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The focus of this article is on the analysis of the release and postrelease fate of the incretin hormones, glucagon-like peptide-1 and glucose-dependent insulinotropic polypeptide. Their actions are dealt with to the extent that they are linked to their secretion. For both hormones, their posttranslational processing is analyzed in detail, because of its importance for the understanding of the molecular heterogeneity of the hormones. Methods of analysis, in particular regarding measurements in plasma from in vivo experiments, are discussed in detail in relation to the molecular heterogeneity of the hormones, and the importance of the designations "total" versus "intact hormones" is explained. Both hormones are substrates for the ubiquitous enzyme, dipeptidyl peptidase-4, which inactivates the peptides with dramatic consequences for their physiological spectrum of activities. The role of endogenous and exogenous antagonists of the receptors is discussed in detail because of their importance for the elucidation of the physiology and pathophysiology of the hormones. Regarding the actual secretion, the most important factors are discussed, including gastric emptying rate and the influence of the different macronutrients. Additional factors discussed are the role of bile, paracrine regulation, the role of the microbiota, pharmaceuticals, and exercise. Finally, the secretion during pathological conditions is discussed. © 2019 American Physiological Society. Compr Physiol 9:1339-1381, 2019., (Copyright © 2019 American Physiological Society. All rights reserved.)

Towards the end of the 20th century, the number of subjects with diabetes and obesity rose exponentially. The discoveries of insulin- and appetite-modulating chemical signals, including glucagon-like peptide-1 (GLP-1), secreted from the gastrointestinal system, led to development of a new group of drugs which now are being used for glucose-lowering therapy and weight loss. Understanding of the physiology of gut derived signals and their pathophysiologi-cal importance requires accurate measurements of their circulat-ing levels. However, the assessment of these gut-derived hor-mones has been hampered by numerous preanalytical and analyti-cal challenges. We focused on three members of the proglucagon family; glucagon, oxyntomodulin and GLP-1, aiming to meet both preanalytical and analytical challenges and to elucidate their implication in diseases including diabetes. First, we studied (Study 1) the preanalytical and storage conditions of GLP-1 and glucagon in humans, demonstrating that inappropriate sample handling may cause up to 50% variation in the RESULTS. Using robust meas-uring METHODS ensuring optimal conditions for preanalytical han-dling of these peptides, we then focused on plasma concentra-tions of glucagon and oxyntomodulin in different clinical condi-tions, including type 2 diabetes and bariatric surgery, because abnormal secretion of these hormones may represent early and specific signs of altered glucose metabolism. To that end, we developed an unbiased mass-spectrometry based platform for detection of low-abundant peptides, including the gut hormones (Study 2). Using the platform, we validated a new method for the measurement of oxyntomodulin, and in a series of in vitro, ex vivo, and clinical studies, we demonstrated that oxyntomodulin is co-distributed and co-secreted in response to glucose with GLP-1 and is degraded by dipeptidyl peptidase 4. Because oxyntomodulin has both GLP-1-like and glucagon-like bioactivity, the secretion of this hormone is of interest in both type 2 diabetes and bariatric sur-gery. Furthermore, using these newly developed METHODS, we subsequently were able to establish that elevated plasma concen-trations of glucagon (hyperglucagonemia) in diseases (Study 3) may be due to either a) increased secretion of fully processed glucagon, as in subjects with diabetes or b) secretion of N-terminally elongated molecular forms (Study 4) in conditions including bariatric surgery and in diseases affecting the kidneys. This glucagon variant may be of importance for glucose homeo-stasis, as we were able to show that it, unexpectedly, activates the glucagon receptor, leading to increased glycogenolysis in hepatocytes and insulin secretion from pancreatic beta-cells. In summary, accurate measurements of gut-derived hormones are indeed crucial for understanding their biology in health and as well in disease. Mass-spectrometry based plasma proteomics is a powerful tool for the validation of these METHODS., (Articles published in the Danish Medical Journal are “open access”. This means that the articles are distributed under the terms of the Creative Commons Attribution Non-commercial License, which permits any non-commercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.)

Background: Roux-en-Y gastric bypass (RYGB) surgery is currently the most effective treatment of obesity, although limited by availability and operative risk. The gut hormones Glucagon-like peptide-1 (GLP-1), Peptide YY (PYY), and Oxyntomodulin (OXM) are elevated postprandially after RYGB, which has been postulated to contribute to its metabolic benefits., Objective: We hypothesized that infusion of the three gut hormones to achieve levels similar to those encountered postprandially in RYGB patients might be effective in suppressing appetite. The aim of this study was to investigate the effect of a continuous infusion of GLP-1, OXM, and PYY (GOP) on energy intake and expenditure in obese volunteers., Methods: Obese volunteers were randomized to receive an infusion of GOP or placebo in a single-blinded, randomized, placebo-controlled crossover study for 10.5 hours a day. This was delivered subcutaneously using a pump device, allowing volunteers to remain ambulatory. Ad libitum food intake studies were performed during the infusion, and energy expenditure was measured using a ventilated hood calorimeter., Results: Postprandial levels of GLP-1, OXM, and PYY seen post RYGB were successfully matched using 4 pmol/kg/min, 4 pmol/kg/min, and 0.4 pmol/kg/min, respectively. This dose led to a mean reduction of 32% in food intake. No significant effects on resting energy expenditure were observed., Conclusion: This is, to our knowledge, the first time that an acute continuous subcutaneous infusion of GOP, replicating the postprandial levels observed after RYGB, is shown to be safe and effective in reducing food intake. This data suggests that triple hormone therapy might be a useful tool against obesity., (Copyright © 2017 Endocrine Society)

Aims Although metformin is widely used for treatment of type 2 diabetes (T2D), its glucose-lowering mechanism remains unclear. Using the glucagon-like peptide 1 (GLP-1) receptor (GLP-1R) antagonist exendin(9-39)NH2, we tested the hypothesis that postprandial GLP-1-mediated effects contribute to the glucose-lowering potential of metformin in T2D. Methods In a randomized, placebo-controlled, double-blind, crossover study, 15 individuals with T2D (median HbA1c 50 mmol/mol [6.7%], body mass index 30.1 kg/m2, age 71 years) underwent, in randomized order, 14 days of metformin and placebo treatment, respectively. Each treatment period was preceded by 14 days without any glucose-lowering medicine and concluded by two 4 h mixed meal tests performed in randomized order and separated by >24 h with either continuous intravenous exendin(9-39)NH2 or saline infusion. Results Compared to placebo, metformin treatment lowered fasting plasma glucose (mean of differences [MD] 1.4 mmol/L × min [95% CI 0.8-2.0]) as well as postprandial plasma glucose excursions during both saline infusion (MD 186 mmol/L × min [95% CI 64-307]) and exendin(9-39)NH2 infusion (MD 268 mmol/L × min [95% CI 108-427]). The metformin-induced improvement in postprandial glucose tolerance was unaffected by GLP-1R antagonization (MD 82 mmol/L × min [95% CI −6564-170]). Metformin treatment increased fasting plasma GLP-1 (MD 1.7 pmol/L × min [95% CI 0.39-2.9]) but did not affect postprandial GLP-1 responses (MD 820 pmol/L × min [95% CI −1750-111]). Conclusions Using GLP-1R antagonization, we could not detect GLP-1-mediated postprandial glucose-lowering effect of metformin in individuals with T2D. We show that 2 weeks of metformin treatment increases fasting plasma GLP-1, which may contribute to metformin's beneficial effect on fasting plasma glucose in T2D. Trial registration: Clinicaltrials.gov NCT03246451 [ABSTRACT FROM AUTHOR]

Bile acids (BA) regulate postprandial metabolism directly and indirectly by affecting the secretion of gut hormones like glucagon-like peptide-1 (GLP-1). The postprandial effects of BA on the secretion of other metabolically active hormones are not well understood. The objective of this study was to investigate the effects of oral ursodeoxycholic acid (UDCA) and chenodeoxycholic acid (CDCA) on postprandial secretion of GLP-1, oxyntomodulin (OXM), peptide YY (PYY), glucose-dependent insulinotropic peptide (GIP), glucagon, and ghrelin. Twelve healthy volunteers underwent a mixed meal test 60min after ingestion of UDCA (12-16mg/kg), CDCA (13-16mg/kg), or no BA in a randomized crossover study. Glucose, insulin, GLP-1, OXM, PYY, GIP, glucagon, ghrelin, and fibroblast growth factor 19 were measured prior to BA administration at ~60 and 0 min (just prior to mixed meal) and 15, 30, 60, 120, 180, and 240min after the meal. UDCA and CDCA provoked differential gut hormone responses; UDCA did not have any significant effects, but CDCA provoked significant increases in GLP-1 and OXM and a profound reduction in GIP. CDCA increased fasting GLP-1 and OXM secretion in parallel with an increase in insulin. On the other hand, CDCA reduced postprandial secretion of GIP, with an associated reduction in postprandial insulin secretion. Exogenous CDCA can exert multiple salutary effects on the secretion of gut hormones; if these effects are confirmed in obesity and type 2 diabetes, CDCA may be a potential therapy for these conditions. NEW & NOTEWORTHY Oral CDCA and UDCA have different effects on gut and pancreatic hormone secretion. A single dose of CDCA increased fasting secretion of the hormones GLP-1 and OXM with an accompanying increase in insulin secretion. CDCA also reduced postprandial GIP secretion, which was associated with reduced insulin. In contrast, UDCA did not change gut hormone secretion fasting or postprandially. Oral CDCA could be beneficial to patients with obesity and diabetes. [ABSTRACT FROM AUTHOR]

Glucagon is secreted by pancreatic α cells in response to hypoglycemia and increases hepatic glucose output through hepatic glucagon receptors (GCGRs). There is evidence supporting the notion of extrapancreatic glucagon but its source and physiological functions remain elusive. Intestinal tissue samples were obtained from patients undergoing surgical resection of cancer. Mass spectrometry analysis was used to detect glucagon from mucosal lysate. Static incubations of mucosal tissue were performed to assess glucagon secretory response. Glucagon concentration was quantitated using a highly specific sandwich enzyme-linked immunosorbent assay. A cholesterol uptake assay and an isolated murine colonic motility assay were used to assess the physiological functions of intestinal GCGRs. Fully processed glucagon was detected by mass spectrometry in human intestinal mucosal lysate. High glucose evoked significant glucagon secretion from human ileal tissue independent of sodium glucose cotransporter and KATP channels, contrasting glucose-induced glucagon-like peptide 1 (GLP-1) secretion. The GLP-1 receptor agonist Exendin-4 attenuated glucose-induced glucagon secretion from the human ileum. GCGR blockade significantly increased cholesterol uptake in human ileal crypt culture and markedly slowed ex vivo colonic motility. Our findings describe the human gut as a potential source of extrapancreatic glucagon and demonstrate a novel enteric glucagon/GCGR circuit with important physiological functions beyond glycemic regulation. [ABSTRACT FROM AUTHOR]

Glucagon is believed to be a pancreas-specific hormone, and hyperglucagonemia has been shown to contribute significantly to the hyperglycemic state of patients with diabetes. This hyperglucagonemia has been thought to arise from α-cell insensitivity to suppressive effects of glucose and insulin combined with reduced insulin secretion. We hypothesized that postabsorptive hyperglucagonemia represents a gut-dependent phenomenon and subjected 10 totally pancreatectomized patients and 10 healthy control subjects to a 75-g oral glucose tolerance test and a corresponding isoglycemic intravenous glucose infusion. We applied novel analytical methods of plasma glucagon (sandwich ELISA and mass spectrometry-based proteomics) and show that 29-amino acid glucagon circulates in patients without a pancreas and that glucose stimulation of the gastrointestinal tract elicits significant hyperglucagonemia in these patients. These findings emphasize the existence of extrapancreatic glucagon (perhaps originating from the gut) in man and suggest that it may play a role in diabetes secondary to total pancreatectomy. [ABSTRACT FROM AUTHOR]

Background: Loss-of-function mutations in hERG (encoding the Kv11.1 voltage-gated potassium channel) cause long-QT syndrome type 2 (LQT2) because of prolonged cardiac repolarization. However, Kv11.1 is also present in pancreatic α and β cells and intestinal L and K cells, secreting glucagon, insulin, and the incretins glucagon-like peptide-1 (GLP-1) and GIP (glucose-dependent insulinotropic polypeptide), respectively. These hormones are crucial for glucose regulation, and long-QT syndrome may cause disturbed glucose regulation. We measured secretion of these hormones and cardiac repolarization in response to glucose ingestion in LQT2 patients with functional mutations in hERG and matched healthy participants, testing the hypothesis that LQT2 patients have increased incretin and β-cell function and decreased α-cell function, and thus lower glucose levels.Methods: Eleven patients with LQT2 and 22 sex-, age-, and body mass index-matched control participants underwent a 6-hour 75-g oral glucose tolerance test with ECG recording and blood sampling for measurements of glucose, insulin, C-peptide, glucagon, GLP-1, and GIP.Results: In comparison with matched control participants, LQT2 patients had 56% to 78% increased serum insulin, serum C-peptide, plasma GLP-1, and plasma GIP responses (P=0.03-0.001) and decreased plasma glucose levels after glucose ingestion (P=0.02) with more symptoms of hypoglycemia (P=0.04). Sixty-three percent of LQT2 patients developed hypoglycemic plasma glucose levels (<70 mg/dL) versus 36% control participants (P=0.16), and 18% patients developed serious hypoglycemia (<50 mg/dL) versus none of the controls. LQT2 patients had defective glucagon responses to low glucose, P=0.008. β-Cell function (Insulin Secretion Sensitivity Index-2) was 2-fold higher in LQT2 patients than in controls (4398 [95% confidence interval, 2259-8562] versus 2156 [1961-3201], P=0.03). Pharmacological Kv11.1 blockade (dofetilide) in rats had similar effect, and small interfering RNA inhibition of hERG in β and L cells increased insulin and GLP-1 secretion up to 50%. Glucose ingestion caused cardiac repolarization disturbances with increased QTc intervals in both patients and controls, but with a 122% greater increase in QTcF interval in LQT2 patients (P=0.004).Conclusions: Besides a prolonged cardiac repolarization phase, LQT2 patients display increased GLP-1, GIP, and insulin secretion and defective glucagon secretion, causing decreased plasma glucose and thus increased risk of hypoglycemia. Furthermore, glucose ingestion increased QT interval and aggravated the cardiac repolarization disturbances in LQT2 patients.Clinical Trial Registration: URL: http://clinicaltrials.gov. Unique identifier: NCT02775513. [ABSTRACT FROM AUTHOR]