Low amplification and fast visual pigment phosphorylation as mechanisms characterizing cone photoresponses

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Abstract
Vertebrate cone photoreceptors are known to show lower light sensitivity and briefer photoresponses than rod photoreceptors. To understand the molecular mechanisms characterizing cone photoresponses, we compared some of the reactions in the phototransduction cascade between rods and cones. For this purpose, rods and cones were obtained in quantities large enough to do biochemical studies. The cells were purified from the retina of carp (Cyprinus carpio) with a stepwise Percoll gradient. The purified rod fraction contained almost no other kinds of cells besides rods, and the purified cone fraction contained a mixture of red-, green-, and blue-sensitive cones in the ratio 3:≈1:≈1. We prepared membrane preparations from the rod and the cone fraction, and in these membranes, we measured activation efficiencies of the reactions in the phototransduction cascade. The results showed that the signal amplification is lower in the cone membranes, which accounts for the lower light sensitivity in cones. Furthermore, we measured the time courses of visual pigment phosphorylation. The result showed that the phosphorylation is much faster in the cone membranes, which also explains the lower light sensitivity and, in addition, the briefer photoresponse in cones.