Regulation of intestinal glucose transport.

The small intestine is capable of adapting nutrient transport in response to numerous stimuli. This review examines several possible mechanisms involved in intestinal adaptation. In some cases, the enhancement of transport is nonspecific, that is, the absorption of many nutrients is affected. Usually, increased transport capacity in these instances can be attributed to an increase in intestinal surface area. Alternatively, some conditions induce specific regulation at the level of the enterocyte that affects the transport of a particular nutrient. Since the absorption of glucose from the intestine is so well characterized, it serves as a useful model for this type of intestinal adaptation. Four potential sites for the specific regulation of glucose transport have been described, and each is implicated in different situations. First, mechanisms at the brush-border membrane of the enterocyte are believed to be involved in the upregulation of glucose transport that occurs in streptozotocin-induced diabetes mellitus and alterations in dietary carbohydrate levels. Also, factors that increase the sodium gradient across the enterocyte may increase the rate of glucose transport. It has been suggested that an increase in activity of the basolaterally located Na(+)-K+ ATPase could be responsible for this phenomena. The rapid increase in glucose uptake seen in hyperglycemia seems to be mediated by an increase in both the number and activity of glucose carriers located at the basolateral membrane. More recently, it was demonstrated that mechanisms at the basolateral membrane also play a role in the chronic increase in glucose transport observed when dietary carbohydrate levels are increased. Finally, alterations in tight-junction permeability enhance glucose absorption from the small intestine.(ABSTRACT TRUNCATED AT 250 WORDS)     

Adaptation of intestinal glucose transport in rats with diabetes mellitus occurs independent of hyperphagia.

Chronic diabetes enhances intestinal absorption of glucose and induces hyperphagia. We examined the enhanced intestinal absorption of glucose in ad libitum-fed rats with streptozocin-induced diabetes mellitus and compared these results with those obtained from pair-fed diabetic animals. Maximal transport capacity (Vmax) and carrier affinity (K0.5) were determined by measuring jejunal and ileal short circuit current (Isc) responses to varying concentrations of 3-O-methyl-D-glucopyranose and D-glucose. Pair-fed diabetic animals maintained the same body weight as animals fed ad libitum, although ad libitum-fed diabetic rats had an increased oral chow intake. Age-matched control rats maintained a constant jejunal and ileal Vmax and K0.5 throughout the study. Diabetic rats fed ad libitum demonstrated an enhanced Vmax and K0.5 in both jejunum and ileum. Pair feeding diabetic animals further enhanced jejunal Vmax while lowering jejunal K0.5 levels. In contrast, pair feeding diabetic animals delayed and blunted changes in ileal Vmax and prevented changes in ileal K0.5. In conclusion, signals other than those of hyperphagia regulate kinetic changes in glucose absorption during diabetes mellitus. Furthermore, these changes have differing effects on jejunum and ileum.     

Use of phlorizin binding to demonstrate induction of intestinal glucose transporters

Summary We used specific binding of phlorizin to the intact intestinal mucosa in order to measure glucose transport site density in intestines of mice fed a high-carbohydrate or no-carbohydrate diet. Nonspecific binding varied with intestinal position but showed only modest dependence on diet. Specific binding to glucose transporters was 1.9 times greater in jejunum of high-carbohydrate mice than of no-carbohydrate mice; this ratio was the same as the ratio for V_max values of actived-glucose uptake between the two diet groups. The gradient in specific binding of phlorizin along the intestine paralleled the gradient in V_max of glucose transport. These results directly demonstrate that the increase in intestinal glucose transport caused by a high-carbohydrate diet is due to induction of glucose transporter. They also indicate that the normal positional graident in glucose transport along the intestine arises from a gradient in transporters, induced by the normal gradient in luminal glucose concentration.     

A method for measuring apical glucose transporter site density in intact intestinal mucosa by means of phlorizin binding

Summary Phlorizin binding has been widely used to estimate the site density of glucose transporters on intestinal and renal brush-border vesicles. Glucose transport measurements in the intact intestinal mucosa show that changes in transport rate postulated to arise from changes in site density occur under many physiological and pathological conditions. Exploring the basis of these regulatory phenomena would be facilitated by comparing changes in transport rate and site density measured in the same preparation. Hence we developed methods for measuring phlorizin binding in everted sleeves of intact mouse intestine. Specific binding of phlorizin to glucose carriers reached an asymptotic value within 120 sec, while nonspecific binding continued to rise thereafter. Hence we used 120-sec incubations. The rate of dissociation of specifically bound phlorizin was accelerated by Na⁺-free solutions and even more by 50mm glucose, while the rate of dissociation of nonspecifically bound phlorizin was independent of these solution changes. Hence we chose a 20-sec rinse in Ringer+50mm mannitol, because it washes out 30–40% of the nonspecifically bound phlorizin but virtually none of the specifically bound phlorizin. Ligand-binding analysis of specific binding against phlorizin concentration suggested two classes of binding sites, of which the one with stronger affinity for phlorizin probably has the higher capacity for glucose transport in mouse jejunum. The calculated affinity and capacity of this component are independent of whether one estimates the specific component of total binding by adding glucose or by removing Na⁺.     

Effects of dietary glucose and sodium chloride on intestinal glucose absorption of common carp (Cyprinus carpio L.)

The co-transport of sodium and glucose is the first step for intestinal glucose absorption. Dietary glucose and sodium chloride (NaCl) may facilitate this physiological process in common carp (Cyprinus carpio L.). To test this hypothesis, we first investigated the feeding rhythm of intestinal glucose absorption. Carps were fed to satiety once a day (09:00 a.m.) for 1 month. Intestinal samples were collected at 01:00, 05:00, 09:00, 13:00, 17:00 and 21:00. Result showed that food intake greatly enhanced sodium/glucose cotransporter 1 (SGLT1) and glucose transporter type 2 (GLUT2) expressions, and improved glucose absorption, with highest levels at 09:00 a.m.. Then we designed iso-nitrogenous and iso-energetic diets with graded levels of glucose (10%, 20%, 30%, 40% and 50%) and NaCl (0%, 1%, 3% and 5%), and submitted to feeding trial for 10 weeks. The expressions of SGLT1 and GLUT2, brush border membrane vesicles (BBMVs) glucose transport and intestinal villus height were determined after the feeding trial. Increasing levels of dietary glucose and NaCl up-regulated mRNA and protein levels of SGLT1 and GLUT2, enhanced BBMVs glucose transport in the proximal, mid and distal intestine. As for histological adaptive response, however, high-glucose diet prolonged while high-NaCl diet shrank intestinal villus height. Furthermore, we also found that higher mRNA levels of SGLT1 and GLUT2, higher glucose transport capacity of BBMVs, and higher intestinal villus were detected in the proximal and mid intestine, compared to the distal part. Taken together, our study indicated that intestinal glucose absorption in carp was primarily occurred in the proximal and mid intestine, and increasing levels of dietary glucose and NaCl enhanced intestinal glucose absorption in carp.     

Effect of size of Trichinella spiralis (Nematode) infections on glucose and ion transport in the rat intestine

An in vivo perfusion technique, using 3 intestinal loops representing the anterior, mid and posterior regions of the rat small intestine, was used to determine intestinal glucose uptake 5 days after infection with Trichinella spiralis. At high levels of infection (3,000 and 6,000 larvae/rat) net glucose absorption by the intestinal mucosa was significantly impaired in all regions of the small intestine when compared to uninfected controls. At low levels of infection (50 larvae/rat) glucose uptake by the mucosa was significantly enhanced in all 3 regions of the small intestine. Intermediate levels of infections (200-1,000 larvae/rat) also enhanced glucose uptake, but only in the anterior regions of the small intestine. When washings from the small intestine of rats infected with 50 larvae/rat were added to the perfusion fluid used on uninfected rats, glucose uptake was also significantly enhanced. These results suggest that at low levels of infection the intestinal lumen contains a metabolite which may affect the mucosal transport of glucose and the related fluxes of H2O, Na+, Cl-, and K+, in the rat intestine. Luminal [H+] and pCO2 decreased from the proximal to distal regions of the small intestine following perfusion; pO2 was significantly decreased in the proximal and distal regions.     

Oxygen availability, dietary restriction and transport of glucose 3-O-methylglucose and fructose in the isolated small intestine of rat.

The influences of PO2 of the incubating medium on glucose, 3-O-methylglucose and fructose transport by everted small intestine sacs in semistarved and rats fed ad libitum (controls) was investigated. Moreover fructose uptake and conversion to glucose by intestinal sacs was also studied. The results showed that intestinal sacs from semistarved rats transported larger amounts of glucose and 3-O-methylglucose and took up more fructose than controls, when PO2 of the incubating medium was 150 mm Hg. There was greater fructose conversion to glucose in the intestine of semistarved rats than in controls at all PO2's considered. The greater functional capacity of intestinal tissue of semistarved rats in comparison to controls has been related to larger O2 availability in their intestinal wall.     

Glutathione involvement on the intestinal Na+-dependent D-glucose active transporter

Glutathione and its related enzymes are present in intestinal epithelium. Depletion or alteration of glutathione levels have been related to different physiological and pathological conditions. Glutathione also seems to be related to the regulation of some protein actvities. The present study, by in vivo experiments. shows a specific relationship between D-glucose Na⁺-dependent active transporter activit in rat intestine brush-border membranes and reduced glutathione/oxidized glutathione ratio levels. Changes of the kinetic parameters show that an increase of this ratio is related to an increase of the affinity of glucose for its binding sites and a higher transport capacity of the transporter. Neither alteration in the activity of other substrate transport systems nor change in the specific activity of the key enzymes related to glutathione and glucose metabolism are found. These findings suggest the possibility that D-glucose transporter activity is modulated through the change in the redox status of glutathione.     

Critical period programming of intestinal glucose transport via alterations in dietary fatty acid composition

Intestinal adaptation occurs in response to physiological or pathological processes that include resection, aging, diabetes, radiation, lactation, chronic alcohol feeding, and feeding diets of varying lipid, protein, or carbohydrate composition. The mechanisms involved in the adaptive response include alterations in morphology, cell kinetics, digestive enzyme activity, transport, membrane lipid composition, and enzymes involved in lipid metabolism. This discussion will review the effect of aging and alterations in dietary fatty acid composition on the small intestine. In addition, it may be possible to program the intestinal response to aging by feeding diets of differing fatty acid composition during the critical period of weaning. Alterations in the ratio of polyunsaturated to saturated fatty acids in the diet modifies the age-associated changes in the intestinal uptake of glucose. The changes occur rapidly, progressively, and irreversibly, suggesting that the intestinal uptake of glucose is subject to critical period programming. The mechanism by which diet may modify the ability of the intestine to up- or down-regulate glucose transport requires further investigation.     

Increase of (Na+ + K+)-activated ATPase activity of basolateral plasma membranes from intestinal mucosa of diabetic rats

A significant increase of the (Na+ + K+)-activated ATPase was found in mucosal homogenates of rat small intestine under conditions of alloxan and streptozotocin diabetes. From studies with isolated plasma membranes it has been shown that the activity changes were caused by that part of the (Na+ + K+)-activated ATPase only which is localized in the basolateral plasma membranes, whereas the enzyme activity in the brush border region remains unchanged. In connection with the enhanced capacity of ion, nonelectrolyte and water absorption in experimental diabetes, our findings support a concept of intestinal transport mechanism which suggest that the basolateral part of the (Na+ + K+)-activated ATPase is responsible for metabolic energy supply. The luminal part of the enzyme may be involved in regulation of passive Na+ influx.     

Pathophysiology of cestode infections: Effect of Hymenolepis diminuta on oxygen tensions,pH and gastrointestinal function

Studies on the normal and parasitized rat intestine were used to investigate the effect of the tapeworm, Hymenolepis diminuta, on in vivo intestinal lumenal oxygen tensions, acid-base balance and mucosal absorption and accumulation of fluid and glucose.The lumenal bulk aqueous phase is considerable, well mixed and aerobic with an oxygen tension of 40–50 mm Hg. Neither the unstirred layers adjacent to the brush border membrane nor the area adjacent to the mucosa (“paramucosal lumen”) are significant barriers to the diffusion of oxygen from the blood to the intestinal lumen. In the uninfected distal ileum and colon anoxic conditions may occur in the central lumen, but, in the parasitized intestine fluid absorption is reduced and anoxic conditions do not occur. Increased H⁺ ion concentration in the parasitized intestine plays a role in increasing the availability of oxygen to intestinal helminths. Concomitant with the lower pH, the pCO₂ in the lumen of the parasitized intestine was twice as high as that found in normal animals. The total CO₂ in the parasitized intestine steadily decreased over a 3-h perfusion period, while in the normal intestine the total CO₂ content increased after an initial fall during the first 30 min of perfusion. When the worms were removed, the ability of the intestine to restore normal acid-base balance was restored. Glucose and fluid absorption in both the infected and uninfected intestine were reduced by an increase in H⁺ ion concentration; both parameters were lower in the parasitized intestine than in the normal animals. Low pH increased fluid and glucose transport by H. diminuta.While the dry weights of both the parasitized and uninfected total small intestine and of the intestinal mucosa were the same, the wet weights were considerably different, indicating defective fluid balance in the infected intestine. Accumulation of glucose by the parasitized mucosa was greater than in control animals and decreased with an increase in H⁺ ion concentration. The glucose transport system in the parasitized gut was therefore affected at two levels, one at the brush border, where transport into the mucosa was decreased by lowering the pH, and secondly at the level of the basal and lateral membranes, where transport out of the mucosal tissue into the circulatory system was also reduced.The above results are discussed in terms of current widely accepted but erroneous concepts relating to the intestinal ‘microcosm’.     

The effect of insulin on glucose transport in rabbit erythrocytes and reticulocytes

Insulin binding and 3-0-Methylglucose transport have been studied in erythrocyte- and reticulocyte-enriched fractions of blood cells in order to determine if the increased number of insulin binding sites in reticulocytes is associated with a glucose transport response to insulin. In these experiments rabbit reticulocytes demonstrate an eightfold increase in total insulin receptors when compared to erythrocytes. Glucose transport activity in the erythrocyte has a K_m of 3.2 mM. Reticulocytes demonstrate a saturable glucose transport activity of lower affinity, K_m 18.9 mM. Neither the erythrocyte, nor the reticulocyte glucose transport activity, was capable of an increased response to insulin. The low affinity glucose transport activity in reticulocytes could allow a fourfold increase in facilitated glucose transport at supraphysiological glucose concentrations that might occur in poorly controlled diabetes mellitus.     

Glucagon-like peptide-2 protects against TPN-induced intestinal hexose malabsorption in enterally refed piglets

Premature infants receiving chronic total parenteral nutrition (TPN) due to feeding intolerance develop intestinal atrophy and reduced nutrient absorption. Although providing the intestinal trophic hormone glucagon-like peptide-2 (GLP-2) during chronic TPN improves intestinal growth and morphology, it is uncertain whether GLP-2 enhances absorptive function. We placed catheters in the carotid artery, jugular and portal veins, duodenum, and a portal vein flow probe in piglets before providing either enteral formula (ENT), TPN or a coinfusion of TPN plus GLP-2 for 6 days. On postoperative day 7, all piglets were fed enterally and digestive functions were evaluated in vivo using dual infusion of enteral ((13)C) and intravenous ((2)H) glucose, in vitro by measuring mucosal lactase activity and rates of apical glucose transport, and by assessing the abundances of sodium glucose transporter-1 (SGLT-1) and glucose transporter-2 (GLUT2). Both ENT and GLP-2 pigs had larger intestine weights, longer villi, and higher lactose digestive capacity and in vivo net glucose and galactose absorption compared with TPN alone. These endpoints were similar in ENT and GLP-2 pigs except for a lower intestinal weight and net glucose absorption in GLP-2 compared with ENT pigs. The enhanced hexose absorption in GLP-2 compared with TPN pigs corresponded with higher lactose digestive and apical glucose transport capacities, increased abundance of SGLT-1, but not GLUT-2, and lower intestinal metabolism of [(13)C]glucose to [(13)C]lactate. Our findings indicate that GLP-2 treatment during chronic TPN maintains intestinal structure and lactose digestive and hexose absorptive capacities, reduces intestinal hexose metabolism, and may facilitate the transition to enteral feeding in TPN-fed infants.     

Gestational diabetes mellitus: a syndrome with phenotypic and genotypic heterogeneity

One hundred ninety-nine gravida with gestational diabetes mellitus (GDM) defined as "carbohydrate intolerance of varying severity with onset or first recognition during pregnancy" have been stratified into subgroups on the basis of fasting plasma glucose and evaluated for further phenotypic and genotypic heterogeneity. A significantly greater proportion of the women in all our groups were older and heavier than in a "control" population of 148 consecutive gravida with documented normal oral glucose tolerance. After correction for age and weight by covariate analysis, absolute insulinopenia in response to oral glucose could be demonstrated in all GDM groups, although exceptions were present in each. The incidence of diabetes in the mothers of our patients with GDM was 8-fold greater than in controls; the incidence in fathers did not deviate from control patterns. HLA-DR3 and DR4 antigens were more frequently present in GDM and the increase was statistically significant in blacks. At the time of diagnosis, cytoplasmic islet cell antibodies (ICA) were significantly more common in GDM associated with elevated fasting plasma glucose than in controls; the frequency of ICA was 18.4% (7/38) in women with fasting plasma glucose greater than or equal to 130 mg/dl. Our findings indicate that GDM entails genotypic as well as phenotypic diversity and may include patients with slowly-evolving Type I diabetes mellitus, as well as patients with Type II diabetes mellitus, and women with asymptomatic diabetes which antedated the pregnancy (i.e. pregestational diabetes mellitus). Appreciation of this heterogeneity should be incorporated into any evaluation of intervention strategies for women with GDM or into prognoses concerning their postpartum metabolic status.     

Role of calcium in the phloretin effects on sugar transport in rat small intestine

The effect of phloretin on D-galactose transport in rat small intestine has been investigated. Phloretin enhanced tissue sugar accumulation and reduced mucosal to serosal D-galactose fluxes. Calcium-deprived bathing solutions and verapamil significantly reduced, but did not abolish, the phloretin-effects on intestinal galactose transport. Furthermore in the presence of the anticalmodulin drugs, RMI 12330A and trifluoperazine, phloretin was without effect on D-galactose transport. These findings suggest that phloretin may reduce serosal sugar permeability via an increase in Ca2+-calmodulin complex.     

The abnormality of glucose transporter in the erythrocyte membrane of Chinese type 2 diabetic patients

Type 2 diabetes mellitus is characterized by impaired glucose uptake. With a photometric method of recording the erythrocyte suspension absorption during the course of glucose transport across the membranes, we observed that the initial rate of glucose zero-trans entry was decreased significantly in 30 Chinese type 2 diabetic patients as compared to 25 healthy controls. The rate of glucose infinite-cis efflux exhibited no difference between the patients and controls. The measurement of temperature dependence of glucose transport showed that the activation energy for glucose entry was increased in diabetic patients. The inhibitory constant of glucose entry by cytochalasin B (CB) in patients was similar to that of the controls. However, we found that the inhibitory constant was increased significantly in the patient erythrocytes after phloretin treatment. After the erythrocytes were made into stripped white ghosts, the fluorescence quenching experiment was performed. Glucose, CB and phloretin can quench the fluorescence of tryptophan residues in the glucose transporter 1, GLUT1. The abnormality of fluorescence quenching in the erythrocyte membranes of patients was observed. The transfer tendency of tryptophan residues from the hydrophilic environment to the hydrophobic environment was decreased in patient ghosts as binding with glucose, and the opposite tendency appeared as CB and phloretin instead of glucose. We conclude that the decreased in glucose entry in the erythrocyte membranes of diabetic patients was due to the GLUT1 change in structure - mostly the outer domain of the glucose transporter.     

Effects of sugars on leucine and lysine uptake by intestinal cells from rats fed sucrose and stock diets.

The effect of various dietary sugars on the uptake of 1 mM leucine and 1 mM lysine by intestinal cells isolated from stock-fed and sucrose-fed rats was determined. Leucine uptake was activated by 10 mM fructose and inhibited by 10 mM glucose or 20 mM sucrose on both diets. The major dietary effect noted was a significant increase in the inhibition of leucine by glucose in the sucrose-fed rats. The uptake of lysine was minimally affected by the sugars irrespective of the diet fed. These results demonstrate an important dichotomy in the properties of glucose and fructose transport in the intestine and suggest that dietary fructose may increase the transport of certain amino acids.     

Enhanced glucose absorption in the rat small intestine following repeated doses of 5-fluorouracil

Many studies demonstrated that 5-fluorouracil (5-FU) treatment of rodents caused the damage of small intestine, resulting in the malabsorption, while we recently found that repeated administration of 5-FU to rats increased Na(+)-dependent glucose absorption in the small intestine. This study investigated the cause of enhanced glucose absorption. 3-O-methyl-d-glucose (3-OMG) absorption was examined using the everted intestine technique. d-Glucose uptake, phlorizin binding, Western blot analysis and membrane fluidity were examined using small intestinal brush-border membrane vesicles (BBMV). Repeated oral administration of 5-FU to rats increased Na(+)-dependent 3-OMG absorption in the small intestine, while alkaline phosphatase activity in the small intestine decreased. Na(+)/K(+)-ATPase activity of 5-FU-treated rats was about three-fold higher than that of control rats. Although the amount of Na(+)-dependent glucose co-transporter (SGLT1) in 5-FU-treated rats decreased, the overshoot magnitude of d-glucose uptake in BBMV was not altered. Maximum binding of phlorizin in 5-FU-treated rats was 1.5-fold larger than that of control rats, but not altered the maximal rate of d-glucose absorption, Michaelis constant of d-glucose and dissociation constant of phlorizin. The membrane fluidity of 5-FU-treated rats increased. The enhanced d-glucose absorption in 5-FU-treated rats seems to occur secondarily due to the activation of Na(+)/K(+)-ATPase activity in basolateral membranes (BLM). Because the amounts of SGLT1 in 5-FU-treated rats decreased, the increase of turnover rate of SGLT1 and/or an expression of unknown Na(+)-dependent glucose co-transporter with high affinity for d-glucose and phlorizin sensitivity would contribute to the enhancement of d-glucose transport in 5-FU-treated rats.     

The effects of insulin on glucose and fluid transport in the isolated small intestine of normal rats

Chronically administered insulin returns enhanced maximal glucose transport capacity induced by diabetes to its normal state. In this study, the direct and acute effects of insulin on glucose transport in different parts of isolated small intestine were investigated. Mucosal Fluid Transport (MFT), Mucosal Glucose Transport (MGT) and Serosal Glucose Transport (SGT) were measured in the presence and absence of insulin in averted sacs, prepared from female Wistar rats. This study shows that the presence of insulin in vitro (40 and 80 microU/mL) can reduce MGT and SGT in different segments of the small intestine (duodenum, jejunum and ileum) after 30 min whereas it had no effect on MFT. Mucosal glucose transfer rates in the duodenum, jejunum and ileum of the controls were 6.07+/-0.4, 6.34+/-0.62 and 6.43+/-0.47 mg/g tissue respectively which were significantly reduced to 3.82+/-0.93, 3.60+/-0.50 and 1.17+/-0.45 in the presence of 80 microU/mL of insulin. Serosal glucose transfer too was decreased significantly from 0.3+/-0.05, 0.57+/-0.07 and 0.43+/-.07 in the duodenum, jejunum and ileum to 0.16+/-0.03, 0.16+/-0.04 and .07+/-.02 respectively. Mucosal fluid transfer was not affected by insulin. Insulin was as effective whether it was added on the mucosal or the serosal side. The results of this study show that insulin can directly affect glucose transport in the small intestine; its physiological role must be examined. Direct effect of insulin deficiency on glucose absorption in diabetic patients may play a role in the pathophysiology of the disease.