Gastric inhibitory polypeptide is the major insulinotropic factor in K(ATP) null mice

Department of Diabetes and Clinical Nutrition, Kyoto University Graduate School of Medicine, Kyoto 606-8507, Japan. tukiyama@metab.kuhp.kyoto-u.ac.jp

OBJECTIVE: ATP-sensitive K(+) (K(ATP)) channels in pancreatic beta-cells are crucial in the regulation of glucose-induced insulin secretion. Recently, K(ATP) channel-deficient mice were generated by genetic disruption of Kir6.2, the pore-forming component of K(ATP) channels, but the mice still showed a significant insulin response after oral glucose loading in vivo. Gastric inhibitory polypeptide (GIP) is a physiological incretin that stimulates insulin release upon ingestion of nutrients. To determine if GIP is the insulinotropic factor in insulin secretion in K(ATP) channel-deficient mice, we generated double-knockout Kir6.2 and GIP receptor null mice and compared them with Kir6.2 knockout mice. METHODS: Double-knockout mice were generated by intercrossing Kir6.2-knockout mice with GIP receptor-knockout mice. An oral glucose tolerance test, insulin tolerance test and batch incubation study of pancreatic islets were performed on double-knockout mice and Kir6.2-knockout mice. RESULTS: Fasting glucose and insulin levels were similar in both groups. After oral glucose loading, blood glucose levels of double-knockout mice became elevated compared with Kir6.2-knockout mice, especially at 15 min (345+/-10 mg/dl vs 294+/-20 mg/dl, P<0.05) and 30 min (453+/-20 mg/dl vs 381+/-26 mg/dl, P<0.05). The insulin response was almost completely lost in double-knockout mice, although insulin secretion from isolated islets was stimulated by another incretin, glucagon-like peptide-1 in the double-knockout mice. Double-knockout mice and Kir6.2-knockout mice were similarly insulin sensitive as assessed by the insulin tolerance test. CONCLUSION: GIP is the major insulinotropic factor in the secretion of insulin in response to glucose load in K(ATP) channel-deficient mice.

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