Energy Metabolism of Photoreceptor Outer Segments
Adenosine triphosphate (ATP) is necessary to drive the reactions that control the ionic current generators as well as the transporters in the outer segment. Because only the inner, and not the outer, segment contains mitochondria, oxidative metabolism is confined to the former. The outer segment is responsible for glycolysis, including the hexose monophosphate pathway and the phosphocreatine shuttle, which produces ATP and GTP and modulates nicotinamide adenine dinucleotide phosphate (NADPH). NADPH reduces retinal to retinol before it is returned to the RPE for isomerization, and it reduces glutathione, which protects against oxidative stress.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.