Na+-H+ exchange activity in taste receptor cells

mRNA for two Na+-H+-exchanger isoforms 1 and 3 (NHE-1 and NHE-3) was detected by RT-PCR in fungiform and circumvallate taste receptor cells (TRCs). Anti-NHE-1 antibody binding was localized to the basolateral membranes, and the anti-NHE-3 antibody was localized in the apical membranes of fungiform and circumvallate TRCs. In a subset of TRCs, NHE-3 immunoreactivity was also detected in the intracellular compartment. For functional studies, an isolated lingual epithelium containing a single fungiform papilla was mounted with apical and basolateral sides isolated and perfused with nominally CO2/HCO3--free physiological media (pH 7.4). The TRCs were monitored for changes in intracellular pH (pH(i)) and Na+ ([Na+](i)) using fluorescence ratio imaging. At constant external pH, 1) removal of basolateral Na+ reversibly decreased pH(i) and [Na+](i); 2) HOE642, a specific blocker, and amiloride, a nonspecific blocker of basolateral NHE-1, attenuated the decrease in pH(i) and [Na+](i); 3) exposure of TRCs to basolateral NH4Cl or sodium acetate pulses induced transient decreases in pH i that recovered spontaneously to baseline; 4) pH(i) recovery was inhibited by basolateral amiloride, 5-(N-methyl-N-isobutyl)-amiloride (MIA), 5-(N-ethyl-N-isopropyl)amiloride (EIPA), HOE642, and by Na+ removal; 5) HOE642, MIA, EIPA, and amiloride inhibited pH(i) recovery with K-i values of 0.23, 0.46, 0.84, and 29 muM, respectively; and 6) a decrease in apical or basolateral pH acidified TRC pH(i) and inhibited spontaneous pH(i) recovery. The results indicate the presence of a functional NHE-1 in the basolateral membranes of TRCs. We hypothesize that NHE-1 is involved in sour taste transduction since its activity is modulated during acid stimulation.