Orexins control intestinal glucose transport by distinct neuronal, endocrine, and direct epithelial pathways

Ceratonia siliqua L. (immature carob bean) inhibits intestinal glucose absorption, improves glucose tolerance and protects against alloxan-induced diabetes in rat

BACKGROUND

This study was designed to investigate the effects of immature carob pod aqueous extract (ICPAE) on intestinal glucose absorption in vitro and in vivo using an oral glucose tolerance test (OGTT) as well as the potential antidiabetic effect in alloxan-induced diabetic rats. OGTT was carried by administration of glucose (2 g kg^-1 , p.o.) and after treatment with extract (50, 100 and 200 mg kg^-1 body weight). Diabetes was induced by single intraperitoneal injection of alloxan (150 mg kg^-1 ). However, the extracts at various doses or glibenclamide (GLB, 10 mg kg^-1 body weight) were given by oral administration for 2 weeks.

RESULTS

ICPAE (50-2000 µg mL^-1 ) exerted dose-dependent reduction of sodium-dependent glucose transport across isolated mice jejunum and the maximal inhibition exceeded 50%.The ICPAE treatment improved glucose tolerance. More importantly, ICPAE at various doses showed a significant reduction in blood glucose and biochemical profiles in diabetic rats.

CONCLUSION

Our findings confirm that the degree of maturity of carob characterized by a different phytochemical composition may be responsible for these actions. Therefore, these compounds may be used as a food supplement in hyperglycemia and diabetes treatments. © 2016 Society of Chemical Industry.

Impact of Orexin-A Treatment on Food Intake, Energy Metabolism and Body Weight in Mice

Orexin-A and -B are hypothalamic neuropeptides of 33 and 28-amino acids, which regulate many homeostatic systems including sleep/wakefulness states, energy balance, energy homeostasis, reward seeking and drug addiction. Orexin-A treatment was also shown to reduce tumor development in xenografted nude mice and is thus a potential treatment for carcinogenesis. The aim of this work was to explore in healthy mice the consequences on energy expenditure components of an orexin-A treatment at a dose previously shown to be efficient to reduce tumor development. Physiological approaches were used to evaluate the effect of orexin-A on food intake pattern, energy metabolism body weight and body adiposity. Modulation of the expression of brain neuropeptides and receptors including NPY, POMC, AgRP, cocaine- and amphetamine related transcript (CART), corticotropin-releasing hormone (CRH) and prepro-orexin (HCRT), and Y2 and Y5 neuropeptide Y, MC4 (melanocortin), OX1 and OX2 orexin receptors (Y2R, Y5R, MC4R, OX1R and OX2R, respectively) was also explored. Our results show that orexin-A treatment does not significantly affect the components of energy expenditure, and glucose metabolism but reduces intraperitoneal fat deposit, adiposity and the expression of several brain neuropeptide receptors suggesting that peripheral orexin-A was able to reach the central nervous system. These findings establish that orexin-A treatment which is known for its activity as an inducer of tumor cell death, do have minor parallel consequence on energy homeostasis control.

The orexin neuropeptide system: physical activity and hypothalamic function throughout the aging process

There is a rising medical need for novel therapeutic targets of physical activity. Physical activity spans from spontaneous, low intensity movements to voluntary, high-intensity exercise. Regulation of spontaneous and voluntary movement is distributed over many brain areas and neural substrates, but the specific cellular and molecular mechanisms responsible for mediating overall activity levels are not well understood. The hypothalamus plays a central role in the control of physical activity, which is executed through coordination of multiple signaling systems, including the orexin neuropeptides. Orexin producing neurons integrate physiological and metabolic information to coordinate multiple behavioral states and modulate physical activity in response to the environment. This review is organized around three questions: (1) How do orexin peptides modulate physical activity? (2) What are the effects of aging and lifestyle choices on physical activity? (3) What are the effects of aging on hypothalamic function and the orexin peptides? Discussion of these questions will provide a summary of the current state of knowledge regarding hypothalamic orexin regulation of physical activity during aging and provide a platform on which to develop improved clinical outcomes in age-associated obesity and metabolic syndromes.

Glucagon regulates orexin A secretion in humans and rodents

AIMS/HYPOTHESIS

Orexin A (OXA) modulates food intake, energy expenditure, and lipid and glucose metabolism. OXA regulates the secretion of insulin and glucagon, while glucose regulates OXA release. Here, we evaluate the role of glucagon in regulating OXA release both in vivo and in vitro.

METHODS

In a double-blind crossover study, healthy volunteers and type 1 diabetic patients received either intramuscular glucagon or placebo. Patients newly diagnosed with type 2 diabetes underwent hyperinsulinaemic-euglycaemic clamp experiments, and insulin-hypoglycaemia tests were performed on healthy volunteers. The primary endpoint was a change in OXA levels after intramuscular glucagon or placebo administration in healthy participants and patients with type 1 diabetes. Secondary endpoints included changes in OXA in healthy participants during insulin tolerance tests and in patients with type 2 diabetes under hyperinsulinaemic-euglycaemic conditions. Participants and staff conducting examinations and taking measurements were blinded to group assignment. OXA secretion in response to glucagon treatment was assessed in healthy and obese mice, the streptozotocin-induced mouse model of type 1 diabetes, and isolated rat pancreatic islets.

RESULTS

Plasma OXA levels declined in lean volunteers and in type 1 diabetic patients injected with glucagon. OXA levels increased during hyperinsulinaemic hypoglycaemia testing in healthy volunteers and during hyperinsulinaemic euglycaemic conditions in type 2 diabetic patients. Plasma OXA concentrations in healthy lean and obese mice and in a mouse model of type 1 diabetes were lower after glucagon treatment, compared with vehicle control. Glucagon decreased OXA secretion from isolated rat pancreatic islets at both low and high glucose levels. OXA secretion declined in pancreatic islets exposed to diazoxide at high and low glucose levels, and after exposure to an anti-insulin antibody. Glucagon further reduced OXA secretion in islets pretreated with diazoxide or an anti-insulin antibody.

CONCLUSIONS/INTERPRETATION

Glucagon inhibits OXA secretion in humans and animals, irrespective of changes in glucose or insulin levels. Through modifying OXA secretion, glucagon may influence energy expenditure, body weight, food intake and glucose metabolism.

Peripheral injections of cholecystokinin, apelin, ghrelin and orexin in cavefish (Astyanax fasciatus mexicanus): effects on feeding and on the brain expression levels of tyrosine hydroxylase, mechanistic target of rapamycin and appetite-related hormones

The effects of intraperitoneal injections of cholecystokinin (CCK), apelin, ghrelin, and orexin on food intake were examined in the blind cavefish Astyanax fasciatus mexicanus. CCK (50ng/g) induced a decrease in food intake whereas apelin (100ng/g), orexin (100ng/g), and ghrelin (100ng/g) induced an increase in food intake as compared to saline-injected control fish. In order to better understand the central mechanism by which these hormones act, we examined the effects of injections on the brain mRNA expression of two metabolic enzymes, tyrosine hydroxylase (TH), and mechanistic target of rapamycin (mTOR), and of appetite-regulating peptides, CCK, orexin, apelin and cocaine and amphetamine regulated transcript (CART). CCK injections induced a decrease in brain apelin injections, apelin injections induced an increase in TH, mTOR, and orexin brain expressions, orexin treatment increased brain TH expression and ghrelin injections induced an increase in mTOR and orexin brain expressions. CART expression was not affected by any of the injection treatments. Our results suggest that the enzymes TH and mTOR and the hormones CCK, apelin, orexin, and ghrelin all regulate food intake in cavefish through a complex network of interactions.

1-deoxynojirimycin inhibits glucose absorption and accelerates glucose metabolism in streptozotocin-induced diabetic mice

We investigated the role of 1-deoxynojirimycin (DNJ) on glucose absorption and metabolism in normal and diabetic mice. Oral and intravenous glucose tolerance tests and labeled ¹³C₆-glucose uptake assays suggested that DNJ inhibited intestinal glucose absorption in intestine. We also showed that DNJ down-regulated intestinal SGLT1, Na⁺/K⁺-ATP and GLUT2 mRNA and protein expression. Pretreatment with DNJ (50 mg/kg) increased the activity, mRNA and protein levels of hepatic glycolysis enzymes (GK, PFK, PK, PDE1) and decreased the expression of gluconeogenesis enzymes (PEPCK, G-6-Pase). Assays of protein expression in hepatic cells and in vitro tests with purified enzymes indicated that the increased activity of glucose glycolysis enzymes was resulted from the relative increase in protein expression, rather than from direct enzyme activation. These results suggest that DNJ inhibits intestinal glucose absorption and accelerates hepatic glucose metabolism by directly regulating the expression of proteins involved in glucose transport systems, glycolysis and gluconeogenesis enzymes.

Cooperative anti-diabetic effects of deoxynojirimycin-polysaccharide by inhibiting glucose absorption and modulating glucose metabolism in streptozotocin-induced diabetic mice

We had previously shown that deoxynojirimycin-polysaccharide mixture (DPM) not only decreased blood glucose but also reversed the damage to pancreatic β-cells in diabetic mice, and that the anti-hyperglycemic efficacy of this combination was better than that of 1-deoxynojirimycin (DNJ) or polysachharide alone. However, the mechanisms behind these effects were not fully understood. The present study aimed to evaluate the therapeutic effects of DPM on streptozotocin (STZ)-induced diabetic symptoms and their potential mechanisms. Diabetic mice were treated with DPM (150 mg/kg body weight) for 90 days and continued to be fed without DPM for an additional 30 days. Strikingly, decrease of blood glucose levels was observed in all DPM treated diabetic mice, which persisted 30 days after cessation of DPM administration. Significant decrease of glycosylated hemoglobin and hepatic pyruvate concentrations, along with marked increase of serum insulin and hepatic glycogen levels were detected in DPM treated diabetic mice. Results of a labeled (13)C6-glucose uptake assay indicated that DPM can restrain glucose absorption. Additionally, DPM down-regulated the mRNA and protein expression of jejunal Na(+)/glucose cotransporter, Na(+)/K(+)-ATPase and glucose transporter 2, and enhanced the activities as well as mRNA and protein levels of hepatic glycolysis enzymes (glucokinase, phosphofructokinase, private kinase and pyruvate decarboxylas E1). Activity and expression of hepatic gluconeogenesis enzymes (phosphoenolpyruvate carboxykinase and glucose-6-phosphatase) were also found to be attenuated in diabetic mice treated with DPM. Purified enzyme activity assays verified that the increased activities of glucose glycolysis enzymes resulted not from their direct activation, but from the relative increase in protein expression. Importantly, our histopathological observations support the results of our biochemical analyses and validate the protective effects of DPM on STZ-induced damage to the pancreas. Thus, DPM has significant potential as a therapeutic agent against diabetes.

Interaction of insulin with prokinetic drugs in STZ-induced diabetic mice

AIM: To study the possible interactions of metoclopramide, domperidone and erythromycin in streptozotocin-induced diabetic mice treated with insulin by various parameters.

METHODS: Effects of the individual as well as combined drugs were studied in diabetic mice via estimation of the blood glucose and serum insulin levels, small intestinal transit (SIT), gastric emptying (GE), xylose absorption and glucose tolerance tests. Groups were given insulin 2 IU/kg s.c., metoclopramide 20 mg/kg p.o., domperidone 20 mg/kg p.o. and erythromycin 6 mg/kg p.o. individually and in combination. There were also normal and diabetic control groups. The first set of experiments was carried out to investigate the subchronic effect on blood glucose and serum insulin levels in diabetic mice of one week of daily dose administration of the tested drugs individually as well as the combination of insulin with each prokinetic drug. The other five sets of experiments were carried out to investigate the acute effect of a single dose of each drug individually and in combination on blood glucose and serum insulin levels, SIT, GE, oral xylose absorption and glucose tolerance tests.

RESULTS: The study included the prokinetic drugs metoclopramide (20 mg/kg), domperidone (20 mg/kg) and erythromycin (6 mg/kg), as well as insulin (2 IU/kg), which was individually effective in decreasing SIT, enhancing GE and increasing xylose absorption significantly in diabetic mice. Erythromycin tended to decrease blood glucose level and increase serum insulin level after 1 wk of daily administration in diabetic mice. Erythromycin potentiated the effect of insulin on blood glucose level and serum insulin level whereas other prokinetic agents failed to do so after repeated dose administration in diabetic mice. Metoclopramide or erythromycin in combination with insulin significantly decreased SIT, in diabetic mice, to lower levels than with insulin alone. Administration of prokinetic drugs along with insulin antagonized the action of insulin on xylose absorption. These combinations also increased the rate of glucose absorption from the gut.

CONCLUSION: The present study suggests that prokinetic drugs could potentially improve glycemic control in diabetic gastroparesis by allowing a more predictable absorption of nutrients, matched to the action of exogenous insulin. The use of prokinetics, such as erythromycin, may be interesting in the clinic in decreasing the need for insulin in diabetic patients. The dose of insulin may be safely decreased with erythromycin in chronic treatments.

Ecklonia cava Inhibits Glucose Absorption and Stimulates Insulin Secretion in Streptozotocin-Induced Diabetic Mice

Aims of study. Present study investigated the effect of Ecklonia cava (EC) on intestinal glucose uptake and insulin secretion. Materials and methods. Intestinal Na⁺-dependent glucose uptake (SGU) and Na⁺-dependent glucose transporter 1 (SGLT1) protein expression was determined using brush border membrane vesicles (BBMVs). Glucose-induced insulin secretion was examined in pancreatic β-islet cells. The antihyperglycemic effects of EC, SGU, and SGLT1 expression were determined in streptozotocin (STZ)-induced diabetic mice. Results. Methanol extract of EC markedly inhibited intestinal SGU of BBMV with the IC₅₀ value of 345 μg/mL. SGLT1 protein expression was dose dependently down regulated with EC treatment. Furthermore, insulinotrophic effect of EC extract was observed at high glucose media in isolated pancreatic β-islet cells in vitro. We next conducted the antihyperglycemic effect of EC in STZ-diabetic mice. EC supplementation markedly suppressed SGU and SGLT1 abundance in BBMV from STZ mice. Furthermore, plasma insulin level was increased by EC treatment in diabetic mice. As a result, EC supplementation improved postprandial glucose regulation, assessed by oral glucose tolerance test, in diabetic mice. Conclusion. These results suggest that EC play a role in controlling dietary glucose absorption at the intestine and insulinotrophic action at the pancreas contributing blood glucose homeostasis in diabetic condition.

Neural regulation of intestinal nutrient absorption

The nervous system and the gastrointestinal (GI) tract share several common features including reciprocal interconnections and several neurotransmitters and peptides known as gut peptides, neuropeptides or hormones. The processes of digestion, secretion of digestive enzymes and then absorption are regulated by the neuro-endocrine system. Luminal glucose enhances its own absorption through a neuronal reflex that involves capsaicin sensitive primary afferent (CSPA) fibres. Absorbed glucose stimulates insulin release that activates hepatoenteric neural pathways leading to an increase in the expression of glucose transporters. Adrenergic innervation increases glucose absorption through α1 and β receptors and decreases absorption through activation of α2 receptors. The vagus nerve plays an important role in the regulation of diurnal variation in transporter expression and in anticipation to food intake. Vagal CSPAs exert tonic inhibitory effects on amino acid absorption. It also plays an important role in the mediation of the inhibitory effect of intestinal amino acids on their own absorption at the level of proximal or distal segment. However, chronic extrinsic denervation leads to a decrease in intestinal amino acid absorption. Conversely, adrenergic agonists as well as activation of CSPA fibres enhance peptides uptake through the peptide transporter PEPT1. Finally, intestinal innervation plays a minimal role in the absorption of fat digestion products. Intestinal absorption of nutrients is a basic vital mechanism that depends essentially on the function of intestinal mucosa. However, intrinsic and extrinsic neural mechanisms that rely on several redundant loops are involved in immediate and long-term control of the outcome of intestinal function.

Relationships between body mass index and short-circuit current in human duodenal and colonic mucosal biopsies

AIM

Retrospectively, to investigate the relationship between body mass index (BMI) and basal electrogenic transport as measured by short-circuit current (SCC) in human duodenal and colonic mucosal biopsies.

METHODS

The study included biopsies from mucosa of normal appearance in the sigmoid colon and/or distal duodenum. Patients were referred for routine endoscopy (predominantly for monosymptomatic abdominal pain) and had normal endoscopic findings. Biopsies were mounted in miniaturized Ussing chambers and basal SCC was recorded. Patients were divided into two groups on the basis of BMI (≤25 and >25 kg m⁻²). Statistical significance was assessed by the unpaired t-test or Wilcoxon rank-sum test. Correlation coefficients were calculated by Pearson product moment correlation.

RESULTS

In colonic biopsies, basal SCC (mean ± standard deviation) was significantly higher in 59 biopsies from 30 patients with low BMI than in 32 biopsies from 23 patients with high BMI (45 ± 29 μA cm⁻² vs. 27 ± 21 μA cm⁻², P = 0.016). In duodenal biopsies, mean basal SCC was numerically lower in 38 biopsies from 15 patients with low BMI than in 46 biopsies from 19 patients with high BMI (54 ± 26 μA cm⁻² vs. 74 ± 39 μA cm⁻², P = 0.069). The correlation coefficient between BMI and SCC was -0.26 (P = 0.06) in colonic biopsies and +0.44 (P = 0.001) in duodenal biopsies.

CONCLUSION

Basal intestinal active electrogenic transport is related to BMI and this relationship may differ in different segments of the intestinal tract.

Role of orexin in the regulation of glucose homeostasis

Orexin-A (hypocretin-1) and orexin-B (hypocretin-2) are hypothalamic neuropeptides that play key roles in the regulation of wakefulness, feeding, reward, autonomic functions and energy homeostasis. To control these functions indispensable for survival, orexin-expressing neurones integrate peripheral metabolic signals, interact with many types of neurones in the brain and modulate their activities via the activation of orexin-1 receptor or orexin-2 receptor. In addition, a new functional role of orexin is emerging in the regulation of insulin and leptin sensitivities responsible for whole-body glucose metabolism. Recent evidence indicates that orexin efficiently protects against the development of peripheral insulin resistance induced by ageing or high-fat feeding in mice. In particular, the orexin receptor-2 signalling appears to confer resistance to diet-induced obesity and insulin insensitivity by improving leptin sensitivity. In fact, the expression of orexin gene is known to be down-regulated by hyperglycaemia in the rodent model of diabetes, such as ob/ob and db/db mice. Moreover, the levels of orexin receptor-2 mRNA have been shown to decline in the brain of mice along with ageing. These suggest that hyperglycaemia due to insulin insensitivity during ageing or by habitual consumption of a high-fat diet leads to the reduction in orexin expression in the hypothalamus, thereby further exacerbating peripheral insulin resistance. Therefore, orexin receptor controlling hypothalamic insulin/leptin actions may be a new target for possible future treatment of hyperglycaemia in patients with type 2 diabetes.

Orexins/hypocretins and orexin receptors in apoptosis: a mini-review

An unexpected and fascinating aspect of the neuropeptides orexins has recently emerged when it was shown that orexins acting at orexin receptors OX1R or OX2R induce dramatic apoptosis resulting in massive reduction in cell growth in various cancer cell lines. This mini-review will provide the reader with recent findings related to the proapoptotic actions of orexins and the entirely novel mechanism whereby the seven membrane-spanning G-protein-coupled receptor (GPCR) OX1R triggers apoptosis. Recent data show that orexins induce tyrosine phosphorylation of the tyrosine-based motifs - immunoreceptor tyrosine-based inhibitory motif and immunoreceptor tyrosine-based switch motif - in OX1R. These phosphorylations result in the recruitment and activation of the phosphotyrosine phosphatase SHP-2 and subsequent cytochrome c-mediated mitochondrial apoptosis. Finally, this mini-review will also speculate on: (1) the potential importance of tyrosine-based motifs in the large family of GPCRs; (2) the interest of orexin receptors as therapeutic targets in cancer therapy; (3) the possible role of orexin receptor-mediated apoptosis in physiology and pathophysiology in the brain (neurodevelopment, neurodegenerative diseases) and in the periphery.

Potential antidiabetic and antioxidant activities of Morus indica and Asystasia gangetica in alloxan-induced diabetes mellitus

Herbal drugs are frequently considered to be less toxic and also free from side effects, than synthetic ones. Hence, the present study was designed to investigate one such combination of herbal drugs, Asystasia gangetica and Morus indica for their antidiabetic and antioxidant potential against alloxan-induced diabetes in albino rats. The effect of both individual and a combination of Asystasia gangetica and Morus indica on blood glucose and liver glycogen were studied in the diabetic rats. The study also assessed for the effect of selected plant extracts for their effect on Superoxide dismutase (SOD), catalase (CAT), reduced glutathione (GSH) and Lipid peroxidation (LPO) in the homogenates of the pancreas. The results of the present study attests significant antidiabetic and antioxidant potential for the selected plants individually and also in combination as a prominent decrease in blood glucose and liver glycogen was observed in the rats treated with the extracts of the selected plants. Similarly, the levels of the protective antioxidant enzymes like SOD, CAT and GSH were increased along with decrease in the LPO levels. The present study provides a scientific evidence for antidiabetic and antioxidant potential of Asystasia gangetica and Morus indica. Further studies to isolate bioactive compounds will pave the way to identify potential lead compounds for developing safe and efficacious antidiabetic agents.

Resistin-like molecule-beta inhibits SGLT-1 activity and enhances GLUT2-dependent jejunal glucose transport

OBJECTIVE

An increased expression of RELM-beta (resistin-like molecule-beta), a gut-derived hormone, is observed in animal models of insulin resistance/obesity and intestinal inflammation. Intestinal sugar absorption is modulated by dietary environment and hormones/cytokines. The aim of this study was to investigate the effect of RELM-beta on intestinal glucose absorption.

RESEARCH DESIGN AND METHODS

Oral glucose tolerance test was performed in mice and rats in the presence and the absence of RELM-beta. The RELM-beta action on glucose transport in rat jejunal sacs, everted rings, and mucosal strips was explored as well as downstream kinases modulating SGLT-1 and GLUT2 glucose transporters.

RESULTS

Oral glucose tolerance test carried out in rodents showed that oral administration of RELM-beta increased glycemia. Studies in rat jejunal tissue indicated that mucosal RELM-beta promoted absorption of glucose from the gut lumen. RELM-beta had no effect on paracellular mannitol transport, suggesting a transporter-mediated transcellular mechanism. In studies with jejunal mucosa mounted in Ussing chamber, luminal RELM-beta inhibited SGLT-1 activity in line with a diminished SGLT-1 abundance in brush border membranes (BBMs). Further, the potentiating effect of RELM-beta on jejunal glucose uptake was associated with an increased abundance of GLUT2 at BBMs. The effects of RELM-beta were associated with an increased amount of protein kinase C betaII in BBMs and an increased phosphorylation of AMP-activated protein kinase (AMPK).

CONCLUSIONS

The regulation of SGLT-1 and GLUT2 by RELM-beta expands the role of gut hormones in short-term AMPK/protein kinase C mediated control of energy balance.

Short food deprivation inhibits orexin receptor 1 expression and orexin-A induced intracellular calcium signaling in acutely isolated duodenal enterocytes

Close intra-arterial infusion of the appetite regulating peptide orexin-A stimulates bicarbonate secretion from the duodenal mucosa. The aim of the present study was to elucidate the ability of orexin-A to induce intracellular calcium signaling in acutely isolated duodenal enterocytes. Freshly isolated clusters of enterocytes, obtained from rat duodenal mucosa or human duodenal biopsies, were loaded with fura 2-AM and mounted in a perfusion chamber. Cryptlike enterocytes were selected (caged), and changes in intracellular calcium concentration ([Ca2+]i) were evaluated by fluorescence imaging. Total RNA was extracted from pellets of enterocytes and reverse transcribed to cDNA, and expression of orexin receptors 1 and 2 (OX1R and OX2R) was measured by quantitative real-time PCR. Orexin-A at all concentrations tested (1-100 nM) increased [Ca2+]i in enterocytes isolated from continuously fed rats, and the OX1R-antagonist SB-334867 (10 nM) attenuated the response. The primary [Ca2+]i response was a slow increase to a sustained plateau persisting after orexin-A removal, and a similar response was observed in enterocytes from human biopsies. In contrast to orexin-A, the OX2R agonist (Ala11,D-Leu15)-orexin-B (1-10 nM) did not induce calcium signaling. There were no significant [Ca2+]i responses in enterocytes from animals food deprived overnight, and overnight fasting decreased (P<0.01) enterocyte OX1R as well as OX2R mRNA. Induction of intracellular calcium signaling in isolated duodenal enterocytes is thus mediated primarily by OX1R receptors. Short (overnight) food deprivation markedly depresses receptor expression and inhibits orexin-A induced increases in [Ca2+]i. Studies of enterocyte signaling and intestinal secretion requires particular evaluation regarding feeding status.

Nigella sativa inhibits intestinal glucose absorption and improves glucose tolerance in rats

AIM OF THE STUDY

Nigella sativa L. (Ranunculaceae) seeds have been used traditionally for centuries, notably for treating diabetes.

MATERIALS AND METHODS

We studied the effects of the crude aqueous extract of Nigella sativa seeds on intestinal glucose absorption in vitro using a short-circuit current technique and in vivo using an oral glucose tolerance test.

RESULTS

The aqueous extract of Nigella sativa (0.1 pg/ml to 100 ng/ml) exerted dose-dependent inhibition of sodium-dependent glucose transport across isolated rat jejunum. Maximal inhibition exceeded 80% and IC50 was close to 10 pg/ml. An oral glucose tolerance test was carried out in rats after the initial dose and after a 6-week treatment of Nigella sativa (2 g/(kg day)), and compared to metformin (300 mg/(kg day)). Chronic Nigella sativa treatment improved glucose tolerance as efficiently as metformin. Nigella sativa and metformin also reduced body weight without any toxic effect.

CONCLUSIONS

To our knowledge, this is the first demonstration that Nigella sativa directly inhibits the electrogenic intestinal absorption of glucose in vitro. Together with the observed improvement of glucose tolerance and body weight in rats after chronic oral administration in vivo, these effects further validate the traditional use of Nigella sativa seeds against diabetes.

Role of orexin in central regulation of gastrointestinal functions

Orexins are neuropeptides that are localized in neurons within the lateral hypothalamus and regulate feeding behavior. The lateral hypothalamus plays an important role in not only feeding but also in the central regulation of gut function. Along this line, accumulating evidence has shown that orexins act in the central nervous system to regulate gastrointestinal functions. The purpose of this review is to summarize recent relevant findings on brain orexins and the digestive system, and discuss the pathophysiological roles of these peptides. Centrally administered orexin or endogenously released orexin in the brain potently stimulates gastric acid secretion in rats. The vagal cholinergic pathway is involved in the orexin-induced stimulation of acid secretion. Because of its stimulatory action on feeding, it can be hypothesized that orexin in the brain is a candidate mediator of cephalic phase gastric secretion. In addition, brain orexin may be involved in the development of depression and functional gastrointestinal disorders, which are frequently accompanied by inhibition of gut function, because lack of orexin activity might cause the inhibition of gastric physiological processes and evoke a depressive state. These lines of evidence suggest that orexin in the brain is a potential molecular target for treatment of functional gastrointestinal disorders.

Glucose-dependent insulinotropic polypeptide (GIP) stimulates transepithelial glucose transport

The purpose of this study was to characterize the effects of glucose-dependent insulinotropic peptide (GIP) on small intestinal glucose transport in vitro. Stripped proximal jejunum from fasted mice was mounted in Ussing chambers. The serosal side was bathed in Regular Ringer solution containing 5 mmol/l glucose, and the mucosal side, with solution containing 10 mmol/l 3-O-methyl glucose (3OMG). Intercellular cyclic adenosine monophosphate (cAMP), mucosa-to-serosa fluxes of 3OMG (J(ms)(3OMG)), and short-circuit current (I(SC)) were measured in the presence and absence of GIP. GIP increased cAMP by 2.5-fold in isolated enterocytes, consistent with a direct effect of GIP on these epithelial cells. GIP also increased I(SC) and J(ms)(3OMG) by 68 and 53%, respectively, indicating that the increase in J(ms)(3OMG) was primarily electrogenic, with a small electroneutral component. The stimulatory effect of GIP on J(ms)(3OMG) was concentration dependent. In addition, 1,000 nmol/l and 10 nmol/l GIP increased J(ms)(3OMG) by 70 and 30% over control, respectively, consistent with receptor activation. Phlorizin (20 mumol/l), an inhibitor of Na(+)-glucose cotransporter (SGLT-1), abolished the increase in I(SC) and decreased J(ms)(3OMG) by approximately 65%. These results indicate that stimulation of SGLT-1 activity by GIP partially accounts for the increase in J(ms)(30MG). These studies are the first to demonstrate direct stimulation of intestinal glucose transport by GIP independent of its insulinotropic properties. GIP stimulates cellular accumulation of cAMP and thereby upregulates glucose transport. The GIP-induced increase in glucose transport appears to be mediated, at least in part, by SGLT-1.