1. The conductance and kinetics of single 3',5'‐cyclic guanosine monophosphate (cGMP)‐activated channels of retinal rod outer segments were studied in inside‐out membrane patches. The size of the single channel currents was increased by using low concentrations of divalent cations. 2. At saturating cGMP concentration, the current flickered at high frequency. Occasionally, the current was interrupted by closures lasting tens or hundreds of milliseconds. At +50 mV the maximum current during an opening was slightly more than 1 pA, but the open channel level was poorly resolved due to the speed of the gating transitions. 3. Amplitude histograms confirmed the presence of a sublevel of current, roughly a quarter the size of the peak current, at low cGMP concentrations. The fraction of time in the sublevel decreased with increasing cGMP concentration, suggesting that the sublevel may be due to opening by the partially liganded channel. 4. Consistent with previous macroscopic current recordings, single channel activation by cGMP had an apparent dissociation constant of 8.6 microM, and a Hill coefficient of 2.8. 5. At saturating cGMP concentrations, the channel was modelled as a two‐state system with the following parameters. The open channel conductance was 25 pS. The opening rate constant, beta, was 1.5 x 10(4) s‐1 at 0 mV, and had a voltage sensitivity equivalent to the movement of 0.23 electronic charges outward through the membrane electric field. The closing rate constant, alpha, was 2.1 x 10(4) s‐1 and was voltage insensitive. Assuming that the open‐state chord conductance was voltage independent, the inferred voltage dependence of beta largely accounted for the outward rectification in the steady‐state macroscopic current‐voltage relation of multichannel patches, at saturating cGMP concentration.
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