Umami Changes Intracellular Ca²⁺ Levels Using Intracellular and Extracellular Sources in Mouse Taste Receptor Cells

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Recently, candidates for umami receptors have been identified in taste cells, but the precise transduction mechanisms of the downstream receptor remain unknown. To investigate how intracellular Ca²⁺ increases in the umami transduction pathway, we measured changes in intracellular Ca²⁺ levels in response to umami stimuli monosodium glutamate (MSG), IMP, and MSG + IMP in mouse taste receptor cells (TRCs) by Ca²⁺ imaging. Even when extracellular Ca²⁺ was absent, 1/3 of umami-responsive TRCs exhibited increased intracellular Ca²⁺ levels. When intracellular Ca²⁺ was depleted, half of the TRCs retained their response to umami. These results suggest that umami-responsive TRCs increase their intracellular Ca²⁺ levels through two pathways: by releasing Ca²⁺ from intracellular stores and by an influx of Ca²⁺ from extracellular sources. We conclude that the Ca²⁺ influx from extracellular source might play an important role in the synergistic effect between MSG and IMP.