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Keisuke Sakurai, Jeannie C. Chen, Vladimir J. Kefalov; Role of NCKX2 in Mouse Cone Phototransduction. Invest. Ophthalmol. Vis. Sci. 2011;52(14):6577.
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The Na+/(Ca2++K+)-exchanger, NCKX, plays an important role in the calcium homeostasis of rod and cone photoreceptors. Calcium continuously flows into their outer segments through cGMP-gated channels and NCKX is responsible for its extrusion from the photoreceptors. While rods and cones express different isoforms of the exchanger, NCKX1 and NCKX2, respectively, it is not known how exchanger properties affect photoreceptor light sensitivity, response kinetics, or adaptation. To determine how NCKX2 modulates cone function, we compared flash responses from wild type and NCKX2 knockout (Nckx2-/-) mouse cones.
We performed transretinal electroretinogram recordings from isolated wild type and Nckx2-/- mouse retinas. Transretinal recordings were done from dorsal mouse retina, with synaptic transmission inhibited pharmacologically to block signaling in second order neurons and isolate the photoreceptor flash response. To remove the rod component of the transretinal flash responses, all recordings were done from mice in rod transducin alpha knockout (Gnat1-/-) background.
Consistent with previous reports, the maximal response amplitude and light sensitivity were similar in control and Nckx2-/- cones. However, the deletion of NCKX2 dramatically slowed down the flash kinetics of dark-adapted mouse cones. The time-to-peak of dim-flash responses increased from 103 ± 3 ms (n = 9) in control cones to 165 ± 11 ms (n = 7) in Nckx2-/- cones and the corresponding integration time increased from 165 ± 17 ms (n = 9) in wild type to 357 ± 33 ms (n = 7) in Nckx2-/- cones. Surprisingly, the background light intensity required to reduce cone maximal response by two fold in Nckx2-/- cones was comparable to that in control cones and the slope of the cone maximal response decline in background light was not altered by the deletion of NCKX2.
Our results demonstrate that while the deletion of NCKX2 dramatically slows down the light response kinetics in mouse cones, it has little effect on the steady state sensitivity of cones in darkness or in constant background light. We conclude that NCKX2 plays an important role in accelerating the extrusion of calcium out of mouse cone outer segments. However, its absence most likely does not affect the steady state level of cone calcium in darkness or in background light.
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