Studies with GIP/Ins cells indicate secretion by gut K cells is K_ATPchannel independent

Abstract

K cells are a subpopulation of enteroendocrine cells that secrete glucose-dependent insulinotropic polypeptide (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 glucose 6-phosphate, fructose 1,6-bisphosphate, 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 tricarboxylic acid cycle. Glucose and methyl pyruvate stimulated 1.4- and 1.6-fold increases, respectively, in the ATP-to-ADP ratio in GIP/Ins cells. Glyceraldehyde profoundly reduced, rather than increased, ATP/ADP. Thus nutrient-regulated secretion is independent of the ATP-dependent potassium (KATP) channel. Antibody staining of mouse intestine demonstrated that enteroendocrine cells producing GIP, glucagon-like peptide-1, CCK, or somatostatin do not express detectable levels of inwardly rectifying potassium (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 that calcium mobilization from intracellular and extracellular sources, independent from KATP channels, regulates secretion from some, but not all, subpopulations of enteroendocrine cells.

Individual Subtypes of Enteroendocrine Cells in the Mouse Small Intestine Exhibit Unique Patterns of Inositol 1,4,5-trisphosphate Receptor Expression

“…The results presented in Figures 1-5, coupled with published observations from our lab (Ramshur et al 2002;Wang et al 2003), indicate that EE cells that produce CGA, but not GIP, express Kir 6.2, IP3R-1, and IP3R-3. Therefore, studies were performed to define the pattern of IP3R expression in additional subpopulations of EE cells.…”

“…Surprisingly, hormone release from GIP-producing cell lines is independent of ATP-regulated potassium channels (Ramshur et al 2002). Consistent with this result, IHC studies revealed that GIP-producing EE cells in the mouse small intestine do not express detectable levels of inward rectifying potassium channel 6.1 (Kir 6.1) or inward rectifying potassium channel 6.2 (Kir 6.2) (Wang et al 2003). Therefore, ATP-regulated potassium channels are not major regulators of cell depolarization and subsequent calcium mobilization in GIPproducing EE cells in vivo.…”

“…Therefore, ATP-regulated potassium channels are not major regulators of cell depolarization and subsequent calcium mobilization in GIPproducing EE cells in vivo. A detailed survey of the mouse small intestinal epithelium revealed that EE cells that produce GLP-1, CCK, or SST also do not express detectable levels of Kir 6.1 or Kir 6.2 (Wang et al 2003). Conversely, most of the EE cells that produce chromogranin A (CGA), serotonin, and/or substance P (sub P) also express Kir 6.2, suggesting that these particular subpopulations of EE cells utilize ATP-regulated potassium channels to control hormone release.…”

Glucagon-Like Peptide 1 Secretion by the L-Cell

“…Taken together, the electrogenic response of the murine L-cell to glucose is similar to the stimulus secretion coupling events that occur during glucose-stimulated insulin secretion, but unlike the ␤-cell, the absence of GLUT2 mRNA expression in the murine L-cell suggests that glucose transport is primarily facilitated by sodium glucose transporters. Nonetheless, a recent report has indicated that the K ATP channel cannot be detected in the mouse L-cell in vivo (69) and K ATP null mice do not show alterations in circulating GLP-1 levels (70). Thus, the physiological importance of the findings made with the murine L-cell in vitro remain to be confirmed.…”

Protein hydrolysate-induced cholecystokinin secretion from enteroendocrine cells is indirectly mediated by the intestinal oligopeptide transporter PepT1

“…G protein-coupled receptors have been implicated in fatty acid sensing (15,22,36), glucose sensing (19,30,33,38,42), and protein/amino acid sensing (3)(4). Interestingly, intestinal transporters may also provide a nutrient-sensing role in the enteroendocrine cell.…”