Modulation of Rat Chorda Tympani NaCl Responses and Intracellular Na⁺ Activity in Polarized Taste Receptor Cells by pH

Mixture interactions between sour and salt taste modalities were investigated in rats by direct measurement of intracellular pH (pH_i) and Na⁺ activity ([Na⁺]_i) in polarized fungiform taste receptor cells (TRCs) and by chorda tympani (CT) nerve recordings. Stimulating the lingual surface with NaCl solutions adjusted to pHs ranging between 2.0 and 10.3 increased the magnitude of NaCl CT responses linearly with increasing external pH (pH_o). At pH 7.0, the epithelial sodium channel (ENaC) blocker, benzamil, decreased NaCl CT responses and inhibited further changes in CT responses induced by varying pH_o to 2.0 or 10.3. At constant pH_o, buffering NaCl solutions with potassium acetate/acetic acid (KA/AA) or HCO₃⁻/CO₂ inhibited NaCl CT responses relative to CT responses obtained with NaCl solutions buffered with HEPES. The carbonic anhydrase blockers, MK-507 and MK-417, attenuated the inhibition of NaCl CT responses in HCO₃⁻/CO₂ buffer, suggesting a regulatory role for pH_i. In polarized TRCs step changes in apical pH_o from 10.3 to 2.0 induced a linear decrease in pH_i that remained within the physiological range (slope = 0.035; r² = 0.98). At constant pH_o, perfusing the apical membrane with Ringer's solutions buffered with KA/AA or HCO₃⁻/CO₂ decreased resting TRC pH_i, and MK-507 or MK-417 attenuated the decrease in pH_i in TRCs perfused with HCO₃⁻/CO₂ buffer. In parallel experiments, TRC [Na⁺]_i decreased with (a) a decrease in apical pH, (b) exposing the apical membrane to amiloride or benzamil, (c) removal of apical Na⁺, and (d) acid loading the cells with NH₄Cl or sodium acetate at constant pH_o. Diethylpyrocarbonate and Zn²⁺, modification reagents for histidine residues in proteins, attenuated the CO₂-induced inhibition of NaCl CT responses and the pH_i-induced inhibition of apical Na⁺ influx in TRCs. We conclude that TRC pH_i regulates Na⁺-influx through amiloride-sensitive apical ENaCs and hence modulates NaCl CT responses in acid/salt mixtures.