Localization of type I inositol 1,4,5-triphosphate receptor in the outer segments of mammalian cones

Calcium enters the outer segment of a vertebrate photoreceptor through a cG MP-gated channel and is extruded via a Na/Ca, K exchanger. We have identifi ed another element in mammalian cones that might help to control cytoplasmi c calcium. Reverse transcription-PCR performed on isolated photoreceptors i dentified mRNA for the SII- splice variant of the type I receptor for inosi tol 1,4,5-triphosphate (IP3), and Western blots showed that the protein als o is expressed in outer segments. Immunocytochemistry showed type I IP3 rec eptor to be abundant in red-sensitive and green-sensitive cones of the tric hromatic monkey retina, but it was negative or weakly expressed in blue-sen sitive cones and rods. Similarly, the green-sensitive cones expressed the r eceptor in dichromatic retina (cat, rabbit, and rat), but the blue-sensitiv e cones did not. Immunostain was localized to disk and plasma membranes on the cytoplasmic face. To restore sensitivity after a light flash, cytoplasm ic cGMP must rise to its basal level, and this requires cytoplasmic calcium to fall. Cessation of calcium release via the IF, receptor might accelerat e this fall and thus explain why the cone recovers much faster than the rod . Furthermore, because its own activity of the IF, receptor depends partly on cytoplasmic calcium, the receptor might control the set point of cytopla smic calcium and thus affect cone sensitivity.