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1 Internal medicine, Virginia Commonwealth University, Richmond, VA, USA
2 Department of Physiology, Virginia Commonwealth University, Richmond, VA, USA; Internal medicine, Virginia Commonwealth University, Richmond, VA, USA
3 Internal medicine, Virginia Commonwealth University, Richmond, VA, USA; Department of Physiology, Virginia Commonwealth University, Richmond, VA, USA
4 Department of Physiology, Virginia Commonwealth University, Richmond, VA, USA
* To whom correspondence should be addressed. E-mail: vlyall{at}hsc.vcu.edu.
Messenger RNA 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 (pHi) and Na+ ([Na+]i) using fluorescence ratio imaging. At constant external pH: (i) Removal of basolateral Na+ reversibly decreased pHi and [Na+]i. (ii) HOE642, a specific blocker, and amiloride, a non-specific blocker of basolateral NHE-1, attenuated the decrease in pHi and [Na+]i. (iii) Exposure of TRCs to basolateral NH4Cl or sodium acetate pulses induced transient decreases in pHi that recovered spontaneously to baseline. (iv) pHi 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. (v) HOE642, MIA, EIPA, and amiloride inhibited pHi recovery with Ki values of 0.23, 0.46, 0.84, and 29 µM, respectively. (vi) A decrease in apical or basolateral pH acidified TRC pHi and inhibited spontaneous pHi 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.
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