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Whitney Cleghorn, Jianhai Du, Martin Sadilek, Viren Govindaraju, James Hurley; Metabolic pathways of photoreceptors in light and dark. Invest. Ophthalmol. Vis. Sci. 2013;54(15):693.
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Rod and cone photoreceptors (PRs) need to produce both ATP and anabolic precursors to survive. The metabolic demands of PRs are different in light and dark: dark adapted PRs use energy to fuel ion exchange, while PRs in the light undergo anabolic metabolism to regenerate rhodopsin and synthesize new membranes. The purpose of this study is to understand how the metabolic pathways differ in PRs under light and dark conditions.
Retinas were isolated from C57BL mice (6-8 weeks old) and cultured in 2 ml KRB buffer with either 5 mM 13C glucose or 5 mM 13C glutamine in 37oC CO2 incubator. The metabolites were analyzed by GC-MS (Agilent 5973 MSD/6890 GC). Mass spectra were collected and isotopomer distributions were corrected for naturally-occurring heavy isotopes using the software, Isocor. The mean enrichment (ME) of each metabolite was calculated.
Carbons from uniformly labeled 13C glucose are incorporated rapidly into glycolytic intermediates and into citrate, but are incorporated more slowly into other TCA cycle intermediates. Interestingly, two of the TCA cycle metabolites, succinate and fumarate, show lower incorporation than surrounding intermediates, suggesting the carbons enter from multiple points into the TCA cycle. The overall incorporation of glucose is qualitatively similar in the dark, however accumulation of labeled intermediates happens faster. Incorporation of 13C labeled glutamine is well isolated from glycolytic intermediates. Carbons from glutamine are incorporated into TCA cycle intermediates at the same rate as glucose, and show higher incorporation in the dark. Interestingly, malate shows the biggest increase in 13C in dark adapted retinas, regardless of the fuel used. In addition, our experiments show that the overall redox state of the retina is stable in glucose but not glutamine, but the reducing power (lac/pyr ratio) is used up faster in the dark.
Glutamine is the major source for formation of TCA cycle intermediates, whereas glucose provides cytosolic reducing power for the photoreceptors. Generally each of these fuels is used similarly in light and dark, but all of the metabolic reactions occur faster in darkness than in light. In particular, pyruvate carboxylase and malic enzyme activity appear to be faster in darkness.
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