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  Low Amplification and Fast Visual Pigment Phosphorylation as Mechanisms Characterizing Cone Photoresponses



In the vertebrate retina, there are two types of photoreceptors, rods and cones. Rods are very sensitive to light and cones are less sensitive. The photoresponse of a cone is much briefer than that of a rod. The molecular mechanisms of these differences between rods and cones have not been known mainly due to inability to purify cones.

In this study, we purified cone photoreceptors using a stepwise Percoll gradient (left panel) and compared the efficiencies of the phototransduction reactions between rods and cones (right panel). The ON reactions studied (blue arrows) were less efficient in cones (the ratio is shown in a circle), and visual pigment phosphorylation (red arrow), an OFF reaction, was much faster in cones. These findings reasonably explain why cones are less sensitive to light and their photoresponses are much briefer.


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