestine. However, the large intestine compensates for the small intestinal Ca absorption decreased by dietary secondary phosphates.