Sodium-coupled Glucose Transporter Research Articles

Chemosensing in enteroendocrine cells: mechanisms and therapeutic opportunities.

Enteroendocrine cells (EECs) are scattered chemosensory cells in the intestinal epithelium that release hormones with a wide range of actions on intestinal function, food intake and glucose homeostasis. The mechanisms by which gut hormones are secreted postprandially, or altered by antidiabetic agents and surgical interventions are of considerable interest for future therapeutic development. EECs are electrically excitable and express a repertoire of G-protein coupled receptors that sense nutrient and nonnutrient stimuli, coupled to intracellular Ca2+ and cyclic adenosine monophosphate. Our knowledge of EEC function, previously developed using mouse models, has recently been extended to human cells. Gut hormone release in humans is enhanced by bariatric surgery, as well as by some antidiabetic agents including sodium-coupled glucose transporter inhibitors and metformin. EECs are important potential therapeutic targets. A better understanding of their chemosensory mechanisms will enhance the development of new therapeutic strategies to treat metabolic and gastrointestinal diseases.

P1010EFFECTS OF SGLT2 INHIBITORS ON DIABETIC NEPHROPATHY: PODOCYTURIA ON FOCUS

Abstract Background and Aims Sodium-glucose co-transporter 2 (SGLT2) inhibitors which are a class of oral anti-diabetic drugs are being used in the treatment of type 2 diabetes (T2D). Basically these drugs reduce blood glucose by blocking glucose reabsorption of primary sodium-coupled glucose transporter in the proximal tubules. Diabetic nephropathy (DN) is a progressive disease with the pathology of glomerular filtration barrier damage and loss of podocytes. In this respect, podocyturia which is the case characterized with presence of podocytes in urine have recently been considered to be a candidate marker for glomerular damage. Therefore, we aimed to investigate effects of SGLT2 inhibitors on glomerulus through the evaluation of podocyturia in DN patients. Method Our study population was composed of two T2D patient groups; one of which received SGLT2 inhibitor (SGLT2 group) and the other one is control which did not received. Patients with T2D for at least 5 years, diagnosed with DN and having microalbuminuria >30 mg/day, glomerular filtration rate (GFR) >60 ml/min and HbA1c< 8.5 were involved. In addition, all patients had received angiotensin converting enzyme inhibitor or angiotension receptor blocker for at least 6 months. Nephrologic parameters of patients were monitored before and 3rd and 6th month of follow up period. Patients’ age, sex, diabetes duration and HbA1c value were obtained from medical charts. GFR was calculated using the abbreviated MDRD formula. Microalbuminuria was measured in 24 hour urine. Number of podocytes in the urine was determined by immunocytochemical staining of two different markers as podocalyxin (podx) and synaptopodin (synp). Statistical analyses were carried out using Statistical Package for the Social Sciences version (SPSS) 24.0 and statistical significance was set as p<0.05. Results We evaluated a total of 29 patients (mean age: 57.59 ± 9.66). The number of patients for SGLT2 and control groups are 18 (mean age: 54.89 ± 7.73) and 11 (mean age: 62.00 ± 11.60) respectively. Between these two groups, there was no statistical difference with respect to durations of diabetes (p=0.875) and HbA1c (p=0.05) values except for age (p=0.023) in which control group is statistically higher than the SGLT receiving group. Among the patients receiving SGLT2 inhibitor, podocyturia and microalbuminuria (mca) levels in the 6th month of follow up period were statistically lower than the initial ones [p=0.039 (synp), p=0.016 (podx) and p=0.011 (mca) respectively] while there was no statistical difference for creatinine clearance rate between the initial and follow up period (p:0.371 and ;p=0.111) (Figure-1). For control group, although podocyturia was significantly reduced in the 6th month of follow up period [p=0.013 (synp) and p:0.005 (podx)], microalbuminuria (p=0.263) in addition to creatinine clearance (p:0.214; p=0.173) did not change significantly (p=0.263). Furthermore, while there was no significant difference for initial microalbuminuria level between the control and SGLT2 groups (p=0.353); in the 3rd and 6th month follow up period, microalbuminuria was significantly lower than the control group accordingly (p=0.011 and p=0.015 respectively). Conclusion According to our preliminary results; patients receiving SGLT2 inhibitors had better microalbuminuria levels than the ones did not and this parameter was correlated with podocyturia. However, even though microalbuminuria of the control group did not change during the follow up period, podocyturia was found out as reduced in the 6th month of follow up period. This discrepancy in the podocyturia level could be resulted from lower number of patients in control so that there is need for evaluation of more patients to emphasize podocyte protective effect of SGLT2 inhibitors.

Rosiglitazone Phosphorylates Akt, Reduces Caspase 3 Activity and Enhances Mitochondrial Function in frozen Thawed Stallion Spermatozoa

In horses, strangulation of the intestine often leads to hypoxia in jejunal loops resulting in irreversible lesions of the epithelium. This contrasts with the common view that this epithelium is able to adapt to different oxygenation conditions well. To get a better understanding of the pathomechanisms of hypoxic injury, our study aimed to investigate functional adaptation processes in equine jejunum epithelium. Therefore, we incubated jejunal epithelia of horses in Ussing chambers and simulated hypoxia by gassing with 1% oxygen or by incubation with the chemical hypoxia-mimetic CoCl2. Because glucose is an important fuel for the cells' metabolism and thus its import is even more important under hypoxia, we assessed glucose transport mechanisms by measuring its electrogenic transport via sodium-coupled glucose transporter (SGLT1) and the transepithelial transport of 14C-glucose and 14C-ortho-methyl-glucose (OMG) as well as mRNA expression of glucose transporters. We could show that electrogenic uptake of glucose into the epithelium via SGLT1 was downregulated under hypoxia as was the mRNA expression of SGLT1 after CoCl2 incubation. However, transepithelial transport of glucose and OMG was not altered. Thus, there must be alterations in transepithelial transport mechanisms securing an energy-independent uptake of glucose into the epithelial cells. We believe that these changes are probably mediated on protein level as we found only effects of prolonged CoCl2 incubation on gene expression while the functional effects observed occurred much earlier.

Coping With Hypoxia: Adaptation of Glucose Transport Mechanisms Across Equine Jejunum Epithelium

In horses, strangulation of the intestine often leads to hypoxia in jejunal loops resulting in irreversible lesions of the epithelium. This contrasts with the common view that this epithelium is able to adapt to different oxygenation conditions well. To get a better understanding of the pathomechanisms of hypoxic injury, our study aimed to investigate functional adaptation processes in equine jejunum epithelium. Therefore, we incubated jejunal epithelia of horses in Ussing chambers and simulated hypoxia by gassing with 1% oxygen or by incubation with the chemical hypoxia-mimetic CoCl2. Because glucose is an important fuel for the cells' metabolism and thus its import is even more important under hypoxia, we assessed glucose transport mechanisms by measuring its electrogenic transport via sodium-coupled glucose transporter (SGLT1) and the transepithelial transport of 14C-glucose and 14C-ortho-methyl-glucose (OMG) as well as mRNA expression of glucose transporters. We could show that electrogenic uptake of glucose into the epithelium via SGLT1 was downregulated under hypoxia as was the mRNA expression of SGLT1 after CoCl2 incubation. However, transepithelial transport of glucose and OMG was not altered. Thus, there must be alterations in transepithelial transport mechanisms securing an energy-independent uptake of glucose into the epithelial cells. We believe that these changes are probably mediated on protein level as we found only effects of prolonged CoCl2 incubation on gene expression while the functional effects observed occurred much earlier.

A microfluidic renal proximal tubule with active reabsorptive function.

In the kidney, the renal proximal tubule (PT) reabsorbs solutes into the peritubular capillaries through active transport. Here, we replicate this reabsorptive function in vitro by engineering a microfluidic PT. The microfluidic PT architecture comprises a porous membrane with user-defined submicron surface topography separating two microchannels representing a PT filtrate lumen and a peritubular capillary lumen. Human PT epithelial cells and microvascular endothelial cells in respective microchannels created a PT-like reabsorptive barrier. Co-culturing epithelial and endothelial cells in the microfluidic architecture enhanced viability, metabolic activity, and compactness of the epithelial layer. The resulting tissue expressed tight junctions, kidney-specific morphology, and polarized expression of kidney markers. The microfluidic PT actively performed sodium-coupled glucose transport, which could be modulated by administration of a sodium-transport inhibiting drug. The microfluidic PT reproduces human physiology at the cellular and tissue levels, and measurable tissue function which can quantify kidney pharmaceutical efficacy and toxicity.

Glucose transport across lagomorph jejunum epithelium is modulated by AMP-activated protein kinase under hypoxia.

The gastrointestinal epithelium possesses adaptation mechanisms to cope with huge variations in blood flow and subsequently oxygenation. Since sufficient energy supply is crucial under hypoxic conditions, glucose uptake especially must be regulated by these adaptation mechanisms. Therefore, we investigated glucose transport under hypoxic conditions. Jejunal epithelia of rabbits were incubated in Ussing chambers under short-circuit current conditions. Hypoxia was simulated by gassing with 1% O2 instead of 100% O2. The activity of sodium-coupled glucose transporter-1 (SGLT-1) was assessed by measuring the increase of short circuit current ( Isc) after the addition of 2 mM glucose to the mucosal buffer solution. We observed decreased activity of SGLT-1 after hypoxia compared with control conditions. To investigate underlying mechanisms, epithelia were exposed to agonists and antagonists of AMP-activated protein kinase (AMPK) before assessment of SGLT-1-mediated transport and the pAMPK/AMPK protein ratio. Preincubation with the antagonist restored SGLT-1 activity under hypoxic conditions to the level of control conditions, indicating an involvement of AMPK in the downregulation of SGLT-1 activity under hypoxia, which was confirmed in Western blot analysis of pAMPK/AMPK. Transepithelial flux studies using radioactively labeled glucose, ortho-methyl-glucose, fructose, and mannitol revealed no changes after hypoxic incubation. Therefore, we could exclude a decreased transepithelial glucose transport rate and increased paracellular conductance under hypoxia. In conclusion, our study hints at a decreased activity of SGLT-1 under hypoxic conditions in an AMPK-dependent manner. However, transepithelial transport of glucose is maintained. Therefore, we suggest other transport mechanisms, especially glucose transporter 1 and/or 2 to substitute SGLT-1 under hypoxia. NEW & NOTEWORTHY To our knowledge, this is the first approach to simulate hypoxia and study its effects in the jejunal epithelium using the Ussing chamber technique. We were able show that AMPK plays a role in the downregulation of SGLT-1 and that there seems to be an upregulation of other glucose transport mechanisms in the apical membrane of lagomorph jejunum epithelium under hypoxia, securing the epithelial energy supply and thus integrity.

The flavanone homoeriodictyol increases SGLT-1-mediated glucose uptake but decreases serotonin release in differentiated Caco-2 cells.

Flavanoids and related polyphenols, among them hesperitin, have been shown to modulate cellular glucose transport by targeting SGLT-1 and GLUT-2 transport proteins. We aimed to investigate whether homoeriodictyol, which is structurally related to hesperitin, affects glucose uptake in differentiated Caco-2 cells as a model for the intestinal barrier. The results revealed that, in contrast to other polyphenols, the flavanon homoeriodictyol promotes glucose uptake by 29.0 ± 3.83% at a concentration of 100 μM. The glucose uptake stimulating effect was sensitive to phloridzin, but not to phloretin, indicating an involvement of the sodium-coupled glucose transporter SGLT-1, but not of sodium-independent glucose transporters (GLUT). In addition, in contrast to the increased extracellular serotonin levels by stimulation with 500 mM D-(+)-glucose, treatment with 100 μM homoeriodictyol decreased serotonin release by –48.8 ± 7.57% in Caco-2 cells via a phloridzin-sensitive signaling pathway. Extracellular serotonin levels were also reduced by –57.1 ± 5.43% after application of 0.01 μM homoeriodictyol to human neural SH-SY5Y cells. In conclusion, we demonstrate that homoeriodictyol affects both the glucose metabolism and the serotonin system in Caco-2 cells via a SGLT-1-meditated pathway. Furthermore, the results presented here support the usage of Caco-2 cells as a model for peripheral serotonin release. Further investigations may address the value of homoeriodictyol in the treatment of anorexia and malnutrition through the targeting of SGLT-1.

Mechanisms underlying glucose-dependent insulinotropic polypeptide and glucagon-like peptide-1 secretion.

The incretin hormones, glucose‐dependent insulinotropic peptide and glucagon‐like peptide‐1, are secreted from intestinal K‐ and L cells, respectively, with the former being most abundant in the proximal small intestine, whereas the latter increase in number towards the distal gut. Although an overlap between K‐ and L cells can be observed immunohistochemically or in murine models expressing fluorescent markers under the control of the two hormone promoters, the majority (>80%) of labeled cells seems to produce only one of these hormones. Transcriptomic analysis showed a close relationship between small intestinal K‐ and L cells, and glucose sensing mechanisms appear similar in both cell types with a predominant role of electrogenic glucose uptake through sodium‐coupled glucose transporter 1. Similarly, both cell types produce the long‐chain fatty acid sensing G‐protein‐coupled receptors, FFAR1 (GPR40) and FFAR4 (GPR120), but differ in the expression/functionality of other lipid sensing receptors. GPR119 and FFAR2/3, for example, have clearly documented roles in glucagon‐like peptide‐1 secretion, whereas agonists for the endocannabinoid receptor type 1 have been found to show largely selective inhibition of glucose‐dependent insulinotropic peptide secretion. In conclusion, although K‐ and L cell populations overlap and share key molecular nutrient‐sensing mechanisms, subtle differences between the responsiveness of the different cell types might be exploited to differentially modulate glucose‐dependent insulinotropic peptide or glucagon‐like peptide‐1 secretion.

Stimulation of incretin secreting cells.

The incretin hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon like peptide-1 (GLP-1) are secreted from enteroendocrine cells in the gut and regulate physiological and homeostatic functions related to glucose control, metabolism and food intake. This review provides a systematic summary of the molecular mechanisms underlying secretion from incretin cells, and an understanding of how they sense and interact with lumen and vascular factors and the enteric nervous system through transporters and G-protein coupled receptors (GPCRs) present on their surface to ultimately culminate in hormone release. Some of the molecules described below such as sodium coupled glucose transporter 1 (SGLT1), G-protein coupled receptor (GPR) 119 and GPR40 are targets of novel therapeutics designed to enhance endogenous gut hormone release. Synthetic ligands at these receptors aimed at treating obesity and type 2 diabetes are currently under investigation.

Extracts and flavonoids from onion inhibit the intestinal sodium-coupled glucose transporter 1 (SGLT1) in vitro but show no anti-hyperglycaemic effects in vivo in normoglycaemic mice and human volunteers

Extracts and flavonoids from onion are described as having anti-diabetic activities. We here demonstrate that an onion extract and individual flavonoids thereof diminish glucose uptake mediated by the intestinal glucose transporter SGLT1 when expressed in oocytes and studied in mouse intestinal segments in vitro. Strongest inhibition of SGLT1 was observed for quercetin-4′-O-glucoside (Q4′glc) in oocytes but with only moderate inhibition in jejunal segments of mice. An oral glucose tolerance test (OGTT) performed in obese/hyperglycaemic mice revealed that the onion extract to reduce blood glucose increases significantly. However, an OGTT performed in healthy volunteers after administration of the onion extract failed to reveal an effect on glucose and insulin levels. Despite its capability to inhibit intestinal glucose uptake via SGLT1 in vitro and in mice in vivo, the onion extract did not alter blood glucose levels during an OGTT in human volunteers and this may predominantly be due to a dosing effect.

SGLT-2 inhibition and glucagon: Cause for alarm?

Recent studies raised the alarm that the inhibition of sodium-coupled glucose transporter type-2 in humans increases endogenous glucose production rates by an unclear mechanism. Surprisingly, a potential explanation may be linked directly to the alpha-cell. Is this a mechanistic spoiler or an added benefit?

Low crude protein diets modulate intestinal responses in weaned pigs challenged withEscherichia coliK88

Opapeju, F. O., Rodriguez-Lecompte, J. C., Rademacher, M., Krause, D. O. and Nyachoti, C. M. 2015. Low crude protein diets modulate intestinal responses in weaned pigs challenged with Escherichia coli K88. Can. J. Anim. Sci. 95: 71–78. Effects of dietary crude protein (CP) content on intestinal indicators of infection and diarrhea in pigs challenged with enterotoxigenic Escherichia coli (ETEC) K88 were investigated. Forty piglets [body weight (BW)=6.96±0.45, mean±SD], housed four per pen, were randomly allotted to two diets (five pens per diet): a 22.2% CP or a 17.3% CP supplemented with amino acids. Diets contained the same amount of standardized ileal digestible Lys, Met+Cys, Thr, Trp based on the ideal amino acid ratio. Isoleucine and Val were added to the 17.3% CP diet up to the level in the 22.2% CP diet. All other nutrients were as per National Research Council (1998) specification. Three piglets per pen were serially slaughtered on days 3, 5, and 7 after weaning for evaluation of intestinal hydrolases (data reported elsewhere). On day 8 post-weaning, the remaining pigs were inoculated with 6 mL of ETEC suspension (1010CFU mL−1) and slaughtered 20 h later. Mucosal-associated ETEC was detected in higher counts (3.17±0.63 log10CFU g−1digesta) in 80% of pigs fed the 22.2% CP diet compared with 20% of those fed the 17.3% CP diet in which the counts were also lower (2.00±log10CFU g−1digesta). Pigs fed the 22.2% CP diet tended (P=0.09) to have fewer goblet cells with sialomucins in jejunal villi compared with those fed the 17.3% CP diet. The expression of toll-like receptors 4 and 5 was unaffected by diet but the expression of sodium-coupled glucose transporter 1 was higher (P=0.04) in the jejunum of pigs fed the 22.2% CP diet compared with those fed the 17.3% CP diet. The results suggest that feeding a low-CP diet decreases ETEC proliferation and attachment in the intestinal mucosa and this is accompanied by a reduced expression of sodium-coupled glucose transporter 1.

Inhibition of the intestinal sodium-coupled glucose transporter 1 (SGLT1) by extracts and polyphenols from apple reduces postprandial blood glucose levels in mice and humans.

There is a growing interest in food constituents that could reduce intestinal glucose absorption to prevent overshooting plasma glucose and insulin levels in patients with prediabetes and diabetes mellitus type 2. We here demonstrate that an extract and individual polyphenols from apple diminish sodium-coupled glucose transporter 1 (SGLT1) mediated glucose uptake in vitro and in vivo. Inhibition of transport of sugars by SGLT1 was shown in Xenopus oocytes and in mice jejunal segments. Strongest inhibition was observed for phlorizin with IC50 values for transport inhibition of 0.46 ± 0.19 and 4.1 ± 0.6 μM in oocytes and intestinal segments, respectively. An oral glucose tolerance test performed in volunteers with prior administration of the apple extract reduced venous blood glucose and plasma insulin levels, similar to findings obtained in C57BL/6N mice. Analysis of human urine samples revealed that the extract increased modestly renal glucose loss that is most likely a result of inhibition of renal glucose reabsorption by phloretin derivatives found in plasma of the volunteers. Although the apple extract substantially decreased intestinal glucose absorption in all test systems, the finding that there are systemic effects that relate to inhibition of glucose transport processes beyond the intestine addresses safety issues that need further exploitation.

Glucose stimulates calcium-activated chloride secretion in small intestinal cells

The sodium-coupled glucose transporter-1 (SGLT1)-based oral rehydration solution (ORS) used in the management of acute diarrhea does not substantially reduce stool output, despite the fact that glucose stimulates the absorption of sodium and water. To explain this phenomenon, we investigated the possibility that glucose might also stimulate anion secretion. Transepithelial electrical measurements and isotope flux measurements in Ussing chambers were used to study the effect of glucose on active chloride and fluid secretion in mouse small intestinal cells and human Caco-2 cells. Confocal fluorescence laser microscopy and immunohistochemistry measured intracellular changes in calcium, sodium-glucose linked transporter, and calcium-activated chloride channel (anoctamin 1) expression. In addition to enhancing active sodium absorption, glucose increased intracellular calcium and stimulated electrogenic chloride secretion. Calcium imaging studies showed increased intracellular calcium when intestinal cells were exposed to glucose. Niflumic acid, but not glibenclamide, inhibited glucose-stimulated chloride secretion in mouse small intestines and in Caco-2 cells. Glucose-stimulated chloride secretion was not seen in ileal tissues incubated with the intracellular calcium chelater BAPTA-AM and the sodium-potassium-2 chloride cotransporter 1 (NKCC1) blocker bumetanide. These observations establish that glucose not only stimulates active Na absorption, a well-established phenomenon, but also induces a Ca-activated chloride secretion. This may explain the failure of glucose-based ORS to markedly reduce stool output in acute diarrhea. These results have immediate potential to improve the treatment outcomes for acute and/or chronic diarrheal diseases by replacing glucose with compounds that do not stimulate chloride secretion.

Glucocorticoids Enhance Intestinal Glucose Uptake Via the Dimerized Glucocorticoid Receptor in Enterocytes

Glucocorticoid (GC) treatment of inflammatory disorders, such as inflammatory bowel disease, causes deranged metabolism, in part by enhanced intestinal resorption of glucose. However, the underlying molecular mechanism is poorly understood. Hence, we investigated transcriptional control of genes reported to be involved in glucose uptake in the small intestine after GC treatment and determined effects of GC on electrogenic glucose transport from transepithelial currents. GR(villinCre) mice lacking the GC receptor (GR) in enterocytes served to identify the target cell of GC treatment and the requirement of the GR itself; GR(dim) mice impaired in dimerization and DNA binding of the GR were used to determine the underlying molecular mechanism. Our findings revealed that oral administration of dexamethasone to wild-type mice for 3 d increased mRNA expression of serum- and GC-inducible kinase 1, sodium-coupled glucose transporter 1, and Na(+)/H(+) exchanger 3, as well as electrogenic glucose transport in the small intestine. In contrast, GR(villinCre) mice did not respond to GC treatment, neither with regard to gene activation nor to glucose transport. GR(dim) mice were also refractory to GC, because dexamethasone treatment failed to increase both, gene expression and electrogenic glucose transport. In addition, the rise in blood glucose levels normally observed after GC administration was attenuated in both mutant mouse strains. We conclude that enhanced glucose transport in vivo primarily depends on gene regulation by the dimerized GR in enterocytes, and that this mechanism contributes to GC-induced hyperglycemia.

Structural and Functional Characterization of the C-terminal Transmembrane Region of NBCe1-A

NBCe1-A and AE1 both belong to the SLC4 HCO(3)(-) transporter family. The two transporters share 40% sequence homology in the C-terminal transmembrane region. In this study, we performed extensive substituted cysteine-scanning mutagenesis analysis of the C-terminal region of NBCe1-A covering amino acids Ala(800)-Lys(967). Location of the introduced cysteines was determined by whole cell labeling with a membrane-permeant biotin maleimide and a membrane-impermeant 2-((5(6)-tetramethylrhodamine)carboxylamino) ethyl methanethiosulfonate (MTS-TAMRA) cysteine-reactive reagent. The results show that the extracellular surface of the NBCe1-A C-terminal transmembrane region is minimally exposed to aqueous media with Met(858) accessible to both biotin maleimide and TAMRA and Thr(926)-Ala(929) only to TAMRA labeling. The intracellular surface contains a highly exposed (Met(813)-Gly(828)) region and a cryptic (Met(887)-Arg(904)) connecting loop. The lipid/aqueous interface of the last transmembrane segment is at Asp(960). Our data clearly determined that the C terminus of NBCe1-A contains 5 transmembrane segments with greater average size compared with AE1. Functional assays revealed only two residues in the region of Pro(868)-Leu(967) (a functionally important region in AE1) that are highly sensitive to cysteine substitution. Our findings suggest that the C-terminal transmembrane region of NBCe1-A is tightly folded with unique structural and functional features that differ from AE1.

Glucose-transporter–mediated positive inotropic effects in human myocardium of diabetic and nondiabetic patients

Insulin causes inotropic effects via Ca 2+-dependent and Ca 2+-independent pathways. The latter one is potentially glucose dependent. We examined inotropic responses and signal transduction of insulin in human atrial myocardium of diabetic and nondiabetic patients to test for the role of glucose transporters. Experiments were performed in isolated atrial myocardium of 88 patients undergoing cardiac surgery and 28 ventricular muscle samples of explanted hearts. Influence of insulin (0.02 μmol/L) on isometric twitch force was examined with and without blocking glucose transporter (GLUT) 4 translocation (latrunculin), sodium-coupled glucose transporter (SGLT) 1 (phlorizin, T-1095A), or PI3-kinase (wortmannin). Experiments were performed in Tyrode solution containing glucose or pyruvate as energetic substrate. Messenger RNA expression of glucose transporters (GLUT1, GLUT4, SGLT1, SGLT2) was analyzed in atrial and ventricular myocardium of both diabetic and nondiabetic patients. Developed force increases after insulin (to 117.8% ± 2.4% and 115.8% ± 1.9%) in trabeculae from patients with and without diabetes. Inotropic effect was reduced after displacing glucose with pyruvate as well as after PI3-kinase inhibition (to 103% ± 2%) or inhibition of glucose transporters GLUT4 (to 105% ± 2%) and SGLT1 (phlorizin to 106% ± 2%, T-1095A to 105% ± 2%), without differences between the 2 groups. In glucose-free pyruvate-containing solution, only inhibition of PI3-kinase but not blocking glucose transporters resulted in further inhibitory effects. Messenger RNA expression did not show significant differences between patients with or without diabetes. Insulin exerts positive inotropic effects in human atrial myocardium. These effects are mediated via a PI3-kinase–sensitive and a glucose-transport–sensitive pathway. Differences in functional effects or messenger RNA expression of glucose transporters were not detectable between patients with and without diabetes.

Flavonoid Glycosides Are Not Transported by the Human Na+/Glucose Transporter When Expressed in Xenopus laevis Oocytes, but Effectively Inhibit Electrogenic Glucose Uptake

There is controversy as to whether intestinal absorption of glycosylated flavonoids, and particularly quercetin glycosides, involves their uptake in intact form via the human sodium-coupled glucose transporter hSGLT1. We here describe studies using Xenopus oocytes that express hSGLT1 and the two-electrode voltage clamp technique to determine the transport characteristics of a variety of flavonoids carrying glucose residues at different positions as well as of their aglycones (altogether 27 compounds). Neither quercetin, luteolin, apigenin, naringenin, pelarginidin, daidzein, or genistein, nor any of their glycosylated derivatives generated significant transport currents. However, the inward current evoked by 1 mM of the hSGLT1 substrate alpha-methyl-D-glucopyranoside was potently reduced by the simultaneous application of not only various flavonoid glycosides but also by some aglycones. The inhibitory potency remained unchanged when the attached glucose was replaced by galactose, suggesting that these residues may bind to SGLT1. Kinetic analysis by Dixon plots revealed inhibition of competitive type with high affinities, particularly when the glucose was attached to the position 4' of the aromatic ring of the flavonoids. The affinities became lower when the glucose was attached to a different position. Unexpectedly, the aglycone form of luteolin also inhibited the transport competitively with high affinity. These data show that hSGLT1 does not transport any of the flavonoids and seems therefore not involved in their intestinal absorption. However, not only glycosylated but also a few nonglycosylated flavonoids show a structure-dependent capability for effective inhibition of SGLT1.

Nephrotoxicity of uranyl acetate: effect on rat kidney brush border membrane vesicles

Since the Gulf war exposure to depleted uranium, a known nephrotoxic agent, there is a renewed interest in the toxic effects of uranium in general and its mechanism of nephrotoxicity which is still largely unknown in particular. In order to investigate the mechanism responsible for uranium nephrotoxicity and the therapeutic effect of urine alkalization, we utilized rat renal brush border membrane vesicles (BBMV). Uranyl acetate (UA) caused a decrease in glucose transport in BBMV. The apparent K (i) of uranyl was 139+/-30 microg uranyl/mg protein of BBMV. Uranyl at 140 microg/mg protein of BBMV reduced the maximal capacity of the system to transport glucose [V (max) 2.2+/-0.2 and 0.96+/-0.16 nmol/mg protein for control and uranyl treated BBMV (P<0.001), respectively] with no effect on the apparent K (m) (1.54+/-0.33 and 1.54+/-0.51 mM for control, and uranyl treated BBMV, respectively). This reduction in V(max) is at least partially due to a decrease in the number of sodium-coupled glucose transporters as apparent from the reduction in phlorizin binding to the uranyl treated membranes, V (max) was reduced from 247+/-13 pmol/mg protein in control BBMV to 119+/-3 pmol/mg protein in treated vesicles (P<0.001). The pH of the medium has a profound effect on the toxicity of UA on sodium-coupled glucose transport in BBMV: higher toxicity at neutral pH (around pH 7.0), and practically no toxicity at alkaline pH (7.6). This is the first report showing a direct inhibitory dose and pH dependent effect of uranyl on the glucose transport system in isolated apical membrane from kidney cortex.

Preterm Birth Affects the Intestinal Response to Parenteral and Enteral Nutrition in Newborn Pigs

Maturation of gastrointestinal (GI) function in neonates is stimulated by enteral nutrition, whereas parenteral nutrition induces GI atrophy and malfunction. We investigated whether preterm birth alters the GI responses to parenteral and enteral nutrition. Pigs were delivered either preterm (107 d gestation) or at term (115 d gestation) and fed total parenteral nutrition (TPN) or enteral sow's milk (ENT) for 6 d after birth. Immaturity of the preterm pigs was documented by reduced blood pH, oxygen saturation and neutrophil granulocyte function, impaired intestinal immunoglobulin G uptake from colostrum, and altered relative weights of visceral organs (small intestine, liver, spleen, pancreas, and adrenals). For both ages at delivery, increases occurred in pancreatic weight (30-75%) and amylase activity (0.5- to 13-fold) after birth, but much more in ENT than in TPN pigs (P < 0.05). Six days of TPN feeding was associated with reduced intestinal weight for both delivery groups (60% of values in ENT, P < 0.001), but only in term TPN pigs was the weight lower than at birth (-20%, P < 0.05). Likewise, it was only in term TPN pigs that intestinal maltase activity increased, compared with ENT, and the absorption of glucose and proline decreased. Only in preterm pigs did TPN feeding increase lactase activity (+50% compared with ENT, P < 0.05). For both delivery ages, the mRNA of lactase-phloridzin hydrolase and sodium-coupled glucose transporter 1 (SGLT-1) were increased in TPN, compared with ENT. In conclusion, the trophic effect of enteral vs. parenteral nutrition on the GI tract is also present after preterm birth, but the postnatal maturation of many GI functions is modified, compared with term birth. The effects of nutritional regimen on the maturation of the gut epithelium in neonates depend on gestational age at birth.