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B. S. Winkler, B. S. Twardy, M. M. Smith; The Battle of Substrates: A Test of the Metabolic Coupling/Trafficking Hypothesis in Retinal Neurons, Glia and RPE. Invest. Ophthalmol. Vis. Sci. 2009;50(13):4169.
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© ARVO (1962-2015); The Authors (2016-present)
Previous work in our laboratory demonstrated that cultured retinal neurons and glial cells preferentially use glucose when they are incubated in media containing a euglycemic level (5 mM) of glucose and 5 mM lactate. This result does not support the metabolic coupling/trafficking hypothesis that links the production of lactate in glial cells to its utilization in neurons. To further test this hypothesis, we have now examined substrate utilization when hypoglycemic concentrations of glucose are paired with 5 mM lactate. These conditions were chosen to simulate an ischemic-like episode.
Confluent cultures of mouse photoreceptors (661W), rat ganglion cells (RGC-5), rat glial (Müller, RMC-1) cells and human RPE cells were incubated for 3 hours in media containing varying low levels (0, 1, or 2 mM) of glucose in the presence and absence of 5 mM lactate (concentration at t=0). Measurements were made of the rate of appearance or disappearance of lactic acid in the media as a function of time. If cells produce lactate, then the concentration of lactate will rise in the media over time; if cells consume lactate, then the concentration of lactate will fall in the media over time. The patterns of LDH isoforms were determined in each cell type by 2-DG electrophoresis.
Neurons, glia and RPE cells produced lactate at linear rates when the incubation media contained 1 or 2 mM glucose as the sole exogenous substrate. Cells incubated in media containing 1 or 2 mM glucose and 5 mM lactate produced lactate at rates similar to those found when incubated with glucose alone. Net consumption of lactate was observed when cells were incubated in glucose-free media containing lactate, though the rate of consumption was a small fraction of the rate of lactate production seen when glucose was paired with lactate. LDH-5 (muscle type) was the dominant isoform in 661W and rMC-1 cells, but in the RPE isoforms other than LDH-5, including LDH-1 (heart type), were dominant.
We conclude that under hypoglycemic conditions retinal neurons, glial cells and the RPE show net utilization of glucose and net production of lactate. This pattern of substrate utilization is similar across cell types, despite differences in the electrophoretic patterns of LDH isoforms. When glucose is not available, the cells can use lactate as an energy source which helps them to survive the insult.
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