Bitter stimuli induce Ca2+ signaling and CCK release in enteroendocrine STC-1 cells:: role of L-type voltage-sensitive Ca2+ channels

We previously demonstrated the expression of bitter taste receptors of the type 2 family (T2R) and the alpha-subunits of the G protein gustducin (G alpha(gust)) in the rodent gastrointestinal (GI) tract and in GI endocrine cells. In this study, we characterized mechanisms of Ca2+ fluxes induced by two distinct T2R ligands: denatonium benzoate (DB) and phenylthiocarbamide (PTC), in mouse enteroendocrine cell line STC-1. Both DB and PTC induced a marked increase in intracellular [Ca2+] ([Ca2+](i)) in a dose- and time-dependent manner. Chelating extracellular Ca2+ with EGTA blocked the increase in [Ca2+](i) induced by either DB or PTC but, in contrast, did not prevent the effect induced by bombesin. Thapsigargin blocked the transient increase in [Ca2+](i) induced by bombesin, but did not attenuate the [Ca2+](i) increase elicited by DB or PTC. These results indicate that Ca2+ influx mediates the increase in [Ca2+](i) induced by DB and PTC in STC-1 cells. Preincubation with the L-type voltage-sensitive Ca2+ channel (L-type VSCC) blockers nitrendipine or diltiazem for 30 min inhibited the increase in [Ca2+](i) elicited by DB or PTC. Furthermore, exposure to the L-type VSCCs opener BAY K 8644 potentiated the increase in [Ca2+](i) induced by DB and PTC. Stimulation with DB also induced a marked increase in the release of cholecystokinin from STC-1 cells, an effect also abrogated by prior exposure to EGTA or L-type VSCC blockers. Collectively, our results demonstrate that bitter tastants increase [Ca2+](i) and cholecystokinin release through Ca2+ influx mediated by the opening of L-type VSCCs in enteroendocrine STC-1 cells.