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Activation by bitter substances of a cationic channel in membrane patches excised from the bullfrog taste receptor cell

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, Ca2+, 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 K½ 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+ (PNa:PK:PCs) was 1:0.48:0.39. The unitary conductance was dependent on the extracellular Ca2+ concentration ([Ca2+]o); 9.2 pS in a nominally Ca2+‐free solution, and 4.5 pS in 1.8 mM [Ca2+]o. 6 The dose dependence, the ion selectivity and the dependence of the unitary conductance on [Ca2+]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.