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Y. Koutalos, Q. Wu; The Mobility of Retinol in Rod Photoreceptor Outer Segments . Invest. Ophthalmol. Vis. Sci. 2006;47(13):2841.
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© ARVO (1962-2015); The Authors (2016-present)
To determine the mobility of all–trans retinol in the outer segments of living rod photoreceptors.
Experiments were carried out with isolated living rod photoreceptors obtained from frog (Rana pipiens) retinas. Cells were placed in chambers on the stage of a microscope that is part of a Non–Linear Optical Confocal Microscope (Zeiss LSM 510 NLO). Retinol mobility was measured with Fluorescence Recovery After Photobleaching (FRAP) using two–photon excitation (720 nm) of retinol fluorescence (emission: 400–650 nm).
Endogenous retinol, generated through the reduction of the all–trans retinal released after rhodopsin bleaching, moved in the disk membrane with an apparent diffusion coefficient of 2.5 ± 0.3 µm2 s–1, similar to the diffusion coefficient of free lipids in cell membranes. The apparent diffusion coefficient of exogenously added retinol was 1.1 ± 0.2 µm2 s–1. Retinol also moved axially, along the length of the outer segment. The time constant for the axial retinol movement was proportional to the square of outer segment length, indicating a process of passive diffusion that does not require energy input. The diffusion coefficient of 0.07 ± 0.01 µm2 s–1 for the axial retinol movement was consistent with free diffusion.
Most of the retinol produced from rhodopsin bleaching in the outer segments of rod photoreceptors moves freely, is not confined to the disks, and is not tightly bound to carrier proteins.
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