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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

Sacubitril/valsartan is a neprilysin-inhibitor/angiotensin II receptor blocker used for the treatment of heart failure. Recently, a post-hoc analysis of a 3-year randomized controlled trial showed improved glycaemic control with sacubitril/valsartan in patients with heart failure and type 2 diabetes. We previously reported that sacubitril/valsartan combined with a dipeptidyl peptidase-4 inhibitor increases active glucagon-like peptide-1 (GLP-1) in healthy individuals. We now hypothesized that administration of sacubitril/valsartan with or without a dipeptidyl peptidase-4 inhibitor would lower postprandial glucose concentrations (primary outcome) in patients with type 2 diabetes via increased active GLP-1.We performed a crossover trial in 12 patients with obesity and type 2 diabetes. A mixed meal was ingested following five respective interventions: (a) a single dose of sacubitril/valsartan; (b) sitagliptin; (c) sacubitril/valsartan + sitagliptin; (d) control (no treatment); and (e) valsartan alone. Glucose, gut and pancreatic hormone responses were measured.Postprandial plasma glucose increased by 57% (incremental area under the curve 0-240 min) (p = .0003) and increased peak plasma glucose by 1.7 mM (95% CI: 0.6-2.9) (p = .003) after sacubitril/valsartan compared with control, whereas postprandial glucose levels did not change significantly after sacubitril/valsartan + sitagliptin. Glucagon, GLP-1 and C-peptide concentrations increased after sacubitril/valsartan, but insulin and glucose-dependent insulinotropic polypeptide did not change.The glucose-lowering effects of long-term sacubitril/valsartan treatment reported in patients with heart failure and type 2 diabetes may not depend on changes in entero-pancreatic hormones. Neprilysin inhibition results in hyperglucagonaemia and this may explain the worsen glucose tolerance observed in this study.gov (NCT03893526).

Glucagon is a key regulator of numerous metabolic functions including glucose, protein and lipid metabolism, and glucagon-based therapies are explored for diabetes, fatty liver disease and obesity. Insight into tissue and cell specific expression of the glucagon receptor (GCGR) is important to understand the biology of glucagon as well as to differentiate between direct and indirect actions of glucagon. However, it has been challenging to accurately localize the GCGR in tissue due to low expression levels and lack of specific methodologies. Immunohistochemistry has frequently been used for GCGR localization, but G-protein-coupled receptors (GPCRs) targeting antibodies are notoriously unreliable. In this study, we systematically evaluated all commercially available GCGR antibodies. Initially, twelve GCGR antibodies were evaluated using HEK293 cells transfected with mouse or human GCGR cDNA. Of the twelve antibodies tested, eleven showed positive staining of GCGR protein from both species. Human liver tissue was investigated using the same GCGR antibodies. Five antibodies failed to stain human liver biopsies (despite explicit claims to the contrary from the vendors). Immunohistochemical (IHC) staining demonstrated positive staining of liver tissue from glucagon receptor knockout (Gcgr−/−) mice and their wild-type littermates (Gcgr+/+) with only one out of the twelve available GCGR antibodies. Three antibodies were selected for further evaluation by western blotting and bands corresponding to the predicted size of the GCGR (62 kDa) were identified using two of these. Finally, a single antibody (no. 11) was selected for specific GCGR localization studies in various tissues. In mouse tissue the most intense immunostainings were found in lever, kidney, ileum, heart, and pancreas. Western blotting, performed on liver tissue from Gcgr+/+ and Gcgr−/− mice, confirmed the specificity of antibody no. 11, detecting a band at high intensity in material from Gcgr+/+and no bands in liver tissue from Gcgr−/−mice. Staining of human kidney tissue, with antibody no. 11, showed GCGR localization to the distal tubules. Autoradiography was used as an antibody-independent approach to support the antibody-based findings, revealing specific binding in liver, pancreas, and kidney. As a final approach, RNA-sequencing and single-cell RNA (scRNA)-sequencing were implemented. RNA-sequencing confirmed GCGR presence within liver and kidney tissue. The GCGR was specifically found to be expressed in hepatocytes by scRNA-sequencing and potentially also in collecting and distal tubule cells in the kidney. Our results clearly indicate the liver and the kidneys as the primary targets of glucagon action.

The impaired glucose tolerance in cirrhosis is poorly understood. We evaluated the influence of gastrointestinal-mediated glucose disposal and incretin effect in patients with cirrhosis.Non-diabetic patients with Child-Pugh A or B cirrhosis (n = 10) and matched healthy controls (n = 10) underwent a 50-g oral glucose tolerance test (OGTT) and an isoglycemic intravenous glucose infusion. We presented data as median ± interquartile range and compared groups using non-parametric analysis of variance.Patients with cirrhosis were glucose intolerant compared with healthy controls (4-h OGTTAUC : 609 ± 458 vs 180 ± 155 min × mmol/L; P = 0.005), insulin resistant (homeostatic model assessment for insulin resistance: 3.7 ± 4.9 vs 2.6 ± 1.4; P = 0.014) and had fasting hyperglucagonemia (8 ± 3 vs 3 ± 4 pmol/L; P = 0.027). Isoglycemia was achieved using 35 ± 12 g of intravenous glucose in patients with cirrhosis compared with 24 ± 10 g in healthy controls (P = 0.003). The gastrointestinal-mediated glucose disposal was markedly lower in patients with cirrhosis (30 ± 23 vs 52 ± 20%; P = 0.003). Despite higher levels of the incretin hormones glucagon-like peptide-1 and glucose-dependent insulinotropic peptide patients with cirrhosis had reduced incretin effect (35 ± 44 vs 55 ± 30%; P = 0.008).Impaired gastrointestinal-mediated glucose disposal and reduced incretin effect may contribute to the glucose intolerance seen in patients with cirrhosis.