The investigators recently demonstrated that blockade of Glucagon-Like Peptide-1's (GLP-1) receptor (GLP1R) results in changes in islet function without changes in circulating GLP-1. This supports other evidence (rodents and humans) that through the (inducible) expression of a prohormone convertase (PC-1/3), the α-cell can process proglucagon to intact GLP-15,6. 'Islet' or 'pancreatic' GLP-1 acts in a paracrine fashion to regulate insulin (basal and 1st phase) and glucagon secretion. These effects are more pronounced in people with early type 2 diabetes (T2DM) in keeping with increased expression of PC-1/3 and GLP-1 that is observed in diabetic islets.
There is evidence that α-cell proglucagon processing is subject to paracrine regulation by the β-cell. β-cell secretion of the signaling peptide 14-3-3-Zeta is decreased by GLP1R agonism (Fig.1), stimulating α-cell production of GLP-1. This is a testable hypothesis in humans; people with type 1 diabetes (T1DM) have dysregulated glucagon secretion and evidence of islet GLP-1. It is unclear if the effects of GLP1R agonism on islet GLP-1 differ compared to T2DM. This experiment will examine the effect of glycemic control ± a GLP1R agonist on islet GLP-1 in people with (T2DM) and without (T1DM) β-cells.
