Intestinal Fructose Transport Research Articles

Molecular characterisation of fructose transport in equine small intestine.

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. 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. 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. 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. Equine small intestine has the capacity to absorb fructose. The molecular probes produced in these studies can be used as diagnostic aids to determine equine intestinal monosaccharide malabsorption.

Cyclic AMP Stimulates Fructose Transport in Neonatal Rat Small Intestine

Intestinal fructose transporter (GLUT5) expression normally increases significantly after completion of weaning in neonatal rats. Increases in GLUT5 mRNA, protein, and activity can be induced in early weaning pups by precocious consumption of dietary fructose or by perfusion of the small intestine with fructose solutions. Little is known about the signal transduction pathway of the dietary fructose-mediated increase in GLUT5 expression during early intestinal development. Recent microarray results indicate that key gluconeogenic enzymes modulated by cAMP are markedly upregulated by fructose perfusion; hence, we tested the hypothesis that cAMP plays an important role in regulating intestinal fructose absorption by simultaneously perfusing adenylyl cyclase, phosphodiesterase, or protein kinase A (PKA) inhibitors along with fructose. Intestinal fructose uptake rates increased by 100% in rat pups perfused with 8-bromo-cAMP. Simultaneous fructose and dideoxyadenosine (DDA; inhibitor of adenylyl cyclase) perfusion completely inhibited increases in fructose uptake rate induced by perfusion with fructose alone. Fructose perfusion increased intestinal mucosal cAMP concentrations by 27%, but simultaneous perfusion of fructose and DDA inhibited the fructose-induced increase in cAMP. However, GLUT5 and sodium-glucose cotransporter (SGLT1) mRNA abundance and glucose transport rates were each not significantly affected by 8-bromo-cAMP and DDA. Moreover, simultaneous perfusion of the small intestine with fructose and PKA inhibitor or N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamid. 2HCl, both inhibitors of PKA, did not prevent the fructose-induced increases in GLUT5 mRNA abundance and fructose uptake rate. Cyclic AMP appears to modulate fructose transport without affecting GLUT5 mRNA abundance, and without involving PKA.

Stimulating effect of glucocorticosteroids on intestinal fructose transport in rats is increased by feeding a saturated fatty acid diet.

Glucocorticosteroids enhance absorptive functions of the intestine. This study was undertaken to assess the influence of budesonide (BUD), prednisone (PRED), or control vehicle in rats fed either a saturated fatty acid diet (SFA) or a polyunsaturated fatty acid diet (PUFA), on the uptake of sugars. Steroids increased the uptake of fructose, and these effects were greater with SFA than PUFA. No effect on the abundance or the expression of the mRNAs of SGLT1, GLUT5, or GLUT2 was observed, and yet immunohistochemistry staining in the jejunum for SGLT1 and GLUT5 was greater in SFA than in PUFA. The early response genes and proglucagon expression was enhanced in the ileum of animals fed SFA and given control vehicle. Steroids increased the ileal proglucagon mRNA. In summary, (1) PRED and BUD enhance the uptake of fructose, (2) this enhancement may be increased further by feeding SFA, and (3) signaling may involve early response genes and proglucagon.

The stimulating effect of locally and systemically acting glucocorticosteroids on intestinal fructose transport in rats is increased by feeding a saturated fatty acid diet

The stimulating effect of locally and systemically acting glucocorticosteroids on intestinal fructose transport in rats is increased by feeding a saturated fatty acid diet

The stimulating effect of locally and systemically acting glucocorticosteroids on intestinal fructose transport in rats is increased by feeding a saturated fatty acid diet

The stimulating effect of locally and systemically acting glucocorticosteroids on intestinal fructose transport in rats is increased by feeding a saturated fatty acid diet

Developmental reprogramming of rat GLUT-5 requires de novo mRNA and protein synthesis.

Fructose transporter (GLUT-5) expression is low in mid-weaning rat small intestine, increases normally after weaning is completed, and can be precociously induced by premature consumption of a high-fructose (HF) diet. In this study, an in vivo perfusion model was used to determine the mechanisms regulating this substrate-induced reprogramming of GLUT-5 development. HF (100 mM) but not high-glucose (HG) perfusion increased GLUT-5 activity and mRNA abundance. In contrast, HF and HG perfusion had no effect on Na(+)-dependent glucose transporter (SGLT-1) expression but increased c-fos and c-jun expression. Intraperitoneal injection of actinomycin D before intestinal perfusion blocked the HF-induced increase in fructose uptake rate and GLUT-5 mRNA abundance. Actinomycin D also prevented the perfusion-induced increase in c-fos and c-jun mRNA abundance but did not affect glucose uptake rate and SGLT-1 mRNA abundance. Cycloheximide blocked the HF-induced increase in fructose uptake rate but not the increase in GLUT-5 mRNA abundance and had no effect on glucose uptake rate and SGLT-1 mRNA abundance. In neonatal rats, the substrate-induced reprogramming of intestinal fructose transport is likely to involve transcription and translation of the GLUT-5 gene.

Regulation of GLUT5, GLUT2 and intestinal brush-border fructose absorption by the extracellular signal-regulated kinase, p38 mitogen-activated kinase and phosphatidylinositol 3-kinase intracellular signalling pathways: implications for adaptation to diabetes

We have investigated the role of the extracellular signal-regulated kinase (ERK), p38 and phosphatidylinositol 3-kinase (PI 3-kinase) pathways in the regulation of intestinal fructose transport. Different combinations of anisomycin, PD98059 and wortmannin had very different effects on fructose transport in perfused isolated loops of rat jejunum. Transport was stimulated maximally by anisomycin (2 microM) and blocked by SB203580 (20 microM), confirming involvement of the p38 pathway. PD98059 (50 microM) alone had little effect on fructose transport. However, it had a dramatic effect on stimulation by anisomycin, diminishing the K(a) 50-fold from 1 microM to 20 nM to show that the ERK pathway restrains the p38 pathway. The K(a) for diabetic jejunum was 30 nM and PD98059 had no effect. Transport in the presence of anisomycin was 3.4-fold that for anisomycin plus PD98059 plus wortmannin. Transport was mediated by both GLUT5 and GLUT2. In general, GLUT2 levels increased up to 4-fold within minutes and with only minimal changes in GLUT5 or SGLT1 levels, demonstrating that GLUT2 trafficks by a rapid trafficking pathway distinct from that of GLUT5 and SGLT1. GLUT2 intrinsic activity was regulated over a 9-fold range. It is concluded that there is extensive cross-talk between the ERK, p38 and PI 3-kinase pathways in their control of brush-border fructose transport by modulation of both the levels and intrinsic activities of GLUT5 and GLUT2. The potential of the intracellular signalling pathways to regulate short-term nutrient transport during the assimilation of a meal and longer-term adaptation to diabetes and hyperglycaemia is discussed.

Dietary modulation of intestinal fructose transport and GLUT5 mRNA expression in hypothyroid rat pups.

Intestinal fructose transport rates or GLUT5 mRNA levels typically show a two- to threefold increase after weaning in rats allowed to wean normally but can be enhanced precociously by high-fructose diets during early weaning. Developmental increases in serum thyroxine levels coincide with the onset of weaning and have been linked to changes in intestinal sucrase and lactase activities. Rat pups were made hypothyroid by giving the dam 0.01% propylthiouracil as drinking water from day 18 of gestation. The hypothyroid pups and age-matched euthyroid control pups were then fed high-fructose or high-glucose solutions by gavage, twice a day starting at 17 days of age for 3 days, and then killed at 20 days of age. Serum thyroxine levels were five times lower in the hypothyroid pups. Rates of intestinal fructose uptake in the proximal and middle small intestine were 2.0 to 2.5 times higher in the hypothyroid and euthyroid pups fed high-fructose solution than in littermates fed high-glucose solution or those allowed to wean normally with the dam. Intestinal glucose uptake also increased in hypothyroid but not in euthyroid pups fed high-fructose or high-glucose solutions. GLUT5 mRNA levels increased in euthyroid and hypothyroid pups fed high fructose and paralleled the increase in fructose uptake. During weaning, dietary fructose can precociously enhance intestinal fructose uptake and GLUT5 mRNA expression, independent of developmental increases in serum thyroxine levels. Modest changes in glucose transport rates indicate that nonspecific mechanisms may provide a minor contribution to diet-induced changes in nutrient absorption in hypothyroid pups.

Oral Dexamethasone Precociously Induces Intestinal Fructose Transport In Suckling Rats 570

We used intestinal fructose transport as model to determine whether transporters expressed late during development, such as fructose and bile acids, can be reprogrammed to appear earlier. Dietary fructose alone can precociously enhance intestinal fructose transport in weaning (≥ 15 d old) but not in suckling (≤ 14 d old) rats. Dexamethasone (DEX) administration at therapeutic dose (0.4 μg/kg body weight (BW), i.p.) to suckling rats, nonspecifically enhanced all sugar transport, allowed specific induction of fructose transport by luminal fructose but resulted in lower growth rates. We minimized the negative systemic effects of DEX by using a lower dose (0.04μg/kg BW, i.p.) which still nonspecifically enhanced nutrient transport and allowed specific induction of fructose transporters by its substrate. In this study, we determined whether oral administration of low-dose DEX(0.04 μg/kg BW) can result in localized effects and still enhance nutrient transport with little or no reduction in growth rate. DEX was administered orally by gavage to 8 d old pups. On d 8 and 9, high fructose (HF) or high glucose (HG) solutions were fed by gavage at a volume of 2% BW 2X per d as in previous work. Pups stayed with dams at other times then were sacrificed on d 10. Compared to those of controls, same diet-litter mates, non DEX-treated pups (phosphate buffer, orally), in vitro fructose uptake per cm and per mg small intestine increased (p 0.5). There was no statistically significant difference in intestinal lengths (p>0.1, one-way ANOVA) or weights (p>0.8) across all treatment groups. In contrast to i.p. administration, oral DEX, therefore, allows specific induction of fructose transport without nonspecific enhancement of other transport systems. Growth rate, however, still lagged behind controls. Future studies will use lower oral doses of DEX in order to minimize systemic effects on growth rates while maintaining its effects on nutrient transport.

DEXAMETHASONE PRECOCIOUSLY INDUCES INTESTINAL SUGAR TRANSPORT IN SUCKLING RATS † 474

During normal development, intestinal fructose transport is enhanced only in postweaning (>28d) rats. Early introduction of a high fructose diet enhances fructose absorption in weaning (14-28d) but not in suckling (<14d) rats. In contrast, glucose absorption is independent of diet in both weaning and suckling stages. In this study, we determined whether Dexamethasone (DEX) would allow dietary induction of sugar transport in suckling rats. DEX, 0.4ug/g body weight (BW), was administered i.p. once daily to rat pups from d 5-9. Pups were gavage-fed on d 8-9 with sugar solutions containing high fructose (HF) and high glucose (HG) at a volume of 2% BW 2X per d. Pups stayed with dams at all other times. Control litter mates were fed similar solutions but received phosphate buffer i.p. instead of DEX. On d 10, pups were sacrificed and intestinal uptake studies using the everted sleeve technique were performed. Two-way analysis of variance (ANOVA) showed that fructose uptake increased 3X in DEX-treated pups fed HF (P<0.0001) but only 2X in DEX-treated pups fed HG (P<0.0001) over control. There was also a 2X significant increase in glucose uptake in DEX-treated pups fed HG(P<0.0001) and HF (P<0.0001). Uptakes were 2-3X greater in the proximal and middle than in the distal small intestine. DEX-treated pups failed to gain weight compared to control (P<0.0001). There was no statistically significant difference in intestinal lengths and weights across treatment groups. DEX therefore non-specifically enhances transport of nutrients and also allows specific induction of fructose transporters by its substrate. The proximate mechanism of this DEX effect remains unclear and is a subject of future research.

Dietary fructose enhances intestinal fructose transport and GLUT5 expression in weaning rats.

Rates of fructose uptake by the small intestine of neonatal rats are typically very low from parturition through weaning but undergo a dramatic increase immediately after weaning is completed. In this study, we used intestinal fructose transport as a model to determine whether nutrient transport, normally enhanced only after completion of weaning, can be enhanced earlier during development. We found that ontogenetic changes in levels of GLUT5 mRNA correlate well with already known ontogenetic changes in rates of intestinal fructose transport: low levels and rates during suckling and weaning, and high levels and rates after weaning. In contrast, levels of GLUT2 and SGLT1 mRNA were relatively more elevated throughout the suckling and weaning periods. We then found that increased expression of GLUT5 mRNA caused by dietary fructose or sucrose paralleled diet-dependent increases in brush-border fructose uptake. Rates of brush-border glucose uptake and levels of SGLT1 and GLUT2 mRNA were not enhanced by dietary fructose, glucose, or sucrose. Finally, we found that rates of fructose uptake, levels of GLUT5 mRNA, and specific sucrase activity each increased with increasing concentrations of dietary fructose given precociously to midweaning rats. In contrast, brush-border glucose uptake was independent of dietary fructose concentration. Thus precocious introduction of dietary fructose causes enhanced expression of fructose transporters earlier during development. This effect is specific: only luminal fructose is effective, and only brush-border fructose transport can be modulated. These results unveil the potential for regulating nutrient transport early in development.

Precocious Enhancement of Intestinal Fructose Uptake by Diet in Adrenalectomized Rat Pups

Intestinal fructose transport typically increases 3-fold after completion of weaning (> 28 d of age) in rats allowed to wean normally. Precocious enhancement of fructose transport has been demonstrated in rats fed high fructose diets during early weaning. To determine the role of corticosterone in the enhancement of fructose uptake by diet, we fed 17-d-old rat pups, previously adrenalectomized or sham-operated at 10 d of age, high (65%) fructose or fructose-free diets for 3 d. Corticosterone levels in 20-d-old sham-operated and unoperated controls were 2.2-3.3-fold higher than those in adrenalectomized littermates and in unoperated 10-d-old pups. Fructose uptake per mg and per cm were each 2.0-2.5-fold higher in adrenalectomized and sham-operated pups fed high fructose diets compared with those in adrenalectomized and sham-operated littermates fed fructose-free diets or to those in unoperated littermates allowed to wean normally with the dam. An increase in levels of GLUT5 mRNA in pups fed high fructose diets paralleled the increase in rates of fructose uptake. Intestinal glucose uptake was independent of corticosterone levels and of diet. Thus, the corticosterone surge is not necessary for the precocious enhancement of intestinal fructose transport and of GLUT5 mRNA expression by dietary fructose during weaning.

Characterization of the rabbit intestinal fructose transporter (GLUT5)

Recent studies suggest that the jejunal/kidney-type facilitative glucose transporter (GLUT5) functions as a high-affinity D-fructose transporter. However, its precise role in the small intestine is not clear. In an attempt to identify the fructose transporter in the small intestine, we measured fructose uptake in Xenopus oocytes expressing jejunal mRNA from five species (rat, mouse, rabbit, hamster and guinea-pig). Only jejunal mRNA from the rabbit significantly increased fructose uptake. We also cloned a rabbit GLUT5 cDNA from a jejunal library The predicted amino acid sequence of the 487-residue rabbit GLUT5 showed 72.3 and 67.1% identity with human and rat GLUT5 respectively. Northern-blot analysis revealed GLUT5 transcripts in rabbit duodenum, jejunum and, to a lesser extent, kidney. After separation of rabbit jejunal mRNA on a sucrose density gradient, the fractions that conferred D-fructose transport activity in oocytes also hybridized with rabbit GLUT5 cDNA. Hybrid depletion of jejunal mRNA with a GLUT5 antisense oligonucleotide markedly inhibited the mRNA-induced fructose uptake in oocytes. Immunoblot analysis indicated that GLUT5 (49 kDa) is located in the brush-border membrane of rabbit intestinal epithelial cells. Xenopus oocytes injected with rabbit GLUT5 cRNA exhibited fructose uptake activity with a Km of 11 mM for D-fructose. D-Fructose transport by GLUT5 was significantly inhibited by D-glucose and D-galactose. D-Fructose uptake in brush-border membrane vesicles shows a Km similar to that of GLUT5, but was not inhibited by D-glucose or D-galactose. Finally, cytochalasin B photolabelled a 49 kDa protein in rabbit brush-border-membrane preparations that was immunoprecipitated by antibodies to GLUT5. Our results suggest that GLUT5 functions as a fructose transporter in rabbit small intestine. However, biochemical properties of fructose transport in Xenopus oocytes injected with GLUT5 cRNA differed from those in rabbit jejunal vesicles.

Fructose and related food carbohydrates. Sources, intake, absorption, and clinical implications.

It is possible to point out subjects consuming considerable quantities of fructose and sorbitol, and the intake seems to be increasing both from added and natural sources. Studies of the absorption of fructose in animals are inconsistent, and the mechanisms of fructose uptake seem to vary in accordance with the species. In most species fructose absorption takes place by a specific carrier (facilitated transport), but it may be active in the rat. In vitro studies of human intestine are very scarce; there is no evidence of active intestinal fructose transport in the human intestine. By means of hydrogen breath tests, a very low absorption capacity for fructose given as the free monosaccharide has been found in humans. Fructose given as sucrose or in equimolar combinations with glucose is well absorbed, and only fructose in excess of glucose is malabsorbed. On this basis it is hypothesized that two different uptake mechanisms for fructose are present in the human intestine. One of these may be a disaccharidase-related uptake system. Sorbitol ingestion may aggravate malabsorption of fructose given as the monosaccharide; it is not known whether a specific mechanism is involved. In children and adults with functional bowel distress the absorption capacities for fructose may not differ from those of healthy individuals, but malabsorption of fructose and/or sorbitol may be the cause of or aggravate abdominal symptoms. Fructose polymers (fructans) are also subject to increasing nutritional interest. Fructans are not absorbed in the small intestine but are strongly fermented in the large bowel. Fructans may be of potential benefit for large-bowel function and blood glucose regulation.

Cadmium induced alterations in the intestinal absorption of glucose and fructose in a freshwater cat fish, Heteropneustes fossilis

The effect of four concentrations (1.0, 0.1, 0.01, and 0.001 mM) of Cd on the rate of absorption of two sugars, glucose and fructose by the intestine of the fresh water cat fish, Heteropneustes fossilis has been studied after 1 h at 23 °C. Intestinal glucose and fructose transport was also assessed in fish exposed to a sublettral concentration (0.0023 mM) of Cd in the ambient water for 15 and 30 days. Control fish showed no marked difference in the rates of absorption of glucose and fructose. All four concentrations of Cd decreased the rate of transport of glucose and fructose significantly. Among the two sugars, decrease in the rate of transport of fructose was more marked than glucose. In fish exposed to Cd in the ambient water, reduction of glucose and fructose absorption was greater after 30 days of exposure than after 15 days.