Molecular biology of intestinal glucose transport

Using Porcine Jejunum Ex Vivo to Study Absorption and Biotransformation of Natural Products in Plant Extracts: Pueraria lobata as a Case Study

Herbal preparations (HPs) used in folk medicine are complex mixtures of natural products (NPs). Their efficacy in vivo after ingestion depends on the uptake of the active ingredient, and, in some cases, their metabolites, in the gastrointestinal tract. Thus, correlating bioactivities measured in vitro and efficacy in vivo is a challenge. An extract of Pueraria lobata rich in different types of isoflavones was used to evaluate the capacity of viable porcine small intestine ex vivo to elucidate the absorption of HP constituents, and, in some cases, their metabolites. The identification and transport of permeants across the jejunum was monitored by liquid chromatography-mass spectrometry (LC-MS), combining targeted and untargeted metabolite profiling approaches. It was observed that the C-glycoside isoflavones were stable and crossed the intestinal membrane, while various O-glycoside isoflavones were metabolized into their corresponding aglycones, which were then absorbed. These results are consistent with human data, highlighting the potential of using this approach. A thorough investigation of the impact of absorption and biotransformation was obtained without in vivo studies. The combination of qualitative untargeted and quantitative targeted LC-MS methods effectively monitored a large number of NPs and their metabolites, which is essential for research on HPs.

Influence of dietary fructose supplementation on visceral organ mass, carbohydrase activity, and mRNA expression of genes involved in small intestinal carbohydrate assimilation in neonatal calves

The hypothesis of this experiment was that dietary fructose would influence visceral organ mass, carbohydrase activity, and mRNA expression of carbohydrases and nutrient transporters in the small intestine in neonatal calves. Therefore, our objective was to use the neonatal calf as a model to evaluate the effects of postruminal fructose supply on small intestinal carbohydrate assimilation. Ten calves (<7 d of age; 41.2 ± 1.46 kg of body weight) were fed milk replacer at 2.0% of body weight daily (816 ± 90.5 g/d; 272 ± 30.1 g/L; dry-matter basis) in 2 equal portions and assigned to the following dietary treatment groups: (1) milk replacer (control; n = 6) or (2) milk replacer + 2.2 g of fructose/kg of body weight (fructose; n = 4). Calves were fed dietary treatments for 28 d, with jugular blood sampled every 7 d before and after the morning feeding. Calves were slaughtered, and visceral weights were recorded. Postruminal carbohydrase activities were assayed. Quantitative real-time PCR was conducted for small intestinal mRNA expression of nutrient transporters [solute carrier family 2 member 5 (GLUT5), solute carrier family 2 member 2 (GLUT2), and solute carrier family 5 member 1 (SGLT1)], carbohydrases (lactase, maltase-glucoamylase, and sucrase-isomaltase), and ketohexokinase (KHK). Data were analyzed using MIXED procedures in SAS version 9.4 (SAS Institute Inc, Cary, NC). Dietary fructose supplementation decreased serum glucose concentration. Small intestinal mass was greater in calves supplemented with fructose. Dietary fructose supplementation did not influence pancreatic α-amylase, small intestinal isomaltase, or maltase activities. Sucrase activity was undetected in the small intestine. Dietary fructose supplementation increased small intestinal glucoamylase activity per gram of tissue by 30% and increased maltase-glucoamylase mRNA expression by 6.8-fold. Dietary fructose supplementation did not influence mRNA expression of GLUT5, SGLT1, GLUT2, or KHK. Dietary fructose supplementation increased small intestinal lactase mRNA expression by 3.1-fold. Sucrase-isomaltase mRNA expression in the small intestine decreased 5.1-fold with dietary fructose supplementation. Dietary fructose supplementation does not induce sucrase activity in neonatal calves; however, sucrase-isomaltase may be transcriptionally regulated by dietary fructose in neonatal calves. More research is needed to compare glucose and fructose at isocaloric intakes to examine effects of dietary fructose at equal metabolizable energy intake.

Effects of long-term exercise training for different durations on pancreatic amylase activity and intestinal glucose transporter content in rats

Long‐term endurance training for a relatively short duration (~1 h) is reported to increase pancreatic amylase activity in rats, suggesting that chronic exercise training enhances carbohydrate digestive capacity. However, it remains unknown whether longer exercise training duration results in greater adaptation in the pancreas and small intestine. Thus, this study aimed to examine the effects of long‐term endurance training for a longer duration on pancreatic amylase activity and intestinal glucose transporter content in rats. Male Sprague–Dawley rats were subjected to swimming exercise training for 1 h (Ex‐1h group) or 6 h (Ex‐6h group, two 3‐h sessions separated by 1 h of rest) each day, 5 days a week, for 6 weeks. Sedentary rats were used as a control (Con group). Total pancreatic amylase activity in the Ex‐6h group was significantly lower than that in the Con and Ex‐1h groups immediately after the last training session. After 24 h of recovery, total pancreatic amylase activity was significantly higher in the Ex‐1h group (~46%) than in the Con group, and a further increase was observed in the Ex‐6h group (~98%). In addition, the Ex‐6h group, but not the Ex‐1h group, showed significantly greater intestinal sodium‐dependent glucose transporter 1 (SGLT1) content compared with the Con group after 24 h of recovery. However, no significant difference was observed in glucose transporter 2 (GLUT2) content among the three groups. In conclusion, chronic endurance exercise training for a longer duration results in larger increases in pancreatic amylase activity and intestinal SGLT1 content in rats.

Nutrient sensing of gut luminal environment

Sensing of nutrients by chemosensory cells in the gastrointestinal tract plays a key role in transmitting food-related signals, linking information about the composition of ingested foods to digestive processes. In recent years, a number of G protein-coupled receptors (GPCR) responsive to a range of nutrients have been identified. Many are localised to intestinal enteroendocrine (chemosensory) cells, promoting hormonal and neuronal signalling locally, centrally and to the periphery. The field of gut sensory systems is relatively new and still evolving. Despite huge interest in these nutrient-sensing GPCR, both as sensors for nutritional status and targets for preventing the development of metabolic diseases, major challenges remain to be resolved. However, the gut expressed sweet taste receptor, resident in L-enteroendocrine cells and responsive to dietary sweetener additives, has already been successfully explored and utilised as a therapeutic target, treating weaning-related disorders in young animals. In addition to sensing nutrients, many GPCR are targets for drugs used in clinical practice. As such these receptors, in particular those expressed in L-cells, are currently being assessed as potential new pathways for treating diabetes and obesity. Furthermore, growing recognition of gut chemosensing of microbial-produced SCFA acids has led further attention to the association between nutrition and development of chronic disorders focusing on the relationship between nutrients, gut microbiota and health. The central importance of gut nutrient sensing in the control of gastrointestinal physiology, health promotion and gut-brain communication offers promise that further therapeutic successes and nutritional recommendations will arise from research in this area.

LPS-squalene interaction on D-galactose intestinal absorption

The dynamic and complex interactions between enteric pathogens and the intestinal epithelium often lead to disturbances in the intestinal barrier, altered fluid, electrolyte, and nutrient transport and can produce an inflammatory response. Lipopolysaccharide (LPS) is a complex polymer forming part of the outer membrane of Gram-negative bacteria. On the other hand, squalene is a triterpene present in high levels in the extra-virgin olive oil that has beneficial effects against several diseases and it has also anti-oxidant and anti-inflammatory properties. The aim of this work was to study whether the squalene could eliminate the LPS effect on D-galactose intestinal absorption in rabbits and Caco-2 cells. The results have shown that squalene reduced the effects of LPS on sugar absorption. High LPS doses increased D-galactose uptake through via paracellular but also decreased the active sugar transport because the SGLT1 levels were diminished. However, the endotoxin effect on the paracellular way seemed to be more important than on the transcellular route. At the same time, an increased in RELM-β expression was observed. This event could be related to inflammation and cause a decrease in SGLT1 levels. In addition, MLCK protein is also increased by LPS which could lead to an increase in sugar transport through tight junctions. At low doses, the LPS could inhibit SGLT1 intrinsic activity. Bioinformatic studies by docking confirm the interaction between LPS-squalene as well as occur through MLCK and SGLT-1 proteins.

Postruminal digestion of starch infused into the abomasum of heifers with or without exogenous amylase administration

The effect of an exogenous amylase on postruminal digestion of starch infused into the abomasum of cattle was studied. Four rumen-cannulated heifers were fed 5.5 kg DM/d of a diet without starch, and assigned randomly to a crossover design. The experiment consisted of 2 periods lasting 23 d each with 10 d for adaptation to the diet followed by 13 d of abomasal infusion and sample collection. During the first 3 d of each infusion phase, isotonic saline solution was infused (1 liter/h) for measurement of baseline values in feces, followed by daily infusions of 880 g DM corn starch (1 kg/10 liters of water) without or with the addition of 2% of amylase. Titanium dioxide (10 g/d) was ruminally administered for estimation of fecal excretion. Digestion of starch in small intestine was calculated as the difference between the amounts of infused starch, disappeared from hindgut and fecal excretion. The apparent disappearance of starch from the hindgut was estimated based on the increment of microbial nitrogen (N) excretion due to starch infusion (1 g microbial N/100 g fermented starch) compared to baseline values. The concentration of purine bases in feces was used to estimate excretion of microbial N. Microbial N excretion increased with starch infusion (P < 0.05) but was not influenced by amylase (P = 0.81). Starch disappearance from the small intestine was not improved by amylase (P = 0.78) and averaged 85%. Amylase affected neither blood concentration of glucose (P = 0.80) nor of insulin (P = 0.26), but glucagon was lower without (P < 0.0001) than with amylase. The infusion of starch increased fecal excretion of total VFA (acetate, propionate, and butyrate) by 53% (P < 0.05), which indicates increased carbohydrate fermentation in the hindgut and incomplete digestion of starch in the small intestine. However, the excretion of total VFA was not affected by amylase (P = 0.66). Lactate excretion was higher at the second day of starch infusion (P < 0.05) without than with amylase, which suggests lower flow of starch from the small intestine to the hindgut due to a possible effect of amylase addition in animals not adapted to starch digestion. However, lactate excretion returned near to baseline values within 2 d, which was probably due to increase of lactate-utilizing bacteria and the adaptation of the microbial population in the hindgut. Further studies with higher starch levels and addition of amylase are recommended.

Glucagon-Like Peptide-2 and the Enteric Nervous System Are Components of Cell-Cell Communication Pathway Regulating Intestinal Na+/Glucose Co-transport

The Na⁺/glucose cotransporter 1, SGLT1 is the major route for transport of dietary glucose from the lumen of the intestine into absorptive enterocytes. Sensing of dietary sugars and artificial sweeteners by the sweet taste receptor, T1R2-T1R3, expressed in the enteroendocrine L-cell regulates SGLT1 expression in neighboring absorptive enterocytes. However, the mechanism by which sugar sensing by the enteroendocrine cell is communicated to the absorptive enterocytes is not known. Here, we show that glucagon-like peptide-2 (GLP-2) secreted from the enteroendocrine cell in response to luminal sugars regulates SGLT1 mRNA and protein expression in absorptive enterocytes, via the enteric neurons. Glucose and artificial sweeteners induced secretion of GLP-2 from mouse small intestine, which was inhibited by the sweet-taste receptor inhibitor, gurmarin. In wild type mice there was an increase in sugar-induced SGLT1 mRNA and protein abundance that was not observed in GLP-2 receptor knockout mice. GLP-2 receptor is expressed in enteric neurons, and not in absorptive enterocytes ruling out a paracrine effect of GLP-2. Electric field stimulation of the intestine resulted in upregulation of SGLT1 expression that was abolished by the nerve blocking agent tetrodotoxin. We conclude that GLP-2 and the enteric nervous system are components of the enteroendocrine-absorptive enterocyte communication pathway regulating intestinal glucose transport.

Hypoglycemic Effects of Pyrodextrins with Different Molecular Weights and Digestibilities in Mice with Diet-Induced Obesity

Pyrodextrin shares some properties of resistant starch, which is metabolically beneficial, and has potential applications as a functional food. In this study, we report that the oral administration of pyrodextrin (50 mg/kg/d for 7 weeks) decreased blood glucose (from 9.18 ± 1.47 to 7.67 ± 0.42 mmol/L), serum HbA1c, triglycerides, adipocyte size, and body weight (from 24.4 ± 1.2 to 22.5 ± 1.2 g) in mice with high-fat-diet-induced obesity. Western-blotting analysis suggested that pyrodextrins decreased intestinal SGLT-1 and GLUT-2 expression to ∼70 and ∼60% of the obese control, respectively, which slowed down glucose transportation from the gut into the blood and tentatively improved hepatic metabolism. Moreover, the pyrodextrin with a lower molecular weight of 44 kDa, a more branched structure, and increased nondigestible starch of 46.2 ± 0.3% showed stronger hypoglycemic activity. This work provides important information for developing pyrodextrins as a functional food and dietary supplement for the management of obesity and diabetes.

Gut Mechanisms Linking Intestinal Sweet Sensing to Glycemic Control

Sensing nutrients within the gastrointestinal tract engages the enteroendocrine cell system to signal within the mucosa, to intrinsic and extrinsic nerve pathways, and the circulation. This signaling provides powerful feedback from the intestine to slow the rate of gastric emptying, limit postprandial glycemic excursions, and induce satiation. This review focuses on the intestinal sensing of sweet stimuli (including low-calorie sweeteners), which engage similar G-protein-coupled receptors (GPCRs) to the sweet taste receptors (STRs) of the tongue. It explores the enteroendocrine cell signals deployed upon STR activation that act within and outside the gastrointestinal tract, with a focus on the role of this distinctive pathway in regulating glucose transport function via absorptive enterocytes, and the associated impact on postprandial glycemic responses in animals and humans. The emerging role of diet, including low-calorie sweeteners, in modulating the composition of the gut microbiome and how this may impact glycemic responses of the host, is also discussed, as is recent evidence of a causal role of diet-induced dysbiosis in influencing the gut-brain axis to alter gastric emptying and insulin release. Full knowledge of intestinal STR signaling in humans, and its capacity to engage host and/or microbiome mechanisms that modify glycemic control, holds the potential for improved prevention and management of type 2 diabetes.

Orally Administered Baker's Yeast β-Glucan Promotes Glucose and Lipid Homeostasis in the Livers of Obesity and Diabetes Model Mice

Baker's yeast glucan (BYG) has been reported to be an anti-diabetic agent. In the work described herein, further study on the effect of orally administered BYG on glucose and lipid homeostasis in the livers of ob/ob mice was performed. It was found that BYG decreased the blood glucose and the hepatic glucose and lipid disorders. Western blotting analysis revealed that BYG up-regulated p-AKT and p-AMPK, and down-regulated p-Acc in the liver. Furthermore, RNA-Seq analysis indicated that BYG down-regulated genes responsible for gluconeogenesis (G6pase and Got1), fatty acid biosynthesis (Acly, Acc, Fas, etc.), glycerolipid synthesis (Gpam and Lipin1/2), and cholesterol synthesis (Hmgcr, Fdps, etc.). Additionally, BYG decreased glucose transporters SGLT1 and GLUT2, fat emulsification, and adipogenic genes/proteins in the intestine to decrease glucose and lipid absorption. All these findings demonstrated that BYG is beneficial for regulating glucose and lipid homeostasis in diabetic mice, and thus has potential applications in anti-diabetic foods or drugs.

Deregulation of transcription factors controlling intestinal epithelial cell differentiation; a predisposing factor for reduced enteroendocrine cell number in morbidly obese individuals

Morbidly obese patients exhibit impaired secretion of gut hormones that may contribute to the development of obesity. After bariatric surgery there is a dramatic increase in gut hormone release. In this study, gastric and duodenal tissues were endoscopically collected from lean, and morbidly obese subjects before and 3 months after laparoscopic sleeve gastrectomy (LSG). Tissue morphology, abundance of chromogranin A, gut hormones, α-defensin, mucin 2, Na⁺/glucose co-transporter 1 (SGLT1) and transcription factors, Hes1, HATH1, NeuroD1, and Ngn3, were determined. In obese patients, the total number of enteroendocrine cells (EEC) and EECs containing gut hormones were significantly reduced in the stomach and duodenum, compared to lean, and returned to normality post-LSG. No changes in villus height/crypt depth were observed. A significant increase in mucin 2 and SGLT1 expression was detected in the obese duodenum. Expression levels of transcription factors required for differentiation of absorptive and secretory cell lineages were altered. We propose that in obesity, there is deregulation in differentiation of intestinal epithelial cell lineages that may influence the levels of released gut hormones. Post-LSG cellular differentiation profile is restored. An understanding of molecular mechanisms controlling epithelial cell differentiation in the obese intestine assists in the development of non-invasive therapeutic strategies.

Effects of isoleucine on glucose uptake through the enhancement of muscular membrane concentrations of GLUT1 and GLUT4 and intestinal membrane concentrations of Na+/glucose co-transporter 1 (SGLT-1) and GLUT2

Knowledge of regulation of glucose transport contributes to our understanding of whole-body glucose homoeostasis and human metabolic diseases. Isoleucine has been reported to participate in regulation of glucose levels in many studies; therefore, this study was designed to examine the effect of isoleucine on intestinal and muscular GLUT expressions. In an animal experiment, muscular GLUT and intestinal GLUT were determined in weaning pigs fed control or isoleucine-supplemented diets. Supplementation of isoleucine in the diet significantly increased piglet average daily gain, enhanced GLUT1 expression in red muscle and GLUT4 expression in red muscle, white muscle and intermediate muscle (P<0·05). In additional, expressions of Na+/glucose co-transporter 1 and GLUT2 were up-regulated in the small intestine when pigs were fed isoleucine-supplemented diets (P<0·05). C2C12 cells were used to examine the expressions of muscular GLUT and glucose uptake in vitro. In C2C12 cells supplemented with isoleucine in the medium, cellular 2-deoxyglucose uptake was increased (P<0·05) through enhancement of the expressions of GLUT4 and GLUT1 (P<0·05). The effect of isoleucine was greater than that of leucine on glucose uptake (P<0·05). Compared with newborn piglets, 35-d-old piglets have comparatively higher GLUT4, GLUT2 and GLUT5 expressions. The results of this study demonstrated that isoleucine supplementation enhanced the intestinal and muscular GLUT expressions, which have important implications that suggest that isoleucine could potentially increase muscle growth and intestinal development by enhancing local glucose uptake in animals and human beings.

Modulation of Small Intestinal Nitric Oxide Synthase by Gum Arabic

Preceding studies have revealed that gum arabic (GA), a natural proteoglycan (≥250, 000 Da), has proabsorptive properties—as shown by increased sodium and water absorption—in normal rats, and especially in two animal models of diarrhea. Because nitric oxide (NO) metabolism is linked to gastrointestinal Physiology, the goals of this study were to determine whether GA modulated NO and to determine intestinal function in vivo when NO production was enhanced by l-arginine (Arg), added at either 1 or 20 mM. Mechanistically, the goal was also to determine whether GA was a NO scavenger and a small intestinal NO synthase (NOS) inhibitor. Using a glucose-electrolyte solution in rat jejunal perfusions we found that GA at ± 10 μM (2.5 g/l) decreased nitrite and nitrate formation, tending to normalize water, sodium, and glucose absorption when modified by Arg addition. In vitro tests, with oxyhemoglobin as a marker, showed that GA at ≥5 μM scavenged NO. For GA effects on NOS, small intestinal homogenate supematants (10, 000 g) from frozen tissues of either adult or 2-day-old rats were incubated for 1 hour at 37°C in the presence of 2 mM Arg and increasing GA concentrations (0-100 μM). GA produced a concentration-dependent inhibition of NOS, reaching approximately 31% inhibition with 5 μM GA and up to 51% with 50 μM GA. GA at 100 μM produced no further inhibition. The data indicate that GA, in addition to its ability to remove NO diffused into the intestinal lumen, may also partially inhibit intestinal NOS end thus modulate intestinal absorption through these mechanisms. Use of GA as a food additive may help in restoring or improving small Intestinal function in conditions where functional damage has occurred.

Role of the apical and basolateral domains of the enterocyte in the regulation of cholesterol transport by a high glucose concentration

We have recently shown that a high glucose (HG) concentration raised intestinal cholesterol (CHOL) transport and metabolism in intestinal epithelial cells. The objective of the present work is to determine whether the stimulus for increased CHOL absorption by glucose originates from the apical site (corresponding to the intestinal lumen) or from the basolateral site (related to blood circulation). We tackled this issue by using differentiated Caco-2/15 cells. Only basolateral medium, supplemented with 25 mmol/L glucose, stimulated [(14)C]-CHOL uptake via the up-regulation of the critical CHOL transporter NPC1L1 protein, as confirmed by its specific ezetimibe inhibitor that abolished the rise in glucose-mediated CHOL capture. No significant changes were noted in SR-BI and CD36. Elevated CHOL uptake was associated with an increase in the transcription factors SREBP-2, LXR-β, and ChREBP, along with a fall in RXR-α. Interestingly, although the HG concentration in the apical medium caused modest changes in CHOL processing, its impact was synergetic with that of the basolateral medium. Our results suggest that HG concentration influences positively intestinal CHOL uptake when present in the basolateral medium. In addition, excessive consumption of diets containing high levels of carbohydrates may strengthen intestinal CHOL uptake in metabolic syndrome, thereby contributing to elevated levels of circulating CHOL and, consequently, the risk of developing type 2 diabetes and cardiovascular disease.

Expression of sweet receptor components in equine small intestine: relevance to intestinal glucose transport

The heteromeric sweet taste receptor T1R2-T1R3 is expressed on the luminal membrane of certain populations of enteroendocrine cells. Sensing of sugars and other sweet compounds by this receptor activates a pathway in enteroendocrine cells, resulting in secretion of a number of gut hormones, including glucagon-like peptide 2 (GLP-2). This subsequently leads to upregulation in the expression of intestinal Na(+)/glucose cotransporter, SGLT1, and increased intestinal glucose absorption. On the basis of the current information available on the horse genome sequence, it has been proposed that the gene for T1R2 (Tas1R2) is absent in the horse. We show here, however, that horses express both the mRNA and protein for T1R2. Equine T1R2 is most closely homologous to that in the pig and the cow. T1R2 protein, along with T1R3, α-gustducin, and GLP-2 proteins are coexpressed in equine intestinal endocrine cells. Intravenous administration of GLP-2, in rats and pigs, leads to an increase in the expression of SGLT1 in absorptive enterocytes and enhancement in blood glucose concentrations. GLP-2 receptor is expressed in enteric neurons, excluding the direct effect of GLP-2 on enterocytes. However, electric stimulation of enteric neurons generates a neural response leading to SGLT1 upregulation, suggesting that sugar in the intestine activates a reflex increase in the functional expression of SGLT1. Horses possess the ability to upregulate SGLT1 expression in response to increased dietary carbohydrates, and to enhance the capacity of the gut to absorb glucose. The gut sweet receptor provides an accessible target for manipulating the equine gut to absorb glucose (and water), allowing greater energy uptake and hydration for hard-working horses.

Glucocorticoids enhance intestinal glucose uptake via the dimerized glucocorticoid receptor in enterocytes

Glucocorticoid (GC) treatment of inflammatory disorders, such as inflammatory bowel disease, causes deranged metabolism, in part by enhanced intestinal resorption of glucose. However, the underlying molecular mechanism is poorly understood. Hence, we investigated transcriptional control of genes reported to be involved in glucose uptake in the small intestine after GC treatment and determined effects of GC on electrogenic glucose transport from transepithelial currents. GR(villinCre) mice lacking the GC receptor (GR) in enterocytes served to identify the target cell of GC treatment and the requirement of the GR itself; GR(dim) mice impaired in dimerization and DNA binding of the GR were used to determine the underlying molecular mechanism. Our findings revealed that oral administration of dexamethasone to wild-type mice for 3 d increased mRNA expression of serum- and GC-inducible kinase 1, sodium-coupled glucose transporter 1, and Na(+)/H(+) exchanger 3, as well as electrogenic glucose transport in the small intestine. In contrast, GR(villinCre) mice did not respond to GC treatment, neither with regard to gene activation nor to glucose transport. GR(dim) mice were also refractory to GC, because dexamethasone treatment failed to increase both, gene expression and electrogenic glucose transport. In addition, the rise in blood glucose levels normally observed after GC administration was attenuated in both mutant mouse strains. We conclude that enhanced glucose transport in vivo primarily depends on gene regulation by the dimerized GR in enterocytes, and that this mechanism contributes to GC-induced hyperglycemia.

Glucose sensing and signalling; regulation of intestinal glucose transport

Epithelial cells lining the inner surface of the intestinal epithelium are in direct contact with a lumenal environment that varies dramatically with diet. It has long been suggested that the intestinal epithelium can sense the nutrient composition of lumenal contents. It is only recently that the nature of intestinal nutrient-sensing molecules and underlying mechanisms have been elucidated. There are a number of nutrient sensors expressed on the luminal membrane of endocrine cells that are activated by various dietary nutrients. We showed that the intestinal glucose sensor, T1R2+T1R3 and the G-protein, gustducin are expressed in endocrine cells. Eliminating sweet transduction in micein vivoby deletion of either gustducin or T1R3 prevented dietary monosaccharide- and artificial sweetener-induced up-regulation of the Na+/glucose cotransporter, SGLT1 observed in wild-type mice. Transgenic mice, lacking gustducin or T1R3 had deficiencies in secretion of glucagon-like peptide 1 (GLP-1) and, glucose-dependent insulinotrophic peptide (GIP). Furthermore, they had an abnormal insulin profile and prolonged elevation of postprandial blood glucose in response to orally ingested carbohydrates. GIP and GLP-1 increase insulin secretion, while glucagon-like peptide 2 (GLP-2) modulates intestinal growth, blood flow and expression of SGLT1. The receptor for GLP-2 resides in enteric neurons and not in any surface epithelial cells, suggesting the involvement of the enteric nervous system in SGLT1 up-regulation. The accessibility of the glucose sensor and the important role that it plays in regulation of intestinal glucose absorption and glucose homeostasis makes it an attractive nutritional and therapeutic target for manipulation.

Expression of Na+/glucose co-transporter 1 (SGLT1) in the intestine of piglets weaned to different concentrations of dietary carbohydrate

Na+/glucose co-transporter 1 (SGLT1) transports dietary sugars from the lumen of the intestine into enterocytes. Regulation of this protein is essential for the provision of glucose to the body and, thus, is important for maintenance of glucose homeostasis. We have assessed expression of SGLT1 at mRNA, protein and functional levels in the intestinal tissue of 28 d old piglets weaned onto isoenergetic diets with differing concentrations of digestible carbohydrate (CHO). We show that expression of SGLT1 remains constant when piglets are fed up to 40 % CHO-containing diets. However, there is a significant increase in SGLT1 expression when the CHO content of the diet is>50 %. Morphometric analyses indicate that the increased expression is not due to a trophic effect. It has been proposed that in rat intestine, in response to a high-CHO diet, GLUT2 (the classical basolateral membrane monosaccharide transporter) is translocated to the luminal membrane of enterocytes to absorb excess dietary glucose. We show, using immunohistochemistry and Western blotting with antibodies raised to amino acids in different epitopes of GLUT2, that under all dietary conditions, low to high CHO, GLUT2 is expressed on the basolateral membrane of pig enterocytes. Furthermore, functional studies indicate that there is no uptake of 2-deoxy-D-glucopyranoside, a specific substrate of Na+-independent glucose transporters into brush-border membrane vesicles isolated from the intestines of piglets either maintained on low- or high-CHO diets. Thus, SGLT1 is the major route for absorption of dietary sugars across the luminal membrane of swine enterocytes.

Active absorption of ginsenoside Rg1 in vitro and in vivo: the role of sodium-dependent glucose co-transporter 1

Objectives

Our previous study suggested that adrenaline (epinephrine) could be an effective absorption enhancer for ginsenoside Rg1 (Rg1). This study focused on the transport mechanism of Rg1 and the role of sodium-dependent glucose co-transporter 1 in the regulation of Rg1 uptake after exposure to adrenaline.

Methods

Caco-2 cells were used as an in-vitro model to assess the absorption mechanism of Rg1. Also the effect of D-glucose on adrenaline-induced absorption of Rg1 was investigated in vivo in rats.

Key findings

Results showed that the uptake of Rg1 was temperature-dependent. The transport from the basolateral side to the apical side was significantly lower compared with that from the apical to the basolateral side (P &lt; 0.01). The transport of Rg1 was concentration dependent (Km was 41.60 mM, Vmax was 353.75 mol/cm2/min). Cells incubated with D-glucose-free medium exhibited significantly greater Rg1 uptake (+ 62.6%) compared with cells in D-glucose-containing medium. The data indicated that sodium-dependent glucose co-transporter 1 was involved in the transport of Rg1. Adrenaline-induced uptake of Rg1 was significantly inhibited in the presence of phlorizin and the absence of Na+. In the in-vivo study in rats, it was found that after co-administration with D-glucose, the adrenaline-induced absorption of Rg1 was inhibited. The area under the concentration-time curve (AUC0→∞) value was significantly decreased from 64.57 ± 27.08 to 1.37 ± 0.42 μg/ml h (P &lt; 0.001).

Conclusions

The data suggested that adrenaline enhanced the absorption of Rg1 by regulating sodium-dependent glucose co-transporter 1.

Effects of basswood honey, honey-comparable glucose-fructose solution, and oral glucose tolerance test solution on serum insulin, glucose, and C-peptide concentrations in healthy subjects

Studies suggest that honey has less influence on serum glucose concentrations than monosaccharides and disaccharides. This study aimed to confirm these findings conclusively by comparing directly the effects of honey, an identical sugar solution, and oral glucose tolerance (OGT) test solution on serum glucose, insulin, and C-peptide values in healthy subjects. Twelve healthy men with a mean age of 27.7 years, a mean body mass index of 23.2 kg/m(2), and no history of metabolic disorders participated in the study. Subjects underwent OGT testing to establish values and exclude preclinical diabetes. One week later they were randomly assigned to basswood honey or a glucose-fructose solution (honey-comparable glucose-fructose solution). The following week subjects were given the other solution. All solutions contained 75 g of glucose. Serum glucose was measured before drinking test solutions and every 10 minutes for 120 minutes afterwards. C-peptide and insulin were measured at 60 and 120 minutes. Serum insulin and C-peptide values at 60 minutes were significantly lower for honey. The mean serum glucose concentration was also lower for honey, but direct comparisons at the various times showed no statistically significant differences between solutions. However, the area under the concentration-time profile for glucose response was lower for the honey than the honey-comparable glucose-fructose solution. Honey had less effect on serum glucose, C-peptide, and insulin values than the honey-comparable glucose-fructose solution. Further study to elucidate underlying mechanisms may be worthwhile, as may investigation of the implications of these findings for diabetic patients.

Adaptive response of equine intestinal Na+/glucose co-transporter (SGLT1) to an increase in dietary soluble carbohydrate

Experimental and epidemiological evidence suggests that consumption of hydrolyzable carbohydrate, hCHO (grain), by horses is an important risk factor for colic, a common cause of equine mortality. It is unknown whether the small intestinal capacity to digest hCHO and/or to absorb monosaccharides is limiting, or even if horses can adapt to increased carbohydrate load. We investigated changes in the brush-border membrane carbohydrate digestive enzymes and glucose absorptive capacity of horse small intestine in response to increased hCHO. Expression of the Na(+)/glucose co-transporter, SGLT1, was assessed by Western blotting, immunohistochemistry, Northern blotting, QPCR, and Na(+)-dependent D-glucose transport. Glucose transport rates, SGLT1 protein, and mRNA expression were all 2-fold higher in the jejunum and 3- to 5-fold higher in the ileum of horses maintained on a hCHO-enriched diet compared to pasture forage. Activity of the disaccharidases was unaltered by diet. In a well-controlled study, we determined SGLT1 expression in the duodenal and ileal biopsies of horses switched, gradually over a 2-month period, from low (<1.0 g/kg bwt/day) to high hCHO (6.0 g/kg bwt/day) diets of known composition. We show that SGLT1 expression is enhanced, with time, 2-fold in the duodenum and 3.3-fold in the ileum. The study has important implications for dietary management of the horse.

Expression of sweet taste receptors of the T1R family in the intestinal tract and enteroendocrine cells

The composition of the intestinal luminal content varies considerably with diet. It is important therefore that the intestinal epithelium senses and responds to these significant changes and regulates its functions accordingly. Although it is becoming evident that the gut epithelium senses and responds to luminal nutrients, little is known about the nature of the nutrient sensing molecule and the downstream cellular events. A prototype example is the modulation in the capacity of the gut to absorb monosaccharides via the intestinal luminal membrane Na(+)/glucose cotransporter, SGLT1. The experimental evidence suggests that luminal sugar is sensed by a glucose sensor residing on the luminal membrane of the gut epithelium and linked to a G-protein-coupled receptor, cAMP/PKA (protein kinase A) pathway, resulting ultimately in modulation of intestinal monosaccharide absorption. Here we report the expression, at mRNA and protein levels, of members of the T1R sweet taste receptors, and the alpha-subunit of the G-protein gustducin, in the small intestine and the enteroendocrine cell line, STC-1. In the small intestine, there is a highly coordinated expression of sweet taste receptors and gustducin, a G-protein implicated in intracellular taste signal transduction, throughout the gut. The potential involvement of these receptors in sugar sensing in the intestine will facilitate our understanding of intestinal nutrient sensing, with implications for better nutrition and health maintenance.

Evaluation of glucose tolerance and intestinal luminal membrane glucose transporter function in horses with equine motor neuron disease

OBJECTIVE

To confirm whether the plasma glucose concentration curve obtained during oral glucose tolerance tests (OGTTs) in horses with equine motor neuron disease (EMND) is decreased, compared with that obtained in clinically normal horses, and determine whether that decrease is a result of defective glucose metabolism or intestinal glucose transport dysfunction.

ANIMALS

8 horses with EMND and 44 matched control horses.

PROCEDURE

Electromyography and OGTTs were performed in all 8 affected horses and 10 control horses. Intravenous GTTs (IVGTTs) were performed in 6 affected horses and another 11 control horses. The activity and levels of jejunal luminal membrane glucose transporter (Na+ / glucose cotransporter isoform 1 [SGLT1]) were measured in 2 affected horses and 23 control horses.

RESULTS

In horses with EMND, generalized neuropathy was detected via quantitative electromyography; the mean increase in plasma glucose concentration during the OGTT was significantly decreased, compared with the value in control horses. During the IVGTT the mean increase in plasma glucose concentration was significantly lower than that of control horses. The activity and levels of SGLT1 in 2 affected horses were similar to those of control horses. Diagnosis of EMND was confirmed postmortem in all affected horses.

CONCLUSIONS AND CLINICAL RELEVANCE

Data suggest that the decreased plasma glucose curve obtained in horses with EMND during OGTTs (compared with control horses) is a result of overall enhanced glucose metabolism or abnormalities in the facilitated glucose transporters; definitive identification of the underlying mechanisms could aid in the development of appropriate treatments of EMND in horses.

Molecular characterisation of fructose transport in equine small intestine

REASONS FOR PERFORMING STUDY

Fructose can be a suitable carbohydrate supplement for horses before and/or during endurance exercise. In comparison to glucose, the ingestion of fructose results in a lower insulin peak and less marked fluctuations in blood glucose during exercise, potentially avoiding hypoglycaemia-induced exhaustion.

OBJECTIVES

To assess the capacity of the equine small intestine to absorb fructose and to determine the mechanism, molecular structure and properties of equine intestinal fructose transport.

METHODS

Using PCR-based strategies, RNA isolated from equine small intestine and primers designed to homologous regions of the fructose transporter, GLUT5, cDNA of other species, we cloned and sequenced equine GLUT5 (eGLUT5). Northern and western blot analyses, in conjunction with immunohistochemistry, utilising eGLUT5 cDNA and antibodies, assessed expression of eGLUT5 along the longitudinal and radial axes of the small intestine. Functional properties of fructose transport in intestinal brush-border membrane vesicles were measured using the rapid-filtration technique.

RESULTS

eGLUT5 is expressed in the villus enterocytes with highest levels in duodenum>jejunum and lowest in the ileum. Kinetic studies indicate eGLUT5 is a low affinity, high capacity transporter.

CONCLUSIONS

Equine small intestine has the capacity to absorb fructose.

POTENTIAL RELEVANCE

The molecular probes produced in these studies can be used as diagnostic aids to determine equine intestinal monosaccharide malabsorption.

Proabsorptive effect of gum arabic in isotonic solutions orally administered to rats: effect on zinc and other solutes

We have previously shown that the addition of gum arabic (GA) to oral rehydration solution (ORS) enhances water and electrolyte absorption during jejunal perfusion in rats under anesthesia. This study investigates whether GA by oral administration could be equally effective in rats. Isotonic solutions containing 25 g/L GA (AG), or without GA (A0) were administered via oral tube to lightly anesthetized adult female rats. Similar experiments were conducted with hypertonic solutions containing no GA (B0), or either 10 (B10) or 50 g/L GA (B50). Blood concentrations of sodium, glucose, glutamate, zinc, and tritiated water were determined at 0, 15, 30, 60, 90, 120, and 180 minutes, and results between treatments were compared. Administration of the isotonic, GA-containing solution (AG) resulted in a higher blood zinc level than with the isotonic GA-free solution (A0) from 15 minutes throughout 180 minutes. Blood zinc at 15 minutes (means +/- SEM) was as follows: for A0: 69.3 +/- 2.0, for AG: 83.4 +/- 3.5 nmol/L, P=0.002. At 180 minutes, A0: 52.6 +/- 1.8; AG: 68.1 +/- 4.6 nmol/L, P=0.004. The corresponding areas under the curve (AUC) were as follows: for A0: 10,737 +/- 214; for AG: 13,919 +/- 765 nmol x min/L, P<0.001). Glucose, glutamate, sodium, and tritiated water body distribution presented no differences in blood concentrations. For sodium and tritiated water body distribution, there was a significant time effect (P<0.0001). In hypertonic solutions, blood zinc levels declined over time, possibly due to their osmotic, counter-absorptive action, thus obscuring possible opposite effects of GA. GA appears to be an effective enhancer of zinc absorption when orally administered in isotonic solutions to laboratory animals. This proabsorptive capacity could be attributed to some of the physicochemical and biochemical properties of GA and suggest possible applications of GA in liquid formulas and solid food preparations.

Zinc as a potential enteroprotector in oral rehydration solutions: its role in nitric oxide metabolism

Zinc has been recognized as an antioxidant with potential for chronic and acute effects. Oxidative damage produced by free radicals, including nitric oxide (NO), is responsible for certain types of intestinal malabsorption syndromes and diarrhea. Under physiologic or mildly stimulatory conditions for NO synthesis, the small intestine characteristically is in a proabsorptive state; however, an excessive production of NO triggers formation of cyclic nucleotides, which cause secretion and malabsorption. In this study, we hypothesized that low-molecular-weight, soluble zinc chelates could modulate the effects of induced NO excess on the small intestine. In vitro experiments demonstrated that zinc-citrate or zinc-histidine at > or =0.66 mM, as well as a known NO scavenger, 2-[carboxyphenyl]-4,4,4,4-tetramethylimidazoline-1-oxyl-3-oxide, at 2 microM, were effective at removing chemically generated NO. In vivo jejunal perfusions, conducted in healthy rats under anesthesia, showed that c-PTIO reduced the proabsorptive effects produced by 1 mM L-arginine, the precursor of NO. In a standard oral rehydration solution, 1 mM zinc-citrate partially reversed the antiabsorptive effects on potassium caused by an excess of NO generated from 20 mM L-arginine but did not alter sodium or water absorption. The data are consistent with the view that soluble zinc compounds incorporated into an oral rehydration solution may deserve further attention as a means to scavenge NO with fluids used for the treatment of chronic or acute diarrhea, especially in malnourished children who are often zinc deficient.

Molecular characterisation of carbohydrate digestion and absorption in equine small intestine

Dietary carbohydrates, when digested and absorbed in the small intestine of the horse, provide a substantial fraction of metabolisable energy. However, if levels in diets exceed the capacity of the equine small intestine to digest and absorb them, they reach the hindgut, cause alterations in microbial populations and the metabolite products and predispose the horse to gastrointestinal diseases. We set out to determine, at the molecular level, the mechanisms, properties and the site of expression of carbohydrate digestive and absorptive functions of the equine small intestinal brush-border membrane. We have demonstrated that the disaccharidases sucrase, lactase and maltase are expressed diversely along the length of the intestine and D-glucose is transported across the equine intestinal brush-border membrane by a high affinity, low capacity, Na+/glucose cotransporter type 1 isoform (SGLT1). The highest rate of transport is in duodenum > jejunum > ileum. We have cloned and sequenced the cDNA encoding equine SGLT1 and alignment with SGLT1 of other species indicates 85-89% homology at the nucleotide and 84-87% identity at the amino acid levels. We have shown that there is a good correlation between levels of functional SGLT1 protein and SGLT1 mRNA abundance along the length of the small intestine. This indicates that the major site of glucose absorption in horses maintained on conventional grass-based diets is in the proximal intestine, and the expression of equine intestinal SGLT1 along the proximal to distal axis of the intestine is regulated at the level of mRNA abundance. The data presented in this paper are the first to provide information on the capacity of the equine intestine to digest and absorb soluble carbohydrates and has implications for a better feed management, pharmaceutical intervention and for dietary supplementation in horses following intestinal resection.

Molecular changes in the expression of human colonic nutrient transporters during the transition from normality to malignancy

Healthy colonocytes derive 60-70% of their energy supply from short-chain fatty acids, particularly butyrate. Butyrate has profound effects on differentiation, proliferation and apoptosis of colonic epithelial cells by regulating expression of various genes associated with these processes. We have previously shown that butyrate is transported across the luminal membrane of the colonic epithelium via a monocarboxylate transporter, MCT1. In this paper, using immunohistochemistry and in situ hybridisation histochemistry, we have determined the profile of MCT1 protein and mRNA expression along the crypt to surface axis of healthy human colonic tissue. There is a gradient of MCT1 protein expression in the apical membrane of the cells along the crypt-surface axis rising to a peak in the surface epithelial cells. MCT1 mRNA is expressed along the crypt-surface axis and is most abundant in cells lining the crypt. Analysis of healthy colonic tissues and carcinomas using immunohistochemistry and Western blotting revealed a significant decline in the expression of MCT1 protein during transition from normality to malignancy. This was reflected in a corresponding reduction in MCT1 mRNA expression, as measured by Northern analysis. Carcinoma samples displaying reduced levels of MCT1 were found to express the high affinity glucose transporter, GLUT1, suggesting that there is a switch from butyrate to glucose as an energy source in colonic epithelia during transition to malignancy. The expression levels of MCT1 in association with GLUT1 could potentially be used as determinants of the malignant state of colonic tissue.

Substrate-induced regulation of the human colonic monocarboxylate transporter, MCT1

Butyrate is the principal source of energy for colonic epithelial cells, and has profound effects on their proliferation, differentiation and apoptosis. Transport of butyrate across the colonocyte luminal membrane is mediated by the monocarboxylate transporter 1 (MCT1). We have examined the regulation of expression of human colonic MCT1 by butyrate, in cultured colonic epithelial cells (AA/C1). Treatment with sodium butyrate (NaBut) resulted in a concentration- and time-dependent upregulation of both MCT1 mRNA and protein. At 2 mM butyrate, the magnitude of induction of mRNA (5.7-fold) entirely accounted for the 5.2-fold increase in protein abundance, and was mediated by both activation of transcription and enhanced mRNA stability. The other monocarboxylates found naturally in the colon, acetate and propionate, had no effect. The properties of butyrate uptake by AA/C1 cells were characteristic of MCT1. Induction of the MCT1 protein resulted in a corresponding increase in the maximal rate of butyrate transport. The V(max) for uptake of [U-(14)C]butyrate was increased 5-fold following pre-incubation with 2 mM NaBut, with no significant change in the apparent K(m). In conclusion, this study is the first to show substrate-induced regulation of human colonic MCT1. The basis of this regulation is a butyrate-induced increase in MCT1 mRNA abundance, resulting from the dual control of MCT1 gene transcription and stability of the MCT1 transcript. We suggest that butyrate-induced increases in the expression and resulting activity of MCT1 serve as a mechanism to maximise intracellular availability of butyrate, to act both as a source of energy and to influence processes maintaining cellular homeostasis in the colonic epithelium.

Expression of monosaccharide transporters in intestine of diabetic humans

Noninsulin-dependent diabetes mellitus (NIDDM) is an increasingly common disease, which brings a number of life-threatening complications. In rats with experimentally induced diabetes, there is an increase in the capacity of the intestine to absorb monosaccharides. We have examined the activity and the expression of monosaccharide transporters in the intestine of patients suffering from NIDDM. Na(+)-dependent D-glucose transport was 3.3-fold higher in brush-border membrane (BBM) vesicles isolated from duodenal biopsies of NIDDM patients compared with healthy controls. Western analysis indicated that SGLT1 and GLUT5 protein levels were also 4.3- and 4.1-fold higher in diabetic patients. This was associated with threefold increases in SGLT1 and GLUT5 mRNA measured by Northern blotting. GLUT2 mRNA levels were also increased threefold in the intestine of diabetic patients. Analysis of other BBM proteins indicated that the activity and abundance of sucrase and lactase were increased by 1.5- to 2-fold and the level of the structural proteins villin and beta-actin was enhanced 2-fold in diabetic patients compared with controls. The increase in the capacity of the intestine to absorb monosaccharides in human NIDDM is due to a combination of intestinal structural change with a specific increase in the expression of the monosaccharide transporters SGLT1, GLUT5, and GLUT2.

Transcriptional regulation of the ovine intestinal Na+/glucose cotransporter SGLT1 gene. Role of HNF-1 in glucose activation of promoter function

Dietary sugars D-glucose and D-galactose are transported across the intestinal brush-border membrane by the Na+/glucose cotransporter, SGLT1. In various species studied, it has been shown that the activity, and expression, of intestinal SGLT1 is regulated by dietary sugars. We report in this paper that regulation of the intestinal SGLT1 gene by lumenal sugar is due, in part, to an increase in transcription. Using deletion analyses of the -66/+21-bp fragment, we have identified the minimal region of the ovine SGLT1 promoter able to support transcription. Site-directed mutagenesis of the hepatic nuclear factor-1 (HNF-1) consensus motif within this domain eliminates basal promoter function. In addition, we show direct evidence for glucose-induced activation of the -66/+21-bp promoter region. There is a co-ordinated decline in the abundance of ovine intestinal HNF-1 and SGLT1 transcripts during transition from preruminant to adult ruminant. This decline is recovered after glucose infusion of adult sheep intestine. Similarly, as shown using DNA mobility-shift assays, the intensity of the HNF-1-binding complex to the target promoter sequence decreases during maturation of the animal; this is restored after intestinal sugar infusion. These data indicate that HNF-1 plays an important role in the glucose responsiveness of the ovine SGLT1 gene. This is the first report of in vitro glucose-induced activation of the intestinal SGLT1 promoter and identification of a glucose-responsive region of the ovine SGLT1 promoter.

Development of the intestinal SGLT1 transporter in rats

Glucose absorption from the small intestine is largely mediated via the sodium-coupled glucose transporter (SGLT1). The goal of this study was to investigate the ontogenesis of the SGLT1, using the rat as an animal model at three stages of development: during lactation, at weaning, and at physiologic maturity. The techniques involved upper small intestinal perfusions with solutions containing 200 mM glucose and 50 mM NaCl, with or without 1 mM phloridzin (Phl), as an inhibitor of SGLT1. Molecular expression of the SGLT1 was also investigated via Western blot analysis from intestinal specimens of the three growth periods. Glucose absorption in weanling rats, in the absence of Phl, was several times higher than in sucklings and approximately double that of mature animals, and the effects of Phl were the greatest in weanlings. Furthermore, the physiologic data correlate to the molecular analysis of the SGLT1 which showed an increase in expression of the SGLT1 in both the weanlings and the adults compared to the sucklings. At all three stages of development Phl abolished Na absorption, and in sucklings there was a net outflow of Na. Due to the coupling between Na and water transport, net water absorption and the influx/efflux ratio, a more sensitive indicator of changes in unidirectional fluid movement, were similarly affected by Phl at the three stages of development. Net water absorption was highest in weanling animals. These findings are consistent with an early development of SGLT1 in rat small intestine and an apparent burst of activity at weaning. Less than complete maturity of other absorptive mechansims is occurring at this time.

Isolation and characterization of a genomic region upstream from the ovine Na+/d-glucose cotransporter (SGLT1) cDNA

d-Glucose and non-metabolisable analogues of D-glucose regulate the expression of intestinal SGLT1 at both transcriptional and post-transcriptional levels. In order to investigate the molecular mechanisms involved in the transcriptional regulation of the ovine intestinal SGLT1 gene, we have isolated an upstream element of about 1 kb in size. This DNA fragment contains a TATA box motif, 48 bp upstream of the transcriptional start site and includes transcription factor binding sites for HNF-1 and AP-2. We have shown that the ovine SGLT1 promoter fragment can drive the transcription of a reporter gene when transfected into the epithelial cell lines STC-1 and LLC-PK1, which endogenously express SGLT1. Deletion analyses of the promoter indicate that -66/+21 bp proximal sequence directs the highest level of reporter gene activity. There are one and possibly two sites of transcriptional suppression.

Modified starch enhances absorption and accelerates recovery in experimental diarrhea in rats

Rice gruels have been used as home remedies to treat dehydration associated with diarrheal illness in developing countries. These preparations have produced conflicting results, most likely due to the heterogeneity of starch used. We investigated whether the modified tapioca starch, Textra (TX), at 5.0 or 10.0 g/L added to a 90 mmol/L Na+-111 mmol glucose oral rehydration solution (ORS) enhanced water and electrolyte absorption in two models of diarrhea. To induce a secretory state (model A), the jejunum of juvenile rats was perfused with 10 mmol/L theophylline (THEO) under anesthesia and then perfused with the solutions indicated above. To produce chronic osmotic-secretory diarrhea (model B), rats had a magnesium citrate-phenolphthalein solution as the sole fluid source for 1 wk, and then were perfused as the THEO-treated rats. Water, electrolyte, and glucose absorption were measured during both perfusions. As an extension of the perfusion studies, we compared how fast rats recovered from chronic osmotic diarrhea by offering them either water, ORS, or ORS containing 5.0 g/L TX along with solid food. Recovery rate markers were measured after 24 h and included weight gain, food and fluid intake, and stool output. In model A, addition of 5.0 g/L TX to ORS reversed Na+ secretion and improved net water as well as K+ and glucose absorption, compared with THEO-treated rats perfused with ORS without TX. In model B, addition of TX to ORS increased water, Na+, K+, and glucose absorption, compared with rats perfused without TX. Increasing TX from 5.0 to 10.0 g/L had no additional benefit. In recovery experiments, animals with free access to ORS with TX had significantly greater weight gain and decreased stool output compared with animals recovering with water or ORS without TX. Our experiments suggest that TX may be a useful additive to standard ORS to promote fluid and electrolyte absorption and may provide additional energy without increasing ORS osmotic load.

Nutrient regulation of the intestinal Na+/glucose co-transporter (SGLT1) gene expression

It is known that dietary carbohydrates regulate the activity of the intestinal SGLT1. We have demonstrated that modifications in SGLT1 activity are due to alterations in SGLT1 expression in response to the sugar content of the diet. To assess the correlation between changes in the activity of SGLT1 and the abundance of SGLT1 protein, we have employed a method for the quantitative measurement of immunoreactive proteins. A calibration curve has been constructed using either a nonadecapeptide (amino acids 402-420), or a recombinant protein corresponding to amino acids 554-640 of the SGLT1 sequence. Immunoblotting the protein samples concurrently with specific quantities of either the peptide or recombinant standard, using antibodies raised against these antigens, enabled accurate quantification of the absolute amounts of immunoreactive protein in the samples. The amount of SGLT1 protein correlates well with measurements of SGLT1 activity. The modulation of the activity of SGLT1 in response to lumenal sugars is due to corresponding changes in the absolute levels of SGLT1 protein.