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Activation by bitter substances of a cationic channel in membrane patches excised from the bullfrog taste receptor cell
Author(s) -
Tsunenari Takashi,
Kurahashi Takashi,
Kaneko Akimichi
Publication year - 1999
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1111/j.1469-7793.1999.0397m.x
Subject(s) - chemistry , biophysics , bullfrog , taste receptor , conductance , inositol , membrane , extracellular , second messenger system , patch clamp , taste , biochemistry , stereochemistry , receptor , endocrinology , biology , mathematics , combinatorics
1 The response to bitter‐tasting substances was recorded in outside‐out membrane patches excised from the taste receptor cell of the bullfrog fungiform papilla. 2 Application of a bitter‐tasting substance, quinine or denatonium, induced channel openings under conditions in which none of the second messenger candidates or their precursors (e.g. cyclic nucleotide, inositol 1,4,5‐trisphosphate, Ca 2+ , ATP and GTP) were present on either side of the membrane. The response could be recorded > 10 min after excision of the patch membrane. These data suggest that the channel was directly gated by the bitter‐tasting substances. 3 No change in response was detected upon addition to the cytoplasmic side of either GDPβS (1 mM) or GTPγS (1 mM), suggesting that the G protein cascade has no direct relation to response generation. 4 The quinine‐induced current was dose dependent. The lowest effective concentration was approximately 0.1 mM, and the saturating concentration was near 1 mM. The dose‐response curve was fitted by the Hill equation with a K ½ of 0.52 mM and a Hill coefficient of 3.8. 5 The single channel conductance measured in 120 mM NaCl solution was 10 pS. The channel was cation selective, and the ratio of the permeabilities for Na + , K + and Cs + ( P Na : P K : P Cs ) was 1:0.48:0.39. The unitary conductance was dependent on the extracellular Ca 2+ concentration ([Ca 2+ ] o ); 9.2 pS in a nominally Ca 2+ ‐free solution, and 4.5 pS in 1.8 mM [Ca 2+ ] o . 6 The dose dependence, the ion selectivity and the dependence of the unitary conductance on [Ca 2+ ] o were almost identical to those of the quinine‐induced whole‐cell current reported previously, indicating that the channel activity observed in the excised membrane is the basis of the whole‐cell current. 7 The present observations suggest the new possibility that the cationic channel directly gated by bitter substances is involved in the bitter taste transduction mechanism.