CONDUCTANCE AND KINETICS OF SINGLE CGMP-ACTIVATED CHANNELS IN SALAMANDER ROD OUTER SEGMENTS

1. The conductance and kinetics of single 3',5'-cyclic guanosine monop hosphate (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 cation s. 2. At saturating cGMP concentration, the current flickered at high frequency. Occasionally, the current was interrupted by closures lasti ng tens or hundreds of milliseconds. At +50 mV the maximum current dur ing 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. Am plitude histograms confirmed the presence of a sublevel of current, ro ughly a quarter the size of the peak current, at low cGMP concentratio ns. The fraction of time in the sublevel decreased with increasing cGM P concentration, suggesting that the sublevel mag be due to opening by the partially liganded channel. 4. Consistent with previous macroscop ic current recordings, single channel activation by cGMP had an appare nt dissociation constant of 8.6 mu M, 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 conducta nce 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. Th e closing rate constant, alpha, was 2.1 x 10(4) s(-1) and was voltage insensitive. Assuming that the open-state chord conductance was voltag e independent, the inferred voltage dependence of beta largely account ed for the outward rectification in the steady-state macroscopic curre nt-voltage relation of multichannel patches, at saturating cGMP concen tration.