Acid detection by taste receptor cells

Sourness is a primary taste quality that evokes an innate rejection response in humans and many other animals. Acidic stimuli are the unique sources of sour taste so a rejection response may serve to discourage ingestion of foods spoiled by acid producing microorganisms. The investigation of mechanisms by which acids excite taste receptor cells (TRCs) is complicated by wide species variability and within a species, apparently different mechanisms for strong and weak acids. The problem is further complicated by the fact that the receptor cells are polarized epithelial cells with different apical and basolateral membrane properties. The cellular mechanisms proposed for acid sensing in taste cells include, the direct blockage of apical K⁺ channels by protons, an H⁺-gated Ca²⁺ channel, proton conduction through apical amiloride-blockable Na⁺ channels, a Cl⁻ conductance blocked by NPPB, the activation of the proton-gated channel, BNC-1, a member of the Na⁺ channel/degenerin super family, and by stimulus-evoked changes in intracellular pH. Acid-induced intracellular pH changes appear to be similar to those reported in other mammalian acid-sensing cells, such as type-I cells of the carotid body, and neurons found in the ventrolateral medulla, nucleus of the solitary tract, the medullary raphe, and the locus coceuleus. Like type-I carotid body cells and brainstem neurons, isolated TRCs demonstrate a linear relationship between intracellular pH (pH_i) and extracellular pH (pH_o) with slope, ΔpH_i/ΔpH_o near unity. Acid-sensing cells also appear to regulate pH_i when intracellular pH changes occur under iso-extracellular pH conditions, but fail to regulate their pH when pH_i changes are induced by decreasing extracellular pH. We shall discuss the current status of proposed acid-sensing taste mechanisms, emphasizing pH-tracking in receptor cells.