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¹ Department of Physiology and Biophysics, Boston University School of Medicine, Boston, MA 02118
² Institute for Evolutionary Physiology and Biochemistry, Russian Academy of Sciences, 194223 St. Petersburg, Russia
³ Department of Ophthalmology, Medical University of South Carolina, Charleston, SC 29425
⁴ Department of Ophthalmology, Agia Sophia Children's Hospital, University of Athens, Athens 11527, Greece
⁵ National Eye Institute, National Institutes of Health, Bethesda, MD 20892

Correspondence to Petri Ala-Laurila: pal{at}bu.edu

The visual cycle is a chain of biochemical reactions that regenerate visual pigment following exposure to light. Initial steps, the liberation of all-trans retinal and its reduction to all-trans retinol by retinol dehydrogenase (RDH), take place in photoreceptors. We performed comparative microspectrophotometric and microfluorometric measurements on a variety of rod and cone photoreceptors isolated from salamander retinae to correlate the rates of photoproduct decay and retinol production. Metapigment decay rate was spatially uniform within outer segments and 50–70 times faster in the cells that contained cone-type pigment (SWS2 and M/LWS) compared to cells with rod-type pigment (RH1). Retinol production rate was strongly position dependent, fastest at the base of outer segments. Retinol production rate was 10–40 times faster in cones with cone pigments (SWS2 and M/LWS) than in the basal OS of rods containing rod pigment (RH1). Production rate was approximately five times faster in rods containing cone pigment (SWS2) than the rate in basal OS of rods containing the rod pigment (RH1). We show that retinol production is defined either by metapigment decay rate or RDH reaction rate, depending on cell type or outer segment region, whereas retinol removal is defined by the surface-to-volume ratio of the outer segment and the availability of retinoid binding protein (IRBP). The more rapid rates of retinol production in cones compared to rods are consistent with the more rapid operation of the visual cycle in these cells.

Abbreviations used in this paper: IRBP, interphotoreceptor retinoid binding protein; IS, inner segment; LED, light-emitting diode; MSP, microspectrophotometry; NADPH, nicotinamide adenine dinucleotide phosphate (reduced form); OS, outer segment; PSB, protonated Schiff base; RAL, retinal; RDH, retinol dehydrogenase; ROL, retinol; ROS, rod outer segment.

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