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A physiological feedback system exists between hepatocytes and the alpha cells, termed the liver-alpha cell axis and refers to the relationship between amino acid-stimulated glucagon secretion and glucagon-stimulated amino acid catabolism. Several reports indicate that non-alcoholic fatty liver disease (NAFLD) disrupts the liver-alpha cell axis, because of impaired glucagon receptor signaling (glucagon resistance). However, no experimental test exists to assess glucagon resistance in humans. The objective was to develop an experimental test to determine glucagon sensitivity with respect to amino acid and glucose metabolism in humans. The proposed glucagon sensitivity test (comprising two elements: 1) i.v. injection of 0.2 mg glucagon and 2) infusion of mixed amino acids 331 mg/hour/kg) is based on nine pilot studies which are presented. Calculation of a proposed glucagon sensitivity index with respect to amino acid catabolism is also described. Secondly, we describe a complete study protocol (GLUSENTIC) according to which the glucagon sensitivity test will be applied in a cross-sectional study currently taking place. 65 participants including 20 individuals with a BMI 18.6–25 kg/m2, 30 individuals with a BMI ≥ 25–40 kg/m2, and 15 individuals with type 1 diabetes with a BMI between 18.6 and 40 kg/m2 will be included. Participants will be grouped according to their degree of hepatic steatosis measured by whole-liver magnetic resonance imaging (MRI). The primary outcome measure will be differences in the glucagon sensitivity index between individuals with and without hepatic steatosis. Developing a glucagon sensitivity test and index may provide insight into the physiological and pathophysiological mechanism of glucagon action and glucagon-based therapies.

IntroductionA physiological feedback system exists between hepatocytes and the alpha cells termed the liver-alpha cell axis and signifies the role between amino acid-stimulated glucagon secretion and glucagon-stimulated amino acid catabolism. Several reports indicate that metabolic diseases such as non-alcoholic fatty liver disease (NAFLD) disrupts this feedback system, because of impaired glucagon receptor (GCGR) signaling (glucagon resistance). However, no experimental test exists to assess glucagon resistance in humans.ObjectiveTo develop and evaluate a test for measuring glucagon sensitivity towards amino acid and glucose metabolism in humans.Methods and analysisThe study protocol is based on several pilot studies presented in this paper. The study will include 65 participants including 20 individuals with a BMI 18.6-25 kg/m2, 30 individuals with a BMI ≥25-40 kg/m2, and 15 individuals with type 1 diabetes with a BMI between 18.6-40 kg/m2. Participants will be grouped according to their percentage of hepatic steatosis measured by whole-liver magnetic resonance imaging (MRI). The primary outcome measure will be differences in a novel ‘glucagon sensitivity’ index between individuals with and without hepatic steatosis (ResultsThese data will be published in peer-reviewed scientific journals and presented at scientific conferences.Ethics and disseminationThe study was approved by the scientific-ethical committee of the Capital region of Denmark (H-20023717) and registered with Danish Data protection Agency (P-2021-39) andClinicalTrials.gov(NCT04907721). Written and oral consent will be obtained from all participants, and the study will adhere to the principles of the Declaration of Helsinki.Strengths and limitations of this studyThe glucagon sensitivity test is based on several pilot experimentsLiver fat is based on whole-liver imaging and not region of interest (ROI)The glucagon sensitivity test may be limited to assess glucagon sensitivity towards amino acid catabolism and glucose productionThe glucagon sensitivity test does not use amino acid or glucose tracers which expands the generalizability of such test but also may impair its accuracy

Glucagon and glucagon-like peptide-1 (GLP-1) have important roles in the development and treatment of diabetes. Their plasma concentrations are currently being measured in more than 500 clinical studies according to clinicaltrial.gov. As glucagon and total GLP-1 (reflecting the secretion and action of GLP-1 and is analytically independent of DPP-4 activity) circulate in the low picomolar range it has been troublesome to accurately measure their plasma concentrations. Equally important are so-called preanalytical considerations including the use of enzyme inhibitors. The latter may also depend on the analytical approach, for example single-antibody assay versus sandwich ELISA. To provide guidelines for measurement of glucagon and GLP-1 in clinical trials we obtained blood samples using four different blood-containers (with either neprilysin inhibitor: sacubitrilat 25 µg/mL; trasylol 500KIU/mL; P800 tubes; or vehicle) from healthy individuals (male/female; 7/4; mean ± SD; age: 32 ± 12 years, BMI; 26 ± 4 kg/m2) after an overnight fast and following an intravenous amino acid (AA) infusion (14 g/L; 331 mg/min/kg body weight. Plasma concentrations of glucagon increased significantly during the AA-infusion independent of blood containers and assays. The sandwich ELISA showed significant lower levels of glucagon with addition of neprilysin inhibitor whereas both trasylol and P800 tubes resulted in increased levels during the AA-infusion. Glucagon levels measured by the two radioimmunoassays (RIA) did not depend on the containers. Levels of total GLP-1 did not depend on the containers using a sandwich ELISA (Mercodia) but increased by a factor of a 2.5 when plasma from P800 containers was assayed using a C-terminal specific RIA. In summary, levels of glucagon and GLP-1 depend on both preanalytical and analytical factors that should be standardized for future clinical studies. Disclosure S. Kjeldsen: None. N.J. Jensen: None. N. Heinz: None. B. Hartmann: Board Member; Bainan Biotech. J.J. Holst: Advisory Panel; Novo Nordisk. Board Member; Antag Therapeutics, Bainan Biotech. N.J. Wewer Albrechtsen: Research Support; Mercodia AB, Novo Nordisk, Regeneron Pharmaceuticals Inc. Speaker's Bureau; Merck & Co., Inc., Mercodia AB. Funding Nicolai J. Wewer Albrechtsen were financed by NNF Excellence Emerging Investigator Grant – Endocrinology and Metabolism (Application No. NNF19OC0055001) , EFSD Future Leader Award (NNF21SA0072746) and DFF Sapere Aude.

Glucagon is essential for glucose control and increased levels of glucagon (hyperglucagonemia) observed in patients with type 2 diabetes contribute to their hyperglycemia. Recently, hyperglucagonemia has also been found in individuals with non-alcoholic fatty liver disease (NAFLD) as well as impaired actions of glucagon on amino acid catabolism. Whether glucagon actions on hepatic glucose production are impaired is unknown. We investigated the acute effects of a single bolus of glucagon (0.2mg) on glucose dynamics in 18 normoglycemic individuals (age: 51±3 years, BMI; 31± 0.8kg/m2, hepatic fat content: 20±2%, fasting glucose: 5.5±0.1mM) with magnetic resonance imaging verified NAFLD and 22 controls (age: 38±3 years, BMI; 24± 0.8kg/m2, hepatic fat content: 4±0.1%, fasting glucose: 5.0±0.1mM) . On a separate day, a mixture of amino acids (14 g/L; 331 mg/min/kg body weight) was infused intravenously for 45min to evaluate the actions of endogenous glucagon on glucose dynamics. Glucose levels (see figure) were significantly increased in individuals with NAFLD 60min after the glucagon bolus and during the amino acid infusion with a maximal difference of 0.5mM 30min into the infusion. These data suggest that the actions of glucagon on hepatic glucose production are not impaired by NAFLD. Therefore, the hyperglucagonemia in patients with NAFLD may constitute a diabetogenic risk factor. Disclosure S.Kjeldsen: None. H.Vilstrup: None. F.V.Schiødt: Advisory Panel; Novo Nordisk. A.Møller: None. E.B.Rashu: None. L.Gluud: Advisory Panel; Novo Nordisk, Consultant; Pfizer Inc., Research Support; Alexion Pharmaceuticals, Inc., Gilead Sciences, Inc., Novo Nordisk, Sobi. S.B.Haugaard: None. J.J.Holst: Advisory Panel; Novo Nordisk, Board Member; Antag Therapeutics, Bainan Biotech. J.Rungby: Advisory Panel; Abbott, Boehringer Ingelheim International GmbH, Speaker’s Bureau; AstraZeneca, Bayer AG, Novo Nordisk, Pfizer Inc. N.J.Wewer albrechtsen: Research Support; Mercodia AB, Novo Nordisk, Regeneron Pharmaceuticals Inc., Speaker’s Bureau; Merck & Co., Inc., Mercodia AB. N.J.Jensen: None. M.Nilsson: None. N.Heinz: None. J.D.Nybing: None. F.H.Linden: None. E.Høgh-schmidt: n/a. M.P.Boesen: None. S.Madsbad: None. Funding NNF Excellence Emerging Investigator Grant – Endocrinology and Metabolism (Application No. NNF19OC0055001) , EFSD Future Leader Award (NNF21SA0072746) and DFF Sapere Aude.

Glucagon is a key regulator of metabolism and is used in the diagnostic of neuroendocrine tumors. Accurate measurement of glucagon requires both extreme sensitivity and specificity since several peptides are derived from the same proglucagon precursor encoding part of the glucagon sequence and given that glucagon circulates in picomolar concentrations. A sandwich ELISA was recently developed and extensively evaluated; however, this method may not be accurate when measuring glucagon in patients with an enhanced production of proglucagon-derived peptides as seen after Roux-en-Y gastric bypass (RYGB). To overcome this, a modified version of the ELISA was developed. In this study, we evaluate an unmodified and a modified version of the ELISA in healthy individuals, individuals with obesity, and finally in two cohorts of patients following RYGB surgery using different nutrient stimuli to assess glucagon dynamics. Finally, in vitro spike-in recoveries using native glucagon and proglucagon-derived peptides were performed in buffer and in plasma. Our data support that both versions of the ELISA accurately capture endogenous and exogenous glucagon in healthy individuals and in individuals with obesity. However, the unmodified version of the assay may overestimate glucagon levels in patients following RYGB in line with minimal but consistent cross-reactivity to oxyntomodulin and glicentin that both are 50-fold increased after RYGB. Importantly, we did not find any changes between the two protocols at fasted conditions and therefore diagnostics of glucagonomas is not affected by the choice of assay procedure nor the surgical history of the patient (RYGB).