The contribution of incretin hormones to the pathogenesis of type 2 diabetes

A Plant-Based Meal Stimulates Incretin and Insulin Secretion More Than an Energy- and Macronutrient-Matched Standard Meal in Type 2 Diabetes: A Randomized Crossover Study

Diminished postprandial secretion of incretins and insulin represents one of the key pathophysiological mechanisms behind type 2 diabetes (T2D). We tested the effects of two energy- and macronutrient-matched meals: A standard meat (M-meal) and a vegan (V-meal) on postprandial incretin and insulin secretion in participants with T2D. A randomized crossover design was used in 20 participants with T2D. Plasma concentrations of glucose, insulin, C-peptide, glucagon-like peptide-1 (GLP-1), amylin, and gastric inhibitory peptide (GIP) were determined at 0, 30, 60, 120, and 180 min. Beta-cell function was assessed with a mathematical model, using C-peptide deconvolution. Repeated-measures ANOVA was used for statistical analysis. Postprandial plasma glucose responses were similar after both test meals (p = 0.64). An increase in the stimulated secretion of insulin (by 30.5%; 95% CI 21.2 to 40.7%; p < 0.001), C-peptide (by 7.1%; 95% CI 4.1 to 9.9%; p < 0.001), and amylin (by 15.7%; 95% CI 11.8 to 19.7%; p < 0.001) was observed following consumption of the V-meal. An increase in stimulated secretion of GLP-1 (by 19.2%; 95% CI 12.4 to 26.7%; p < 0.001) and a decrease in GIP (by −9.4%; 95% CI −17.3 to −0.7%; p = 0.02) were observed after the V-meal. Several parameters of beta-cell function increased after the V-meal, particularly insulin secretion at a fixed glucose value 5 mmol/L, rate sensitivity, and the potentiation factor. Our results showed an increase in postprandial incretin and insulin secretion, after consumption of a V-meal, suggesting a therapeutic potential of plant-based meals for improving beta-cell function in T2D.

A Plant-Based Meal Increases Gastrointestinal Hormones and Satiety More Than an Energy- and Macronutrient-Matched Processed-Meat Meal in T2D, Obese, and Healthy Men: A Three-Group Randomized Crossover Study

Gastrointestinal hormones are involved in regulation of glucose metabolism and satiety. We tested the acute effect of meal composition on these hormones in three population groups. A randomized crossover design was used to examine the effects of two energy- and macronutrient-matched meals: a processed-meat and cheese (M-meal) and a vegan meal with tofu (V-meal) on gastrointestinal hormones, and satiety in men with type 2 diabetes (T2D, n = 20), obese men (O, n = 20), and healthy men (H, n = 20). Plasma concentrations of glucagon-like peptide -1 (GLP-1), amylin, and peptide YY (PYY) were determined at 0, 30, 60, 120 and 180 min. Visual analogue scale was used to assess satiety. We used repeated-measures Analysis of variance (ANOVA) for statistical analysis. Postprandial secretion of GLP-1 increased after the V-meal in T2D (by 30.5%; 95%CI 21.2 to 40.7%; p < 0.001) and H (by 15.8%; 95%CI 8.6 to 23.5%; p = 0.01). Postprandial plasma concentrations of amylin increased in in all groups after the V-meal: by 15.7% in T2D (95%CI 11.8 to 19.6%; p < 0.001); by 11.5% in O (95%CI 7.8 to 15.3%; p = 0.03); and by 13.8% in H (95%CI 8.4 to 19.5%; p < 0.001). An increase in postprandial values of PYY after the V-meal was significant only in H (by 18.9%; 95%CI 7.5 to 31.3%; p = 0.03). Satiety was greater in all participants after the V-meal: by 9% in T2D (95%CI 4.4 to 13.6%; p = 0.004); by 18.7% in O (95%CI 12.8 to 24.6%; p < 0.001); and by 25% in H (95%CI 18.2 to 31.7%; p < 0.001). Our results indicate there is an increase in gut hormones and satiety, following consumption of a single plant-based meal with tofu when compared with an energy- and macronutrient-matched processed-meat meat and cheese meal, in healthy, obese and diabetic men.

Akkermansia muciniphila can reduce the damage of gluco/lipotoxicity, oxidative stress and inflammation, and normalize intestine microbiota in streptozotocin-induced diabetic rats

This study aimed to investigate how Akkermansia muciniphila can implicate type 2 diabetes mellitus and the mechanisms underlying the effects A. muciniphila on type 2 diabetes mellitus. Normal and streptozotocin-induced diabetic Sprague-Dawley rats were orally administered with A. muciniphila and solvent. After 4 weeks of treatment, diabetic rats orally administered with live or pasteurized A. muciniphila exhibited significant increase in the blood concentration of high-density lipoprotein, and decrease in the hepatic glycogen, serum plasminogen activator inhibitor-1, tumor necrosis factor-α, lipopolysaccharide, malondialdehyde and total glucagon-like peptide-1. Moreover, diabetic rats orally administered with A. muciniphila showed significantly increased species alpha diversity and gene function in gut microbes. These results indicated that A. muciniphila can improve liver function, reduce gluco/lipotoxicity, alleviate oxidative stress, suppress inflammation and normalize intestine microbiota of the host animal, thereby ameliorating type 2 diabetes mellitus. Akkermansia muciniphila might be considered as one of the ideal new probiotics used in the management of type 2 diabetes mellitus in future.

A new hypoglycemic mechanism of catalpol revealed by enhancing MyoD/MyoG-mediated myogenesis

AIMS

Enhancing myogenesis has been identified as a possible target to improve insulin sensitivity and protect against metabolic diseases. Catalpol, an iridoid glycoside, has been shown to exert a hypoglycaemic effect by improvement of insulin sensitivity; however, the underlying mechanism remains unknown. In this study, we tested whether catalpol has the potential to improve insulin sensitivity by augmenting myogenesis.

MAIN METHODS

We examined the hypoglycaemic mechanism of catalpol in db/db mice and C2C12 cells. db/db mice were treated with catalpol (200 mg/kg) for 8 consecutive weeks. Serum analysis, skeletal muscle performance and histology, and gene and protein expression were performed. In vitro glucose uptake, gene and protein expression were determined, and small interfering RNA was used to identify the underlying hypoglycaemic mechanism of catalpol.

KEY FINDINGS

In this study, we tested whether catalpol has the potential to improve skeletal insulin sensitivity by augmenting myogenesis, in which we found that, catalpol treatment in db/db mice lowered blood glucose and improved insulin sensitivity via activation of phosphatidylinositol‑3‑Kinase (PI3K)/protein kinase B (AKT) pathway. Moreover, catalpol-treated mice exhibited enhanced myogenesis, as evidenced by increased myogenic differentiation (MyoD), myogenin (MyoG) and myosin heavy chain (MHC) expressions. The in vitro experimental results showed that both catalpol and metformin enhanced glucose uptake via activation of PI3K/AKT pathway. However, unlike metformin, the PI3K/AKT pathway activation by catalpol was dependent on enhanced MyoD/MyoG-mediated myogenesis.

SIGNIFICANCE

Improvement of insulin sensitivity by enhancing MyoD/MyoG-mediated myogenesis may constitute a new therapeutic approach for treating type 2 diabetes.

The Cells of the Islets of Langerhans

Islets of Langerhans are islands of endocrine cells scattered throughout the pancreas. A number of new studies have pointed to the potential for conversion of non-β islet cells in to insulin-producing β-cells to replenish β-cell mass as a means to treat diabetes. Understanding normal islet cell mass and function is important to help advance such treatment modalities: what should be the target islet/β-cell mass, does islet architecture matter to energy homeostasis, and what may happen if we lose a particular population of islet cells in favour of β-cells? These are all questions to which we will need answers for islet replacement therapy by transdifferentiation of non-β islet cells to be a reality in humans. We know a fair amount about the biology of β-cells but not quite as much about the other islet cell types. Until recently, we have not had a good grasp of islet mass and distribution in the human pancreas. In this review, we will look at current data on islet cells, focussing more on non-β cells, and on human pancreatic islet mass and distribution.

The effect of meal frequency in a reduced-energy regimen on the gastrointestinal and appetite hormones in patients with type 2 diabetes: A randomised crossover study

Background

Appetite and gastrointestinal hormones (GIHs) participate in energy homeostasis, feeding behavior and regulation of body weight. We demonstrated previously the superior effect of a hypocaloric diet regimen with lower meal frequency (B2) on body weight, hepatic fat content, insulin sensitivity and feelings of hunger compared to the same diet divided into six smaller meals a day (A6). Studies with isoenergetic diet regimens indicate that lower meal frequency should also have an effect on fasting and postprandial responses of GIHs. The aim of this secondary analysis was to explore the effect of two hypocaloric diet regimens on fasting levels of appetite and GIHs and on their postprandial responses after a standard meal. It was hypothesized that lower meal frequency in a reduced-energy regimen leading to greater body weight reduction and reduced hunger would be associated with decreased plasma concentrations of GIHs: gastric inhibitory peptide (GIP), glucagon-like peptide-1(GLP-1), peptide YY(PYY), pancreatic polypeptide (PP) and leptin and increased plasma concentration of ghrelin. The postprandial response of satiety hormones (GLP-1, PYY and PP) and postprandial suppression of ghrelin will be improved.

Methods

In a randomized crossover study, 54 patients suffering from type 2 diabetes (T2D) underwent both regimens. The concentrations of GLP-1, GIP, PP, PYY, amylin, leptin and ghrelin were determined using multiplex immunoanalyses.

Results

Fasting leptin and GIP decreased in response to both regimens with no difference between the treatments (p = 0.37 and p = 0.83, respectively). Fasting ghrelin decreased in A6 and increased in B2 (with difference between regimens p = 0.023). Fasting PP increased in B2with no significant difference between regimens (p = 0.17). Neither GLP-1 nor PYY did change in either regimen. The decrease in body weight correlated negatively with changes in fasting ghrelin (r = -0.4, p<0.043) and the postprandial reduction of ghrelin correlated positively with its fasting level (r = 0.9, p<0.001). The postprandial responses of GIHs and appetite hormones were similar after both diet regimens.

Conclusions

Both hypocaloric diet regimens reduced fasting leptin and GIP and postprandial response of GIP comparably. The postprandial responses of GIHs and appetite hormones were similar after both diet regimens. Eating only breakfast and lunch increased fasting plasma ghrelin more than the same caloric restriction split into six meals. The changes in fasting ghrelin correlated negatively with the decrease in body weight. These results suggest that for type 2 diabetic patients on a hypocaloric diet, eating larger breakfast and lunch may be more efficient than six smaller meals during the day.

Red wine enhances glucose-dependent insulinotropic peptide (GIP) and insulin responses in type 2 diabetes during an oral glucose tolerance test

BACKGROUND

Ingestion of ethanol before a glucose challenge enhances the insulin response by an unknown mechanism. In addition, epidemiological studies consistently indicate that moderate alcohol consumption reduces the risk of developing type 2 diabetes (T2D). The purposes of this study were to evaluate the potential involvement of glucose-dependent insulinotropic peptide (GIP) and glucagon-like peptide 1 (GLP-1) in alcohol-induced augmentation of the insulin response and to determine if red wine acutely improves glucose tolerance during an oral glucose tolerance test (OGTT).

METHODS

Nine subjects (eight T2D and one pre-diabetes) completed two OGTT 30 min after consumption of 263 ml water or red wine (28 g ethanol). Blood samples were obtained for 3 h and analyzed for glucose, insulin, C-peptide, GIP, and GLP-1.

RESULTS

Compared with water, consumption of red wine increased the incremental area under the curve (iAUC) for insulin by 50 % (14,837 ± 4759 vs. 9885 ± 2686 µU/ml × min; p < 0.05) and for GIP by 25 % (7729 ± 1548 vs. 6191 ± 1049 pmol/l × min; p < 0.05). Glucose and GLP-1 responses were not affected by red wine.

CONCLUSION

Wine consumption before an OGTT augments the insulin response, which may be partially driven by a greater GIP response. Because glucose levels were not reduced, acute wine consumption may not be effective treatment for enhancing glycemic control or may need to be combined with therapy that improves insulin sensitivity.

Bioavailability of milk protein-derived bioactive peptides: a glycaemic management perspective

Milk protein-derived peptides have been reported to have potential benefits for reducing the risk of type 2 diabetes. However, what the active components are and whether intact peptides exert this bioactivity has received little investigation in human subjects. Furthermore, potentially useful bioactive peptides can be limited by low bioavailability. Various peptides have been identified in the gastrointestinal tract and bloodstream after milk-protein ingestion, providing valuable insights into their potential bioavailability. However, these studies are currently limited and the structure and sequence of milk peptides exerting bioactivity for glycaemic management has received little investigation in human subjects. The present article reviews the bioavailability of milk protein-derived peptides in human studies to date, and examines the evidence on milk proteins and glycaemic management, including potential mechanisms of action. Areas in need of advancement are identified. Only by establishing the bioavailability of milk protein-derived peptides, the active components and the mechanistic pathways involved can the benefits of milk proteins for the prevention or management of type 2 diabetes be fully realised in future.

Accumulation of intestinal tissue 3-deoxyglucosone attenuated GLP-1 secretion and its insulinotropic effect in rats

BACKGROUND

Our recent findings support the idea that 3-deoxyglucosone (3DG), a dietary composition, has been suggested as an independent factor for the development of prediabetes. Secretion of glucagon-like peptide-1 (GLP-1) has been suggested to be impaired in T2DM and in conditions associated with hyperglycemia. Since low oral bioavailability of 3DG has been indicated in a single administration study, in the present study we examined if 3DG is capable of accumulating in intestinal tissue of rats after 2-week administration of 3DG, and the 3DG treatment affects GLP-1 secretion and glucose tolerance.

METHODS

Rats were administered by gastric gavage for 2 weeks. We measured 3DG contents of intestinal tissues (by HPLC), plasma levels of total GLP-1 (by ELISA), insulin and glucagon (both by radioimmunoassay) and blood glucose concentrations. The expressions of the sweet receptor subunits (TAS1R2, TAS1R3) and its downstream molecule TRPM5 in duodenum and colon tissues of rats were quantified by WB. We examined GLP-1 secretion in enteroendocrine STC-1 cells exposured to 3DG.

RESULTS

3DG treatment for 2 weeks increased 3DG content of intestinal tissues, fasting blood glucose concentration, and reduced plasma concentrations of GLP-1 and insulin at fasting and 15 and 180 min after the glucose load and oral glucose tolerance in conjunction with increased plasma glucagon concentrations. The expressions of TAS1R2, TAS1R3 and TRPM5 were shown to be reduced whereas 3DG treatment did not affect plasma dipeptidyl peptidase-4 activity, indicating an impaired GLP-1 secretion in 3DG-treated rats. This idea was further supported by the fact that exposure to 3DG directly decrease GLP-1 secretion in STC-1.

CONCLUSION

It is the first demonstration that 3DG was capable of accumulating in intestinal tissue and thereby decreased secretion of GLP-1 and insulin in a similar manner. 3DG-treated rats developed impaired glucose regulation (IGR) with obviously pancreatic islet cell dysfunction. It is further concluded that a decrease in the biological function of GLP-1 resulting from the decreased GLP-1 secretion is the most likely mechanism for the impaired insulin secretion, which ultimately promoted the development of IGR. These results will also contribute to a better understanding of the significance for restoring physiological GLP-1 secretion.

Modulation of glucose transporter protein by dietary flavonoids in type 2 diabetes mellitus

Diabetes mellitus (DM) is a metabolic diseases characterized by hyperglycemia due to insufficient or inefficient insulin secretory response. This chronic disease is a global problem and there is a need for greater emphasis on therapeutic strategies in the health system. Phytochemicals such as flavonoids have recently attracted attention as source materials for the development of new antidiabetic drugs or alternative therapy for the management of diabetes and its related complications. The antidiabetic potential of flavonoids are mainly through their modulatory effects on glucose transporter by enhancing GLUT-2 expression in pancreatic β cells and increasing expression and promoting translocation of GLUT-4 via PI3K/AKT, CAP/Cb1/TC10 and AMPK pathways. This review highlights the recent findings on beneficial effects of flavonoids in the management of diabetes with particular emphasis on the investigations that explore the role of these compounds in modulating glucose transporter proteins at cellular and molecular level.

Glucagon and type 2 diabetes: the return of the alpha cell

In normal physiology, glucagon from pancreatic alpha cells plays an important role in maintaining glucose homeostasis via its regulatory effect on hepatic glucose production. Patients with type 2 diabetes suffer from fasting and postprandial hyperglucagonemia, which stimulate hepatic glucose production and, thus, contribute to the hyperglycemia characterizing these patients. Although this has been known for years, research focusing on alpha cell (patho)physiology has historically been dwarfed by research on beta cells and insulin. Today the mechanisms behind type 2 diabetic hyperglucagonemia are still poorly understood. Preclinical and clinical studies have shown that the gastrointestinal hormone glucose-dependent insulinotropic polypeptide (GIP) might play an important role in this pathophysiological phenomenon. Furthermore, it has become apparent that suppression of glucagon secretion or antagonization of the glucagon receptor constitutes potentially effective treatment strategies for patients with type 2 diabetes. In this review, we focus on the regulation of glucagon secretion by the incretin hormones glucagon-like peptide-1 (GLP-1) and GIP. Furthermore, potential advantages and limitations of suppressing glucagon secretion or antagonizing the glucagon receptor, respectively, in the treatment of patients with type 2 diabetes will be discussed.

GLP-1 responses are heritable and blunted in acquired obesity with high liver fat and insulin resistance

OBJECTIVE Impaired incretin response represents an early and uniform defect in type 2 diabetes, but the contributions of genes and the environment are poorly characterized. RESEARCH DESIGN AND METHODS We studied 35 monozygotic (MZ) and 75 dizygotic (DZ) twin pairs (discordant and concordant for obesity) to determine the heritability of glucagon-like peptide 1 (GLP-1) responses to an oral glucose tolerance test (OGTT) and the influence of acquired obesity to GLP-1, glucose-dependent insulinotropic peptide (GIP), and peptide YY (PYY) during OGTT or meal test. RESULTS The heritability of GLP-1 area under the curve was 67% (95% CI 45-80). Cotwins from weight-concordant MZ and DZ pairs and weight-discordant MZ pairs but concordant for liver fat content demonstrated similar glucose, insulin, and incretin profiles after the OGTT and meal tests. In contrast, higher insulin responses and blunted 60-min GLP-1 responses during the OGTT were observed in the heavier as compared with leaner MZ cotwins discordant for BMI, liver fat, and insulin sensitivity. Blunted GLP-1 response to OGTT was observed in heavier as compared with leaner DZ cotwins discordant for obesity and insulin sensitivity. CONCLUSIONS Whereas the GLP-1 response to the OGTT is heritable, an acquired unhealthy pattern of obesity characterized by liver fat accumulation and insulin resistance is closely related to impaired GLP-1 response in young adults.

A randomized cross-over trial of the postprandial effects of three different diets in patients with type 2 diabetes

BACKGROUND

In the clinic setting both fasting levels of glucose and the area under the curve (AUC) of glucose, by determination of HbA1c levels, are used for risk assessments, in type 2 diabetes (NIDDM). However little is known about postprandial levels, and hence AUC, regarding other traditional risk factors such as insulin and blood-lipids and how this is affected by different diets.

OBJECTIVE

To study postprandial effects of three diets, during a single day, in NIDDM.

METHODS

A low-fat diet (45-56 energy-% from carbohydrates), and a low-carbohydrate diet (16-24 energy-% from carbohydrates) was compared with a Mediterranean-style diet (black coffee for breakfast and the same total-caloric intake as the other two diets for lunch with red wine, 32-35 energy-% from carbohydrates) in a randomized cross-over design. Total-caloric intake/test-day at the clinic from food was 1025-1080 kCal in men and 905-984 kCal in women. The test meals were consumed at a diabetes ward under supervision.

RESULTS

Twenty-one participants were recruited and 19 completed the studies. The low-carbohydrate diet induced lower insulin and glucose excursions compared with the low-fat diet (p<0.0005 for both AUC). The insulin-response following the single Mediterranean-style lunch-meal was more pronounced than during the low-fat diet lunch (insulin increase-ratio of the low-fat diet: 4.35 ± 2.2, of Mediterranean-style diet: 8.12 ± 5.2, p = 0.001) while postprandial glucose levels were similar. The increase-ratio of insulin correlated with the elevation of the incretin glucose-dependent insulinotropic-polypeptide following the Mediterranean-style diet lunch (Spearman, r = 0.64, p = 0.003).

CONCLUSIONS

The large Mediterranean-style lunch-meal induced similar postprandial glucose-elevations as the low-fat meal despite almost double amount of calories due to a pronounced insulin-increase. This suggests that accumulation of caloric intake from breakfast and lunch to a single large Mediterranean style lunch-meal in NIDDM might be advantageous from a metabolic perspective.

TRIAL REGISTRATION

ClinicalTrials.gov NCT01522157 NCT01522157.

Glucose-dependent insulinotropic polypeptide regulates dipeptide absorption in mouse jejunum

Glucose-dependent insulinotropic polypeptide (GIP) secreted from jejunal mucosal K cells augments insulin secretion and plays a critical role in the pathogenesis of obesity and Type 2 diabetes mellitus. In recent studies, we have shown GIP directly activates Na-glucose cotransporter-1 (SGLT1) and enhances glucose absorption in mouse jejunum. It is not known whether GIP would also regulate other intestinal nutrient absorptive processes. The present study investigated the effect of GIP on proton-peptide cotransporter-1 (PepT1) that mediates di- and tripeptide absorption as well as peptidomimetic drugs. Immunohistochemistry studies localized both GIP receptor (GIPR) and PepT1 proteins on the basolateral and apical membranes of normal mouse jejunum, respectively. Anti-GIPR antibody detected 50-, 55-, 65-, and 70-kDa proteins, whereas anti-PepT1 detected a 70-kDa proteins in mucosal homogenates of mouse jejunum. RT-PCR analyses established the expression of GIPR- and PepT1-specific mRNA in mucosal cells of mouse jejunum. Absorption of Gly-Sar (a nondigestible dipeptide) measured under voltage-clamp conditions revealed that the imposed mucosal H(+) gradient-enhanced Gly-Sar absorption as an evidence for the presence of PepT1-mediated H(+):Gly-Sar cotransport on the apical membranes of mouse jejunum. H(+):Gly-Sar absorption was completely inhibited by cephalexin (a competitive inhibitor of PepT1) and was activated by GIP. The GIP-activated Gly-Sar absorption was completely inhibited by RP-cAMP (a cAMP antagonist). In contrast to GIP, the ileal L cell secreting glucagon-like peptide-1 (GLP-1) did not affect the H(+):Gly-Sar absorption in mouse jejunum. We conclude from these observations that GIP, but not GLP-1, directly activates PepT1 activity by a cAMP-dependent signaling pathway in jejunum.

Routine OGTT: a robust model including incretin effect for precise identification of insulin sensitivity and secretion in a single individual

In order to provide a method for precise identification of insulin sensitivity from clinical Oral Glucose Tolerance Test (OGTT) observations, a relatively simple mathematical model (Simple Interdependent glucose/insulin MOdel SIMO) for the OGTT, which coherently incorporates commonly accepted physiological assumptions (incretin effect and saturating glucose-driven insulin secretion) has been developed. OGTT data from 78 patients in five different glucose tolerance groups were analyzed: normal glucose tolerance (NGT), impaired glucose tolerance (IGT), impaired fasting glucose (IFG), IFG+IGT, and Type 2 Diabetes Mellitus (T2DM). A comparison with the 2011 Salinari (COntinuos GI tract MOdel, COMO) and the 2002 Dalla Man (Dalla Man MOdel, DMMO) models was made with particular attention to insulin sensitivity indices IS_COMO, IS_DMMO and k_xgi (the insulin sensitivity index for SIMO). ANOVA on k_xgi values across groups resulted significant overall (P<0.001), and post-hoc comparisons highlighted the presence of three different groups: NGT (8.62×10⁻⁵±9.36×10⁻⁵ min⁻¹pM⁻¹), IFG (5.30×10⁻⁵±5.18×10⁻⁵) and combined IGT, IFG+IGT and T2DM (2.09×10⁻⁵±1.95×10⁻⁵, 2.38×10⁻⁵±2.28×10⁻⁵ and 2.38×10⁻⁵±2.09×10⁻⁵ respectively). No significance was obtained when comparing IS_COMO or IS_DMMO across groups. Moreover, k_xgi presented the lowest sample average coefficient of variation over the five groups (25.43%), with average CVs for IS_COMO and IS_DMMO of 70.32% and 57.75% respectively; k_xgi also presented the strongest correlations with all considered empirical measures of insulin sensitivity. While COMO and DMMO appear over-parameterized for fitting single-subject clinical OGTT data, SIMO provides a robust, precise, physiologically plausible estimate of insulin sensitivity, with which habitual empirical insulin sensitivity indices correlate well. The k_xgi index, reflecting insulin secretion dependency on glycemia, also significantly differentiates clinically diverse subject groups. The SIMO model may therefore be of value for the quantification of glucose homeostasis from clinical OGTT data.

Bariatric surgery and T2DM improvement mechanisms: a mathematical model

Background

Consensus exists that several bariatric surgery procedures produce a rapid improvement of glucose homeostasis in obese diabetic patients, improvement apparently uncorrelated with the degree of eventual weight loss after surgery. Several hypotheses have been suggested to account for these results: among these, the anti-incretin, the ghrelin and the lower-intestinal dumping hypotheses have been discussed in the literature. Since no clear-cut experimental results are so far available to confirm or disprove any of these hypotheses, in the present work a mathematical model of the glucose-insulin-incretin system has been built, capable of expressing these three postulated mechanisms. The model has been populated with critically evaluated parameter values from the literature, and simulations under the three scenarios have been compared.

Results

The modeling results seem to indicate that the suppression of ghrelin release is unlikely to determine major changes in short-term glucose control. The possible existence of an anti-incretin hormone would be supported if an experimental increase of GIP concentrations were evident post-surgery. Given that, on the contrary, collected evidence suggests that GIP concentrations decrease post-surgery, the lower-intestinal dumping hypothesis would seem to describe the mechanism most likely to produce the observed normalization of Type 2 Diabetes Mellitus (T2DM) after bariatric surgery.

Conclusions

The proposed model can help discriminate among competing hypotheses in a context where definitive data are not available and mechanisms are still not clear.

Effects of diet soda on gut hormones in youths with diabetes

OBJECTIVE

In patients with type 2 diabetes, but not type 1 diabetes, abnormal secretion of incretins in response to oral nutrients has been described. In healthy youths, we recently reported accentuated glucagon-like peptide 1 (GLP-1) secretion in response to a diet soda sweetened with sucralose and acesulfame-K. In this study, we examined the effect of diet soda on gut hormones in youths with diabetes.

RESEARCH DESIGN AND METHODS

Subjects aged 12-25 years with type 1 diabetes (n = 9) or type 2 diabetes (n = 10), or healthy control participants (n = 25) drank 240 mL cola-flavored caffeine-free diet soda or carbonated water, followed by a 75-g glucose load, in a randomized, cross-over design. Glucose, C-peptide, GLP-1, glucose-dependent insulinotropic peptide (GIP), and peptide Tyr-Tyr (PYY) were measured for 180 min. Glucose and GLP-1 have previously been reported for the healthy control subjects.

RESULTS

GLP-1 area under the curve (AUC) was 43% higher after ingestion of diet soda versus carbonated water in individuals with type 1 diabetes (P = 0.020), similar to control subjects (34% higher, P = 0.029), but was unaffected by diet soda in patients with type 2 diabetes (P = 0.92). Glucose, C-peptide, GIP, and PYY AUC were not statistically different between the two conditions in any group.

CONCLUSIONS

Ingestion of diet soda before a glucose load augmented GLP-1 secretion in type 1 diabetic and control subjects but not type 2 diabetic subjects. GIP and PYY secretion were not affected by diet soda. The clinical significance of this increased GLP-1 secretion, and its absence in youths with type 2 diabetes, needs to be determined.

New type 2 diabetes risk genes provide new insights in insulin secretion mechanisms

Type 2 diabetes results from the inability of beta cells to increase insulin secretion sufficiently to compensate for insulin resistance. Insulin resistance is thought to result mainly from environmental factors, such as obesity. However, there is compelling evidence that the decline of both insulin sensitivity and insulin secretion have also a genetic component. Recent genome-wide association studies identified several novel risk genes for type 2 diabetes. The vast majority of these genes affect beta cell function by molecular mechanisms that remain unknown in detail. Nevertheless, we and others could show that a group of genes affect glucose-stimulated insulin secretion, a group incretin-stimulated insulin secretion (incretin sensitivity or secretion) and a group proinsulin-to-insulin conversion. The most important so far type 2 diabetes risk gene, TCF7L2, interferes with all three mechanisms. In addition to advancing knowledge in the pathophysiology of type 2 diabetes, the discovery of novel genetic determinants of diabetes susceptibility may help understanding of gene-environment, gene-therapy and gene-gene interactions. It was also hoped that it could make determination of the individual risk for type 2 diabetes feasible. However, the allelic relative risks of most genetic variants discovered so far are relatively low. Thus, at present, clinical criteria assess the risk for type 2 diabetes with greater sensitivity and specificity than the combination of all known genetic variants.

Nitric oxide/reactive oxygen species generation and nitroso/redox imbalance in heart failure: from molecular mechanisms to therapeutic implications

Adaptation of the heart to intrinsic and external stress involves complex modifications at the molecular and cellular levels that lead to tissue remodeling, functional and metabolic alterations, and finally to failure depending upon the nature, intensity, and chronicity of the stress. Reactive oxygen species (ROS) have long been considered as merely harmful entities, but their role as second messengers has gradually emerged. At the same time, our comprehension of the multifaceted role of nitric oxide (NO) and the related reactive nitrogen species (RNS) has been upgraded. The tight interlay between ROS and RNS suggests that their imbalance may implicate the impairment in physiological NO/redox-based signaling that contributes to the failing of the cardiovascular system. This review initially provides basic concepts on the role of nitroso/oxidative stress in the pathophysiology of heart failure with a particular focus on sources of ROS/RNS, their downstream targets, and endogenous modulators. Then, the role of NO/redox regulation of cardiomyocyte function, including calcium homeostasis, electrogenesis, and insulin signaling pathways, is described. Finally, an overview of old and emerging therapeutic opportunities in heart failure is presented, focusing on modulation of NO/redox mechanisms and discussing benefits and limitations.

Initial combination therapy for type 2 diabetes mellitus: is it ready for prime time?

The increased prevalence of type 2 diabetes mellitus is primarily being driven by the increasing global rates of overweight/obesity. Given the magnitude of this epidemic, we can expect these metabolic abnormalities to play an increasing role in the development of cardiovascular disease. In a pathophysiologic sense, type 2 diabetes is a multiorgan, multifactorial condition, characterized by β-cell dysfunction, insulin resistance in peripheral tissues and the liver, defective incretin activity, and elevated levels of free fatty acids and proinflammatory mediators. Despite the considerable burden of disease associated with type 2 diabetes, most patients are not at, or are unable to achieve, recommended glycemic control guideline targets. In part, this is because of the relentlessly progressive nature of the disease, but it may also be attributable to the current diabetes treatment paradigm, which is characterized by ineffective lifestyle interventions, followed by monotherapy and frequent early treatment failure with prolonged periods of elevated glucose as a consequence of clinical inertia. Thus, it is most appropriate to rethink the current treatment paradigm for type 2 diabetes in the context of a more aggressive initial therapy; specifically with early initiation of combination therapy. Our current understanding of the complex pathophysiology of the disease and the progressive deterioration in glycemic control over time supports the philosophy of earlier intervention with a more comprehensive initial therapy. Thus, while control of hyperglycemia remains the paramount goal, focusing on the underlying pathophysiology of type 2 diabetes is increasingly becoming the therapeutic strategy, with the aim of potentially providing disease modification. Although this is a logical approach, it remains to be demonstrated that early combination therapy will result in disease modification in a clinical setting. Not surprisingly, the incretin-based therapies have gained a great deal of attention in the context of being a component of initial combination therapy, given their potential beneficial effects on β-cell function with lowered risk of weight gain and hypoglycemia.

Diabetic gastroparesis-backwards and forwards

Diabetic gastroparesis was once thought to be rare, associated with a poor prognosis, and to affect only patients with type 1 diabetes and irreversible autonomic neuropathy. A landmark study conducted by Horowitz et al. and published in JGH in 1986 paved the way for further studies to examine the pathophysiology, natural history and prognosis of diabetic gastroparesis, as well as its optimal management. This review summarizes the developments in knowledge gained over the last ∼25 years that have led to understanding about normal and disordered gastric emptying in diabetes, with a particular emphasis on the inter-relationship between the rate of gastric emptying and the regulation of blood glucose.

Incretin-based therapies for type 2 diabetes mellitus: properties, functions, and clinical implications

The incretin hormones, glucose-dependent insulinotropic polypeptide (GIP) and glucagonlike peptide-1 (GLP-1), which are secreted by cells of the gastrointestinal tract in response to meal ingestion, exercise important glucoregulatory effects, including the glucose-dependent potentiation of insulin secretion by pancreatic β-cells. Research on the defective incretin action in type 2 diabetes mellitus suggests that the observed loss of insulinotropic activity may be due primarily to a decreased responsiveness of β-cells to GIP. GLP-1 does retain efficacy, albeit not at physiologic levels. Accordingly, augmentation of GLP-1 is a logical therapeutic strategy to ameliorate this deficiency, although the short metabolic half-life of the native hormone renders direct infusion impractical. GLP-1 receptor agonists that resist degradation by the enzyme dipeptidyl peptidase-4 (DPP-4) and have protracted-action kinetics have been developed, and DPP-4 inhibitors that slow the enzymatic cleavage of native GLP-1 provide alternative approaches to enhancing incretin-mediated glucose control. However, GLP-1 receptor agonists and DPP-4 inhibitors are premised on highly divergent mechanisms of action. DPP-4 is ubiquitously expressed in many tissues and is involved in a wide range of physiologic processes in addition to its physiologic influence on incretin hormone biological activity. GLP-1 receptor agonists provide a pharmacologic level of GLP-1 receptor stimulation, whereas DPP-4 inhibitors appear to increase levels of circulating GLP-1 to within the physiologic range. This article examines the physiology of the incretin system, mechanistic differences between GLP-1 receptor agonists and DPP-4 inhibitors used as glucose-lowering agents in the treatment of type 2 diabetes, and the implications of these differences for treatment. The results of recent head-to-head trials are reviewed, comparing the effects of incretin-based therapies on a range of clinical parameters, including glycemia, β-cell function, weight, and cardiovascular function.

Diabetic gastroparesis and its impact on glycemia

Diabetes is the most common cause of gastroparesis and it is now recognized that the relationship between gastric emptying and glycemia is complex and intertwined. Postprandial blood glucose levels influence, and are influenced by, the rate of gastric emptying, highlighting the difficulty in determining which is the cause and which is the effect. Novel diagnostic techniques and therapeutic strategies have been developed for the management of diabetic gastroparesis. This article highlights recent advances in knowledge about diabetic gastroparesis, with an emphasis on the inter-relationships between disordered gastric motor function on glycemia and vice versa, as well as therapeutic strategies for optimizing glycemic control using modulation of gastric emptying.

An emerging role for NAADP-mediated Ca2+ signaling in the pancreatic β-cell

Several recent reports, including one in this journal, have reignited the debate about whether the calcium-mobilizing messenger, nicotinic adenine nucleotide diphosphate (NAADP) plays a central role in the regulation of calcium signalling in pancreatic β-cell. These studies have highlighted a role for NAADP-induced Ca(2+) mobilization not only in mediating the effects of the incretin, GLP-1 and the autocrine proliferative effects of insulin, but also possibly a fundamental role in glucose-mediated insulin secretion in the pancreatic β-cell.