Background: Circulating plasma glucagon level is known to increase in diabetic state. We found that postprandial plasma glucagon level significantly increased in diabetic mice more than that in the control group. However, it’s still unknown which nutrient stimulates glucagon secretion as well as the underlying mechanism of glucagon secretion enhancement in diabetic state. Therefore, we aimed to answer these questions by using diabetic mouse models and isolated islets.
Methods: Plasma glucagon levels were measured by sandwich ELISA in several diabetic model mice after oral administration of various nutrients. The effects of nutrients on glucagon secretion were assessed using isolated islets from diabetic mice. The expression levels of nutrient catabolism-related factors were analyzed in diabetic mice compared with control mice.
Results and Discussions: Rather than carbohydrate or lipid, protein increased plasma glucagon levels in all diabetic models including high-fat diet mice, STZ mice, and db/db mice. Among amino acids, branched-chain amino acids (BCAA), but not the other essential or nonessential amino acids, increased plasma glucagon. BCAA directly stimulated glucagon secretion in isolated islets from diabetic mice, which was suppressed by amino acid transporter inhibitor (BCH) or branched-chain alpha-ketoacid dehydrogenase (BCKDK) inhibitor (BT2). This suggests that BCAA’s stimulation of glucagon secretion is mediated by BCAA catabolism in alpha cells. We also found that the expression level of BCKDK was higher, while branched-chain amino acid transaminase 2 (BCAT2) was lower, in diabetic islets than control islets. Considering that dysfunction of BCAA catabolism has been reported in both adipocytes and hepatocytes of diabetic mice, the dysfunction of BCAA catabolism might be a common pathogenesis for metabolic disorders observed in diabetes.
E. Wada: None. M. Kobayashi: None. O. Kikuchi: None. D. Kohno: None. T. Kitamura: None.