The effect of gastric inhibitory polypeptide on intestinal glucose absorption and intestinal motility in mice

Gastric inhibitory polypeptide (GIP) is released from the small intestine upon meal ingestion and increases insulin secretion from pancreatic β cells. Although the GIP receptor is known to be expressed in small intestine, the effects of GIP in small intestine are not fully understood. This study was designed to clarify the effect of GIP on intestinal glucose absorption and intestinal motility. Intestinal glucose absorption in vivo was measured by single-pass perfusion method. Incorporation of [¹⁴C]-glucose into everted jejunal rings in vitro was used to evaluate the effect of GIP on sodium-glucose co-transporter (SGLT). Motility of small intestine was measured by intestinal transit after oral administration of a non-absorbed marker. Intraperitoneal administration of GIP inhibited glucose absorption in wild-type mice in a concentration-dependent manner, showing maximum decrease at the dosage of 50 nmol/kg body weight. In glucagon-like-peptide-1 (GLP-1) receptor-deficient mice, GIP inhibited glucose absorption as in wild-type mice. In vitro examination of [¹⁴C]-glucose uptake revealed that 100 nM GIP did not change SGLT-dependent glucose uptake in wild-type mice. After intraperitoneal administration of GIP (50 nmol/kg body weight), small intestinal transit was inhibited to 40% in both wild-type and GLP-1 receptor-deficient mice. Furthermore, a somatostatin receptor antagonist, cyclosomatostatin, reduced the inhibitory effect of GIP on both intestinal transit and glucose absorption in wild-type mice. These results demonstrate that exogenous GIP inhibits intestinal glucose absorption by reducing intestinal motility through a somatostatin-mediated pathway rather than through a GLP-1-mediated pathway.     

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.     

Correlation of LUMO localization with the alpha-amylase inhibition constant in a Tendamistat-based series of linear and cyclic peptides

The glycosidase alpha-amylase is responsible for the hydrolysis of alpha(1-->4) glycosidic linkages found in dietary starch as one means for controlling blood sugar level. The effect of alpha-amylase is detrimental, however, in the disease state diabetes mellitus, where blood glucose levels are elevated due to a biochemical defect. Inhibition of the enzyme's activity would reduce glucose absorption by the small intestine. Our objective was to develop small peptides based on essential binding elements of the natural protein inhibitor, Tendamistat. These smaller analogs may be better studied structurally and conformationally to help us understand molecular-level interactions. In addition, we have been able to correlate the activity of our compounds with the lowest unoccupied molecular orbital (LUMO) localization in energy-minimized conformations. The positive charge/LUMO of most active inhibitors is localized on the central Arg residue of the required triplet. This provides a predictive model for the design of active molecules.     

In vivo glucose metabolism in individual tissues of the rat. Interaction between epinephrine and insulin

The interaction between epinephrine and insulin in modulating in vivo glucose metabolism within individual tissues of the body has not previously been examined. This was investigated using the euglycemic hyperinsulinemic (120 milliunits/liter) clamp combined with administration of [3H]2-deoxyglucose and D-[U-14C]glucose. Epinephrine produced whole body insulin resistance due to increased hepatic glucose output and reduced peripheral glucose disposal. Despite elevated insulin levels liver glycogen content was reduced by 50% during epinephrine infusion (5 nM). However, this effect was transient, occurring predominantly during the initial 60 min of study. These effects were prevented during beta-adrenergic blockade with propranolol and potentiated during alpha 1-adrenergic blockade with prazosin. The most significant effect of epinephrine in peripheral tissues was increased glycogenolysis in both oxidative and glycolytic skeletal muscle. A significant reduction in insulin-mediated [3H]2-deoxyglucose uptake (30%) was evident in 5 of 9 muscles tested during epinephrine infusion. This effect was most pronounced in the more insulin-sensitive oxidative muscles. The latter effect was probably indirectly mediated via increased glycogenolysis--increased accumulation of metabolites--inhibition of hexokinase. In addition, it is evident that insulin-mediated glycogen synthesis occurred during epinephrine infusion. All effects of epinephrine on muscle glucose metabolism were prevented by propranolol but not prazosin. Similar effects to that observed in muscle were not evident in adipose tissue. It is concluded that epinephrine may override many of the actions of insulin in vivo, and most of these effects are mediated via the beta-adrenergic receptor. In the intact rat there may be a complex interaction between alpha- and beta-adrenergic effects in regulating hepatic glucose output.     

Effect of phloretin and phloridzin on properties of digestion and absorption in the rat small intestine

Experiments in vitro on everted sacs of rat small intestine have shown that phloretin (an inhibitor of basolatheral glucose GLUT2 transporter) added from mucosal side of the sacs decreases release of glucose from enterocytes into serosal fluid without changing glucose accumulation in tissue of the preparations. Addition of phloridzin (an inhibitor of Na⁺ and glucose co-transporter SGLT1) from mucosal side inhibited both glucose accumulation in the tissue and its release into serosal fluid. Unspecific effects of phloretin and phloridzin on activities of several digestive enzymes (in particular, alkaline phosphatase, amino peptidase, and glycyl-L-leucine dipeptidase) has been revealed in homogenates of the rat small intestine mucosa. In chronic experiments on rats, absorption of glycine from the isolated small intestinal loop was inhibited in the presence of phloretin in perfusate. The obtained results indicate that the experimental approach of inhibition of glucose absorption by phloretin used from mucosal side in vitro appears to give a significant overestimation of contribution of facilitated diffusion (with participation of the GLUT2 transporter inserted in the apical enterocyte membrane) to glucose transport across this membrane. Thus, the role of the GLUT2 transporter in the mechanism of glucose absorption in the small intestine under its physiological conditions does not seem to be as great as it is thought by the authors of the recently proposed hypothesis.     

Evaluation of Glucose Absorption Level in the Small Intestine of Different Rat Strains under Natural Conditions

The peculiarities of carbohydrate metabolism were studied in seven rat strains under conditions maximally approximating natural ones. The glucose absorption level in the small intestine was evaluated using a method based on ad libitum drinking of concentrated glucose solutions by prefasted (18–20 h) rats. It was shown that in the steady-state regime the volume-normalized uptake rate of glucose solution (mL/min) was constant and inversely proportional to the glucose concentration in the solution, while the uptake rate of glucose itself (μmol/min) was independent of the substrate concentration in quite a wide range, being mainly determined by the absorptive capacity of the small intestine. A significant difference was revealed between the tested rat strains in terms of the rate of glucose absorption from its solution (200 g/L). In the daytime (10 AM–4 PM), the highest rates were observed in Sprague Dawley rats (116.7 ± 3.1 μmol/min) while the lowest—in Wistar Kyoto rats (35.6 ± 1.1 μmol/min). In the evening (4–10 PM), rates of glucose absorption in different rat strains were 1.3–2.2 times higher than in the daytime. Apparently, the increased absorptive capacity of the small intestine in the evening is due to enhanced SGLT1-mediated active glucose transport and reflects the peculiarities of carbohydrate metabolism regulation in different rat strains.     

Dietary fructose vs glucose lowers copper solubility in the digesta in the small intestine of rats

The hypothesis was tested that dietary fructose vs glucose lowers copper solubility in the digesta in the small intestine of rats, which in turn causes a decreased copper absorption. Male rats were fed adequate-copper (5 mg Cu/kg) diets containing either fructose or glucose (709.4 g monosaccharide/kg) for a period of 5 wk. Fructose vs glucose significantly lowered copper concentrations in plasma and the liver, but did not alter hepatic copper mass. Fructose feeding resulted in a significantly lesser intestinal solubility of copper as based on either a smaller soluble fraction of copper in the liquid phase of small intestinal contents or a lower copper concentration in the liquid phase. The latter fructose effect can be explained by the observed fructose-induced increase in volume of liquid phase of intestinal digesta. After administration of a restricted amount of diet extrinsically labeled with⁶⁴Cu, rats fed fructose also had significantly lower soluble⁶⁴Cu fraction in the digesta of the small intestine. Although this study shows that fructose lowered intestinal copper solubility, only a slight reduction of apparent copper absorption was observed. It is suggested that the fructose-induced lowering of copper status in part counteracted the fructose effect on copper absorption at the level of the intestinal lumen.     

Nippostrongylus brasiliensis: malabsorption in experimentally infected rats

Glucose absorption and net small intestinal water movement were examined in rats infected with Nippostrongylus brasiliensis at Days 4, 6, 9, 13, and 19 after inoculation. Rats were infected with 4 X 10(3) N. brasiliensis third stage larvae. The entire small intestine was divided into three segments and each segment perfused simultaneously in vivo with Krebs-Ringer phosphate buffer containing 80 mM glucose, 6 X 10(5) dpm/ml [3H]glucose, and 6.2 X 10(3) dpm/ml [14C]polyethylene glycol. Rats perfused on Days 6, 9, 13, and 19 after inoculation showed a significant (P less than 0.05) decrease in glucose absorption rates from all three segments of the small intestine when compared to uninfected controls. In the three segments of uninfected rat small intestine and those perfused on Days 4, 13, and 19 after inoculation, net absorption of water occurred. However, in the proximal and distal segments perfused on Day 6 and the proximal segment perfused on Day 9, net water movement into the lumen occurred. This is the first report of depressed glucose absorption along the entire length of the small intestine during nippostrongylosis and contradicts previous reports of unaltered net glucose absorption in response to this parasite.     

The influence of protein starvation on hydrolytic and transport characteristics of the rat small intestine in chronic experiments

Time dynamics of maltose, glycylglycine, glucose, and glycine hydrolysis and absorption in isolated loop of the small intestine was studied in chronic experiments on Wistar rats (group 1) after their transition from the standard diet to the protein-free one with enhanced content of carbohydrates. During protein starvation, there were different changes in the rates of glucose and glycine absorption, and glycylglycine hydrolysis and absorption in isolated intestinal loop, but to the end of the 2nd week they returned to the initial levels (for glucose and glycylglycine) or increased (for glycine). The rates of maltose hydrolysis and derived glucose absorption remained at the initial levels for the first days of protein starvation, decreased on the 5th day, and did not change afterwards. Maltase, alkaline phosphatase, and amino peptidase M activities, determined in homogenates of the small intestinal mucosa (per g of the tissue) after 2 weeks of protein starvation, were lower in the rats of group 1 in comparison with the rats of group 2, kept on the standard diet. Thus, under protein deficiency the hydrolytic and absorptive capacities of the small intestine correspond to both ingested food composition, and body requirements.     

Studies of the interaction between glucagon and alpha-adrenergic agonists in the control of hepatic glucose output

alpha-Adrenergic stimulation of hepatocytes prevented, in a dose-dependent manner, the stimulation of [U-14C]lactate conversion to [14C]glucose by glucagon and exogenously added cAMP and Bt2cAMP. The inhibition was referable to an interaction with adrenergic receptors which resulted in a small decrease in hepatic cAMP levels. Low concentrations of epinephrine (10 nM) were able to inhibit phosphorylase activation and glucose output elicited by low doses of glucagon (5 X 10(-11) M to 2 X 10(-10) M). The ability of epinephrine (acting via alpha 1-adrenergic receptors), vasopressin, and angiotensin II to elicit calcium efflux was inhibited by glucagon, suggesting that intracellular redistributions of Ca2+ are importantly involved in the gluconeogenic process. It is proposed that vasopressin, angiotensin II, and catecholamines, acting primarily via alpha 1-adrenergic receptors, are responsible for inhibition of glucagon mediated stimulation of gluconeogenesis by altering subcellular calcium redistribution and decreasing cAMP levels.     

Alpha 2-adrenergic inhibition of pancreatic islet glucose utilization is mediated by an inhibitory guanine nucleotide regulatory protein

The rate of glucose utilization in isolated pancreatic islets of the rat was inhibited by the alpha 2-adrenoceptor agonists clonidine and epinephrine. Yohimbine reversed the inhibition. alpha 1 or beta-adrenoceptor agonists had little or no effect on glucose utilization. Stimulation of muscarinic receptors by carbamylcholine reversed the effect of clonidine. Pertussis toxin blocked the effect of clonidine on glucose utilization, and potentiated the response to carbamylcholine. 8-Bromo-cAMP did not affect glucose utilization in the presence of clonidine. Thus, alpha 2-adrenoceptors negatively modulate glucose utilization, and the effect is mediated by an inhibitory guanine nucleotide regulatory protein, but not by cAMP.     

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.     

Study of glucose concentrated solution consumption in rats and simulation of glucose distribution along the intestine

Free ingestion of glucose solution (200 or 400 g/l) by Wistar rats, previously starved for 18-20 Hrs, was investigated in two groups of the animals: with intact small intestine (group 1, n = 9), and a shortened small intestine following the Thiry-Wella isolation of its one third proximal part (group 2, n = 9). In the rats of the group 2, the isolated intestinal loops were perfused in chronic experiments with soulutions of different glucose concentrations to estimate a permeability of the pre-epithelial ("unstirred") layer and "true" kinetic constants of glucose active transport. The rate of glusouse ingestion was found to be 1.3-fold as high in the of rats fgroup 1 than in the rats of group 2 (p < 0.01). According to results of mathematical modeling, the rate of glucose ingestion by rats corresponds to glucose concentration in the initial solutions and to the absorbing capacity of the small intestine due to the substrate regulation of gastric emptying. The model predicts that, during free ingestion by rats of 400 g/l (2200 mM) glucose solution, the substrate concentration in the intestinal lumen under steady state conditions hardly exceeds 75 mM. This fact contradicts a recently proposed hypothesis about a facilitated transport mediated by GLUT2 as the main mechanism of glucose absorption in the small intestine under normal conditions.     

Islet cyclic AMP levels are not lowered during alpha 2-adrenergic inhibition of insulin release

Epinephrine-induced changes in insulin release and cyclic AMP levels were measured simultaneously in isolated rat islets. Forskolin was used to enhance islet cyclic AMP levels. Forskolin (30 microM) stimulated adenylate cyclase activity 10-fold in islet homogenates and raised cyclic AMP levels 5-fold in intact islets (both at low and high glucose). Insulin release was enhanced by forskolin only at high glucose. Epinephrine (0.1 microM) inhibited glucose- and forskolin-induced insulin release to basal rates. At the same time epinephrine potentiated forskolin-elevated cyclic AMP levels. In contrast epinephrine attenuated forskolin-stimulated adenylate cyclase activity in islet homogenates. At low glucose, both alpha 2- and beta-adrenergic blockade counteracted the epinephrine potentiation, each by 50%. At high glucose the effect was mainly beta-adrenergic in nature. The actions of epinephrine in the presence of a beta-blocker were mimicked by the alpha 2-agonist clonidine. Despite the variations in cyclic AMP levels stimulated insulin release was always inhibited by activation of alpha 2-receptors. Finally, insulin release stimulated by exogenous cyclic AMP was abolished by epinephrine. These results suggest that epinephrine inhibits insulin release at a step distal to the generation of cyclic AMP.     

Effect of DG5128 on epinephrine and glucagon induced glucose output from the isolated perfused rat liver

The effect of a specific alpha 2-adrenergic antagonist 2-[2-(4,5-dihydro-1.H-imidazol-2-yl)-1-phenyl-ethyl] pyridine dihydrochloride sesquihydrate (DG5128), on the glucose output by epinephrine and/or glucagon was studied using the perfused rat liver. The administration of DG5128 alone did not affect the glucose output. However, DG5128 produced a significant inhibition of the increased glucose output when induced by 10(-6) M epinephrine alone or 10(-6) M epinephrine plus 1.4 x 10(-10) M glucagon. There were no significant changes of the glucose output by 1.4 x 10(-10) M or 7.0 x 10(-11) M glucagon alone. On the other hand, addition of 1 mU/ml insulin to the perfusate suppressed the 7.0 x 10(-11) M glucagon-induced glucose output, but failed to decrease the 1.4 x 10(-10) M glucagon effect. DG5128 suppressed further the glucagon (7.0 x 10(-11) M)-induced increase of glucose output in the presence of insulin. These results suggest that DG5128 produces a hypoglycemic effect partly through an inhibition of the increased hepatic glucose output elicited by epinephrine and glucagon.     

Kinetics of intestinal sugar transport, in vivo

Sugar absorption by the small intestine has been studied in rat and hamster in vivo, with luminal perfusion, during 1 minute successive periods. Transport is calculated as the difference between absorption and diffusion. The diffusion component is evaluated in the presence of phlorizin or as absorption of sorbose. The resulting KT values for glucose and galactose (rat: 7.7 and 10 mM; hamster: 10 and 14 mM) and 3-0-methyl-glucose (hamster: 25-33 mM) are quite lower than those previously obtained in vivo, but still higher than those in vitro. The physiological levels of glucose in the intestine of normally fed animals imply that the diffusion component plays an important role in the proximal regions of the small intestine, especially in rat.     

The acute regulation of glucose absorption, transport and metabolism in rat small intestine by insulin in vivo.

The effect of acute changes in insulin concentrations in vivo on the absorption, transport and metabolism of glucose by rat small intestine in vitro was investigated. Within 2 min of the injection of normal anaesthetized rats with anti-insulin serum, lactate production and glucose metabolism were respectively diminished to 28% and 21% of normal and the conversion of glucose into lactate became quantitative. These changes correlated with the inhibition of two mucosal enzymes, namely the insulin-sensitive enzyme pyruvate dehydrogenase, and phosphofructokinase, which was shown by cross-over measurements to be the rate-limiting enzyme of glycolysis in mucosa. The proportion of glucose translocated unchanged from the luminal perfusate to the serosal medium was simultaneously increased from 45% to 80%. All the changes produced by insulin deficiency were completely reversed with 2 min when antiserum was neutralized by injection of insulin in vivo. The absorption and transport of 3-O-methylglucose were unaffected by insulin. It is concluded that glucose metabolism in rat small intestine is subject to short-term regulation by insulin in vivo and that glucose absorption and transport are regulated indirectly in response to changes in metabolism. Moreover, transport and metabolism compensate in such a way as to deliver the maximal 'effective' amount of glucose to the blood, whether as glucose itself or as lactate for hepatic gluconeogenesis.     

Effect of pea and lentil lectins on in vitro absorption of nutrients

In vitro absorption of nutrients like glucose, leucine, protein hydrolysate and Ca2+ by ligated loops of small intestine was significantly affected in presence of lectins from peas and lentils. Except for sucrose, all other nutrients showed significant decrease in their absorption in presence of lectins. Lentil lectins had a greater inhibitory effect than pea lectins.     

Non-starch polysaccharides extracted from seaweed can modulate intestinal absorption of glucose and insulin response in the pig

We have investigated the possible effects of algal polysaccharides on postprandial blood glucose and insulin responses in an animal model, the pig. Three seaweed fibres of different viscosities, extracted from Palmaria palmata (PP), Eucheuma cottonii (EC), or Laminaria digitata (LD), were compared to purified cellulose (CEL). Blood glucose and plasma insulin levels were monitored and intestinal absorption quantified for 8 h following a high carbohydrate test-meal supplemented with 5% fibre. Digestive contents were also sampled, 5 h postprandial. As compared to CEL, PP had no effect on glucose and insulin responses. The latter decreased with EC, but glucose absorption balance was not modified. LD addition resulted in a dramatically reduced glucose absorption balance, accompanied by a higher amount of starch left in the small intestine. Among polysaccharides tested, only the highly viscous alginates could affect intestinal absorption of glucose and insulin response.