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

The complementary DNA encoding gustatory cyclic nucleotide--gated ion channel (or gustCNG channel) cloned from rat tongue epithelial tissue was expressed in Xenopus oocytes, and its electrophysiological characteristics were investigated using tight-seal patch-clamp recordings of single and macroscopic 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 was estimated as 28 pS in 130 mM of symmetric Na(+). Single-channel current recordings revealed fast open-close transitions and longer lasting closure states. The distribution of both open and closed events could be well fitted with two exponential components and intracellular cGMP increased the open probability (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(+) approximately K(+) > Rb(+) > Li(+) > Cs(+) with the permeability ratio of 1:0.95:0.74:0.63:0.49. Magnesium ion blocked Na(+) currents through gustCNG channels from both intracellular and extracellular sides in voltage-dependent manners. The inhibition constants (K(i)s) of intracellular Mg(2+) were determined as 360 +/- 40 microM at 70 mV and 8.2 +/- 1.5 mM at -70 mV with z delta value of 1.04, while K(i)s of extracellular Mg(2+) were as 1.1 +/- 0.3 mM at 70 mV and 20.0 +/- 0.1 microM at -70 mV with z delta of 0.94. Although 100 microM l-cis-diltiazem blocked significant portions 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.