The Insulin Secretory Granule Is the Major Site of KATP Channels of the Endocrine Pancreas

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Abstract
With ATP sites on Kir6.2 that inhibit activity and ADP sites on SUR1 that antagonize the inhibition, ATP-sensitive potassium channels (KATP channels) are designed as exquisite sensors of adenine nucleotide levels that signal changes in glucose metabolism. If pancreatic KATP channels localize to the insulin secretory granule, they would be well positioned to transduce changes in glucose metabolism into changes in granule transport and exocytosis. Tests for pancreatic KATP channels localized to insulin secretory granules led to the following observations: fluorescent sulfonylureas that bind the pancreatic KATP channel specifically label intracellular punctate structures in cells of the endocrine pancreas. The fluorescent glibenclamides colocalize with Ins-C-GFP, a live-cell fluorescent reporter of insulin granules. Expression of either SUR1-GFP or Kir6.2-GFP fusion proteins, but not expression of GFP alone, directs GFP fluorescence to insulin secretory granules. An SUR1 antibody specifically labels insulin granules identified by anti-insulin. Two different Kir6.2 antibodies specifically label insulin secretory granules identified by anti-insulin. Immunoelectron microscopy showed Kir6.2 antibodies specifically label perimeter membrane regions of the secretory granule. Relatively little or no labeling of other structures, including the plasma membrane, was found. Our results demonstrate that the insulin secretory granule is the major site of KATP channels of the endocrine pancreas.