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1 Internal Medicine, Washington University School of Medicine, Saint Louis, MO, USA
2 Obstetrics and Gynecology, Washington University School of Medicine, Saint Louis, MO, USA
* To whom correspondence should be addressed. E-mail: bwice{at}im.wustl.edu.
K-cells are a sub-population of enteroendocrine cells that secrete GIP- a hormone that promotes glucose homeostasis and obesity. Therefore, it is important to understand how GIP secretion is regulated. GIP-producing (GIP/Ins) cell lines secreted hormones in response to many GIP secretagogues except glucose. In contrast, glyceraldehyde and methyl-pyruvate stimulated hormone release. Measurements of intracellular G-6-P, F-1,6-P2, and pyruvate levels, as well as glycolytic flux, in glucose-stimulated GIP/Ins cells indicated that glycolysis was not impaired. Analogous results were obtained using glucose-responsive MIN6 insulinoma cells. Citrate levels increased similarly in glucose-treated MIN6 and GIP/Ins cells. Thus, pyruvate entered the Krebs cycle. Glucose and methyl pyruvate stimulated 1.4- and 1.6-fold increases, respectively, in the ATP/ADP ratio in GIP/Ins cells. Glyceraldehyde profoundly reduced, rather than increased, ATP/ADP. Thus, nutrient-regulated secretion is KATP channel-independent. Antibody staining of mouse intestine demonstrated that enteroendocrine cells producing GIP, GLP-1, cholecystokinin, or somatostatin do not express detectable levels of Kir 6.1 or Kir 6.2 indicating that release of these hormones in vivo may also be KATP channel independent. Conversely, nearly all cells expressing chromogranin A or Substance P, and ~50% of the cells expressing secretin or serotonin, exhibited Kir 6.2 staining. Compounds that activate calcium mobilization were potent secretagogues for GIP/Ins cells. Secretion was only partially inhibited by verapamil suggesting calcium mobilization from intracellular as well as extracellular sources, independent from KATP channels, regulates secretion from some, but not all, sub-populations of enteroendocrine cells.
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