Light-induced calcium release by intact retinal rods.

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Abstract
Brief illumination of the isolated toad [Bufo marinus] retina produces a transient increase in the extracellular free Ca concentration in the photoreceptor layer. This change in concentration arises from the release of Ca2+ by the rod outer segments. The release begins within 200 ms after the stimulus flash. The peak amplitude of the concentration change increases linearly with light intensity up to approximately 100 absorbed photons/rod and saturates at about 107 absorbed photons/rod. In the linear range the amount of Ca released corresponds to about 2 .times. 104 Ca2+/absorbed photon per rod. The high stoichiometry of this release, in relation to the low free intracellular Ca concentration, implies the release reflects a light-dependent increase in the intracellular free Ca concentration. This light-activated Ca release occurs in the absence of a detectable receptor potential (in low-Na Ringer''s solution) and must reflect an initial stage in the transduction process.