Monovalent cation selectivity has been characterized for the 3',5'-cyclic guanosine monophosphate (cGMP)-activated channel in vertebrate photoreceptor outer segment plasma membranes without divalent cations. Macroscopic currents in excised, inside-out patches were activated with saturating concentrations of cGMP (200 microM). Using a bi-ionic protocol with symmetrical 120 mM ion concentrations across the membrane, alkali metal ions and certain organic cations were substituted for sodium on the cytoplasmic face. The relative permeabilities, determined from shifts in the reversal potential (Erev), were NH4 much greater than Na greater than guanidinium greater than K greater than Li greater than Rb greater than Cs (3.34: 1.0: 0.97: 0.93: 0.92: 0.74: 0.50, respectively). Erev's were also measured as a function of [Na], [NH4], and [Cs], and the slope of the relation was -59.8, -52.1, and -49.1 mV/decade, respectively. The slopes for NH4 and Cs differ significantly from the Nernst-Planck prediction of -58.2 mV/decade expected for a single ion channel. Relative permeabilities were also determined for the alkali metal series of ions with 20 mM ionic concentrations on both sides of the membrane. The permeability sequence at 20 mM was unchanged, but the relative permeability for NH4 and Cs deviated significantly from the measurements at 120 mM with 1.46 and 0.75 ratios, respectively. The dependence of Erev on absolute concentrations and the deviation from Nernst-Planck predictions are best explained by multi-ion occupancy of the cGMP-activated channel. Selectivity was also examined by comparing the conductance ratios as a function of potential.(ABSTRACT TRUNCATED AT 250 WORDS)

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