Regulation of sodium glucose co-transporter SGLT1 through altered glycosylation in the intestinal epithelial cells

Empagliflozin attenuates intestinal inflammation through suppression of nitric oxide synthesis and myeloperoxidase activity in in vitro and in vivo models of colitis

Inflammatory bowel diseases (IBD) are characterized by chronic and relapsing inflammation affecting the gastrointestinal (GI) tract. The incidence and prevalence of IBD are relatively high and still increasing. Additionally, current therapeutic strategies for IBD are not optimal. These facts urge todays' medicine to find a novel way to treat IBD. Here, we focused on the group of anti-diabetic drugs called gliflozins, which inhibit sodium glucose co-transporter type 2 (SGLT-2). Numerous studies demonstrated that gliflozins exhibit pleiotropic effect, including anti-inflammatory properties. In this study, we tested the effect of three gliflozins; empagliflozin (EMPA), dapagliflozin (DAPA), and canagliflozin (CANA) in in vitro and in vivo models of intestinal inflammation. Our in vitro experiments revealed that EMPA and DAPA suppress the production of nitric oxide in LPS-treated murine RAW264.7 macrophages. In in vivo part of our study, we showed that EMPA alleviates acute DSS-induced colitis in mice. Treatment with EMPA reduced macro- and microscopic colonic damage, as well as partially prevented from decrease in tight junction gene expression. Moreover, EMPA attenuated biochemical inflammatory parameters including reduced activity of myeloperoxidase. We showed that SGLT-2 inhibitors act as anti-inflammatory agents independently from their hypoglycemic effects. Our observations suggest that gliflozins alleviate inflammation through their potent effects on innate immune cells.

Functional characterization of SGLT1 using SSM-based electrophysiology: Kinetics of sugar binding and translocation

Beside the ongoing efforts to determine structural information, detailed functional studies on transporters are essential to entirely understand the underlying transport mechanisms. We recently found that solid supported membrane-based electrophysiology (SSME) enables the measurement of both sugar binding and transport in the Na⁺/sugar cotransporter SGLT1 (Bazzone et al, 2022a). Here, we continued with a detailed kinetic characterization of SGLT1 using SSME, determining K_M and K_D ^app for different sugars, k_obs values for sugar-induced conformational transitions and the effects of Na⁺, Li⁺, H⁺ and Cl^- on sugar binding and transport. We found that the sugar-induced pre-steady-state (PSS) charge translocation varies with the bound ion (Na⁺, Li⁺, H⁺ or Cl^-), but not with the sugar species, indicating that the conformational state upon sugar binding depends on the ion. Rate constants for the sugar-induced conformational transitions upon binding to the Na⁺-bound carrier range from 208 s^-1 for D-glucose to 95 s^-1 for 3-OMG. In the absence of Na⁺, rate constants are decreased, but all sugars bind to the empty carrier. From the steady-state transport current, we found a sequence for sugar specificity (V_max/K_M): D-glucose > MDG > D-galactose > 3-OMG > D-xylose. While K_M differs 160-fold across tested substrates and plays a major role in substrate specificity, V_max only varies by a factor of 1.9. Interestingly, D-glucose has the lowest V_max across all tested substrates, indicating a rate limiting step in the sugar translocation pathway following the fast sugar-induced electrogenic conformational transition. SGLT1 specificity for D-glucose is achieved by optimizing two ratios: the sugar affinity of the empty carrier for D-glucose is similarly low as for all tested sugars (K_D,K ^app = 210 mM). Affinity for D-glucose increases 14-fold (K_D,Na ^app = 15 mM) in the presence of sodium as a result of cooperativity. Apparent affinity for D-glucose during transport increases 8-fold (K_M = 1.9 mM) compared to K_D,Na ^app due to optimized kinetics. In contrast, K_M and K_D ^app values for 3-OMG and D-xylose are of similar magnitude. Based on our findings we propose an 11-state kinetic model, introducing a random binding order and intermediate states corresponding to the electrogenic transitions detected via SSME upon substrate binding.

Ascaris suum Nutrient Uptake and Metabolic Release, and Modulation of Host Intestinal Nutrient Transport by Excretory-Secretory and Cuticle Antigens In Vitro

Ascaris suum, the most important pig parasite, also infects humans as a zoonotic pathogen. Malabsorption upon infection probably results from impaired nutrient transport, presumably mediated by the parasite’s excretory-secretory (ES) or cuticle somatic (CSO) antigens. The present study investigated the electrogenic transport (ΔI_sc) of glucose, alanine and the dipeptide glycyl-l-glutamine (glygln), as well as glucose net flux rates in pig jejunal tissue after in vitro exposure to adult A. suum total ES or CSO antigens in Ussing chambers. ΔI_sc of glucose, alanine and glucose net flux rate were significantly decreased after one hour of exposure to total ES antigen. In contrast, CSO antigens increased the transport of glygln. Additionally, nutrient uptake and ES antigen pattern were compared in culture medium from untreated adult worms and those with sealed mouth and anal openings. Untreated worms completely absorbed glucose, while cuticular absorption in sealed worms led to 90% reduction. Amino acid absorption was 30% less effective in sealed worms, and ammonia excretion decreased by 20%. Overall, the results show that A. suum total ES antigen rapidly impairs nutrient transport in vitro. Future studies confirming the results in vivo, narrowing down the ES components responsible and investigating underlying molecular mechanisms are needed.

Unique Regulation of Coupled NaCl Absorption by Inducible Nitric Oxide in a Spontaneous SAMP1/YitFc Mouse Model of Chronic Intestinal Inflammation

In the small intestine, Na:H (NHE3) and Cl:HCO3 (DRA or PAT1) exchangers present in the brush border membrane (BBM) of absorptive villus cells are primarily responsible for the coupled absorption of NaCl, the malabsorption of which causes diarrhea, a common symptom of inflammatory bowel disease (IBD). Inducible nitric oxide (iNO), a known mediator of inflammation, is increased in the mucosa of the chronically inflamed IBD intestine. An SAMP1/YitFc (SAMP1) mouse, a spontaneous model of chronic ileitis very similar to human IBD, was used to study alterations in NaCl absorption. The SAMP1 and control AKR mice were treated with I-N(6)-(1-Iminoethyl)-lysine (L-NIL) to inhibit iNO production, and DRA/PAT1 and NHE3 activities and protein expression were studied. Though Na:H exchange activity was unaffected, Cl:HCO3 activity was significantly decreased in SAMP1 mice due to a reduction in its affinity for Cl, which was reversed by L-NIL treatment. Though DRA and PAT1 expressions were unchanged in all experimental conditions, phosphorylation studies indicated that DRA, not PAT1, is affected in SAMP1. Moreover, the altered phosphorylation levels of DRA was restored by L-NIL treatment. Inducible NO mediates the inhibition of coupled NaCl absorption by decreasing Cl:HCO3 but not Na:H exchange. Specifically, Cl:HCO3 exchanger DRA but not PAT1 is regulated at the level of its phosphorylation by iNO in the chronically inflamed intestine.

Inducible Nitric Oxide Regulates Na-Glucose Co-transport in a Spontaneous SAMP1/YitFc Mouse Model of Chronic Ileitis

In mammalian small intestine, glucose is primarily absorbed via Na-dependent glucose co-transporter (SGLT1) on the brush border membrane (BBM) of absorptive villus cells. Malabsorption of nutrients (e.g., glucose) leads to malnutrition, a common symptom of inflammatory bowel disease (IBD), where the mucosa is characterized by chronic inflammation. Inducible nitric oxide (iNO) is known to be elevated in IBD mucosa. SAMP1/YitFc (SAMP1) mouse is a spontaneous model of chronic ileitis that develops lesions in its terminal ileum, very similar to human IBD. How SGLT1 may be affected in SAMP1 model of chronic ileitis is unknown. Ten-week-old SAMP1 mice with AKR mice as control were treated with N6-(1-iminoethyl)-L-lysine dihydrochloride (L-NIL) to inhibit iNO production. Intracellular NO levels were found to be increased in villus cells from SAMP1 mice. Moreover, SGLT1 and Na^+/K⁺-ATPase activities and BBM SGLT1 expression were significantly decreased. However, L-NIL treatment reduced the intracellular iNO production, and reversed both downregulated SGLT1 and Na⁺/K⁺-ATPase activities in SAMP1 mice. Inhibition of iNO by L-NIL treatment also significantly reversed the BBM SGLT1 protein expression in SAMP1 mice. L-NIL reversed the inflammation mediated downregulation of SGLT1 activity by restoring the BBM SGLT1 expression. Thus, regulation of SGLT1 in chronic ileitis is likely mediated by iNO.

Moderate Alcohol Consumption Uniquely Regulates Sodium-Dependent Glucose Co-Transport in Rat Intestinal Epithelial Cells In Vitro and In Vivo

BACKGROUND

Chronic alcohol use often leads to malnutrition. However, how the intestinal absorption of nutrients such as glucose may be affected during moderate ethanol use has not been investigated. Glucose is absorbed via sodium (Na)-dependent glucose co-transport (SGLT1; SLC5A1) along the brush border membrane (BBM) of intestinal absorptive villus cells.

OBJECTIVE

The aim of this study was to investigate how moderate alcohol consumption affects the absorption of glucose via SGLT1.

METHODS

Intestinal epithelial cells (IEC-18; rat) were exposed to 8.64 mM ethanol over 1, 3, 6, and 12 h. Rats (16-wk-old, male, Sprague-Dawley) were administered 2 g/kg ethanol over 1, 3, and 6 h. Na-dependent 3H-O-methyl-d-glucose uptake was measured to assess SGLT1 activity. Na-K-ATPase activity was measured as a function of inorganic phosphate release. Protein expression was analyzed by Western blot analysis and immunohistochemical staining.

RESULTS

Ethanol significantly decreased Na-dependent glucose absorption in enterocytes in vitro (ethanol treatment: 48.4% of controls at 1 h; P < 0.01) and in vivo (ethanol treatment: 60.0% of controls at 1 h; P < 0.01). Na-K-ATPase activity was significantly inhibited in vitro (ethanol treatment: 36.9% of controls at 1 h; P < 0.01) and in vivo (ethanol treatment: 42.1% of controls at 1 h; P < 0.01). Kinetic studies showed that the mechanism of inhibition of Na-glucose co-transport was secondary to a decrease in the affinity (1/Km) of the co-transporter for glucose both in vitro and in vivo. Western blots and immunohistochemistry further demonstrated unaltered amounts of SGLT1 after ethanol treatment.

CONCLUSIONS

Moderate ethanol significantly decreases glucose absorption in IEC-18 cells and in villus cells of Sprague-Dawley rats. The inhibition of SGLT1 is secondary to an altered Na gradient at the cellular level and secondary to diminished affinity of the co-transporter for glucose at the protein level in the BBM. These observations may, at least in part, explain 1 possible mechanism of the onset of malnutrition associated with alcohol consumption.

Mechanisms of Regulation of Transporters of Amino Acid Absorption in Inflammatory Bowel Diseases

Intestinal absorption of dietary amino acids/peptides is essential for protein homeostasis, which in turn is crucial for maintaining health as well as restoration of health from significant diseases. Dietary amino acids/peptides are absorbed by unique transporter processes present in the brush border membrane of absorptive villus cells, which line the entire length of the intestine. To date, the only nutrient absorptive system described in the secretory crypt cells in the mammalian intestine is the one that absorbs the amino acid glutamine. Majority of the amino acid transporters are sodium dependent and therefore require basolateral membrane Na-K-ATPase to maintain an efficient transcellular Na gradient for their activity. These transport processes are tightly regulated by various cellular and molecular mechanisms that facilitate their optimal activity during normal physiological processes. Malabsorption of amino acids, recently described in pathophysiological states such as in inflammatory bowel disease (IBD), is undoubtedly responsible for the debilitating symptoms of IBD such as malnutrition, weight loss and ultimately a failure to thrive. Also recently, in vivo models of IBD and in vitro studies have demonstrated that specific immune-inflammatory mediators/pathways regulate specific amino acid transporters. This provides possibilities to derive novel nutrition and immune-based treatment options for conditions such as IBD. © 2020 American Physiological Society. Compr Physiol 10:673-686, 2020.

Microarray analysis reveals the inhibition of intestinal expression of nutrient transporters in piglets infected with porcine epidemic diarrhea virus

Porcine epidemic diarrhea virus (PEDV) infection can induce intestinal dysfunction, resulting in severe diarrhea and even death, but the mode of action underlying these viral effects remains unclear. This study determined the effects of PEDV infection on intestinal absorption and the expression of genes for nutrient transporters via biochemical tests and microarray analysis. Sixteen 7-day-old healthy piglets fed a milk replacer were randomly allocated to one of two groups. After 5-day adaption, piglets (n = 8/group) were orally administrated with either sterile saline or PEDV (the strain from Yunnan province) at 10^4.5 TCID₅₀ (50% tissue culture infectious dose) per pig. All pigs were orally infused D-xylose (0.1 g/kg BW) on day 5 post PEDV or saline administration. One hour later, jugular vein blood samples as well as intestinal samples were collected for further analysis. In comparison with the control group, PEDV infection increased diarrhea incidence, blood diamine oxidase activity, and iFABP level, while reducing growth and plasma D-xylose concentration in piglets. Moreover, PEDV infection altered plasma and jejunal amino acid profiles, and decreased the expression of aquaporins and amino acid transporters (L-type amino acid transporter 1, sodium-independent amino acid transporter, B(°^,+)-type amino acid transport protein, sodium-dependent neutral amino acid transporter 1, sodium-dependent glutamate/aspartate transporter 3, and peptide transporter (1), lipid transport and metabolism-related genes (lipoprotein lipase, apolipoprotein A1, apolipoprotein A4, apolipoprotein C2, solute carrier family 27 member 2, solute carrier family 27 member 4, fatty acid synthase, and long-chain acyl-CoA synthetase (3), and glucose transport genes (glucose transporter-2 and insulin receptor) in the jejunum. However, PEDV administration increased mRNA levels for phosphoenolpyruvate carboxykinase 1, argininosuccinate synthase 1, sodium/glucose co-transporter-1, and cystic fibrosis transmembrane conductance regulator in the jejunum. Collectively, these comprehensive results indicate that PEDV infection induces intestinal injury and inhibits the expression of genes encoding for nutrient transporters.

Moderate Alcohol Consumption Inhibits Sodium-Dependent Glutamine Co-Transport in Rat Intestinal Epithelial Cells in Vitro and Ex Vivo

Malnutrition is present in chronic alcoholics. However, how moderate alcohol consumption affects the absorption of nutrients like glutamine has not been investigated. Glutamine, an amino acid, is vital to gastrointestinal health. Glutamine is absorbed via sodium-dependent glutamine co-transport (B0AT1; SLC6A19) along the brush border membrane of absorptive villus cells. Rat intestinal epithelial cells (IEC-18) and sixteen-week-old Sprague Dawley rats were administered the equivalent of a 0.04% blood alcohol content of ethanol (8.64 mM; 2 g/kg) to investigate the effect of moderate alcohol on sodium-glutamine co-transport. Sodium-dependent ³H-glutamine uptakes were performed to measure B0AT1 activity. Inorganic phosphate was measured as a function of Na-K-ATPase activity. Protein expression was analyzed by immunohistochemical and Western blot analysis. Ethanol significantly inhibited sodium-dependent glutamine absorption and Na-K-ATPase activity in enterocytes in vitro and ex vivo. Kinetic studies suggested that the mechanism of inhibition was due to decreased maximal rate of uptake (V_max) of the B0AT1 co-transporter, corresponding to decreased B0AT1 protein expression and secondary to an inhibited sodium-gradient at the cellular level in vitro and ex vivo. In all, moderate ethanol significantly inhibited glutamine absorption at the level of decreased B0AT1 expression at the brush border membrane and a reduced sodium gradient, which may contribute to malnutrition present in chronic alcoholics.

Stimulation of constitutive nitric oxide uniquely and compensatorily regulates intestinal epithelial cell brush border membrane Na absorption

In the mammalian small intestine, sodium is primarily absorbed by Na+ /H+ exchange (NHE3) and Na-glucose cotransport (SGLT1) in the brush border membrane (BBM) of villus cells. However, how enhanced cellular constitutive nitric oxide (cNO) may affect NHE3 and SGLT1 remains unclear. Both in vivo in rabbit intestinal villus cells and in vitro IEC-18 cells, administration of NO donor, GSNAP, modestly increased cNO. GSNAP stimulated SGLT1 in villus and IEC-18 cells. The mechanism of stimulation was secondary to an increase in the affinity of SGLT1 for glucose. The change in SGLT1 was not secondary to altered Na-extruding capacity of the cell since Na+ /K+ -ATPase was decreased by GSNAP treatment. In contrast, GSNAP inhibited NHE3 activity in villus cell BBM. The mechanism of NHE3 inhibition was secondary to reduced BBM transporter numbers. These studies demonstrated that the physiological increase in cNO uniquely regulates mammalian small intestinal NHE3 and SGLT1 to maintain Na homeostasis.

Elevated glucose concentration in culture media decreases membrane trafficking of SGLT2 in LLC-PK1 cells via a cAMP/PKA-dependent pathway

Na⁺-dependent glucose reabsorption in the renal proximal tubule is dynamically regulated by changes in blood glucose levels. There is, however, a disparity in reports studying the relationship between hyperglycemia and Na⁺-glucose-linked transporter (SGLT) function and expression. Similarly, manipulation of the glucose content in growth media of cultured renal cells has been shown to influence SGLT activity. In this investigation, SGLT activity was significantly lower in proximal tubule LLC-PK₁ cells cultured in medium containing 17.5 than 5 mM glucose. α-Methyl d-glucopyranoside (AMG) transport kinetics showed reduced apparent V_max and K_m in cells grown in 17.5 mM glucose. SGLT2 was identified as the isoform responsible for glucose transport, and protein expression analyses showed decreased apical membrane localization of SGLT2 in cells grown in 17.5 mM glucose, explaining the reduced activity. Multiple signaling pathways have been implicated in regulation of SGLT activity and trafficking. Elevated media glucose decreased intracellular cAMP and PKA activation, leading to decreased SGLT2 trafficking into the plasma membrane, which was reversed after treatment with 1 µM forskolin. The effects of media glucose on SGLT activity were found to be dependent on p38 MAPK activation due to PKA-mediated signaling. Glucose-modulated AMG uptake is reversible and was associated with altered SGLT2 membrane trafficking and cAMP alterations. In summary, elevated glucose concentrations in culture medium decrease SGLT activity in LLC-PK₁ cells by reducing membrane trafficking of SGLT2 via decreasing intracellular cAMP, resulting in a lowered PKA-dependent phosphorylation of p38 MAPK.

Renal tubule insulin receptor modestly promotes elevated blood pressure and markedly stimulates glucose reabsorption

Although the cause of hypertension among individuals with obesity and insulin resistance is unknown, increased plasma insulin, acting in the kidney to increase sodium reabsorption, has been proposed as a potential mechanism. Insulin may also stimulate glucose uptake, but the contributions of tubular insulin signaling to sodium or glucose transport in the setting of insulin resistance is unknown. To directly study the role of insulin signaling in the kidney, we generated inducible renal tubule-specific insulin receptor-KO mice and used high-fat feeding and mineralocorticoids to model obesity and insulin resistance. Insulin receptor deletion did not alter blood pressure or sodium excretion in mice on a high-fat diet alone, but it mildly attenuated the increase in blood pressure with mineralocorticoid supplementation. Under these conditions, KO mice developed profound glucosuria. Insulin receptor deletion significantly reduced SGLT2 expression and increased urinary glucose excretion and urine flow. These data demonstrate a direct role for insulin receptor-stimulated sodium and glucose transport and a functional interaction of insulin signaling with mineralocorticoids in vivo. These studies uncover a potential mechanistic link between preserved insulin sensitivity and renal glucose handling in obesity and insulin resistance.

The Nutrient-Sensing Hexosamine Biosynthetic Pathway as the Hub of Cancer Metabolic Rewiring

Alterations in glucose and glutamine utilizing pathways and in fatty acid metabolism are currently considered the most significant and prevalent metabolic changes observed in almost all types of tumors. Glucose, glutamine and fatty acids are the substrates for the hexosamine biosynthetic pathway (HBP). This metabolic pathway generates the “sensing molecule” UDP-N-Acetylglucosamine (UDP-GlcNAc). UDP-GlcNAc is the substrate for the enzymes involved in protein N- and O-glycosylation, two important post-translational modifications (PTMs) identified in several proteins localized in the extracellular space, on the cell membrane and in the cytoplasm, nucleus and mitochondria. Since protein glycosylation controls several key aspects of cell physiology, aberrant protein glycosylation has been associated with different human diseases, including cancer. Here we review recent evidence indicating the tight association between the HBP flux and cell metabolism, with particular emphasis on the post-transcriptional and transcriptional mechanisms regulated by the HBP that may cause the metabolic rewiring observed in cancer. We describe the implications of both protein O- and N-glycosylation in cancer cell metabolism and bioenergetics; focusing our attention on the effect of these PTMs on nutrient transport and on the transcriptional regulation and function of cancer-specific metabolic pathways.

Direct and specific inhibition of constitutive nitric oxide synthase uniquely regulates brush border membrane Na-absorptive pathways in intestinal epithelial cells

Pharmacological manipulations of constitutive nitric oxide (cNO) levels have been shown to have variable effects on Na absorption in vivo and in vitro in different tissues. Species differences, untoward in vivo effects (e.g. ENS, blood flow) and pharmacological non-specificity may account for these confounding observations. Thus, to directly and specifically determine the effect of cNO on brush border membrane Na/H exchange (NHE3) and Na-dependent glucose co-transport (SGLT-1), we inhibited cNO synthase (NOS3) with its siRNA in rat small intestinal epithelial cells (IEC-18) in vitro. As expected, intracellular cNO levels were reduced in siRNA NOS3 transfected cells. In these cells, SGLT-1 was significantly reduced compared to control. In contrast, NHE3 was significantly increased in siRNA NOS3 transfected cells. To determine if SGLT-1 changes were secondary to altered Na/K-ATPase, its activity was measured and found to be increased in NOS3 silenced cells. The mechanism of inhibition of SGLT-1 was secondary to diminished affinity of the co-transporter for glucose in NOS3 silenced cells. In contrast, the mechanism of stimulation of NHE3 is by increasing BBM exchanger numbers in siRNA NOS3 cells while the affinity was unaffected. Western blot studies of immunoreactive BBM proteins also confirmed the kinetic studies. All these data indicates that direct and specific inhibition of NOS3 with its siRNA inhibits SGLT-1 while stimulating NHE3 in the BBM. Thus, cNO uniquely and compensatorily regulates BBM NHE3 and SGLT-1 to maintain cellular Na homeostasis and these unique alterations by cNO are mediated by its intracellular 2nd messenger cGMP.

6q12 and 11p14 variants are associated with postnatal exhaled nitric oxide levels and respiratory symptoms

BACKGROUND

Exhaled nitric oxide (eNO) is a biomarker of airway inflammation and seems to precede respiratory symptoms, such as asthma, in childhood. Identifying genetic determinants of postnatal eNO levels might aid in unraveling the role of eNO in epithelial function or airway inflammation and disease.

OBJECTIVE

We sought to identify genetic determinants of early postnatal eNO levels and subsequent respiratory symptoms during the first year of life.

METHODS

Within a population-based birth cohort, eNO levels were measured in healthy term infants aged 5 weeks during quiet tidal breathing in unsedated sleep. We assessed associations of single nucleotide polymorphisms with eNO levels in a genome-wide association study and subsequent symptoms of lower respiratory tract infections during the first year of life and asked whether this was modified by prenatal and early-life environmental factors.

RESULTS

We identified thus far unknown determinants of infant eNO levels: rs208515 (P = 3.3 × 10^-8), which is located at 6q12, probably acting in "trans" and explaining 10.3% of eNO level variance, and rs1441519 (P = 1.6 × 10^-6), which is located at 11p14, potentially affecting nitric oxide synthase 3 (NOS3) expression, as shown by means of in vitro functional analyses. Moreover, the 6q12 locus was inversely associated with subsequent respiratory symptoms (P < .05) and time to recovery after first respiratory symptoms during the first year of life (P < .05).

CONCLUSION

The identification of novel genetic determinants of infant eNO levels might implicate that postnatal eNO metabolism in healthy infants before first viral infections and sensitization is related to mechanisms other than those associated with asthma, atopy, or increased risk thereof later in life.

Chronic Hyperinsulinaemic Hypoglycaemia in Rats Is Accompanied by Increased Body Weight, Hyperleptinaemia, and Decreased Neuronal Glucose Transporter Levels in the Brain

The brain is vulnerable to hypoglycaemia due to a continuous need of energy substrates to meet its high metabolic demands. Studies have shown that severe acute insulin-induced hypoglycaemia results in oxidative stress in the rat brain, when neuroglycopenia cannot be evaded despite increased levels of cerebral glucose transporters. Compensatory measures in the brain during chronic insulin-induced hypoglycaemia are less well understood. The present study investigated how the brain of nondiabetic rats copes with chronic insulin-induced hypoglycaemia for up to eight weeks. Brain level of different substrate transporters and redox homeostasis was evaluated. Hyperinsulinaemia for 8 weeks consistently lowered blood glucose levels by 30-50% (4-6 mM versus 7-9 mM in controls). The animals had increased food consumption, body weights, and hyperleptinaemia. During infusion, protein levels of the brain neuronal glucose transporter were decreased, whereas levels of lipid peroxidation products were unchanged. Discontinued infusion was followed by transient systemic hyperglycaemia and decreased food consumption and body weight. After 4 weeks, plasma levels of lipid peroxidation products were increased, possibly as a consequence of hyperglycaemia-induced oxidative stress. The present data suggests that chronic moderate hyperinsulinaemic hypoglycaemia causes increased body weight and hyperleptinaemia. This is accompanied by decreased neuronal glucose transporter levels, which may be leptin-induced.

Intracellular Na+ Concentration ([Na+]i) Is Elevated in Diabetic Hearts Due to Enhanced Na+-Glucose Cotransport

Background

Intracellular Na⁺ concentration ([Na⁺]_i) regulates Ca²⁺ cycling, contractility, metabolism, and electrical stability of the heart. [Na⁺]_i is elevated in heart failure, leading to arrhythmias and oxidative stress. We hypothesized that myocyte [Na⁺]_i is also increased in type 2 diabetes (T2D) due to enhanced activity of the Na⁺–glucose cotransporter.

Methods and Results

To test this hypothesis, we used myocardial tissue from humans with T2D and a rat model of late-onset T2D (HIP rat). Western blot analysis showed increased Na⁺–glucose cotransporter expression in failing hearts from T2D patients compared with nondiabetic persons (by 73±13%) and in HIP rat hearts versus wild-type (WT) littermates (by 61±8%). [Na⁺]_i was elevated in HIP rat myocytes both at rest (14.7±0.9 versus 11.4±0.7 mmol/L in WT) and during electrical stimulation (17.3±0.8 versus 15.0±0.7 mmol/L); however, the Na⁺/K⁺-pump function was similar in HIP and WT cells, suggesting that higher [Na⁺]_i is due to enhanced Na⁺ entry in diabetic hearts. Indeed, Na⁺ influx was significantly larger in myocytes from HIP versus WT rats (1.77±0.11 versus 1.29±0.06 mmol/L per minute). Na⁺–glucose cotransporter inhibition with phlorizin or glucose-free solution greatly reduced Na⁺ influx in HIP myocytes (to 1.20±0.16 mmol/L per minute), whereas it had no effect in WT cells. Phlorizin also significantly decreased glucose uptake in HIP myocytes (by 33±9%) but not in WT, indicating an increased reliance on the Na⁺–glucose cotransporter for glucose uptake in T2D hearts.

Conclusions

Myocyte Na⁺–glucose cotransport is enhanced in T2D, which increases Na⁺ influx and causes Na⁺ overload. Higher [Na⁺]_i may contribute to arrhythmogenesis and oxidative stress in diabetic hearts.

Chronic and selective inhibition of basolateral membrane Na-K-ATPase uniquely regulates brush border membrane Na absorption in intestinal epithelial cells

Na-K-ATPase, an integral membrane protein in mammalian cells, is responsible for maintaining the favorable intracellular Na gradient necessary to promote Na-coupled solute cotransport processes [e.g., Na-glucose cotransport (SGLT1)]. Inhibition of brush border membrane (BBM) SGLT1 is, at least in part, due to the diminished Na-K-ATPase in villus cells from chronically inflamed rabbit intestine. The aim of the present study was to determine the effect of Na-K-ATPase inhibition on the two major BBM Na absorptive pathways, specifically Na-glucose cotransport and Na/H exchange (NHE), in intestinal epithelial (IEC-18) cells. Na-K-ATPase was inhibited using 1 mM ouabain or siRNA for Na-K-ATPase-α1 in IEC-18 cells. SGLT1 activity was determined as 3-O-methyl-D-[(3)H]glucose uptake. Na-K-ATPase activity was measured as the amount of inorganic phosphate released. Treatment with ouabain resulted in SGLT1 inhibition at 1 h but stimulation at 24 h. To further characterize this unexpected stimulation of SGLT1, siRNA silencing was utilized to inhibit Na-K-ATPase-α1. SGLT1 activity was significantly upregulated by Na-K-ATPase silencing, while NHE3 activity remained unaltered. Kinetics showed that the mechanism of stimulation of SGLT1 activity was secondary to an increase in affinity of the cotransporter for glucose without a change in the number of cotransporters. Molecular studies demonstrated that the mechanism of stimulation was not secondary to altered BBM SGLT1 protein levels. Chronic and direct silencing of basolateral Na-K-ATPase uniquely regulates BBM Na absorptive pathways in intestinal epithelial cells. Specifically, while BBM NHE3 is unaffected, SGLT1 is stimulated secondary to enhanced affinity of the cotransporter.