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Incorporation of a Retinal Rod cGMP-Dependent Conductance into Planar Bilayers
Jacqueline C. Tanaka, Roy E. Furman, Walter H. Cobbs and Paul Mueller
Proceedings of the National Academy of Sciences of the United States of America
Vol. 84, No. 3 (Feb. 1, 1987), pp. 724-728
Published by: National Academy of Sciences
Stable URL: http://www.jstor.org/stable/28800
Page Count: 5
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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 <50 μ M. Washout of cGMP rapidly reversed the effect. The apparent half-saturating concentrations were between 12 and 27 μ M. Sodium, lithium, cesium, and potassium supported current in the presence of low concentrations of Ca2+, Mg2+, and 100 μ M 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.
Proceedings of the National Academy of Sciences of the United States of America © 1987 National Academy of Sciences