Development and Evaluation of a Glucagon Sensitivity Test in Individuals With and Without Hepatic Steatosis

Amino acids administered orally or intravenously stimulate glucagon secretion from the pancreas and in turn, glucagon is a powerful stimulus for hepatic amino acid turnover through transcriptional (long-term) and non- transcriptional (acute) mechanisms. Several groups including the investigators have linked glucagon secretion to hepatic amino acid metabolism suggesting a mutual feedback cycle, termed the liver-alpha cell axis. A disruption of this axis, which has been shown both pharmacologically using glucagon receptor antagonists and genetically in glucagon receptor knockout mouse models, leads to increased glucagon (hyperglucagonemia) and amino acid (hyperaminoacidemia) concentrations. This phenotype is also evident in subjects with biopsy-verified metabolic dysfunction-associated steatotic liver disease (MASLD) independent of type 2 diabetes suggesting reduced hepatic glucagon sensitivity in the presence of hepatic steatosis. Glucagon increases amino acid catabolism by potentiating ureagenesis, a pathway exclusive to the liver, and hepatic accumulation of triglycerides may reduce glucagon's ability to augment amino acid turnover. Some amino acids are more potent in stimulating glucagon secretion (glucagonotropic amino acids), including alanine, and the glucagon-alanine index is currently used as a surrogate marker for the hepatic actions of glucagon on ureagenesis. The fate of amino acid turnover depends on both glucagon and insulin dynamics by stimulating amino acid catabolism and synthesis, respectively. Studying the effects of glucagon in individuals with type 1 diabetes will allow one to differentiate between the combined effects of glucagon and insulin compared to the effects of glucagon alone. This study aims to explore hepatic glucagon sensitivity towards amino acid metabolism in individuals with and without hepatic steatosis (based on magnetic resonance imaging (MRI)). The investigators hypothesize that the effect of endogenous and exogenous glucagon on plasma amino acid levels are impaired in individuals with MASLD (based on hepatic steatosis measured by MRI) compared to controls. The nomenclature for MASLD has recently been updated from non-alcoholic fatty liver disease (NAFLD) (https://pubmed.ncbi.nlm.nih.gov/37363821/).