Expression of amiloride-sensitive epithelial sodium channels in mouse taste cells after chorda tympani nerve crush

Segregated Expression of ENaC Subunits in Taste Cells

Salt taste is one of the 5 basic taste qualities. Depending on the concentration, table salt is perceived either as appetitive or aversive, suggesting the contribution of several mechanisms to salt taste, distinguishable by their sensitivity to the epithelial sodium channel (ENaC) blocker amiloride. A taste-specific knockout of the α-subunit of the ENaC revealed the relevance of this polypeptide for low-salt transduction, whereas the response to other taste qualities remained normal. The fully functional ENaC is composed of α-, β-, and γ-subunits. In taste tissue, however, the precise constitution of the channel and the cell population responsible for detecting table salt remain uncertain. In order to examine the cells and subunits building the ENaC, we generated mice carrying modified alleles allowing the synthesis of green and red fluorescent proteins in cells expressing the α- and β-subunit, respectively. Fluorescence signals were detected in all types of taste papillae and in taste buds of the soft palate and naso-incisor duct. However, the lingual expression patterns of the reporters differed depending on tongue topography. Additionally, immunohistochemistry for the γ-subunit of the ENaC revealed a lack of overlap between all potential subunits. The data suggest that amiloride-sensitive recognition of table salt is unlikely to depend on the classical ENaCs formed by α-, β-, and γ-subunits and ask for a careful investigation of the channel composition.

Is the Amiloride-Sensitive Na+ Channel in Taste Cells Really ENaC?

Among the 5 taste qualities, salt is the least understood. The receptors, their expression pattern in taste cells, and the transduction mechanisms for salt taste are still unclear. Previous studies have suggested that low concentrations of NaCl are detected by the amiloride-sensitive epithelial Na+ channel (ENaC), which in other systems requires assembly of 3 homologous subunits (α, β, and γ) to form a functional channel. However, a new study from Lossow and colleagues, published in this issue of Chemical Senses, challenges that hypothesis by examining expression levels of the 3 ENaC subunits in individual taste cells using gene-targeted mice in combination with immunohistochemistry and in situ hybridization. Results show a lack of colocalization of ENaC subunits in taste cells as well as expression of subunits in taste cells that show no amiloride sensitivity. These new results question the molecular identity of the amiloride-sensitive Na+ conductance in taste cells.

Does ENaC Work as Sodium Taste Receptor in Humans?

Taste reception is fundamental for the proper selection of food and beverages. Among the several chemicals recognized by the human taste system, sodium ions (Na⁺) are of particular relevance. Na⁺ represents the main extracellular cation and is a key factor in many physiological processes. Na⁺ elicits a specific sensation, called salty taste, and low-medium concentrations of table salt (NaCl, the common sodium-containing chemical we use to season foods) are perceived as pleasant and appetitive. How we detect this cation in foodstuffs is scarcely understood. In animal models, such as the mouse and the rat, the epithelial sodium channel (ENaC) has been proposed as a key protein for recognizing Na⁺ and for mediating preference responses to low-medium salt concentrations. Here, I will review our current understanding regarding the possible involvement of ENaC in the detection of food Na⁺ by the human taste system.

Sodium Imbalance in Mice Results Primarily in Compensatory Gene Regulatory Responses in Kidney and Colon, but Not in Taste Tissue

Renal excretion and sodium appetite provide the basis for sodium homeostasis. In both the kidney and tongue, the epithelial sodium channel (ENaC) is involved in sodium uptake and sensing. The diuretic drug amiloride is known to block ENaC, producing a mild natriuresis. However, amiloride is further reported to induce salt appetite in rodents after prolonged exposure as well as bitter taste impressions in humans. To examine how dietary sodium content and amiloride impact on sodium appetite, mice were subjected to dietary salt and amiloride intervention and subsequently analyzed for ENaC expression and taste reactivity. We observed substantial changes of ENaC expression in the colon and kidney confirming the role of these tissues for sodium homeostasis, whereas effects on lingual ENaC expression and taste preferences were negligible. In comparison, prolonged exposure to amiloride-containing drinking water affected β- and αENaC expression in fungiform and posterior taste papillae, respectively, next to changes in salt taste. However, amiloride did not only change salt taste sensation but also perception of sucrose, glutamate, and citric acid, which might be explained by the fact that amiloride itself activates bitter taste receptors in mice. Accordingly, exposure to amiloride generally affects taste impression and should be evaluated with care.

Expression of protocadherin-20 in mouse taste buds

Taste information is detected by taste cells and then transmitted to the brain through the taste nerve fibers. According to our previous data, there may be specific coding of taste quality between taste cells and nerve fibers. However, the molecular mechanisms underlying this coding specificity remain unclear. The purpose of this study was to identify candidate molecules that may regulate the specific coding. GeneChip analysis of mRNA isolated from the mice taste papillae and taste ganglia revealed that 14 members of the cadherin superfamily, which are important regulators of synapse formation and plasticity, were expressed in both tissues. Among them, protocadherin-20 (Pcdh20) was highly expressed in a subset of taste bud cells, and co-expressed with taste receptor type 1 member 3 (T1R3, a marker of sweet- or umami-sensitive taste cells) but not gustducin or carbonic anhydrase-4 (markers of bitter/sweet- and sour-sensitive taste cells, respectively) in circumvallate papillae. Furthermore, Pcdh20 expression in taste cells occurred later than T1R3 expression during the morphogenesis of taste papillae. Thus, Pcdh20 may be involved in taste quality-specific connections between differentiated taste cells and their partner neurons, thereby acting as a molecular tag for the coding of sweet and/or umami taste.

Expression of Renin-Angiotensin System Components in the Taste Organ of Mice

The systemic renin-angiotensin system (RAS) is an important regulator of body fluid and sodium homeostasis. Angiotensin II (AngII) is a key active product of the RAS. We previously revealed that circulating AngII suppresses amiloride-sensitive salt taste responses and enhances the responses to sweet compounds via the AngII type 1 receptor (AT1) expressed in taste cells. However, the molecular mechanisms underlying the modulation of taste function by AngII remain uncharacterized. Here we examined the expression of three RAS components, namely renin, angiotensinogen, and angiotensin-converting enzyme-1 (ACE1), in mouse taste tissues. We found that all three RAS components were present in the taste buds of fungiform and circumvallate papillae and co-expressed with αENaC (epithelial sodium channel α-subunit, a salt taste receptor) or T1R3 (taste receptor type 1 member 3, a sweet taste receptor component). Water-deprived mice exhibited significantly increased levels of renin expression in taste cells (p < 0.05). These results indicate the existence of a local RAS in the taste organ and suggest that taste function may be regulated by both locally-produced and circulating AngII. Such integrated modulation of peripheral taste sensitivity by AngII may play an important role in sodium/calorie homeostasis.

Insights on modulators in perception of taste modalities: a review

A major challenge in taste research is to overcome the flavour imperfections in food products and to build nutritious strategies to combat against obesity as well as other related metabolic syndromes. The field of molecular taste research and chemical senses has contributed to an enormous development in understanding the taste receptors and mechanisms of taste perception. Accordingly, the development of taste-modifying compounds or taste modulators that alter the perception of basic taste modalities has gained significant prominence in the recent past. The beneficial aspects of these substances are overwhelming while considering their potential taste-modifying properties. The objective of the present review is to provide an impression about the taste-modulating compounds and their distinctive taste-modifying properties with reference to their targets and proposed mechanisms of action. The present review also makes an effort to discuss the basic mechanism involved in oro-gustatory taste perception as well as on the effector molecules involved in signal transduction downstream to the activation of taste receptors.

Recent Advances in Molecular Mechanisms of Taste Signaling and Modifying

The sense of taste conveys crucial information about the quality and nutritional value of foods before it is ingested. Taste signaling begins with taste cells via taste receptors in oral cavity. Activation of these receptors drives the transduction systems in taste receptor cells. Then particular transmitters are released from the taste cells and activate corresponding afferent gustatory nerve fibers. Recent studies have revealed that taste sensitivities are defined by distinct taste receptors and modulated by endogenous humoral factors in a specific group of taste cells. Such peripheral taste generations and modifications would directly influence intake of nutritive substances. This review will highlight current understanding of molecular mechanisms for taste reception, signal transduction in taste bud cells, transmission between taste cells and nerves, regeneration from taste stem cells, and modification by humoral factors at peripheral taste organs.

Effect of chorda tympani nerve transection on salt taste perception in mice

Effects of gustatory nerve transection on salt taste have been studied extensively in rats and hamsters but have not been well explored in the mouse. We examined the effects of chorda tympani (CT) nerve transection on NaCl taste preferences and thresholds in outbred CD-1 mice using a high-throughput phenotyping method developed in our laboratory. To measure taste thresholds, mice were conditioned by oral self-administration of LiCl or NaCl and then presented with NaCl concentration series in 2-bottle preference tests. LiCl-conditioned and control NaCl-exposed mice were given bilateral transections of the CT nerve (LiCl-CTX, NaCl-CTX) or were left intact as controls (LiCl-CNT, NaCl-CNT). After recovery from surgery, mice received a concentration series of NaCl (0–300 mM) in 48-h 2-bottle tests. CT transection increased NaCl taste thresholds in LiCl-conditioned mice and eliminated avoidance of concentrated NaCl in control NaCl-exposed mice. This demonstrates that in mice, the CT nerve is important for detection and recognition of NaCl taste and is necessary for the normal avoidance of high concentrations of NaCl. The results of this experiment also show that the method of high-throughput phenotyping of salt taste thresholds is suitable for detecting changes in the taste periphery in mouse genetic studies.

Ghrelin is produced in taste cells and ghrelin receptor null mice show reduced taste responsivity to salty (NaCl) and sour (citric acid) tastants

Background

The gustatory system plays a critical role in determining food preferences, food intake and energy balance. The exact mechanisms that fine tune taste sensitivity are currently poorly defined, but it is clear that numerous factors such as efferent input and specific signal transduction cascades are involved.

Methodology/Principal Findings

Using immunohistochemical analyses, we show that ghrelin, a hormone classically considered to be an appetite-regulating hormone, is present within the taste buds of the tongue. Prepro-ghrelin, prohormone convertase 1/3 (PC 1/3), ghrelin, its cognate receptor (GHSR), and ghrelin-O-acyltransferase (GOAT , the enzyme that activates ghrelin) are expressed in Type I, II, III and IV taste cells of mouse taste buds. In addition, ghrelin and GHSR co-localize in the same taste cells, suggesting that ghrelin works in an autocrine manner in taste cells. To determine a role for ghrelin in modifying taste perception, we performed taste behavioral tests using GHSR null mice. GHSR null mice exhibited significantly reduced taste responsivity to sour (citric acid) and salty (sodium chloride) tastants.

Conclusions/Significance

These findings suggest that ghrelin plays a local modulatory role in determining taste bud signaling and function and could be a novel mechanism for the modulation of salty and sour taste responsivity.

Responses of the hamster chorda tympani nerve to sucrose+acid and sucrose+citrate taste mixtures

Studies of taste receptor cells, chorda tympani (CT) neurons, and brainstem neurons show stimulus interactions in the form of inhibition or enhancement of the effectiveness of sucrose when mixed with acids or citrate salts, respectively. To investigate further the effects of acids and the trivalent citrate anion on sucrose responses in hamsters (Mesocricetus auratus), we recorded multifiber CT responses to 100 mM sucrose; a concentration series of HCl, citric acid, acetic acid, sodium citrate (with and without amiloride added), potassium citrate, and all binary combinations of acids and salts with 100 mM sucrose. Compared with response additivity, sucrose responses were increasingly suppressed in acid + sucrose mixtures with increases in titratable acidity, but HCl and citric acid were more effective suppressors than acetic acid. Citrate salts suppressed sucrose responses and baseline CT neural activity to a similar degree. Citrate salts also elicited prolonged, concentration-dependent, water-rinse responses. The specific loss in sucrose effectiveness as a CT stimulus with increasing titratable acidity was confirmed; however, no increase in sucrose effectiveness was found with the addition of citrate. Further study is needed to define the chemical basis for effects of acids and salts in taste mixtures.

Effects of dietary Na+ deprivation on epithelial Na+ channel (ENaC), BDNF, and TrkB mRNA expression in the rat tongue

BACKGROUND

In rodents, dietary Na+ deprivation reduces gustatory responses of primary taste fibers and central taste neurons to lingual Na+ stimulation. However, in the rat taste bud cells Na+ deprivation increases the number of amiloride sensitive epithelial Na+ channels (ENaC), which are considered as the "receptor" of the Na+ component of salt taste. To explore the mechanisms, the expression of the three ENaC subunits (alpha, beta and gamma) in taste buds were observed from rats fed with diets containing either 0.03% (Na+ deprivation) or 1% (control) NaCl for 15 days, by using in situ hybridization and real-time quantitative RT-PCR (qRT-PCR). Since BDNF/TrkB signaling is involved in the neural innervation of taste buds, the effects of Na+ deprivation on BDNF and its receptor TrkB expression in the rat taste buds were also examined.

RESULTS

In situ hybridization analysis showed that all three ENaC subunit mRNAs were found in the rat fungiform taste buds and lingual epithelia, but in the vallate and foliate taste buds, only alpha ENaC mRNA was easily detected, while beta and gamma ENaC mRNAs were much less than those in the fungiform taste buds. Between control and low Na+ fed animals, the numbers of taste bud cells expressing alpha, beta and gamma ENaC subunits were not significantly different in the fungiform, vallate and foliate taste buds, respectively. Similarly, qRT-PCR also indicated that Na+ deprivation had no effect on any ENaC subunit expression in the three types of taste buds. However, Na+ deprivation reduced BDNF mRNA expression by 50% in the fungiform taste buds, but not in the vallate and foliate taste buds. The expression of TrkB was not different between control and Na+ deprived rats, irrespective of the taste papillae type.

CONCLUSION

The findings demonstrate that dietary Na+ deprivation does not change ENaC mRNA expression in rat taste buds, but reduces BDNF mRNA expression in the fungiform taste buds. Given the roles of BDNF in survival of cells and target innervation, our results suggest that dietary Na+ deprivation might lead to a loss of gustatory innervation in the mouse fungiform taste buds.

NaCl responsive taste cells in the mouse fungiform taste buds

Previous studies have demonstrated that rodents' chorda tympani (CT) nerve fibers responding to NaCl can be classified according to their sensitivities to the epithelial sodium channel (ENaC) blocker amiloride into two groups: amiloride-sensitive (AS) and -insensitive (AI). The AS fibers were shown to respond specifically to NaCl, whereas AI fibers broadly respond to various electrolytes, including NaCl. These data suggest that salt taste transduction in taste cells may be composed of at least two different systems; AS and AI ones. To further address this issue, we investigated the responses to NaCl, KCl and HCl and the amiloride sensitivity of mouse fungiform papilla taste bud cells which are innervated by the CT nerve. Comparable with the CT data, the results indicated that 56 NaCl-responsive cells tested were classified into two groups; 25 cells ( approximately 44%) narrowly responded to NaCl and their NaCl response were inhibited by amiloride (AS cells), whereas the remaining 31 cells ( approximately 56%) responded not only to NaCl, but to KCl and/or HCl and showed no amiloride inhibition of NaCl responses (AI cells). Amiloride applied to the basolateral side of taste cells had no effect on NaCl responses in the AS and AI cells. Single cell reverse transcription-polymerase chain reaction (RT-PCR) experiments indicated that ENaC subunit mRNA was expressed in a subset of AS cells. These findings suggest that the mouse fungiform taste bud is composed of AS and AI cells that can transmit taste information differently to their corresponding types of CT fibers, and apical ENaCs may be involved in the NaCl responses of AS cells.

Cracking taste codes by tapping into sensory neuron impulse traffic

Insights into the biological basis for mammalian taste quality coding began with electrophysiological recordings from "taste" nerves and this technique continues to produce essential information today. Chorda tympani (geniculate ganglion) neurons, which are particularly involved in taste quality discrimination, are specialists or generalists. Specialists respond to stimuli characterized by a single taste quality as defined by behavioral cross-generalization in conditioned taste tests. Generalists respond to electrolytes that elicit multiple aversive qualities. Na(+)-salt (N) specialists in rodents and sweet-stimulus (S) specialists in multiple orders of mammals are well characterized. Specialists are associated with species' nutritional needs and their activation is known to be malleable by internal physiological conditions and contaminated external caloric sources. S specialists, associated with the heterodimeric G-protein coupled receptor T1R, and N specialists, associated with the epithelial sodium channel ENaC, are consistent with labeled line coding from taste bud to afferent neuron. Yet, S-specialist neurons and behavior are less specific than T1R2-3 in encompassing glutamate and E generalist neurons are much less specific than a candidate, PDK TRP channel, sour receptor in encompassing salts and bitter stimuli. Specialist labeled lines for nutrients and generalist patterns for aversive electrolytes may be transmitting taste information to the brain side by side. However, specific roles of generalists in taste quality coding may be resolved by selecting stimuli and stimulus levels found in natural situations. T2Rs, participating in reflexes via the glossopharynygeal nerve, became highly diversified in mammalian phylogenesis as they evolved to deal with dangerous substances within specific environmental niches. Establishing the information afferent neurons traffic to the brain about natural taste stimuli imbedded in dynamic complex mixtures will ultimately "crack taste codes."

Amiloride-sensitive channels in type I fungiform taste cells in mouse

Background

Taste buds are the sensory organs of taste perception. Three types of taste cells have been described. Type I cells have voltage-gated outward currents, but lack voltage-gated inward currents. These cells have been presumed to play only a support role in the taste bud. Type II cells have voltage-gated Na⁺ and K⁺ current, and the receptors and transduction machinery for bitter, sweet, and umami taste stimuli. Type III cells have voltage-gated Na⁺, K⁺, and Ca²⁺ currents, and make prominent synapses with afferent nerve fibers. Na⁺ salt transduction in part involves amiloride-sensitive epithelial sodium channels (ENaCs). In rodents, these channels are located in taste cells of fungiform papillae on the anterior part of the tongue innervated by the chorda tympani nerve. However, the taste cell type that expresses ENaCs is not known. This study used whole cell recordings of single fungiform taste cells of transgenic mice expressing GFP in Type II taste cells to identify the taste cells responding to amiloride. We also used immunocytochemistry to further define and compare cell types in fungiform and circumvallate taste buds of these mice.

Results

Taste cell types were identified by their response to depolarizing voltage steps and their presence or absence of GFP fluorescence. TRPM5-GFP taste cells expressed large voltage-gated Na⁺ and K⁺ currents, but lacked voltage-gated Ca²⁺ currents, as expected from previous studies. Approximately half of the unlabeled cells had similar membrane properties, suggesting they comprise a separate population of Type II cells. The other half expressed voltage-gated outward currents only, typical of Type I cells. A single taste cell had voltage-gated Ca²⁺ current characteristic of Type III cells. Responses to amiloride occurred only in cells that lacked voltage-gated inward currents. Immunocytochemistry showed that fungiform taste buds have significantly fewer Type II cells expressing PLC signalling components, and significantly fewer Type III cells than circumvallate taste buds.

Conclusion

The principal finding is that amiloride-sensitive Na⁺ channels appear to be expressed in cells that lack voltage-gated inward currents, likely the Type I taste cells. These cells were previously assumed to provide only a support function in the taste bud.

Amiloride-sensitive channels in type I fungiform taste cells in mouse

BACKGROUND

Taste buds are the sensory organs of taste perception. Three types of taste cells have been described. Type I cells have voltage-gated outward currents, but lack voltage-gated inward currents. These cells have been presumed to play only a support role in the taste bud. Type II cells have voltage-gated Na+ and K+ current, and the receptors and transduction machinery for bitter, sweet, and umami taste stimuli. Type III cells have voltage-gated Na+, K+, and Ca2+ currents, and make prominent synapses with afferent nerve fibers. Na+ salt transduction in part involves amiloride-sensitive epithelial sodium channels (ENaCs). In rodents, these channels are located in taste cells of fungiform papillae on the anterior part of the tongue innervated by the chorda tympani nerve. However, the taste cell type that expresses ENaCs is not known. This study used whole cell recordings of single fungiform taste cells of transgenic mice expressing GFP in Type II taste cells to identify the taste cells responding to amiloride. We also used immunocytochemistry to further define and compare cell types in fungiform and circumvallate taste buds of these mice.

RESULTS

Taste cell types were identified by their response to depolarizing voltage steps and their presence or absence of GFP fluorescence. TRPM5-GFP taste cells expressed large voltage-gated Na+ and K+ currents, but lacked voltage-gated Ca2+ currents, as expected from previous studies. Approximately half of the unlabeled cells had similar membrane properties, suggesting they comprise a separate population of Type II cells. The other half expressed voltage-gated outward currents only, typical of Type I cells. A single taste cell had voltage-gated Ca2+ current characteristic of Type III cells. Responses to amiloride occurred only in cells that lacked voltage-gated inward currents. Immunocytochemistry showed that fungiform taste buds have significantly fewer Type II cells expressing PLC signalling components, and significantly fewer Type III cells than circumvallate taste buds.

CONCLUSION

The principal finding is that amiloride-sensitive Na+ channels appear to be expressed in cells that lack voltage-gated inward currents, likely the Type I taste cells. These cells were previously assumed to provide only a support function in the taste bud.

Amiloride-sensitive NaCl taste responses are associated with genetic variation of ENaC alpha-subunit in mice

An epithelial Na(+) channel (ENaC) is expressed in taste cells and may be involved in the salt taste transduction. ENaC activity is blocked by amiloride, which in several mammalian species also inhibits taste responses to NaCl. In mice, lingual application of amiloride inhibits NaCl responses in the chorda tympani (CT) gustatory nerve much stronger in the C57BL/6 (B6) strain than in the 129P3/J (129) strain. We examined whether this strain difference is related to gene sequence variation or mRNA expression of three ENaC subunits (alpha, beta, gamma). Real-time RT-PCR and in situ hybridization detected no significant strain differences in expression of all three ENaC subunits in fungiform papillae. Sequences of the beta- and gammaENaC subunit genes were also similar in the B6 and 129 strains, but alphaENaC gene had three single nucleotide polymorphisms (SNPs). One of these SNPs resulted in a substitution of arginine in the B6 strain to tryptophan in the 129 strain (R616W) in the alphaENaC protein. To examine association of this SNP with amiloride sensitivity of CT responses to NaCl, we produced F(2) hybrids between B6 and 129 strains. Amiloride inhibited CT responses to NaCl in F(2) hybrids with B6/129 and B6/B6 alphaENaC R616W genotypes stronger than in F(2) hybrids with 129/129 genotype. This suggests that the R616W variation in the alphaENaC subunit affects amiloride sensitivity of the ENaC channel and provides evidence that ENaC is involved in amiloride-sensitive salt taste responses in mice.

Recovery of two independent sweet taste systems during regeneration of the mouse chorda tympani nerve after nerve crush

In rodents, section of the taste nerve results in degeneration of the taste buds. Following regeneration of the cut taste nerve, however, the taste buds reappear. This phenomenon can be used to study the functional reformation of the peripheral neural system responsible for sweet taste. In this study we examined the recovery of sweet responses by the chorda tympani (CT) nerve after nerve crush as well as inhibition of these responses by gurmarin (Gur), a sweet response inhibitor. After about 2 weeks of CT nerve regeneration, no significant response to any taste stimuli could be observed. At 3 weeks, responses to sweet stimuli reappeared but were not significantly inhibited by Gur. At 4 weeks, Gur inhibition of sweet responses reached statistically significant levels. Thus, the Gur-sensitive (GS) component of the sweet response reappeared about 1 week later than the Gur-insensitive (GI) component. Moreover, single CT fibers responsive to sucrose could be classified into distinct GS and GI groups at 4 weeks. After 5 weeks or more, responses to sweet compounds before and after treatment with Gur became indistinguishable from responses in the intact group. During regeneration, the GS and GI components of the sucrose response could be distinguished based on their concentration-dependent responses to sucrose. These results suggest that mice have two different sweet-reception systems, distinguishable by their sensitivity to Gur (the GS and GI systems). These two sweet-reception systems may be reconstituted independently during regeneration of the mouse CT nerve.

Taste receptors in the gastrointestinal tract III. Salty and sour taste: sensing of sodium and protons by the tongue

Taste plays an essential role in food selection and consequently overall nutrition. Because salt taste is appetitive, humans ingest more salt than they need. Acids are the source of intrinsically aversive sour taste, but in mixtures with sweeteners they are consumed in large quantities. Recent results have provided fresh insights into transduction and sensory adaptation for the salty and sour taste modalities. The sodium-specific salt taste receptor is the epithelial sodium channel whereas a nonspecific salt taste receptor is a taste variant of the vanilloid receptor-1 nonselective cation channel, TRPV1. The proximate stimulus for sour taste is a decrease in the intracellular pH of a subset of acid-sensing taste cells, which serves as the input to separate transduction pathways for the phasic and tonic parts of the sour neural response. Adaptation to sour arises from the activation of the basolateral sodium-hydrogen exchanger isoform-1 by an increase in intracellular calcium that sustains the tonic phase of the sour taste response.