Excitation and Adaptation in the Detection of Hydrogen Ions by Taste Receptor Cells: A Role for cAMP and Ca²⁺

Abstract

The role of intracellular cAMP and Ca(2+) in the excitation and adaptation of taste responses by HCl was investigated by direct measurement of intracellular pH (pH(i)) in polarized taste receptor cells (TRCs) and by chorda tympani (CT) nerve recordings. Stimulating the tongue with HCl concentrations between 1 and 30 mM caused a dose-dependent increase in CT responses that were insensitive to voltage clamp of the lingual receptive field and to amiloride. At a fixed HCl concentration (20 mM) topical lingual application of 8-chlorophenylthio(CPT)-cAMP increased the magnitude of HCl-induced CT response by twofold under zero current clamp. The magnitude of the CT response increased further at -60 mV and decreased at +60 mV lingual voltage clamp but remained amiloride insensitive. In untreated polarized TRCs, apical stimulation with HCl concentrations between 1 and 30 mM HCl induced sustained decreases in TRC pH(i). The magnitude of pH(i) decrease increased with increasing HCl concentration. Following treatment of the basolateral membrane with 8-CPT-cAMP the decrease in pH(i) due to apical 1 mM HCl application was significantly increased. Treatment with cAMP alone decreased resting TRC pH(i) and inhibited the recovery of pH(i) from a basolateral NH4Cl pulse by 46%. Topical lingual application of ionomycin, a Ca(2+) ionophore, did not affect the initial CT response to 20 mM HCl +10 mM CaCl2, but the response declined rapidly to 50% of its initial level within 2 min. In polarized TRCs, basolateral exposure to ionomycin increased TRC pH(i) and activated pH(i) recovery from NH4Cl pulse by 388%. Apical HCl stimulation induced a transient decrease in resting TRC pH(i) followed by spontaneous recovery. The data suggest that cAMP enhances the sour taste of strong acids by activating a Ca(2+)- and amiloride-insensitive H(+) conductance and inhibiting pH(i) recovery in TRCs. However, an increase in [Ca(2+)]i stimulates pH(i) recovery, which, in turn, increases sensory adaptation to acids.

Na+-H+Exchange Activity in Taste Receptor Cells

“…An increase in intracellular cAMP enhanced the sour taste of strong acids by activating a Ca 2ϩ -and amiloride-insensitive apical H ϩ conductance and inhibited pH i recovery in TRC membranes. In contrast, an increase in intracellular Ca 2ϩ concentration ([Ca 2ϩ ] i ) stimulated pH i recovery in TRCs, which increased sensory adaptation to acids (Lyall et al 2002a). Overall, the data indicate that pH recovery mechanisms in TRC membranes play an important role in sour taste transduction and adaptation, but these have not been characterized so far.…”

“…The lingual epithelium was isolated by collagenase treatment (Lyall et al 1997;Stewart et al 1998). A small piece of the anterior lingual epithelium containing a single fungiform papilla was mounted in a special microscopy chamber (Chu et al 1995) as described before (DeSimone et al 2001b;Lyall et al 2001Lyall et al , 2002a. For the measurement of pH i , TRCs within the taste bud were loaded with BCECF, and for Na ϩ measurement ([Na ϩ ] i ), the taste cells were loaded with either 1,3-benzenedicarboxylic acid, 4,4Ј- [1,4,10-trioxa-7,13-diazacyclopentadecane-7, 13-diylbis(5-methoxy-6,12-benzofurandiyl]bis-, tetrakis[(acetyloxy) methyl] ester (SBFI) or Na-green.…”

“…TRCs are polarized neuro-epithelial cells that can be studied in vitro while maintaining their natural polarity in the lingual epithelium. Stimulating the apical membranes of TRCs with acids induced sustained decreases in intracellular pH (pH i ) (Lyall et al , 2002a. During acid stimulation, a decrease in TRC pH i , rather than a decrease in extracellular pH (pH o ), is the stimulus intensity variable that correlates specifically with increased chorda tympani (CT) taste nerve activity.…”

“…These results indicate that a decrease in TRC pH i is the proximate stimulus for sour taste. Further studies indicate that, in the case of strong mineral acids (HCl), both apical H ϩ entry and H ϩ exit across the basolateral membrane of TRCs are regulated by second messengers, which also modulate CT responses to HCl (Lyall et al 2002a). An increase in intracellular cAMP enhanced the sour taste of strong acids by activating a Ca 2ϩ -and amiloride-insensitive apical H ϩ conductance and inhibited pH i recovery in TRC membranes.…”

“…Because TRCs are epithelial cells, with distinct apical and basolateral membrane domains, pH regulatory mechanisms can differ in kind and function between membrane domains (Noel and Pouysségur 1995;Ritter et al 2001). We have therefore used a method that allows us to make measurements on a single fungiform taste bud while maintaining a normal polarized epithelial environment (Lyall et al , 2002aSimon 2002). Using pH imaging, Na ϩ imaging, RT-PCR, and immunocytochemical methods, we have identified both Na ϩ -H ϩ exchanger isoform 1 (NHE-1) and isoform 3 (NHE-3) in fungiform and circumvallate TRCs.…”