Hyperglucagonaemia (in the fasting as well as in the postprandial state) is considered a core pathophysiological component of diabetes and is found to contribute substantially to the hyperglycaemic state of diabetes. Hyperglucagonaemia is usually viewed upon as a consequence of pancreatic alpha cell insensitivity to the glucagon-suppressive effects of glucose and insulin. Since we observed that the well-known hyperglucagonaemic response to oral glucose in patients with type 2 diabetes is exchanged by normal suppression of plasma glucagon levels following isoglycaemic intravenous glucose administration in these patients, we have been focusing on the gut and gut-derived factors as potential mediators of diabetic hyperglucagonaemia. In a series of clinical experiments, we have elucidated the role of gut-derived factors in diabetic hyperglucagonaemia and shown that glucose-dependent insulinotropic polypeptide promotes hyperglucagonaemia and that glucagon, hitherto considered a pancreas-specific hormone, may also be secreted from extrapancreatic tissues – most likely from proglucagon-producing enteroendocrine cells. Furthermore, our observation that fasting hyperglucagonaemia is unrelated to the diabetic state, but strongly correlates with obesity, liver fat content and circulating amino acids, has made us question the common ‘pancreacentric’ and ‘glucocentric’ understanding of hyperglucagonaemia and led to the hypothesis that steatosis-induced hepatic glucagon resistance (and reduced amino acid turnover) and compensatory glucagon secretion mediated by increased circulating amino acids constitute a complete endocrine feedback system: the liver–alpha cell axis. This article summarises the physiological regulation of glucagon secretion in humans and considers new findings suggesting that the liver and the gut play key roles in determining fasting and postabsorptive circulating glucagon levels.
Filip K Knop
David P Sonne, Morten Hansen and Filip K Knop
Bile acid sequestrants have been used for decades for the treatment of hypercholesterolaemia. Sequestering of bile acids in the intestinal lumen interrupts enterohepatic recirculation of bile acids, which initiate feedback mechanisms on the conversion of cholesterol into bile acids in the liver, thereby lowering cholesterol concentrations in the circulation. In the early 1990s, it was observed that bile acid sequestrants improved glycaemic control in patients with type 2 diabetes. Subsequently, several studies confirmed the finding and recently – despite elusive mechanisms of action – bile acid sequestrants have been approved in the USA for the treatment of type 2 diabetes. Nowadays, bile acids are no longer labelled as simple detergents necessary for lipid digestion and absorption, but are increasingly recognised as metabolic regulators. They are potent hormones, work as signalling molecules on nuclear receptors and G protein-coupled receptors and trigger a myriad of signalling pathways in many target organs. The most described and well-known receptors activated by bile acids are the farnesoid X receptor (nuclear receptor) and the G protein-coupled cell membrane receptor TGR5. Besides controlling bile acid metabolism, these receptors are implicated in lipid, glucose and energy metabolism. Interestingly, activation of TGR5 on enteroendocrine L cells has been suggested to affect secretion of incretin hormones, particularly glucagon-like peptide 1 (GLP1 (GCG)). This review discusses the role of bile acid sequestrants in the treatment of type 2 diabetes, the possible mechanism of action and the role of bile acid-induced secretion of GLP1 via activation of TGR5.
David P Sonne, Jens F Rehfeld, Jens J Holst, Tina Vilsbøll and Filip K Knop
Recent preclinical work has suggested that postprandial flow of bile acids into the small intestine potentiates nutrient-induced glucagon-like peptide 1 (GLP1) secretion via bile acid-induced activation of the G protein-coupled receptor TGR5 in intestinal L cells. The notion of bile-induced GLP1 secretion combined with the findings of reduced postprandial gallbladder emptying in patients with type 2 diabetes (T2DM) led us to speculate whether reduced postprandial GLP1 responses in some patients with T2DM arise as a consequence of diabetic gallbladder dysmotility.
Design and methods
In a randomised design, 15 patients with long-standing T2DM and 15 healthy age-, gender- and BMI-matched control subjects were studied during 75-g oral glucose tolerance test (OGTT) and three isocaloric (500 kcal) and isovolaemic (350 ml) liquid meals: i) 2.5 g fat, 107 g carbohydrate and 13 g protein; ii) 10 g fat, 93 g carbohydrate and 11 g protein; and iii) 40 g fat, 32 g carbohydrate and 3 g protein. Basal and postprandial plasma concentrations of glucose, insulin, C-peptide, glucagon, GLP1, glucose-dependent insulinotropic polypeptide (GIP), cholecystokinin and gastrin were measured. Furthermore, gallbladder emptying and gastric emptying were examined.
Gallbladder emptying increased with increasing meal fat content, but no intergroup differences were demonstrated. GIP and GLP1 responses were comparable among the groups with GIP levels being higher following high-fat meals, whereas GLP1 secretion was similar after both OGTT and meals.
In conclusion, patients with T2DM exhibited normal gallbladder emptying to meals with a wide range of fat content. Incretin responses were similar to that in controls, and an association with postprandial gallbladder contraction could not be demonstrated.
Caroline C Øhrstrøm and Filip K Knop
Roux-en-Y gastric bypass (RYGB) is one of the most common and successful bariatric surgeries. However, more than half of RYGB-operated individuals may suffer from post-bariatric hypoglycaemia (PBH) characterised by traditional hypoglycaemic symptoms occurring 1 to 4 hours after meal intake. The mechanisms underlying PBH most likely relate to accelerated delivery of nutrients to the small intestine resulting in unretarded nutrient absorption, large elevations in postprandial plasma glucose concentrations (constituting a potent insulin secretory stimulus), and grossly elevated postprandial plasma levels of the insulinotropic gut-derived hormone glucagon-like peptide 1 (GLP-1) potentiating glucose-stimulated insulin secretion. Based on previous findings that circulating GLP-1 concentrations increased by ~100% during insulin-induced hypoglycaemia before but not after RYGB, Almby et al. explored whether exogenous GLP-1 may protect against PBH. They performed hyperinsulinaemic hypoglycaemic clamps with concomitant infusion of the GLP-1 analogue exenatide and saline, respectively, in individuals who had undergone RYGB surgery. Infusion with exenatide during hypoglycaemia had no plasma glucose-raising effects, did not increase the counterregulatory glucagon response, and did not affect symptom scores. In the present commentary, potentially important implications derived from the study by Almby et al. published in the August issue of EJE, are discussed in the light of previous observations on GLP-1 receptor agonist treatment in PBH. While the findings by Almby et al. do not provide a solution for patients with PBH, they contribute to the knowledge base needed to address the growing problem of PBH.
Astrid Plamboeck, Simon Veedfald, Carolyn F Deacon, Bolette Hartmann, André Wettergren, Lars B Svendsen, Søren Meisner, Claus Hovendal, Filip K Knop, Tina Vilsbøll and Jens J Holst
Glucagon-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 methods
Truncally 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.
Isoglycaemia 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.
GIGD 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.
Monika J Bak, Nicolai Wewer Albrechtsen, Jens Pedersen, Bolette Hartmann, Mikkel Christensen, Tina Vilsbøll, Filip K Knop, Carolyn F Deacon, Lars O Dragsted and Jens J Holst
To determine the specificity and sensitivity of assays carried out using commercially available kits for glucagon and/or oxyntomodulin measurements.
Ten different assay kits used for the measurement of either glucagon or oxyntomodulin concentrations were obtained. Solutions of synthetic glucagon (proglucagon (PG) residues 33–61), oxyntomodulin (PG residues 33–69) and glicentin (PG residues 1–69) were prepared and peptide concentrations were verified by quantitative amino acid analysis and a processing-independent in-house RIA. Peptides were added to the matrix (assay buffer) supplied with the kits (concentration range: 1.25–300 pmol/l) and to human plasma and recoveries were determined. Assays yielding meaningful results were analysed for precision and sensitivity by repeated analysis and ability to discriminate low concentrations.
Results and conclusion
Three assays were specific for glucagon (carried out using the Millipore (Billerica, MA, USA), Bio-Rad (Sundbyberg, Sweden), and ALPCO (Salem, NH, USA) and Yanaihara Institute (Shizuoka, Japan) kits), but none was specific for oxyntomodulin. The assay carried out using the Phoenix (Burlingame, CA, USA) glucagon kit measured the concentrations of all three peptides (total glucagon) equally. Sensitivity and precision were generally poor; the assay carried out using the Millipore RIA kit performed best with a sensitivity around 10 pmol/l. Assays carried out using the BlueGene (Shanghai, China), USCN LIFE (Wuhan, China) (oxyntomodulin and glucagon), MyBioSource (San Diego, CA, USA) and Phoenix oxyntomodulin kits yielded inconsistent results.