The cGMP-dependent channel of vertebrate rod photoreceptors exists in two forms of different cGMP sensitivity and pharmacological behavior

Channel modulators affect PGE(2) binding to bovine aortic endothelial cells

PGE(2), PGF(2alpha) and the thromboxane agonist U-46619 bind to bovine aortic endothelial cells and compete on the same binding site with similar affinity. In addition, binding remains unaffected by prolonged exposure to the ligand. These characteristics differ significantly from those of any known G-coupled prostaglandin receptor. Binding of PGE(2) to the cells is reduced in the presence of the cyclic nucleotides cGMP and cAMP, and is unaffected by protein kinase inhibitors. Removal of permeable cyclic nucleotides from the cell medium results in a fast and complete restoration of PGE(2) binding to the cells, suggesting that both cyclic nucleotides reduce PGE(2) binding by a reversible interaction with the prostaglandin-binding site, without the involvement of second messenger-activated protein kinases. Our data further show that binding of prostaglandins to bovine aortic endothelial cells is sensitive to heavy metals and to activators and blockers of calcium, ATP-sensitive K(+) and chloride channels. Nickel, a specific cyclic nucleotide-gated (CNG) channel activator, decreases PGE(2) binding and so do the CNG channel activators Rp-8-Br-PET-cGMPS and Sp-8-Br-PET-cGMPS. On the other hand, the calcium channel blockers pimozide, diltiazem as well as LY-83,583, a guanylate cyclase inhibitor, which were reported to block CNG channels, enhance PGE(2) binding. The sensitivity of PGE(2) binding to selective CNG channel modifying agents, as well as the rapid and reversible interaction with cyclic nucleotides, may suggest that the common low-affinity prostanoid-binding site on bovine aortic endothelial cells is associated with a molecular entity, which possess several properties of a CNG channel.

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 approximately 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 approximately 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 alpha and beta subunits. The maximal PCa/PNa of channels formed from alpha subunits of bovine rod channels is less than that of heteromeric channels formed from alpha and beta subunits. In addition, Ca2+ is a more effective blocker of channels formed by alpha subunits than of channels formed by alpha and beta subunits. The cGMP-dependent shift in divalent cation selectivity is a property of alphabeta channels and not of channels formed from alpha subunits alone.

Calcium release from rod outer segments: evidence for a cGMP-sensitive calcium binding protein

Numerous studies investigating the cGMP-gated cation conductance in rod disk membranes have purported to measure efflux of Ca2+ entrapped in rod disk membrane vesicles. We have utilized sonication and osmotic shock as additional tests for sensitivity of cGMP- and A23187-induced Ca2+ release to elimination of the transvesicular Ca2+ gradient. We find that 1) Treatment with sonication or osmotic shock in low Ca2+ medium does not release Ca2+ from either native cGMP/Ca2(+)-loaded vesicles or solubilized, reconstituted "Ca2(+)-loaded" vesicles, 2) 70-100% of the cGMP-induced "flux" and 90-100% of the A23187-induced Ca2+ "flux" is insensitive to elimination of the Ca2+ gradient by sonication or osmotic shock in low Ca2+ medium, and 3) total amount of releasable Ca2+ is related to membrane surface area rather than vesicle entrapment volume. We conclude that 1) A23187 disrupts binding of Ca2+ to proteins and phospholipids as well as releasing entrapped Ca2+ and 2) a large fraction of the cGMP-induced release observed in rod disk vesicles is due to release of bound Ca2+.

Voltage-dependent gating and block of the cyclic-GMP-dependent current in bovine rod outer segments

The properties of the cyclic-GMP-activated conductance in the plasma membrane of bovine rod outer segments were studied in excised membranes. Multiple-channel and single-channel currents were recorded by the patch-clamp technique in symmetrical NaCl solutions which were free of divalent cations. The current-voltage relationship for the current, recorded when a large population of channels was activated, exhibited outward rectification. Rectification decreased as the concentration of cyclic-GMP was increased, and the concentration of cyclic-GMP required for half maximal activation of the channel decreased with depolarization. At a concentration of 1-3 microM cyclic-GMP, single-channel activity could be observed from these excised patches. The conductance of the open channel was 6 pS and was independent of the membrane potential. These results are consistent with the interpretation that under these conditions, the mechanism responsible for the outward rectification is due to an increase in the probability of an open channel as the membrane is depolarized. The cyclic-GMP-activated current could be blocked by L-cis-diltiazem. Block was voltage and time dependent. The time constant for the onset of block and its steady state level increased with depolarization. The extent of block by diltiazem was not enhanced as the cyclic-GMP concentration was increased, suggesting that the channel is not required to be open for block to occur. Complete block was never attained even for high concentrations of diltiazem. However, the diltiazem-resistant component of the cyclic-GMP-activated current could be blocked by tetracaine.

Silver ions induce a rapid Ca2+ release from isolated intact bovine rod outer segments by a cooperative mechanism

Micromolar concentrations of silver ion activate large Ca2+ fluxes across the plasma membrane of intact rod outer segments isolated from bovine retinas (intact ROS). The rate of Ag+-induced Ca2+ efflux from intact ROS depended on the Ag+ concentration in a sigmoidal manner suggesting a cooperative mechanism with a Hill coefficient between 2 and 3. At a concentration of 50 microM Ag+ the rate of Ca2+ efflux was 7 x 10(6) Ca2+/outer segment/sec; this represents a change in total intracellular Ca2+ by 0.7 mM/outer segment/sec. Addition of the nonselective ionophore gramicidin in the absence of external alkali cations greatly reduced the Ag+-induced Ca2+ efflux from intact ROS, apparently by enabling internal alkali cations to leak out. Adding back alkali cations to the external medium restored Ag+-induced Ca2+ efflux when gramicidin was present. In the presence of gramicidin, Ag+-induced Ca2+ efflux from intact ROS was blocked by 50 microM tetracaine or L-cis diltiazem, whereas without gramicidin both blockers were ineffective. Both L-cis diltiazem and tetracaine are blockers of one kinetic component of cGMP-induced Ca2+ flux across ROS disk membranes. The ion selectivity of the Ag+-induced pathway proved to be broad with little discrimination between the alkali cations Li+, Na+, K+, and Cs+ or between Ca2+ and Mg2+. The properties of the Ag+-induced pathway(s) suggest that it may reflect the cGMP-dependent conductance opened in the absence of cGMP by silver ions.