Electrophysiological characteristics of rat gustatory cyclic nucleotide-gated channel expressed in Xenopus oocytes

The complementary DNA encoding gustatory cyclic nucleotide-gated ion channe l (or gustCNG channel) cloned from rat tongue epithelial tissue was express ed in Xenopus oocytes, and its electrophysiological characteristics were in vestigated using tight-seal patch-clamp recordings of single and macroscopi c channel currents. Both cGMP and cAMP directly activated gustCNG channels but with markedly different affinities. No desensitization or inactivation of gustCNG channel currents was observed even in the prolonged application of the cyclic nucleotides. Single-channel conductance of gustCNG channel wa s estimated as 28 pS in 130 mM of symmetric Nat. Single-channel current rec ordings revealed fast open-close transitions and longer lasting closure sta tes. The distribution of both open and closed events could be well fitted w ith two exponential components and intracellular cGMP increased the open pr obability (P-o) of gustCNG channels mainly by increasing the slower opening rate. Under bi-ionic conditions, the selectivity order of gustCNG channel among divalent cations was determined as Na+ similar to K+ > Rb+ > Li+ > Cs + with the permeability ratio of 1:0.95:0.74:0.63:0.49. Magnesium ion block ed Na+ currents through gustCNG channels from both intracellular and extrac ellular sides in voltage-dependent manners. The inhibition constants (K(i)s ) of intracellular Mg2+ were determined as 360 +/- 40 muM at 70 mV and 8.2 +/- 1.5 mM at -70 mV with z delta value of 1.04, while K(i)s of extracellul ar Mg2+ were as 1.1 +/- 0.3 mM at 70 mV and 20.0 +/- 0.1 muM at -70 mV with z delta of 0.94. Although 100 muM I-cis-diltiazem blocked significant port ions of outward Na+ currents through both bovine rod and rat olfactory CNG channels, the gustCNG channel currents were minimally affected by the same concentration of the drug.