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The Journal of Neurophysiology Vol. 87 No. 1 January 2002, pp. 399-408
Copyright ©2002 by the American Physiological Society
Department of Physiology, Virginia Commonwealth University, Richmond, Virginia 23298-0551
Lyall, Vijay,
Rammy I. Alam,
Tam-Hao T. Phan,
Duy Q. Phan,
Gerard L. Heck, and
John A. DeSimone.
Excitation and Adaptation in the Detection of Hydrogen Ions
by Taste Receptor Cells: A Role for cAMP and Ca2+. J. Neurophysiol. 87: 399-408, 2002. The role of intracellular cAMP and Ca2+ in the
excitation and adaptation of taste responses by HCl was investigated by
direct measurement of intracellular pH (pHi) 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
pHi. The magnitude of pHi
decrease increased with increasing HCl concentration. Following
treatment of the basolateral membrane with 8-CPT-cAMP the decrease in
pHi due to apical 1 mM HCl application was
significantly increased. Treatment with cAMP alone decreased resting
TRC pHi and inhibited the recovery of
pHi from a basolateral
NH4Cl pulse by 46%. Topical lingual application
of ionomycin, a Ca2+ 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 pHi and
activated pHi recovery from NH4Cl pulse by 388%. Apical HCl stimulation
induced a transient decrease in resting TRC pHi
followed by spontaneous recovery. The data suggest that cAMP enhances
the sour taste of strong acids by activating a
Ca2+- and amiloride-insensitive
H+ conductance and inhibiting
pHi recovery in TRCs. However, an increase in
[Ca2+]i stimulates
pHi recovery, which, in turn, increases sensory adaptation to acids.
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