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Frans Vinberg, Jeannie Chen, Vladimir Kefalov; Studying GCAP-independent Ca2+ Feedback in Mammalian Cones. Invest. Ophthalmol. Vis. Sci. 2013;54(15):2486.
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© ARVO (1962-2015); The Authors (2016-present)
Differences in GCAP- or Rec-mediated Ca2+-feedbacks are not sufficient to explain the differences between mouse rod and cone light adaptation. We sought to test the importance of GCAP- and Rec-independent Ca2+ -feedback in mammalian cones. This was done by mimicking light-induced drop in [Ca2+]i in cones with reduced or absent cGMP synthesis modulation to prevent a high increase of cGMP in darkness.
Cone responses were recorded with transretinal ERG from mice in GNAT1-/- background. Retinas were perfused with bicarbonate-buffered Locke’s solution that contained either 1.2 mM or low EGTA-buffered ~30 nM free [Ca2+] at 37 oC. To isolate the photoreceptor component of the ERG response 20 μM D, L-AP4 and 50 μM BaCl2 were used. The change in the phototransduction gain caused by low Ca2+ exposure was defined as the factor by which the calculated single-photon response in low Ca2+ had to be scaled in order to match its leading edge with that in normal Ca2+.
When control retinas were switched from normal to low Ca2+ the maximum cone response amplitude (rsat) first increased several-fold and then declined together with significant deceleration and desensitization of flash responses producing 2-fold increase in dim flash response time-to-peak (tp) and ~30-fold decrease in gain (n = 3). In GCAP-/- cones tp increased 15% and the gain decreased 10-fold when switched from normal to low Ca2+ (n = 4). Exposure of Rec-/- cones to low Ca2+ caused several-fold transient increase in rsat and 2.3-fold increase in tp together with ~45-fold decrease in gain (n = 4). Rec-/- cones in GCAP+/- background showed 30% increase in tp and 30-fold decrease in gain when they were exposed to low Ca2+ (n = 7). All mice except GCAP-/- showed a prominent response recovery after light step onset and overshoot at light step offset in normal Ca2+. Both response features disappeared in low Ca2+.
Reducing expression level of GCAPs in mouse cones prevented excessive deceleration and desensitization of flash responses and enabled long stable recordings in low Ca2+. In addition, the degree of low Ca2+- induced decline in the gain of mouse cone phototransduction decreased with reduced expression of GCAPs. Thus, manipulation of [Ca2+]o in GCAP-/- background mice can be used to study GCAP-independent Ca2+ -feedback, such as CNG channel modulation, and Ca2+-independent feedback in mammalian cones.
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