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Purpose: There is general agreement that some dietary polyphenols block non-haem iron uptake, but the mechanisms by which they achieve this action are poorly understood. Since the polyphenol quercetin is ingested daily in significant amounts, we have investigated the effect of quercetin on duodenal non-haem iron absorption in vivo, as well as its effect on factors known to be involved in systemic iron metabolism., Methods: Rats were subject to gastric gavage and systemic quercetin administration. Treatments were followed with uptake studies using radiolabeled iron, serum iron and transferrin saturation measurements, LC-MS/MS analysis of quercetin metabolites in serum, determination of tissue non-haem iron content and analysis of gene expression of iron-related proteins., Results: Both oral and intraperitoneal (IP) quercetin caused serum and tissue iron depletion by two means, first by increasing mucosal iron uptake and inhibiting iron efflux from duodenal mucosa, and second by decreasing levels of duodenal DMT1, Dcytb and FPN. Additionally, IP quercetin induced highly significant increased liver expression of hepcidin, a hormone known to inhibit intestinal iron uptake., Conclusions: Oral quercetin significantly inhibited iron absorption, while IP quercetin significantly affected iron-related genes. These results could lead to development of new effective ways of preventing and treating iron deficiency anaemia, the most widespread nutritional disorder in the world.

Artificial sweeteners are extensively used by the food industry to replace sugar in food and beverages and are widely considered to be a healthy alternative. However, recent data suggest that artificial sweeteners may impact intestinal glucose absorption and that they might lead to glucose intolerance. Moreover, chronic consumption of artificial sweeteners has also been linked to detrimental changes in renal function. Using an in vivo approach, our study aimed to determine if short-term infusion of the artificial sweetener saccharin can alter renal function and renal glucose absorption. We show that saccharin infusion does not induce any major change in GFR or urine flow rate at either the whole kidney or single nephron level, suggesting that any reported change in renal function with artificial sweeteners must depend on chronic consumption. As expected for a nondiabetic animal, glucose excretion was low; however, saccharin infusion caused a small, but significant, decrease in fractional glucose excretion. In contrast to the whole kidney data, our micropuncture results did not show any significant difference in fractional glucose reabsorption in either the proximal or distal tubules, indicating that saccharin does not influence renal glucose handling in vivo under euglycemic conditions. In keeping with this finding, protein levels of the renal glucose transporters SGLT1 and SGLT2 were also unchanged. In addition, saccharin infusion in rats undergoing a glucose tolerance test failed to induce a robust change in renal glucose excretion or renal glucose transporter expression. In conclusion, our results demonstrate that saccharin does not induce acute physiologically relevant changes in renal function or renal glucose handling., (© 2018 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of The Physiological Society and the American Physiological Society.)

New Findings: What is the central question of this study? Does a previously hypothesized signalling mechanism, believed to detect postprandial increases in intestinal phosphate and that can stimulate the kidneys to rapidly excrete phosphate, operate under physiological conditions? What is the main finding and its importance? Contrary to earlier reports, rapid signalling between the small intestine and kidney mediated by a gut-derived phosphaturic factor in response to a physiological intestinal phosphate load is not supported by the present findings; moreover, hyperphosphataemia and increased parathyroid hormone concentrations are likely to be the underlying factors responsible for the phosphaturia following a supraphysiological intestinal phosphate load. To date, the role of the small intestine in the regulation of postprandial phosphate homeostasis has remained unclear and controversial. Previous studies have proposed the presence of a gut-derived phosphaturic factor that acts independently of changes in plasma phosphate concentration or parathyroid hormone (PTH) concentration; however, these early studies used duodenal luminal phosphate concentrations in the molar range, and therefore, the physiological relevance of this is uncertain. In the present study, we used both in vivo and in vitro approaches to investigate the presence of this putative 'intestinal phosphatonin'. Instillation of 1.3 m phosphate into the duodenum rapidly induced phosphaturia, but in contrast to previous reports, this was associated with significant hyperphosphataemia and elevated PTH concentration; however, there was not the expected decrease in abundance of the renal sodium-phosphate cotransporter NaPi-IIa. Instillation of a physiological (10 mm) phosphate load had no effect on plasma phosphate concentration, PTH concentration or phosphate excretion. Moreover, phosphate uptake by opossum kidney cells was unaffected after incubation with serosal fluid collected from intestinal segments perfused with different concentrations of phosphate. Taken together, these findings do not support the concept of a gut-derived phosphaturic factor that can mediate rapid signalling between the gut and kidney, leading to increased urinary phosphate excretion, as part of normal phosphate homeostasis., (© 2017 The Authors. Experimental Physiology © 2017 The Physiological Society.)

What is the central question of this study? Although SGLT2 inhibitors represent a promising treatment for patients suffering from diabetic nephropathy, the influence of metabolic disruption on the expression and function of glucose transporters is largely unknown. What is the main finding and its importance? In vivo models of metabolic disruption (Goto-Kakizaki type II diabetic rat and junk-food diet) demonstrate increased expression of SGLT1, SGLT2 and GLUT2 in the proximal tubule brush border. In the type II diabetic model, this is accompanied by increased SGLT- and GLUT-mediated glucose uptake. A fasted model of metabolic disruption (high-fat diet) demonstrated increased GLUT2 expression only. The differential alterations of glucose transporters in response to varying metabolic stress offer insight into the therapeutic value of inhibitors. SGLT2 inhibitors are now in clinical use to reduce hyperglycaemia in type II diabetes. However, renal glucose reabsorption across the brush border membrane (BBM) is not completely understood in diabetes. Increased consumption of a Western diet is strongly linked to type II diabetes. This study aimed to investigate the adaptations that occur in renal glucose transporters in response to experimental models of diet-induced insulin resistance. The study used Goto-Kakizaki type II diabetic rats and normal rats rendered insulin resistant using junk-food or high-fat diets. Levels of protein kinase C-βI (PKC-βI), GLUT2, SGLT1 and SGLT2 were determined by Western blotting of purified renal BBM. GLUT- and SGLT-mediated d-[(3) H]glucose uptake by BBM vesicles was measured in the presence and absence of the SGLT inhibitor phlorizin. GLUT- and SGLT-mediated glucose transport was elevated in type II diabetic rats, accompanied by increased expression of GLUT2, its upstream regulator PKC-βI and SGLT1 protein. Junk-food and high-fat diet feeding also caused higher membrane expression of GLUT2 and its upstream regulator PKC-βI. However, the junk-food diet also increased SGLT1 and SGLT2 levels at the proximal tubule BBM. Glucose reabsorption across the proximal tubule BBM, via GLUT2, SGLT1 and SGLT2, is not solely dependent on glycaemic status, but is also influenced by diet-induced changes in glucose metabolism. We conclude that different metabolic disturbances result in complex adaptations in renal glucose transporter protein levels and function., (© 2016 The Authors. Experimental Physiology © 2016 The Physiological Society.)

The liver expresses batteries of cytoprotective genes that confer cellular resistance to oxidative stress and xenobiotic toxins, and protection against cancer and other stress-related diseases. These genes are mainly regulated by Nrf2, making this transcription factor a target for small molecule discovery to treat such diseases. In this report, we identified dietary polyphenolic antioxidants that not only activated these genes but also relieved Nrf2 repression by Keap1, a Cul3-dependent ubiquitin ligase adaptor protein that mediates its degradation. Analysis of postprandial liver RNA revealed a marked activation of both genes by all test polyphenols compared with controls. Nrf2 inhibition by RNA interference reduced polyphenol effects on its target gene expression. Our data suggest that polyphenols may induce cellular defense genes by derepressing Nrf2 inhibition by Keap1. We posit that this ability to derepress Nrf2 and reactivate its target genes may underlie the protection conferred by polyphenols against oxidative stress-related diseases., (Copyright © 2015 Elsevier Inc. All rights reserved.)

Despite the importance of extracellular phosphate in many essential biological processes, the mechanisms of phosphate transport across the epithelium of different intestinal segments remain unclear. We have used an in vitro method to investigate phosphate transport at the brush border membrane (BBM) of intact intestinal segments and an in vivo method to study transepithelial phosphate absorption. We have used micromolar phosphate concentrations known to favor NaPi-IIb-mediated transport, and millimolar concentrations that are representative of the levels we have measured in luminal contents, to compare the extent of Na(+)-dependent and Na(+)-independent phosphate transport along the rat duodenum, jejunum, ileum, and proximal and distal colon. Our findings confirm that overall the jejunum is the main site of phosphate absorption; however, at millimolar concentrations, absorption shows ~30% Na(+)-dependency, suggesting that transport is unlikely to be mediated exclusively by the Na(+)-dependent NaPi-IIb co-transporter. In the ileum, studies in vitro confirmed that relatively low levels of phosphate transport occur at the BBM of this segment, although significant Na(+)-dependent transport was detected using millimolar levels of phosphate in vivo. Since NaPi-IIb protein is not detectable at the rat ileal BBM, our data suggest the presence of an as yet unidentified Na(+)-dependent uptake pathway in this intestinal segment in vivo. In addition, we have confirmed that the colon has a significant capacity for phosphate absorption. Overall, this study highlights the complexities of intestinal phosphate absorption that can be revealed using different phosphate concentrations and experimental techniques., (© 2015 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of the American Physiological Society and The Physiological Society.)

Balancing systemic iron levels within narrow limits is critical for maintaining human health. There are no known pathways to eliminate excess iron from the body and therefore iron homeostasis is maintained by modifying dietary absorption so that it matches daily obligatory losses. Several dietary factors can modify iron absorption. Polyphenols are plentiful in human diet and many compounds, including quercetin--the most abundant dietary polyphenol--are potent iron chelators. The aim of this study was to investigate the acute and longer-term effects of quercetin on intestinal iron metabolism. Acute exposure of rat duodenal mucosa to quercetin increased apical iron uptake but decreased subsequent basolateral iron efflux into the circulation. Quercetin binds iron between its 3-hydroxyl and 4-carbonyl groups and methylation of the 3-hydroxyl group negated both the increase in apical uptake and the inhibition of basolateral iron release, suggesting that the acute effects of quercetin on iron transport were due to iron chelation. In longer-term studies, rats were administered quercetin by a single gavage and iron transporter expression measured 18 h later. Duodenal FPN expression was decreased in quercetin-treated rats. This effect was recapitulated in Caco-2 cells exposed to quercetin for 18 h. Reporter assays in Caco-2 cells indicated that repression of FPN by quercetin was not a transcriptional event but might be mediated by miRNA interaction with the FPN 3'UTR. Our study highlights a novel mechanism for the regulation of iron bioavailability by dietary polyphenols. Potentially, diets rich in polyphenols might be beneficial for patients groups at risk of iron loading by limiting the rate of intestinal iron absorption.

Purpose of Review: For a number of years, there has been increasing interest in the concept of directly targeting intestinal phosphate transport to control hyperphosphatemia in chronic kidney disease. However, progress has been slow due to the paucity of information on the mechanisms involved in intestinal phosphate absorption. This editorial highlights the most recent developments in our understanding of this process and the role of the intestine in the maintenance of phosphate balance., Recent Findings: Recent studies in NaPi-IIb knockout mice have confirmed that this transport protein plays a significant role in intestinal phosphate absorption and is critical in the proposed feed-forward mechanism between the small intestine and kidney, which helps to maintain normal phosphate balance and steady-state plasma phosphate concentrations. In addition, renal failure-induced hyperphosphatemia is attenuated in NaPi-IIb knockout mice, confirming that NaPi-IIb is a suitable target in the prevention and treatment of hyperphosphatemia., Summary: Recent findings suggest that consumption of processed foods containing phosphate preservatives may lead to excessive phosphate exposure (if not overload), toxicity, and cardiovascular disease in the general population, as well as in patients with declining renal function. Therefore, establishing more effective ways of targeting the intestine to limit dietary phosphate absorption could have wide-reaching health benefits.

The inhibitory effects of the angiotensin-converting enzyme (ACE)-ANG II-angiotensin type 1 (AT₁) receptor axis on jejunal glucose uptake and the reduced expression of this system in type 1 diabetes mellitus (T1DM) have been documented previously. The ACE2-ANG-(1-7)-Mas receptor axis is thought to oppose the actions of the ACE-ANG II-AT₁ receptor axis in heart, liver, and kidney. However, the possible involvement of the ACE2-ANG-(1-7)-Mas receptor system on enhanced jejunal glucose transport in T1DM has yet to be determined. Rat everted jejunum and Caco-2 cells were used to determine the effects of ANG-(1-7) on glucose uptake and to study the ACE2-ANG-(1-7)-Mas receptor signaling pathway. Expression of target gene and protein in jejunal enterocytes and human Caco-2 cells were quantified using real-time PCR and Western blotting. T1DM increased jejunal protein and mRNA expression of ACE2 (by 59 and 173%, respectively) and Mas receptor (by 55 and 100%, respectively) in jejunum. One millimolar ANG-(1-7) reduced glucose uptake in jejunum and Caco-2 cells by 30.6 and 30.3%, respectively, effects that were abolished following addition of 1 μM A-779 (a Mas receptor blocker) or 1 μM GF-109203X (protein kinase C inhibitor) to incubation buffer for jejunum or Caco-2 cells, respectively. Finally, intravenous treatment of animals with ANG-(1-7) significantly improved oral glucose tolerance in T1DM but not control animals. In conclusion, enhanced activity of the ACE2-ANG-(1-7)-Mas receptor axis in jejunal enterocytes is likely to moderate the T1DM-induced increase in jejunal glucose uptake resulting from downregulation of the ACE-ANG II-AT₁ receptor axis. Therefore, altered activity of both ACE and ACE2 systems during diabetes will determine the overall rate of glucose transport across the jejunal epithelium.

The secoiridoids 3,4-dihydroxyphenylethanol-elenolic acid (3,4-DHPEA-EA) and 3,4-dihydroxyphenylethanol-elenolic acid dialdehyde (3,4-DHPEA-EDA) account for approximately 55 % of the phenolic content of olive oil and may be partly responsible for its reported human health benefits. We have investigated the absorption and metabolism of these secoiridoids in the upper gastrointestinal tract. Both 3,4-DHPEA-EDA and 3,4-DHPEA-EA were relatively stable under gastric conditions, only undergoing limited hydrolysis. Both secoiridoids were transferred across a human cellular model of the small intestine (Caco-2 cells). However, no glucuronide conjugation was observed for either secoiridoid during transfer, although some hydroxytyrosol and homovanillic alcohol were formed. As Caco-2 cells are known to express only limited metabolic activity, we also investigated the absorption and metabolism of secoiridoids in isolated, perfused segments of the jejunum and ileum. Here, both secoiridoids underwent extensive metabolism, most notably a two-electron reduction and glucuronidation during the transfer across both the ileum and jejunum. Unlike Caco-2 cells, the intact small-intestinal segments contain NADPH-dependent aldo-keto reductases, which reduce the aldehyde carbonyl group of 3,4-DHPEA-EA and one of the two aldeydic carbonyl groups present on 3,4-DHPEA-EDA. These reduced forms are then glucuronidated and represent the major in vivo small-intestinal metabolites of the secoiridoids. In agreement with the cell studies, perfusion of the jejunum and ileum also yielded hydroxytyrosol and homovanillic alcohol and their respective glucuronides. We suggest that the reduced and glucuronidated forms represent novel physiological metabolites of the secoiridoids that should be pursued in vivo and investigated for their biological activity.

Erythropoietin is produced by the kidney and stimulates erythropoiesis; however, in chronic renal disease its levels are reduced and patients develop anemia that is treatable with iron and recombinant hormone. The mechanism by which erythropoietin improves iron homeostasis is still unclear, but it may involve suppression of the iron regulatory peptide hepcidin and/or a direct effect on intestinal iron absorption. To investigate these possibilities, we used the well-established 5/6th nephrectomy rat model of chronic renal failure with or without human recombinant erythropoietin treatment. Monolayers of human intestinal Caco-2 cells were also treated with erythropoietin to measure any direct effects of this hormone on intestinal iron transport. Nephrectomy increased hepatic hepcidin expression and decreased intestinal iron absorption; these effects were restored to levels found in sham-operated rats on erythropoietin treatment of the rats with renal failure. In Caco-2 cells, the addition of erythropoietin significantly increased the expression of apical divalent metal transporter 1 (DMT1) and basolateral ferroportin and, consequently, iron transport across the monolayer. Taken together, our results show that erythropoietin not only exerts a powerful inhibitory action on the expression of hepcidin, thus permitting the release of iron from reticuloendothelial macrophages and intestinal enterocytes, but also acts directly on enterocytes to increase iron absorption.

Transport of phosphate across intestinal and renal epithelia is essential for normal phosphate balance, yet we know less about the mechanisms and regulation of intestinal phosphate absorption than we do about phosphate handling by the kidney. Recent studies have provided strong evidence that the sodium-phosphate cotransporter NaPi-IIb is responsible for sodium-dependent phosphate absorption by the small intestine, and it might be that this protein can link changes in dietary phosphate to altered renal phosphate excretion to maintain phosphate balance. Evidence is also emerging that specific regions of the small intestine adapt differently to acute or chronic changes in dietary phosphate load and that phosphatonins inhibit both renal and intestinal phosphate transport. This review summarizes our current understanding of the mechanisms and control of intestinal phosphate absorption and how it may be related to renal phosphate reabsorption; it also considers the ways in which the gut could be targeted to prevent, or limit, hyperphosphatemia in chronic and end-stage renal failure.

Streptozotocin-induced (Type 1) diabetes mellitus (T1DM) in rats promotes jejunal glucose transport, but the trigger for this response remains unclear. Our recent work using euglycemic rats has implicated the enterocyte renin-angiotensin system (RAS) in control of sodium-dependent glucose transporter (SGLT1)-mediated glucose uptake across the jejunal brush border membrane (BBM). The aim of the present study was to examine whether expression of enterocyte RAS components is influenced by T1DM. The effects of mucosal addition of angiotensin II (AII) on [(14)C]-D-glucose uptake by everted diabetic jejunum was also determined. Two-week diabetes caused a fivefold increase in blood glucose level and reduced mRNA and protein expression of AII type 1 (AT(1)) and AT(2) receptors and angiotensin-converting enzyme in isolated jejunal enterocytes. Angiotensinogen expression was, however, stimulated by diabetes while renin was not detected in either control or diabetic enterocytes. Diabetes stimulated glucose uptake into everted jejunum by 58% and increased the BBM expression of SGLT1 and facilitated glucose transporter 2 (GLUT2) proteins, determined by Western blotting by 25% and 135%, respectively. Immunohistochemistry confirmed an enhanced BBM expression of GLUT2 in diabetes and also showed that this was due to translocation of the transporter from the basolateral membrane to BBM. AII (5 microM) or L-162313 (1 microM), a nonpeptide AII analog, decreased glucose uptake by 18% and 24%, respectively, in diabetic jejunum. This inhibitory action was fully accountable by an action on SGLT1-mediated transport and was abolished by the AT(1) receptor antagonist losartan (1 microM). The decreased inhibitory action of AII on in vitro jejunal glucose uptake in diabetes compared with that noted previously in jejunum from normal animals is likely to be due to reduced RAS expression in diabetic enterocytes, together with a disproportionate increase in GLUT2, compared with SGLT1 expression at the BBM.

The role of putative humoral factors, known as phosphatonins, in phosphate homeostasis and the relationship between phosphate handling by the kidney and gastrointestinal tract are incompletely understood. Matrix extracellular phosphoglycoprotein (MEPE), one of several candidate phosphatonins, promotes phosphaturia, but whether it also affects intestinal phosphate absorption is unknown. Here, using the in situ intestinal loop technique, we demonstrated that short-term infusion of MEPE inhibits phosphate absorption in the jejunum but not the duodenum. Simultaneous measurement of urinary phosphate excretion suggests that the phosphaturic action of MEPE correlates with a significant reduction in the protein levels of the renal sodium-phosphate co-transporter NaPi-IIa in the proximal convoluted tubules of the outer renal cortex, assessed by Western blotting and immunohistochemistry. This short-term inhibitory effect of MEPE on renal and intestinal phosphate handling occurred without any changes in circulating levels of parathyroid hormone, 1,25-dihydroxyvitamin D(3), or fibroblast growth factor 23. Taken together, these findings suggest that MEPE is a candidate phosphatonin involved in phosphate homeostasis, acting in both the kidney and the gastrointestinal tract.

The concept of a regulatory link between the gastrointestinal tract and kidneys is not new. The idea that dietary intake and composition can affect renal function is perhaps self-evident, but defining this relationship, especially in terms of sensors and effectors, is proving more difficult. That the gastrointestinal tract can exert some control over renal function was strengthened by the early observation that oral ingestion of a sodium chloride load has a greater natriuretic effect than when the same amount is given intravenously. This effect was subsequently shown to be independent of changes in aldosterone and atrial natriuretic peptide, although not necessarily angiotensin-II. However, the nature of any intestinal natriuretic peptide remains uncertain, despite suggestions that various gut-derived hormones, including guanylin and uroguanylin, may be involved. There is also an emerging concept of gastrointestinal taste and taste-like receptor mechanisms present throughout the gastrointestinal tract, which may govern the excretion of other key electrolytes, including potassium and phosphate. The evidence for gut sensors of nutrients such as proteins, amino acids, glucose, and acid is now becoming more established. Thus, we can anticipate the existence and eventual characterization of several gut ion sensors.

There is increasing evidence that locally produced angiotensin AII (AII) regulates the function of many tissues, but the involvement of enterocyte-derived AII in the control of intestinal transport is unknown. This study examined whether there is a local renin-angiotensin system (RAS) in rat villus enterocytes and assessed the effects of AII on SGLT1-dependent glucose transport across the brush border membrane (BBM). Gene and protein expression of angiotensinogen, ACE, and AT(1) and AT(2) receptors were studied in jejunal and ileal enterocytes using immunocytochemistry, Western blotting and RT-PCR. Mucosal uptake of d-[(14)C]glucose by everted intestinal sleeves before and after addition of AII (0-100 nm) to the mucosal buffer was measured in the presence or absence of the AT(1) receptor antagonist losartan (1 microm). Immunocytochemistry revealed the expression of angiotensinogen, ACE, and AT(1) and AT(2) receptors in enterocytes; immunoreactivity of AT(1) receptor and angiotensinogen proteins was especially pronounced at the BBM. Expression of angiotensinogen and AT(1) and AT(2) receptors, but not ACE, was greater in the ileum than the jejunum. Addition of AII to mucosal buffer inhibited phlorizin-sensitive (SGLT1-dependent) jejunal glucose uptake in a rapid and dose-dependent manner and reduced the expression of SGLT1 at the BBM. Losartan attenuated the inhibitory action of AII on glucose uptake. AII did not affect jejunal uptake of l-leucine. The detection of RAS components at the enterocyte BBM, and the rapid inhibition of SGLT1-dependent glucose uptake by luminal AII suggest that AII secretion exerts autocrine control of intestinal glucose transport.

Aims/hypothesis: GLUT2 is the main renal glucose transporter upregulated by hyperglycaemia, when it becomes detectable at the brush border membrane (BBM). Since glucose-induced protein kinase C (PKC) activation in the kidney is linked to diabetic nephropathy, we investigated the effect of glycaemic status on the protein levels of PKC isoforms alpha, betaI, betaII, delta and epsilon in the proximal tubule, as well as the relationship between them and changes in GLUT2 production at the BBM., Methods: Plasma glucose concentrations were modulated in rats by treatment with nicotinamide 15 min prior to induction of diabetes with streptozotocin. Levels of GLUT2 protein and PKC isoforms in BBM were measured by western blotting. Additionally, the role of calcium signalling and PKC activation on facilitative glucose transport was examined by measuring glucose uptake in BBM vesicles prepared from proximal tubules that had been incubated either with thapsigargin, which increases cytosolic calcium, or with the PKC activator phorbol 12-myristate,13-acetate (PMA)., Results: Thapsigargin and PMA enhanced GLUT-mediated glucose uptake, but had no effect on sodium-dependent glucose transport. Diabetes significantly increased the protein levels of GLUT2 and PKC-betaI at the BBM. Levels of GLUT2 and PKC-betaI correlated positively with plasma glucose concentration. Diabetes had no effect on BBM levels of alpha, betaII, delta or epsilon isoforms of PKC., Conclusions/interpretation: Enhanced GLUT2-mediated glucose transport across the proximal tubule BBM during diabetic hyperglycaemia is closely associated with increased PKC-betaI. Thus, altered levels of GLUT2 and PKC-betaI proteins in the BBM may be important factors in the pathogenic processes underlying diabetic renal injury.

Hyperphosphatemia is an important consequence of chronic renal failure (CRF). Lowering of the plasma phosphate concentration is believed to be critical in the management of patients with CRF, especially those on dialysis. Reports of the effect of CRF on the intestinal handling of phosphate in vitro have been conflicting; but what happens in vivo has not been studied. What effect a reduction in the dietary phosphate intake has on intestinal phosphate absorption in CRF in vivo is unclear. In this study, we have used the in situ intestine loop technique to determine intestinal phosphate absorption in the 5/6-nephrectomy rat model of CRF under conditions of normal and restricted dietary phosphate intake. In this model of renal disease, we found that there is no significant change in the phosphate absorption in either the duodenum or jejunum regardless of the dietary phosphate intake. There was also no change in the expression of the messenger RNA of the major intestinal phosphate carrier the sodium-dependent-IIb transporter. Furthermore, we found no change in the intestinal villus length or in the location of phosphate uptake along the villus. Our results indicate that in CRF, unlike the kidney, there is no reduction in phosphate transport across the small intestine. This makes intestinal phosphate absorption a potential target in the prevention and treatment of hyperphosphatemia.

We have conducted a detailed investigation into the absorption, metabolism and microflora-dependent transformation of hydroxytyrosol (HT), tyrosol (TYR) and their conjugated forms, such as oleuropein (OL). Conjugated forms underwent rapid hydrolysis under gastric conditions, resulting in significant increases in the amount of free HT and TYR entering the small intestine. Both HT and TYR transferred across human Caco-2 cell monolayers and rat segments of jejunum and ileum and were subject to classic phase I/II biotransformation. The major metabolites identified were an O-methylated derivative of HT, glucuronides of HT and TYR and a novel glutathionylated conjugate of HT. In contrast, there was no absorption of OL in either model. However, OL was rapidly degraded by the colonic microflora resulting in the formation of HT. Our study provides additional information regarding the breakdown of complex olive oil polyphenols in the GI tract, in particular the stomach and the large intestine.

Previously, it was thought that intestinal phosphate transport occurred exclusively in the proximal small intestine of rodents and humans. However, a recent study has demonstrated that the ileum of mice contributes significantly to the absorption of dietary phosphate, but it is not known whether this region is also an important site of phosphate absorption in the rat. In the present study, we have investigated the mRNA and protein levels of the sodium-phosphate cotransporter, NaPi-IIb, in three regions of rat and mouse small intestine, and related its expression levels to the rate of net phosphate absorption, as measured using the in situ intestinal loop technique. 1,25-Dihydroxyvitamin D3 is an important physiological regulator of intestinal phosphate absorption that increases phosphate transport in both the duodenum and jejunum of the rat. Based on the recently proposed regional profile of phosphate absorption along the mouse small intestine, we have re-evaluated the effects of 1,25-dihydroxyvitamin D3 using three distinct regions of the mouse and rat small intestine. Our studies have revealed important differences in the intestinal handling of phosphate between mice and rats. In mice, maximal phosphate absorption occurs in the ileum, which is paralleled by the highest expression levels of NaPi-IIb mRNA and protein. In contrast, in rats maximal absorption occurs in the duodenum with very little absorption occurring in the ileum, which is similar to the pattern reported in humans. However, in both rodent species only the jejunum shows an increase in phosphate absorption in response to treatment with 1,25-dihydroxyvitamin D3.

Studies have suggested that diets rich in polyphenols such as flavonoids may lead to a reduced risk of gastrointestinal cancers. We demonstrate the ability of monomeric and dimeric flavanols to scavenge reactive nitrogen species derived from nitrous acid. Both epicatechin and dimer B2 (epicatechin dimer) inhibited nitrous acid-induced formation of 3-nitrotyrosine and the formation of the carcinogenic N-nitrosamine, N-nitrosodimethylamine. The reaction of monomeric and dimeric epicatechin with nitrous acid led to the formation of mono- and di-nitroso flavanols, whereas the reaction with hesperetin resulted primarily in the formation of nitrated products. Although, epicatechin was transferred across the jejunum of the small intestine yielding metabolites, its nitroso form was not absorbed. Dimer B2 but not epicatechin monomer inhibited the proliferation of, and triggered apoptosis in, Caco-2 cells. The latter was accompanied by caspase-3 activation and reductions in Akt phosphorylation, suggesting activation of apoptosis via inhibition of prosurvival signaling. Furthermore, the dinitroso derivative of dimer B2, and to a lesser extent the dinitroso-epicatechin, also induced significant toxic effects in Caco-2 cells. The inhibitory effects on cellular proliferation were paralleled by early inhibition of ERK 1/2 phosphorylation and later reductions in cyclin D1 levels, indicating modulation of cell cycle regulation in Caco-2 cells. These effects highlight multiple routes in which dietary derived flavanols may exert beneficial effects in the gastrointestinal tract.

Background: Only 10% of dietary iron is absorbed in the duodenum which implies that 90% (approximately 9 mg day(-1)) reaches the lower small intestine and colon. Therefore the purpose of this study was to assess the iron transport capacity of the rat proximal colon and to determine whether iron absorption is regulated by changes in dietary iron content., Materials and Methods: Rats were fed for 14 days on either iron adequate (44 mg Fe kg(-1) diet) or iron-deficient (< 0.5 mg Fe kg(-1) diet) diets. The 59Fe transport across the colonic epithelium and its subsequent appearance in the blood were measured in vivo. In separate studies the colon was excised and used to measure divalent metal transporter expression., Results: Divalent metal transporter (DMT1) was expressed at the apical membrane of the surface epithelium in rat proximal colon. In animals fed an iron-deficient diet, DMT1 mRNA and protein expression were increased. This was accompanied by a significant increase in tissue 59Fe uptake., Conclusions: The proximal colon can absorb non-haem iron from the intestinal lumen. The purpose of this mechanism remains to be elucidated.

Background: Despite the requirement for increased iron delivery for erythropoiesis during hypoxia, there is very little information on how duodenal iron uptake and its transfer to the blood adapts to this condition., Aims: To assess the effects of 30 days of chronic hypoxia in rats on luminal iron uptake and transfer of the metal to blood, together with gene expression of hepcidin, a proposed negative regulator of iron transport., Methods: 59-Fe uptake by isolated duodenum and its transfer to blood by in vivo duodenal segments was measured after exposure of rats to room air or 10% oxygen for four weeks. Liver hepcidin expression was measured by real time reverse transcription-polymerase chain reaction. The effects of hypoxia on hepcidin gene expression by HepG2 cells was also determined., Results: Hypoxia did not affect villus length but enhanced (+192.6%) luminal iron uptake by increasing the rate of uptake by all enterocytes, particularly those on the upper villus. Hypoxia promoted iron transfer to the blood but reduced mucosal iron accumulation in vivo by 66.7%. Hypoxia reduced expression of hepcidin mRNA in both rat liver and HepG2 cells., Conclusions: Prolonged hypoxia enhances iron transport from duodenal lumen to blood but the process is unable to fully meet the iron requirement for increased erythropoiesis. Reduced secretion of hepcidin may be pivotal to the changes in iron absorption. The processes responsible for suppression of hepcidin expression are unknown but are likely to involve a direct effect of hypoxia on hepatocytes.

The effect of the putative iron regulatory peptide hepcidin on iron absorption was investigated in mice. Hepcidin peptide was synthesized and injected into mice for up to 3 days, and in vivo iron absorption was measured with tied-off segments of duodenum. Liver hepcidin expression was measured by reverse transcriptase-polymerase chain reaction. Hepcidin significantly reduced mucosal iron uptake and transfer to the carcass at doses of at least 10 microg/mouse per day, the reduction in transfer to the carcass being proportional to the reduction in iron uptake. Synthetic hepcidin injections down-regulated endogenous liver hepcidin expression excluding the possibility that synthetic hepcidin was functioning by a secondary induction of endogenous hepcidin. The effect of hepcidin was significant at least 24 hours after injection of hepcidin. Liver iron stores and hemoglobin levels were unaffected by hepcidin injection. Similar effects of hepcidin on iron absorption were seen in iron-deficient and Hfe knockout mice. Hepcidin inhibited the uptake step of duodenal iron absorption but did not affect the proportion of iron transferred to the circulation. The effect was independent of iron status of mice and did not require Hfe gene product. The data support a key role for hepcidin in the regulation of intestinal iron uptake.

The metabolism of chlorogenic acid, naringin, and rutin, representative members of three common families of dietary polyphenols, the hydroxycinnamates, the flavanones, and the flavonols, respectively, was studied in an in vitro mixed culture model of the human colonic microflora. Time- and concentration-dependent degradation of all three compounds was observed, which was associated with the following metabolic events after cleavage of the ester or glycosidic bond: reduction of the aliphatic double bond of the resulting hydroxycinnamate caffeic acid residue; dehydroxylation and ring fission of the heterocyclic C-ring of the resulting deglycosylated flavanone, naringenin, and of the deglycosylated flavonol, quercetin (which differed depending on the substitution). The metabolic events, their sequences, and major phenolic end products, as identified by GC-MS or LC-MS/MS, were elucidated from the structural characteristics of the investigated compounds. The major phenolic end products identified were 3-(3-hydroxyphenyl)-propionic acid for chlorogenic acid, 3-(4-hydroxyphenyl)-propionic acid and 3-phenylpropionic acid for naringin, and 3-hydroxyphenylacetic acid and 3-(3-hydroxyphenyl)-propionic acid for rutin. The degree of degradation of the compounds studied was significantly influenced by the substrate concentration as well as individual variations in the composition of the fecal flora. The results support extensive metabolism of dietary polyphenols in the colon, depending on substrate concentration and residence time, with resultant formation of simple phenolics, which can be considered biomarkers of colonic metabolism if subsequently absorbed. It is also apparent that a relatively small number of phenolic degradation products are formed in the colon from the diverse group of natural polyphenols.

The mechanism of renal glucose transport involves the reabsorption of filtered glucose from the proximal tubule lumen across the brush border membrane (BBM) via a sodium-dependent transporter, SGLT, and exit across the basolateral membrane via facilitative, GLUT-mediated, transport. The aim of the present study was to determine the effect of streptozotocin-induced diabetes on BBM glucose transport. We found that diabetes increased facilitative glucose transport at the BBM by 67.5 % (P < 0.05)--an effect that was abolished by overnight fasting. Western blotting and immunohistochemistry demonstrated GLUT2 expression at the BBM during diabetes, but the protein was undetectable at the BBM of control animals or diabetic animals that had been fasted overnight. Our findings indicate that streptozotocin-induced diabetes causes the insertion of GLUT2 into the BBM and this may provide a low affinity/high capacity route of entry into proximal tubule cells during hyperglycaemia.

Hereditary hemochromatosis is a common iron-loading disorder found in populations of European descent. It has been proposed that mutations causing loss of function of HFE gene result in reduced iron incorporation into immature duodenal crypt cells. These cells then overexpress genes for iron absorption, leading to inappropriate cellular iron balance, a persistent iron deficiency of the duodenal mucosa, and increased iron absorption. The objective was to measure duodenal iron content in Hfe knock-out mice to test whether the mutation causes a persistent decrease in enterocyte iron concentration. In both normal and Hfe knock-out mice, duodenal nonheme iron content was found to correlate with liver iron stores (P <.001, r = 0.643 and 0.551, respectively), and this effect did not depend on dietary iron levels. However, duodenal iron content was reduced in Hfe knock-out mice for any given content of liver iron stores (P <.001).

Glucagon is known to affect glomerular filtration rate and renal tubular solute and fluid transport, although it is only thought to act directly on the thick ascending limb (TAL) and collecting duct (CD). Indeed, previous studies have detected glucagon-sensitive adenylate cyclase exclusively in these nephron segments, suggesting the presence of glucagon receptors. In the present study, we have demonstrated for the first time that glucagon receptor mRNA is expressed in the rat proximal tubule, as well as in the TAL and CD. By autoradiography, we have also shown that specific binding of glucagon occurs in both the renal cortex and medulla. In addition, using proximal tubule brush-border membrane (BBM) vesicles for studies of glucose transport, we have established that glucagon stimulates glucose uptake via a facilitative GLUT-mediated transport process (by 58%; P < 0.005), whereas cAMP stimulates only the sodium glucose-linked transporter ('SGLT')-mediated glucose uptake (by 53%; P < 0.05). Taken together, these findings suggest that glucagon could have a role in controlling proximal tubular transport function, including glucose reabsorption, but unlike in the TAL and CD, the proximal tubule glucagon receptor might not be coupled primarily to adenylate cyclase.

Duodenal Fe2+ uptake is essential to body Fe2+ homeostasis, but the interaction of metals with the uptake process remains unclear. The present study compared the effects of four essential trace metals (Mn2+, Zn2+, Co2+ and Ni2+) with two toxic metals (Pb2+ and Cd2+) on Fe2+ uptake across the brush border membrane of villus-attached duodenal enterocytes. Everted rat duodenum was exposed to buffer containing 0.2 mm-59Fe2+-ascorbate with or without the competing metal (2 mm) and the tissue was then processed for autoradiography allowing Fe2+ uptake to be determined at specific crypt-villus regions. The quantification method ensured that uptake by cells, rather than Fe2+ binding to the tissue surface, was measured. Fe2+ uptake was significantly inhibited by Cd2+ in upper villus enterocytes only and Pb2+ was without effect on Fe2+ uptake. The inhibition by Cd2+ was not due to general cell damage as judged by the release of lactate dehydrogenase from tissue into incubation fluid. Essential divalent trace metals reduced uptake significantly along the whole length of the crypt-villus axis. Cd2+ uptake, measured separately, took place at all regions of the villus-crypt axis, highest uptake being into crypt enterocytes. The very different uptake profiles for Cd2+ and Fe2+ suggests that the divalent metal transporter 1 is not the principal transporter of Cd2+. The addition of Fe2+ to incubation buffer inhibited Cd2+ uptake by both crypt and villus enterocytes. The possibility that the inhibitory actions of Fe2+ and Cd2+ on the uptakes of Cd2+ and Fe2+ respectively can be explained by a non-competitive action or the involvement of an additional metal transporter is discussed.

The success of nasoenteral nutrition support can be limited by intestinal impairment. In particular, reduced absorptive area, mucosal atrophy and abnormal motility may reduce absorption of macronutrients and micronutrients, and diarrhoea remains a commonly encountered complication. We review how basic physiological techniques can be used to investigate such pathophysiology. Lumenal nutrients control mucosal growth, expression of mucosal transporters and regional gut motility. Cell biology techniques now complement classical intestinal perfusion methods in determining the 'safety factor' of excess absorptive capacity. The controversial role of the sodium-glucose linked transporter in dietary glucose assimilation is described in terms of its control, its true function and its role in uptake of other solutes. Techniques that involve brush-border membrane vesicles, Caco-2 cells, mucosal immunohistochemistry and gene expression probes are described. Together, these techniques describe a picture of an organ with remarkable ability to maintain digestive and absorptive function in response to a wide variety of nutritional intakes, often in the face of inflammatory illness.

Perfusion of isolated small intestine with the procyanidin dimers B2 and B5 extracted from cocoa indicated that both forms of dimer are transferred to the serosal side of enterocytes but only to a very small extent (<1% of the total transferred flavanol-like compounds). However, perfusion of dimer mainly resulted in large amounts of unmetabolised/unconjugated epicatechin monomer being detected on the serosal side (95.8%). The cleavage of dimer during transfer seemed to be energy-dependent, requiring an intact cell system, as incubation with jejunal homogenates failed to yield epicatechin. Low levels methylated dimer were also detected (3.2%), but no conjugates and metabolites of epicatechin indicating that metabolism of monomer and dimer is limited during dimer cleavage/translocation. The methylation of dimer may be by catechol-O-methyltransferase, however, at high concentrations of dimer COMT activity is reduced leading to an inhibition of both monomer and dimer O-methylation., (Copyright 2001 Academic Press.)

There is considerable interest in the bioavailability of polyphenols and their bioactivity in vivo. We have studied the absorption and metabolism of catechin and epicatechin in the small intestine and the comparative transfer across the jejunum and ileum. Perfusion of isolated jejunum with the flavanols resulted in glucuronidation ( approximately 45%), O-methylation: 3'-O-Methyl- and 4'-O-methyl- ( approximately 30%), and O-methyl-glucuronidation ( approximately 20% of total flavanols identified) during transfer across the enterocytes to the serosal side. This demonstrates the activity of catechol-O-methyl transferases in the metabolism of flavanols and suggests that these metabolites and conjugates are likely to enter the portal vein. In contrast, in the case of the ileum, the majority of the flavanols appeared on the serosal side unmetabolised and the total percentage of flavanols transferred was higher than that in the jejunum ( approximately fivefold)., (Copyright 2000 Academic Press.)

We have studied the absorption and metabolism of resveratrol in the jejunum in an isolated rat small intestine model. Only small amounts of resveratrol were absorbed across the enterocytes of the jejunum and ileum unmetabolised. The major compound detected on the serosal side was the glucuronide conjugate of resveratrol (96.5% +/- 4.6 of the amount absorbed) indicating the susceptibility of resveratrol to glucuronidation during transfer across the rat jejunum. The presence of the glucuronide was confirmed using HPLC-PDA and nanoES-MS/MS techniques. These findings suggest that resveratrol is most likely to be in the form of a glucuronide conjugate after crossing the small intestine and entering the blood circulation. This will have important implications for the biological functions of resveratrol in vivo.

Iron deficiency induced in rats maintained on a commercial diet with a low iron content has been used to investigate adaptive mechanisms that enhance duodenal iron uptake. These adaptive changes have been divided into those that result from changes in villus surface area (structural adaptation) and those that reflect changes in the way individual enterocytes express iron transport function (cellular adaptation). Cellular adaptation was assessed by carrying out microdensitometry of autoradiographs prepared from duodenal tissue previously incubated for 5 min in 200 micromol/l 59Fe2+-ascorbate. Structural adaptation was studied by performing image analysis of microdissected and sectioned villi. Cellular adaptation involved increased iron uptake by enterocytes present in the lower villus. Thus iron deficiency resulted in a threefold enhanced expression of uptake in the lower 100 microm villus (3.9+/-2.4 versus 12.6+/-1.5 arbitrary units, P<0.001). Maximal uptake was reached in the upper region of both control and iron-deficient villi, but iron deficiency had no effect on cellular uptake at this part of the villus. Structural adaptation involved the lengthening (+16%, P<0.05) and broadening (+14%) of villi in the duodenum of iron-deficient rats. The resultant expansion in villus area caused a further increase in uptake that was mostly expressed in the upper villus. Maximal uptake corrected for structure occurred in the middle third of villi from control and iron-deficient rats. Cellular plus structural adaptation produced a twofold increase in iron uptake. More than half of this effect was caused by changes in villus structure. [3H]Thymidine labelling experiments revealed a slightly earlier expression of enterocyte iron uptake in iron deficiency.

We have studied the perfusion of the jejunum and ileum in an isolated rat intestine model with flavonoids and hydroxycinnamates and the influence of glycosylation on the subsequent metabolism. Flavone and flavonol glucosides and their corresponding aglycones are glucuronidated during transfer across the rat jejunum and ileum and this glucuronidation occurs without the need for gut microflora. Furthermore, this suggests the presence of glycosidases as well as UDP-glucuronyl transferase in the jejunum. In contrast, quercetin-3-glucoside and rutin are mainly absorbed unmetabolised. The results suggest that the more highly reducing phenolics are absorbed predominantly as glucuronides (96.5%+/-4.6) of the amount absorbed, whereas monophenolic hydroxycinnamates and monophenolic B-ring flavonoids are less predisposed to glucuronidation and higher levels of aglycone (88.1%+/-10.1) are detected on absorption through both the jejunum and ileum.

Background: The intestinal handling of dextran, an alpha-1,6-linked glucose polymer, is poor compared with starch, and some ingested dextran might therefore reach the lower small intestine. As luminal sugar up-regulates SGLT1 (sodium-dependent glucose transporter) locally, we report the effects of a dextran-enriched diet on jejunal and ileal brush border membrane (BBM) glucose uptake., Methods: Rats were maintained on a diet containing 65% maltodextrin or 32.5% maltodextrin + 32.5% dextran (10 kD or 40 kD) for 8-10 days, and the kinetics of phlorizin-sensitive [3H]-glucose uptake by purified BBM vesicles was determined., Results: Ingestion of 40-kD but not 10-kD dextran increased Vmax for jejunal and ileal glucose uptake (+64.3% and +61.8% respectively, both P < 0.02). The transport response to 40-kD dextran was in keeping with lower levels of expired H2 at the end of the feeding period. High-performance liquid chromatography (HPLC) analysis of luminal contents indicated extensive hydrolysis of ingested dextran. Finally, 3-h jejunal exposure to 40-kD dextran in vivo increased the Vmax for glucose uptake by jejunal BBM., Conclusion: It is likely that increased SGLT1-mediated glucose uptake after short or longer term mucosal exposure to dextran results from luminal dextran per se or a hydrolysis product. The clinical implications of this up-regulation are discussed.

The effects of 1 day of streptozotocin-induced diabetes in rats on glucose transport across the brush border membrane (BBM) and basolateral membrane (BLM) prepared from jejunal enterocytes has been studied. The effects on glucose transport of treatment of diabetic animals with insulin to reduce to normal the elevated blood glucose levels has also been assessed. The maximum capacity (Vmax) for SGLT1-mediated glucose uptake by BBM vesicles was unaffected by diabetes or insulin treatment of diabetic rats. In contrast, Vmax for BLM glucose uptake was increased by 206% in diabetes, a response that could not be reversed by treatment with insulin. Western blotting of BBM for SGLT1 protein revealed a single band with a molecular weight of 73 kDa and the intensity of this band was unaffected by diabetes. However, an increased level of GLUT2 was noted in diabetic BLM and this was not a consequence of changes in glycaemic or insulin status. Diabetes hyperpolarised the BBM, implying an increased driving force for Na(+)-sugar co-transport but insulin treatment only partially reversed this enhanced potential difference. Benzamil (2 microns), an epithelial Na+ channel blocker, hyperpolarised the BBM of control but not diabetic enterocytes, implying that a reduced Na+ permeability was responsible for the diabetic hyperpolarisation. It was concluded that in early diabetes, before the onset of hyperphagia, a greater driving force for Na(+)-dependent BBM sugar transport together with increased GLUT2 activity at the BLM promotes sugar movement across the enterocyte. Possible triggers for the transport responses are discussed.

1. Chronic hypoxia enhances intestinal iron transport but the cellular processes involved are poorly understood. In order to assess the effects of 3 days of hypoxia on iron uptake across the duodenal brush-border membrane, we have measured the membrane potential difference (Vm) of villus-attached enterocytes by direct microelectrode impalement and have used semi-quantitative autoradiography to study changes in expression of iron uptake during enterocyte maturation. 2. Hypoxia increased duodenal Vm (-57.7 vs. -49.3 mV, P < 0.001). Ion substitution experiments revealed that hyperpolarization was due, at least in part, to a reduction in brush-border Na+ permeability. 3. Autoradiography revealed that hypoxia increased by 6-fold the rate of iron accumulation during enterocyte transit along the lower villus and enhanced by 3-fold the maximal accumulation of iron. Depolarization of the brush border, using a high-K(+)-containing buffer, caused a proportionally greater reduction in iron uptake in control compared with hypoxic tissue suggesting that the raised iron uptake is only partly driven by brush-border hyperpolarization. 4. We conclude that hypoxia increases the expression of iron transport in duodenal brush-border membrane and an enhanced electrical driving force may be involved in this response.

Changes in membrane expression of sodium-dependent glucose transporter (SGLT1) and glucose transporter isoform (GLUT2) protein have been implicated in the increased intestinal glucose transport in streptozotocin-diabetes. The possible involvement of GLUT1 in the transport response, however, has not previously been studied. Using confocal microscopy on tissue sections and Western blotting of purified brush border membrane (BBM) and basolateral membrane (BLM), we have examined enterocyte expression of GLUT1 in untreated and in 1 and 21 day streptozotocin diabetic rats. In control enterocytes, GLUT1 was absent at the BBM and detected at low levels at the BLM. Diabetes resulted in a 4- to 5-fold increased expression of GLUT1 at the BLM and the protein could also be readily detected at the BBM. Insulin treatment of diabetic rats increased GLUT1 level at the BBM but was without effect on expression of the protein at the BLM.

Background: Increased jejunal glucose transport after ingestion of carbohydrate rich diets may reflect higher concentrations of lumenal glucose. Normal processing of carbohydrate causes wide fluctuations in glucose concentration in the jejunal lumen and this raises the question of whether the high lumenal concentrations seen at peak digestion affect glucose uptake., Aims: To study the effects of 30 minute exposure of rat jejunal mucosa to glucose on sodium-glucose transporter (SGLT1) mediated glucose transport across the brush border membrane., Methods: Jejunal mucosa was exposed in vitro or in vivo to 25 mM glucose or 25 mM mannitol for 30 minutes. In addition, isolated villus enterocytes were incubated with mannitol or glucose for the same time. Brush border membrane vesicles were isolated from these preparations and phlorizin sensitive 3H-D-glucose accumulation was measured., Results: Lumenal glucose in vivo significantly enhanced SGLT1 mediated glucose uptake by 49.2-57.2%. For jejunal loops in vitro, the increase was 32.0-85.2%. Kinetic analysis disclosed a 50% greater Vmax for glucose uptake in each preparation. The facilitated and passive components of uptake were, however, unaffected by prior exposure to glucose. Incubation of villus enterocytes with 25 mM glucose did not influence glucose uptake by brush border membranes. Finally, exposure of intact mucosa to 20 mM galactose, a nonmetabolised sugar also transported by SGLT1, did not alter glucose transport., Conclusions: Lumenal glucose promotes glucose transport by brush border membrane within 30 minutes. An intact mucosa is necessary for upregulation and evidence suggests that the response is mediated by locally acting mechanisms.