Incorporation of a retinal rod cGMP-dependent conductance into planar bilayers

The light-modulated current of vertebrate retinal rods flows through a 3',5'-cyclic GMP-dependent conductance located in the outer segment plasma membrane. We report the incorporation into planar bilayers of a conductance derived from vertebrate rod outer segment membranes specifically activated by cGMP but not by cAMP, 5'-GMP, GTP, or 5'-AMP. When the mean currents were measured as a function of increasing cGMP concentration, maximal activation occurred at concentrations less than 50 microM. Washout of cGMP rapidly reversed the effect. The apparent half-saturating concentrations were between 12 and 27 microM. Sodium, lithium, cesium, and potassium supported current in the presence of low concentrations of Ca2+, Mg2+, and 100 microM cGMP; choline did not. Removal of the divalent cations reversibly increased the currents. When calcium was the only current-carrying cation, attenuated currents were seen. These experiments support the hypothesis that calcium is a permeant blocker of the conductance. At low concentrations of cGMP in solutions also containing 0.5 mM EDTA, brief current spikes occurred with amplitudes from 0.5 to 4 pA at 50 mV. These spikes differed from the well-defined, unitary conductance steps usually associated with the opening and closing of ion channels. Occasionally we saw longer-lasting channel-like events; however, amplitude histograms did not resolve discrete conductance levels.