Effect of extracellular Ca2+ on the quinine-activated current of bullfrog taste receptor cells

1. The bitter substance quinine activates a cation current from the frog ta ste receptor cell. We have analysed the noise associated with this current, and the effect of extracellular Ca2+ on the current, using whole-cell reco rding on single dissociated cells. 2. Quinine induced an inward current from the taste receptor cell near the resting potential. The response was accompanied by an increase in current f luctuations. From the variance/mean ratio of the quinine-activated current, the single-channel conductance was estimated to be 12 pS in the nominal ab sence of extracellular Ca2+. In the presence of 1.8 mM Ca2+, this conductan ce decreased to 5 pS. These values broadly agree with those previously obta ined from excised, outside-out membrane patches. 3. The dependence of the current on quinine concentration had a K-1/2 of 0. 48 mM in the absence of extracellular Ca2+, consistent with measurements fr om excised patches. The K-1/2 value increased to 2.8 mM in 1.8 mM external Ca2+. The maximum current induced by quinine was also reduced by about 20% by Ca2+. 4. The spectral power density distribution of the quinine-activated current could be described by the sum of two Lorentzian functions, with corner fre quencies not substantially different in the absence and presence of 1.8 mM external Ca2+. 5. The above results lend further support to the notion that the major comp onent of the response of frog taste receptor cells to quinine comes from an ion channel directly activated by quinine.