Dysregulation of
glucagon is associated with the pathophysiology of
type 2 diabetes. We previously reported that postprandial hyperglucagonemia is more obvious than fasting hyperglucagonemia in
type 2 diabetes patients. However, which nutrient stimulates
glucagon secretion in the diabetic state and the underlying mechanism after nutrient intake are unclear. To answer these questions, we measured plasma
glucagon levels in diabetic mice after
oral administration of various nutrients. The effects of nutrients on
glucagon secretion were assessed using islets isolated from diabetic mice and
palmitate-treated islets. In addition, we analyzed the expression levels of
branched chain amino acid (BCAA) catabolism-related
enzymes and their metabolites in diabetic islets. We found that
protein, but not
carbohydrate or
lipid, increased plasma
glucagon levels in diabetic mice. Among
amino acids, BCAAs, but not the other essential or nonessential
amino acids, increased plasma
glucagon levels. BCAAs also directly increased the intracellular
calcium concentration in α cells. When BCAAs transport was suppressed by an inhibitor of system L-
amino acid transporters,
glucagon secretion was reduced even in the presence of BCAAs. We also found that the expression levels of BCAA catabolism-related
enzymes and their metabolite contents were altered in diabetic islets and
palmitate-treated islets compared to control islets, indicating disordered BCAA catabolism in diabetic islets. Furthermore, BCKDK inhibitor BT2 suppressed BCAA-induced hypersecretion of
glucagon in diabetic islets and
palmitate-treated islets. Taken together, postprandial hypersecretion of
glucagon in the diabetic state is attributable to disordered BCAA catabolism in pancreatic islet cells.