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Regulation of Glucagon Secretion at Low Glucose Concentrations: Evidence for Adenosine Triphosphate-Sensitive Potassium Channel Involvement

Departments of Medicine (L.A.-B., A.S.R.) and Molecular and Cellular Biology (A.M., M.H., J.B., L.A.-B.), Baylor College of Medicine, Houston, Texas 77030; and The Institute of Child Health (K.H.), Biochemistry Endocrinology and Metabolism Unit, University College London, London WC1N 1EH United Kingdom

Address all correspondence and requests for reprints to: Arun S. Rajan, One Baylor Plaza, BCMA 700B, Houston, Texas 77030. E-mail: arajan{at}bcm.edu.

Glucagon is a potent counterregulatory hormone that opposes the action of insulin in controlling glycemia. The cellular mechanisms by which pancreatic -cell glucagon secretion occurs in response to hypoglycemia are poorly known. SUR1/K_IR6.2-type ATP-sensitive K⁺ (K_ATP) channels have been implicated in the glucagon counterregulatory response at central and peripheral levels, but their role is not well understood. In this study, we examined hypoglycemia-induced glucagon secretion in vitro in isolated islets and in vivo using Sur1KO mice lacking neuroendocrine-type K_ATP channels and paired wild-type (WT) controls. Sur1KO mice fed ad libitum have normal glucagon levels and mobilize hepatic glycogen in response to exogenous glucagon but exhibit a blunted glucagon response to insulin-induced hypoglycemia. Glucagon release from Sur1KO and WT islets is increased at 2.8 mmol/liter glucose and suppressed by increasing glucose concentrations. WT islets increase glucagon secretion approximately 20-fold when challenged with 0.1 mmol/liter glucose vs. approximately 2.7-fold for Sur1KO islets. Glucagon release requires Ca²⁺ and is inhibited by nifedipine. Consistent with a regulatory interaction between K_ATP channels and intra-islet zinc-insulin, WT islets exhibit an inverse correlation between ß-cell secretion and glucagon release. Glibenclamide stimulated insulin secretion and reduced glucagon release in WT islets but was without effect on secretion from Sur1KO islets. The results indicate that loss of -cell K_ATP channels uncouples glucagon release from inhibition by ß-cells and reveals a role for K_ATP channels in the regulation of glucagon release by low glucose.

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