Succinate is a mitochondrial metabolite well known for its ability to stimulate respiration through succinate dehydrogenase. Data from multiple studies have implied that succinate localized to mitochondria and does not cross tissue boundaries. We tested this hypothesis by infusing 13C-labeled succinate into the bloodstream of awake, moving C57BL6/J mice through a jugular catheter. Following the infusion we probed intermediates of glycolytic and Krebs cycle metabolism to determine how different tissues utilize succinate. We found that retina and eyecup metabolism appeared unique in their handling of succinate. The retina appeared to be the least permeant to succinate, and succinate that was taken up was not well integrated into the Krebs cycle and was rather directed to become glycolytic intermediates. In the eyecup, 13C originating from succinate populated Krebs cycle intermediates particularly well. We also found that ex vivo, succinate stimulates mitochondrial uncoupling in eyecup tissue, which may be particularly relevant in the biology of the eye, as retina tissue secretes succinate.

PurposeCharacterize how metabolic function in the murine retina and retinal pigment epithelium-choroid-sclera (eyecup) complex is impacted by natural aging.MethodsWe examined scotopic and photopic visual function of young (3-6 months) and aged (23-26 months) C57Bl/6J mice using electroretinograms (ERGs). Metabolic changes in retina and eyecup explants were characterized by measuring uptake and usage of U-13C-glucose or U-13C-glutamine at different timepoints by gas chromatography-mass spectrometry (GC-MS), measuring oxygen consumption rate (OCR) using a perifusion apparatus, and determining ATP levels with a bioluminescence assay.ResultsScotopic and photopic ERG responses declined in aged mice. Glucose metabolism, glutamine metabolism, OCR, and ATP pools in retinal explants were mostly unaffected by the age of the mouse. In eyecups, glutamine usage in the Krebs Cycle decreased while glucose metabolism, OCR, and ATP pools remained stable.ConclusionsThe ex vivo approach in our study to examine aging glucose and glutamine metabolism in retina and RPE showed negligible impact of age on retina and an impairment of glutamine anaplerosis in eyecups. The surprising metabolic stability of these tissues ex vivo suggests age-related metabolic alterations in these tissues may not be intrinsic. Future experiments should focus on determining whether external factors including nutrient supply, oxygen availability, or other structural changes influence ocular metabolism in vivo.