Divalent Cation Selectivity Is a Function of Gating in Native and Recombinant Cyclic Nucleotide–gated Ion Channels from Retinal Photoreceptors

The selectivity of Ca²⁺ over Na⁺ is ∼3.3-fold larger in cGMP-gated channels of cone photoreceptors than in those of rods when measured under saturating cGMP concentrations, where the probability of channel opening is 85–90%. Under physiological conditions, however, the probability of opening of the cGMP-gated channels ranges from its largest value in darkness of 1–5% to essentially zero under continuous, bright illumination. We investigated the ion selectivity of cGMP-gated channels as a function of cyclic nucleotide concentration in membrane patches detached from the outer segments of rod and cone photoreceptors and have found that ion selectivity is linked to gating. We determined ion selectivity relative to Na⁺ (PX/PNa) from the value of reversal potentials measured under ion concentration gradients. The selectivity for Ca²⁺ over Na⁺ increases continuously as the probability of channel opening rises. The dependence of PCa/PNa on cGMP concentration, in both rods and cones, is well described by the same Hill function that describes the cGMP dependence of current amplitude. At the cytoplasmic cGMP concentrations expected in dark-adapted intact photoreceptors, PCa/PNa in cone channels is ∼7.4-fold greater than that in rods. The linkage between selectivity and gating is specific for divalent cations. The selectivity of Ca²⁺ and Sr²⁺ changes with cGMP concentration, but the selectivity of inorganic monovalent cations, Cs⁺ and NH₄⁺, and organic cations, methylammonium⁺ and dimethylammonium⁺, is invariant with cGMP. Cyclic nucleotide–gated channels in rod photoreceptors are heteromeric assemblies of α and β subunits. The maximal PCa/PNa of channels formed from α subunits of bovine rod channels is less than that of heteromeric channels formed from α and β subunits. In addition, Ca²⁺ is a more effective blocker of channels formed by α subunits than of channels formed by α and β subunits. The cGMP-dependent shift in divalent cation selectivity is a property of αβ channels and not of channels formed from α subunits alone.