Project Summary/Abstract Glucagon and glucagon-like peptide 1 (GLP-1) are proglucagon gene products that regulate blood glucose levels through insulin-dependent and insulin–independent mechanisms. Canonically, glucagon and GLP-1 are believed to have opposing effects on pancreatic β-cell function, with glucagon from pancreatic α-cells functioning as a counterregulatory hormone to increase blood glucose during fasting and GLP-1 augmenting glucose-stimulated insulin secretion in the fed state to lower blood glucose. Glucagon receptor antagonism, which should decrease hepatic glucose production, has been explored as a means to lower blood glucose during diabetes. However, this has not translated to the clinic. More recently, drugs incorporating glucagon receptor agonism actually improved glycemia—highlighting that glucagon’s physiological role in regulating glycemia is likely more complex. Our preclinical data suggest that glucagon action depends on ambient glycemia, i.e., at fasting glucose concentrations, glucagon acts canonically at the liver to maintain hepatic glucose output, while at elevated (e.g., postprandial) glucose concentrations, glucagon acts in an incretin manner to directly stimulate insulin secretion from pancreatic β-cells. We hypothesize that this is also true in humans. Moreover, there appears to be intra-islet interplay between glucagon, GLP-1, and their respective receptors depending on nutrient intake and metabolic stress. We further hypothesize that α-cell glucagon and GLP-1—not gut-derived GLP-1— regulate nutrient-stimulated insulin secretion. To address these hypotheses, we will test whether glucagon’s ability to increase insulin secretion is dependent on glycemia in humans (aim 1) and the relative contributions of islet GLP-1 and glucagon toward facilitating insulin secretion in preclinical models of normal physiology and metabolic stress (aim 2). This translational approach offers insight into novel paracrine relationships between α- and β-cells, which has the potential to profoundly revolutionize our understanding of islet biology and offer new treatment approaches for diabetes.