The T allele of rs7903146 TCF7L2 is associated with impaired insulinotropic action of incretin hormones, reduced 24?h profiles of plasma insulin and glucagon, and increased hepatic glucose production in young healthy men

Aims/hypothesis  We studied the physiological, metabolic and hormonal mechanisms underlying the elevated risk of type 2 diabetes in carriers of TCF7L2 gene. Methods  We undertook genotyping of 81 healthy young Danish men for rs7903146 of TCF7L2 and carried out various beta cell tests including: 24 h glucose, insulin and glucagon profiles; OGTT; mixed meal test; IVGTT; hyperglycaemic clamp with co-infusion of glucagon-like peptide (GLP)-1 or glucose-dependent insulinotropic polypeptide (GIP); and a euglycaemic–hyperinsulinaemic clamp combined with glucose tracer infusion to study hepatic and peripheral insulin action. Results  Carriers of the T allele were characterised by reduced 24 h insulin concentrations (p < 0.05) and reduced insulin secretion relative to glucose during a mixed meal test (beta index: p < 0.003), but not during an IVGTT. This was further supported by reduced late-phase insulinotropic action of GLP-1 (p = 0.03) and GIP (p = 0.07) during a 7 mmol/l hyperglycaemic clamp. Secretion of GLP-1 and GIP during the mixed meal test was normal. Despite elevated hepatic glucose production, carriers of the T allele had significantly reduced 24 h glucagon concentrations (p < 0.02) suggesting altered alpha cell function. Conclusions/interpretation  Elevated hepatic glucose production and reduced insulinotropic effect of incretin hormones contribute to an increased risk of type 2 diabetes in carriers of the rs7903146 risk T allele of TCF7L2. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorised users.     

Glucokinase, the pancreatic glucose sensor, is not the gut glucose sensor

Aims/hypothesis  The incretin hormones glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotrophic peptide (GIP) are released from intestinal endocrine cells in response to luminal glucose. Glucokinase is present in these cells and has been proposed as a glucose sensor. The physiological role of glucokinase can be tested using individuals with heterozygous glucokinase gene (GCK) mutations. If glucokinase is the gut glucose sensor, GLP-1 and GIP secretion during a 75 g OGTT would be lower in GCK mutation carriers compared with controls. Methods  We compared GLP-1 and GIP concentrations measured at five time-points during a 75 g OGTT in 49 participants having GCK mutations with those of 28 familial controls. Mathematical modelling of glucose, insulin and C-peptide was used to estimate basal insulin secretion rate (BSR), total insulin secretion (TIS), beta cell glucose sensitivity, potentiation factor and insulin secretion rate (ISR). Results  GIP and GLP-1 profiles during the OGTT were similar in GCK mutation carriers and controls (p = 0.52 and p = 0.44, respectively). Modelled variables of beta cell function showed a reduction in beta cell glucose sensitivity (87 pmol min⁻¹ m⁻² [mmol/l]⁻¹ [95% CI 66–108] vs 183 pmol min⁻¹ m⁻² [mmol/l]⁻¹ [95% CI 155–211], p < 0.001) and potentiation factor (1.5 min [95% CI 1.2–1.8] vs 2.2 min [95% CI 1.8–2.7], p = 0.007) but no change in BSR or TIS. The glucose/ISR curve was right-shifted in GCK mutation carriers. Conclusions/interpretation  Glucokinase, the major pancreatic glucose sensor, is not the main gut glucose sensor. By modelling OGTT data in GCK mutation carriers we were able to distinguish a specific beta cell glucose-sensing defect. Our data suggest a reduction in potentiation of insulin secretion by glucose that is independent of differences in incretin hormone release.     

Four weeks of near-normalisation of blood glucose improves the insulin response to glucagon-like peptide-1 and glucose-dependent insulinotropic polypeptide in patients with type 2 diabetes

Objective  The incretin effect is attenuated in patients with type 2 diabetes mellitus, partly as a result of impaired beta cell responsiveness to glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1). The aim of the present study was to investigate whether 4 weeks of near-normalisation of the blood glucose level could improve insulin responses to GIP and GLP-1 in patients with type 2 diabetes. Methods  Eight obese patients with type 2 diabetes with poor glycaemic control (HbA_1c 8.6 ± 1.3%), were investigated before and after 4 weeks of near-normalisation of blood glucose (mean blood glucose 7.4 ± 1.2 mmol/l) using insulin treatment. Before and after insulin treatment the participants underwent three hyperglycaemic clamps (15 mmol/l) with infusion of GLP-1, GIP or saline. Insulin responses were evaluated as the incremental area under the plasma C-peptide curve. Results  Before and after near-normalisation of blood glucose, the C-peptide responses did not differ during the early phase of insulin secretion (0–10 min). The late phase C-peptide response (10–120 min) increased during GIP infusion from 33.0 ± 8.5 to 103.9 ± 24.2 (nmol/l) × (110 min)⁻¹ (p < 0.05) and during GLP-1 infusion from 48.7 ± 11.8 to 126.6 ± 32.5 (nmol/l) × (110 min)⁻¹ (p < 0.05), whereas during saline infusion the late-phase response did not differ before vs after near-normalisation of blood glucose (40.2 ± 11.2 vs 46.5 ± 12.7 [nmol/l] × [110 min]⁻¹). Conclusions  Near-normalisation of blood glucose for 4 weeks improves beta cell responsiveness to both GLP-1 and GIP by a factor of three to four. No effect was found on beta cell responsiveness to glucose alone. ClinicalTrials.gov ID no.: NCT 00612950 Funding: This study was supported by The Novo Nordisk Foundation, The Medical Science Research Foundation for Copenhagen.     

Glucagon receptor antagonism improves islet function in mice with insulin resistance induced by a high-fat diet

Aims/hypothesis Increased glucagon secretion predicts deterioration of glucose tolerance, and high glucagon levels contribute to hyperglycaemia in type 2 diabetes. Inhibition of glucagon action may therefore be a potential novel target to reduce hyperglycaemia. Here, we investigated whether chronic treatment with a glucagon receptor antagonist (GRA) improves islet dysfunction in female mice on a high-fat diet (HFD). Materials and methods After 8 weeks of HFD, mice were treated with a small molecule GRA (300 mg/kg, gavage once daily) for up to 30 days. Insulin secretion was studied after oral and intravenous administration of glucose and glucagon secretion after intravenous arginine. Islet morphology was examined and insulin secretion and glucose oxidation were measured in isolated islets. Results Fasting plasma glucose levels were reduced by GRA (6.0 ± 0.2 vs 7.4 ± 0.5 mmol/l; p = 0.017). The acute insulin response to intravenous glucose was augmented (1,300 ± 110 vs 790 ± 64 pmol/l; p < 0.001). The early insulin response to oral glucose was reduced in mice on HFD + GRA (1,890 ± 160 vs 3,040 ± 420 pmol/l; p = 0.012), but glucose excursions were improved. Intravenous arginine significantly increased the acute glucagon response (129 ± 12 vs 36 ± 6 ng/l in controls; p < 0.01), notably without affecting plasma glucose. GRA caused a modest increase in alpha cell mass, while beta cell mass was similar to that in mice on HFD + vehicle. Isolated islets displayed improved glucose-stimulated insulin secretion after GRA treatment (0.061 ± 0.007 vs 0.030 ± 0.004 pmol islet⁻¹ h⁻¹ at 16.7 mmol/l glucose; p < 0.001), without affecting islet glucose oxidation. Conclusions/interpretation Chronic glucagon receptor antagonism in HFD-fed mice improves islet sensitivity to glucose and increases insulin secretion, suggesting improvement of key defects underlying impaired glucose tolerance and type 2 diabetes.     

The ageing entero-insular axis

Summary Ageing is one of the major risk factors for glucose intolerance including impaired glucose tolerance and Type II (non-insulin-dependent) diabetes mellitus. Reduced insulin secretion has been described as part of normal ageing although there is no information on age-related changes in the secretion of the major insulinotropic hormones, glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide (7–36 amide) (GLP-1). We assessed the entero-insular axis in 6 young premenopausal and 6 older postmenopausal women following treatment with oral carbohydrate. Insulin and glucose integrated responses were similar in the younger and older groups. Total integrated responses for GIP and GLP-1 were considerably greater in the older subjects. A positive correlation between age and total integrated responses for glucose (r = 0.65; p < 0.02) as well as GLP-1 (r = 0.85; p < 0.001) was seen. We hypothesise that an age-related impairment of insulin secretion to insulinotropic hormones, GIP and GLP-1, contributes to a reduction in glucose tolerance in this age group. The pronounced compensatory increase in postprandial secretion of GIP and GLP-1 provides further evidence not only for the negative feedback relation between incretin and insulin secretion but also for the importance of the entero-insular axis in the regulation of insulin secretion. [Diabetologia (1998) 41: 1309–1313] Received: 14 January 1998 and in revised form: 12 June 1998     

Secretion of incretin hormones and the insulinotropic effect of gastric inhibitory polypeptide in women with a history of gestational diabetes

Aims/hypothesis The insulinotropic effect of gastric inhibitory polypeptide (GIP) is reduced in patients with type 2 diabetes and around 50% of their first-degree relatives under hyperglycaemic conditions. It is unknown whether this is a result of a specific defect in GIP action or of a general reduction in beta cell function. Moreover, impaired secretion of glucagon-like peptide 1 (GLP-1) has been described in patients with type 2 diabetes. Therefore, we studied the insulinotropic effect of GIP in women with previous gestational diabetes (pGDM) under euglycaemic fasting conditions and during a hyperglycaemic clamp experiment. The secretion of GIP and GLP-1 was assessed following oral glucose ingestion.Materials and methods On separate occasions we performed an OGTT and administered an i.v. bolus of 20 pmol GIP/kg body weight in 20 women with pGDM and 20 control women. An additional hyperglycaemic clamp experiment (140 mg/dl [7.8 mmol/l] over 120 min) with i.v. infusion of GIP (2 pmol kg^–1 min^–1; 30–90 min) was performed in 14 women in each group. Capillary and venous blood samples were drawn for the measurement of glucose (glucose oxidase), insulin, C-peptide, GIP and GLP-1 (specific immunoassays). Indices of insulin sensitivity and beta cell function were calculated. Statistical analyses were carried out using repeated measures ANOVA.Results Following oral glucose ingestion, plasma glucose, insulin and C-peptide concentrations increased to higher levels in the women with pGDM than in the control women (p<0.05). The women with pGDM were characterised by a higher degree of insulin resistance than the control women (p=0.007 for the Matsuda index), but showed no overt defects in glucose-stimulated insulin secretion (p=0.40 for the insulinogenic index following i.v. glucose). The secretion of GLP-1 and GIP was not different between the groups (p=0.87 and p=0.57, respectively). The insulin secretory response to GIP administration was similar in the two groups both after GIP bolus administration and during the hyperglycaemic clamp experiment (p=0.99 and p=0.88, respectively). A hyperbola-like relationship was found between the degree of insulin sensitivity (Matsuda index) and the insulin secretory response to GIP and i.v. glucose administration.Conclusions/interpretation These results do not support the hypothesis of an early defect in GIP action as a risk factor for subsequent development of diabetes in women with previous gestational diabetes. The inverse relationship between insulin resistance and the insulin secretory response to glucose or GIP suggests that beta cell secretory function in response to different stimuli increases adaptively when insulin sensitivity is diminished.     

Near normalisation of blood glucose improves the potentiating effect of GLP-1 on glucose-induced insulin secretion in patients with type 2 diabetes

Aims/hypothesis The ability of glucagon-like peptide-1 (GLP-1) to enhance beta cell responsiveness to i.v. glucose is impaired in patients with type 2 diabetes mellitus compared with healthy individuals. We investigated whether 4 weeks of near normalisation of blood glucose (BG) improves the potentiation of glucose-stimulated insulin secretion by GLP-1. Methods Nine obese patients with type 2 diabetes and inadequate glycaemic control (HbA_1c 8.0 ± 0.4%) were investigated before and after 4 weeks of near normalisation of BG using insulin treatment (mean diurnal blood glucose 6.4 ± 0.3 mmol/l, HbA_1c 6.6 ± 0.3%). Nine matched healthy participants were also studied. Beta cell function was investigated before and after insulin treatment using stepwise glucose infusions and infusion of saline or GLP-1 (1.0 pmol kg⁻¹ min⁻¹), resulting in supraphysiological total GLP-1 concentrations of approximately 200 pmol/l. The responsiveness to glucose or glucose+GLP-1 was expressed as the slope of the linear regression line relating insulin secretion rate (ISR) and plasma glucose concentration (pmol kg⁻¹ min⁻¹ [mmol/l]⁻¹). Results In the diabetic participants, the slopes during glucose+saline infusion did not differ before and after insulin treatment (0.33 ± 0.07 and 0.39 ± 0.04, respectively; p = NS). In contrast, near normalisation of blood glucose improved beta cell sensitivity to glucose during glucose+GLP-1 infusion (1.27 ± 0.2 before vs 1.73 ± 0.31 after; p < 0.01). In the healthy participants, the slopes during the glucose+saline and glucose+GLP-1 infusions were 1.01 ± 0.14 and 4.79 ± 0.53, respectively. Conclusions/interpretation A supraphysiological dose of GLP-1 enhances beta cell responses to glucose in patients with type 2 diabetes, and 4 weeks of near normalisation of blood glucose further improves this effect. ClinicalTrials.gov ID no.: NCT00612625     

Impaired fasting glycaemia vs impaired glucose tolerance: similar impairment of pancreatic alpha and beta cell function but differential roles of incretin hormones and insulin action

Aims/hypothesis The impact of strategies for prevention of type 2 diabetes in isolated impaired fasting glycaemia (i-IFG) vs isolated impaired glucose tolerance (i-IGT) may differ depending on the underlying pathophysiology. We examined insulin secretion during OGTTs and IVGTTs, hepatic and peripheral insulin action, and glucagon and incretin hormone secretion in individuals with i-IFG (n = 18), i-IGT (n = 28) and normal glucose tolerance (NGT, n = 20). Methods Glucose tolerance status was confirmed by a repeated OGTT, during which circulating insulin, glucagon, glucose-dependent insulinotrophic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) levels were measured. A euglycaemic–hyperinsulinaemic clamp with [3–³H]glucose preceded by an IVGTT was performed. Results Absolute first-phase insulin secretion during IVGTT was decreased in i-IFG (p = 0.026), but not in i-IGT (p = 0.892) compared with NGT. Hepatic insulin sensitivity was normal in i-IFG and i-IGT individuals (p ≥ 0.179). Individuals with i-IGT had peripheral insulin resistance (p = 0.003 vs NGT), and consequently the disposition index (DI; insulin secretion×insulin sensitivity) during IVGTT (DI_IVGTT)) was reduced in both i-IFG and i-IGT (p < 0.005 vs NGT). In contrast, the DI during OGTT (DI_OGTT) was decreased only in i-IGT (p < 0.001), but not in i-IFG (p = 0.143) compared with NGT. Decreased levels of GIP in i-IGT (p = 0.045 vs NGT) vs increased levels of GLP-1 in i-IFG (p = 0.013 vs NGT) during the OGTT may partially explain these discrepancies. Basal and post-load glucagon levels were significantly increased in both i-IFG and i-IGT individuals (p ≤ 0.001 vs NGT). Conclusions/interpretation We propose that differentiated preventive initiatives in prediabetic individuals should be tested, targeting the specific underlying metabolic defects.     

Pharmacokinetic,insulinotropic, and glucagonostatic properties of GLP-1 [7–36 amide] after subcutaneous injection in healthy volunteers. Dose-response-relationships

Summary Intravenous infusions of glucagon-like peptide 1 (GLP-1) [7–36 amide] are glucose-dependently insulinotropic and glucagonostatic and normalize plasma glucose concentrations in non-insulin-dependent diabetic patients. It was the aim of this study to investigate whether subcutaneous GLP-1 [7–36 amide] also has an influence on insulin and glucagon secretion, and which doses are required for significant effects. Therefore, eight healthy volunteers (24±2 years, body mass index [BMI] 21.9±2.3 kg/ m²) were studied in the fasting state on five occasions in randomized order. Placebo (0.9% NaCl with 1% human serum albumin) or GLP-1 [7–36 amide] in doses of 0.15, 0.5, 1.5 or 4.5 nmol/kg body weight (volume 1 ml or, at the highest dose, 2 ml) was administered subcutaneously. An intravenous glucose bolus (0.33 g/kg body weight) was injected 30 min later. Blood was drawn for the measurement of glucose, insulin, C-peptide, GLP-1 [7–36 amide], and glucagon using specific radioimmunoassays. There were dose-related increments in GLP-1 [7–36 amide] concentrations (p<0.0001). However, basal values were reached again after 90–120 min. Before glucose administration, insulin (p<0.0001) and C-peptide (p<0.0004) increased, whereas glucagon (p = 0.0018) and glucose (p<0.0001) decreased in a dose-dependent manner. After glucose stimulation, integrated increments in insulin (p=0.0007) and C-peptide (p=0.02) were augmented and k_G-values increased (p<0.0001) in a dose-related fashion. The extent of reactive hypoglycaemia was related to the GLP-1 [7–36 amide] dose. With the highest GLP-1 [7–36 amide] dose, at the time of peak plasma concentrations, most volunteers felt unwell, and nausea and vomiting were observed in four subjects. In conclusion, subcutaneous GLP-1 [7–36 amide] is also able to stimulate insulin and inhibit glucagon secretion, thereby altering glucose assimilation. However, with unmodified GLP-1 [7–36 amide], the duration of action is short, and with high doses side effects are common.Abbreviations BMI Body mass index - GLP-1 glucagon-like peptide-1 - RIA radioimmunoassay - RM-ANOVA repeated-measures analysis of variance - GIP gastric inhibitory polypeptide     

Exaggerated secretion of glucagon-like peptide-1 (GLP-1) could cause reactive hypoglycaemia

Summary The plasma concentrations of the insulinotropic incretin hormone, glucagon-like peptide-1 (GLP-1) are abnormally high after oral glucose in partially gastrectomised subjects with reactive hypoglycaemia, suggesting a causal relationship. Because of the glucose-dependency of its effects, it is impossible to induce hypoglycaemia in normal subjects in the basal state by exogenous GLP-1, regardless of dose. To further assess the role of the incretin hormones in reactive hypoglycaemia, we reproduced the glucose and hormone profiles of the patients with reactive hypoglycaemia in 8 healthy volunteers in 4 separate protocols: 1) i. v. infusion of glucose (25 g) alone, 2) glucose together with i. v. GLP-1 infusion, and 3) and 4) glucose together with i. v. infusion of the other incretin hormone, glucose-dependent insulinotropic polypeptide (GIP), at two different infusion rates. The plasma glucose, GLP-1 and GIP concentrations (low dose) obtained were comparable with those of the patients. With GLP-1, infusion of a total of 33.4 ± 1.3 g glucose was required to obtain plasma glucose concentrations similar to those obtained by glucose infusion alone; with low GIP, 28.0 ± 1.2 g and with high GIP 38.4 ± 3.5 g. Insulin concentrations increased 10-fold with GLP-1 compared with i. v. glucose alone, but less with high and low GIP. In contrast, C-peptide concentrations were similar after GLP-1 and high GIP. After termination of i. v. glucose the lowest glucose concentrations were 4.5 (3.7–4.9) (median, range) for glucose alone; 2.4 (1.9–2.8) mmol/l with GLP-1; 3.7 (2.6–4.0) with low GIP and 3.3 (2.1–4.2 ) with high GIP. Thus, the exaggerated GLP-1 response to nutrients in patients with accelerated gastric emptying could be responsible for their high incidence of postprandial reactive hypoglycaemia. [Diabetologia (1998) 41: 1180–1186] Received: 23 February 1998 and in revised form: 18 May 1998     

A DPP-IV-resistant triple-acting agonist of GIP, GLP-1 and glucagon receptors with potent glucose-lowering and insulinotropic actions in high-fat-fed mice

Aims/hypothesis

We designed a chemically modified, enzyme-resistant peptide with triple-acting properties based on human glucagon with amino acid substitutions aligned to strategic positions in the sequence of glucose-dependent insulinotropic polypeptide (GIP).

Methods

Y¹-dA²-I¹²-N¹⁷-V¹⁸-I²⁷-G^28,29-glucagon (termed YAG-glucagon) was incubated with dipeptidylpeptidase IV (DPP-IV) to assess stability, BRIN-BD11 cells to evaluate insulin secretion, and receptor-transfected cells to examine cAMP production. Acute glucose-lowering and insulinotropic properties of YAG-glucagon were assessed in National Institutes of Health (NIH) Swiss mice, while longer-term actions on glucose homeostasis, insulin secretion, food intake and body weight were examined in high-fat-fed mice.

Results

YAG-glucagon was resistant to DPP-IV, increased in vitro insulin secretion (1.5–3-fold; p?<?0.001) and stimulated cAMP production in GIP receptor-, glucagon-like peptide-1 (GLP-1) receptor- and glucagon receptor-transfected cells. Plasma glucose levels were significantly reduced (by 51%; p?<?0.01) and insulin concentrations increased (1.2-fold; p?<?0.01) after acute injection of YAG-glucagon in NIH Swiss mice. Acute actions were countered by established GIP, GLP-1 and glucagon antagonists. In high-fat-fed mice, twice-daily administration of YAG-glucagon for 14 days reduced plasma glucose (40% reduction; p?<?0.01) and increased plasma insulin concentrations (1.8-fold; p?<?0.05). Glycaemic responses were markedly improved (19–48% reduction; p?<?0.05) and insulin secretion enhanced (1.5-fold; p?<?0.05) after a glucose load, which were independent of changes in insulin sensitivity, food intake and body weight.

Conclusions/interpretation

YAG-glucagon is a DPP-IV-resistant triple agonist of GIP, GLP-1 and glucagon receptors and exhibits beneficial biological properties suggesting that it may hold promise for treatment of type 2 diabetes.     

The evolving story of incretins (GIP and GLP-1) in metabolic and cardiovascular disease: A pathophysiological update

The incretin hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) have their main physiological role in augmenting insulin secretion after their nutrient-induced secretion from the gut. A functioning entero-insular (gut-endocrine pancreas) axis is essential for the maintenance of a normal glucose tolerance. This is exemplified by the incretin effect (greater insulin secretory response to oral as compared to “isoglycaemic” intravenous glucose administration due to the secretion and action of incretin hormones). GIP and GLP-1 have additive effects on insulin secretion. Local production of GIP and/or GLP-1 in islet α-cells (instead of enteroendocrine K and L cells) has been observed, and its significance is still unclear. GLP-1 suppresses, and GIP increases glucagon secretion, both in a glucose-dependent manner. GIP plays a greater physiological role as an incretin. In type 2-diabetic patients, the incretin effect is reduced despite more or less normal secretion of GIP and GLP-1. While insulinotropic effects of GLP-1 are only slightly impaired in type 2 diabetes, GIP has lost much of its acute insulinotropic activity in type 2 diabetes, for largely unknown reasons. Besides their role in glucose homoeostasis, the incretin hormones GIP and GLP-1 have additional biological functions: GLP-1 at pharmacological concentrations reduces appetite, food intake, and—in the long run—body weight, and a similar role is evolving for GIP, at least in animal studies. Human studies, however, do not confirm these findings. GIP, but not GLP-1 increases triglyceride storage in white adipose tissue not only through stimulating insulin secretion, but also by interacting with regional blood vessels and GIP receptors. GIP, and to a lesser degree GLP-1, play a role in bone remodelling. GLP-1, but not GIP slows gastric emptying, which reduces post-meal glycaemic increments. For both GIP and GLP-1, beneficial effects on cardiovascular complications and neurodegenerative central nervous system (CNS) disorders have been observed, pointing to therapeutic potential over and above improving diabetes complications. The recent finding that GIP/GLP-1 receptor co-agonists like tirzepatide have superior efficacy compared to selective GLP-1 receptor agonists with respect to glycaemic control as well as body weight has renewed interest in GIP, which previously was thought to be without any therapeutic potential. One focus of this research is into the long-term interaction of GIP and GLP-1 receptor signalling. A GLP-1 receptor antagonist (exendin [9-39]) and, more recently, a GIP receptor agonist (GIP [3-30] NH₂) and, hopefully, longer-acting GIP receptor agonists for human use will be helpful tools to shed light on the open questions. A detailed knowledge of incretin physiology and pathophysiology will be a prerequisite for designing more effective incretin-based diabetes drugs.     

Inappropriate suppression of glucagon during OGTT but not during isoglycaemic i.v. glucose infusion contributes to the reduced incretin effect in type 2 diabetes mellitus

Aims/hypothesis We investigated glucagon responses during OGTT and isoglycaemic i.v. glucose infusion, respectively, to further elucidate the mechanisms behind the glucose intolerance in patients with type 2 diabetes. Materials and methods Ten patients (eight men) with type 2 diabetes (age: 64 [51–80] years; BMI: 23 [21–26] kg/m²; HbA_1c: 6.9 [6.2–8.7]%, values mean [range]) and ten control subjects matched for sex, age and BMI were studied. Blood was sampled on two separate days following a 4-h 50-g OGTT and an isoglycaemic i.v. glucose infusion, respectively. Results Isoglycaemia during the 2 days was obtained in both groups. In the control subjects no difference in glucagon suppression during the first 45 min of OGTT and isoglycaemic i.v. glucose infusion (−36 ± 12 vs −64 ± 23 mmol/l × 45 min; p = NS) was observed, whereas in the group of patients with type 2 diabetes significant glucagon suppression only occurred following isoglycaemic i.v. glucose infusion (−63 ± 21 vs 10 ± 16 mmol/l × 45 min; p = 0.002). The incretin effect was significantly reduced in patients with type 2 diabetes compared with control subjects, but no significant differences in the secretion of glucagon-like peptide-1 or glucose-dependent insulinotropic polypeptide between the two groups during OGTT or isoglycaemic i.v. glucose infusion, respectively, could explain this. Conclusions/interpretation Attenuated and delayed glucagon suppression in patients with type 2 diabetes occurs after oral ingestion of glucose, while isoglycaemic i.v. administration of glucose results in normal suppression of glucagon. We suggest that this phenomenon contributes both to the glucose intolerance and to the reduced incretin effect observed in patients with type 2 diabetes.     

Effect of glucagon antibodies on plasma glucose,insulin and somatostatin in the fasting and fed rat

Summary A potent high-titre glucagon antibody pool was used to induce a state of acute glucagon deficiency in order to investigate the importance of glucagon in maintaining euglycaemia in the fed and fasted anaesthetised rat. Binding characteristics of the antiserum and evidence of its neutralisation of the biological effects of exogenous glucagon are described. The amount of antibody administered was capable of neutralising up to 12 times the total content of glucagon (approximately 1nmol) in the rat pancreas. The hyperglycaemic response to 1.43 nmol exogenous glucagon was significantly inhibited in the rat by glucagon antibodies given intravenously or intraperitoneally (p < 0.001). However, no changes in plasma glucose occurred in rats fasted 16 h (4.35±0.1 mmol/l or 24 h (4.0±0.05 mmol/l) after antibody administration. The same dose of glucagon antibodies produced no change in plasma glucose (6.1±0.2 mmol/l), immunoreactive insulin (1.85±0.05 g/l) or immunoreactive somatostatin (110±30 ng/l) in rats after antibody administration. Antibody excess, equivalent to a binding capacity for glucagon of 40 nmol/l in the plasma of recipient animals, was demonstrable at all times after passive immunisation. The absence of any affect on glucose concentrations following immunoneutralisation of glucagon suggests that glucagon secretion may not be a major factor in the maintenance of euglycaemia in the rat.     

Circadian rhythms of GIP and GLP1 in glucose-tolerant and in type 2 diabetic patients after biliopancreatic diversion

Aims/hypothesis  We tested the hypothesis that the reversibility of insulin resistance and diabetes observed after biliopancreatic diversion (BPD) is related to changes in circadian rhythms of gastrointestinal hormones. Methods  Ten morbidly obese participants, five with normal glucose tolerance (NGT) and five with type 2 diabetes, were studied before and within 2 weeks after BPD. Within-day variations in glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide 1 (GLP1) levels were assessed using a single cosinor model. Insulin sensitivity was assessed by euglycaemic–hyperinsulinaemic clamp. Results  Basal GLP1 relative amplitude (amplitude/mesor × 100) was 25.82–4.06% in NGT; it increased to 41.38–4.32% after BPD but was unchanged in diabetic patients. GLP1 and GIP mesor were shifted in time after surgery in diabetic patients but not in NGT participants. After BPD, the GLP1 AUC significantly increased from 775 ± 94 to 846 ± 161 pmol l⁻¹ min in NGT, whereas GIP AUC decreased significantly from 1,373 ± 565 to 513 ± 186 pmol l⁻¹ min in diabetic patients. Two-way ANOVA showed a strong influence of BPD on both GIP (p = 0.010) and GLP1 AUCs (p = 0.033), which was potentiated by the presence of diabetes, particularly for GIP (BPD × diabetes, p = 0.003). Insulin sensitivity was markedly improved (p < 0.01) in NGT (from 9.14 ± 3.63 to 36.04 ± 8.55 μmol [kg fat-free mass]⁻¹ min⁻¹) and diabetic patients (from 9.49 ± 3.56 to 38.57 ± 4.62 μmol [kg fat-free mass]⁻¹ min⁻¹). Conclusions/interpretation  An incretin circadian rhythm was shown for the first time in morbid obesity. The effect of BPD on the 24 h pattern of incretin differed between NGT and diabetic patients. GLP1 secretion impairment was reversed in NGT and could not be overcome by surgery in diabetes. On the other hand, GIP secretion was blunted after the operation only in diabetic patients, suggesting a role in insulin resistance and diabetes.     

Evidence that autonomic mechanisms contribute to the adaptive increase in insulin secretion during dexamethasone-induced insulin resistance in humans

Aims/hypothesis This study examined whether autonomic mechanisms contribute to adaptively increased insulin secretion in insulin-resistant humans, as has been proposed from studies in animals. Methods Insulin secretion was evaluated before and after induction of insulin resistance with or without interruption of neural transmission. Insulin resistance was induced by dexamethasone (15 mg given over 3 days) in nine healthy women (age 67 years, BMI 25.2 ± 3.4 kg/m², fasting glucose 5.1 ± 0.4 mmol/l, fasting insulin 46 ± 6 pmol/l). Insulin secretion was evaluated as the insulin response to intravenous arginine (5 g) injected at fasting glucose and after raising glucose to 13 to15 mmol/l or to >28 mmol/l. Neural transmission across the ganglia was interrupted by infusion of trimethaphan (0.3–0.6 mg kg⁻¹ min⁻¹). Results As an indication of insulin resistance, dexamethasone increased fasting insulin (to 75 ± 8 pmol/l, p < 0.001) without significantly affecting fasting glucose. Arginine-induced insulin secretion was increased by dexamethasone at all glucose levels (by 64 ± 12% at fasting glucose, by 80 ± 19% at 13–15 mmol glucose and by 43 ± 12% at >28 mmol glucose; p <0.001 for all). During dexamethasone-induced insulin resistance, trimethaphan reduced the insulin response to arginine at all three glucose levels. The augmentation of the arginine-induced insulin responses by dexamethasone-induced insulin resistance was reduced by trimethaphan by 48 ± 6% at fasting glucose, 61 ± 8% at 13–15 mmol/l glucose and 62 ± 8% at >28 mmol/l glucose (p < 0.001 for all). In contrast, trimethaphan did not affect insulin secretion before dexamethasone was given. Conclusions/interpretations Autonomic mechanisms contribute to the adaptative increase in insulin secretion in dexamethasone-induced insulin resistance in healthy participants.     

Incretins and the development of type 2 diabetes

The incretin hormones gastric inhibitory polypeptide (GIP) and glucagon-like peptide 1 (GLP-1) are released in response to nutrient ingestion and potentiate glucosestimulated insulin secretion from pancreatic β cells. The augmentation of postprandial insulin secretion by such gastrointestinal hormones is called the incretin effect. The incretin effect is almost completely absent in patients with type 2 diabetes. This is due to 1) an approximate 15% reduction in postprandial GLP-1 secretion and 2) a near total loss of insulinotropic activity of GIP. This review article summarizes clinical studies on abnormalities in the secretion and insulinotropic effects of GIP and GLP-1 in patients with type 2 diabetes as well as in individuals at high risk. A significant proportion of first-degree relatives are characterized by a reduced insulinotropic response to exogenous GIP. Nevertheless, this phenomenon does not predispose to a more rapid deterioration in glucose tolerance or conversion to impaired glucose tolerance or diabetes. Therefore, although there are hints of early abnormalities in incretin secretion and action in prediabetic populations, it has not been proven that such phenomena are central to the pathogenesis of type 2 diabetes.     

Regulation of glucagon secretion by incretins

Glucagon secretion plays an essential role in the regulation of hepatic glucose production, and elevated fasting and postprandial plasma glucagon concentrations in patients with type 2 diabetes (T2DM) contribute to their hyperglycaemia. The reason for the hyperglucagonaemia is unclear, but recent studies have shown lack of suppression after oral but preserved suppression after isoglycaemic intravenous glucose, pointing to factors from the gut. Gastrointestinal hormones that are secreted in response to oral glucose include glucagon-like peptide-1 (GLP-1) that strongly inhibits glucagon secretion, and GLP-2 and GIP, both of which stimulate secretion. When the three hormones are given together on top of isoglycaemic intravenous glucose, glucagon suppression is delayed in a manner similar to that observed after oral glucose. Studies with the GLP-1 receptor antagonist, exendin 9-39, suggest that endogenous GLP-1 plays an important role in regulation of glucagon secretion during fasting as well as postprandially. The mechanisms whereby GLP-1 regulates glucagon secretion are debated, but studies in isolated perfused rat pancreas point to an important role for a paracrine regulation by somatostatin from neighbouring D cells. Clinical studies of the antidiabetic effect of GLP-1 in T2DM suggest that the inhibition of glucagon secretion is as important as the stimulation of insulin secretion.     

Defective amplification of the late phase insulin response to glucose by GIP in obese Type II diabetic patients

AIMS/HYPOTHESIS: Glucagon-like-peptide-1 (GLP-1) is strongly insulinotropic in patients with Type II (non-insulin-dependent) diabetes mellitus, whereas glucose-dependent insulinotropic polypeptide (GIP) is less effective. Our investigation evaluated "early" (protocol 1) - and "late phase" (protocol 2) insulin and C-peptide responses to GLP-1 and GIP stimulation in patients with Type II diabetes. METHODS: Protocol 1: eight Type II diabetic patients and eight matched healthy subjects received i.v. bolus injections of GLP-1(2.5 nmol) or GIP(7.5 nmol) concomitant with an increase of plasma glucose to 15 mmol/l. Protocol 2: eight Type II diabetic patients underwent a hyperglycaemic clamp (15 mmol/l) with infusion (per kg body weight/min) of either: 1 pmol GLP-1 (7-36) amide (n=8), 4 pmol GIP (n=8), 16 pmol GIP (n=4) or no incretin hormone (n=5). For comparison, six matched healthy subjects were examined. RESULTS: Protocol 1: Type II diabetic patients were characterised by a decreased "early phase" response to both stimuli, but their relative response to GIP versus GLP-1 stimulation was exactly the same as in healthy subjects [insulin (C-peptide): patients 59+/-9% (74+/-6%) and healthy subjects 62+/-5% (71+/-9%)]. Protocol 2, "Early phase" (0-20 min) insulin response to glucose was delayed and reduced in the patients, but enhanced slightly and similarly by GIP and GLP-1. GLP-1 augmented the "late phase" (20-120 min) insulin secretion to levels similar to those observed in healthy subjects. In contrast, the "late phase" responses to both doses of GIP were not different from those obtained with glucose alone. Accordingly, glucose infusion rates required to maintain the hyperglycaemic clamp in the "late phase" period (20-120 min) were similar with glucose alone and glucose plus GIP, whereas a doubling of the infusion rate was required during GLP-1 stimulation. CONCLUSION/INTERPRETATION: Lack of GIP amplification of the late phase insulin response to glucose, which contrasts markedly to the normalising effect of GLP-1, could be a key defect in insulin secretion in Type II diabetic patients.