Production and Clearance of All–trans Retinol in Bleached Rods and Cones Depend on Opsin Type and Photoreceptor Morphology

Abstract

Abstract: : Purpose: These experiments were designed to determine the effects of photoreceptor structure and visual pigment type on the all–trans retinal to all–trans retinol reduction and the clearance of the retinol from the cell following bleaching. Methods: Measurements were performed on red and blue cones, and red and green rods isolated from the dark–adapted retinae of the tiger salamander (Ambystoma tigrinum) and rods isolated from the dark–adapted retinae of the Tokay gecko (Gekko gekko). The spatial and temporal distribution of all–trans retinol was measured by microfluorometry after an intense bright flash bleaching (> 99%) of the visual pigment. Retinol fluorescence was recorded at different temperatures in the range 5–38 ^oC. Cells were treated with interphotoreceptor retinol binding protein (IRBP, 100 µM) to determine the effect of the presence of an extracellular lipid binding protein. Simultaneous electrophysiological recordings were carried out to identify the photoreceptor types. Results: Green rods and blue cones have been shown previously (Ma et al., Neuron 32, 451, 2001) to contain identical pigments. However, this pigment resides in internal disks in green rods and in the plasma membrane in blue cones. Gecko rods contain cone–like visual pigments (Kojima et al., Proc. Natl. Acad. Sci. 89, 6841, 1992) and the pigments reside in internal disks. All–trans retinol is produced ca. 30 times faster in salamander blue and red cones than in red rods. In both gecko rods and salamander green rods, which contain cone–type visual pigments in a rod–type morphology, the production of all–trans retinol is ca. 4–8 times faster than in salamander red rods. The clearance of all–trans retinol is ca. 5–20 times faster in the photoreceptors with cone–type morphology than in the photoreceptors with rod–type morphology. Retinol clearance from both rods and cones is substantially accelerated in the presence of IRBP. Conclusions: Our results are consistent with a model in which the rate of the reduction of all–trans retinal to all–trans retinol is defined by the visual pigment type, i.e. its metarhodopsin II decay rate, as well as the availability of the reducing agent. Retinol clearance can be modeled as a simple diffusion process, the rate of which is defined by the photoreceptor morphology.

Keywords: photoreceptors • retinoids/retinoid binding proteins • electrophysiology: non-clinical