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Taste receptor cells express pH-sensitive leak K ϩ channels.

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Fatty acidinduced stimulation of enteroendocrine cells leads to release of the hormones such as cholecystokinin (CCK) that contribute to satiety. Recently, the fatty acid activated G protein-coupled receptor GPR120 has been shown to mediate long-chain unsaturated free fatty acidinduced CCK release from the enteroendocrine cell line, STC-1, yet the downstream signaling pathway remains unclear. Here we show that linoleic acid (LA) elicits membrane depolarization and an intracellular calcium rise in STC-1 cells and that these responses are significantly reduced when activity of G proteins or phospholipase C is blocked. LA leads to activation of monovalent cation-specific transient receptor potential channel type M5 (TRPM5) in STC-1 cells. LA-induced TRPM5 currents are significantly reduced when expression of TRPM5 or GPR120 is reduced using RNA interference. Furthermore, the LA-induced rise in intracellular calcium and CCK secretion is greatly diminished when expression of TRPM5 channels is reduced using RNA interference, consistent with a role of TRPM5 in LA-induced CCK secretion in STC-1 cells. GPR120; cholecystokinin; calcium; patch-clamp recording INGESTED FAT, SPECIFICALLY in the form of free fatty acids (FFAs), activates enteroendocrine cells (EECs) in the proximal small intestine, which initiates a number of physiological functions including satiety, potentiating insulin release, and delaying gastric emptying (7,9,14). FFA-induced satiety is mainly mediated by secretion of satiety peptides including cholecystokinin (CCK) and glucagon-like peptide-1 (GLP-1) by specialized EECs (type I) present in preabsorptive sites in the intestine (6,7,14,24,28). Recently, the fatty acid-activated G protein-coupled receptor, GPR120, has been shown to mediate long-chain unsaturated FFA-induced CCK and GLP-1 release from the enteroendocrine cell line, STC-1, yet the downstream signaling pathway from GPR120 activation to CCK secretion remains unclear (9, 26). Several groups have shown that FFA-induced CCK release in STC-1 cells is dependent on increased intracellular calcium. Moreover, removal of extracellular calcium and/or application of the L-type voltage-gated calcium channel (VGCC) blocker nicardipine significantly reduced the FFA-induced intracellular calcium rise and CCK secretion in STC-1 cells (23,26). Thus, it is reasonable to postulate from these observations that polyunsaturated fatty acids (PUFAs) may depolarize STC-1 cells by activating GPR120 and initiating membrane depolarization thereby activating L-type VGCCs leading to an elevation of intracellular calcium concentration and CCK secretion. The link between GPR120 and the development of the receptor potential (i.e., depolarization) remains unknown.One candidate for producing the FFA-induced receptor potential in nutrient-responsive cells is transient receptor potential channel type M5 (TRPM5). TRPM5 is a calcium-activated, nonselective, monovalent cation-permeable channel that is highly expressed in taste receptor cells (TRCs) and is required for sweet, bitter...