Aim/hypothesis Hyperglucagonaemia is a characteristic of several clinical conditions (e.g. end-stage renal disease (ESRD), type 2 diabetes, obesity before and after Roux-en-Y gastric bypass (RYGB) and vagotomy with pyloroplasty), but the molecular nature of 'immunoreactive' glucagon is poorly characterised. The specific determination of fully processed, intact glucagon requires a 'sandwich' assay employing a combination of antibodies directed against both N-and C-termini. We compared a novel assay for intact glucagon with a highly sensitive C-terminal RIA (hitherto considered specific) to determine the extent to which the hyperglucagonaemia measured in clinical samples was caused by authentic glucagon. Methods We examined the performance of three commercial glucagon 'sandwich' ELISAs. The ELISA with the best overall performance was selected to compare glucagon measurements in clinical samples with an established glucagon RIA. ResultsThe first assay performed poorly: there was high cross-reactivity with glicentin (22%) and a lack of sensitivity for glucagon. The second and third assays showed minor cross-reactivity (1-5%) with oxyntomodulin and glicentin; however, the second assay had insufficient sensitivity for glucagon in plasma (>10-20 pmol/l). Thus, only the third assay was suitable for measuring glucagon concentrations in clinical samples. The ELISA and RIA measured similar glucagon levels in healthy individuals. Measurements of samples from individuals with abnormally high (type 2 diabetes or obese) or very elevated (post vagotomy with pyloroplasty, post-RYGB) glucagon levels were also similar in both assays. However, glucagon levels in participants with ESRD were much lower when measured by ELISA than by RIA, indicating that the apparent hyperglucagonaemia is not caused by fully processed intact glucagon.

The incretin hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide 1 (GLP-1) are secreted postprandially and contribute importantly to postprandial glucose tolerance. In this study, we assessed the individual and combined contributions of endogenous GIP and GLP-1 to the postprandial changes in glucose and glucoregulatory hormones using the novel GIP receptor antagonist GIP(3-30)NH2 and the well-established GLP-1 receptor antagonist exendin(9-39)NH2. During 4-h oral glucose tolerance tests (75 g) combined with an ad libitum meal test, 18 healthy men received on four separate days in randomized, double-blinded order intravenous infusions of A) GIP(3-30)NH2 (800 pmol/kg/min) plus exendin(9-39)NH2 (0–20 min: 1,000 pmol/kg/min; 20–240 min: 450 pmol/kg/min), B) GIP(3-30)NH2, C) exendin(9-39)NH2, and D) saline, respectively. Glucose excursions were significantly higher during A than during B, C, and D, while glucose excursions during B were higher than during C and D. Insulin secretion (assessed by C-peptide/glucose ratio) was reduced by 37 ± 16% (A), 30 ± 17% (B), and 8.6 ± 16% (C) compared with D (mean ± SD). A and C resulted in higher glucagon levels and faster gastric emptying. In conclusion, endogenous GIP affects postprandial plasma glucose excursions and insulin secretion more than endogenous GLP-1, but the hormones contribute additively to postprandial glucose regulation in healthy individuals.

Rapid degradation of glucagon-like peptide-1 (GLP-1) by dipeptidyl peptidase-4 suggests that endogenous GLP-1 may act locally before being degraded. Signaling via the vagus nerve was investigated in 20 truncally vagotomized subjects with pyloroplasty and 10 matched healthy controls. Subjects received GLP-1 (7-36 amide) or saline infusions during and after a standardized liquid mixed meal and a subsequent ad libitum meal. Despite no effect on appetite sensations, GLP-1 significantly reduced ad libitum food intake in the control group but had no effect in the vagotomized group. Gastric emptying was accelerated in vagotomized subjects and was decreased by GLP-1 in controls but not in vagotomized subjects. Postprandial glucose levels were reduced by the same percentage by GLP-1 in both groups. Peak postprandial GLP-1 levels were approximately fivefold higher in the vagotomized subjects. Insulin secretion was unaffected by exogenous GLP-1 in vagotomized subjects but was suppressed in controls. GLP-1 significantly reduced glucagon secretion in both groups, but levels were approximately twofold higher and were nonsuppressible in the early phase of the meal in vagotomized subjects. Our results demonstrate that vagotomy with pyloroplasty impairs the effects of exogenous GLP-1 on food intake, gastric emptying, and insulin and glucagon secretion, suggesting that intact vagal innervation may be important for GLP-1's actions.

ObjectiveGlucagon-like peptide 1 (GLP1) is rapidly inactivated by dipeptidyl peptidase 4 (DPP4), but may interact with vagal neurons at its site of secretion. We investigated the role of vagal innervation for handling of oral and i.v. glucose.Design and methodsTruncally vagotomised subjects (n=16) and matched controls (n=10) underwent 50 g-oral glucose tolerance test (OGTT)±vildagliptin, a DPP4 inhibitor (DPP4i) and isoglycaemic i.v. glucose infusion (IIGI), copying the OGTT without DPP4i.ResultsIsoglycaemia was obtained with 25±2 g glucose in vagotomised subjects and 18±2 g in controls (P<0.03); thus, gastrointestinal-mediated glucose disposal (GIGD) – a measure of glucose handling (100%×(glucoseOGTT−glucoseIIGI/glucoseOGTT)) – was reduced in the vagotomised compared with the control group. Peak intact GLP1 concentrations were higher in the vagotomised group. Gastric emptying was faster in vagotomised subjects after OGTT and was unaffected by DPP4i. The early glucose-dependent insulinotropic polypeptide response was higher in vagotomised subjects. Despite this, the incretin effect was equal in both groups. DPP4i enhanced insulin secretion in controls, but had no effect in the vagotomised subjects. Controls suppressed glucagon concentrations similarly, irrespective of the route of glucose administration, whereas vagotomised subjects showed suppression only during IIGI and exhibited hyperglucagonaemia following OGTT. DPP4i further suppressed glucagon secretion in controls and tended to normalise glucagon responses in vagotomised subjects.ConclusionsGIGD is diminished, but the incretin effect is unaffected in vagotomised subjects despite higher GLP1 levels. This, together with the small effect of DPP4i, is compatible with the notion that part of the physiological effects of GLP1 involves vagal transmission.

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Type 2 diabetes mellitus (T2DM) is an increasingly prevalent chronic condition, characterized by abnormally elevated blood glucose concentrations and, as a consequence, increased risk of micro‐ and macrovascular complications. Metformin is usually the first‐line glucose‐lowering medication in T2DM; however, despite being used for more than 60 years, the mechanism underlying the glucose‐lowering action of metformin remains incompletely understood. Although metformin reduces hepatic glucose production, there is persuasive evidence that the gastrointestinal tract is crucial in mediating this effect, particularly via secretion of the incretin hormone glucagon‐like peptide 1 (GLP‐1). It is now well recognized that bile acids, in addition to their established function in fat digestion and absorption, are important regulators of glucose metabolism. Exposure of the small and large intestine to bile acids induces GLP‐1 secretion, modulates the composition of the gut microbiota, and reduces postprandial blood glucose excursions in humans with and without T2DM. Metformin reduces intestinal bile acid resorption substantially, such that intraluminal bile acids may, at least in part, account for its glucose‐lowering effect. The present review focuses on the conceptual shift in our understanding as to how metformin lowers blood glucose in T2DM, with a particular emphasis on the role of intestinal bile acids.

Context The actions of both endogenous incretin hormones during a meal have not previously been characterized. Objective Using specific receptor antagonists, we investigated the individual and combined contributions of endogenous glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide 1 (GLP-1) to postprandial glucose metabolism, energy expenditure, and gallbladder motility. Design Randomized, double-blinded, placebo-controlled, crossover design. Setting On four separate days, four liquid mixed meal tests (1894 kJ) over 270 minutes (min). Patients or Other Participants Twelve healthy male volunteers. Interventions Infusions of the GIP receptor antagonist GIP(3–30)NH2 (800 pmol/kg/min), the GLP-1 receptor antagonist exendin(9–39)NH2 (0–20 min: 1000 pmol/kg/min; 20–270 min: 450 pmol/kg/min), GIP(3–30)NH2+exendin(9–39)NH2, or placebo/saline. Main Outcome Measure Baseline-subtracted area under the curve (bsAUC) of C-peptide. Results Infusion of GIP(3–30)NH2+exendin(9–39)NH2 significantly increased plasma glucose excursions (bsAUC: 261 ± 142 mmol/L × min) during the liquid mixed meals compared with GIP(3–30)NH2 (180 ± 141 mmol/L × min; P = 0.048), exendin(9–39)NH2 (171 ± 114 mmol/L × min; P = 0.046), and placebo (116 ± 154 mmol/L × min; P = 0.015). Correspondingly, C-peptide:glucose ratios during GIP(3–30)NH2+exendin(9–39)NH2 infusion were significantly lower than during GIP(3–30)NH2 (P = 0.0057), exendin(9–39)NH2 (P = 0.0038), and placebo infusion (P = 0.014). GIP(3–30)NH2 resulted in significantly lower AUCs for glucagon than exendin(9–39)NH2 (P = 0.0417). Gallbladder ejection fraction was higher during GIP(3–30)NH2 compared with placebo (P = 0.004). For all interventions, energy expenditure and respiratory quotient were similar. Conclusions Endogenous GIP and GLP-1 lower postprandial plasma glucose excursions and stimulate insulin secretion but only endogenous GIP affects gallbladder motility. The two incretin hormones potentiate each other’s effects in the control of postprandial glycemia in healthy men.

Enteropancreatic hormone secretion is thought to include a cephalic phase, but the evidence in humans is ambiguous. We studied vagally induced gut hormone responses with and without muscarinic blockade in 10 glucose-clamped healthy men (age: 24.5 ± 0.6 yr, means ± SE; body mass index: 24.0 ± 0.5 kg/m(2); HbA1c: 5.1 ± 0.1%/31.4 ± 0.5 mmol/mol). Cephalic activation was elicited by modified sham feeding (MSF, aka "chew and spit") with or without atropine (1 mg bolus 45 min before MSF + 80 ng·kg(-1)·min(-1) for 2 h). To mimic incipient prandial glucose excursions, glucose levels were clamped at 6 mmol/l on all days. The meal stimulus for the MSF consisted of an appetizing breakfast. Participants (9/10) also had a 6 mmol/l glucose clamp without MSF. Pancreatic polypeptide (PP) levels rose from 6.3 ± 1.1 to 19.9 ± 6.8 pmol/l (means ± SE) in response to MSF and atropine lowered basal PP levels and abolished the MSF response. Neither insulin, C-peptide, glucose-dependent insulinotropic polypeptide (GIP), nor glucagon-like peptide-1 (GLP-1) levels changed in response to MSF or atropine. Glucagon and ghrelin levels were markedly attenuated by atropine prior to and during the clamp: at t = 105 min on the atropine (ATR) + clamp (CLA) + MSF compared with the saline (SAL) + CLA and SAL + CLA + MSF days; baseline-subtracted glucagon levels were -10.7 ± 1.1 vs. -4.0 ± 1.1 and -4.7 ± 1.9 pmol/l (means ± SE), P < 0.0001, respectively; corresponding baseline-subtracted ghrelin levels were 303 ± 36 vs. 39 ± 38 and 3.7 ± 21 pg/ml (means ± SE), P < 0.0001. Glucagon and ghrelin levels were unaffected by MSF. Despite adequate PP responses, a cephalic phase response was absent for insulin, glucagon, GLP-1, GIP, and ghrelin.