Affiliation:
1. From the Department of Physiology and Graduate Program in Biophysics, School of Medicine, University of California at San Francisco, San Francisco, California 94143
Abstract
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.
Publisher
Rockefeller University Press
Cited by
46 articles.
订阅此论文施引文献
订阅此论文施引文献,注册后可以免费订阅5篇论文的施引文献,订阅后可以查看论文全部施引文献