Amiloride-insensitive currents of the acid-sensing ion channel-2a (ASIC2a)/ASIC2b heteromeric sour-taste receptor channel.
Acid-sensing ion channel-2a (ASIC2a) is an amiloride-blockable proton-gated cation channel, probably contributing to sour-taste detection in rat taste cells. To isolate another subtype of the sour-taste receptor, we screened a rat circumvallate papilla cDNA library and identified ASIC2b, an N-terminal splice variant of ASIC2a. Reverse transcription-PCR analyses confirmed the expression of ASIC2b transcripts in the circumvallate papilla and, moreover, demonstrated its expression in the foliate and fungiform papillae. Immunohistochemical analyses revealed that ASIC2b, as well as ASIC2a, was expressed in a subpopulation of taste cells in the circumvallate, foliate, and fungiform papillae, and some of the cells displayed both ASIC2a and ASIC2b immunoreactivities. Subsequent coimmunoprecipitation studies with circumvallate papillae extracts indicated that ASIC2b associated with ASIC2a to form assemblies and, together with our immunohistochemical findings, strongly suggested that both ASIC2 subunits formed heteromeric channels in taste cells in the circumvallate, foliate, and fungiform papillae. Oocyte electrophysiology demonstrated that the ASIC2a/ASIC2b channel generated maximal inward currents at a pH of < or =2.0, which is in agreement with the in vivo pH sensitivity of rat taste cells, and that the amiloride sensitivity of the heteromer decreased with decreasing pH and was almost completely abolished at a pH of 2.0. These findings provide persuasive explanations for the amiloride insensitivity of acid-induced responses of rat taste cells.
Department of Anatomy II, Nagoya City University Medical School, Nagoya 467-8601, Japan. ugawa@med.nagoya-cu.ac.jp
Shinya
S
Ugawa
Takashi
T
Yamamoto
Takashi
T
Ueda
Yusuke
Y
Ishida
Akira
A
Inagaki
Makoto
M
Nishigaki
Shoichi
S
Shimada
D062565:Acid Sensing Ion Channels; D000143:Acids / Q000494:pharmacology; D000584:Amiloride / Q000494:pharmacology*; D000818:Animals; D015153:Blotting, Western; D062626:Degenerin Sodium Channels; D004847:Epithelial Cells / Q000378:metabolism; D053503:Epithelial Sodium Channels; D015723:Gene Library; D006863:Hydrogen-Ion Concentration; D007150:Immunohistochemistry; D007473:Ion Channels / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D046911:Macromolecular Substances; D008297:Male; D009419:Nerve Tissue Proteins / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D009865:Oocytes / Q000187:drug effects / Q000378:metabolism; D018408:Patch-Clamp Techniques; D011233:Precipitin Tests; D020033:Protein Isoforms / Q000187:drug effects / Q000235:genetics / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013268:Stimulation, Chemical; D013650:Taste Buds / Q000166:cytology / Q000378:metabolism; D014059:Tongue / Q000166:cytology / Q000378:metabolism; D014982:Xenopus laevis
D062565:Acid Sensing Ion Channels; D000143:Acids; C568425:Asic2 protein, rat; D062626:Degenerin Sodium Channels; D053503:Epithelial Sodium Channels; D007473:Ion Channels; D046911:Macromolecular Substances; D009419:Nerve Tissue Proteins; D020033:Protein Isoforms; D012333:RNA, Messenger; D000584:Amiloride