In Intact Cone Photoreceptors, a Ca2+-dependent, Diffusible Factor Modulates the cGMP-gated Ion Channels Differently than in Rods

Abstract

We investigated the modulation of cGMP-gated ion channels in single cone photoreceptors isolated from a fish retina. A new method allowed us to record currents from an intact outer segment while controlling its cytoplasmic composition by superfusion of the electropermeabilized inner segment. The sensitivity of the channels to agonists in the intact outer segment differs from that measured in membrane patches detached from the same cell. This sensitivity, measured as the ligand concentration necessary to activate half-maximal currents, K 1/2, also increases as Ca2+ concentration decreases. In electropermeabilized cones, K 1/2 for cGMP is 335.5 ± 64.4 μM in the presence of 20 μM Ca2+, and 84.3 ± 12.6 μM in its absence. For 8Br-cGMP, K 1/2 is 72.7 ± 11.3 μM in the presence of 20 μM Ca2+ and 15.3 ± 4.5 μM in its absence. The Ca2+-dependent change in agonist sensitivity is larger in extent than that measured in rods. In electropermeabilized tiger salamander rods, K 1/2 for 8Br-cGMP is 17.9 ± 3.8 μM in the presence of 20 μM Ca2+ and 7.2 ± 1.2 μM in its absence. The Ca2+-dependent modulation is reversible in intact cone outer segments, but is progressively lost in the absence of divalent cations, suggesting that it is mediated by a diffusible factor. Comparison of data in intact cells and detached membrane fragments from cones indicates that this factor is not calmodulin. At 40 μM 8Br-cGMP, the Ca2+-dependent change in sensitivity in cones is half-maximal at K Ca = 286 ± 66 nM Ca2+. In rods, by contrast, K Ca is ∼50 nM Ca2+. The difference in magnitude and Ca2+ dependence of channel modulation between photoreceptor types suggests that this modulation may play a more significant role in the regulation of photocurrent gain in cones than in rods.

Circadian Regulation of cGMP-Gated Channels of Vertebrate Cone Photoreceptors: Role of cAMP and Ras

“…Circadian modulation of CNGCs entails a post-translational modification of the channel complex, and it qualitatively resembles effects that others have observed after binding of calmodulin and related proteins, or phosphorylation on specific CNGC tyrosine residues (Rebrik and Korenbrot, 1998;Kramer and Molokanova, 2001;Molokanova and Kramer, 2001). Modulation of photoreceptor CNGCs is predicted to evoke complex effects on the dynamics of phototransduction (Pugh et al, 1999;Savchenko et al, 2001).…”

Altered Expression of Genes of the Bmp/Smad and Wnt/Calcium Signaling Pathways in the Cone-only Nrl-/- Mouse Retina, Revealed by Gene Profiling Using Custom cDNA Microarrays

“…Furthermore, we observed altered expression of several Ca 2ϩ -dependent genes, such as Calm2, Camk2b, Mtap6, Myo5a, and Myo7a. Because these genes are expressed in both rods and cones (57,58), we hypothesize that the expression changes may reflect differences in Ca 2ϩ homeostasis in rods versus cones (59,60). We suggest that the Wnt/Ca 2ϩ pathway may modulate intracellular Ca 2ϩ concentration and may have more direct control on cone function or homeostasis.…”

Molecular Basis of an Inherited Form of Incomplete Achromatopsia

“…Although we cannot predict whether the dark current and [Ca 2ϩ ] i are increased because of the single-channel properties or decreased because of a lower channel density, Ca 2ϩ -regulated negative feedback on the synthesis and degradation of cGMP (for review, see Pugh and Lamb, 2000), on the activity of the cone CNG channel, and on the activity of the Na ϩ /Ca 2ϩ -K ϩ exchanger (Korenbrot and Rebrik, 2002) Koch et al, 2002). Moreover, at high [Ca 2ϩ ] i , the cGMP sensitivity of photoreceptor CNG channels is lowered (Hsu and Molday, 1993;Rebrik and Korenbrot, 1998;Korenbrot and Rebrik, 2002;Peng et al, 2003). Therefore, we favor the idea that enhanced current noise is the principal defect in the outer segment caused by the mutant CNG channel.…”