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, α-gustducin and transducin, to activate phospholipase C β2 and increase intracellular calcium concentration. Intestinal brush cells or solitary chemosensory cells (SCCs) have a structure similar to lingual taste cells and strongly express α-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 α-gustducin, transducin or phospholipase C β2 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 ∼ 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 α-gustducin versus T1R1, transducin and phospholipase C β2. 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.     

A diurnal rhythm in the absorption of glucose and water by isolated rat small intestine.

1. Glucose and water absorption by isolated small intestine from rats which have had unrestricted access to food is 50-60% higher at night than during the daytime. 2. When the feeding time is restricted to 06.00-09.00 hr G.M.T. glucose and water absorption rates in the period from 3 to 7 hr after withdrawal of food are almost as high as the rates observed at night-time in the animals with unrestricted feeding. 3. These changes in absorption rates appear to be associated with feeding time and not with the pattern of illumination.     

Water absorption and swelling in the rat small intestine in vitro

Summary Net mucosal-to-serosal water transport and water retention within the tissue were simultaneously measured in the isolated, everted rat jejunum and ileum along with time.The swelling of the epithelium (w/DW-v/DW) precedes the absorption of fluid and then it remains constant throughout the experiment.The water uptake, determined as a weight increase, (w/DW) and the net water transport determined as a serosal volume increase (v/DW) were higher in Krebs-Ringer-bicarbonate than in a Krebs-Ringer-phosphate medium.In the jejunum glucose markedly stimulated both the tissue retention and net transport of water, but such effect was not seen in the ileum.Replacement of chloride with sulphate in the medium diminishes both water transport and retention, but both were markedly stimulated by the addition of glucose to such medium.As a conclusion it seems that during water transport the epithelial layer swells and the swelling increases when the amount of water transported is increased.     

Sucrose absorption by the rat small intestine in vivo and in vitro

1. The absorption of glucose and fructose derived from sucrose has been studied using in vitro and in vivo loops of the rat jejunum.     

Simple-sugar meals target GLUT2 at enterocyte apical membranes to improve sugar absorption: a study in GLUT2-null mice

The physiological significance of the presence of GLUT2 at the food-facing pole of intestinal cells is addressed by a study of fructose absorption in GLUT2-null and control mice submitted to different sugar diets. Confocal microscopy localization, protein and mRNA abundance, as well as tissue and membrane vesicle uptakes of fructose were assayed. GLUT2 was located in the basolateral membrane of mice fed a meal devoid of sugar or containing complex carbohydrates. In addition, the ingestion of a simple sugar meal promoted the massive recruitment of GLUT2 to the food-facing membrane. Fructose uptake in brush-border membrane vesicles from GLUT2-null mice was half that of wild-type mice and was similar to the cytochalasin B-insensitive component, i.e. GLUT5-mediated uptake. A 5 day consumption of sugar-rich diets increased fructose uptake fivefold in wild-type tissue rings when it only doubled in GLUT2-null tissue. GLUT5 was estimated to contribute to 100 % of total uptake in wild-type mice fed low-sugar diets, falling to 60 and 40 % with glucose and fructose diets respectively; the complement was ensured by GLUT2 activity. The results indicate that basal sugar uptake is mediated by the resident food-facing SGLT1 and GLUT5 transporters, whose mRNA abundances double in long-term dietary adaptation. We also observe that a large improvement of intestinal absorption is promoted by the transient recruitment of food-facing GLUT2, induced by the ingestion of a simple-sugar meal. Thus, GLUT2 and GLUT5 could exert complementary roles in adapting the absorption capacity of the intestine to occasional or repeated loads of dietary sugars.     

Na+ and pH dependence of proline and beta-alanine absorption in rat small intestine

Aims: Early characterization of intestinal absorption of imino acids in mammals has demonstrated the existence of a Na⁺‐dependent, Cl^?‐independent transport system in rat small intestine, which is the only carrier for β‐alanine. Based on the substrate selectivity, it was proposed that the Proton Amino Acid Transporter 1 (PAT1) could be the same as this imino acid carrier. The present study characterizes the pH and Na⁺ dependence of proline and β‐alanine uptake in rat small intestine. Methods: Intestinal uptake of radiolabelled l ‐proline or β‐alanine was measured in brush border membrane vesicles and everted intestinal rings, in the presence and absence of Na⁺ and at different pH values. Results: The existence of an inwardly directed H⁺ gradient in the absence of Na⁺ enhanced the initial entry of proline and β‐alanine in brush border membrane vesicles, that reached a transient overshoot with maximal value around 30 s. In the absence of pH gradient, no overshoot was shown. In entire tissue, there was an increase of proline and β‐alanine uptake at acidic pH that was higher in the presence of Na⁺ than in its absence. This ion dependence and pH effect of the amino acids uptake also increased with the incubation period. Substrate inhibition studies confirmed that intestinal proline absorption in rat occurs mainly by system B and PAT1‐like transporter. Conclusions: There is a Na⁺‐independent, H⁺‐dependent transporter of amino acids at the apical membrane of the rat enterocytes.     

The absorption of a mixture of amino acids by rat small intestine

1. The absorption of a mixture of amino acids by the small intestine has been studied in vitro with mucosal slices from rat jejunum. The mixture contained eighteen amino acids and was used at a concentration of 9.5 mM in the incubation medium. The uptake of fifteen of the amino acids was followed in each sample with the aid of an amino acid analyser.2. The endogenous amino acid content of the slices decreased during preparation and further substantial losses occurred when the slices were preincubated for 30 sec before the addition of the amino acid mixture to the incubation medium.3. When the slices were incubated with the amino acid mixture for 4.5 min all of the amino acid studied were accumulated to give concentration ratios of approximately 2.0.4. The amino acids were taken up rapidly by the tissue so that on average 82% of the amino acids absorbed in 4.5 min had already accumulated after one min of incubation.5. The data for individual amino acids revealed no obvious competition between the various amino acids, and the extent to which the various amino acids were accumulated by the tissue was proportional to their concentrations in the mixture.6. When the Na(+) in the incubation medium was replaced by Li(+), the slices accumulated all of the amino acids studied to concentrations significantly higher than those in the incubation medium. However, the concentration ratios obtained with the Li(+) medium were all lower than those obtained with the Na(+) medium and averaged slightly more than 1.5.7. When the Na(+) in the incubation medium was replaced by K(+) there was little amino acid accumulation, although concentration ratios significantly greater than one were achieved for aspartic acid, glycine, histidine and methionine.     

大鼠全身辐射损伤后小肠吸收功能变化的研究

目的：研究大鼠全身辐射损伤后小肠吸收Ｌ－酪氨酸（Ｔｙｒ）的变化。方法：用在体门静脉取血法研究了６Ｇｙ６０Ｃｏγ线全身一次照射后不同时间以及不同剂量照射后第３ｄ大鼠小肠吸收Ｔｙｒ的变化，并测定相应的小肠上皮细胞计数、小肠粘膜钠－钾－ＡＴＰ酶（Ｎａ＋－Ｋ＋－ＡＴＰａｓｅ）活性以及Ｎａ＋的跨膜转运。结果：６Ｇｙγ线全身照射后第３ｄ出现小肠对Ｔｙｒ的吸收障碍，以后很快恢复，并持续在正常水平。随着照射剂量的加大，吸收障碍加重，但３Ｇｙ以下剂量照射，则不出现Ｔｙｒ的吸收障碍。小肠上皮细胞计数、Ｎａ＋－Ｋ＋－ＡＴＰａｓｅ活性和Ｎａ＋的跨膜转运的变化与小肠吸收Ｔｙｒ的变化基本一致。结论：辐射损伤后小肠对Ｔｙｒ的吸收障碍与小肠上皮细胞数量减少导致的吸收面积的下降以及Ｎａ＋－Ｋ＋－ＡＴＰａｓｅ活性下降导致的Ｎａ＋跨膜转运能力的减弱有关。     

Disaccharide absorption by amphibian small intestine in vitro

1. An account is given of the absorption of disaccharides by the small intestine of Rana temporaria, R. pipiens and Bufo vulgaris perfused in vitro through the vascular system. Maltase and trehalase activity are found in the intestine of all three species; very small amounts of sucrase are present in the intestine of R. pipiens but there is no evidence for the presence of lactase in any of the animals studied.2. During maltose absorption free glucose appears in the vascular effluent and in the intestinal lumen. Only very small quantities of disaccharide are found in the vascular effluent. The concentration of free glucose in the intestinal lumen during maltose absorption is not high enough to account for the rates of glucose transport observed. The rate of appearance of glucose in the vascular effluent is determined by the concentration of disaccharide in the luminal fluid, and hexose, free in solution in the lumen, is not an obligatory intermediate in the process of disaccharide absorption.3. For R. pipiens more than 90% of the maltase activity in the system is present in the intestinal wall and the rate of maltose hydrolysis by maltase, free in the intestinal lumen, is found to be inadequate to account for the rates of appearance of glucose observed to occur in the lumen and in the vascular effluent. It is not possible to wash away maltase activity from the intestinal wall.4. The kinetic properties of maltase and trehalase acting in situ are of the Michaelis-Menten type; the apparent K(m) is 2 mM for maltase, and 3 mM for trehalase.5. The relationship which exists between the rate of absorption of glucose and the concentration in the luminal fluid of either disaccharide or free glucose is of the Michaelis-Menten type. Expressed in molar units, the apparent K(m) for the glucose transport is about one fifth that of the disaccharidase. The maximum rate of glucose transport observed is less than the maximum rate of disaccharide hydrolysis. In R. pipiens equimolar concentrations in the intestinal lumen of the monomer free glucose, or of the dimer, maltose, yield approximately equal rates of transport of the free hexose.6. It is concluded that in the amphibian, either intestine disaccharide hydrolysis and glucose transport are functions of separate subcellular systems which spatially are very closely related, or that the hydrolysis and transport are different facets of the activity of a common system.     

Thiamine Countertransport in rat small intestine

Summary Countertransport experiments on rings of everted jejunum of rats suggest that thiamine is actively transported by a mobile carrier in intestinal epithelial cells and that pyrithiamine, but not oxythiamine or deshydroxythiamine, has the common carrier with thiamine.This investigation was partially supported by a grant (No. 115. 2162. O 1323) from Consiglio Nazionale delle Ricerche, Rome, Italy.     

Effect of Vilon and Epithalon on glucose and glycine absorption in various regions of small intestine in aged rats

Vilon (Lys-Glu) and Epithalon (Ala-Glu-Asp-Gly) administered orally for 1 month improved transport characteristics of the small intestine in aged rats. Vilon enhanced passive glucose accumulation in the serous fluid in inverted sac made from the distal region of the small intestine, while Epithalon enhanced this process in the medial region. Vilon stimulated active glucose accumulation in the serous sac of the medial small intestine, Epithalon - in the proximal and distal small intestinal segments. Glycine absorption increased only in the proximal intestinal segment under the effect of Epithalon.     

Metabolic energy dependence of glucose, water and sodium absorption in the presence and absence of 'downhill' sodium gradients across isolated rat small intestine.

1. Absorption of glucose and water was studied in isolated perfused rat small intestine with arterial infusion under normal conditions and with a 'downhill' sodium gradient across the mucosa. 2. The effects of metabolic inhibitors, iodoacetate phenformin and dinitrophenol, on absorption were determined in the presence of normal and 'downhill' sodium gradients. 3. Glucose absorption was inhibited by each inhibitor, and the inhibition was independent of the direction of the sodium gradient. 4. Net sodium flux across the mucosa was also inhibited under 'downhill' sodium gradient conditions. 5. The simultaneous and parallel inhibition of both glucose and sodium transport is consistent with, but does not prove, the sodium gradient theory.