Apical GLUT2 Research Articles

Reproducibility Study of Computational Modelling of Glucose Uptake by SGLT1 and GLUT2 in the Enterocyte

Many researchers in the last couple of decades have suggested that glucose absorption in the small intestine depends on both constitutively expressed SGLT1 and translocated GLUT2 in the brush border membrane, especially in the presence of a high concentration of luminal glucose. Afshar et al. generated a computational model of non-isotonic glucose uptake by small intestinal epithelial cells. The model incorporates apical uptake via SGLT1 and GLUT2, basolateral efflux into the blood via GLUT2 and cellular volume changes in response to non-isotonic conditions. The dependence of glucose absorption on luminal glucose, blood flow rate, and inlet blood glucose concentration is studied, and the results explain more about the role of apical GLUT2 in intestinal cell glucose absorption. Here, we used the CellML file provided by the model authors, together with SED-ML files and Python scripts, to demonstrate the reproduction of the figures in the original paper by using the associated model.

Reproducibility Study of Computational Modelling of Glucose Uptake by SGLT1 and GLUT2 in the Enterocyte

Many researchers in the last couple of decades have suggested that glucose absorption in the small intestine depends on both constitutively expressed SGLT1 and translocated GLUT2 in the brush border membrane, especially in the presence of a high concentration of luminal glucose. Afshar et al. generated a computational model of non-isotonic glucose uptake by small intestinal epithelial cells. The model incorporates apical uptake via SGLT1 and GLUT2, basolateral efflux into the blood via GLUT2 and cellular volume changes in response to non-isotonic conditions. The dependence of glucose absorption on luminal glucose, blood flow rate, and inlet blood glucose concentration is studied, and the results explain more about the role of apical GLUT2 in intestinal cell glucose absorption. Here, we used the CellML file provided by the model authors, together with SED-ML files and Python scripts, to demonstrate the reproduction of the figures in the original paper by using the associated model.

Enterocyte HKDC1 Modulates Intestinal Glucose Absorption in Male Mice Fed a High-fat Diet.

Hexokinase domain containing protein-1, or HKDC1, is a widely expressed hexokinase that is genetically associated with elevated 2-hour gestational blood glucose levels during an oral glucose tolerance test, suggesting a role for HKDC1 in postprandial glucose regulation during pregnancy. Our earlier studies utilizing mice containing global HKDC1 knockdown, as well as hepatic HKDC1 overexpression and knockout, indicated that HKDC1 is important for whole-body glucose homeostasis in aging and pregnancy, through modulation of glucose tolerance, peripheral tissue glucose utilization, and hepatic energy storage. However, our knowledge of the precise role(s) of HKDC1 in regulating postprandial glucose homeostasis under normal and diabetic conditions is lacking. Since the intestine is the main entry portal for dietary glucose, here we have developed an intestine-specific HKDC1 knockout mouse model, HKDC1Int–/–, to determine the in vivo role of intestinal HKDC1 in regulating glucose homeostasis. While no overt glycemic phenotype was observed, aged HKDC1Int–/– mice fed a high-fat diet exhibited an increased glucose excursion following an oral glucose load compared with mice expressing intestinal HKDC1. This finding resulted from glucose entry via the intestinal epithelium and is not due to differences in insulin levels, enterocyte glucose utilization, or reduction in peripheral skeletal muscle glucose uptake. Assessment of intestinal glucose transporters in high-fat diet–fed HKDC1Int–/– mice suggested increased apical GLUT2 expression in the fasting state. Taken together, our results indicate that intestinal HKDC1 contributes to the modulation of postprandial dietary glucose transport across the intestinal epithelium under conditions of enhanced metabolic stress, such as high-fat diet.

Computational Modelling of Glucose Uptake in the Enterocyte

We describe an implemented model of glucose absorption in the enterocyte, as previously published by Afshar et al. (2019), The model used mechanistic descriptions of all the responsible transporters and was built in the CellML framework. It was validated against published experimental data and implemented in a modular structure which allows each individual transporter to be edited independently from the other transport protein models. The composite model was then used to study the role of the sodium-glucose cotransporter (SGLT1) and the glucose transporter type 2 (GLUT2), along with the requirement for the existence of the apical Glut2 transporter, especially in the presence of high luminal glucose loads, in order to enhance the absorption. Here we demonstrate the reproduction of the figures in the original paper by using the associated model. EDITOR'S NOTE (v3): Instructions within the manuscript changed, in order to properly execute the model files. Spelling of author's name corrected in filenames. (v4): Abstract fixes.

Computational Modelling of Glucose Uptake in the Enterocyte

We describe an implemented model of glucose absorption in the enterocyte, as previously published by Afshar et al. Afshar et al. (2019), The model used mechanistic descriptions of all the responsible transporters and was built in the CellML framework. It was validated against published experimental data and implemented in a modular structure which allows each individual transporter to be edited independently from the other transport protein models. The composite model was then used to study the role of the sodium-glucose cotransporter (SGLT1) and the glucose transporter type 2 (GLUT2), along with the requirement for the existence of the apical Glut2 transporter, especially in the presence of high luminal glucose loads, in order to enhance the absorption. Here we demonstrate the reproduction of the figures in the original paper by using the associated model. EDITOR'S NOTE (v2): Instructions within the manuscript changed, in order to properly execute the model files. Spelling of author's name corrected in filenames.

Computational Modelling of Glucose Uptake in the Enterocyte

We describe an implemented model of glucose absorption in the enterocyte, as previously published by Afshar et al. (2019), The model used mechanistic descriptions of all the responsible transporters and was built in the CellML framework. It was validated against published experimental data and implemented in a modular structure which allows each individual transporter to be edited independently from the other transport protein models. The composite model was then used to study the role of the sodium-glucose cotransporter (SGLT1) and the glucose transporter type 2 (GLUT2), along with the requirement for the existence of the apical Glut2 transporter, especially in the presence of high luminal glucose loads, in order to enhance the absorption. Here we demonstrate the reproduction of the figures in the original paper by using the associated model. EDITOR'S NOTE (v3): Instructions within the manuscript changed, in order to properly execute the model files. Spelling of author's name corrected in filenames. (v4): Abstract fixes.

Computational Modelling of Glucose Uptake by SGLT1 and Apical GLUT2 in the Enterocyte.

It has been suggested that glucose absorption in the small intestine depends on both constitutively expressed SGLT1 and translocated GLUT2 in the brush border membrane, especially in the presence of high levels of luminal glucose. Here, we present a computational model of non-isotonic glucose uptake by small intestinal epithelial cells. The model incorporates apical uptake via SGLT1 and GLUT2, basolateral efflux into the blood via GLUT2, and cellular volume changes in response to non-isotonic conditions. The dependence of glucose absorption on luminal glucose, blood flow rate, and inlet blood glucose concentration is studied. Uptake via apical GLUT2 is found to be sensitive to all these factors. Under a range of conditions, the maximum apical GLUT2 flux is about half of the SGLT1 flux and is achieved at high luminal glucose (> 50 mM), high blood flow rates, and low inlet blood concentrations. In contrast, SGLT1 flux is less sensitive to these factors. When luminal glucose concentration is less than 10 mM, apical GLUT2 serves as an efflux pathway for glucose to move from the blood to the lumen. The model results indicate that translocation of GLUT2 from the basolateral to the apical membrane increases glucose uptake into the cell; however, the reduction of efflux capacity results in a decrease in net absorption. Recruitment of GLUT2 from a cytosolic pool elicits a 10–20% increase in absorption for luminal glucose levels in the a 20–100 mM range. Increased SGLT1 activity also leads to a roughly 20% increase in absorption. A concomitant increase in blood supply results in a larger increase in absorption. Increases in apical glucose transporter activity help to minimise cell volume changes by reducing the osmotic gradient between the cell and the lumen.

Intestinal Absorption of Fructose.

Increased understanding of fructose metabolism, which begins with uptake via the intestine, is important because fructose now constitutes a physiologically significant portion of human diets and is associated with increased incidence of certain cancers and metabolic diseases. New insights in our knowledge of intestinal fructose absorption mediated by the facilitative glucose transporter GLUT5 in the apical membrane and by GLUT2 in the basolateral membrane are reviewed. We begin with studies related to structure as well as ligand binding, then revisit the controversial proposition that apical GLUT2 is the main mediator of intestinal fructose absorption. The review then describes how dietary fructose may be sensed by intestinal cells to affect the expression and activity of transporters and fructolytic enzymes, to interact with the transport of certain minerals and electrolytes, and to regulate portal and peripheral fructosemia and glycemia. Finally, it discusses the potential contributions of dietary fructose to gastrointestinal diseases and to the gut microbiome.

A model explaining protracted weight loss after Roux en Y gastric by-pass bariatric surgery

Introduction: Despite evidence that Roux en Y gastric by-pass (RYGB) does not generally lead to a sustained decrease in dietary intake or appetite loss, this form of treatment is the most effective method of producing sustained weight loss in morbidly obese subjects. Animal and clinical studies have shown that RYGB induces metabolic changes enhancing energy expenditure at rest and particularly in the post-prandial period, so-called âÂ?Â?diet-induced thermogenesisâÂ? (DIT) and also to increased faecal energy loss via both fat and undigested protein. A key question is what is the cause of the enhanced DIT following RYGB? Saeidi et al and Cavin et al. have reported that after RYGB in obese rats, the alimentary limb (AL) becomes hyperplastic and hypertrophic showing up-regulation of the glucose transporter GLUT1 in enterocyte basolateral membranes, also cytoplasmic hexokinase 2 and increased intraluminal metabolism of glucose as demonstrated by 2-deoxy-2-[18F] fluoro-D-glucose (18FDG) positron emission tomography. Sustained weight loss due to Roux en Y gastric bypass surgery (RYGB) may either be the result of improved incretin response to or a change in energy balance. A recent computer model of human glucose absorption and metabolism has been adapted to provide an explanation of both the increased DIT and weight-loss after RYGB. There are three likely mechanisms explaining the RYGB induced sustained weight loss: Increased sensitivity to incretins gastric inhibitory polypeptide GIP and glucagon-like peptide 1 GLP-1; increased leakage of glucose from the splanchnic circulation back into the small intestinal lumen via either transcellular route. This results from increased expression of glucose transporters at both the enterocyte basolateral and apical membranes, i.e. GLUT1 and GLUT2, or to inflammation-induced increased paracellular leakage. Overgrowth of facultative aerobic bacteria within the small intestine leads to enhanced luminal conversion of glucose to CO2 with consequent depletion of net energy absorption. The computer model simulates all of the combinations of these conditions and the observed decreases in plasma glucose and insulin concentrations along with increase in post prandial and fasting metabolic rates following RYGB. A combination of all three factors is likely to be the explanation for the success of RYGB. This raises the interesting consideration as to whether a non-surgical intervention which simulates these effects might be as efficacious as RYGB in producing sustained weight loss in morbidly obese subjects.

Jejunal T Cell Inflammation in Human Obesity Correlates with Decreased Enterocyte Insulin Signaling

In obesity, insulin resistance is linked to inflammation in several tissues. Although the gut is a very large lymphoid tissue, inflammation in the absorptive small intestine, the jejunum, where insulin regulates lipid and sugar absorption is unknown. We analyzed jejunal samples of 185 obese subjects stratified in three metabolic groups: without comorbidity, suffering from obesity-related comorbidity, and diabetic, versus 33 lean controls. Obesity increased both mucosa surface due to lower cell apoptosis and innate and adaptive immune cell populations. The preferential CD8αβ Tcell location in epithelium over lamina propria appears a hallmark of obesity. Cytokine secretionby Tcells from obese, but not lean, subjects blunted insulin signaling in enterocytes relevant to apical GLUT2 mislocation. Statistical links between Tcell densities and BMI, NAFLD, or lipid metabolism suggest tissue crosstalk. Obesity triggers T-cell-mediated inflammation and enterocyte insulin resistance in the jejunum with potential broader systemic implications.

Does apical membrane GLUT2 have a role in intestinal glucose uptake?

It has been proposed that the non-saturable component of intestinal glucose absorption, apparent following prolonged exposure to high intraluminal glucose concentrations, is mediated via the low affinity glucose and fructose transporter, GLUT2, upregulated within the small intestinal apical border. The evidence that the non-saturable transport component is mediated via an apical membrane sugar transporter is that it is inhibited by phloretin, after exposure to phloridzin. Since the other apical membrane sugar transporter, GLUT5, is insensitive to inhibition by either cytochalasin B, or phloretin, GLUT2 was deduced to be the low affinity sugar transport route. As in its uninhibited state, polarized intestinal glucose absorption depends both on coupled entry of glucose and sodium across the brush border membrane and on the enterocyte cytosolic glucose concentration exceeding that in both luminal and submucosal interstitial fluids, upregulation of GLUT2 within the intestinal brush border will usually stimulate downhill glucose reflux to the intestinal lumen from the enterocytes; thereby reducing, rather than enhancing net glucose absorption across the luminal surface. These states are simulated with a computer model generating solutions to the differential equations for glucose, Na and water flows between luminal, cell, interstitial and capillary compartments. The model demonstrates that uphill glucose transport via SGLT1 into enterocytes, when short-circuited by any passive glucose carrier in the apical membrane, such as GLUT2, will reduce transcellular glucose absorption and thereby lead to increased paracellular flow. The model also illustrates that apical GLUT2 may usefully act as an osmoregulator to prevent excessive enterocyte volume change with altered luminal glucose concentrations.

Diet effects on glucose absorption in the small intestine of neonatal calves: Importance of intestinal mucosal growth, lactase activity, and glucose transporters

Colostrum (C) feeding in neonatal calves improves glucose status and stimulates intestinal absorptive capacity, leading to greater glucose absorption when compared with milk-based formula feeding. In this study, diet effects on gut growth, lactase activity, and glucose transporters were investigated in several gut segments of the small intestine. Fourteen male German Holstein calves received either C of milkings 1, 3, and 5 (d 1, 2, and 3 in milk) or respective formulas (F) twice daily from d 1 to d 3 after birth. Nutrient content, and especially lactose content, of C and respective F were the same. On d 4, calves were fed C of milking 5 or respective F and calves were slaughtered 2h after feeding. Tissue samples from duodenum and proximal, mid-, and distal jejunum were taken to measure villus size and crypt depth, mucosa and brush border membrane vesicles (BBMV) were taken to determine protein content, and mRNA expression and activity of lactase and mRNA expression of sodium-dependent glucose co-transporter-1 (SGLT1) and facilitative glucose transporter (GLUT2) were determined from mucosal tissue. Additionally, protein expression of SGLT1 in BBMV and GLUT2 in crude mucosal membranes and BBMV were determined, as well as immunochemically localized GLUT2 in the intestinal mucosa. Villus circumference, area, and height were greater, whereas crypt depth was smaller in C than in F. Lactase activity tended to be greater in C than in F. Protein expression of SGLT1 was greater in F than in C. Parameters of villus size, lactase activity, SGLT1 protein expression, as well as apical and basolateral GLUT2 localization in the enterocytes differed among gut segments. In conclusion, C feeding, when compared with F feeding, enhances glucose absorption in neonatal calves primarily by stimulating mucosal growth and increasing absorptive capacity in the small intestine, but not by stimulating abundance of intestinal glucose transporters.

Live imaging of GLUT2 glucose-dependent trafficking and its inhibition in polarized epithelial cysts.

GLUT2 is a facilitative glucose transporter, expressed in polarized epithelial cells of the liver, intestine, kidney and pancreas, where it plays a critical role in glucose homeostasis. Together with SGLT1/2, it mediates glucose absorption in metabolic epithelial tissues, where it can be translocated apically upon high glucose exposure. To track the subcellular localization and dynamics of GLUT2, we created an mCherry–hGLUT2 fusion protein and expressed it in multicellular kidney cysts, a major site of glucose reabsorption. Live imaging of GLUT2 enabled us to avoid the artefactual localization of GLUT2 in fixed cells and to confirm the apical GLUT2 model. Live cell imaging showed a rapid 15 ± 3 min PKC-dependent basal-to-apical translocation of GLUT2 in response to glucose stimulation and a fourfold slower basolateral translocation under starvation. These results mark the physiological importance of responding quickly to rising glucose levels. Importantly, we show that phloretin, an apple polyphenol, inhibits GLUT2 translocation in both directions, suggesting that it exerts its effect by PKC inhibition. Subcellular localization studies demonstrated that GLUT2 is endocytosed through a caveolae-dependent mechanism, and that it is at least partly recovered in Rab11A-positive recycling endosome. Our work illuminates GLUT2 dynamics, providing a platform for drug development for diabetes and hyperglycaemia.

Transepithelial glucose transport and Na+/K+ homeostasis in enterocytes: an integrative model

The uptake of glucose and the nutrient coupled transcellular sodium traffic across epithelial cells in the small intestine has been an ongoing topic in physiological research for over half a century. Driving the uptake of nutrients like glucose, enterocytes must have regulatory mechanisms that respond to the considerable changes in the inflow of sodium during absorption. The Na-K-ATPase membrane protein plays a major role in this regulation. We propose the hypothesis that the amount of active Na-K-ATPase in enterocytes is directly regulated by the concentration of intracellular Na(+) and that this regulation together with a regulation of basolateral K permeability by intracellular ATP gives the enterocyte the ability to maintain ionic Na(+)/K(+) homeostasis. To explore these regulatory mechanisms, we present a mathematical model of the sodium coupled uptake of glucose in epithelial enterocytes. Our model integrates knowledge about individual transporter proteins including apical SGLT1, basolateral Na-K-ATPase, and GLUT2, together with diffusion and membrane potentials. The intracellular concentrations of glucose, sodium, potassium, and chloride are modeled by nonlinear differential equations, and molecular flows are calculated based on experimental kinetic data from the literature, including substrate saturation, product inhibition, and modulation by membrane potential. Simulation results of the model without the addition of regulatory mechanisms fit well with published short-term observations, including cell depolarization and increased concentration of intracellular glucose and sodium during increased concentration of luminal glucose/sodium. Adding regulatory mechanisms for regulation of Na-K-ATPase and K permeability to the model show that our hypothesis predicts observed long-term ionic homeostasis.

Comment on: Gorboulev et al. Na+-D-glucose Cotransporter SGLT1 Is Pivotal for Intestinal Glucose Absorption and Glucose-Dependent Incretin Secretion. Diabetes 2012;61:187–196

The elegant paper from Koepsell and colleagues (1) demonstrates the apical GLUT2 mechanism, but appears at first sight to question its significance. However, when set in a physiological context, the results are those expected from the three roles of SGLT1 and the differences between fed rat and starved mouse. SGLT1 acts as 1 ) a transporter and 2 ) a powerful scavenger. In 2000, my laboratory proposed that SGLT1 also has 3 ) a pivotal regulatory role, controlling the rapid insertion and activation of GLUT2 at the enterocyte apical membrane so that absorptive capacity matches glucose load. …

GLUT2 Accumulation in Enterocyte Apical and Intracellular Membranes

OBJECTIVEIn healthy rodents, intestinal sugar absorption in response to sugar-rich meals and insulin is regulated by GLUT2 in enterocyte plasma membranes. Loss of insulin action maintains apical GLUT2 location. In human enterocytes, apical GLUT2 location has not been reported but may be revealed under conditions of insulin resistance.RESEARCH DESIGN AND METHODSSubcellular location of GLUT2 in jejunal enterocytes was analyzed by confocal and electron microscopy imaging and Western blot in 62 well-phenotyped morbidly obese subjects and 7 lean human subjects. GLUT2 locations were assayed in ob/ob and ob/+ mice receiving oral metformin or in high-fat low-carbohydrate diet–fed C57Bl/6 mice. Glucose absorption and secretion were respectively estimated by oral glucose tolerance test and secretion of [U-14C]-3-O-methyl glucose into lumen.RESULTSIn human enterocytes, GLUT2 was consistently located in basolateral membranes. Apical GLUT2 location was absent in lean subjects but was observed in 76% of obese subjects and correlated with insulin resistance and glycemia. In addition, intracellular accumulation of GLUT2 with early endosome antigen 1 (EEA1) was associated with reduced MGAT4a activity (glycosylation) in 39% of obese subjects on a low-carbohydrate/high-fat diet. Mice on a low-carbohydrate/high-fat diet for 12 months also exhibited endosomal GLUT2 accumulation and reduced glucose absorption. In ob/ob mice, metformin promoted apical GLUT2 and improved glucose homeostasis. Apical GLUT2 in fasting hyperglycemic ob/ob mice tripled glucose release into intestinal lumen.CONCLUSIONSIn morbidly obese insulin-resistant subjects, GLUT2 was accumulated in apical and/or endosomal membranes of enterocytes. Functionally, apical GLUT2 favored and endosomal GLUT2 reduced glucose transepithelial exchanges. Thus, altered GLUT2 locations in enterocytes are a sign of intestinal adaptations to human metabolic pathology.

Hypoglycemic properties of banana pseudo-stems

Water extract of banana (Musa sapientum L.) pseudo-stems has been claimed by Lebanese herbalists to be efficient in the treatment of diabetes mellitus. This work aimed at verifying the alleged effect and at elucidating its possible mode of action. Administration of the extract in replacement of drinking water to streptozotocin-induced diabetic rats, did reduce significantly blood glucose levels. The mechanism of action of the extract was studied by investigating its involvement in intestinal glucose absorption and its effect on the Na+/K+ ATPase and glucose transporters SGLT1 and GLUT2 in the rat jejunum. Rat jejuna were perfused in situ with Krebs Ringer buffer containing [14C] 3-O-methyl-D-glucose, and the activity of the Na+/K+ ATPase in jejunal homogenates was assayed in vitro, by measuring the amount of inorganic phosphate released in presence and absence of inhibitors of the ATPase. The extract induced a significant reduction in glucose absorption and Na+/K+ ATPase activity, but did not affect the protein expression of SGLT1 and GLUT2 glucose transporters. It was concluded that the extract acts by reducing the Na+/K+ ATPase activity and consequently the sodium gradient required for sugar transport by SGLT1. Reduced activity of SGLT1 leads to a decrease in intracellular glucose and in the number of apical GLUT2 [1], which contributes to the observed hypoglycemic effect. The current on going work focuses on identifying the active ingredient(s) in the extract.

An energy supply network of nutrient absorption coordinated by calcium and T1R taste receptors in rat small intestine

T1R taste receptors are present throughout the gastrointestinal tract. Glucose absorption comprises active absorption via SGLT1 and facilitated absorption via GLUT2 in the apical membrane. Trafficking of apical GLUT2 is rapidly up-regulated by glucose and artificial sweeteners, which act through T1R2 + T1R3/alpha-gustducin to activate PLC beta2 and PKC betaII. We therefore investigated whether non-sugar nutrients are regulated by taste receptors using perfused rat jejunum in vivo. Under different conditions, we observed a Ca(2+)-dependent reciprocal relationship between the H(+)/oligopeptide transporter PepT1 and apical GLUT2, reflecting the fact that trafficking of PepT1 and GLUT2 to the apical membrane is inhibited and activated by PKC betaII, respectively. Addition of L-glutamate or sucralose to a perfusate containing low glucose (20 mM) each activated PKC betaII and decreased apical PepT1 levels and absorption of the hydrolysis-resistant dipeptide L-Phe(PsiS)-L-Ala (1 mM), while increasing apical GLUT2 and glucose absorption within minutes. Switching perfusion from mannitol to glucose (75 mM) exerted similar effects. c-glutamate induced rapid GPCR internalization of T1R1, T1R3 and transducin, whereas sucralose internalized T1R2, T1R3 and alpha-gustducin. We conclude that L-glutamate acts via amino acid and glucose via sweet taste receptors to coordinate regulation of PepT1 and apical GLUT2 reciprocally through a common enterocytic pool of PKC betaII. These data suggest the existence of a wider Ca(2+) and taste receptor-coordinated transport network incorporating other nutrients and/or other stimuli capable of activating PKC betaII and additional transporters, such as the aspartate/glutamate transporter, EAAC1, whose level was doubled by L-glutamate. The network may control energy supply.

Sweet taste receptors in rat small intestine stimulate glucose absorption through apical GLUT2

Natural sugars and artificial sweeteners are sensed by receptors in taste buds. T2R bitter and T1R sweet taste receptors are coupled through G-proteins, alpha-gustducin and transducin, to activate phospholipase C beta2 and increase intracellular calcium concentration. Intestinal brush cells or solitary chemosensory cells (SCCs) have a structure similar to lingual taste cells and strongly express alpha-gustducin. It has therefore been suggested over the last decade that brush cells may participate in sugar sensing by a mechanism analogous to that in taste buds. We provide here functional evidence for an intestinal sensing system based on lingual taste receptors. Western blotting and immunocytochemistry revealed that all T1R members are expressed in rat jejunum at strategic locations including Paneth cells, SCCs or the apical membrane of enterocytes; T1Rs are colocalized with each other and with alpha-gustducin, transducin or phospholipase C beta2 to different extents. Intestinal glucose absorption consists of two components: one is classical active Na+-glucose cotransport, the other is the diffusive apical GLUT2 pathway. Artificial sweeteners increase glucose absorption in the order acesulfame potassium approximately sucralose > saccharin, in parallel with their ability to increase intracellular calcium concentration. Stimulation occurs within minutes by an increase in apical GLUT2, which correlates with reciprocal regulation of T1R2, T1R3 and alpha-gustducin versus T1R1, transducin and phospholipase C beta2. Our observation that artificial sweeteners are nutritionally active, because they can signal to a functional taste reception system to increase sugar absorption during a meal, has wide implications for nutrient sensing and nutrition in the treatment of obesity and diabetes.

Apical GLUT2 and Cav1.3: regulation of rat intestinal glucose and calcium absorption.

We have proposed a model of intestinal glucose absorption in which transport by SGLT1 induces rapid insertion and activation of GLUT2 in the apical membrane by a PKC betaII-dependent mechanism. Since PKC betaII requires Ca(2+) and glucose is depolarizing, we have investigated whether glucose absorption is regulated by the entry of dietary Ca(2+) through Ca(v)1.3 in the apical membrane. When rat jejunum was perfused with 75 mM glucose, Ca(2+)-deplete conditions, or perfusion with the L-type antagonists nifedipine and verapamil strongly diminished the phloretin-sensitive apical GLUT2, but not the phloretin-insensitive SGLT1 component of glucose absorption. Western blotting showed that in each case there was a significant decrease in apical GLUT2 level, but no change in SGLT1 level. Inhibition of apical GLUT2 absorption coincided with inhibition of unidirectional (45)Ca(2+) entry by nifedipine and verapamil. At 10 mM luminal Ca(2+), (45)Ca(2+) absorption in the presence of 75 mM glucose was 2- to 3-fold that in the presence of 75 mM mannitol. The glucose-induced component was SGLT1-dependent and nifedipine-sensitive. RT-PCR revealed the presence of Ca(v)beta(3) in jejunal mucosa; Western blotting and immunocytochemistry localized Ca(v)beta(3) to the apical membrane, together with Ca(v)1.3. We conclude that in times of dietary sufficiency Ca(v)1.3 may mediate a significant pathway of glucose-stimulated Ca(2+) entry into the body and that luminal supply of Ca(2+) is necessary for GLUT2-mediated glucose absorption. The integration of glucose and Ca(2+) absorption represents a complex nutrient-sensing system, which allows both absorptive pathways to be regulated rapidly and precisely to match dietary intake.