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Frans Vinberg, Vladimir J Kefalov; Calcium Modulates Mammalian Cone Phototransduction in the Absence of GCAP And Recoverin. Invest. Ophthalmol. Vis. Sci. 2016;57(12):No Pagination Specified.
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© ARVO (1962-2015); The Authors (2016-present)
Calcium sensor proteins Guanylyl Cyclase Activating Protein (GCAP) and recoverin (Rv) can adjust the sensitivity and kinetics of rod and cone phototransduction in response to changing background light intensity. Whether other Ca2+-dependent mechanisms contribute to the function and light adaptation capacity of mammalian cone photoreceptors is not known. Here we determine how Ca2+ and light modulate phototransduction of mammalian cones lacking both GCAP and Rv.
We studied phototransduction and light adaptation of cones in WT and GCAP-/- Rv-/- (DKO) mice that were on Gnat1-/- background to facilitate cone recordings. Cone light responses were recorded from isolated retinas in standard (1.2 mM [Ca2+]) and low Ca2+ (~30 nM free [Ca2+]) Locke’s media at 37 oC. Dark-adapted flash response properties and light adaptation were compared between normal and low Ca2+ conditions in DKO mice to determine the contribution of GCAP/Rv-independent Ca2+ feedback mechanisms to the mammalian cone phototransduction.
Exposure of WT cones to low Ca2+ led to >3-fold transient increase of maximal response amplitudes (Rmax). At steady state Rmax of WT cones was comparable between normal and low Ca2+ but response kinetics became very slow and cones desensitized by ~10-fold when switched to low Ca2+. The large transient increase of Rmax upon low Ca2+ exposure was absent in DKO cones but their Rmax increased by 52 (±15, SEM)% when switched to low Ca2+. Time-to-peak of cone dim flash responses decreased 21 (±7)% and their sensitivity decreased by 49 (±10)% when switched to low Ca2+. In normal Ca2+, DKO cones responded to light step onset by initial hyperpolarization followed by relaxation to a plateau level, and a prominent overshoot at the step offset. Both, the relaxation and overshoot disappeared in low Ca2+. Whereas WT cones showed prototypical Weber-like adaptation, light adaptation of the DKO cones was severely compromised. However, some feedbacks extending the operating range of cones were still present, contributing more to light adaptation in normal than in low Ca2+.
Ca2+-dependent photoransduction feedback is still operational in mammalian cones even in the absence of GCAP and Rv, potentially through Ca2+ modulation of the transduction channels. Our results also reveal novel Ca2+-independent modulation of cone phototransduction via an unknown mechanism.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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