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T. Szikra, K. Cusato, D. Krizaj; Store–Operated Channels Regulate Baseline Ca in Light–Adapted Photoreceptors . Invest. Ophthalmol. Vis. Sci. 2006;47(13):3715.
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We have previously described store–operated calcium (SOC) channel activity in rod and cone photoreceptors. In the present study we test the hypothesis that SOC channels modulate baseline Ca concentration in photoreceptors, we characterize modulation of SOC channel activity by the endoplasmic reticulum (ER) and determine the molecular identity of SOC channels.
High–resolution calcium imaging was performed using a digital CCD camera in isolated salamander photoreceptor cells loaded with the high–affinity Ca indicator dye fura–2. Ca imaging was performed in control and in TRPC1 siRNA– loaded cells.
The driving force for Ca entry in unstimulated cells was modulated by exposing light–adapted cells to 1, 2, 5 and 10 mM external Ca in the presence of the L–type Ca channel blocker nifedipine. [Ca2+]o elevation triggered a moderate dose–dependent increase in cytosolic [Ca2+]. The influx was potentiated following depletion of Ca stores with the SERCA blocker thapsigargin. Ca elevations were blocked by SKF–96365 (5 uM) or La (10 uM) and were partially blocked by MRS–1845 (15 uM). TRPC isoform 1 was expressed in rod, but not cone, photoreceptors. Incubation of cells with siRNA designed to block TRPC1 expression reduced the amplitude of the overshoot by ∼4 % in cones and ∼48 % in rods. Rods, but not cones, were labeled with the TRPC1 antibody.
Sustained influx of Ca into the inner segments of light–adapted photoreceptors occurs in part via SOC channels. In rods, these channels belong to the TRPC1 family. Ca influx via SOCs is potentiated by depletion of ER Ca stores. ER stores thus regulate the dynamic range of cytosolic Ca. SOC channels may represent an important mode of plasma membrane Ca influx in light–adapted vertebrate photoreceptors.
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