Sour Taste Stimuli Evoke Ca2+ and pH Responses in Mouse Taste Cells

摘要

Sour taste is elicited by acids. How taste cells transduce sour taste is controversial because acids (specifically protons) have diverse effects on cell membranes. Consequently, it is difficult to differentiate between events related to sour taste transduction per se and unrelated effects of protons. We have studied acid taste transduction in mouse taste buds using a lingual slice preparation where it is possible to measure changes in pH and [Ca²⁺]i simultaneously in taste cells. Focal application of citric acid or HCl to the apical tips of taste buds produced widespread acidification of the entire taste bud. Citric acid was effective at a pH of ∼4, but HCl only at a pH of ∼1.5. Despite acidification of the whole taste bud, only a select few taste cells exhibited Ca²⁺ responses. Acid-evoked Ca²⁺ responses were dose dependent in a range consistent with them being sour-taste responses. Cells exhibiting acid-evoked Ca²⁺ responses also responded to KCl depolarization. Acid-evoked Ca²⁺ responses were blocked by Ba²⁺ (2 mm) and Cd²⁺ (500 μm), suggesting that acid responses are generated by Ca²⁺ influx through depolarization-gated Ca²⁺ channels. Removing extracellular Ca²⁺ reduced acid-evoked Ca²⁺ responses, but depleting intracellular Ca²⁺ stores with thapsigargin had no effect, suggesting that acid taste responses are generated by an influx of extracellular Ca²⁺. Neither Cs⁺ (500 μm) nor amiloride (100 μm) affected acid-evoked Ca²⁺ responses, suggesting that neither hyperpolarization-activated cyclic nucleotide-gated cation (pacemaker) channels nor epithelial Na⁺ channels, respectively, transduce sour taste. Collectively, the results indicate that acids, especially weak acids, acidify the taste bud and evoke depolarization-induced Ca²⁺ entry into a select subset of taste cells. The primary transducer protein(s) for sour taste remain undiscovered.