Gating properties of the cAMP-gated channel in toad olfactory receptor cells.

1. Inside-out membrane patches were excised from different parts of isolated olfactory receptor cells of the toad, and the gating properties of cAMP-gated channels were investigated under low concentrations of divalent cations. 2. At +50 mV, an outward current was observed when 1 mM cAMP was applied to the cytoplasmic side of membrane patches excised from cilia. cGMP had a similar effect. The dose-response relation for the cAMP-induced current could be fitted with the Hill equation with a coefficient of 1.5 for both cAMP and cGMP, and half-maximal concentration (K1/2) values of 19 microM (cAMP) and 16 microM (cGMP). 3. cAMP at low concentration (1 microM) induced step-like currents representing the opening of individual channels with a unitary conductance of about 30 pS. The I-V relation for the unitary events showed a weak outward rectification. 4. The relation between variance and mean current amplitude was well described with a parabola. Even at a saturating ligand concentration (1 mM cAMP) the current fluctuations did not disappear, indicating that fully liganded channels still switched between open and closed states. The maximum open probability was about 80% (+40 to +60 mV). 5. The current fluctuations at 1 mM cAMP were analysed with power spectral analysis. At +50 mV, frequency components lower than 10 Hz were well described with a Lorentzian function with a corner frequency of 1.7 +/- 0.2 Hz (+/- S.D.). In addition, higher flat frequency components were observed. At -50 mV the corner frequency became 0.6 +/- 0.1 Hz. 6. Membrane patches having a single cAMP-gated channel were obtained from the dendro-somatic membrane. These channels were very similar to the ciliary channels in electrophysiological characteristics. 7. The single cAMP-gated channel current showed flickering bursts that were interrupted with gaps. In saturating cAMP condition (1 mM), both burst- and gap-time histograms were fitted with single-exponential functions with time constants of 148 and 141 ms, respectively. 8. The channels were present at a high density of around 1750 microns-2 on the ciliary plasma membrane, as compared to 6 microns-2 on dendro-somatic membrane.